JOEL NUORA

ASSEMBLY LINE BALANCING FOR HIGH-MIX, LOW-VOLUME

PRODUCTION

Master’s Thesis

Examiners: Professor Miia Martinsuo and lecturer Ilkka Kouri Examiners and topic approved by the Faculty Council of the Faculty of Business and Built Environment on 6th March 2013

ii

ABSTRACT TAMPERE UNIVERSITY OF TECHNOLOGY Master’s Degree Programme in Industrial Engineering and Management NUORA, JOEL: Assembly line balancing for high- mix, low-volume production Master of Science Thesis, 93 pages, 1 Appendix page November 2013 Major: Industrial Management Examiners: Professor Miia Martinsuo, lecturer Ilkka Kouri Keywords: Assembly line balancing, production scheduling, high-mix, low-volume, takt time, production levelling This master’s thesis presents assembly line balancing methods, which aim to

improve continuous material flow in a variable environment. The most important

purpose of assembly line balancing is to continuously equalize the workload

between employees. Moreover, aspects related to production scheduling and

control methods for high-mix, low volume assembly lines are also discussed in

this work. The thesis is made as an action research so that available infor-

mation from literature is used and evaluated with the viewpoint of the needs and

problems of the case company. The goal of this thesis is to improve the produc-

tivity of the assembly line of power series hooklifts, with balancing methods and

a more organised production scheduling system.

Nine different assembly line balancing methods are presented, which are all

applied for the operation in the case company to improve the flow of materials.

The most significant method is the conventional way to first divide the total

workload to workstations as equally as possible and then allocate employees

based on average standard times. The first balancing action provides a good

starting point for the application of the methods which focus more on variable

standard times. The other balancing methods include multi-skilled workforce,

pre-assembly stations, different routings, production levelling, in-process inven-

tory, work time arrangements, task assignment variations and waste elimination

from bottleneck stations.

As a result of this thesis standard times of work tasks are used systemically for

assembly line balancing and production scheduling. Applying assembly line

balancing methods has equalized the workloads between employees, de-

creased waiting times and provided a good potential for productivity improve-

ment. For production scheduling the thesis presents a plan based on production

rate oriented system that aims at a more precise target setting. Related to the

scheduling system, a new visual assembly control system has been taken in

use, which has significantly improved target setting practices and real time pro-

duction control.

iii

TIIVISTELMÄ

TAMPEREEN TEKNILLINEN YLIOPISTO Tuotantotalouden koulutusohjelma NUORA, JOEL: Kokoonpanolinjan tasapainottaminen varioivassa ja matalan volyymin tuotannossa Diplomityö, 93 sivua, 1 liitesivu Marraskuu 2013 Pääaine: Teollisuustalous Tarkastajat: professori Miia Martinsuo, lehtori Ilkka Kouri Avainsanat: Tuotannon tasapainottaminen, tahtiaika, hienokuormitus, varioiva kokoonpano Diplomityö esittää kokoonpanolinjan tasapainottamismenetelmiä, joiden pää-

määränä on tarjota paremmat edellytykset materiaalien tasaiselle virtaukselle

varioivassa tuotannossa. Kokoonpanolinjan tasapainottamisen tärkeimpänä

tarkoituksena on jakaa työkuorma jatkuvasti tasaisesti työntekijöiden kesken.

Työssä käsitellään myös hienokuormitukseen liittyviä käsitteitä sekä kontrolloin-

timenetelmiä varioivalle ja matalan volyymin kokoonpanolinjalle. Työ tehdään

toimintatutkimuksena, jossa kirjallisuudesta löytyvää tietoa pyritään hyödyntä-

mään sekä arvioimaan kohdeyrityksen tarpeiden ja ongelmien kautta. Työn ta-

voitteena on parantaa vaihtolavalaitteiden kokoonpanolinjan tuottavuutta tasa-

painotusmenetelmien ja järjestelmällisemmän hienokuormituksen avulla.

Työssä esitetään yhdeksän erilaista tuotannon tasapainotusmenetelmää, joita

kaikkia on sovellettu kohdeyrityksen toimintaan asennuslinjan tasaisen virtauk-

sen edistämiseksi. Merkittävimpänä tasapainotusmenetelmänä voidaan pitää

tavanomaista tapaa jakaa ensin työmäärät keskiarvojen mukaan mahdollisim-

man tasaisesti työpisteille, minkä jälkeen työntekijät sijoitetaan eri työpisteisiin

standardiaikojen keskiarvojen mukaan. Tämä ensimmäinen toimenpide antaa

hyvän lähtökohdan muiden enemmän varioivan tuotannon huomioon ottavien

menetelmien soveltamiselle. Muut esitetyt menetelmät ovat monitaitoiset työn-

tekijät, esiasennus, vaihtoehtoiset reititykset, työjonon tasapainotus, välivaras-

tot, liikkuvat työtehtävät, työaikajärjestelyt sekä pullonkaulatyöpisteiden kehitys.

Työn tuloksena kohdeyrityksen kokoonpanolinjan työvaiheiden standardiaikoja

käytetään järjestelmällisesti kokoonpanolinjan tasapainottamisessa ja hieno-

kuormituksessa. Tasapainotusmenetelmien soveltaminen on tasoittanut kuormi-

tuksia työntekijöiden välillä, vähentänyt odotusaikoja ja tarjonnut edellytykset

tuottavuuden parantamiselle. Hienokuormituksen osalta tuloksena on suunni-

telma laitemääriin perustuvasta tavoitteenasettelusta, jolla pyritään tarkempaan

tuotannon ajoitukseen. Tähän liittyen kohdeyritykselle on myös laadittu uusi vi-

suaalinen asennuksenohjausjärjestelmä, jonka avulla tavoitteenasettelua ja re-

aaliaikaista tuotannonohjausta on pystytty parantamaan huomattavasti.

iv

ACKNOWLEDGEMENTS

This master’s thesis is made for Cargotec Finland Oy, Multilift Raisio factory, in

collaboration with Tampere University of Technology, Department of Industrial

Engineering. I would like to express my gratitude to my thesis supervisors Ilkka

Kouri and Miia Martinsuo for the guidance with the project.

I would also like to thank the company for this very motivating and interesting

project. I am deeply grateful for the assistance given to me by the whole Raisio

factory personnel and it has been a privilege working with you. Special thanks

for Esko Kleemola, Asko Nevalainen and Seppo Kantola for the support and the

discussions related to the development actions of this thesis work.

Finally I would like to thank my family and friends for their encouragement and

support through the whole studentship.

Turku, 25.10.2013

Joel Nuora

v

CONTENTS

1 Introduction .................................................................................................. 1

1.1 Foreword .............................................................................................. 1

1.2 Objectives and scope ........................................................................... 2

1.3 Methodologies ...................................................................................... 3

1.4 Company presentation .......................................................................... 5

2 Assembly line balancing and control ............................................................ 7

2.1 Definition and purpose of assembly line balancing ............................... 7

2.2 Assembly line balancing key terminology ............................................. 9

2.2.1 Time standards ........................................................................ 10

2.2.2 Production scheduling ............................................................. 11

2.2.3 Takt time and production rate .................................................. 12

2.3 Assembly line balancing for variable environment .............................. 16

2.4 Assembly line balancing methods ...................................................... 19

2.4.1 Assembly line balancing based on average station times ....... 19

2.4.2 Flexible multi-skilled workforce ................................................ 24

2.4.3 Pre-assembly for optional modules ......................................... 27

2.4.4 Different routings for variable products .................................... 28

2.4.5 Sequence planning to level out the workload .......................... 30

2.4.6 In-process inventory to avoid idle time .................................... 33

2.4.7 Assignment of identical tasks to different stations ................... 34

2.4.8 Work time arrangements ......................................................... 35

2.4.9 Continuous improvement of current bottleneck station ............ 36

2.5 Synthesis of assembly line balancing for high-mix, low-volume

production .................................................................................................. 38

3 Analysis of demountables assembly line ................................................... 41

3.1 Production system in the case company ............................................ 41

3.2 Work analysis ..................................................................................... 46

3.3 Interview analysis ............................................................................... 48

3.4 Assembly line time study .................................................................... 49

4 Demountables assembly line development................................................ 53

4.1 Assembly line balancing ..................................................................... 53

4.1.1 Assembly line balancing based on average station times ....... 53

4.1.2 Flexible multi-skilled workforce ................................................ 55

4.1.3 Pre-assembly for optional modules ......................................... 58

4.1.4 Different routings for variable products .................................... 59

4.1.5 Work queue levelling ............................................................... 61

4.1.6 In-process inventory to avoid idle time .................................... 63

4.1.7 Assignment of identical tasks to different stations ................... 64

4.1.8 Work time arrangements ......................................................... 65

4.1.9 Continuous improvement of current bottleneck station ............ 66

vi

4.2 Production control ............................................................................... 67

4.2.1 Production scheduling and target setting ................................. 67

4.2.2 Visual management tools for production control ...................... 68

4.2.3 Restrictions and problem solving ............................................. 71

5 Testing and implementation ....................................................................... 74

5.1 Implementation of assembly line balancing methods.......................... 74

5.1.1 Average load percentage towards ideal situation .................... 75

5.1.2 Increased use of multi-skilled employees ................................ 76

5.1.3 New pre-assembly station ....................................................... 77

5.1.4 Different routings for complex products ................................... 78

5.1.5 Sequence planning to support production flow ........................ 79

5.1.6 More detailed in-process inventory planning ........................... 79

5.1.7 Flexible tasks between workstations ....................................... 80

5.1.8 Change to one shift system ..................................................... 80

5.1.9 Problem solving and 5S for bottleneck stations ....................... 81

5.2 Implementation of new production scheduling system ....................... 82

6 Discussion ................................................................................................. 85

6.1 Result analysis ................................................................................... 85

6.2 Subjects for further studies ................................................................. 87

6.3 Conclusions ........................................................................................ 88

7 References ................................................................................................ 90

Appendix 1: Example of time and period dependent variances in a high-mix

production

Appendix 2: Product and workstation dependent variances in the power series

demountable assembly line

Appendix 3: Power series demountables assembly line balancing actions

Appendix 4: Power series demountables assembly line balancing calculations

with average target times

Appendix 5: Productivity of the demountable assembly line during 2013

Appendix 6: MAU Raisio value stream map

Appendix 7: Interview

Appendix 8: Temporary production scheduling system

vii

TERMS AND DEFINITIONS

Cycle time The operation time required to complete one process

in the value stream.

Station time The cycle time of one workstation, which is a sum of

task times based on product specifications.

Total cycle time The sum of all cycle times in a process from the

scope’s first station to the completion of the scope’s

last station of the scope.

Lead time The total amount of time elapsed from the start of the

first phase to the completion of last station.

Takt time The amount of time between two consecutive unit

completions in order to exactly meet the demand.

Formula for calculating the takt time: available produc-

tion time divided by demand.

Planned cycle time The amount of time between two consecutive unit

completions in order to meet the demand, taking into

account unplanned downtime or problems with allow-

ance time.

Production rate The number of completed units or throughput of an

assembly line, which is an inverse ratio for takt time

for same or longer period.

Takt-driven system Aims to synchronous movement of units using takt

time based scheduling.

Production rate The number of completed units during a predeter-

oriented system mined period used as a primary scheduling criteria.

1

1 INTRODUCTION

In this thesis assembly line balancing methods are studied and evaluated for

variable environments. The objective is to find solutions for creating a smooth

and organised production flow. This is a big challenge for high-mix assembly

lines. Assembly line balancing is directly connected to productivity and efficien-

cy of the operation by reducing work overloads and idle time. The aim of this

introductory chapter is to presents the purpose of this thesis, the research

method and the case company of the study

1.1 Foreword

Since the times of Henry Ford’s conveyor-based mass production to today’s

more flexible assembly systems, assembly lines have been an active field of

research. The first assembly line balancing related studies were made in the

1950s and the core idea was only to assign tasks equally to workstations. For

several decades the research concentrated on these simple assembly line bal-

ancing problems, which have many restricting assumptions making them appli-

cable only for single model assembly lines. Today’s more complex product re-

quirements and more variable assembly systems require also more extensions

for assembly line balancing. More research has been recently conducted to

solve more realistic and variable balancing problems. However, there is still a

clear gap between theories and practice, because studies often take into ac-

count only a single or just a few extensions for assembly line balancing prob-

lems. Real-world variable assembly systems require a lot of these extensions in

a combined manner. Thus, there is a need for more flexible assembly line bal-

ancing practices that are applicable for various kinds of flexible assembly lines.

(Boysen et al, 2008; Becker & Scholl 2006)

This thesis will concentrate on the assembly line balancing problems of a real-

world high-mix, low-volume environment. The work is made for Cargotec Fin-

land Oy Multilift factory in Raisio, by focusing on a demountables mixed-model

assembly line. The idea is to study many different balancing methods simulta-

neously first as an alternative development ideas and then in practice. The sec-

ond chapter will concentrate on production balancing and scheduling theories,

which are related to variable low-volume type production needs. Chapters 3, 4

and 5 concentrate on the practical side of this work by introducing the current

situation, the implementation plan as well as implemented development actions.

2

The final chapter focuses on the theoretical and practical views from a com-

parative angle and also presents the conclusions of the thesis.

1.2 Objectives and scope

The main objective of this master’s thesis is to plan alternative solution ideas for

assembly line balancing in high-mix, low-volume production at the Raisio facto-

ry. Based on the balancing related study also production scheduling for the de-

mountable assembly line is analysed. The theory part is mainly focused on

build-to-order type of production in a variable environment. It also examines the

production of complete equipment rather than individual parts. These kinds of

production environments are normally very customer oriented and can be found

in business such as industrial machines, trucks or airplanes.

The research question is: How can the Hiab Raisio factory create a balanced

and organised material flow in a low-volume and high-mix type of demountable

machine assembly line? For Raisio demountable factory the main goals are to

improve productivity and shorten the lead time in assembly line. The objective is

to create a smooth, well planned and organized production flow. There are no

ready-made solutions or proposals for line balancing or takt time, so a master’s

thesis study on this topic is needed. The sub-objectives for balancing are to

minimize waiting times in demountable assembly line and to create a clear tar-

get system which would be based on standard times. The target system will

require visual management improvements and some clarification for production

planning. Other fundamental aims are to increase the overall Lean manufactur-

ing awareness among the employees and to emphasize the importance of elim-

ination of non-value added activities from demountables production.

The development work is reconfiguration of the already existing production sys-

tem rather than developing totally new assembly line. The scope of this thesis is

assembly line of power series demountables from the output area of the paint

shop to the final workstation before testing. Subassemblies are also covered,

because they work with the same pace with the main assembly line. The scope

of the thesis is also presented in the assembly line flowchart in figure 3.2 with

bolded workstations. Development of the outsourced paint shop is left out of the

scope because it does not follow the same production system with the assem-

bly line, and because work time arrangements are not the same. Pipe bending,

which is made as a pre-assembly, is also not covered due to its batch type of

production and different scheduling periods. Final testing is not in the scope,

because its scheduling is based more on quality problems, delivery times and

current product mix of all demountables, rather than standard times of power

series hooklifts. The main focus is on material flow within the assembly line,

3

while inbound and outbound material logistics are studied only in case of re-

strictions, problem solving and production scheduling. Employee engagement

and change management are closely related to the development of assembly

line, but they are not deeply discussed in this thesis. For example in balance

calculations all employees are perceived to have the same competence, moti-

vation and capacity regarding to workloads, which does not reflect to real world

assembly work. In case of standard times the scope is to only use available ma-

terial from ERP system and not to make any detailed stop-watch time studies.

There are no complex mathematical formulations or algorithms in the thesis that

exist in many assembly line balancing theories. It was acknowledged that the

source data is not reliable enough for that kind of statistical research and the

production system is too flexible for very accurate calculations. The idea was to

get a rough balance situation by recognizing the assembly line bottleneck and

other production flow restrictions.

1.3 Methodologies

The methodology of this thesis is an action research, which is aimed at to use

appropriate knowledge to improve practices in an organisation’s context.

Throughout the project, theories from assembly line planning related literature

were used to support decisions in balancing and controlling activities for de-

mountable assembly line. Figure 1.1 illustrates the methodology of the project,

which is also used as a structure of the thesis. The first phase is a development

of different alternative ideas to solve the research question. Ideas are generated

through the literature review and an analysis of the current situation. In the next

phase, these ideas are evaluated with empirical data and logical thinking, which

will result in an implementation plan with selected alternative ideas. In testing

and piloting the plan is implemented in practice for demountable assembly line

and the consequences of different changes are analysed. Finally the results and

empirical work are compared to the literature review in the framework of the

discussion chapter.

Figure 1.1. Methodology of the thesis.

4

The analysis of the current situation was made with empirical participative ob-

servation, interviews, data collection and daily discussions with personnel. The

analysis of the production line was made at the beginning of the year 2013.

The idea behind the observation period was to learn the assembly line better, to

get to know the employees and to find development ideas for production. The

observation was conducted through a two-day hands-on line-work and daily

visits in the assembly area. The findings were first listed as notes, which were

used as checklist for comprehensive report of current situation analysis. Waiting

and idle time were detected to be the most significant inefficiencies of the as-

sembly work and it emphasizes the need of this assembly line balancing work.

Data collection was the biggest part of the empirical work done for this thesis. In

the analysis of the current situation the most important task was to determine

the workstation balance situation by dividing target times to workstations and

calculating of capacity requirements. The data source was the ERP -system

and the work was mainly done through Spreadsheet software calculations. The

source data included total current order book of highly variable power series

demountables. The main analysed factors where cycle times of each work-

station and the differences of standard times between products.

The development project for balancing and production scheduling was made

during spring 2013. The most critical issues were recognized based on the

analysis of current situation. Action plans were planned through meetings, a few

trainings and various tests within the assembly line. There were meetings held

for definition of target times, sequence planning, visual management and gen-

eral development meetings of factory’s lean team. The actual changes in the

assembly were made together with employees, supervisors and managers.

Small changes were usually based on statistical data and discussions with dif-

ferent responsible persons. The test weeks were based on the changes in the

assembly line balancing, but concentrated more on new production scheduling

and a target setting system. The development work was documented mainly to

weekly report made by the author of this thesis. The report included information

of the results previous week, completed hours, productivity, differences com-

pared to targets, report of different changes and author’s opinion of next short-

term development objects. The overall idea of the development project is to

create a plan for future ways of working and it is not aimed implementing all the

changes presented during the thesis project. The most significant assembly line

balancing and controlling actions are made in the long term after having been

well planned, tested and all consequences are recognized. The thesis will pro-

vide an analysis of current situation, a study of the subject, balancing methods,

as well as the first steps in implementing changes. The purpose is also to create

an environment for continuous improvement for assembly line balancing.

5

1.4 Company presentation

This work has been done for Hiab’s Raisio factory, where Hiab Multilift de-

mountables are assembled, designed and managed. The roots of Multilift are

already in the year 1947, when Terho brothers patented demountable working

with cables. This cable lift enabled the founding of the Raisio Multilift factory in

1961 and it is currently the only production facility of Cargotec in Finland. The

Multilift brand name has gone through many acquisitions and owners. It was

first bought by Sponsor Oy in 1968, followed by Partek in 1977. In year 2002

Kone Oyj acquired Partek and made Cargotec as one of its business area for

load handling solutions. Cargotec Corporation demerged from Kone and be-

came an independent stock listed company in 2005. (Teräväinen 2005)

Cargotec improves the efficiency of cargo flows around the world in over 120

countries with an extensive product portfolio. Cargotec’s turnover was 3.3 billion

euros and the average number of personnel was 10 500 in 2012. Cargotec is

composed of three well-known brands MacGregor, Kalmar and Hiab which are

now working as individual business areas. This work is done for Hiab business

area of which sales was 840 million euro with 3038 people in 2012. Hiab pro-

vides different on-road load handling solutions for various transport and delivery

sectors. Its offering contains loader cranes, forestry and recycling cranes, truck

mounted forklifts, tail lifts and demountables. Hiab products are used, for in-

stance, on construction sites, forestry, warehousing, waste handling as well as

by the Defence forces. (Cargotec Oyj, 2013a 3, p.73)

Figure 1.2. Hiab Multilift S-model.

6

Demountables are now sold as Hiab products and Multilift is regarded as a well-

known product name for global market leader demountable solutions. The core

idea of Hiab Multilift demountables is that the truck can be driving all the time

and carry out multiple tasks because containers can be loaded and unloaded

separately. Demountables are used, for example, in waste handling and recy-

cling businesses as well as by fire brigades and defence applications.

(Teräväinen 2005, p.19)

There are three different product families of demountable products: hooklifts

(figure 1.2), cablelifts, and skip loaders. Hooklift is the most important Multilift

product family and it is divided into power series, small hooks and special prod-

ucts. The scope in this thesis is assembly line of power series hooklifts. All the

products are designed modular and assembled from options chosen by cus-

tomers so that there can be thousands of different kinds of variations of hook-

lifts. There is an assembly line for power series hooklifts and assembly cells for

small hooklifts, cablelifts and defence products. Today all welding and part

manufacturing is made by suppliers and Raisio factory only assembles the de-

mountables. More detailed presentation of the demountables production system

is in chapter 3.1.

7

2 ASSEMBLY LINE BALANCING AND CON-

TROL

Assembly line balancing and production balancing are not totally unequivocal

terms, because they are presented in at least in three different kinds of con-

cepts. The most common viewpoint is to balance the speed and volumes of the

production to meet customer demand as closely as possible. Another very

common perspective for balancing is the workload balance based on a certain

time period, which is also called production levelling or known through the Jap-

anese term heijunka. However, in this study production or assembly line balanc-

ing means process design for workloads between assembly line workstations

and employees. The core purpose is to equalize the amount of work between

employees and to improve material flow in the assembly line. In this chapter

there is first an introduction to assembly line balancing and its purposes. After

that the assembly line key terminology and concepts related to production

scheduling are presented. The final sections focus on assembly line balancing

methods and solution ideas for high-mix low-volume environment.

2.1 Definition and purpose of assembly line balancing

An assembly line is a flow-oriented production system where the productive

units performing the operations, referred to as stations, are aligned in a serial

manner. The workpieces visit stations successively as they are moved along

the line. Assembly line balancing was first introduced by Salveson in 1955 in his

pioneer work where production design problems were analysed with prece-

dence graphs and a planned cycle time together with a mathematical formula-

tion. The assembly line balancing problem consists in determining a set of tasks

for every workstation so that precedence relation requirements between single

tasks are not violated and operation time does not exceed the planned cycle

time. In a classical time-oriented assembly line balancing the objective is to min-

imise the manpower needed to assemble one product and the number of sta-

tions which also leads to minimal idle time. (Salveson 1955; Baybards 1986)

Assembly line balancing consists of scheduling and controlling the production in

order to meet the required production rate and to achieve a minimum amount of

idle time. In assembly line balancing all tasks are assigned to workstations so

8

that each station has approximately same amount of work at all times. An un-

balanced line may lead to overburden in some stations, high variation in output,

waiting times and poor efficiency. Instead, well balanced assembly line has to-

tally opposite effects and it promotes a one piece flow for the assembly line.

(Konnully 2013)

The purposes of assembly line balancing are to:

Equalize the workload among the assemblers

Establish the speed of the assembly line

Identify the bottleneck operation

Assist in plant layout

Determine the number of workstations

Determine the labour cost of assembly

Establish the percentage workload of each operator

Reduce production cost. (Stephens & Mayers 2010, p.111)

The most important objective of assembly line balancing is to give each opera-

tor as close to the same amount of work as possible. The workstation with the

largest time requirement is designated to be 100% workstation and is the limit of

output of assembly line. The station is a bottleneck station and it should be the

first priority for development actions. Through a well-balanced assembly line

idle time is minimized and a continuous production is enabled. This leads to a

better productivity of the assembly line. Also speed of the assembly line is a

consequence of balancing calculations, because the amount of workstations

and workers influence on cycle time, which determines the speed of production.

(Stephens & Mayers. 2010, p.111)

Production balancing requires a lot of calculations of production related indica-

tors like cycle times, lead times, standard times and resources. The inputs for

assembly line balancing problems are precedence constraints based on product

and time requirements. These elements can be visualized with precedence

graphs, which contain a node for each task of the assembly system. Figure 2.1

shows a precedence diagram for 10 tasks having task times between 1 and 10

time units. Nodes weight for task times and lines for the sequence constraints.

In this example the precedence constraints require tasks 1 and 4 to be com-

pleted before processing task 5. The tasks are assigned to different stations as

equally as possible so that precedence and capacity constraints are fulfilled at

all times. (Becker & Scholl 2006, p.695)

9

Figure 2.1. Precedence graph. (Becker & Scholl 2006, p.695)

Production balancing may often influence to the number of workstations and

layout changes. This is more common in mass-production type of assembly

where operations are planned in seconds and where there is only one worker

per station, whereas in low-volume production issues related to space and prob-

lem solving, among others, can lead to changes. The assembly line balance

situation is normally visualized through column charts. These charts represent

the differences between workloads between workstations and they are used as

a main visualization tool in this thesis. Examples of column charts can be found

in figures 2.3, 2.4, and 2.6.

The final listed purpose of assembly line balancing is to reduce production

costs. The main improvement comes from the equalized workload, because the

non-productive idle and waiting times are used for assembly work instead. This

leads to a better productivity because the available time is used more effectively

to standard times instead of waiting. The cost savings gained through a better

productivity thus come from more standard hours sold or reduced number of

employees. Another perspective, alternative for the usual time-oriented balanc-

ing, is called cost-oriented assembly line balancing. The objective of this ap-

proach is to minimise the unit costs by giving a value for each task and then

minimise labour and capital costs by reducing idle time by prioritizing most ex-

pensive tasks (Amen 2006, p.749).

2.2 Assembly line balancing key terminology

In this chapter different assembly line control methods are analysed briefly for

high-mix, low-volume assembly line. The definitions for standard time, cycle

time, lead time and other main production scheduling terms are explained brief-

ly to avoid misunderstandings. The terms are presented because they are nec-

essary for production balancing which is the main subject of this thesis. This

chapter also assesses the possibilities and readiness to implement a takt time –

based production system for demountable assembly line which was the initial

vision in the beginning of thesis project. The focus will then be on comparison of

10

takt-driven and production rate oriented system which will be defined and ana-

lysed focusing on high-mix, low-volume assembly lines.

2.2.1 Time standards

Time standards have many informational purposes in an organisation. They are

the most basic yet very important sources for production planning, cost alloca-

tion and control, inventory management, performance evaluation, incentive

pays and decisions for alternative methods of operation. The main idea of time

standards is to determine how much time it takes to conduct one operation. For

a facilities planner, the standard time is the primary input for determining the

required resources and capacities to meet the production schedule. Time

standards are also the main source for assembly line balancing. (Stephens &

Mayers 2010, p.51)

Cycle time is the time required to complete one process in a value stream or the

time between two discrete units of production. Cycle time alone describes the

time in one workstation and this time can also be called station time. In the de-

mountable production the definition for station time is also station’s target time.

In this thesis, the total cycle time refers in this thesis to the operation time of all

stations from the first assembly station to the last phase including all pre- and

subassemblies. Planned cycle time shown also in figure 2.4 is the desired sta-

tion time, which is usually higher than real cycle time and lower than demand

rate. The difference between the planned cycle time and the station time can be

perceived to be idle time, waiting or slowed pace of work (Rother 2013).

Productivity is a measure of output divided by input and the sources can be ei-

ther number of units or earned hours. Number of units produced per period can

be good indicators for plant or whole industries but not for smaller divisions.

Therefore, without time standards it is impossible to calculate productivity for

individuals in a reliable way, especially in variable environments. Already in the

1980’s it was discovered in a 400 plant study that an operation that is not work-

ing towards time standards typically works only 60% of time. Those operations

working with time standards work at 85% of time. In a plant of 100 people this

improvement equals to 41 extra people, or about million dollars per year in sav-

ings. (Stephens & Mayers. 2010, p.62) More recent outlook from Greg Lane

(2007) suggests a productivity increase from 10 to 15 per cent if time is associ-

ated with all work and if it is visually compared to actual time.

11

2.2.2 Production scheduling

The production planning and control function of an organisation is responsible

for ensuring that production activities are as efficient as possible. Its purpose is

to find the best and the cheapest methods to produce the required quantity and

quality at the right time. Production planning is the choice from several alterna-

tives how to utilise the resources available to achieve the desired objectives.

Control is monitoring performance by comparing the results achieved with the

planned targets so that operations can be improved through proper corrective

actions. (Aswathappa & Shridharabhat 2009, p.208)

The purpose of production scheduling is to make a detailed plan for the produc-

tion processes. The basis for production scheduling is the longer term rough cut

planning. Planning the schedule for different tasks requires the knowledge of

standard times and of the current situation in production. The timeframe for pro-

duction scheduling is normally kept as short as possible which typically means

from one week to one day. With a short timeframe it is possible to get more

specific information and reliable plan. Good delivery accuracy and high produc-

tivity are common goals of production scheduling. (Haverila et al. 2009, p.417)

In a lean environment, the production control department plays an absolutely

vital role and it is responsible for very detailed planning. It includes capacity

planning down to a process level. Getting all the right parts to the right point on

time is probably the biggest issue. Production planning department should

make a daily or an hourly plan for each process and compare them with pro-

cess capabilities and realization. (Lane 2007, p.46)

All workstations should have a schedule of what will be occurring during the

day. In high-mix, low-volume environment, where cycle times are normally cal-

culated in several minutes, standard times may not be particularly precise. Cy-

cle times must be close but not necessarily exact. For example 410 minutes can

be counted as seven hours. A continuous updating of standard times is neces-

sary in order to ensure reliability of assembly line balance calculation and prod-

uct costing. (Larco et al. 2008, 74, p.106)

The production planning for different phases in assembly can be done with

backwards or forwards scheduling. Frontwards scheduling starts from the start-

ing time of production and when resources become available to determine due

date. The starting time of the second phase is calculated by adding the time

required to complete the first phase. The next phases are scheduled with the

same system until all phases and the finishing time is calculated. Backwards

scheduling starts from the planned due date so that the starting time of the final

12

phase is calculated backwards in time. The same system is used to calculate

the beginning time of the second last phase and then finally continued to the

first phase. This is the most common system in production planning programs.

(Haverila et al. 2009, p.419)

There are various different charts and tables to visually manage production

schedules. The most popular tool to display schedules is the Gantt chart, which

is used to graphically display the workloads of each work centre. There are two

types of Gantt charts: the workload chart as well as the scheduling chart. In

both charts time elapses on vertical axis. In the Gantt workload chart the hori-

zontal axis shows the amount of work while the vertical bars depict workloads

for different periods. In Gantt scheduling chart different workload groups are on

the vertical axel and tasks are shown with different colours with horizontal bars,

which length depicts time required to complete the phase. (Aswathappa &

Shridharabhat 2009, p.312)

Computer systems are the best for monitoring production control, because as

the data is available as soon as it is entered to the system. Old fashion cards

are slow in comparison and they are subjects to even more errors (Larco 2004,

p.108). The programs that are used for production scheduling are based on dif-

ferent kinds of algorithms that will solve optimisation problems and generate

alternative plans, which are used to support the final decisions made by the

planner. (Haverila et al. 2009, p.419)

2.2.3 Takt time and production rate

Takt is a German word meaning a musical beat, stroke of an engine or a regular

rhythm. These are natural extensions to think of takt time as the time between

beats of the pace of production. Takt time is the average amount of time that

must be elapsed between the completions of two units in order to meet the de-

mand. A takt time based system is transferred also as paced production in

many references which mean that the all stations have common cycle time. This

time matches to the rate of how customers require finished units. This pace is

calculated with demand and net available production time, which means the

working time without breaks. (Baudin 2002, p.42)

Takt time can be likened to conductor’s baton keeps the orchestra in synchro-

nized order (Rother & Harris 2001, p.13). Liker (2004, p.94) compares takt time

to the heart beat of one-piece flow or the person in key position of coxswain

13

coordinating the pace for rowing so that any rower would not under or underper-

form. Analogy of takt time for high-mix products can be compared to chairlift

system presented in figure 2.2 where the workload can be different but the time

between chairs is constant. If there is a heavy load the lift just needs more pow-

er but the frequency will not be affected (Baudin 2002, p.43).

Figure 2.2. The chairlift analogy for takt time in mixed-flow line (Baudin 2002,

p.43)

Takt time provides a good picture of customer demand over a period of time.

The customer takt should be reviewed for example every two weeks because of

demand changes. Effective operation time is calculated by subtracting breaks

and planned downtime from the total available time. When net available time is

divided by the demand for the same period the result is takt time. Takt time itself

is not enough for production scheduling and to be used for cycle time because

there are always problems occurring in production. That is why production is

scheduled for planned cycle time, which is the desired pace of the production.

Planned cycle time is faster than takt time because it accommodates changeo-

vers, downtime and possibly some other non-value added activities. (Rother

2013, p.18)

Lane (2007) calls takt time as pure takt time and planned cycle time as actual

takt time. In actual takt time the basis for calculations is the overall equipment

effectiveness rate. It is more preferred in part manufacturing rather than assem-

bly, but the system is the same. The actual takt time should be compared to

standard times and cycle times for each task. The result is usually showed with

assembly line balancing graphs which are discussed in the next chapter. Takt

time, planned cycle time and standard times are used for production scheduling

to plan activities as efficiently as possible. However, takt time based production

scheduling cannot be applied to all assembly line environments. In low-volume

14

build to order environment, where processes are managed rather with day-by-

hour boards or Gantt’s scheduling charts, takt time is not used. (Lane 2007,

p.36)

The takt time allows defining an ideal state for production one-piece flow with

exactly matching station times. This ideal state can be called as takt-driven pro-

duction, where all deviations are translated to different inefficiencies or wastes.

In takt-driven production takt time gives the direction for operation, but in real-

world assembly lines it is never perfectly realized. Time per demand calculation

is the way to calculate takt time, but it does not tell the rules of how to use the

number or how it maps to shop floor. Takt-driven operation is not relevant for

example in business with non-repetitive operations, where it becomes more dif-

ficult to balance the work among stations with broaden mix of products. In many

production plants the inverse ratio is used which will give the same information

with production rate over a period. Demand per time calculation gives mathe-

matically the equivalent result, but the shop floor operation may be totally differ-

ent. Working at a takt time of 1 minute and making 60 units per hour gives the

same throughput during an hour, but the scheduling system may differentiate

significantly. In terms of units per hour it does not matter if nothing comes out

for the first 59 minutes of an hour as long as all 60 units are completed in the

end. In takt-driven operation unit will come out every minute according to

planned cycle time. (Baudin 2012)

As introduced, the alternative approach for takt-driven operation is to concen-

trate on completed units over a predetermined period. This system does not

have well-established definition and it is called with many different terms like

production rate -oriented system, takt rate -system or throughput -oriented pro-

duction planning. In this thesis the approach is called with production rate ori-

ented system. It is not paced production because the time between two prod-

ucts are completed can fluctuate. Production rate for certain predetermined pe-

riod is much more flexible in variable assembly compared to takt time, because

different products take different time to be completed. Production rate -oriented

system will smooth difficulties in capacity allocation because the requirements

can be divided for longer timeframes than in takt-driven operation.

Production rate or using day-by-hour boards is good especially in shared pro-

cesses where work is done without a solid forecast. The rate and schedule will

serve as clear targets for assembly for a certain period when all different pro-

jects should be completed. Standard times and available capacity are used in

target setting for the rates. The system will help in capacity planning because it

is easier to see where production is late when compared to the targets. The cur-

rent status can be visualized versus plans and ability to prioritize different tasks

15

will increase. With good plans, targets and visualization the current imbalance is

indicated clearly and it is easier to make corrective actions faster. A clear

schedule will also encourage operators to list problems that cause delays.

(Lane 2007, p.36)

In production rate oriented system cycle time is not always the same for all sta-

tions so the control system is normally unpaced. The system can be either un-

paced asynchronous or unpaced synchronous. In asynchronous movement the

products are transferred forward to other works station as soon as they are

completed. In order to balance workloads buffers are needed to avoid waiting

times. Under synchronous system all stations would wait for the slowest station

to finish before the work pieces are transferred. This will cause waiting times but

buffers are not necessary (Boysen et al. 2008, p.8).

The target production rate is calculated based on demand for certain time peri-

od. Takt time calculations may support the scheduling decisions but are not di-

rectly used because of variable product cycle times. The period for the rate is

decided based on product specifications and the required accuracy of plans.

The minimum for the period is planned cycle time of one product which is then

practically the same than takt time based production. The period can also be

the average cycle time to assemble two products. The normal system is to plan

the rate for a longer period such as half a day, day or even a week.

Figure 2.3. Comparison of takt time and production rate based systems with

variable cycle times.

Comparison of takt- driven and production rate -oriented systems20 units, 8h production , one station, cycle times vary from 12min to 28min, 15% of allowance time

0

5

10

15

20

25

30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

min

units

0

50

100

150

200

250

Period 1 (1. half day) Period 2 (2. half day)

min

10 units

TAKT-DRIVEN

Takt time = 480min / 20 = 24min

Planned cycle time = (1-0,15) x 24min = 20,4min

PRODUCTION RATE ORIENTED

Production rate for half a day periods =>

10 units per 240min period

Actual period time = (1-0,15) x 240= 204 min

cycle times for each product

16

Figure 2.3 is an example of a situation of one station’s work for 8 hours. The

target is to complete 20 products, which is the demand of the period for the sta-

tion. There are both takt-driven and production rate oriented systems illustrated

in a high-mix, low-volume production where cycle times vary significantly from

12 minutes to 29 minutes. In both cases the amount of work is the same 380

minutes which is 100 minutes less than 8 hours. In both cases this 15% allow-

ance percentage can be subtracted from the available time to get the planned

cycle time or actual scheduled period time.

In takt-driven production all the cycle times should be as equal as possible but

in variable production it is not necessarily possible. In the example many prod-

ucts cross the planned cycle time and also takt time. In these products more

resources or better productivity is needed to reach takt time. Additionally in-

process inventory can also be used to even out the workload so that the next

stations do not need to wait for products. There is much more unevenness cre-

ated if variable cycle times are tried to fit to the takt-driven system without very

detailed scheduling. In the production rate oriented system these variable fac-

tors are divided into longer periods when the workload seems to be much more

even and short term balancing problems are avoided. The operator only needs

to complete all the required parts during predetermined period while the sched-

uler is responsible that the total cycle time fits to the demand and allowance

rates. In the product rate oriented system product variances fade because of

longer time periods and because it is much easier to reach the targets. In both

systems it is important to aim at to decrease variances in station times and

there are different methods presented in chapter 2.4 for this purpose.

2.3 Assembly line balancing for variable environment

Originally assembly lines were developed for a cost effective mass production

of standardized products and it was also the focus on production planning relat-

ed literature. Since the first mathematical formulation of assembly line balancing

by Salveson 1955 the research focused for many decades on the core problem

to assign tasks to different stations evenly. This was usually done with numer-

ous simplifying assumptions which can only be generalized to mass-production

environment of homogeneous products. When the results were tried to apply in

real world production systems it was understood that product requirements do

not often reflect with the assembly line balancing calculations. These simplified

formulations are today labeled as simple assembly line balancing problems

(SALB) and they have only two constraints considered. In SALB cycle time con-

straint means that station time of any station cannot exceed the planned cycle

time and precedence constraint means that the requirements of assembly order

must be carefully observed. SALB characteristics are applicable for a single

17

model assembly line, which is paced with a fixed cycle time and has no assign-

ment restrictions. In the simple assembly line all the stations are equally

equipped and the idea is to maximize the line efficiency with station times that

are as near to the planned common cycle time as possible. (Baybards 1986,

p.150; Scholl & Becker 2006, p.667)

As mentioned in the first chapter the scope is in high-mix, low-volume assembly

line for complete equipment. The standard assumption for assembly line bal-

ancing is the traditional single model production and many publications study

this perspective. Today’s assembly lines have changed dramatically since the

early versions due to more complex product requirements and diversified cus-

tomer needs. Companies have to be able to individualize their products with

modularisation or mass-customization. For example car manufacturer BMW

offers various optional features that in theory would allow 1032 different models

which are produced in one assembly line. Better production techniques and

production planning enable efficient flow-line systems also for varying low-

volume assembly-to-order production. The main principles are the same in sim-

ple assembly line balancing and mixed-model assembly line balancing but in

the latter all the calculations, problem solving and restrictions are more com-

plex. (Boysen et al. 2008, p.1-3)

More flexible assembly line requirements have also attracted the attention of

researchers and a great amount of different extensions of basic assembly line

balancing studies have been made. Assembly line balancing research evolved

towards formulating and solving generalized problems (GALBP) with different

additional characteristics such as cost functions, equipment selection, U-shaped

line layout and mixed-model production (Scholl & Becker 2006, p.667). The last

one of these characteristics, the mixed-model assembly line balancing, is the

most important extension for this study that concentrates on high-mix produc-

tion of built-to-order products.

In mixed-model assembly line (MALB) the models may differ from each other

with respect to size, color, tasks, task times, precedence relations and many

other variables. Consequently it is almost impossible to find a line balance when

workloads of different stations have the same station time and equipment re-

quirements for all models. In these kinds of environments the conventional con-

straints are no longer relevant, because there can be flexibility in local cycle

time violations and also employees need to be flexible. Cycle time is no longer

the implicit maximum station time because the primary station time must be de-

fined from the average cycle time. Employees must be flexible enough to qualify

several tasks in order to balance the line. The analogy of MALB consists of find-

ing the optimized number of station, cycle times and line balance such as in

18

SALB. However, the work is a lot more complex because of the large amount of

variable factors while the station time must be smoothened for each station

separately. (Becker & Scholl 2006, p.706)

In simple assembly line balancing problems the capacity of the line is defined

from the amount of workstations, because workplaces and operators can be

perceived as the same attribute. In a more variable environment this definition is

not necessarily applicable because many products manufactured on assembly

line are large enough to be worked at several workers simultaneously on one

workstation. Moreover, the stations are often designed merely based on product

structures than on common cycle time and workload may also differ between

stations. In these kinds of variable environments the productive capacity is not

defined by the number of workplaces but by the number of employees required.

Because station times may significantly vary between workstations significantly,

the stations are balanced with the amount of employees. However, it is often

proposed to distribute the total work content as evenly as possible among the

stations because it promises better product quality due to a more standardised

work system. (Becker & Scholl 2009, p.359-361)

In a variable environment it can be challenging to allocate and calculate accu-

rate real workloads of workstations because the cycle times are not the same

for every product or model. For high-mix, low-volume line where standard times

fluctuate, determining average standard time per process is more accurate for

determining resources. The balance of the assembly line is then calculated by

dividing the resources equally based on average standard times. The resource

calculation is straight forward but the resource allocation may not be as simple

and accurate because there is so much variance in times. (Hobbs 2011, p.236)

There are two aspects in assembly line balancing for mixed-model assembly

lines. The first aspect is the equal allocation of the total workload to all employ-

ees based on average station times. This is called vertical balancing and it is

described more in detail in chapter 4.3.1. The other one is more horizontal bal-

ancing, which aims to decrease the variability of station times in order to avoid

occasional work overload or idle time. This method is described more in detail in

chapter 4.3.3 where the method used is to decrease variability by assigning op-

tional modules to pre-assembly. Vertical balancing is important for all kinds of

assembly lines but horizontal balancing is a characteristic only for mixed-model

assembly with variable station times. (Merengo et al. 1999, p.2839)

One of the objectives of assembly line balancing is to determine bottleneck sta-

tion, which is the slowest operation or the most loaded station that is constrain-

ing the assembly line throughput. In high volume plants, a bottleneck can be

19

determined also visually from predetermined buffers before and after work-

station. For example if the buffer before is full and the one after is empty, the

workstation is likely to be a bottleneck or at least a local constraint, and no

deeper analysis is needed. For low-volume production the bottleneck can be

less obvious because the bottleneck can change place depending on certain

condition. (Lane 2007, p.71)

Assembly line balancing for more complex mixed-model lines has regarded as a

tactical level problem. It can be solved by dividing tasks equally to different sta-

tions, assigning unlimited buffers and determination of production sequence of

all models for each station separately. However, competitive markets require

more flexible production systems that respond rapidly to changes in the market

conditions. Then unlimited buffers are not a solution in assembly line systems

and workloads must be planned more in detail in order to avoid unbalance. In

flexible systems with limited buffers mixed-model assembly line balance prob-

lem becomes an operational problem, because task assignment and operations

scheduling must be considered simultaneously with a shorter timeframe.

(Öztürk et al. 2013, p.436)

Larco et al. (2008, p.56) has come to a conclusion that assembly line balancing

and designing layout in a variable environment is more like an art than basic

production planning, because there are so many different factors to be consid-

ered simultaneously. Multi-skilled employees, different routings, scheduling

problems and determination of bottlenecks are just a few extensions compared

to a mass-production environment. These kinds of environments require skilled

planners and self-management from employees in order to operate the facility in

an efficient manner. (Larco et al. 2008, 56; p.91)

2.4 Assembly line balancing methods

Various optimization methods have been introduced and discussed in literature

for assembly line balancing. The methods aim to support decision makers to

configure the assembly systems as efficiently as possible (Boysen et al. 2008).

In this chapter nine different methods presented. They are also perceived as

alternative solution ideas to be implemented in practice for the case company’s

needs. All these alternatives can be used in parallel. However in mixed-model

line at least two methods must be used because both vertical and horizontal

balancing aspects need to be considered.

2.4.1 Assembly line balancing based on average station times

The purpose of this first method is simply to equalize the workload for all em-

ployees based on the workstation planning, capacities and average station

20

times. This is the most common and almost compulsory method to balance as-

sembly lines. It is also presented in all sources that present how assembly line

should be designed and it fits to all kinds of productions. In a high-mix produc-

tion line some other methods must also be used but balancing according to av-

erage workloads is the basis and starting point for actions and for the use of

other methods. This will define the normal situation, which balances the work-

loads on a very long term period, but also considers short-term variations in

production.

There are many factors that affect the production balancing based on workloads

and a lot of calculation is needed. Values that need to be considered in assem-

bly line balancing are, for instance, all standard times, the available working

time, number of workstations, number of workers, routings and demand. The

current production set-up normally defines the most important factors to be

evaluated for reconstructive assembly line balancing. For example the product

structure, the employees’ skills as well as available space can be restrictions

that define the perspective for the plans and actions.

In assembly line balancing the first thing is to evaluate and compare the total

cycle time with the theoretical takt time. In simple assembly line balancing prob-

lems it will give a rough estimation for the number of employees needed and

speed of the line. There are big differences in allocation of these values in dif-

ferent production systems. In simple assembly line balancing problems for mass

production requirements the station times are always the same. The calculated

cycle time is divided equally between workstations, which are usually defined to

match the takt time as presented in chapter 2.2.3. Furthermore, an early study

for mixed-model assembly line by Thomopoulos (1970) attains for equality of

workloads across all workstations and models to enable synchronous move-

ment in assembly line.

The first step in transformation from simple assembly line balancing problem to

mixed model balancing is to compute average task times for workstations.

Becker & Scholl (2006, p.707) call this process as a reduction to single-model

problem. The next step is the minimization of cycle time differences from aver-

age station time and to aim for synchronous takt-driven production. For high

volume assembly line Baudin (2001, p.54) proposes that the cycle time of the

bottleneck station should be equal or multiple of other stations. Then resource

allocation would be pretty simple too because the resources are divided with the

same share than the multiples of station times. To achieve such accurate and

detailed station times, a very comprehensive production planning and schedul-

ing for assembly line must be conducted.

21

These traditional viewpoints presented above indicating that all stations must be

equally equipped with respect to machines and workers is not often applicable

in real-world variable assembly lines. The average task time ensures that the

cycle time is sufficient to perform all tasks on average but even in an optimal

solution considerable inefficiencies such as work overload or idle time may oc-

cur. There are also many restrictions and constraints related, such as flexibility

requirements, problem solving, technological capabilities or position in assem-

bly work. (Becker & Scholl 2006, p.697)

Table 2.1 shows an example of assembly line balancing problem and a tech-

nique for capacity calculation for every workstation. First average standard

times for all existing workstations must exist and takt time needs to be calculat-

ed based on demand. Additionally, allowance percentage or desired productivity

is needed in order to get the planned cycle time for a certain available time pe-

riod. In the example, daily demand is 20 for the day’s production. The allowance

percentage compared to the takt time is set to 80% so that 20% of time is re-

served for problem solving, training or other inefficiencies which are not taken

into account in standard times. In comparison Toyota usually balances their

highly efficient high-volume facility to 95% of allowance time but there process-

es are stabilized and leaders are taught to solve problems efficiently (Lane

2007, p.144).

Table 2.1. Assembly line balance calculation (modified from Stephens & May-

ers. 2010, p.111)

The system above is modified for low-volume environment and manual assem-

bly work. In this example the system is very inflexible because only average

workloads are used and other balancing options are not handled. The times are

presented in minutes and hours instead of seconds which are usually used in a

Daily demand 20 18,5

Time available (min) 480 4

Desired allowance/productivity percent 80 % 74

Takt time (min) 24,0 1

Planned takt time (min) 19,2 100,0 %

0,308

Operation

No.

Average time

standard for

one product

Number of

workers,

stations or

machines

Rounded

up

Cycle time

per station

or machine

Load

Hours per

unit per

worker

Max

units

per

day

Total

productivity

(compared to

100% time)

A1 102 5,31 6 17,0 91,9 % 1,850 22 71 %

A2 99 5,16 6 16,5 89,2 % 1,850 23 69 %

A3 74 3,85 4 18,5 100,0 % 1,233 20 77 %

SA1 80 4,17 5 16,0 86,5 % 1,542 24 67 %

SA3 77 4,01 5 15,4 83,2 % 1,542 24 64 %

SA4 50 2,60 3 16,7 90,1 % 0,925 23 69 %

A7 118 6,15 7 16,9 91,1 % 2,158 22 70 %

T ota l 600 36 11,100 69 %

22

conveyer based production. The number of stations is presented also in number

of employees on one station which is more common in industrial low-volume

assembly work. With these values it is possible to calculate the number of sta-

tions, machines or employees in workstation. (Stephens & Mayers 2010, p.111)

The assembly line set up in this example is the same as in case company, but

the values are made to demonstrate assembly line balance problem. Sub-

assemblies are presented with SA and main assembly line stations with A, and

the sequence of the assembly is from top to down. The average time standard

is presented in the second column for all stations and in variable imbalanced

production those can vary significantly, because normally the layout is planned

more according to product structure than equal amount of work for every sta-

tion. In this example the total cycle time is 600 minutes, which means that it

takes 10 active hours to assemble an average product. Number of workers is

calculated by dividing the average time standard by planned cycle time for each

station. In the next column the computed amount of workers is rounded up to

the next whole number because the idea is to seek for the right head count and

if rounded down the demand or rate targets would not be reached. The assem-

bly line cycle time is presented in the fifth column by dividing the time standard

by the number of workers.

Workstation A3 has the highest cycle time and it is the bottleneck station of the

example. Bottleneck stations are marked as 100% station in balance calcula-

tions which present the place of the current maximum workload of the assembly

line. However, it does not mean 100% productivity because it would be calcu-

lated from the total time available and actualized working hours and here actual-

ized work hours are not concerned. The balance load percentages of the other

stations are calculated based on the workload of the 100% station and the

numbers tell how busy each workstation is compared to the bottleneck station

(Stephens & Mayers. 2010, p.116). The idea of this table is to determine the

amount of employees needed for workstations with given starting values and

the balance situation of the assembly line. The result seeks the minimum num-

ber of employees in order to balance the assembly line with current process

setup by using only average standard times. The numbers can be compared to

actual current situation for indicative action plans for changes. In the last col-

umn we can see that if the demand target is reached with given values the

productivity of the bottleneck station is 77% which is 3% lower than desired.

The total productivity of the assembly line would be only 69% (11% below de-

sired) when actualized standard times are divided by total day’s hours of the

employees. Even theoretical calculations cannot reach to better maximum val-

ues and it underlines the complexity of assembly line balancing for variable en-

vironments.

23

In the example we can see that assembly station SA3 employees work only

83% of time compared to the bottleneck station and the difference represents in

most cases waiting time or slowed pace of work. According to Stephens & May-

ers (2010, p.112) the cost of balancing is calculated from the difference of the

most loaded station compared to the least loaded or slowest activity. In the ex-

ample table the lowest load percentage is 83,2 % and the hours per unit is

1,542. The cost of balancing calculation is presented in table 2.2 with starting

values of volume for one year 10000 and the hourly rate 20€.

Table 2.2. Cost of balancing (modified from Stephens & Mayers. 2010, p.112).

There are many ways to develop the balance situation and productivity of the

presented situation in the example and as discussed before the first priority

should concern on actions for bottleneck station. If there are more employees

added to 100% station when A1 with the second highest load of 92% will turn to

100% station. This improvement will affect all stations with an approximately 8%

increase in load percentage (except A3), and the assembly line will be more

balanced and faster. By adding that one extra person to the 100% station would

save approximately 8% for 32 workers, which is equal to the workload of 2.6

employees. The best balance with these kinds of calculations is the lowest total

number of hours per unit and not the productivity because it is related to com-

pleted standard hours. Another method is to make bottleneck operations more

effective by decreasing the amount of inefficient non-value added activities.

(Stephens & Mayers. 2010, p.113)

The traditional form of presenting assembly line balance situation is histogram

graphs. Figure 2.4 presents the balance state of the previous example based on

the cycle times. The pillars can easily be compared to each other as well as

both to the customer takt and the planned cycle time. (Rother 2013, p.18)

Balanced cost (hours per unit

for the lowest loaded station) 1,54 hours

Individual cost (83,2% x 1,54) - 1,33 hours

Hour per unit savings 0,21 hours

Units per year x 10000 pieces

Hours per year 2083 hours

Cost of an hour x 20 euros

Savings per year (euros) = 41667 euros

24

Figure 2.4. Balance state based on cycle times. (Rother 2013, p.18)

Quite many restrictions exist in the traditional assembly line balancing based on

average cycle times, because it is almost impossible to analyse all influential

attributes related to the real-world work. Issues such as problem solving, devia-

tions in employee skills, lacking parts and demand fluctuations are not normally

analysed together. In many assembly line methods the purpose is also to find

the exact and most suited number of workers for assembly line without any dis-

cussions of excess of capacity or instant hiring of people. It must also be re-

membered that in Lean manufacturing environment, employees should not be

laid-off for cost savings or based on short term economic logic, because it

would make more harm for productivity actions than advantage (Liker 2004,

p.77). In this thesis the number of employees on the assembly line is perceived

to be fixed even though the traditional assembly line balancing calculations

would support other decisions.

Another restriction is that in unpaced an asynchronous lines throughput can

often be improved if less workload is assigned to central stations compared to

those located at the beginning or the end of the line. This concept which partial-

ly challenges traditional assembly line balancing is known as “bowl phenome-

non” and the effect seems to be stronger when the deviations in processing

times are higher (Hillier et all. 1993, p.1-2). Usually studies and publications

analyse only some isolated parts of the assembly line balancing problems. Ac-

cording to literature covering made by Boysen et al. (2008, p.15) only 15 out of

312 assembly line balancing articles deal with real-world assembly line prob-

lems.

2.4.2 Flexible multi-skilled workforce

In a high-mix production environment multi-skilled workforce is an extremely

valuable resource and as for the employees it is one of the key requirements in

Lean manufacturing. Multi-skilled workforce gives flexibility for production plan-

ning and capacity calculations. It is also a way to balance uneven workload in

25

different workstations when employees change places according to the needs

from fluctuations in workloads. In chapter 3.1 assembly line balancing was pre-

sented based on average workloads of workstations but this method does not

recognize the need of flexibility of high-mix production.

There is an example figure 2.5 which represents workloads of four different

workstations during four periods in a paced assembly line. The example is pre-

sented in a paced and synchronised system without buffers so that the figure

highlights differences of variable station times. All the stations have as much

work but it is unevenly divided during the 4 periods. The assembly line is bal-

anced between average workloads as presented in the previous chapter, but

here time and period dependent variances are presented as well. The workload

can fluctuate at least in four different ways which are a) product mix, b) work-

stations differences, c) variances in workstation cycle times and d) differences

in period total times. Of course changes in demand, problems and many other

factors can also influence on balance situation. A more detailed table with

source numbers for this example is presented in appendix 1.

Figure 2.5. Time depended variances in high-mix production.

a) Variances in product mix mean the differences between total cycle times.

Some models with many customized options are simply much more labour-

intensive than basic products. However, in the example figure every product

has the same total cycle time of 12 but only unit 4 is demonstrated from the be-

ginning to the end. b) Workstation differences mean the variances between cy-

cle times compared to other stations. As mentioned in this example this factor is

also simplified so that the sum of all stations is 12 hours for these 4 periods. c)

Variance in workstation cycle times mean the range from the minimum time to

maximum time of the workstation, which normally depends on the optional at-

26

tributes. In the example presented, both the difference compared to other sta-

tions and the workstation variance range is from 2 to 4. d) Fluctuation in work-

load between periods is also a very important factor which will be analysed

more in-depth in chapter 2.4.5 production levelling. In the example figure it is

clearly visual that different time periods have high cycle time variances from 9 to

15 time units.

When workstation workloads considerably fluctuate between products it makes

little sense to daily rearrange or remove physical workstations. Instead, a more

logical solution is to adjust the number of flexible labour resources so that no

idle time is generated and productivity increases (Hobbs 2011, p.233). In these

kinds of cases presented in figure 2.5 multi-skilled employees are an excellent

way to balance the workload between all employees. For example during period

1 all stations should have an equal amount of employees which is 25% per sta-

tion. When products move forward to period 2 it must be ensured that work-

station 2 has more employees than average because of the higher workload.

During period 3 the total workload is very high but workstation 3 has only 3 time

units and its multi-skilled employees can be allocated to other stations with

more labour-intensive products. During period 4 workstation 3 should instead

have double the amount of resources compared to workstation 2. These balanc-

ing actions described above in highly variable environment would not be possi-

ble without multi-skilled employees who change place based on the standard

time requirements.

Planning a system in order to manage a flexible workforce can be difficult and it

has a lot of restrictions. There must be right standard times and enough time for

each job so that tasks can be performed. The employee must also have the

right skills for the specific job while materials also need to be available when

labour resources are changed between workstations. The most challenging part

is creating a culture of self-management so that people know what tasks are to

be fulfilled and that they are aware of the boundaries. Employees need to be

able to move from one workstation to another without missing a beat. (Lane

2007, p.74)

In order to promote self-management among workers, leaders must work to-

ward becoming leaders, coaches, mentors or advisors rather than remaining in

the role of authoritarian bosses. They need to be ready to step in and help. That

is the way how operators and line leaders learn how to balance their own work

as different products ought to flow through production. The workers in the work-

stations where the complex or variable tasks are completed must be multi-

skilled so that they can perform whatever special or unusual tasks are called

for. One option for managing resources is to create a team of “floaters” who are

27

always ready to help the currently highest loaded station. (Larco et al. 2008,

p.48, p.60, p.86)

There are many restrictions in using multi-skilled workforce as a balancing

method. First of all management must have a proper competence matrix so that

they know who have abilities and willingness to do different tasks (Lane 2004,

p.146). When there is more than one worker in a workplace performing tasks

related to the same workpiece simultaneously, the workers obstructing each

other should be avoided. This can be achieved by a detailed production plan-

ning or subdividing the workpiece to responsibility areas. (Becker & Scholl

2009, p.361)

There are also differences in skills and all employees are not able to perform

tasks in the required standard time. When using multi-skilled employees the

worker should always be able to meet the standard time. Coromias et al. (2008)

suggests that if a task is done by a skilled worker the normal standard time

should be used but if it is assigned to unskilled worker the standard time should

be multiplied by a factor greater than 1. Another solution to this problem is that

the employee’s capacity is calculated by a factor under 1 person in the case of

an unskilled or a temporary worker.

Another restriction consists also of the employees’ change resistance and of

motivational factors. For these issues an adequate awarding system should be

in place so that multi-skilled workers would truly be motivated to change places

and improve their skills. If multi-skilled employees are used as one of the bal-

ancing methods a good controlling system is necessary to support the deci-

sions. However, establishing such awarding system is a true challenge for a

high-mix environment where the production situation is quite unstable and hard

to measure reliably. Problems related to instant “hiring or firing” also prevail

when it comes to capacity requirements as was already discussed in the previ-

ous chapter.

2.4.3 Pre-assembly for optional modules

In order to forward products near to the same pace on mixed-model assembly

line all the station times must be matching at each station. This can be done by

changing more work to subassembly lines from the products which standard

times are over takt time (Baudin 2002, p.113). Pre-assembly is a balancing al-

ternative to level out the peaks in the workload so that optional modules are

assembled already beforehand and the workload in the main assembly line

would be as smooth as possible at all times. Pre-, and subassemblies create

more flexibility in production scheduling because the assembly does not have to

be performed at the same time with the main assembly line. Of course just-in-

28

time principles with minimum inventory and work-in-process must be planned,

but it is not that exact if the parts are only available on time for the main assem-

bly line. When workload is more even it is much easier to create a flow for the

main assembly line.

Assigning tasks to preassemblies is known also as horizontal balancing for

mixed-model assembly lines. The idea is to minimize variances in station times

over all models. This will reduce difficulties in sequence planning and reduce

overloads or idle time in the assembly line. (Merengo et al. 1999, p.2839) There

are three different methods to perform and measure horizontal balancing in the

mixed-model assembly line. The first alternative objective is to minimize the

sum of absolute differences compared to the average station time (Thomopou-

los 1970). A second alternative is to minimize the maximal deviation of station

time of any model compared to the average station time. A third option is to

minimize the sum of cycle time violations of all models in all stations. (Becker &

Scholl 2008, p.708)

The assembly line needs to be loaded so that all stations are always full and

subassembly stations make no exception. These must be scheduled by calcu-

lating backward from the time each subassembly will be needed in final assem-

bly. This creates a cascading linkage backward time from main line to sub-

assembly stations and their possible subassemblies. On the other hand it may

be possible to plan subassemblies without affecting the final assembly as long

as they are completed before the time they are needed. In order to secure that

subassemblies are available when needed the work must begin far enough in

advance. Software used in production scheduling must be capable of making

the calculations for subassemblies too. The controlling of preassemblies can be

compared to making a menu by a chef who needs to start cooking servings at

different times so that they are all served at the same time when needed. (Larco

et al. 2008, p.79, p.100)

When assembly is moved to be preassembled from the assembly line it is im-

portant to also think about make-or-buy decisions. For example outsourcing can

be the best alternative for the subassembly work. Pre-assembly can also be

used by totally opposite way by returning some tasks from pre-assembly to

main assembly line. If there is a low workload at any assembly line main station

it is possible to enrich he workload with additional work from pre-assembly or

from suppliers to the main line. (Baudin 2002, p.113)

2.4.4 Different routings for variable products

Routing is referred to be the sequence of steps required to assemble a single

product. The product is routed from the first assembly station to the second sta-

29

tion and further until the product is finished. Assembly charts are used to show

the sequence of these steps. The sequence of assembly may have several dif-

ferent routing alternatives and time standards are required in order to decide

which assembly sequence is the best. (Stephens & Mayers 2010, p.107)

One balancing method is to change the layout so that it enables assembly line

to adapt to variable standard times of products. This can be done by arranging

different routings or even a totally separate part of the assembly line for more

special product modifications. The main line would do all the standard work and

the alternative routing would operate on more customized versions. Alternative

routings will mitigate the fluctuation of standard times and help in production

planning if takt times of all workstations would constantly be more even regard-

ing workloads. Products that require extra steps are sent to alternative paths

and then they rejoin the main line later when customized options are assem-

bled. This could be compared to scheduling local trains that stop every station

and express trains that stop only in large cities (Larco et al. 2008, p.51).

The different routings can be arranged for example by duplication of work-

stations so that they work parallel side by side in an assembly line. In duplicate

stations the work does not usually start at the same time because of varieties in

processing times, random problems and repair times. In this case buffers are

needed in front of and behind of duplicated stations because it is extremely hard

to schedule the production so that all assembly areas would be full. With paral-

lel stations it is possible to decrease the unproductive portion of the planned

cycle time. (Becker & Scholl 2006, p.701)

The method is pretty much the same than in the pre-assembly alternative pre-

sented before but different routings come to in case when the assembly must

be done directly to the product when pre-assembly is not possible. Different

routings or new secondary assembly line may require lot of planning for layout

and scheduling. It can also be done with very simple decisions on how the work

is arranged in workstations. For example it is sometimes possible to arrange

more space inside a workstation so that more products are assembled at the

same time and enables a situation that a standard product can overtake more

complex one when the waiting time is minimized. If there is already a parallel

assembly in a workstation it can be decided that the other workplace only con-

centrates on more complex products and the other one on standard versions.

The risks related lacking parts or quality problems do not affect the assembly

line so much when there is more than one unit in-process in production.

30

2.4.5 Sequence planning to level out the workload

This chapter discusses production levelling, production smoothing, mixed-model

sequencing and other terms referring to production queue’s sequence planning

as one assembly line balancing method. Planning production order is in many

assembly line balancing theories presented as the only method to balance pro-

duction for the variable production or mixed-model assembly line, but the topics

often are then more often concentrated on high volume production. Assembly

line balancing and sequencing problems are closely interrelated. However,

probably due to the computational complexities involved, these two problems

are usually addressed independently of each other. (Fernandes & Groover

1995)

There are two basic objectives in model sequence planning when studied from

different perspectives. The first objective is to minimize work overloads or idle

time, which occur when there are fluctuations in station times. For this objective

the approach is mixed-model sequencing, which aims to avoid sequence de-

pended work overloads based on detailed scheduling. This approach studies

operation times, worker movements, station requirements and other operational

characteristics. The second objective is to level part usages in order to support

just-in-time objectives because of deviations in material requirements. The ap-

proach for this problem is level scheduling according to demands and material

needs following lean and just-in-time principles. Model sequences are planned

with such a manner that material usages are as smooth as possible. In this

study the focus is on the first objective, mixed-model sequencing, but some

techniques from just-in-time principles do also apply for smoothen capacity utili-

zation. (Boysen et al. 2009, p.350)

The first assembly line balancing method presented demonstrates that cycle

times need to be determined by observing average station times over all mod-

els. This is also labelled as a reduction to single-model problem. As a conse-

quence, the station times of some models are longer than the planned cycle

time, whereas those of others are shorter. Whenever multiple labour-intensive

models, follow each other in direct succession at a specific station, a work over-

load situation occurs. In such situations, workers are not able to finish the prior

products in time and the cycle time or the planned cycle time might be exceed-

ed. Line stoppage, utility workers, off-line repair or higher local production

speed at the station are examples of reactions to compensate the overload. A

more proactive way to avoid overload is to find a sequence of models which

balance the workload by altering high station times to less work intensive ones

at each station. Planning the production sequence for a short term product mix

is a way to minimize the overloads in workstations and better assembly line bal-

31

ance can be achieved. The amount of overloads by itself is also one measure of

efficiency of the assembly line balance. (Boysen et al. 2008, p.4; Boysen et al.

2011, p.4736)

As discussed in chapter 2.4.3 related to pre-assembly, horizontal balancing has

a great influence on sequence planning. The better the horizontal planning,

meaning less variance in mixed-model line station times, works the better re-

sults are possible from short term sequence planning. The objectives of mixed-

model sequence problems arise per shift, day or week with particular demand

and volume of different models. (Becker & Scholl 2006, p.707)

The next four different types of methods with different timeframes to level out

the workload are discussed for a built-to-order environment. The methods do

not exclude each other but are just used in different occasions in queue plan-

ning. The first queue planning method starts already before the orders and the

last is used already when the product is on assembly line. The sequence plan-

ning systems for different timeframes are:

1. The product is allocated with predetermined slot-based levelled sequence

2. The production sequence is levelled according to time of delivery

3. Short term daily production and sequence planning

4. Self-management of the next chosen product from the buffer

In the first timeframe the sequence decisions are already made before the actu-

al order is received. Products are scheduled according to predetermined pro-

duction sequence and received orders are allocated with a slot-based system to

the next available free slot. At the same time the predetermined slot-based sys-

tem defines the capacity and the resource constraints. The slots can be based

on a specific product model or the total cycle time of the product.

Production levelling is planning with the aim to get a balanced total workload,

volume and product mix for production. In lean manufacturing production level-

ling is known also with Japanese term “heijunka”. In levelled production prod-

ucts are not built according to the actual flow of the customer orders but it takes

the total volume of orders of a certain period and levels them out so that the

same mix is made each period. Achieving heijunka is fundamental to eliminating

unevenness (mura), which is, in turn, fundamental for eliminating overburden

(muri) and non-value adding activities (muda). When production levelling is

planned and executed effectively the assembly line will theoretically balance

itself after the planned period and resource calculations will thus become sim-

pler. Through this method, flexibility also increases for customers and demand

is smoothened for upstream processes for suppliers creating less inventory.

(Liker 2004, p.114-116)

32

The next timeframe for the sequence planning is set to after the orders are re-

ceived. The queue should be planned according to received orders for a given

time period. In variable production environment this system is called mixed-

model sequencing. The purpose is to find a sequence where work overload and

idle time is minimized. The basic idea is to allocate labour-intensive and more

simple products consecutively. In a mixed-model line this can simply be done

with a total cycle time or by taking into account all variations in all workstations.

This sequence planning is done for a certain time period and it aims to balance

the sequencing periods compared to each other. There is a vast number of pub-

lications for calculating the most effective way to sequence mixed-model as-

sembly lines but in some fields of business the product variety is simply too

large to allow reliable calculations. The only reliable estimation in this field is a

prognosis of single customized options which influences the most for assembly.

Following this prognosis or the determination of the option occurrences, a joint

precedence graph must be made to imply how it really affects the assembly line

workstations. This mixed-model sequencing method based on estimations of

option occurrences is not necessarily the most efficient but it is the most reliable

for very large varieties of products. (Boysen et al. 2008, p.5)

The third timeframe for queue planning is just before the production starts. The

input for this timeframe is the planned sequence, but in real-world assembly

systems there are always exceptions and restrictions compared to ideal se-

quence. This short term production planning takes available material, quality

problems and current production situation into account. The idea is to re-

schedule the sequence according restrictions with best possible way. For this

purpose a flexible scheduling programs are very advantageous.

The last chance to arrange the sequence is when the products are already in

production. The idea is that supervisors and employees would have self-

management to choose correct products from the buffer so that the workstation

is not overloaded with labour-intensive products for long time. The system is

applicable only if there are more than one product in buffer. This system re-

quires that standard times are visually available and employees would have

basic production planning knowledge.

There are many restrictions in using production levelling for the high-mix –low-

volume production system because it is so vulnerable for problems. The chang-

es in work queue because of lacking parts or quality problems must be easily

recalculated. Assembly should not be started if all parts are not been received

from suppliers and then the work queue must be changed. That will mix up the

well planned sequence and then queue levelling will not work as a balancing

method anymore.

33

In production levelling for mixed model assembly line it is not only the total cycle

time which needs to be concentrated but the whole mix. As presented in 2.2 the

total cycle time is not normally divided evenly to workstations but the custom-

ized options define the real situation workstation specifically. For example in

some cases the station time can be very low even though the total assembly

cycle time would indicate very work-intensive product. In next section in-process

inventory is discussed and that can be used together with production levelling

effectively because it reduces the need of accurate calculations. Together

mixed-model sequencing and buffers are an effective way to balance assembly

line and in creating flow in high-mix environment.

2.4.6 In-process inventory to avoid idle time

In unpaced and asynchronous assembly lines workpieces are always moved as

soon as the operations are completed at a station. After transference the station

starts to work with the next unit, unless the preceding station is unable to deliver

it. To minimize waiting times in asynchronous lines, buffers needs to be in-

stalled in-between stations, which can temporally store workpieces for in-

process inventory. Synchronous assembly line works with the same beat and

in-process inventories are needed only for exceptions and flexibility. (Boysen et

al. 2008, p.9)

Using in-process inventory in the assembly line is more like traditional mass

production thinking than lean, but in mixed-model assembly line balancing it is a

good way to smooth peaks in cycle times and it gives flexibility in case of prob-

lems. Buffers can also be used as a visual production controlling method. Buffer

places helps to visualize work-in-process workloads and identify where too

much capacity or manpower is available (Lane 2007, p.92). The inventory can

be used to maintain the targeted takt time when a process is incapable of

achieving the takt time rate (Hobbs 2011, p.232). There is also a trade- off be-

tween installation costs (productivity) and achievable throughput when installing

buffers, because the latter usually increases when more buffers are installed

(Boysen et al. 2008, p.9) Buffers naturally increase work in process level but at

the same time it ensures that all workstations have work to do and decreases

waiting times. In a highly variable production environment buffers can together

with production levelling reduce overloads and improve smooth material move-

ment. Naturally, the most important thing is to create a flow for production.

There are two restrictions related to buffers between workstations when they

are used as a balancing alternative for an unpaced mixed-model assembly line.

The first one is blocking, which occurs when the downstream buffer is full and

the station cannot move completed units forward. Another problem is starving,

34

which occurs when idle time is generated because upstream buffer is empty.

These problems can be solved by assigning more buffer places or by concen-

trating on more detailed production scheduling. (Merengo et al. 1999, p.2843)

In the lean environment it is important to define rules for the buffer places so

that no excess inventory and overproduction is generated. In lean manufactur-

ing this is normally controlled with kanban -systems which indicate the material

needs for products. In a mixed model assembly line another way to indicate the

needs is a constant work in process –system (CONWIP) which is based on

more queue sequence than the amount of certain parts or materials. In a

CONWIP -system the in-process inventory is controlled by the consumption by

a demand. The production of the next unit in queue is triggered only when the

next station has finished its work. The CONWIP systems have been found to

have superior performance especially in variable environments compared to

other systems with respect to the average work-in-process level, variability of

processing times. CONWIP has also been identified to be easier to control and

have a shorter lead time than kanban systems due to the better management of

customized work in-process products. (Pettersen & Segerstedt 2009, p.206)

2.4.7 Assignment of identical tasks to different stations

On multi-product or mixed-model assembly line the normal system to assign

tasks to different workstations is to examine precedence diagrams and product

structures. Normally there are common tasks between products that are always

performed in the same stations. However in case of optional features it is possi-

ble to seek the shortest-route formulation and assign tasks to different stations

in order to optimize current production balance. In this method identical tasks

are performed in different stations so that the assembly line balancing would be

done in a product specific way. The objective is to decrease the station time

variances in a high-mix assembly line. The method for this system is to use

combined precedence diagrams and the optional modules would be assigned to

the lowest loaded station based on production sequence. Another option is to

find the best possible task assignment solutions separately for all the different

models by using computational minimization of variances between station

times. (Erel & Gokcen 1999, p.195)

One suggestion is also to plan standard times based on time-slots by dividing

and combining different task times to fixed standard time for modules. For ex-

ample with fixed 10 minutes module times it is easier to assign tasks to different

stations and balance the line simultaneously. This would require lots of standard

time planning, possible layout changes and strict modularity from the products.

However, it is reminded (Boysen et al. 2008) that investments made for assign-

ing similar tasks to different stations can be considered an improved balance,

35

for instance in form of lower cycle time from the bottleneck stations. There is a

trade-off between higher investment costs and potentially higher output that

should be regarded in mixed model assembly line balancing.

In order to eliminate the idle time, it is also important to consider tasks which

are totally different from the usual ones and not directly related to the product.

If waiting time occurs due to the variances in station times some other tasks

could be offered for the employees. The tasks should necessary, yet not de-

pendent on where or when their fulfillment should take place. There should al-

ways be an alternative plan in order to avoid decrease in productivity. For in-

stance, one could think of many small tasks related to material handling which

would ease the work of assemblers.

According to Becker & Scholl (2006, p.707) the method of assigning identical

tasks to different stations is not usually a desired alternative, because of its var-

ious restrictions. There are station related constraints, assignment restrictions,

additional facility requirements, loss of specialization effects, complicated pro-

duction control and setup inefficiencies. It is also very hard to allocate resources

when tasks can change workstations. Planning identical tasks is yet another

attribute to an already complex production scheduling and the factor must be

monitored carefully. Moreover, restrictions exist in relation to the planning of

efficient logistics for different workstations. The internal logistics should be

planned product specifically so that parts are always provided to the correct

workstation.

2.4.8 Work time arrangements

In some cases it is easier and less costly to manage only one shift, particularly if

running a second shift means an extra support structure and time premiums. An

additional bonus is that the waste of waiting is easier to see and eliminate when

takt time is shorter. (Rother & Harris 2001, p.16)

When labour force or demand is very low the employee allocation can be diffi-

cult because there are not enough people to every workstation in multiple shifts.

Some of the tasks may even require more than one person in order to be per-

formed, which further complicates the capacity calculations. It is more difficult to

match the assembly line balance calculations to the real-world requirements in

practice if the task requires that it is made in pairs.

For example if there are only 10 people in an assembly line of five equal sta-

tions, but one of the stations always requires a minimum of two people for a

safe performance. It is very hard to balance the line equally for all employees in

a two shift system because the special system would require in total two people

36

in the first shift and two people in the second shift. Only 6 employees would re-

main for 4 stations worked with two shifts. This could naturally be balanced with

flexible multi-skilled employees or better workstation planning, but it is not al-

ways possible. In a one shift- operation, all stations would constantly have peo-

ple and the line would thus easily be in balance.

2.4.9 Continuous improvement of current bottleneck station

Developing a bottleneck station should always be the first priority in assembly

line balancing and production planning in general. It is the most crucial work-

station and determines the maximum speed of the whole assembly line. Waste

elimination from a bottleneck station is a very effective way to improve the

productivity of the whole production. This section will concentrate on these key

Lean manufacturing principles as a balancing alternative to improve the flow of

assembly line. (Stephens & Mayers 2010, p.111)

It is important to concentrate very carefully on bottleneck stations in production

scheduling. Furthermore, it is necessary to plan the sequences and the sched-

ules so that the workload is high and stoppages in other stations would not ef-

fect on bottleneck station. Waiting time and lost production in the bottleneck

station is directly decreases the whole factory’s production. (Haverila et al.

2009, p.418)

This ninth alternative is like a combination of all other alternatives, but the focus

of balancing is only on the bottleneck stations. Actions related to bottlenecks

were discussed in the previous balancing alternatives too, but many authors

consider waste elimination also as a way to improve balancing situation of pro-

duction. The idea of this alternative is to have continuous actions to eliminate

waste from the present the bottleneck station so that it meets takt time targets

or the level of average target time. After bottleneck station development the cy-

cle time is reduced and the bottleneck station may change place. The develop-

ment projects are then continued at the next bottleneck station in order to make

it more effective. After several development projects at different bottleneck sta-

tions the production will almost automatically become more balanced. The

analogy of this assembly line method is presented in figure 2.6.

37

Figure 2.6. Developing bottleneck stations as an assembly line balancing

method.

A few examples on how to calculate productivity improvement and cost savings

were presented in chapter 2.4.1. In the example the bottleneck time decreased

5 minutes in total, which means that all workstations need to complete the task

5 minutes faster. If there were 12 workers on the assembly line of the example

the saved time would total 60 minutes for each product. If the volume can be

increased at the same time to new maximum capacity defined by the bottleneck

cycle time, the cost savings could be calculated by multiplying the saved time

with the amount of products. For instance, this 60 minutes saving for 5000

products in a year would be 5000 hours. The saving in time would mean better

throughput or with a 20€ hourly rate 100 000€ savings, again if the throughput is

kept at the maximum level. This is done simply by decreasing the bottleneck

cycle time which will at the same time eliminate waiting time from the other sta-

tions.

A bottleneck can be easily determined in a high volume assembly line if visual

buffers have predetermined sizes. In low-volume it is not that clear. The first

thing to do is to gather some data to clarify the situation or to discover how the

bottleneck changes depending on certain quantifiable or measurable conditions.

(Lane 2007, p.71)

38

There are many ways to improve the efficiency of bottleneck station. The core

idea is to minimize non-value adding value of the work content and to maximise

the work, that the customer is really willing to pay for. In lean manufacturing this

is called waste elimination and there are various different techniques and tools

available presented by numerous authors and benchmarked companies. One of

the most important principles is that assemblers should concentrate only on as-

sembling and materials should be handled by logistics. The cycle time of the

bottleneck station may be shortened dramatically if material handling is totally

changed for responsibility of logistics. Baudin (2002, p.113) suggests that as-

sembly line should be balanced by adding more automation to the bottleneck

station of mixed-flow lines especially if the workloads fluctuate much. Further-

more, improving modularity and the structure of products in order to ensure the

most efficient assembly as possible is also a good way to shorten cycle time of

bottleneck station.

Development actions for bottleneck stations have many advantages but also

some restrictions as an assembly line balancing method. The most significant

positive aspect in developing bottleneck stations is that it really improves effi-

ciency of the assembly line. The other methods concentrate on equalizing work-

load between stations but do not effect on the real work content and non-value

added activities instead of waiting waste. The restriction of this alternative as an

assembly line balancing method is that in variable environment the bottleneck

may change place for different products. The alternative suits best for dedicated

lines where there is normally only one constraint. Even good results from devel-

opment actions can cause more waiting and idle time if they are concentrated

on station which is not bottleneck station. For instance in graph 2.6 actions fo-

cused on AS 4 will not improve the efficiency of the line and will only cause

more waiting for the station if any other balancing actions are not made.

2.5 Synthesis of assembly line balancing for high-mix, low-volume production

Nine different assembly line balancing methods were presented in chapter 2.4

and they are also used as alternative solution ideas for the case company. The

most important finding was that assembly line balancing for the high-mix as-

sembly line must be a combination of both vertical and horizontal balancing.

Another finding is that most of the balancing methods are closely interrelated

and it is difficult to specify the boundaries for each balancing action introduced.

Table 2.3 summarizes all these nine methods with a short description and a list

of main sources used.

39

Table 2.3. Assembly line balancing methods

Assembly line balancing method Summary of the alternative idea Main sources

Assembly line balancing based on average sta-tion times

Calculation of workstation average cycle time and allocation of correct number of employees to workstations. This is also called as a reduction to single model as-sembly line balancing problem because it does not take account any variability of different models.

Stephens & Mayers 2010 Thomopoulos 1970 Becker & Scholl 2006

Flexible multi-skilled workforce

Employees are used as flexible resource to balance the assembly line by changing workstations according to fluctuating work-loads.

Larco et al. 2008 Coromias et al. 2008 Lane 2004 Hobbs 2011

Pre-assembly for optional modules

Assembly line standard times are smooth-ened with horizontal balancing, which aims to reduce variances in station times. Op-tional modules are assigned to pre-assembly stations so that work overloads and idle time would be minimized.

Merengo et al. 1999 Baudin 2002 Larco et al. 2008

Different routings for variable products

Different routings are designed for products with variable waiting times so that waiting times are minimized. Solutions include par-allel workplaces and separate assembly lines for complex and standard products.

Stephens & Mayers 2010 Larco et al. 2008 Becker & Scholl 2006

Sequence planning to level out the workload

Occasional work overloads and idle time are minimized with leveled production se-quence. Mixed-model sequencing aims to situation where models with high pro-cessing times alternate with less work-intensive ones at each station.

Boysen et al. 2008, 2009, 2011 Liker 2004 Karabati & Sayin 2003 Merengo et al 1999

In-process inventory to avoid idle time

Buffers are used to avoid idle time in un-paced and asynchronous mixed model assembly line. Buffers can also be used as a production control method with CONWIP system, which supports continuous flow and assembly line balance.

Boysen et al. 2008 Pettersen & Segerstedt 2009

Assignment of identical tasks to different stations

Idea is to assign optional tasks to different stations based on the best possible line-balance situation. Shortest-path formulation and additional tasks to minimize idle time are included to this method.

Boysen et al 2008 Becker & Scholl 2006 Erel & Gokcen 1998

Work time arrangements

Design the most efficient work time ar-rangement so that constant assembly line balance situation is considered. For exam-ple imbalance caused by safety require-ment or task restrictions can be solved by changing work times.

Rother & Harris 2001

Continuous development actions to current bottleneck station

Continuous elimination of inefficient working practices from current bottleneck station will eventually step by step balance the assem-bly line.

Lane 2007 Baudin 2002

40

The first balancing method is the vertical assembly line balancing activity that

divides the workload for employees for a longer period. There are numerous

articles related to different aspects for single model assembly line balancing that

aims to assign tasks and employees equally to workstations. For variable pro-

duction this vertical balancing must be made with average station times and the

method is called, in many sources, reduction to single model problem. The next

four methods aim to support horizontal balancing, where the idea is to decrease

the variability of standard times and station times in the assembly line. These

methods apply in shorter timeframes and require good controlling systems. The

last two methods are more production planning and scheduling related subjects

that impact upon the assembly line balance as well.

Many factors must be considered in the variable assembly line balancing and

no single solution exists that would directly fit to any real world assembly sys-

tem. Different theories are not able to provide any holistic or comprehensive

solution procedure that would take all variables into account. There must be a

sense of discretion when applying different solution ideas from the literature,

because the scope of the studies rarely matches with the case company. Even

research papers for variable environments have often very restricting assump-

tions regarding the studied assembly system attributes. The ideal situation is

normally presented as takt-driven production where product variances are min-

imized from the main assembly line. It is also difficult to generalize studies that

focus on only some specific product or part type. Most of the research papers

suggest only one or two approaches to balance the line, but it is often not

enough in case of reconfiguration of high-mix assembly lines. This conclusion

from the literature review reinforces the impression that the solution for the case

company of this thesis is going to be a combination of multiple balancing ac-

tions.

41

3 ANALYSIS OF DEMOUNTABLES ASSEMBLY

LINE

In this chapter the power series demountables assembly line is introduced and

analysed. The main advantages and problems are presented based on work

analysis and ten interviews. The most important topic of the current situation

analysis is to describe the balance situation of different workstations and of the

whole assembly line. Results and findings presented in this chapter are used as

a basis for actions for assembly line balancing together with assembly line bal-

ancing methods.

3.1 Production system in the case company

This thesis concentrates only on the assembly line of power series hooklifts.

Power series hooklifts can be further divided to five different product types: slide

(S), slide-low (SL), tilt (T), slide and tilt (Z) and slide-tilt-low (ZL). There are 10

different loading capacities between 14 to 30 tons. Demountables have also

many other variable options that can be categorized based on, for example,

length, valves, control system, oil tanks or container locking but the lifting type is

the most significant differentiation.

The Hiab Multilift power series demountables assembly line consists of ten

workstations. The production is based on thousands of variable modules and

their assembly standard times are calculated in several minutes. The work-

stations usually have their own product structure based responsibilities and the

amount of work significantly varies between products. Also the average work-

loads may differ between different workstations and can cause unequal amount

of work between employees. These attributes in high-mix, low-volume assembly

can easily cause problems for smooth production flow and lead to occasional

waiting in some stations to hurry next. Power series demountables production is

controlled by a sequence method where assembly queue determines the needs

and operation priorities for the whole supply chain.

42

Figure 3.1. Hiab Multilift Z-model.

Figure 3.1 presents Hiab Multilift Z –model and the numbers presents different

frames that also defines the assembly system of demountables:

1. Subframe

2. Rear frame

3. Middle frame

4. Hook frame

5. Slide frame, only in Z- models

The current demountable assembly line in Raisio was initially planned in 2005

and the layout was updated in 2008 and 2012. The scope of this study is the

bolded area of figure 3.2 that presents power range hooklifts production

flowchart. The assembly line consist four main assembly stations and three sub

assembly stations. There is also hydraulic pipe bending pre-assembly station

which is not on scope of this thesis because the pipes are bended more in a

batch production type and not in the same schedule as the products in assem-

bly line. The scope starts from buffer, which is after outsourced paint shop that

is located in the same facility right next to assembly area. Paint shop company

should always plan their production so that there are frames available according

to work queue of demountables assembly line. The first three main stations

(5101-5103) assemble all customer specific modules onto subframes and at the

same time subassemblies (5104-5106) prepare rear-, middle- and hook frames

which are then assembled together in the final assembly line (5107). Assembly

line ends to final testing where all products go through detailed inspection and

reporting procedures. Final testing where all hooklift models are tested is not on

43

scope because it does not always work with the same pace with power series

hooklift assembly line.

Figure 3.2. Power series hook lifts assembly line (modified from Stephens &

Mayers 2001, p.108)

Figure 3.3 describes the different variables in real world assembly lines, which

affects in assembly line balancing methods (Boysen et al. 2008, p.3). The main

aspects of demountable assembly line are presented in the scheme with five

different attributes. Mixed-model assembly line is based on the idea that all dif-

ferent options of big hooks are assembled on the same line. The line control

method is currently unpaced and asynchronous because there is so much vari-

ance between products. Unpaced line means that the workpieces are trans-

ferred when the required task is ready for the workstation and asynchronous

means that workstations decide on transferences individually (Boysen et al.

2008, p.8). This thesis also evaluates other alternatives of line control options in

production scheduling related sections. In this context, the frequency means

that the scope is reconfiguration of already existing line instead of planning a

new assembly line. The business is demountables production which has some

differences compared to the more commonly referred automobile industry.

44

Figure 3.3. Classification of scope from investigated kinds of assembly lines

(modified from Boysen et al. 2008, p.3)

The strategy of Cargotec and Hiab is to concentrate only on the final assembly

of products. In recent years part manufacturing and welding operations have

been outsourced and the focus of the factory is now on the efficiency of logis-

tics, quality and the final assembly of products. For this purpose the Hiab pro-

duction system (HPS) was launched in 2013 to assist as a roadmap for adding

value for Hiab’s customers. HPS is based on Lean philosophy and Toyota Pro-

duction system (TPS), but it is tailored for the needs of Hiab. (Cargotec Oyj

2013b, p.6)

The thesis is one of Raisio factory’s lean and HPS projects. Lean is for eliminat-

ing waste, reducing cycle times, increasing capacity, reducing inventory, in-

creasing customer satisfaction, eliminating bottlenecks and improving commu-

nications. This thesis concentrates on Lean manufacturing principles which are

more closely related to production and offers some tools to be implemented for

the demountable assembly line. The lean manufacturing tools that have already

been vastly used are 5S and just-in-time (JIT) methods. 5S is a housekeeping

method that aims at organizing, standardising and setting in order of everything

at workplaces. JIT is making available the right part at precisely at the right

time, and in right quantity, to go into the assembly. The JIT-production aims to

achieve minimum inventories with a pull system that is based only on material

needs. (Ohno 1982) The thesis focuses on assembly line balancing and produc-

tion scheduling, which both play an important role in Lean manufacturing. Lean

tools are used as a primary guideline for example subjects that relate to produc-

tion levelling, multi-skilled employees, visual management or takt time.

In lean production developing generally culminates to the elimination of three

different kinds of elements that affect all the inefficiencies in production. These

are called with three Japanese words that all start with M –letter; muda, muri

and mura. Muda is all the non-value-added activity that is further divided to

eight different inefficiencies called wastes. One of those wastes is waiting,

which is closely related to assembly line balancing and its elimination is also

45

one of the objectives of this study. The second “M” stands for muri, which is

overburdening people or equipment. Assembly line balancing affects this M too,

because the idea is to create a more stable and well-planned work environment.

The last M is for mura which means unevenness resulted from irregular produc-

tion schedule or fluctuation in volumes per time. Mura is the most interesting of

the three M’s for this thesis because the assembly line balance is improved

simply by eliminating unevenness and variation. There is mura whenever a

smooth flow of work, parts or production schedule is interrupted. All these M’s

affect each other and good results are achieved only by the elimination all them

together. (Liker 2004, p.114-115; Imai 2012, p.90)

Hiab is implementing lean in all of its operations. HPS aims to provide value for

customers and eliminate all forms of inefficiencies in Hiab operation with partici-

pation of all employees. It is done through continuous improvement and lean

philosophy offers the main tools to achieve the targets. HPS house (see figure

3.4) cornerstones safety, waste elimination, standard work and continuous im-

provement are based on these main Lean principles. Quality and on-time are

presented in pillars which enable and support customer satisfaction presented

in the roof. People are set to the centre of the house to present that they are the

enablers that make it all happen. (Cargotec Oyj 2013b, p.1)

Figure 3.4. HPS house (Cargotec Oyj 2013b, p.1).

The most significant part of this thesis concentrates on production balancing

and scheduling issues, which means that the on-time –pillar is the main focus

46

on the HPS house. In HPS booklet (Cargotec Oyj 2013b, p.28) on-time -pillar is

about managing value stream of the supply chain and just-in-time production

initially introduced by Toyota. In this thesis the scope is only in the assembly

line where the customer can be perceived to be the next station in the process.

The idea is to deliver the right product at the right time in the most efficient way.

There have been successive projects in the Raisio factory related to logistics

and outsourcing. In development projects for logistics especially order-handling

procedures were improved. This has led to a more just-in-time material handling

system. Certain key performance indicators show that the situation has signifi-

cantly improved in the long run and now also a more specific production control

is possible when material handling is more reliable. Outsourcing projects have

enabled production planning to concentrate more on the assembly line im-

provement. One big improvement action from recent times was the layout up-

date in 2012. The new layout supports the flow of the materials with more neat,

spacious and organised set up. Together all these recent projects offer a very

auspicious starting situation for this thesis, which concentrates on the produc-

tion planning issues of the assembly line.

3.2 Work analysis

Analysis of the current situation started with a two day hand-on observation pe-

riod in power series assembly line in the beginning of the year 2013. The objec-

tive was to understand the assembly line stations and differences of assembly

work between different products. The idea was also to make a superficial analy-

sis for the most important development subjects in the assembly line and get

familiarised with the people in the production. There were many good conversa-

tions and questioning about the improvement possibilities of the assembly line

with the line personnel. The line work was mainly a follow up of different work

tasks in all stations presented in flowchart in figure 3.2.

The most significant inefficiency or waste in the demountables assembly was

clearly waiting times, which also underlines the importance of this work. The

most important reason for waiting was the imbalance of workloads between

workstations, which is described later more in detail. Waiting was also caused

for example by the inefficient production sequence planning, which was mostly

managed by line managers instead of the master planner. The sequence was

handled more based on experience than hard data and standard times. The

product sequence was written to one flap board where employees needed

come to check the production plan. No planned procedure existed for maintain-

ing and updating the sequence and sometimes it caused problems if it was

somehow changed. For example valves are partly preassembled and if the se-

47

quence is messed up a situation might occur where the pre-assembled valve is

not available for the product in-process.

During the thesis project it could be also clearly noticed that the outsourced

paint shop has a too strong influence to the efficiency of the power series hook-

lifts assembly line. In several occasions employees at the first assembly station

did not have any subframes available in the buffer after the paint shop. This

caused waiting which is also often cumulated to the whole assembly line when

the flow is interrupted already from the first station onwards. The paint shop al-

so had too much influence on the assembly sequence and determined too

much the production planning activities. There are many different factors and

restrictions that influence the sequence and it is hard to make corrective actions

that would not affect other places in a negative way. It was found in the analysis

that with the current system the queue planning creates waiting in many places

and a new system for the queue sequence planning should be created.

There were also various other reasons for waiting which were not that easy to

point out with a data analysis. One of them was that two workstations partially

shared an assembly area, causes continuously short waiting times when the

other task is ready sooner than the other and the product cannot be moved for-

ward on the line. There were no buffers between the stations and the other sta-

tion was able to start assembly work on the product earlier if they made some

operations already in the previous station. This illusion that starting work earlier

would decrease waiting times was hard to break, because the system had been

in use for such a long time. The procedure was in advance for shorter lead time

because the product was completed earlier from the final assembly station, but

at the same time it decreased the productivity due to continuous waiting times.

One reason for waiting was also the fact that parts were lacking and the real

problem was that they were often noticed after the assembly had already start-

ed. The logistic is not the scope of this thesis, but problem solving of these

kinds issues is related to assembly line balancing. In many occasions there was

no substitutive work to do when some parts were missing which caused waiting

problems for many stations. In these cases foremen should be able to quickly

decide what alternative tasks employees should do instead of waiting for miss-

ing parts.

Work analysis also revealed that employees do not have enough information

about the daily or weekly targets that are set for the production. The pace of

operation was more based on the current market situation or the time limits of

individual products. Employees did not have access for the module specific time

standards and they were not used for production planning. The work was more

48

based on the number of products and the total average standard time, even

though the operation is highly variable.

3.3 Interview analysis

Interviews were held for ten assembly line related white collar employees from

production, logistics, production engineering, quality and information manage-

ment. There was an hour booked for each interview but few turned to be more

like production development discussions rather than interviews and took even

two hours. The purpose was to get a comprehensive outlook of the current situ-

ation of the assembly line by canvassing pros and cons, main problems and

development ideas from different departments. The interview question sheet is

presented in appendix 7 in Finnish and it had the same semi-structured ques-

tions for every interviewee. Open questions were about employees’ responsibili-

ties, different development ideas and opinions. The statistical data from the in-

terviews were gathered with likert-scale of four scales questionnaires about

workstation arrangements, time targets and visual management.

The results of the interview showed that logistics and material handling are the

competitive advantages of the Raisio factory. Also the good trends and num-

bers in key performance indicators endorse that result, but those may have also

affected the opinions of the interviewees even too much. The most important

development subject was capacity management related to the assembly line.

The interviewer may have influenced to the interviewees in this question, be-

cause that subject was also discussed generally during the interviews. Other

main development subjects were to improve problem solving and information

flow about the issues in production.

Figure 3.5. Results from structured questions.

49

The results for the structured questions are presented in the figure 3.5 above.

Interviewees though that workstations are defined and arranged pretty well. The

main reason for this is the overall cleanliness of the assembly line, at least

when compared the situation before layout changes. All the tasks and responsi-

bilities are generally working well even though the system is mostly modified by

itself. In an average of the opinions were on the positive side of the scale, but

there were also several small development objects mentioned.

Time targets were regarded as a clear development subject and the average

opinion was on a negative side of the scale. Most of the critics were pointed to

the availability of standard times and how they are used for the guidance for

assembly. There has been a continuous updating of standard times with many

different timing methods for demountables assembly, but the general overview

has not been analysed. Standard times were used more on pricing and capacity

planning than production scheduling. Also definitions and meanings of different

time standards seemed not to be clear enough.

Visual control was perceived to be on a quite good level and on a positive side

of the scale. It polarized the interviewees’ opinions because some perceived it

how different areas are visually defined and others how visual managing helps

to control production flow. Also inconsistent opinions occurred: some thought

that there is enough information available and other thought that the only infor-

mation is the flap board of production sequence to support production control.

3.4 Assembly line time study

Collecting, verifying and analysing relevant data is the starting point for all con-

tinuous improvement projects. The current status hard data helps to understand

where to really focus in problem solving and improving operations. If there is no

data all the decisions are made more like relying on hunches and feelings in-

stead of scientific or objective approach. (Imai 2012, p.7)

The standard time study was probably the most important task of this whole

thesis. The idea of time study was to make a survey of the balancing situation of

the big hook assembly line. The data analysis was based on the work made in

2012 when assembly modules were allocated to correct workstations and ena-

bled to make statistical calculations for the balancing situation. Before it was

possible only to calculate the total assembly line cycle time and productivity, but

now this study was able to reveal more specific standard times for the assembly

line.

50

The methodology was to collect time data of the products in backlog from ERP

system to spreadsheet software sheets and make various calculations and

graphs to present the balancing situation of the production. Another input for the

analysis was capacity information collected from supervisors and daily visits in

the assembly area. The information from workers helped to correct some mis-

takes of standard times and to solve misunderstandings. Also definitions of dif-

ferent standard time calculation methods used in production needed to be de-

fined, because there where many different purposes for different methods. After

brief analysis it was agreed that the standard time called “target time” would be

the correct meter to use for the assembly line balancing actions. With the target

times and capacity allocations it was easy to calculate the balance situation of

assembly line.

The system was to first calculate the average work amount of each workstation

by using target times. The results showed that some workstations have much

more work to do than other stations and the tasks are really divided to work-

stations based on product structure rather than amount of work. The differences

were smoothened by dividing the total workstation average target time by the

current normal number of employees per station. The results show the average

cycle time per product for each workstation and also the differences between

workers from different workstations. The load percentage is then calculated by

comparing each station to the 100% bottleneck station. All station times are di-

vided by the highest average time and multiplied with 100% to get the work-

station workloads (Stephens & Mayers 2010, p.116).

Figure 3.6. Workload percentage situation in the beginning of the project.

The balance situation with load percentages in the beginning of the project is

shown in figure 3.6 and more in detail in the appendix 4 with red colour. The

analysis shows that workstation 5101 is a clear bottleneck station of the assem-

bly line according to the target times. The workload of the second loaded station

5104 is 86% and the least loaded employees in station 5105 have only 36,3 %

workload compared to employees in the 100% bottleneck station when ana-

51

lysed only based on target times. The average workload of workstations is 63%

and average workload of individual employee is only 61% compared to bottle-

neck employees.

The first calculations for takt times and planned cycle times are very often

wrong but they serve as a starting point for setting targets. The initial numbers

provides information of the current pattern of operation so that the first target

condition can be established. With the current situation and targets it is possible

to know exactly where to concentrate on and strive forward by using continuous

improvement methods. (Rother 2013, p.5)

The main restriction relates to the source of time standards and many small

mistakes were corrected during the data analysis. As described above the time

study was done mostly by using only target times. However, this is not the most

reliable way to do time study. The best way would have been to use actual

times and avoid using existing standard times because there are multiple re-

strictions related to ERP –based times (Larco et al. 2004, p.52). However, time

study made by stopwatch based on actual cycle times was not included to the

scope of this thesis. Because of ERP-based target times all the results from

balancing calculations are just indicative rough information which cannot be

used towards any radical changes as a primary source or tool.

The next phase was to analyse the variances between product target times.

The purpose was to evaluate how much different products really affect the as-

sembly line work and the production flow. The analysis was made for all the

orders in backlog to get as comprehensive and reliable source material as pos-

sible. The first research was made in relation to total product target times to get

a picture of how much variance there really is between products. After that the

second research focused on the main options and modules that most affect the

most on production and imbalance.

The analysis of total target times revealed that the scale was very large from the

minimum to the maximum target times. The most labour-intensive product mod-

el require even double amount of work compared to the simplest option of de-

mountables. Based on the total target times, products were divided to three cat-

egories; easy, normal and hard. These categories help to identify the capacity

requirements for different products or planning of production sequence. The

category names are somewhat misleading because for example a hard product

is in fact more labour-intensive but not necessarily any more complicated. In

case of station times the differences were even more significant. In average the

minimum station time is three times shorter than maximum time depending on

workstation.

52

A more detailed analysis based on different product related specification also

showed clear differences in all analysed factors. Target time variances between

product models were as significant as expected. The data analysis convinced

that the most complex low slide-tilt model (ZL) is much more labour-intensive

than the most standard slide model (S). The target times of all other models

gradually increased from S to ZL. The analysis of the lifting capacity, which can

also be considered as the size of the product, showed that usually more capaci-

ty means also longer target time. However, this direct correlation was not as

significant as expected and the product type is much more determining factor.

In analysis of the customized options there were many important findings made

about options that influence to clear peaks in target times in some workstation.

The information was used in several assembly line balancing actions for exam-

ple in planning of new pre-assembly station, routings or production sequence.

The assembly line product variance graph is presented in appendix 2.

53

4 DEMOUNTABLES ASSEMBLY LINE DE-

VELOPMENT

The development areas, balance situation and bottleneck stations were pre-

sented in the last chapter. In this chapter the focus is to create a concrete plan

to balance low-volume high-mix assembly line for demountable production. The

first phase introduces an action plan to balance the workload by re-engineering

part of the assembly and equalizing the amount of work between assemblers.

The second phase presents a plan how to continuously control the balance and

target times. Throughout the process, literature was used as guidance and all

changes were reflected to different theories.

4.1 Assembly line balancing

Based on the time study and balancing situation presented in chapter 3.1 it was

clear that there was a need to make development actions for the assembly line.

As discussed in chapter 2 there are many ways and alternatives to balance as-

sembly lines and the theory was used to give guidance for the development ac-

tions. The first thing in balancing is to recognize the bottleneck station. Data

analysis shows that the bottleneck station with current manpower is clearly

5101 Subframe assembly. Bottleneck is always the constraint for productions

and most of the development ideas and planning should be addressed to the

bottleneck station (Stephens & Mayers 2002). After the plans and first actions

for bottleneck station, all other workstations were also re-evaluated. Next all

these different variables are studied with the alternative balancing methods pre-

sented in table 2.3.

4.1.1 Assembly line balancing based on average station times

The current situation analysis revealed that there are significant differences be-

tween workloads of employees in different workstations when calculated with

average product target times. The most important reason that causes imbal-

ance for assembly line is the uneven workload for different stations and em-

ployees. This can be noticed from figure 3.6 where workstation 5101 is a clear

bottleneck station with over 16% per cent higher workload than the second

highest workstation. It was obvious that there was a need to use this assembly

line balancing method to allocate correct amount of employees to workstation

based on average standard times. Using the method of reduction to a single

54

model problem does not alone solve the assembly line balancing problems for

high-mix, low volume production but it will provide the basis for other balancing

methods.

The development action is to calculate the ideal balance situation by using the

assembly line balancing table presented by Stephens & Mayers (2011, p.111) in

chapter 2.4.1. The table was modified for the needs of case company and the

source numbers were allocated based on current demand and production situa-

tion. The average standard times for each workstation were already calculated

in the current situation analysis so the re-calculation was pretty simple.

The ideal balance situation with a fixed amount of employees is presented in

figure 4.1. The base number of employees of workstations is different in 6 out of

7 stations in the ideal balance situation of demountables assembly line. The

plan is do these changes one by one during a longer period. First there must be

a survey on who are the right individuals to change the base workstation based

on competence matrix and personal affections. A possible training period must

be also planned for workers that do not have enough experience or competence

for a new workstation. All the development actions are made by production

managers and foremen based on ideal situation calculations. After the changes

there is a follow up of the consequences. In appendix 4 the ideal balance situa-

tions are presented with green colour.

Figure 4.1. Ideal assembly line load % with fixed amount of employees.

Because all the assembly line balancing calculations are made only with unreli-

able standard times the ideal situation is not necessarily the best balance set-

ting. There are differences in employees competences and motivation so

standard times do not always correlate with actual times. This stresses the im-

portance of researching the actual cycle times because it is complicated to

make further decisions with unreliable target times that are not verified frequent-

ly. In the future it will be important to follow the balance situation and continu-

ously update the target times. There are still quite many small corrections to be

55

made for the allocation of target times to the right workstations, because some

parts are assembled in wrong place or have totally wrong target times.

4.1.2 Flexible multi-skilled workforce

The demountable assembly line has seven workstations and they are all dedi-

cated to slightly different types of assembly work. There has been a team cul-

ture for workstations and all the employees belong to some workstation team.

However, as may notice from the previous chapter, all target times were in “av-

erage” –format, which indicates that the assembly times of different products

are really very variable. The workstations where employees were placed at

were in turn on “base” –format which indicates that the workstation allocations

are not fixed in variable environment. Some multi-skilled employees have been

changing places according to the needs of fluctuating workloads or to replace

other workers in case of sick days. In the future this will be a clear target to con-

tinuously increase this flexibility factor and encourage employees for self-

management in case of changing places when there is fluctuating workloads.

Competence matrix and motivation methods for changing places flexibility have

already also been in use also before but not very effectively. During the assem-

bly line balancing project the previous subjects and the controlling system for

multi-skilled workforce where analysed and planned.

There are clear advantages in using multi-skilled employees in the demountable

assembly line. It is possible to prevent waiting times, shorten lead time and

equalize the amount of work between assemblers in variable environment. As

presented in chapter 4, the target times fluctuate in average times between sta-

tions but also between different products in one station. For example low model

hook lifts need much more assembly work in piping and valve assembly but not

in the final assembly line. However Z hooks which have both slide and tilt op-

tions need more work at final assembly line but not that much in the first sta-

tions. The time depended factors that effect on fluctuations in capacity require-

ments in short term production scheduling are presented in chapter 2.4.2. All

these variables are relevant in demountables assembly line and they significant-

ly affect the short term employee requirements in different assembly line sta-

tions.

The first variance attribute is the differences in total cycle time. The product mix

and the total standard times of assemble one demountable is the baseline for

variable employee requirements. The difference between the most laborious

and the most simple product is clearly over a double in total cycle time. The

second factor is the difference in station times between each other from differ-

ent stations. As discussed in chapter 2 the purpose of simple assembly line bal-

ancing in mass production is to equalize the workload between workstations,

56

but in high mix industrial assembly the tasks are divided to different station

more based on product structure and the type of different tasks and then bal-

anced with employees. In demountables assembly line there are significant dif-

ferences in the amount of work between workstations. The average target time

of the most loaded station is even 6 times longer than in the least loaded work-

station. This average difference is balanced with employees as discussed in

previous chapter. There are one to eight employees in one work station simul-

taneously. In variable environment, where optional modules affect the amount

of work on different workstations, the range from the most loaded and the least

loaded station at the same time can temporary fluctuate much more. If a very

labour-intensive product in the most loaded station is compared to an easy op-

tion in the least loaded station assembled at the same time, the difference in

standard times can even be over 15 times higher. The number can also be con-

verted to the employee requirements which would indicate 15 more employees

to the most loaded station compared to least loaded station for the product. If

the assembly line works in a strict takt time system it is impossible to balance

this high differences by changing the employees workstation based on period

depended requirements calculated directly from standard times.

Target time variance in one station is the third factor discussed in chapter 2.4.2.

In the demountable assembly line there are often over 300% difference be-

tween consecutive product models in assembly line. In the pre-assembly of

hook frames the difference between two consecutive station times can be even

ten times higher for a laborious product compared to the simplest option. Ac-

cording to theoretical calculations it is not unusual that the difference of capacity

requirement of two consecutive periods in takt-driven production would be over

four employees for one station. There are also practical limits in changing the

employees’ workstations based on each unit and time period because this

would require a very good and reliable controlling system and larger work-

stations. There is an example of the different attributes affecting the station

times described in chapter 2.4.2 and appendix 1.

The fourth source of variance is the period depended fluctuation in total cycle

time, which is a consequence of the production sequence. There is a conflict

between the takt time based production system and capacity requirements if

there are only complex products on the line because there are not enough em-

ployees. In turn when there are only simpler products on the line there is an ex-

cess of capacity if takt time kept the same for the period. This overburdening,

unevenness and waste can be avoided by developing production levelling which

is discussed in chapter 4.1.5.

57

As presented above there are considerable time depended differences and fluc-

tuations in demountables assembly, which affect the capacity planning and pro-

duction flow. It was analysed that it is not possible to make a detailed plan to

manage and control the all the variances affecting on employee allocation for

every single product. That is why employee allocation and use of multi-skilled

employees are based more on average workloads for some period. At the be-

ginning, the period is one day and the plan is to continuously shorten the period.

To manage these short term variations and resource requirements, a system

was designed to give an indicative number for each workstation for certain peri-

od based on target times. At first the system works with spreadsheet software

calculations which convert target times to employee requirements, but in the

future idea is to design even more automatic method. The first step in the calcu-

lation is to download production sequence and all target times from ERP-

system. The next phase is to determine the scheduling timeframe and in 2.2.3

there were two different methods presented for that purpose. Takt-driven sys-

tem and production rate oriented system have the same output but can have a

totally different production system.

In the takt-driven system the period would match with the planned cycle time

and the amount of products would only be one for every short term scheduling

period. In a production rate oriented system the period and number of products

is in more free discretion based on the longer term targets, current capacity or

normal pace of work. Production rate for one day has been the conventional

target setting method for power series demountables assembly line and the sys-

tem is presented more in detail in production scheduling related chapters.

The next phase in employee requirement calculation is to define the available

amount of employees. The spreadsheet software file will then calculate the em-

ployee requirements by first calculating the total target time for each workstation

and then dividing the available employees based on the shares of target times.

This will generate an indicative number of employees expressed to one decimal

place. The employees can’t be divided to decimals but the decimal place will

help foremen to consider the real employee allocation more accurately. For ex-

ample, the indicative number of 3.5 will leave more discretionary power based

on the actual situation than number 4. However, the number of workers should

be quite near to the same value that is calculated based on average workloads

in previous chapter. Workers should be moved to other stations only when there

is a bigger gap between the indicative number and base amount of the employ-

ees in a workstation. The same file works as a reporting system for productivity,

targets and results for certain production indicators. The plan is to start using

the new production scheduling file on testing weeks and to use it as assistance

for employee allocation. The time period during the test weeks is half a day in

58

one shift system and spreadsheet software calculates the employee require-

ments based on target times for each station. The information is then provided

for production foremen who will make employee changes if needed. Idea is that

foremen can proactively react on variable standard times and not only after

some stations are late from targets. The file is available for foremen so that they

can change values and print out the plans also themselves.

There has been a system that a worker who is assigned to another workstation

is regarded to be only 50% of the capacity in the new location. This is because

of possible training requirements, motivational factors and competence differ-

ences compared to base workers of the workstation. This system is considered

to be re-taken in use now when the new production planning system and work-

station based productivity calculation is created. This could help the workstation

team to accept help from people from other stations without the concern of los-

ing in workstation’s productivity so much. There is also an option that a small

team of “floaters” (Larco et al. 2008) would be established for the demountable

assembly line. These employees would have the skills to help workstations that

have higher workload than average or left behind from production targets.

4.1.3 Pre-assembly for optional modules

The customized options assembled in the bottleneck station cause temporary

overload situations in workloads. The labour-intensive products with tool con-

sole or hydraulic front locking options take much longer to assemble compared

to stations average target time in bottleneck station. This creates a lot of fluctua-

tion and disturbs the production flow. This has been conventionally balanced

with buffers after the first station and work queue levelling, but it is not always

enough. When the buffer is already empty or multiple labour-intensive products

one after another are in queue the next workstation will be waiting.

The solution to balance the workload in variable workstation is to add a new

pre-assembly station for these special modules that increases target times. The

plan is to have one pre-assembly worker whose task is to prepare these mod-

ules ready for assembly line and the target times are allocated for the pre-

assembly instead of bottleneck station. There is a figure 4.2 to illustrate how the

new pre-assembly station will balance the workloads between the different

models.

59

Figure 4.2. Analogy in moving optional modules to pre-assembly.

This change requires planning of a new pre-assembly station and load calcula-

tions for the responsible operator. It includes time studies for the modules which

assembly place will be changed and possible planning for tooling issues. The

idea is that the new pre-assembly workload station is also capable of doing

many other kinds of tasks which are not necessarily related to the pace of as-

sembly line. The pre-assembly station provides flexibility and improved planning

capabilities for assembly line planning and balancing.

4.1.4 Different routings for variable products

The idea of different routings can be applied if very labour-intensive products

causes continuously temporary overload situations in a workstation. The op-

tional features can take much longer to assemble, which may lead to situation

that the next station end up to wait for the product to be completed. In de-

mountable production this kind of feature is fast lowering option for valves that

takes much longer to assemble. In this case it was not possible to change any

modules to pre-assembly because the tasks are made directly to the hooklift. In

the beginning situation this optional module causes big problems and even

stoppage of the assembly line flow due to the extra piping that do not always fit

to place as designed. These problems can easily stop the production for quite a

long time and in-process inventories are generated before the station and the

next workstations need to wait. The waiting problem for the waiting assembly

station had conventionally avoided by starting the assembly work already in ad-

vance at the previous station, which will also shorten lead time. This problem

was described already in chapter the 3.1. The assembly in workstation 5103

seemed occasionally more like a cell assembly type production because two

workstations were working parallel at a small area.

60

To solve these problems some small layout changes were planned for 5103

workstation (see figure 4.3). The workstation is enlarged so that up to three

equipment fit to the area. The first one is for cylinder assembly, second for con-

trol systems and the new place is optionally a buffer or an assembly station.

The new 3/5103 workplace is used for assembly when there is a more challeng-

ing product in-process in place 2/5103 and extra personnel there would not help

to speed up the process. This way the employees can avoid waiting by assem-

bling the next product in place 3/5103. The place can also be used as buffer

when 5107 is not ready with the previous product in line. The new layout should

ensure more reliable production flow forward to workstation 5107 and waiting

times are decreased.

Figure 4.3. Layout change for workstation 5103.

In the new layout final assembly station 5107 does not need to start assembly in

advance in previous workstation 5103. Instead they will not assemble rear

frame until the product is completed in workstation 5103 and that is why also

the rear frame buffer place is moved next to the correct assembly area in this

layout plan. The final assembly line workers will no longer disturb the team 5013

anymore with parallel assembly work because all the tasks are done in the right

assembly areas dedicated to each teams. The new enlarger workstation layout

is much more flexible because in case of very variable production or quality

problems it is easier to find alternative solutions to add value to the products

instead of waiting.

61

In the future, the similar kind of change to routings is also possible for the work-

station 5107 where certain models are much more labour-intensive than other

models. The current layout does not allow overtaking of other products in work-

station 5107 so the slowest model will pace the speed of the station. The as-

sembler of the following standard product after labour-intensive product will then

end up having long waiting times. There is a possibility to create a side assem-

bly place like in 5103 but with controversial system. The more complex products

would go to the new place so that other products could pass it. This method is

also compared (Larco et al. 2008, p.51) to local trains that stop at all stations

and express trains that stop only in larger cities. Alternative routings could even

be used in a larger scale by creating another assembly line for complex prod-

ucts. The current assembly line would focus only on standard products and an-

other line for products with more variability. Also defence products could be in-

cluded in the assembly line of complex products. This arrangement would bene-

fit current assembly line because of horizontal balancing, which would decrease

the variability in assembly line. The new line with a good flexibility could for

one’s part improve the efficiency of defence products assembly.

4.1.5 Work queue levelling

Workload levelling is one of the major challenges in the high-mix production

where all workstations have their specific tasks and customized options are as-

sembled at different stations in different times. The cycle times can also fluctu-

ate so that a product that is very laborious for some station can be a very low

loaded for another station. For example assembly of low model products have

higher target times for every workstation but some optional modules for exam-

ple in cylinders cause higher workload for only one station. It is extremely hard

to calculate and evaluate the exact sequence in such a highly fluctuating envi-

ronment where also problems will mix the sequence. As mentioned in the as-

sembly line analysis there is a clear habit of concentrating more on production

rates than standard hours. This perspective is very challenging for production

planning and especially in production levelling because the number of products

is not totally comparable to the amount of work.

There was a meeting held in order to improve the production levelling in the as-

sembly with the responsible people of queue sequence planning activities in

different timeframes. The production engineering manager is responsible for

order receiving practices, Master planner for the weekly sequence planning and

finally the assembly line supervisors are responsible for daily scheduling activi-

ties. The four different options presented in chapter 2.3.5 were analysed for

demountables mixed model sequence planning. The first step in production se-

quence planning is already done in order receiving. The system of using slots in

order receiving was perceived to be too complicated compared to the impend-

62

ing advantages and restrictions. The idea to reserve slot for products is not ap-

plicable for the demountable production because the assembly work is not that

standardized that sequence would rebalance itself with predetermined se-

quence. Furthermore, there are quite many real world production related re-

strictions in the demountable assembly line that the well planned sequence

would not remain the same. In the current system the order receiving depart-

ment sets the delivery date based on available weeks with enough capacity and

customer needs, but variances in power series hooklifts are not covered. The

available capacity is calculated with weekly rates, but standard hours are not

checked in that point. During the follow up period total weekly standard hours

were pretty stable but there is a big risk for high fluctuations. In the future the

available capacity should be based on standard hours instead of product rates.

Some practices related to delivery dates for certain countries should also be

loosened so that the focus would be more on flow of production. However, it

was agreed that these order receiving related sequence planning actions are

not planned during the thesis project.

The actual sequence planning starts when orders are arranged by the Master

planner and the system has been working well. However, there are differences

between daily workloads based on the weekday, because of the time of delivery

requirements by customers. The balance period that sequence planning and

production levelling covers is one week. It was agreed that the workload would

be levelled out for the work week more evenly based on target times. Now this

process is easier than before, because target times are visually available at the

same time the sequence is finalised.

The main restriction is related to mixed-model sequence planning are the re-

quirements of the outsourced paint shop, which also influences also the produc-

tion scheduling of the assembly line. The short-term solution was to increase

the buffer places after the paint shop in order to give flexibility for production

levelling. With extra buffer places it is now easier to plan the assembly se-

quence based more on production levelling than the requirements of paint shop.

There are still lots of room for development regarding sequence planning and

production levelling but the actions would need some structural reforms. First

there should be certain practices agreed with the paint shop about the most im-

portant requirements from both sides. This will be discussed more in chapter

5.3 subjects for further studies. When the process for sequence planning is

more standardised for both parties the next step could be optimizing the se-

quence. The Raisio factory does not have resources to make complicated man-

ual calculations presented in many articles but there is a wide variety of produc-

tion planning programs that should be considered. The programs should be

able to calculate the most optimal sequence and which is flexible enough for

63

instant changes. However, the near future implementation plan is to focus more

on assembly line levelling for the weekly load and to stabilize the queue with

larger buffer after paint shop.

Short term production scheduling concentrates on fine-tuning the sequence and

problem solving. The most significant reasons for changes in sequence are

lacking parts or problems with paint shop. Now production levelling in problem

solving cases is much easier when the target times are presented in reports that

foremen use for production planning. Foremen use mainly total times to adjust

the sequence with the best possible way so that no waiting would occur in the

assembly line. The next step would be to teach employees for self-management

for production levelling so that they would rebalance the assembly line by se-

lecting products in the right sequence from buffers. One possible restriction for

this self-management is that employees would select only the most convenient

products to assemble and ignore the delivery dates.

4.1.6 In-process inventory to avoid idle time

Buffers have been used as a production controlling method in demountable

production to level out the fluctuations, to reduce waiting times and to ensure

that every worker has equipment to assemble at their station. During this thesis

project these buffer places were defined more specifically and started to be

used as one of the balancing methods to smooth the flow in the assembly line.

Buffers can provide flexibility and decrease the risk that problems will affect the

production. Buffers are also increasing work in process value, but in the Raisio

factory’s case this is not seen as to be a big problem because in any case the

parts are in the inventory value without structural changes in logistics. In lean

manufacturing one idea to keep minimum inventories is to ensure that problems

cannot be hidden in work-in-process-inventories. Increasing WIP will also in-

crease lead time and that will have a negative effect on one of the objectives of

this thesis, which was totally controversially to shorten lead time. This can be

calculated by using Little’s law where throughput will remain the same regard-

less of buffers. (Little & Graves 2008)

However, during the thesis project it became clearer that minimizing waiting

times is much more important than short lead time for the assembly line. Short-

ening lead time will not give as good results as minimizing waiting times, which

64

improves productivity. That is one reason which supports the decision to set

buffers between demountable assembly stations.

Suggestions on new buffer places are presented in the updated value stream

map in appendix 6. There are 7 buffer palaces after paint shop and before as-

sembly line. The high amount will increase the flexibility for production levelling

options and there should not even be so much messing up of sequence when

the correct frame is more likely available. The big buffer will enable flexibility

schedule changes already before the assembly process starts if some product

is lacking parts. The buffer places also decreases the risk that assembly line

would wait for frames that have been occurred every now and then. In the main

assembly line there are one or two buffers between on each station which are

used to avoid waiting in asynchronous assembly. Between the subassemblies

and the final assembly line buffers are not as important because the primary

issue is to ensure that there is correct frame always available.

In order to keep the balance between flexibility and minimum in-process inven-

tory there should be a clear control practices. The demountable assembly line is

ideal for CONWIP system where products are pulled to buffer place whenever it

is empty, which is indicating the need for a new unit. The pull starts from the last

assembly station or final testing that are producing products according to de-

mand. In fact this system is already partly in practice but it is just not used as a

standard procedure. Implementation of CONWIP system starts with marking of

buffer places. The idea is to mark red and yellow floor paintings, where red indi-

cates need and yellow is additional buffer place to provide flexibility.

Training on main production planning principles should be arranged for workers.

The main issues in training would be the idea of CONWIP system and self-

management for using buffers as a production control method. The objective of

the training would be that all workers really know what needs to be done for

production flow. Today the follow-up of the amount of products in the assembly

line is much easier with the new real time assembly screen, which will be pre-

sented in 4.2.3 visual managing chapter. The new system will help in measuring

the current situation but the actual controlling would be best to arrange by as-

semblers self-managed CONWIP system. Then computer systems would sup-

port the production planning and workers would do the actual controlling and

balancing of the assembly line.

4.1.7 Assignment of identical tasks to different stations

Changing work responsibilities between workstations has not been widely used

in the demountable production. There have not been plans to use this balancing

method in the same manner as presented in literature. Methods presented in

65

chapter 2.4.7 were not applicable for demountable production. It was analysed

that moving tasks from station to station would require too much standard time

calculation and changes for logistics. It was also acknowledged that almost all

restrictions presented in the literature review would realize if task responsibili-

ties are changed.

The only exceptions are some modifications of this method such as pre-

assembly for another main assembly station or extra tasks. Pre-assembly made

by another main assembly station is classified to this method because the task

is not done by separate pre-assembly station but by another main line station.

The idea is that idle time is minimized by assigning the task another station. The

workload is temporary balanced between the helping station and the station that

is behind of schedule. This kind of a situation has been noticed to occasionally

exist between the workstation 5103 cylinder assembly and the workstation 5107

final assembly line. As presented already in 4.1.4 Different routings method

there is sometimes a situation that the final assembly line needs to wait prod-

ucts that have a complex valve option and this causes temporary higher work-

load. One solution is that the workstation 5107 preassembles the valve as ready

as possible in order to avoid this waiting time. The implementation plan is to

assign pre-assembly of the complex valve for final assembly line 5107 in case

they have significantly lower temporary workload. This valve cannot be pre-

assembled in a dedicated pre-assembly station because of logistics reasons

and changing employee places is not possible because of space problems. In-

troduction of this method will be done together with layout change in the area

presented in 4.1.4. There will not be any standard time corrections made for this

case and idea is only to serve one more method to prevent waiting time in final

assembly station.

Another method that closely relates to moving tasks is alternative tasks, which

must be done in some point. Extra tasks such as material handling, packing,

cleaning repairing can be done instead of waiting. These kinds of tasks should

always be available. However there are no implementation plans in scope of

this thesis for alternative or extra tasks.

4.1.8 Work time arrangements

In the demountables assembly line some stations always require a minimum of

two workers because of safety and practicality issues of the tasks. This causes

problems in the evening shift, because based on balance calculations only one

worker would be enough. There is a clear contradiction compared to capacity

requirements. The second, almost mandatory employee is then decreasing

productivity and disturbs production flow with overproduction. In the next day

66

the imbalanced employee allocation leads to almost always to a situation where

some stations need to wait for the first product and others need to hurry.

During the thesis project demountable production changed from two shift sys-

tem to one-shift system. Balance calculations support this decision even though

it was not the most important factor for the change. In a one-shift system it is

theoretically possible to allocate just the correct number of employees to work-

stations because there are no safety related restrictions anymore. This solves

the assembly line problem presented above. In theory the one-shift system

supports the flow of products through assembly line because synchronous

movements are enabled during the whole working day due to the balanced

workload at all times. The most important issue in the change from two-shift

system to the one-shift system is the easier production scheduling which has

direct impact also to production balance. A test week was arranged together

with the change towards one shift system. The plan for the test weeks are pre-

sented more in detail in production scheduling related chapters.

4.1.9 Continuous improvement of current bottleneck station

There are two workstations in the demountable assembly which are defined as

bottleneck station. The other is clearly the workstation with the highest workload

based on standard times and the other because station is affected by problems

that cause delays and disturbance. Main balancing actions were already in the

presented in previous chapters, but they concentrated only on performing the

existing workload as efficiently as possible. Operation as well as standard times

include both value-adding and non-value adding activities. In this method the

focus is to eliminate the tasks that are not adding value by improving the work-

ing practices. Different methods to eliminate these inefficiencies were presented

briefly in chapter 2.4.9. After the development actions for bottleneck station the

tasks should be able to be performed more efficiently and also station time can

then be decreased.

The implementation plan is to concentrate more on these bottleneck stations in

continuous improvement actions. There were no bigger projects planned for

waste elimination during this thesis project but the idea is to make small correc-

tions towards working practices and problem solving. For the first station, the

idea is to evaluate operation and development objects in 5S audits and make

various small corrective actions in order to ease the working practices. For the

other bottleneck station the plan is to improve problem solving methods so that

quality problems, lacking parts or other problems that disturb production would

be solved faster.

67

4.2 Production control

In the beginning of the thesis project the idea was to make a takt time study and

plan changes for implementing takt-driven system for demountable assembly

line. The subject was re-evaluated to focus on assembly line balancing but also

production control development actions were made. Most of these activities

were based on an assembly line balancing study and its implementation plans,

which required changes for production control methods too. In this section those

plans and changes for production scheduling and control are presented based

on the research made in chapter 2 and requirements from balancing activities.

4.2.1 Production scheduling and target setting

The plan is to schedule the assembly line with production rate oriented system.

The scheduling period will be in the beginning one work day for each station.

The follow up is made continuously so that all problems, overproduction and

leaving behind can be noticed as soon as possible. When the control system for

follow up and the target system is standardized the period for production rate

can be shortened to half day. Change to half day scheduling period will require

minor changes to computer systems so that the monitoring would be as practi-

cal as possible. The objective is to smooth the flow for assembly line and create

flexibility with buffers for the variable product mix. In the future this scheduling

period can be further shortened more from a half day and be even few hours.

However, takt-driven or paced production will not be implemented during this

project.

The reason to use production rate oriented system instead of takt-driven opera-

tion is the high variety and easier production planning for demountable produc-

tion. Scheduling production rates for a period is much more flexible and it

doesn’t need as exact standard times than as assembly based on takt time. It

will also divide the workloads to longer timeframe than takt time and then ca-

pacity calculations can be done for average workloads instead of unreliable in-

dividual standard hours. The complexity of takt-driven capacity planning is pre-

sented in chapter 4.1.2. The production rate and the target setting are based on

the current situation in production and delivery dates of products. The target

rate is set to all workstation so that the assembly line balance would be main-

tained. In the target situation all workstations and buffer places will always be in

the situation presented in chapter 4.1.6 and value stream map in appendix 6.

Improving the target setting was one of the objectives of this thesis. The produc-

tion rate oriented system will not make any revolutionary change to the previous

system where the daily targets were based on the amount of completed prod-

ucts from final testing. Now the target rate is also set for every workstation,

68

which will define production controlling for the whole assembly line. Another

way to control targets is to set production rate targets only for the final work-

station and control the flow with buffers. The current situation and the targets

are now visually presented for all employees and it is easier to react on prob-

lems proactively.

4.2.2 Visual management tools for production control

Visual management is a powerful method to provide information and help in a

clearly visible manner. It means to stabilize and to improve the process by iden-

tifying problems and highlighting discrepancies between targets compared to

current realities. Visual management is used to direct, organize and standardize

operations with many ways. Visual management can help in making problems

visible, managing complexity, instructing work and setting targets. It also moti-

vates and provides opportunities to for improvement to both workers and man-

agers to achieve goals when all the operations are as visual as possible. (Imai

2012, p.103-113)

In this context visual management is discussed only regarding assembly line

balancing activities and production control practices. There were various differ-

ent assembly line balancing techniques and an implementation plan made for

each method in chapter 4.1. These plans include things such as controlling mul-

ti-skilled employees, follow up of buffer places, sequence planning, and problem

solving. The production rate oriented system presented in chapter 4.2.1 for

one’s part handles issues like target setting, standard times and current produc-

tion real time situation. In the beginning situation, all these factors above were

controlled only with paper reports, flap board for work sequence and experi-

enced supervisors. Obviously these methods were not enough to support all the

assembly line balancing activities and more detailed production scheduling.

That was the main reason for planning a totally new visual control system for

demountable production. A working solution for logistics already existed and the

new control system for the assembly was made with the same program and

with nearly the same techniques. The new visual assembly line control system

was named assembly screen.

The purpose of the new assembly screen is to be a visual tool for production

control activities. It is a visual display of information entered into different infor-

mation systems. It provides real time assembly line information product and

workstation specifically for power series hooklifts assembly line. The objectives

of different attributes to be displayed in assembly screen are presented in table

4.1. Also the relation to the implementation plans of assembly line balancing

and scheduling are presented in the table. Part of the implementation plans also

serve as requirements for the assembly screen. The assembly screen was

69

planned and created during spring 2013, launched in week 19 and it has been

continuously improved after that. The assembly situation and sequence is

shown to everybody with large televisions in assembly area and all the comput-

ers connected to Multilift Intranet.

Table 4.1. Objectives and requirements of the new assembly screen

Displayed in assembly screen Relation to assembly line implementation plan (related chapter number and short explanation)

1. Assembly sequence of the products 4.1.5: tool for planning production levelling

2. Current real-time production situation

4.2.1: overall production scheduling 4.1.6: tool to control CONWIP- system

3. Target times for each product for all workstations

4.1.2: tool for short term employee allocation

4. Target product rate for each work-station for current scheduling period

4.2.1: basis for production rate oriented system

5. Signal for problems

4.1.9: faster problem solving in bottleneck stations 4.1.5: help for flexible re-sequencing of queue

6. Amount of employees in each station

4.1.2: follow-up for current employee allocation

7. Workstation specific productivity 4.1.1: easier follow-up of overall balance situation

The first step in the new control system was to start using the real assembly

sequence situation in IT -systems so that the first priority of the planned se-

quence is updated to computer systems instead of flap boards. This queue is

the backbone for the whole supply chain from order to delivery of products and

now it can also be used for assembly line as a controlling method more easily.

The queue is arranged so that production levelling would support production

balance. The queue sequence is now more consistent for power series hooks

production because the same sequence is shown for logistics, assembly and

managers via internal IT-system. Flap boards or papers are no longer used to

show the sequence for employees anymore, except some exceptional problem

solving cases. This change to update the assembly sequence to IT-systems

was also a requirement for the new assembly screen.

The second phase in the assembly screen planning to design how current pro-

duction situation should be presented. The solution was to create a cell system

where the product queue is presented vertically and workstations horizontally

from left to right. There is an illustration figure 4.4 of the assembly screen,

where workstations are presented in the first row. Workstation numbers are

marked from pre-assembly 5100 to testing 5108 without 51-marking in the front.

The progress of each unit presented with green colour and the latest green cell

of the unit depicts also the current location of any specific unit on assembly line.

70

It is also clearly visual how many items there are in buffers between each sta-

tion and that can be used as a controlling tool to support CONWIP system pre-

sented in chapter 4.1.6. Reporting from the workstations is done by the assem-

blers with an electrical stick, which is also used for many other purposes. Work-

station checking enables also more detailed automatic time studies in the fu-

ture.

Figure 4.4. Simplified illustration of assembly screen.

The third objective and purpose of the assembly screen is to present target

times product and workstation specifically. At the beginning, it was found out

that standard times were not easily available for employees and they were not

used for production scheduling purposes. In the assembly screen these target

times for workstations and different products are set to each cell, where they

are visually available and all workers. As was discussed already in the time

standards chapter, a study made for 400 industrial plants show that an opera-

tion that is not working toward time standards typically works 60% of time and

operations working with time standards work at 85% of time (Stephens & May-

ers 2010, p.64). According to this research there should now be better possibili-

ties to increase productivity in demountable assembly line because of better

target time availability. Workstation specific target times are important also for

assembly line balancing activities. They provide important information for short

term production scheduling, better production levelling and employee allocation.

Most of these actions are done already beforehand, but the assembly screen

support decision especially when there are quick flexible changes made to the

original plans.

PA A A SA SA SA A A T

Project Typ Model Ctry QNr PA HP KE 00 01 02 04 05 06 03 07 08 Comments

527782 BH XR21S59 -W-ITLF- FX 001 complited

522345 BH XR21S59 -WM-FL-- BE 002 complited during current period

523324 BH XR21Z59 -WMO-L-- AT 003 target rate for the period

528863 BH XR18S53 -P-O-LF- FX 004 0-series late from previous period

528844 BH XR26Z51 -S-ITLF- SE 005 problem ("simple andon" -signal)

528845 BH XR26S55 -DMO-LF- ES 006 started

528442 BH XR24SL59 -D-OCLBT AT 007 see ECR 66297

528847 BH XR24SL56 -W-OTLAT FX 008

528848 BH XR26S55 -DMOTLB- UK 009

528998 BH XR26S55 -DMOTLB- UK 010

524621 BH XR26S55 -DMOCLB- UK 011

528851 BH XR16T59 -WMI-L-- UK 012

528852 BH XR16S59 -WMI-L-- NL 013 lacking part 221334

528853 BH XR21S56 -WM-TLF- FX 014

528210 BH XR21S56 -W-OTLAH CH 015

528855 BH XR18ZL56 -WMO-L-- CH 016

528448 BH XR20SL52 -DMO-L-T BE 017

528272 BH XR18S56 -WM--L-- SE 018

528747 BH XR24SL58 -W-OTE-T BE 019

71

Target rate setting is the fourth factor for the new assembly screen and it is the

basis of production rate oriented system that will be implemented for demount-

ables assembly line. Target rates are set with blue colours to assembly screen

for each workstation for the scheduled period. Currently the period is one day

so all workstations have the day’s target rate clearly presented with blue colour

in assembly screen illustrated in figure 4.4. If the targets rates are not reached

the colour will change to light red and it is reset to include the next period tar-

gets. The blue colour illustrates the targets for the current period. Blue and light

red target cells are then used for capacity calculation (see chapter 5.1.2),

productivity measures and general production controlling. The longer the period

the closer the employee amount is to the average amount of employees. During

one day it is in normal situation very near to the amount calculated with the av-

erage target times presented in appendix 4. However, during a half-day period

the number of employees can already significantly vary compared to the longer

term average base values.

The fifth objective was to create a create signal for problems occurring in the

assembly line. For this purpose there was a stop feature added to the electronic

sticks that were used for workstation phase checking. The button is pressed

when there is some kind of issue, which stops, disturbs or interrupts the produc-

tion of a product. This stop button is shown as a red colour in assembly screen

as a signal for foremen to see and solve the problem. The red signal is a clear

visual help for all assembly line related workers and helps production control.

The sixth and seventh objectives were to show the amount of employees and

productivity of each workstation. The plan is to make another productivity

screen where different assembly line key indicators are displayed. The devel-

opment of the productivity screen is important because most of the calculations

are now made manually with Spreadsheet software and automatic system

would save lots of time. However, the development and implementation of these

features will not be done during this thesis project. Other development plans for

visual management and target setting is to implement tablet computer for pro-

duction scheduling activities. The idea is that if foremen can make changes to

the assembly screen already in the assembly area they do not need to return for

their computers in the office. The tablet will also work as camera, reporting de-

vice for problems and source for work instructions so that all information is easi-

ly available electrically.

4.2.3 Restrictions and problem solving

There were quite many restrictions in the demountables production compared to

calculations, plans and other production scheduling issues. In this chapter the

most critical restrictions are briefly described and analysed. Most of the re-

72

strictions were already mentioned earlier in this thesis and this chapter summa-

rizes these factors from a daily real-world assembly work.

The most significant restriction compared to theoretical assembly line balancing

methods and the real-world actions is that employees do not work like ma-

chines. This thesis does not discuss much about change management or

change resistance, but there could be another research made for those. The

project was first understood as a productivity program which does not always

have the best acceptance among employees, especially during a period of low-

er average demand. As described in the introduction chapter the idea was to

make the work easier by a better organisation and assembly line balancing.

During the project this message needed to be communicated to the employees

in the beginning of every development action performed. One restriction related

to the employee engagement is also that it is difficult to evaluate or measure

production balance and efficiency of scheduling when the productivity rate is

low. In such cases, it depends more on problems or individuals than the current

production system. Employees do not work with the same pace in all demand

situations. In low volume situation the work is done slowly so that it seems diffi-

cult and in high volume the possible bonuses encourages for better productivity.

It has also been recognized that it is easier to manage higher volumes than low

compared to average demand in demountable assembly. All these change re-

sistance and people engagement issues must be carefully considered in all de-

velopment actions and good communication is the best solution in most cases.

The most significant measurable factor that influenced on assembly line balanc-

ing activities and control was the output of units from the outsourced paint shop.

There were several occasions when assembly needed to wait for the painted

subframes and couldn’t work according to the plans. This naturally leads to

plenty of problems such as late production, idle time, uneven workload and pro-

duction planning difficulties. Problem solving is more difficult because of lack of

buffers which leads to decreased flexibility in variable environment. Further-

more, production sequence can be messed up in the paint shop and that affects

directly affects the production levelling. The first assembly workstation must on-

ly take the available frames for production to prevent waiting if the buffer after

paint shop is empty. The lack of frames enforces the management to make

quick actions so that employees would not need to wait and it can be challeng-

ing to organize 5S audits, team meetings, extra work or employees to change

workstations in that situation.

Evaluation and study is made only with current ERP target times and actual

times were not studied properly because that subject was not in the scope of

this thesis. The validation of target times was made by spot checks with ques-

73

tioning employees and foremen. The estimations of different target times of

modules where surprisingly close to the calculated standard times and it verified

that target times where applicable for rough balance calculations. During the

analysis, target times were continuously reassessed by comparing to specific

product type and modules. Even though there were not many major errors

found from standard times, the project stresses the impression that there should

be more specific analysis and time studies performed.

The production control calculations are currently made with an application that

require some amount of knowledge and experience before the system can be

used efficiently. There are already plans to develop more automatic reporting

system but it will require some amount of time and work before the system is

available. In order to control the employee allocation, assembly line balance

situation or workstation specific production indicators there must be resources

and knowhow for manual production planning calculations. There are many re-

strictions related the reliability of computer systems. The assembly screen is

currently the only tool to manage the sequence of units for the assemblers and

especially for the first workstation. The system may go down every once in a

while so there should always be alternative plan ready. The most common prob-

lem, although easily repairable, has been that the when queue is updated the

sequence changes from 1,2,3,4... to 1,10,100, 101… -format and the root rea-

son for the bug has not been found. The easiest alternative plan is to use the

old paint board system that can be set up rather quickly. If the assembly screen

is down the target setting must be done manually with printed papers and good

communication. However, target settings are not that critical information in de-

mountable production that short term computer breakdowns would disturb op-

eration much.

Lacking parts or quality issues are endless problems in high-mix assembly, and

they are also restrictions for reliable and detailed production planning. It is nor-

mal that there are always some problems to be handled in variable production.

In lean manufacturing one idea of small inventories is to force solve these

emerging problems quickly and effectively. The environment for problem solving

should be organised as flexible as possible so that productivity would stay at a

good level. For example work sequence and capacities in different workstations

must be able to be changed when needed to avoid idle time. This requires lot of

self-management from employees and foremen. IT -systems should also be

flexible enough to always changing plans. In this thesis all these restrictions

were handled in different chapters. In the case of assembly line balancing multi-

skilled employees and buffers are the most important flexibility factors for prob-

lem solving.

74

5 TESTING AND IMPLEMENTATION

Testing and implementation of assembly line balancing and production schedul-

ing activities were performed during the thesis project in spring and summer

2013. The idea of this chapter is to describe how the development plans pre-

sented in previous chapter were tested and implemented to demountables

power series assembly line.

5.1 Implementation of assembly line balancing methods

The assembly line balancing actions were practically tested together with the

actual implementation and they were compared to the beginning situation of the

project. Table 5.1 summarizes the implementation plan for different assembly

line balancing methods.

Table 5.1. Implementation plan for demountable assembly line balancing

Assembly line balancing method Summary of the implementation plan Assembly line balanc-ing based on average workloads

Equalize the workload between employees according to average standard times and aim to ideal balance situation

Flexible multi-skilled workforce

Increase changing employees between workstations according to current workloads and create a controlling system to support decisions related to it.

Pre-assembly for op-tional modules

Create a pre-assembly station where few optional modules are prepared for main assembly line in advance in order to smooth fluctuations.

Different routings for variable products

Plan a new layout for routings in cylinder assembly so that responsibilities are clearer and more complex products would not stop the flow.

Sequence planning to level out the workload

Plan is to even out the weekly workload more evenly between workdays and focus more on assembly line than paint shop in production leveling activities

In-process inventory to avoid idle time

Define buffer places more in detail with value stream mapping and train to use them as production control method with CONWIP -system.

Assign identical tasks to different stations

Plan is to assign part of the fast lowering valve assembly for another station when there is overload in cylinder assembly, which would cause waiting.

Work time arrange-ments

Change to one-shift system to develop production scheduling and balance situation during the whole day in order to improve constant flow.

Continuous develop-ment actions to current bottleneck station

Various development actions for bottleneck stations in order to decrease workload or avoid problems that are the constraints for higher productivity.

75

5.1.1 Average load percentage towards ideal situation

The idea of this method was to allocate the correct number of employees to the

workstations and equalize the workload between employees. The main goal

was to improve productivity and material flow in the assembly line. The plan

was to aim for the calculated ideal situation presented in figure 4.1 by making

employee base workstation changes one-by-one and follow the consequences

carefully. One input was also to follow in-process inventories and analyse the

operation performance of workstations. This information helped to analyse the

competence and skill level in a real world production system in order to analyse

the correct amount of required employees to perform the tasks. It worked as a

validation method for ERP- based target times, which were corrected continu-

ously. All the development actions were made by line supervisors who have

experience to handle these situations.

The first balancing action was to add more employees to the highest loaded

bottleneck station from the least loaded station. The next move was to add

more employees to the station which seemed to always be late and had above

average load percentage. These kinds of employee changes were made to six

workstations during the project to balance assembly line based on the average

station times. Only one station has totally same amount of employees than be-

fore.

Figure 5.1. New load percentage of power series assembly line.

The new normal load percentage based on average target times after three

months of the first balancing actions is presented in figure 5.1, which can be

compared to beginning situation presented in figure 3.6. The average of the

new load percentages is in 80% when in the beginning situation it was 63%. In

ideal situation the load percentage would be 85%, with current task assign-

ments. It would be possible to increase the average load percentage with an-

other 5%, by changing even more employees’ base workstations but there are

currently quite many practical restrictions on that. A comparison of the begin-

76

ning situation, the current situation and the ideal situation can be found in ap-

pendix 4. The most significant result was that it was clearly noticeable that wait-

ing times were decreased in stations where the amount of employees was re-

duced.

The theoretical calculations indicates 22% shorter total cycle time, maximum

capacity increase of 29% and improvement of 31% in average load percentage

of assembly line balancing. These results come only from equalizing the same

work more effectively between fixed amounts of employees and decrease the

amount of waiting times from assembly line. However, there are many con-

straints when calculating these kinds of numbers only based on average work-

loads, because of errors in standard times, differentiations in employees and

problems that may define the production very much. It must also be remem-

bered that there are no other assembly line balancing development methods

recognised in these values.

5.1.2 Increased use of multi-skilled employees

During the test weeks target time based employee allocation was taken in use.

There was a spreadsheet software file made to calculate the target times for

each unit and workstation. With these values it generates the indicating em-

ployee requirements for the scheduling period. The system is presented in ap-

pendix 8. The information provided for the foremen did help them to make pro-

active decisions for employee assignment to workstations and follow the pro-

duction situation more deeply. There was also an idea to start using “floaters”

as a method to control to balance workloads with employees changing work-

stations. The method was tested in a limited way with good feedback during this

thesis project. At the beginning it was mostly because of practical consequence,

because there was an employee who did not have a base workstation so with

“floater” status he balanced the most loaded station. The idea could be system-

atized and the team of floaters would have skills to help all workstations if they

are facing problems or lack of capacity. Floaters would be responsible for bal-

ancing employee capacity in assembly line.

The challenges for this system are that there are some problems when employ-

ees and products are examined only based on numbers. Employees have dif-

ferent skills and motivation to change places. There has been a competence

matrix in use for employees, but nonetheless assembly work for highly variable

products is not black- and-white regarding abilities to do the task. For example

some employees have skills to do all assembly tasks for demountables but they

will do it slower and inferior quality than colleagues. Another worker for instance

can be extremely motivated to do only one task but does it very well. In this kind

of environment it is extremely hard to lead personnel and plan the correct re-

77

sources to each station. Some workstations also require lots of experience be-

fore a substitutive or an extra worker is really productive. The extra worker may

even slow down the speed if the worker must be trained much. There is also a

change that the motivation level of employees drops if their good work is

awarded with a workstation change to a station, which is left behind from

schedule. Because of these people engagement factors presented above it is

very difficult to present results in numerical formats.

The conclusion for this method is that employees were assigned to different

workstations more than before and it was analysed to be in advantage for the

assembly line operation. It was decided that the use of new capacity allocation

system will be continued and developed. The Spreadsheet software based sys-

tem requires lots of manual work, which was pretty time consuming. The next

step is to develop these calculations and inputs to be more automatic and inte-

grated to Multilift Intranet’s assembly screen. In the future the purpose is to

measure workstations productivity more closely and maybe design some award

systems for reaching productivity targets. At the same time some kind of an

award system should be generated for multi-skilled employees who are willing

to change workstation.

5.1.3 New pre-assembly station

Because of very fluctuating workloads in the first assembly stations, there was a

new pre-assembly station established for tool console and hydraulic front lock-

ing assemblies. The pre-assembly station is located near to the stations where

the preassembled parts are needed to ensure better communication of needs

and ease of material handling. The new pre-assembly station was numbered as

5100. Also pre-assembly of valves will be changed to be part of pre-assembly

station in the future because now it is part of workstation 5102. There were time

studies made for the changed modules by the person responsible of standard

times. Based on the results of time studies also workload calculations were

made for the needed capacity allocation. Planning mixed-model sequence for

the assembly line is now much easier when the most significant fluctuations are

removed to pre-assembly station and buffers are not essential between main

assembly stations.

The implementation of the new system was successful but there could have

been much better communication and definition of responsibilities. The worker

in the pre-assembly station should have gotten better instructions and sched-

ules in the beginning of the test of new workstation. The pre-assembly could be

used also for many other modules assembled in other stations and that option

should be researched in the future to level out the workload even more. It is

very effective way to smooth peaks in station times. The station times on main

78

assembly line would be more stable if optional modules would be continuously

moved to be preassembled. If there are more these kinds of horizontal balanc-

ing activities made to reduce variances from main assembly line, it may be pos-

sible to adopt takt-driven operation in the future.

5.1.4 Different routings for complex products

The layout change presented in figure 4.3 was implemented during summer

2013. New layout divides the responsibilities between 5103 and 5107, which is

a clear advantage for the operation of both workstations. The new layout is

much more flexible because cylinder assembly can use an extra workplace in

case of quality problems or very complex products. The extra station prevents

that there are not too many operators working around one product and disturb-

ing each other’s work.

The challenge in the layout change was to convince its advantages for employ-

ees. It was difficult to prove that the task time and possible waiting times remain

the same but the responsibilities become clearer. The lead time from the begin-

ning of 5103 to the end of 5107 may be longer but at the same time productivity

should increase because of more effective assembly work. There are no numer-

ical results, but the comments by the employees are mainly positive. There

have been some practical difficulties to use the extra workplace but in general

the new set up has worked well. There could have been better communication

about the purposes of the change and the finalising of the new layout could

have been done in shorter time period. Employees have also understood them-

selves that it is more productive to assemble more than one product simultane-

ously when employees do not disturb each other around one product. Partly

because of the layout change 5103 station has had the biggest productivity im-

provement of assembly stations during the project. Rear frames are assembled

to subframes only in final assembly line area and the waiting time has de-

creased. According to employees on 5107 the production flow has been surpris-

ingly much better than in the previous layout and long waiting times do not exist

so often anymore.

Use of different routings was analysed to be a working method for the variable

demountables assembly line. The method could be implemented to other sta-

tions to differentiate complex and standard products so that with small respon-

sibility definitions assembly line balance would be controlled more easily. Differ-

ent routings could also be used in a larger scale to divide standard models from

more complex products in demountable assembly line to create more equalized

balance for the lines.

79

5.1.5 Sequence planning to support production flow

After the meeting held in spring 2013 for production levelling it was agreed to

concentrate more on weekly sequence planning for assembly line instead of the

paint shop requirements. The levelling is based on total cycle times, because

workstation specific levelling is almost impossible without a program that would

calculate all variances and fluctuations.

During the thesis project there are no clear computational evidence of better

production levelling based on weekly workload, but empirical observation and

according to foremen opinion the situation has improved a little. The real devel-

opment for production levelling was achieved by increasing buffer places after

paint shop. The problem that the paint shop messes up the sequence was

avoided by controlling the output form the outsourced paint shop. Bigger in-

process inventory enabled operators to choose the correct product according to

initial production sequence made by master planner. The buffer rate was fol-

lowed in production daily meetings by recording the amount of frames in buffers

and proactive production planning. The target value was set to seven sub-

frames and if the rate is below the target, corrective actions are made as soon

as possible.

After the implementation of more assembly focused production levelling and

stabilized buffer after paint shop, there are better possibilities to improve the

situation further. All in all sequence planning was identified to be very difficult to

use efficiently for demountable production because of so many variable factors.

In the future it is important to start optimization of production levelling and aim to

decrease in-process inventories with more detailed paint shop queue planning.

5.1.6 More detailed in-process inventory planning

In-process inventories are used to smooth the fluctuations in variable station

times and avoid idle time as an assembly line balancing method. The imple-

mentation plan was to make more detailed and controlled system for inventories

at assembly line area. The plan was also to implement CONWIP system and

control assembly line with buffer places, which would be used as pull signals for

the need of the next product in sequence.

More detailed buffer places were defined with value stream mapping, which is

presented in appendix 6. The buffer values serve also as targets for production

scheduling in production rate oriented system. Foremen use the new defined

buffer places in daily target setting and the ideal situation after a planned period

is usually according to value stream mapping work-in-process values.

80

The values should also be used as indicative number for CONWIP based pro-

duction control system. The system requires a good training for assemblers in

order to work properly. This training was not arranged during the thesis project,

but it will be held together with overall Lean and HPS training. Because of de-

layed training also implementation of CONWIP system is moved forwards. The

next plan is to mark buffer places with different colours that would indicate dif-

ferent pull signals for next products. Red colour is a CONWIP signal for previ-

ous workstation to fill the empty buffer place as soon as possible in order to

avoid idle time and starving for the next station. Additionally there will be yellow

buffer places, which will provide flexibility in high-mix assembly line. If all buffers

are full next product should not be moved to buffer place because it is overpro-

duction.

When these changes above have been implemented, the next focus will be on

decreasing buffer places with more detailed scheduling. The purpose of mini-

mizing in-process inventory is to achieve better value-adding rate percentage

which is calculated by dividing the value adding working time by lead time. In

the future there will be only CONWIP buffers used as balancing method and

additional buffers should be removed.

5.1.7 Flexible tasks between workstations

Balancing method of assigning identical task to different stations was not widely

used for demountable production, because of its various restrictions and chal-

lenges. The only exception was that final assembly line started to preassemble

fast lowering valves for cylinder assembly because it’s high workload. Idea was

to assign the task to another assembly station in order to balance the workload

among these stations and avoid idle time. The method is planned to be used

only when final assembly line does not have enough workload.

The implementation of this change turned out to be pretty difficult because em-

ployees did not have any agreed procedure to move to help the previous station

and supervisors needed to always arrange the process. It was also noticed in

some point that the method became unnecessary when other balancing meth-

ods were taken in use. The final assembly line did not have so much idle time

anymore and they were not able to help in tasks of the previous workstation. It

was finally analysed that this method was not very useful for demountable as-

sembly line except in special occasions.

5.1.8 Change to one shift system

During the thesis project the demountable assembly was changed to a one shift

system, which mainly affected on the assembly line balancing with a more

81

straightforward production planning. As a balancing method the change enabled

to allocate the correct number of workers for each workstation at all times. The

station where minimum of two workers are required because of safety reasons

should not have over capacity compared to other stations anymore during any

shift.

Before the change to one-shift system there was an informative meeting held

for all production employees. The main focus was to explain the reasons for the

change and introduce production planning activities related to it. Employees in

final assembly line had sceptical presumptions that the speed of the line would

not be high enough to feed them enough products. They thought that balancing

activities and improvement of productivity would not succeed. The change to

the one-shift system was made in week 21 and there was a two week test peri-

od arranged when production indicators were monitored more carefully.

The operation in the assembly line worked very well during the test weeks and

the balance situation was clearly improved. Employees from final assembly line

also changed their opinion to support new one-shift system because the flow of

products was more stable and workload more balanced during the work day.

Various computational balance calculations and employees opinions both show

that one-shift system has improved the assembly line operation and decreased

waiting times.

5.1.9 Problem solving and 5S for bottleneck stations

The development plan of this final method was to improve the working practices

in bottleneck stations of demountable assembly. There are two bottleneck sta-

tions in the assembly line for two different reasons. The other one is simply be-

cause of highest workload and the other one is workstation with most of the

problems that stops production.

In the first bottleneck station the focus was more on other balancing actions like

right allocation of employees, but also some waste elimination was made. The

most important tool was 5S audits where working practises and operation were

examined. There were lots of inefficient working detected and corrective actions

were made systematically. Small development actions include for example bet-

ter tools, material markings and safety instructions. Those have decreased

searching times and standardized operations to be more productive.

The other bottleneck station had enough resources but problem solving was not

efficient. The new system of simple stop signal and engineering change request

(ECR) top list where taken in use. Stop-signal, which was presented in the visu-

al management chapter, helped to react on problems faster. This signal was

82

especially practical for bottleneck station due to the amount of problems. During

the test weeks the use of stop signal and it root causes were analysed and the

station pressed stop signal 26% of the occurrences which was more than a

double compared to the average percentage. After the follow-up period this bot-

tleneck station has also clearly been the most active user of the stop-signal.

There are continuous problem solving actions made to prevent these problems

and stop signal has made the solving process more visual so that it is easier to

re-arrange production. Another action for problem solving was establishing of a

top list of production related engineering change requests (ECR) for R&D de-

partment. The purpose was to prioritize design problems so that critical issues

would be solved faster. This has made the communication between production,

engineering department and R&D department more transparent and system-

ised.

The development actions of this method of eliminating inefficiencies from bot-

tleneck stations was not analysed with target times so there is not statistical

evidence of development. However, it was clear that all these small actions

helped operation and improved problem solving processes. In the future it is

important to continue improvement actions and in some point also reflect the

improvements in target times.

5.2 Implementation of new production scheduling sys-tem

For the new production scheduling system there a test period was arranged for

weeks 21 and 22 at the same time when assembly screen was taken in use and

production changed to one shift system. During the test weeks production rate

oriented system with target times were analysed and more accurate measure-

ments were performed for assembly line. Production rates were planned for a

half day scheduling period so that each workstation has near to the same

amount of products to be assembled. The production scheduling system for tar-

get setting is presented in appendix 8. The objective was to find out how new

the more detailed scheduling system works together with balancing activities in

practice.

The original input for the target rates is the weekly target amount of completed

units which is based on orders and rough cut production planning. The weekly

target is then divided to daily target rates for each workstation and then sched-

uled for morning and evening rates separately. The current rate is coloured blue

in assembly screen so that it is visually presented for all employees. The next

step was to calculate the total target hours for each station and define the cor-

83

rect employee allocation. For this purpose a report in attachment 8 was used as

guidance for the foremen to balance employees equally to workstations.

During the first test week there were lots of different problems in production,

which disturbed the flow and balance of assembly line. It was very difficult to

follow how well different balancing actions and target setting works because

production sequence was messed up and products needed to take aside from

assembly line. However, employees and supervisors were pleased how well

one shift system generally worked, because there had been doubts about all

employees working together at same time. The space requirements and almost

double speed of the line did not cause any troubles and there were positive

feedback given for new time arrangements.

During the second test week there were not as much lacking parts or quality

problems in production so the starting situation for test of implementation plan

was better than during the first week. Also the follow up and measuring was

easier because production sequence was pretty stable after the first assembly

station. The second test week succeeded very well and the productivity was

13% higher than the average based on completed number of units. Overall the

assembly line balance was near to the ideal situation and production flow was

according to employees much better than before. The only problem during the

second test week was that paint shop was not able to provide enough frames

for assembly line. The buffer before first assembly station 5101 was empty con-

tinuously so it was impossible to implement any production levelling method.

The first assembly station was the only station that could not achieve the target

rate of the whole week. Other main assembly workstations 5102, 5103 and

5107 were capable for a very stable production flow, which indicated that new

more detailed production scheduling works well. Especially workers in the final

assembly station perceived that there is not as much idle time than before bal-

ancing actions. Subassemblies 5104, 5105 and 5106 did not follow targets that

were presented on assembly screen as well as main stations and their workload

was much more unstable. These fluctuations were balanced with buffers after

the subassembly so that waiting was not occurred in final assembly line.

After the test weeks there was a comprehensive report made for the results and

findings that were used for further balancing actions. The production rate ori-

ented system was analysed to be working method for current demountable as-

sembly, but there are still restrictions to implement shorter period than one day

target rates. There is quite much manual work to do in target setting and calcu-

lating employee allocation in variable production. Especially when assembly

sequence needs to be updated there should be more automatic calculation sys-

tem in order to reschedule production effectively for half only day period. After

84

the test weeks it was decided to continue production rate oriented system with

one day scheduling period. The restriction of a longer period is that workers

complete more products on morning or evening and there are not enough buff-

ers to smooth the temporary unbalance. CONWIP system must be used more

strictly to avoid waiting when completion of products is monitored only once a

day.

In the future the plan is to shorten the scheduling period from one day to half

day and then step by step even shorter. In order to shorten this predetermined

time period there must be continuous improvement in assembly line balancing,

problem solving and employee engagement. Currently the balance calculations

are made with the Spreadsheet software and if the scheduling period is short-

ened also employee calculation should be done automatically. In the future it is

possible that the time period is shortened to even match the planned cycle time,

when the system is changed to takt-driven scheduling method, which was the

initial goal of this thesis.

The assembly screen did helped workers to understand the current production

situation and the targets. Production target rates were displayed in televisions

for all workstations so that everybody knew what they should assemble during

the scheduled period. However there were some problems occurred during the

test weeks because of changes in product sequences and few technical unreli-

ability issues. After the test weeks there were more detailed responsibilities and

standardized updating methods were planned so that the assembly screen

could be used reliably. The assembly screen has worked very well after these

improvements very well and it is fulfils the objectives and requirements present-

ed in chapter 4.2.2 for the flexible target setting system. Production sequence,

current production situation, standard times, target rates for the period and

problem signals are visually presented in real time in televisions and computer

screens connected to Multilift Intranet. The next phase is to implement the cal-

culation of workstation specific employee allocation and productivities to the

program. The assembly screen was originally planned only for demountable

power series production scheduling but now also material handling department

has begun using the tool and there has been interest from other Hiab factories

too.

85

6 DISCUSSION

In this discussion chapter the theoretical and empirical studies are summarized

and compared to each other. The idea of the result analysis chapter is to ana-

lyse how well the thesis answers to the research question and fulfils the objec-

tives set in the introductory chapter. The purpose of the conclusion chapter is to

evaluate and compare how the different theories and assembly line balancing

methods are applicable in practice for the Hiab’s Raisio factory.

6.1 Result analysis

The objective was to create smooth, well planned and organized production

flow, and sub-objectives were to decrease waiting times and create a target

system for assembly line. There were nine different assembly line balancing

methods presented and all these methods were used development actions for

demountable assembly line.

Table 6.1. Assembly line balancing methods for demountable assembly line.

Method

Location/ place

Wh

ole

Ra

isio

pro

du

ctio

n

All a

sse

mb

ly

sta

tio

ns

51

00

Pre

asse

mb

ly

sta

tio

n

51

01

Su

bfr

am

e a

sse

mb

ly

51

02

Ele

ctr

icity a

nd

va

lve

asse

mb

ly

51

03

Cylin

de

r a

nd

ho

sin

g a

sse

mb

ly

51

04

Re

ar

fra

me

pre

asse

mb

ly

51

05

Mid

dle

fra

me

pre

asse

mb

ly

51

06

Ho

ok fra

me

pre

asse

mb

ly

51

07

Fin

al

asse

mb

ly lin

e

sum

1x x x x x x x x 8

2x x x x x 5

3x x x 3

4x x x 3

5x x 2

6x x x x x x 6

7x 1

8x x 2

9x x x x 4

sum 5 3 2 5 2 5 3 1 2 6

Continuous development actions to

present bottleneck station

Production planning and assembly line

balancing based on average work loads

Using multi-skilled work force

Preassembly for optional modules

Different routings for variable products

Sequence planning to level out the

workload

In-process inventory to avoid waiting

Balance assembly line with moving tasks

per station

Work time arrangements

86

Table 6.1 summarizes all the assembly line balancing actions used in different

workstations in demountable assembly. The X -marking indicates of some kind

of change that has been made with any specific method to some location. More

detailed description of each mark is presented in appendix 3. The most signifi-

cant results came from assigning near to the correct amount of employees to

different stations. This gave theoretically 31% better average load percentage

for demountables assembly line, which also decreased the waiting time signifi-

cantly. Other results related to assembly line balancing included an increased

use of flexible multi-skilled employees, new pre-assembly station, development

of routings and more detailed buffer places to control production flow. Most of

the balancing actions focused on 5107 final assembly line, but the most signifi-

cant changes were made to 5101 subframe assembly and 5103 cylinder and

hosing assembly workstations.

Workstation specific target setting and visual assembly screen were the biggest

improvement steps related to scheduling activities. More organised production

planning and all assembly line balancing actions cut idle time and provided

much better changes for even further productivity improvement. The knowhow

and planning practices improved significantly compared to the beginning situa-

tion. All together these development actions and results meet all the objectives

and expectations set for this project.

The research question aims to find answers for creation of a more balanced and

organised material flow for high-mix, low-volume type of assembly line in Hiab

Raisio factory. Literature review provides methods for high-mix assembly line

balancing and they were used as alternative development ideas (table 2.3).

There was an implementation plan made for each method (table 5.1) and sev-

eral different improvement actions were performed to balance the line (table

6.1). The other part of the research question was about organisation of material

flow and the solution was to implement production rate oriented system, which

is controlled with new visual assembly screen. The literature review and imple-

mentation plan do answer to the research question and the results have proved

improvement of the operation of demountable assembly line.

The goals of this thesis in the long run were to improve productivity and shorten

the lead time. During the project assembly line productivity increased over 15%

due to increased production volume calculated by average daily target hours

and decreased amount of employees (see appendices 4 and 5). It is difficult to

measure how much various improvement actions contributed to this increase

because of excess capacity in the beginning situation. The most significant noti-

fication was that this productivity ramp up succeeded exceptionally well and it

did not cause almost any problems. Also according to employees and supervi-

87

sors the efficiency and production flow of the assembly line has improved signif-

icantly compared to the beginning situation. These observations indicate that

the implemented balancing and scheduling actions have really improved the

assembly line operation and increased its capacity constraints. In case of short-

er lead time the thesis did not provide much result during the project. Part of the

balancing was to specify more detailed in-process inventory plans, which had a

slightly negative influence on lead time but positive effect on productivity. How-

ever with more organised production and better knowledge of current situation it

is possible to make improvements also to shorten the lead time in the future.

The updated value stream map will be used for that purpose. The scope will

also be larger than only assembly because it was analysed that the actual

benefits and development objects of shorter lead time are in logistics more than

in the assembly operation.

6.2 Subjects for further studies

There are many issues mentioned that should be researched in the future. The

most critical subject for further study is the production scheduling of the out-

sourced paint shop so that the operation would really support the flow of pro-

duction. The next project should concentrate on sequence of frames loaded to

the painting. Various different factors have an impact on the best possible se-

quence. Available frames, delivery date, paint colour, amount of layers, produc-

tion levelling for assembly line and many other. The current way to manage the

variable sequence is the buffer of seven frames after paint shop. Next develop-

ment subjects would be to plan the system so transparent and reliable that

there would be always correct frames and the buffer could be smaller. The paint

shop is the input for the assembly line so it must be perceived more as part of

assembly line in the future.

In an ideal situation there are seven sub-frames in the buffer after the paint

shop. Consequently, there are also many more other frames in the buffer before

subassemblies. The number is roughly seven plus all the work-in-process prod-

ucts in assembly line before the final assembly station. This will create space

problems for middle frames and hook frames. There should be a layout change

designed for the area to improve the assembly work flow. At the same time as-

sembly line balancing alternatives presented in this thesis could be analysed so

that the workloads of these stations would be more balanced.

The third subject for further study is to analyse possibilities for alternative tasks

for employees working in the assembly line. Research for additional tasks would

support balancing method of 3.7 assignment of identical tasks to different sta-

tions. The tasks could relate to 5S housekeeping program, continuous im-

88

provement, helping logistics or any activity which need to be done anyway in

some point. If there is excess capacity it is good to re-evaluate make or buy de-

cisions of different subassemblies. Furthermore, planning totally alternative

business ideas and work positions could be also thought. The objective is that

there would always be a plan B with extra tasks for idle workers in order to

reach best possible productivity and changes in demand would not affect it so

much.

6.3 Conclusions

The perspective of this study was to analyse different assembly line balancing

problems as alternative solution ideas for demountable production. There were

lots of information and theory available regarding assembly line balancing and

most of them were linked to lean manufacturing, which suited well for the case

company. However, it was very difficult to find theories or examples from litera-

ture, which would fit to the scope of this thesis. Also Boysen et al (2008, p.18)

insist that more research should be done for reconfiguration of real-world as-

sembly line balancing problems. There is a clear gab between the practical line

balancing problems and theoretical research, which lead to situation that there

are not enough widely approved methods to be applied for practice. The most

significant reason for the gap is the amount of different attributes that must be

considered in a real-world assembly line balancing problems and research pa-

pers typically concentrates on only few extensions in an isolated manner. For

instance, it is very hard to evaluate optimization of production levelling, assem-

bly line balancing and multi-skilled employees in one context and make compu-

tational experiments for all attributes simultaneously.

The main difference of this study compared to a literature review is that none of

the research papers or other sources included as many as nine different as-

sembly line balancing alternatives. Most of the researchers have studied only

two or three different methods at a time and the methods are often presented in

an isolated manner compared to each other. Also the style was somewhat dif-

ferent than in many research papers, which were more based on problem defi-

nition and characteristics than problem solutions. The most in-depth research

for assembly line balancing problems were made by Nils Boysen and Armin

Scholl, who had studied the subject for many different kinds of production envi-

ronments.

In this thesis all the nine different alternative solution ideas, implementation plan

and results were analyzed in one context together with an action research

methodology. The research was produced from a large variety of problem defi-

nitions without a coherent direction to any straightforward single solution. The

89

methods were characterized by the author and there were some inconsistency

between the methods presented in literature when compared to the categoriza-

tion of various attributes. As mentioned in many research papers it is extremely

difficult to evaluate the results of any specific action when more than one bal-

ancing methods are used simultaneously. There are also many restrictions and

problems which cannot be connected to any specific method and analysis. Due

to the complexity the balancing methods and real-world assembly system it is

impossible to create any generalized computational models or conclusions of

the results of this thesis compared to different theoretical formulations.

The most important phase of this thesis was the data analysis of the beginning

situation of the demountables assembly line. There were also many other anal-

yses made during the thesis, such as various time studies, comparisons,

productivity follow up and weekly production report by the author of this thesis.

It was interesting to recognise how much hard data and statistics influence on

the production planning decisions even though some issues had been noticed

and known beforehand. It clearly proved that it is much easier for supervisors to

make decisions based on analysed numbers, providing a rational explanation to

lean on. It is much harder to explain changes made based on feelings or obser-

vations made by leaders.

The thesis project clearly increased the knowledge of the operation in the de-

mountables power series assembly line. This study provides a good basis for

continuous improvement actions in the future. The assembly line balance situa-

tion should be continuously re-evaluated and assessed together with standard

times to achieve the best possible productivity. There are still lots of further

studies to be performed in order to achieve better balance and production con-

trol practices. The author of the thesis considers the project as a very interest-

ing, challenging and educational project. The objectives of this thesis were ful-

filled and the author believes that the thesis project has brought The Raisio fac-

tory a few steps closer to a more modern production planning environment.

90

7 REFERENCES

Amen, M. 2006. Cost-oriented assembly line balancing: Mole formulation, solu-

tion difficulty, upper and lower bounds. European Journal of Operational Re-

search 168, 747–770. 24p.

Aswathappa, K. Shridharabhat. K. 2009. Production and Operations Manage-

ment. Global Media. India. 665p.

Baudin, M. 2002. Lean Assembly - The Nuts and Bolts of Making Assembly

Operations Flow. Productivity Press. USA. 274p.

Baudin, M. 2012. Takt time: can it be universally applied to all types of produc-

tion? Michel Baudin’s Blog - Ideas from manufacturing operations. [WWW].

[Accessed on 28.9.2013]. Available at: http://michelbaudin.com/2012/10/31/takt-

time-can-it-be-universally-applied-to-all-types-of-production/.

Baybards, I. 1986. An Efficient heuristic method for the simple assembly line

balancing problem. International Journal of Production Research 24, 149–166.

18p.

Becker, C. Scholl, A. 2006. A survey on problems and methods in generalized

assembly line balancing. European Journal of Operational Research 168, 694-

715. 22p.

Becker, C. Scholl, A. 2009. Balancing assembly line with variable parallel work-

places: Problem definition and effective solution procedure. European Journal

of Operational Research 199, 359-374. 16p.

Boysen, N. Fliedner, M. Scholl, A. 2007. A Classification of assembly line bal-

ancing problems European Journal of Operational Research 183, 674–693.

20p.

Boysen, N. Fliedner, M. Scholl, A. 2008. Assembly line balancing: Which model

to use when?. International Journal Of Production Economies 111, 509-528.

24p.

91

Boysen, N. Fliedner, M. Scholl, A. 2009. Sequencing mixed-model assembly

lines: Survey, classification and model critique. European Journal of Operational

Research 192, 349–373. 25p.

Boysen, N. Kiel, M. Scholl, A. 2011. Sequencing mixed-model assembly lines to

minimise the number of work overload situations. International Journal of Pro-

duction Research Volume 49, Number 16, 4735–4760, 36p.

Cargotec Oyj. 2013a. Annual report 2012. Cargotec. Helsinki. 243p.

Cargotec Oyj. 2013b. Hiab Production system booklet. Cargotec. Helsinki. 60p.

Coromias, A. Pastor, R. Plans, J. 2008. Balancing assembly line with skilled

and unskilled workers. IOC Research Institute. Omega 38, 1126-1132. 6p.

Erel, E. Gokcen H. 1999. Shortest-route formulation of mixed-model assembly

line balancing problem. European Journal of Operational Research 116, 194-

204. 11p.

Fernandes, C. Groover, M. 1995. Mixed-model assembly line balancing and

sequencing: A survey. Engineering Design and Automation 1,33–42. 10p.

Haverila, J. Uusi-Rauva, E. Kouri, I. Miettinen, A. 2009. Teollisuustalous. Infacs

Johtamistekniikka Oy. Tampere. 243p.

Hillier, F. So, K. Boling, R. 1993. Toward Characterizing the Optimal Allocation

of Storage Space in Production Line Systems with Variable Processing Times.

The Institute of Management Sciences. Management Science Vol 39. USA. 9p.

Hobbs, D. 2011. Applied Lean Business Transformation. J. Ross Publishing.

USA. 510p.

Imai, M. 2012. Gemba Kaizen: A Commonsense Approach to a Continuous Im-

provement Strategy. Second edition. McGraw-Hill. USA. 426p.

Karabati, S. Sayin, S. 2003. Assembly line balancing in a mixed-model se-

quencing environment with synchronous transfers. European Journal of Opera-

tional Research 149, 417–429. 13p.

Konnully, J. 2013. Assembly-Line Balancing: An effective tool for improving

productivity. SlideShare Inc. [WWW] [Accessed on 29.7.2013] Available at:

http://www.slideshare.net/JosephKonnully/assembly-line-balancing.

92

Lane, G. 2007. Made-To-Order Lean Excellence in a high-mix, low-volume envi-

ronment. Productivity Press. USA. 209p.

Larco, J. Bortolan, E. Studley, M. 2008. Lean manufacturing in Build to Order,

Complex and Variable Environments. The Oeklea Press. USA. 192p.

Liker, J. 2004. The Toyota Way: 14 Management Priciples From The World’s

Greatest Manufacturer. McGraw-Hill. USA. 330p.

Little, J. Graves, S. 2008. "Little's Law". Building Intuition. International Series in

Operations Research & Management Science 115. 81p.

Merengo, C. Nava, F. Pozetti, A. 1999. Balancing and sequencing manual

mixed-model assembly lines. International Journal of Production Research 37,

2835–2860. 26p.

Ohno, T. 1982. How the Toyota Production System was Created. Japanese

Economy. Volume 10, Number 4, 83–101. 18p.

Pettersen, J. Segerstedt, A. 2009. Restricted work-in-process: a study of differ-

ences between Kanban and CONWIP. International Journal of Production Eco-

nomics 118, 199–207. 8p.

Rother, M. 2013. Improvement Kata Handbook – Grasp the Current Condition.

Version 17. 53p. [WWW] [Accessed on 1.8.2013] Available at: http://www-

personal.umich.edu/~mrother/Materials_to_Download.html.

Rother, M. Harris, R. 2001. Creating Continuous Flow. The Lean Enterprice In-

stitute. USA. 105p.

Salveson, M. 1955. The assembly line balancing problem. Journal of Industrial

Engineering 6, 18-25. 8p.

Scholl, A. Becker, C 2006. State-of-the-art exact and heuristic solution proce-

dures for simple assembly line balancing. European Journal of Operational Re-

search 168, 666–693. 28p.

Stephens P, Mayers F. 2010. Manufacturing Facilities Design and Material

Handling. Fourth edition. Pearson Education International. USA. 486p.

93

Thomopoulos, N.1970. Mixed model line balancing with smoothed station as-

signments, Management Science 16, 593–603. 11p.

Teräväinen, E. Cargotec – Siellä missä tavara liikkuu. Cargotec. 135p.

Öztürk, C. Tunalı, S. Hnich, B. Örnek, A. Balancing and scheduling of flexible

mixed model assembly lines. Constraints 18, 434–469. 36p.

Example of time and period dependent variances in high-mix production

Total cycle time

for the period

Unit 1 12

Unit 2 12

Unit 3 12

Unit 4 12

Unit 5 12

Unit 6 12

Unit 7 12

Total cycle time

for the period

Period -2 unit 1 5 n/a n/a n/a n/a

Period -1 unit 2 2 unit 1 2 n/a n/a n/a

Period 0 unit 3 3 unit 2 5 unit 1 2 n/a n/a

Period 1 unit 4 3 unit 3 3 unit 2 3 unit 1 3 12

Period 2 unit 5 2 unit 4 3 unit 3 2 unit 2 2 9

Period 3 unit 6 4 unit 5 4 unit 4 3 unit 3 4 15

Period 4 unit 7 3 unit 6 2 unit 5 4 unit 4 3 12

Period 5 n/a unit 7 3 unit 6 4 unit 5 2 n/a

Period 6 n/a n/a unit 7 2 unit 6 2 n/a

Period 7 n/a n/a n/a unit 7 4 n/a

period 1-4 total 12 12 12 12

total

Period 1 100 %

Period 2 100 %

Period 3 100 %

Period 4 100 %

33 %

27 %

17 %

Work station 1

25 %

22 %

27 %

25 %

22 %

27 %

25 %

Work station 3

25 %

22 %

20 %

33 %

Period cycle times in paced assembly without buffers

Unit cycle times

Percentage of workers needed to balance the paced assembly line

Work station 4

25 %

Work station 2

25 %

4

2

3

2

4

3

2

2

4

2

3

2

3

4

4

3

2

5

3

3

4

2

3

Work station 1 Work station 2 Work station 3 Work station 4

Work station 1 Work station 2 Work station 3 Work station 4

5

2

3

3

2

3 24 3

3

2

3 4

3

3

42

3

2

4

3

0

2

4

6

8

10

12

14

16

Period 1 Period 2 Period 3 Period 4

Tota

l ass

em

bly

tim

e p

er

pe

rio

d

Periods in paced and synchronous assembly without buffers

Time and period depended variances in high-mix production

Work station 1

Work station 2

Work station 3

Work station 4unit 4

All units has equal

total cycle time

Work station cycle

time range is 2-4

hours

High variance in

total cycle time per

periods althought all

the products have

equal total cycle time

Equal amount of work

for every work station

for periods 1-4.

APPENDIX 1: EXAMPLE OF TIME AND PERIOD DEPENDENT VARIANCES IN A HIGH-MIX PRODUCTION