chp 4

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Work Systems and the Methods, M easurement, and Management of W orkby Mi kell P. Groover, ISBN 0-13-140650- 7.

©2007 Pearson Education, Inc., U pper Saddle Ri ver, NJ. All rights r eser ved.

Manual Assembly Lines

Sections:1. Fundamentals of Manual Assembly Lines2. Analysis of Single Model Assembly Lines3. Line Balancing Algorithms4. Other Considerations in Assembly Line

Design5. Alternative Assembly Systems

Chapter 4



Manual Assembly Lines

Work systems consisting of multiple workers organized to produce a single product or a limited range of products

Assembly workers perform tasks at workstations located along the line-of-flow of the product

Factors favoring the use of assembly lines: High or medium demand for product Similar or identical products Total work content can be divided into work

elements Not possible to automate assembly tasks

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Why Assembly Lines are Productive

Specialization of labor Learning curve

Interchangeable parts Components made to close tolerances

Work flow Products are brought to the workers

Line pacing Workers must complete their tasks within

the cycle time of the line



Manual Assembly Line

A production line that consists of a sequence of workstations where assembly tasks are performed by human workers

Products are assembled as they move along the line At each station a portion of the total work

content is performed on each unit Base parts are launched onto the beginning of

the line at regular intervals (cycle time) Workers add components to progressively

build the product

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Manual Assembly Line

Configuration of an n-workstation manual assembly line



Two assembly operators working on an engine assembly line (photo courtesy of Ford Motor Company)

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Assembly Workstation

A designated location along the work flow path at which one or more work elements are performed by one or more workers

Typical operations performed at manual assembly stations

Adhesive application

Sealant application

Arc welding

Spot welding

Electrical connections

Component insertion

Press fitting

Riveting

Snap fitting

Soldering

Stitching/stapling

Threaded fasteners



Work Transport Systems

Manual methods Work units are moved between stations by

the workers without powered conveyor Problems: Starving of stations Blocking of stations

Mechanized work transport - types: Continuously moving conveyor Synchronous transport Asynchronous transport

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Coping with Product Variety

Single model assembly line (SMAL) Every work unit is the same

Batch model assembly line (BMAL) Two or more different products Products are so different that they must be

made in batches with setup between Mixed model assembly line (MMAL) Two or more different models Differences are slight so models can be

made simultaneously with no downtime



Analysis of Single Model Lines

Annual demand Da must be reduced to an hourly production rate Rp

where Sw = number of shifts/week, and Hsh = number of hours/shift

shw

ap HS

DR50

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Determining Cycle Time

Production rate Rp is converted to a cycle time Tc, accounting for line efficiency E

where 60 converts hourly production rate to cycle time in minutes, and E = proportion uptime on the line

pc R

ET 60



Number of Workers Required

The theoretical minimum number of workers on the line is determined as:

w* = Minimum Integer

where Twc = work content time, min; and Tc = cycle time, min/worker

c

wc

TT

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Theoretical Minimum Not Possible

Two reasons why theoretical minimum number of workers cannot be achieved in practice:

Repositioning losses – Some time will be lost at each station every cycle for repositioning the worker or the work unit; thus, the workers will not have the entire Tc each cycle

Line balancing problem – It is not possible to divide the work content time evenly among workers, and some workers will have an amount of work that is less than Tc



Repositioning Losses

Repositioning losses occur on a production line because some time is required each cycle to reposition the worker, the work unit, or both Repositioning time = Tr

Service time = time available each cycle for the worker to work on the product Service time Ts = Tc – Tr

Repositioning efficiency Er = c

rc

c

s

TTT

TT

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Cycle Time on an Assembly Line

Components of cycle time at several stations on a manual assembly line



Line Balancing Problem

Given: The total work content consists of many distinct

work elements The sequence in which the elements can be

performed is restricted The line must operate at a specified cycle timeThe Problem: To assign the individual work elements to

workstations so that all workers have an equal amount of work to perform

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Work Element Times

Total work content time Twc

Twc =

where Tek = work element time for element kWork elements are assigned to station i that add up to the service time for that station

Tsi =

The station service times must add up to the total work content time

Twc =

en

kekT

1

ik

ekT

n

isiT

1



Precedence Constraints

Restrictions on the order in which work elements can be performed

Precedence diagram

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Measures of Balance Efficiency

Line balance efficiency Eb

Eb =

Balance delay d

d =

Note that Eb + d = 1

s

wc

wTT

s

wcs

wTTwT



Worker Requirements

The actual number of workers on the assembly line is given by:

w = Min Int sb

wc

cbr

wc

br

wcp

TET

TEET

EEETR

60

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Workstation Manning Level

Defined as the number of workers per station For a single station, station i, Mi = wi

For the line, M =

where w = number of workers, and n = number of stations on the line

nw



Tolerance Time

Defined as the time a work unit spends inside the boundaries of the workstation

Provides a way to allow for product-to-product variations in task times at a station

Tt =

where Tt = tolerance time, min; Ls = station length, m (ft); and vc = conveyor speed, m/min (ft/min)

c

s

vL

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Line Balancing Objective

To distribute the total work content on the assembly line as evenly as possible among the workers

Minimize (wTs – Twc) or

Minimize (Ts - Tsi)

Subject to: (1) Ts

(2) all precedence requirements are obeyed

∑∈ ik

ekT

∑1=

w

i



Line Balancing Algorithms

1. Largest candidate rule2. Kilbridge and Wester method3. Ranked positional weights method, also

known as the Helgeson and Birne method

In the following descriptions, assume one worker per workstation

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Largest Candidate RuleList all work elements in descending order based

on their Tek values; then,1. Start at the top of the list and selecting the first

element that satisfies precedence requirements and does not cause the total sum of Tek to exceed the allowable Ts valueWhen an element is assigned, start back at the top of the list and repeat selection process

2. When no more elements can be assigned to the current station, proceed to next station

3. Repeat steps 1 and 2 until all elements have been assigned to as many stations as needed



Largest Candidate Rule

1,20.1430.116110.1212-0.21

6,7,80.27920.3530.327

5,80.3810-0.42

9,100.5113,40.6810.73

Preceded byTekWork element

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Solution for Largest Candidate Rule



Solution for Largest Candidate Rule

Physical layout of workstations and assignment of elements to stations using the largest candidate rule

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Kilbridge and Wester Method

Arrange work elements into columns according to their positions in the precedence diagram

Work elements are then organized into a list according to their columns, starting with the elements in the first column

Proceed with same steps 1, 2, and 3 as in the largest candidate rule



Kilbridge & Wester Method

Arrangement of elements into columns for the K&W algorithm

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Ranked Positional Weights Method

A ranked position weight (RPW) is calculated for each work element

RPW for element k is calculated by summing the Te values for all of the elements that follow element k in the diagram plus Tek itself

Work elements are then organized into a list according to their RPW values, starting with the element that has the highest RPW value

Proceed with same steps 1, 2, and 3 as in the largest candidate rule



Other Considerations in Line Design

Methods analysis To analyze methods at bottleneck or other

troublesome workstations Utility workers To relieve congestion at stations that are

temporarily overloaded Preassembly of components Prepare certain subassemblies off-line to

reduce work content time on the final assembly line

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Other Considerations - continued

Storage buffers between stations To permit continued operation of certain

sections of the line when other sections break down

To smooth production between stations with large task time variations

Parallel stations To reduce time at bottleneck stations that

have unusually long task times



Alternative Assembly Systems

Single-station manual assembly cell A single workstation in which all of the

assembly work is accomplished on the product or on some major subassembly

Common for complex products produced in small quantities, sometimes one of a kind

Assembly by worker teams Multiple workers assigned to a common

assembly task Advantage: greater worker satisfaction Disadvantage: slower than line production

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