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Retractable Boarding Step to Scania Crew Cab
Product Development, Design, FEM Simulations and Verification
Utfällbart insteg till Scania Crew Cab lastbilar
Produktutveckling, konstruktion, FEM simulering och verifikation
Nadine Möllberg
Faculty of health, Science and Technology
Degree Project for master of Science in Engineering, Mechanical Engineering
30 hp
Supervisor: JanErik Odhe and Henrik Jackman
Examiner: Jens Bergström
2017-05-23
Serial number: 1
Abstract
This master thesis treats the development of a new solution for a retractable boarding step to Scania
trucks, which is a part of the modular system.
Some customers have the need to transport additional passengers. For such applications, Scania
provides trucks featuring a Crew Cab, which is an extended cab with rear doors. Easy exit through the
rear doors is important for many customers who use this type of cabin. Therefore, there is a possibility
to get a retractable boarding step, equipped with an upper step and a lower foldable, that enables easy
entry and exit. This function is especially important for fire fighters carrying heavy equipment and
therefore has more difficulties exiting the truck. The robustness and dependability of the function is
critical to ensure the safety. If it fails, the legal requirements are not met while driving or even worse,
injury may occur. Pneumatics is used for the fold out of the step and a spring folds it in.
The current boarding step needs improvement in order for it to be dependable and robust. If the
boarding step have not been folded out, the entry and exit of the cab is not possible.
This thesis covers the product development, simulations and verification of a new boarding step
concept that shall improve the entry- and exit function, making it more robust and dependable.
Through problem identification, a product specification and a thorough concept generation and
development process a final concept has produced.
Simulations were made in order to verify that the step could be stepped on when entering and exiting
the cab.
The result was a lower step, sliding on linear bearings in a linear motion. This enabled egress and
ingress independent of the extraction or retraction of the step. This makes it more reliable than the
current product. The sliding mechanism need to be tested in order to ensure the robustness. A
prototype was made in order for the function to be tested.
Keyword: Product development, FEM Simulations, Catia GAS, Boarding Step, Crew Cab, Scania CV
AB
Sammanfattning
Detta examensarbete behandlar utvecklingen av en ny lösning för ett fällbart insteg till Scania lastbilar,
som är ett modular product
Scania utvecklar lastbilar till ett brett spektrum av tillämpningar. Behov av att transportera ytterligare
passagerare kan finnas. För sådana tillämpningar erbjuder Scania specialfordon med manskapshytt, en
förlängd hytt med bakdörrar. Enkel urstigning genom bakdörrarna är viktigt för många kunder som
utnyttjar denna typ av hytt. Därför finns möjligheten att få ett utfällbart steg som förenklar in- och
urstigning, utrustad med ett fast övre steg och ett fällbart nedre. Detta är särskilt viktigt för brandmän
som bär tung utrustning och därför har svårare att ta sig ur bilen. Det utfällbara steget fälls ut för enkel
åtkomst när det behövs och viks in under körning. Detta för att uppfylla lagkrav för fordonsbredd.
Tryckluft används för att fälla ut steget och en fjäder används för att fälla in det.
Flera fall där problem gällande in- och utfällningsmekanism har påvisats, fälls inte steget ut är det inte
möjligt att stiga i och ur hytten. Förbättring av robusthet och pålitlighet behövs genomföras.
Examensarbetet ska förbättra instignings- och urstigningsfunktionen och se till att den är robust och
pålitlig.Genom att identifiera problemet, upprätta en produktspecifikation och genomföra en grundlig
konceptgenerering och utvecklingsprocess har ett slutgiltigt koncept tagits fram.
FEM simuleringar gjordes för att kunna verifiera att insteget håller för att kliva på.
Resultatet blev ett undre steg, som glider på linjärlager. Detta möjliggör i och urstigning oberoende av
i vilket läge steget är. Det gör den mer tillförlitlig än dagens produkt. Glidmekanismen måste testas för
att kunna säkerställa dess robusthet. En prototyp har tillverkats för att testa detta.
Nyckelord: Produktutveckling, FEM Simulering, Catia GAS, Insteg, Crew Cab, Scania CV AB
Table of content
1 Introduction ..................................................................................................................................... 1
1.1 The company ........................................................................................................................... 1
1.2 The modular way of thinking .................................................................................................. 1
1.3 Crew Cab ................................................................................................................................. 1
1.4 Retractable boarding step ........................................................................................................ 2
1.5 Dependability and robustness .................................................................................................. 2
1.6 Goals ........................................................................................................................................ 2
1.7 Purpose .................................................................................................................................... 3
1.8 Delimitations ........................................................................................................................... 3
2 Methods ........................................................................................................................................... 3
2.1 Analysis of current product ..................................................................................................... 3
2.2 Problem definition ................................................................................................................... 3
2.3 Regulations by law .................................................................................................................. 4
2.4 Corporate standards ................................................................................................................. 4
2.5 Product requirements ............................................................................................................... 4
2.5.1 Identification of customer requirements .......................................................................... 4
2.6 Product specification ............................................................................................................... 4
2.7 Comparison with others ........................................................................................................... 5
2.8 Concept generation .................................................................................................................. 5
2.9 Concept development .............................................................................................................. 5
2.10 Concept selection .................................................................................................................... 5
2.11 Detailed engineering ................................................................................................................ 6
2.11.1 Carry over ........................................................................................................................ 6
2.11.2 Quality ............................................................................................................................. 6
2.11.3 Design for Safety and Reliability .................................................................................... 9
2.11.4 Geometry assurance......................................................................................................... 9
2.11.5 DFA – Design For Assembly ........................................................................................ 10
2.11.6 Aftermarket and maintenance ........................................................................................ 11
2.12 Verification and FEM ............................................................................................................ 11
2.12.1 Simplifications ............................................................................................................... 11
2.12.2 Model ............................................................................................................................. 12
2.12.3 Materials ........................................................................................................................ 12
2.12.4 Mesh and mesh elements ............................................................................................... 12
2.12.5 Boundary conditions ...................................................................................................... 12
2.12.6 Load condition ............................................................................................................... 12
2.12.7 Structural stiffness ......................................................................................................... 16
2.12.8 Bolts and bolt pretension ............................................................................................... 16
2.13 Prototype ............................................................................................................................... 20
3 Results ........................................................................................................................................... 21
3.1 Analysis of current product ................................................................................................... 21
3.1.1 Function and design ....................................................................................................... 21
3.1.2 Materials ........................................................................................................................ 23
3.1.3 Fail causes ..................................................................................................................... 24
3.1.4 Simulation results .......................................................................................................... 25
3.2 Problem definition ................................................................................................................. 29
3.3 Regulations by law ................................................................................................................ 29
3.4 Corporate standards ............................................................................................................... 29
3.5 Product requirements ............................................................................................................. 29
3.5.1 Identification of customer requirements ........................................................................ 29
3.6 Product specification ............................................................................................................. 31
3.7 Comparison with others ......................................................................................................... 33
3.8 Concept generation ................................................................................................................ 33
3.9 Concept development ............................................................................................................ 33
3.10 Concept selection .................................................................................................................. 33
3.11 Detailed engineering .............................................................................................................. 35
3.11.1 Carry over ...................................................................................................................... 35
3.11.2 Upper step plate ............................................................................................................. 36
3.11.3 Lower step plate ............................................................................................................ 36
3.11.4 Sliding rails .................................................................................................................... 37
3.11.5 Side brackets .................................................................................................................. 38
3.11.6 Lower bracket ................................................................................................................ 38
3.11.7 Pneumatic cylinder ........................................................................................................ 38
3.11.8 Quality ........................................................................................................................... 39
3.11.9 Design for Safety and Reliability .................................................................................. 40
3.11.10 Geometry assurance................................................................................................... 40
3.11.11 DFA – Design For Assembly .................................................................................... 41
3.12 Verification and FEM ............................................................................................................ 44
3.12.1 Simplifications ............................................................................................................... 44
3.12.2 Model ............................................................................................................................. 45
3.12.3 Materials ........................................................................................................................ 45
3.12.4 Mesh and mesh elements ............................................................................................... 47
3.12.5 Boundary conditions ...................................................................................................... 51
3.12.6 Load condition ............................................................................................................... 53
3.12.7 Structural stiffness ......................................................................................................... 60
3.12.8 Bolts and bolt pretension ............................................................................................... 61
3.13 Prototype ............................................................................................................................... 64
4 Discussion ..................................................................................................................................... 67
4.1 Final concept ......................................................................................................................... 67
4.2 Fulfillment of goal ................................................................................................................. 67
4.3 Methods and processes .......................................................................................................... 67
4.4 Simulations ............................................................................................................................ 68
4.5 Prototype ............................................................................................................................... 70
5 Future work ................................................................................................................................... 70
6 Conclusions ................................................................................................................................... 71
7 Acknowledgements ....................................................................................................................... 72
8 Bibliography .................................................................................................................................. 73
Appendices
Appendix A - Variable definition
Appendix B - Deflection and stiffness
Appendix C - QFD matrix
Appendix D - Comparison with others
Appendix E - Concepts
Appendix F - Concept development
Appendix G - Concept selection
Appendix H - Friction force
Appendix I - Spring and Cylinder
Appendix J - FMEA
Appendix K - FEM model verification
Abbreviations
2D - Two Dimensional
3D - Three Dimensional
CAD - Computer Aided Design
Catia GAS - Catia Generative Structural Analysis
CP - Cab P
DFA - Design for Assembly
FEM - Finite Element Method
FMEA - Failure Mode and Effect Analysis
FTA - Fault Three Analysis
PEEQ - Equivalent Plastic Strain
UHMW PE - Ultra High Molecular Weight Polyethylene
Secrecy omissions
- Fatigue load: Acceptance criterion for fatigue load is replaced
- Static load: Acceptance criterion for static load is removed
- Tilted load: Acceptance criterion for tilted load is removed
- g-loads: Acceptance criteria for accelerating forces are removed
- Failure probability: Acceptance criterion for failure probability is removed
- Fatigue cycles: Acceptance criterion for fatigue cycles is removed
- Structural stiffness: Acceptance criterion for structural stiffness is removed
- General user weight: General user weight is removed
- Plastic deflection: Acceptance criterion for plastic deflection is removed
1
1 Introduction
This chapter will give a short description of the company and background to the problem.
The reader will be introduced to the assignment and will be followed by the purpose and
goals of the project.
1.1 The company Scania is a globally leading manufacturer of heavy trucks, busses and industry- and marine engines.
The truck program provides delivery vehicles, long haulage transports, coaches, local busses and so on.
For each need, there needs to be a solution.
1.2 The modular way of thinking The modular system is the link between the customer demand and the demands of Scania. By having a
modular system, there is the possibility to fulfill each customer need without developing a completely
new vehicle for each application or customer. Modular building blocks enable an almost infinite
number of combinations tailored for each customer need, the principle is shown in Figure 1-1.
Figure 1-1. Schematic picture of the module system and its performance step theory.
1.3 Crew Cab
Figure 1-2. CP28 and CP 31 respectively.
2
Scania is developing vehicles for a broad spectrum of applications. The cabins, transporting drivers
and passengers, are modular and available in different shapes and sizes.
Some customers have the need to transport additional passengers. For such applications, Scania
provides trucks featuring a Crew Cab, which is an extended cab with rear doors. It is possible to get it
in two different lengths, either the CP28 or the CP31 measuring lengths of 2.8 m and 3.1 m
respectively. The CP28 can house five to six persons while the CP31 can house six up to eight persons.
These types of trucks are often used as fire trucks, rescue vehicles, tow trucks and other public
vehicles.
1.4 Retractable boarding step Easy exit through the rear doors is important for many customers who use this type of cabin. Therefore,
there is a possibility to get a retractable boarding step that enables easy entry and exit. The steps low,
wide and comfortable configuration allows for this. This function is especially important for fire
fighters carrying heavy equipment and therefore has more difficulties exiting the truck. The step
enables exiting the truck facing forward instead of going backwards. The step folds out for easy access
when needed and is folded in while driving, this to meet the legal requirements for vehicle width and
safety.
The robustness and dependability of the function is critical to ensure the safety. If it fails, the legal
requirements are not met while driving or even worse, injury may occur.
1.5 Dependability and robustness The dependability of a technical system can be defined as the measure of the system availability,
reliability and maintainability. It is a way of describing the functional safety and the characteristics,
linked to the fulfillment of it. [1]
Scania adopts the dependability definition from SS-EN 13306:2010.
Dependability is the ability of an item to perform a required function under given conditions for a
given time interval.
The robustness of a technical system can be defined as a system with low downtime, failure rate,
variability and insensitivity to continuous changing external environment. [2]
1.6 Goals The project should improve the entry- and exit functions and ensure that the functions will not fail.
This by identifying and delimiting problems and apply knowledge obtained in studies for a master’s
degree at the Karlstad University.
The project treats detail design, dimensioning and improvement of the entry and exit mechanism. The
concept generation has been performed in another thesis work [3].
With the help of computer-aided functions like Computer Aided Design and Finite Element Method
and with calculations the function is to be validated.
The final function should then be further evaluated with a prototype with respect to treated objectives.
The main requirements are:
o Durable to stand on
o Robust
o Dependable
o Easily mounted
o Fulfill European standards
o Fit in the interface
3
1.7 Purpose This master thesis work has been carried out at Scania CV AB as a part of the Master of Science
degree in mechanical engineering at Karlstad University.
The aim of the project is to be of value for the development of special vehicles. The facilitation of
entry and exit from special vehicles will be evaluated by analysis and synthesis.
1.8 Delimitations The extent of the project is set to 20 working weeks. The project will cover the entry and exit function
with respect to what the passengers set their feet on but will not include surrounding functions like
handles, lighting and other parts that simplify boarding.
Following delimitations are made:
o The interface of the step is not altered.
o No other functions of the boarding than the step function are treated.
o European standards are to be fulfilled.
o Specification of manufacturing of the product is not included. However, it is taken into
consideration when designing the product.
o The interfacing systems are not taken into account regarding placement and dimensioning.
o The lifecycle of the product is not analyzed. However, an overall approach regarding the
production, lifetime and end of life is taken into throughout the project.
2 Methods
The following section describes all methods used in the project. For development of the new
retractable boarding step, the product development methods described in [4] were used.
Only the steps in the method considered relevant were used and some methods were
modified to meet the need of this thesis. For verification, Scania specific methods were used.
2.1 Analysis of current product Analysis of current product is done by interviews, footage and finding documentation. Aspects like
function, material, fail causes, environmental impact and cost are taken into consideration.
Current boarding step have been verified through a stress analysis and structural stiffness analysis
made in Abaqus. To get consistent simulation results an analysis of the structural stiffness is also made
in Catia GAS.
2.2 Problem definition A first step in the process of the product development is to formulate the problem. The complexity of
design problems often requires systematic approaches when formulating the problem. At first, the
level of approach needs to be considered, if the level should be broad with a more general approach or
if it should be narrow with a more concretely problem formulation. This determines the suitable
starting point of the problem definition.
In order to formulate the underlying problem with the current product the question method is used. It
includes answering a questionnaire consisting of a series of questions that will lead to the root cause of
the problem. The question method can be used on both a broad level handling a specific solution or on
a more specific level with a broad perspective. [4]
4
2.3 Regulations by law In order to be able to sell the product, it needs to fulfill the regulations and standards of the intended
market. These can be provided from the marketing section that knows where the product is to be sold
and what need to be taken in consideration at each market.
2.4 Corporate standards Scania provides standards, documents containing corporate approved standardization results in the
form of specifications, rules and recommendations for application within Scania. This will assist in the
development of new products and the choices made.
2.5 Product requirements In the product requirement definition phase, additional information that is not stated in the terms of
reference at the project start is provided. The product requirements are used both in the development
of solutions and later as references in the evaluation of these solutions. It concretizes the problem,
engages the stakeholders in the process, and therefore simplifies the development [4]. The product
requirements are later compiled into a product specification.
The requirements can be classified into two categories, solution operating and solution limiting. The
former is related to the required functions of the solution and the latter is related to fulfillment of
standards, demands etc. In the synthesis phase, the solution operating requirements are used to
generate solutions and the solution limiting ones are used to evaluate and eliminate solutions.
2.5.1 Identification of customer requirements
Identification of customer requirements is a vital part in establishing a product specification.
Stakeholders are persons, groups or organizations that in some way are affected by the project or could
provide information and requirements [4]. By defining the stakeholders, their connections and
contribution to the requirements a complete product specification is made possible.
Examples on different stakeholders are customers, service organization, manufacturing section,
marketing section, assembly section, society.
It is also of importance to quantify the different requirements, needs, wants and expectations in order
to prioritize.
2.6 Product specification The content of the product specification is of great importance in the development of products. The
product specification is developed and refined continuously through the development process as
knowledge about the solution is improved.
Requirements should be stated in a product specification as follows:
o Complete – all identified stakeholders and aspects should be considered.
o Requirements shall be formulated independent on the solution and be unambiguously.
o Requirements shall be measurable and controllable, if possible.
o The specification shall be non-redundant.
It may not be entirely possible to achieve measurable results, experienced characteristics like
ergonomics, form and feel. It is essential for the customer and can be what differentiates the product
from its competitors [4]. The document is governing and changes are approved through
communication with the client.
To be able to translate the needs of the customer into requirements and wishes a QFD matrix is used.
The terminology behind it is described in the thesis work of Cederlöf [3].
5
2.7 Comparison with others Comparison with others is done by investigation of competitors and their services. The benefits and
disadvantages with solutions present on the market are addressed. This will help as a support in the
further concept generation and development.
2.8 Concept generation The concept generations aim is to reach a number of concepts that fulfills the product requirements. It
is focused on creative methods and evaluation methods, explained in another master thesis work [3].
2.9 Concept development After completion of concept generation and concept selection, done in the master thesis carried out by
Cederlöf [3], the chosen concept should be further developed into a functioning product that satisfies
the requirements of the product specification. Functional and operational properties are in focus but
other properties needs to be considered, like production, aesthetic, environmental impact, and
economical functions.
The product concept is a first approach on a solution and should contain [4]:
o A preliminary product layout with estimations of space.
o A preliminary weight estimate.
o Descriptions in text or sketches.
o Description of the properties of the solution relative to the product specification.
o Motivation of the choice of the input part solutions
o Summary of the calculations, analyzes and experiment results.
2.10 Concept selection The concept development will lead up to a final concept selection. This is done with a reduction
method called concept scoring. This is done with Pug’s relative decision matrix, where the selection is
based on the relative comparison between different concepts. [4]
Information produced in the concept development phase is used to do the comparison. After one round,
one or more concept with lower rankings are removed.
Table 2-1. Concept scoring table
Before moving on to the next evaluation round it should be investigated if new stronger concepts
could be created by:
o Modifying already existing strong concepts so that their minus assessments will be eliminated.
o Combination of concepts with different strong sides so that the combined concept gets mostly
positive assessments.
1 (ref) 2 3 4 5
Wish A 1 D + 0 0 -
Wish B 2 A - + + -
Wish C 3 T 0 + 0 -
Demand D 5 U + + - 0
Wish E 4 M + - - 0
10 10 2 0
7 1 4 9
2 4 9 6
0 8 6 -7 -6
3 1 2 5 4
yes yes yes no no
Total value
Ranking
Proceed with concept
Requirement WeightConcepts
Sum +
Sum 0
Sum -
6
Next round of the scoring method will contain the remaining concept and new ones added in the
process. A new reference is used. The iteration will continue until convergence is reached, when there
is no change in the result. [4]
2.11 Detailed engineering The concept development process will generate the best possible solution to the problem relative to the
product specification. In order to do further assessment and development of the solution into a realized
design a detailed construction phase is followed. Through methods stated below the design is being
improved regarding, cost, assembly, quality, robustness, service etcetera.
2.11.1 Carry over
Information about the product stated in the concept description needs to be concretized and be made
detailed. The different parts of the product can be divided into [4]:
o Standard components, available internally or externally.
o Unique parts, produced internally or externally.
The parts or modules that can be reused from one generation of a product to another are called
carryovers. The benefits of carryovers are that they already have been quality assured and do not need
extensive development. [4] Reducing in logistic cost is also a result of carry over since spare parts
becomes identical.
2.11.2 Quality
Quality requirements have been one of the key drivers in the automotive industry for the last 30 years.
As customers increase their expectations regarding reliability and robustness and reduced maintenance,
the quality aspect is imposed. At the same time, eventual complaints will quickly spread among users
as the communications improves and will affect the image of the company. Vehicles are becoming
more compact and fuel-efficient by using new materials and complex components. These features lead
to an increased probability of contact or clash between parts, causing quality issues like noise, wear
and in worst-case injuries. [5]
For a profitable company it is also important to get the right quality for the use, called “fitness for use”
[6]. Both the customer and the company need to be satisfied.
The quality of the product is determined already during the design phase; it therefore needs to be built
into the product. The right choice of material, production method, dimensioning method and a design
solution is therefore crucial to achieve a product with desirable quality, and function.
Some methods for managing identified risks are [7]:
o Eliminate - Change strategy. Change the goals or reschedule
o Prevent - Reduce the probability of the event occurring
o Prepare - Have a contingency plan to minimize the consequence
o Ignore - Ignore the risks of low probability and/or mild consequence
This can be done with different kind of methods, two treated below.
2.11.2.1 FMEA – Failure Modes and Effects Analysis
FMEA, Failure Mode and Effect Analysis, is a method for systematic identification of possible
failures and the estimation of the related risks and is a way to prevent and prioritize quality problems.
FMEA can be divided into Design FMEA and Process FMEA and the latter will be executed [4].
Different failure modes, failure causes and the failure effects are defined. By doing an assessment
about their Occurrence (O), Severity (S) and Detectability (D) with a weighting, a probability of the
component failing through the specified failure can be made. The values of O, S and D range from 1 to
7
10 (rating shown below) and by multiplying these, Risk Priority Number, RPN quantifies the risk
effects of component failures. [4]
The estimation of the weighting is based on following information [4]:
o Occurrence
1 = Very small occurrence
4 = Certain occurrence
10 = High occurrence
o Severity
1 = Negligible effect on the function
4-6 = Quite severe errors
10 = Serious errors affecting person safety and/or legislation
o Detectability
1 = The error is almost certainly discovered
4-6 = The error is possibly discovered
10 = Difficult to detect and is almost certainly not detected
This is conducted in an FMEA report configured as in Figure 2-1. It is used to get an overview of the
risks and a way of prioritizing what problem to handle first.
To know when the risk is low enough to be accepted, the rule of thumb is that the Risk Priority
Number should be lower than 100.
Together with a Fault Three Analysis described later it makes an exceptional way of increase
robustness of the solution and decrease failures due to design.
Figure 2-1. Choice of FMEA, changes from original by Johannesson. [4]
2.11.2.2 FTA – Fault Tree Analysis
Fault Tree Analysis is a method to identify the relationship between failures on a system level and root
causes on a subsystem- and component level. A top-down method starting with a top event and
working backwards of the tree to determine the root causes of the top event. The different
dependencies of the errors and events are presented with logical gates. The dependencies may be
different in character and therefore are represented with graphical symbols shown in Figure 2-2. [8]
Figure 2-2 Fault tree logic gate symbols
Function Component Failure mode Failure cause Failure effect O S D RPN Solution Responsible Action taken O S D RPN
A 1
B 2
C 4
8
The AND gate is used to illustrate that all of the underlying events must have occurred for the parent
event to take place. Hence the output of an AND gate is true if all of its input events are true. For an
OR gate only one of the underlying events needs to occur. The output event occurs if at least one of
the input events occurs. [8]
The XOR gate is a special case of the OR gate, true if one and only one of its input events are true. It
is considered a two input gate where only one of two inputs is true. The INHIBIT gate is a special case
of the AND gate, it produces an output only when its input event is true in the presence of a
conditioning. [8]
Sohag 2017 illustrates the FTA approach by showing an example of a fire detection system, Figure 2-3.
“A fire detection system can fail if both smoke detector unit and heat detector unit fail but not by the
failure of just one unit. Similar to the OR gate there may be any number of input events to an AND
gate but in contrast to the OR gate, the AND gate usually represents a causal relationship between its
inputs and outputs.“ [8]
Figure 2-3 FTA analysis of fire protection system, circles illustrate root causes.
The analysis can be either qualitative or quantitative by principally studying the dependencies or by
introducing failure intensities respectively [4]. The qualitative analysis is usually performed by
reducing the fault tree to minimal cut basic events that are sufficient to cause the top event.
The FTA model can be further developed by categorizing the events that can be of importance with
complex and highly safety critical systems. The events can be categorized with system fails,
component fails, function fails and root causes. These can be kept apart and categorized with different
symbols; the one chosen can be seen in Figure 2-4.
9
Figure 2-4 Fault tree event symbols.
2.11.3 Design for Safety and Reliability
There are some principles for dimensioning for fatigue [4]:
o Infinite life
Applies when large amount of loading cycles and at low stress levels beneath the
fatigue limit.
o Safe life
The construction is dimensioned in such way that cracks are not allowed to be
initiated or propagated to a critical size during the lifetime of the product
o Fail safe
For a statically undetermined structure, there are alternative load paths that can be
used in order to accept some local fatigue damages. Non-significant parts of the
structure can have fatigue damages without the construct failing. Inspections are
needed in order to repair or change components.
o Damage tolerance
The construction is dimensioned for a finite service life span, with alternative load
paths at local fatigue damages.
The functional safety of a design is described with a failure rate λ. If the design consists of several
cooperating parts or systems, the fault intensity hence become the sum of the failure rates of the sub
systems shown in Equation (1). [4]
(1)
The sub systems can be derived from a Fault Tree Analysis, later described in Chapter 2.11.2.2.
Therefore, the reliability of a system rapidly decreases when a system has several functional
components in a series dependent of each other. By eliminating multifunctional components in series,
the fault rate will decrease. To have few functions and one part for each function are favored.
Another way of increasing the reliability is to introduce parallel components that can assume the
function, called redundancy. It can either be active where both components works at the same time or
passive, where the parallel function is activated first the main function fails. [4]
2.11.4 Geometry assurance
When mechanical products are synthesized, modeled and analyzed, the geometrical restrictions are
nominal [4].When the product later is realized by manufacturing and assembly processes it results in
geometrical variations. Propagation and accumulation of these variations can lead to products that do
10
not fulfill functional, assembly or esthetical conditions and quality problem is followed [6]. The
variations cannot be eliminated but kept at a minimum and transferred from critical regions of
dimensions. This control of geometrical variations will lead to non-sensitive and robust constructions,
defined as designs insensitive to variation.
The factors affecting a construction are divided into control factors and noise factors, easy to control
and hard to control respectively. These factors determine whether the variation will be amplified
(sensitive) or suppressed (robust) [6].
Methods for controlling the geometrical variation are to:
o Control where the geometrical contacts between different parts are located to assure a robust
construction.
o Control what tolerances input parts should have.
2.11.5 DFA – Design For Assembly
Following questions needs to be answered [6]:
o Could the detail be eliminated or combined with other detail?
o Does the detail need to be mobile relatively other details?
o Is it required to have a separate component with respect to:
- Specific material?
- Possibility to install other details?
Today the assembly of the whole step module is preassembled externally, mainly in order to reduce
lead-time in the main production line. Therefore, the assembly of the step as a whole is important for
Scania, but the preassembly is not of same importance regarding time. 1 The cost, which is a main
driver in Design for Assembly is important and this is affected by the assembly time.
Principles and processes for design for Assembly [4]:
o Minimize number of parts
o Design parts with self-locating features
o Design parts with self-fastening features
o Minimize reorientation of parts during assembly
o Standardize parts
o Modular design
2.11.5.1 DFA2 analysis
The principles can be analyzed with a DFA2, a simplified method that is based on qualitative
assessments instead of actual assembly time studies. A table sheet, Figure 2-5, is used to score an
article or activity according to 12 different assembly aspects. [9]
For each aspect, a value is set:
o 9 = best possible solution
o 3 = acceptable solution
o 1 = undesirable solution
1 Production, Johan Z Karlsson.
11
Figure 2-5. Table sheet for FTA2 analysis.
The scores are summated for each article and activity and for the whole assembly. The ease of
assembly as a whole is assessed with an index, the ratio between the point sum and the maximum
possible sum shown in Equation (2). [9]
( )
( ) (2)
2.11.6 Aftermarket and maintenance
The product is not included in the general service of the truck and therefore a service interval cannot
be taken into consideration. However, the fire fighters service the truck to a minor extent. Some
service including greasing and cleaning can be accepted, the aim is although an almost maintenance
free solution. Today’s solution has bushings to enable the fold out movement with minimal wear.
These and other minor parts can be changed when worn out or failed.
2.12 Verification and FEM Verification is made in order to ensure the result of the produced solution. The process is done with a
number of analyses to get the whole picture of the conditions.
2.12.1 Simplifications
In order to do time effective analyzes but still getting reliable results, careful considerations regarding
the simplifications of the model needs to be made. The present solution is modeled with no contact
and without bolt tightening forces. The bolts are excluded and replaced with beam spiders. Excluding
the preload will make for a conservative result and therefore is an acceptable simplification.2 The
material models are defined as linear elastic and the system will therefore never experience plastic
deformation. To be able to analyze a true static load case this needs to be included due to high loads
that will almost certainly induce plasticity. A first step is to investigate the static load and if it will
reach the yielding point of the materials.
2 Simulation section.
Nu
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Do
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1
2
3
4
Total sum
M Ease of assembly
12
A step load is applied exiting. Due to static simulations, the load is applied quasi statically and is
therefore not entirely consistent with reality.
2.12.2 Model
The model should be built in a way, corresponding as closely to reality.
2.12.3 Materials
In order to evaluate the behavior at static loading, the plastic behavior of the materials needs to be
described thoroughly. Scania have material models describing this.
2.12.4 Mesh and mesh elements
The mesh is checked for convergence by reducing the mesh size until the stresses and deflections
stagnates.
2.12.5 Boundary conditions
The boundary conditions aim is to hold the parts of the assembly together and at the same time
illustrate the reality of the product as much as possible.
2.12.6 Load condition
The general step load that the step plate is subjected to is derived from the 95th
percentile obese male
weighting m kg provided from guidelines. [10]
The design should also cope with acceleration loads [11]:
The reference coordinate system is located according to Figure 2-6. [12]
Figure 2-6. Standard reference coordinate system.
2.12.6.1 Static load case
According to Scania standards, the step shall be able to withstand the weight of two users without it
heavily deforming. With a safety factor, the step shall withstand a static load, without
considerable plastic deformation; mm is suggested as an acceptable magnitude according to
Lagergren. To verify the requirement of a magnitude of plastic deformation not exceeding the
magnitude, the static load is applied and then removed. The magnitude is then measured and compared
to the requirement. [10]
13
Additionally the strain must not exceed the ultimate strain of the material. Present solution has not
undergone this type of test and therefore the acceptance criteria for this specific application have not
been ensured [10].
2.12.6.2 Thermal stresses and strength
Since the module is positioned next to the silencer, it will be exposed to high temperatures, exceeding
80° Celsius. Differences in expansion and contraction between materials can cause thermal stresses.
This is not being analyzed, instead the construction and material choice is emphasized on using
compatible materials regarding Poisson’s ratio and constructing a solution insensitive to thermal
stresses. [3]
2.12.6.3 Fatigue load case
When analyzing the fatigue properties of a construction it is crucial to understand the conditions of the
application. Material, load case and initiations points of fatigue needs to be considered.
The number of load cycles that a normal ingress and regress application shall withstand is C cycles,
according to Scania regulations [10]. However, there are indications that the rear boarding steps of the
crew cab is not used as often as the steps to the front cab. For some demanding applications like sugar
cane vehicles in Brazil, it needs to be considered. The conclusion is that if there are no different
performance levels for the steps, they shall all be designed for C cycles. [10]
The acceptance criteria are that the steps shall withstand the aforementioned loads and number of
cycles with a P % failure probability. Most fatigue data is evaluated at 100 000 cycles, however the
difference can be seen as a safety factor. [10]
Fatigue tests that pertain as closely as possible to the application are preferable to use when deducing
an acceptance criteria. Whenever this is not possible, generalized empirical factors can be used to get a
good approximation of the lifetime of a part. Ingress and egress generates pulsating loads on the step,
the load is applied and then entirely removed. In the contrary load case, alternating load the load varies
from a minimum to a maximum. [10]
Factor reduction of amplitude stress to compensate for pulsating load:
Fatigue data is often based on stress amplitudes of alternating loads. For pulsating loads, the allowed
stress amplitude will be reduced and hence the data is not applicable, see Figure 2-7. It needs to be
reformulated in order to get a reliable acceptance criterion. [10]
Figure 2-7. Illustration of a pulsating load together with the range and amplitude of the load cycle
Amplitude:
To take into account for the pulsating load, a factor ( ) is used on the fatigue strength data,
according to Equation (3). [10]
14
( ) (3)
If no fatigue data for the material is found, a share of the yield stress for the material can be used as an
acceptance criterion, Equation (4). [10]
(4)
The two approximations will lead to an expression of the acceptance criteria of the pulsating fatigue
load, Equation (5).
( )
( ) (5)
Range:
It is the stress range that is the dimensioning factor, see Figure 2-7. Hence, the acceptance criteria
needs to multiplied by 2, Equation (6).
( ) (6)
The factors and ( ) differs from material to material, if not known
and ( ) can be used. [10]
2.12.6.4 Positioning of load
When stepping in and out of the cab, not the entire foot is in contact with the step. An approximation
of the area of contact is made by evaluation of the boot size and the stepping area, see Figure 2-8. A
circle with a 100 mm diameter is an acceptable approximation. [13]
15
Figure 2-8. Measurements in mm of a standard fire fighter boot.
Depending on type of application and if entering or exiting the cab, the operator is more or less likely
to use certain step areas and ergonomics. Ergonomic testing shows that when entering the cab the
stepping is tilted. The load can be derived from measuring load levels in the handrails, used when
entering the cab. This will give another distribution of the load, more likely a line load applied along
the edge of the step. [13]
From methods mentioned above, the step position is shown to be tilted at an angle of 30 degrees
relative to the vertical axle see Figure 2-9. Tilted step load.. The force applied at this type of stepping
is derived to be N. [14]
Figure 2-9. Tilted step load.
Due to the position of the boarding step relative to the doorway there are areas more likely to be used
when entering and exiting the cab, however all areas are handled in the same way when looking at
fatigue loads and static loads. Four stepping positions are being used when evaluating the concept,
three on the upper step and one on the upper. [13]
16
2.12.7 Structural stiffness
The structural stiffness of the solution shall be, N/mm according to guidelines [10]. The method
used to measure the structural stiffness is to measure the deflection of the system when applying a load.
By dividing the force applied with the deflections measured, the structural stiffness is obtained. All
components from the frame to the boarding step are considered in this type of calculations, see Figure
2-10. Scania uses Abaqus to determine the stiffness of the system. In order to get comparable results,
todays design needs to be evaluated using the same method as the evaluation of the new design. When
doing this only the boarding step structure is analyzed. The contribution of the inner components to
the structural stiffness will be the same in both cases and therefore they are excluded to save analysis
time. In order to get the stiffness of the structure as a whole, results from Abaqus simulations are used
to add on to the results from Catia GAS.
Figure 2-10. Left and right boarding step unit including mounting to frame member.
The structural stiffness is derived from Equation (7).
(7)
2.12.8 Bolts and bolt pretension
Scania has standardized torques for threaded joints and values for normal tightening torques. About 50%
of the torque is used to overcome friction in contact surface and about 40 % of the torque is used to
overcome friction in the thread. 10% of the torque remains to provide the preload of the joint, seen in
Figure 2-11 [15]. The torque will affect the joint, the strength and fatigue strength of both the bolt and
the clamped material and needs to be evaluated.
17
Figure 2-11. Contribution to preload, thread and contact surface from the tightening torque applied
The bolts are not included in the FEM analysis and therefore they are not evaluated through
simulations. Instead, they can be evaluated theoretically through joint theory. The fatigue strength of
the bolts can be estimated through equations shown below with supporting information from Figure
2-12. [16]
The stiffness of the clamped material should be larger than the stiffness of the clamped screw. For
pulsating loads, the strength of the joint is determined through the stresses in the bold caused by the
external load applied. [16]
Following information is needed to do the evaluation:
: Preload
: Minimum allowed preload in order for the structure not to be fully unloaded
: External load
: Screw clamping length
: Unthreaded length of screw
: Threaded length of screw
: Screw elongation
: Young’s modulus of screw
: Clamped material clamping length
: First clamped material length
: Second clamped material length
: Screw stiffness.
: Clamped material contraction
: Clearance hole diameter
: Tension diameter of screw
: Outer diameter of screw
: Inner diameter of screw
: Young’s moduli of clamped materials
: Requisite preload force in order to cope with radial force
18
: Radial force
: Coefficient of friction between parting planes with lowest COF
Figure 2-12. Schematic figure of the dimensions and loads of a screw joint.
Equations (8) to (10) are used to calculate the screw stiffness and elongation :
(8)
(9)
(10)
Equations (11) to (14) are used to calculate the clamped material stiffness and elongation:
(11)
When the clamped parts have different material properties, and therefore different stiffness is a
summation according to Equation (12) can be made. [16]
19
(12)
The stress carrying area of the clamped materials can be defined according to Equation (13). [16]
[(
)
] (13)
(14)
The force in the joint that arises from the radial force can be derived through equation (15). [16]
(15)
The result from the equations can then be conducted into at Load-Deformation diagram shown in
Figure 2-13. The diagram will show how much of the screw and how much of the joined material that
will take up the imposed load. [16]
Figure 2-13. Force-Displacement diagrams.
When dealing with dynamic loads a risk of fatigue of the screw. The fatigue strength of the screw
needs to be sufficient in order to not risk fatigue failure, see Table 2-1. If the applied force is
pulsating, the screw will be affected by a pulsating force according to Figure 2-14.The criteria to
avoid fatigue is stated in Equation (16). [16]
(16)
20
Table 2-2. Maximum allowed stress amplitude for pulsating loads [2]
Figure 2-14 Force-Displacement diagram for pulsating loads.
2.13 Prototype In order to further verify the concept and ensure that the function of the fold out mechanism is
satisfactory a prototype will be manufactured. The previous phases acted as a support for the prototype
execution.
Basis in form of drawings of each component were sent to the Scania mechanical workshop for
manufacturing. The assembling was made by the project team members.
done by the project team.
The following simplifications and changes were made on the prototype:
o The lower step plate has a simplified geometry with no anti slip teeth and a simplified pattern
in order for it to be jet cut instead of sand casted. This will make it heavier than the actual step
design.
o The sliding rails are machined instead of extruded.
o The plastic cover is taken from the current boarding step and modified in order for the rails
and step to be fitted.
o The upper step plate is taken from a former version of the step and has a different pattern.
o The plastic bearings are glued on to the aluminum rails with a two-component adhesive that is
not optimal to adhere these materials.
Grade Condition Allowed fatigue stress amplitude, pulating load
Rolled thread, untreated 50-60
8.8 Rolled thread, galvanized 35
Cut thread, untreated 35
21
3 Results
Following chapter presents the results of the project, leading up to a new boarding step solution. This
includes product specification, concepts, concept selection, detailed engineering, final design,
simulations and verifications.
3.1 Analysis of current product An analysis of the current product is made in order to get knowledge about the difficulties and the
possibilities of the part, see Figure 3-1.
Figure 3-1. Scania Crew Cab CP31.
3.1.1 Function and design
The boarding step consists of two steps, the upper one is fixed and the lower one is able to be folded
and unfolded. The lower boarding step is retracted during driving to fulfill the legislation of truck
width. The fold out step creates a stair effect, enabling exit facing forward out of the cab, see Figure
3-3. The lower boarding step is folded and unfolded by a pneumatic cylinder, activated by a
mechanically controlled sensor in the door. When in the folded position, the step is kept fixed by a
spring. The step units are not fully symmetric between left and right hand side, as can be seen in
Figure 3-2. Front view of right hand side and left hand side step unit.
22
Figure 3-3. Left hand side step unit in lowered position and unfolded position.
All of the ingoing parts of the boarding step unit assembly can be viewed in Figure 3-4. [17]
Figure 3-2. Front view of right hand side and left hand side step unit.
23
Figure 3-4. Exploded view of the boarding step unit.
3.1.2 Materials
The two fixed step boards are made of pressure die casted aluminum, EN AC-4430 DF. The foldable
step board is made of sand casted aluminum, EN AC-43300 ST6 according to standard STD4279.
Brackets and consoles are made of 35 hot-rolled steel, STD 755. The plastic panel and covers are
made of polypropylene with 20% talc. The summary can be seen in Table 3-1. Variable list in
Appendix A. [11] [17] [18]
Table 3-1. Material specification of components
Material Part
Young
modulus
[GPa]
Poisson’s
ratio
Density
[kg/m3]
Yield
strength
[MPa]
Tensile
strength
[MPa]
Domex 35
Levers
207 0.3 7880 355 430-550 Spacers
Side brackets
Sand casted
Aluminum EN-
AC-43100 ST6
Outer lower
step plate 70 0.33 2700 190 150-240
Aluminum EN-
AC-44300 DF
Upper step
plate 70 0.3 2700 130 240
Inner lower
step plate
Plastic STD4376-
2-A Cover 2.1 0.3 1200 - -
Domex 35:
The factors below together with Equation (17) are used to estimate the fatigue acceptance criteria for
the side brackets, made of Domex 35 steel. [10]
( )
24
Domex 35 steel:
( )
(17)
Sand casted aluminum:
The factors below together with Equation (18) are used to estimate the fatigue acceptance criteria for
the lower step, made of sand casted aluminum. [10]
( )
( )
(18)
3.1.3 Fail causes
The fail causes on the boarding step unit arises from problems with the fold out mechanism of the
retractable boarding step. This causes severe consequences; the lower step is not able to be used if the
retractable step is not folded out.
Reported function fail causes of the fold out mechanism are3:
o Pneumatic cylinder does not function due to lack of air pressure.
o Corrosion on the hinge causes the retractable step to be stuck.
o Dirt in the pneumatic cylinder due to torn cover. Causes problems with the fold out.
o Due to frequent washing of the vehicle, serving the pneumatic cylinder (through greasing) has
little effect.
These fail causes will be remedied through the product specification and an FMEA.
3 Interviews with stakeholders.
25
3.1.4 Simulation results
The simulations that have been done on the current product are fatigue, von Mises stresses and
structural stiffness. All simulations fulfill the acceptance criterion except the structural stiffness. The
structural stiffness is measured from the frame beam to the step of the boarding step unit illustrated in
Figure 3-5. Left and right boarding step unit including mounting to frame member.
Figure 3-5. Left and right boarding step unit including mounting to frame member.
The structural stiffness that should be N/mm is not fulfilled for all loading cases [11]. Table 3-2
show the structural stiffness and corresponding deflection at a number of points [10]. Variable list in
Appendix A. Only one has fulfilled the structural stiffness acceptance criteria, the remaining ones have
failed (marked in red). Exceptions from the acceptance criteria have been made due to the mounting of
the boarding step units. The right one is mounted on the battery box and left one, directly on the frame
member. Deflection in these components will propagate to the boarding step unit and therefore the
criterion is near impossible to fulfill.
Table 3-2. Calculated structural stiffness and the respective deformation for the boarding steps with
different loading points. Value marked in red means it have not fulfilled the acceptance criterion.
Loading point
Structural
stiffness and deformations
Lower left Lower
middle
Lower right Upper
middle
[N/mm] 0.075
0.075
0.066
0.147
[mm] 13.4 13.3 15.1 6.8
[N/mm] 0.066
0.074
0.076
0.149
[mm] 15.1 13.5 13.2 6.7
26
The lever assembly of the boarding step, whose task is to suspend the outer step and to transfer and
stop its motion, is divided into four parts. It consists of two side brackets, holding the step, one rod
enabling the rotation and one bar stopping the motion when the step is fully unfolded. These are
assembled by welding at different points. To simulate this, a rigid seam welding connection property
has been used. The seam welding connection connects the components chosen, by lines. Otherwise,
the model is meshed and connected in the same way as for the new concept. More information
regarding the boundary condition is available in Chapter 3.12.5.
When performing a static load case simulation, it shows that the bracket suspension reaches local
stresses well over yielding, the maximum ones about , Figure 3-6. V.M Stress
for static load case, in on the left hand side of the lower step.
Figure 3-6. V.M Stress for static load case, in on the left hand side of the lower step.
Figure 3-7, Figure 3-8 and Figure 3-9 show the Von Mises stress for the simulations done in Catia
GAS on the current boarding step.
27
Figure 3-8. V.M Stress for fatigue load, 𝐹𝐹𝑎𝑡𝑖𝑔𝑢𝑒 case on the right side of the lower step.
Figure 3-7. V.M Stress for fatigue load case, 𝐹𝐹𝑎𝑡𝑖𝑔𝑢𝑒 in the middle of the lower step.
28
The deformation of the boarding step and the corresponding stiffness for the different load cases are
conducted in Table 3-3.
Table 3-3. Calculated structural stiffness and the respective deformation for the boarding step with
different loading points
Loading point
Structural
stiffness and deformations
Lower left Lower
middle
Lower right Upper
middle
[N/mm] 0.407 0.30 0.29 -
[mm] 2.46 3.29 3.50 -
[N/mm] 0.27 0.30 0.41 -
[mm] 3.69 3.35 2.44 -
In order to calculate the stiffness and deformations from chassis out to the step the deformation of the
components from the frame member out to the step is applied, Appendix B. The results can be viewed
in Table 3-4.
Table 3-4. Calculated structural stiffness and the respective deformation from chassis to the boarding
step in Catia GAS
Loading point
Structural
stiffness and deformations
Lower left Lower middle Lower right Upper
middl
e
[N/mm] 0.0787
0.0817
0.0712
-
[mm] 12.71 12.24 14.05 -
[N/mm] 0.0702
0.0813
0.0801
-
[mm] 14.24 12.30 12.49 -
Figure 3-9. V.M Stress for fatigue load case, 𝐹𝐹𝑎𝑡𝑖𝑔𝑢𝑒 on the left side of the lower step.
29
3.2 Problem definition Through research of, and interviews with the stakeholders, a problem definition can be conducted.
The problem can then be divided into several components. The analysis shows following components
to the problem:
o Something to stand on/step on.
o Certain staircase effect.
o Space limitations.
o Dynamic functions.
o Laws and standards.
o Ergonomics.
3.3 Regulations by law Standards and regulations that are considered are:
o SS-EN 1846-2:2009+A1:2013 Firefighting and Rescue Service Vehicles [19]
o Commission Regulation (EU) No 1230/2012 Masses & Dimensions [20]
o Commission Regulation (EU) No 130/2012 Vehicle Access and Maneuverability [21]
o UNECE ECE R61 External Projections [22]
o
3.4 Corporate standards
STD4323 - SES – Scania Ergonomic Standard for Design, Ergonomic Load Evaluation Manual [23]
STD755 - Property classes - Flat products of steel [18]
STD4279 - Cast aluminum [17]
STD4290 - Wrought aluminum [24]
STD4463 - Coordinate system – Reference Coordinate System (RCS) [12]
STD3637 - Tightening torque – Normal [25]
3.5 Product requirements
3.5.1 Identification of customer requirements
Through investigations, an amount of stakeholders have been identified, both internal and external.
Figure 3-10 illustrates the connections between the design department and different stakeholders. The
arrows show how and where the information is transmitted. For example, the arrow from styling to
design indicates that styling has input to design, but ergonomics have their input to the styling work. If
the arrow points in both ways, the inputs are, inter functional.
30
Figure 3-10. Stakeholders and their connection, SQUARED boxes for internal and OVAL for external
stakeholders.
In order to utilize the stakeholders when constructing the product specification, the different stake
holders are being mapped and their respective input parts are stated,
Table 3-5. List of stakeholders, their respective input and the method used for containing it. How the
input is to be identified is also mentioned.
Table 3-5. List of stakeholders, their respective input and the method used for containing it
Stakeholders Input Method
Fire fighters Customer needs and functions Field trips and interviews
Society Standards and laws SIS
Marketing Needs of the customer and how the
customer perceive the products
Interviews
Ergonomics Knowledge about how the product is
used and what is of importance
regarding ergonomics.
Interviews
Styling Opinions about the looks and how
the product is perceived.
Interviews
Service Knowledge about service intervals,
spare parts and what needs to be
considered in order to service the
product and it surrounding parts.
Interviews
Production The product needs to be able to be
easily assembled and mounted.
Interviews and research
Purchase Knowledge about what the suppliers
are able to provide.
Interviews
Body builder Knowledge about how the truck is
altered and how Scania can enable
these changes.
Interviews and research
Manufacturing Knowledge about what the company
manages to produce.
Interviews
SOCIETY
R
MARKETING
CUSTOMER
ERGONOMICS
STYLING
SERVICE
PRODUCTION
MANUFACTURING
DESIGN
PURCHASE BODY BUILDER
31
3.6 Product specification Through of processes research and interviews the demands (D) and wishes (W) of the different stake
holders can be conducted into a Products specification with the help of a QFD, available in Appendix
C [10] [13] [14] [19] [21]:
Table 3-6. Product Specification
1 General
Standard EN1846 regarding firefighting and rescue service vehicles – safety and
performance shall be fulfilled D
UN ECE Regulation No 61 regarding external projections shall be fulfilled D
Commission Regulation (EU) no 130/2012 regarding vehicle access shall be
fulfilled D
Vehicle width demands according to Commission Regulation 1230/2012 shall be
fulfilled D
2 Functions and Dependability
The function shall allow egress facing forward D
The design shall be dependable in all weather conditions D
The design shall be robust D
It shall always be possible to get in and out D
Plan B if the function fails W
The function shall work when the truck is not on D
The time to function shall be minimized W
Low risk of pinching if moving parts W
The solution shall be slip resistant D
3 Performance
The design shall be able to carry a static load of N D
The fatigue strength shall be cycles at N D
The design shall withstand temperatures between to °C D
The structural stiffness shall be N/mm W
4 General Design
Ergonomically configured W
The solution shall not lead to altering of the interface W
The design shall be simple e.g. simple geometries and as few pats as possible W
No sharp corners or edges D
The design shall be module based e.g. existing components shall be used if possible W
Low weight W
Minimized sound from the function W
Symmetric solution, the same solution shall be applicable on both left and right side
of the truck. W
No black listed materials D
No grey listed materials W
Cost effective W
Quality and appearance that match with the interface and rest of the truck W
Existing technical system shall be utilized if possible W
32
5 Dimensions, supporting information in Figure 3-11. Example of access to
crew compartments.
Horizontal distance c1≤150 mm, c2>150 mm D
Height of first step from ground level, d≤550 mm D
Height between steps, b1≤400 mm, b2≤450 mm D
Depth of foot space, a1≥150 mm, a2≥150 mm D
Step width≥300 mm D
Step angle, α1≤25°, α2≤25° W
Total size W
Not exceed total vehicle width of 2500 mm D
6 Assembly and Maintenance
The design shall be maintenance free if possible W
It shall be easy to perform service W
Standard parts shall be used when possible W
Number of components shall be kept low W
The design shall be easy to assemble W
Not possible to be assembled incorrect W
Figure 3-11. Example of access to crew compartments. (19)
33
3.7 Comparison with others Through comparison with others, the advantages and disadvantages of each solution can be converted
to suggestions on solutions for Scania. The brands investigated can be seen in Appendix D.
3.8 Concept generation The concept generation phase together with a concept screening, [3] have lead up to a number of
concept that can be proceeded with.
A concept screening is performed in order to distinguish which concepts that fulfill the requirements.
A first concept screening is then made to reduce the number of concept to a manageable amount. All
results are available in the thesis work of Cederlöf [3].
The evaluation continues with a concept development phase in order to do a concept selection based
on knowledge.
3.9 Concept development After the first evaluative methods are made, the selection has been reduced to a number of concepts. [3]
[9] To further evaluate the concepts, analyzes in layout, cost, forces, dimensioning and folding
mechanisms are done as stated in the method for concept development, Chapter 2.9.
The first sketches of the concept are to be viewed in Appendix E.
All analyzes will be made on the lower step plate since the upper one is expected to be carried over
from the existing solution.
All concepts fulfill the layout- and interface requirements with some minor alterations. This has been
ensured with layout tests in 2D CAD.
A preliminary weight estimate have been performed, only the distinguishing parts of the different
concepts have been included since the nonspecific parts will not change the comparison.
An estimation of the forces present in the constructions together with dimensioning will be used to
evaluate realizability of the concepts. The fold out mechanism and the locking mechanism are being
developed and evaluated. This is done with a 3D representation and dynamic motion simulation in
order to fully understand the behavior of the mechanisms. A preliminary weight estimate have been
performed, only the distinguishing parts of the different concepts have been included since the
nonspecific parts will not change the comparison.. The results will serve as support in the final concept
selection. A list of advantages and disadvantages will sum up this, available in Appendix F.
3.10 Concept selection With the help of the previous concept development together with a concept scoring, Appendix G, a
concept selection is made.
Concept, 13 c, Figure 3-12, a slide out concept is chosen with motivations shown below.
Figure 3-12. Concept 13 c.
34
The concept 13 c is chosen because it is:
o Robust
The concept is considered to be the most robust one of the concepts. It consists of few
components and moving parts. The linear motion of the step is created of a linear acting
pneumatic cylinder which is considered being more robust than a cylinder having to perform
angular motions, needed for the other concepts. This should give a system with low down time,
failure rate, insensitive to continuous changing external environment. The hidden pneumatic
cylinder, protected from damage due to use will also increase the robustness.
o Safe
The safety of the function has been improved from the present design with a solution that
enables ingress and egress independently of the lower step extraction. However without a stair
effect
o Ergonomic
A stair effect with provides with a step angle of 60°. The design has a large clean step surface,
where no holes for cylinders or struts take space from the actual step area, compared to others.
o Qualitative
The smooth linear motion of only one lower step gives a premium and quality appearance.
o Simple
Since the concept only consists of one lower step plate and less parts in total than the concepts
it was evaluated against it is considered simple. The step also slides in a simple linear motion.
35
3.11 Detailed engineering The detailed construction phase’s aim is to develop the chosen concept into a product, the final result
can be seen in Figure 3-13. Boarding step unit, extracted to the left and retracted to the right.
Figure 3-13. Boarding step unit, extracted to the left and retracted to the right.
3.11.1 Carry over
The interface of the attachment points between battery box and the boarding step is kept the same.
This to maintain the layout of and interface to the rest of the truck. This means that the step is to be
suspended with four M10 screws. The side brackets, whose task is to hold up the boarding steps and to
suspend the whole design onto the truck is carried over from previous design with some small
alternations.
The upper step is reused from earlier generation. Since the customer can choose between a retractable
boarding step and a fixed one, the design will be more modularized if the upper step is the same on
both solutions. In addition, the step is dye casted; a design change would result in purchasing of new
molds, a costly investment, especially due to the small series of retractable boarding step.
The plastic cover that will improve the looks of the boarding step is divided into two parts, the outer
cover that will cover the boarding step and the inner well that will hide the components behind, Figure
3-14. The outer cover can be carried over with some minor changes. It is manufactured through
vacuum forming so a change in the design will be relatively easy to make regarding cost. The inner
well will be made deeper in order for the step to be able to slide in when driving. The design will
otherwise be quite similar to the old one.
36
Figure 3-14. Plastic cover, divided into two parts, outer cover and inner well.
3.11.2 Upper step plate
The upper step plate is carried over from the current boarding step design. Figure 3-15. Upper step
plate. show the design.
Figure 3-15. Upper step plate.
3.11.3 Lower step plate
The lower step plate, Figure 3-16, is a development of the upper boarding step. It is extended to 340
mm in order to form the required stair effect. It has also been equipped with sliding areas in order to
be able to slide of the linear bearings.
37
Figure 3-16. Lower step plate.
3.11.4 Sliding rails
The lower step is extracted and retracted through sliding on two sliding rails, Figure 3-17. The sliding
rails consists of an aluminum profile with Ultra High Molecular weight Poly Ethylene, UHMW PE,
glued onto the sliding surfaces, see Figure 3-17. This to lower the wear rate of the step. The reason for
choosing the UHMW PE is due to its superior properties. The design should have good wear
properties, be dependable in all weather conditions and cope with the static and fatigue load case,
and , according to the product specification. The plastic should also cope with high
local stresses, due to the using of the step. In order for it to do so it has a high hardness, low
absorbency and relatively high yield strength. More about the material choice in the thesis work of
Cederlöf [3].
The sliding material does not only function as a low wear rate material but do also have to have a
coefficient of friction high enough to be able to keep the step from sliding when stepped on. Through
calculations Appendix H, the COF should be at least 0.3.
The asymmetrical shape of the screw holes and the rail profile is due to the load case acting on the rail.
The lower flange is designed to cope with higher loads and the screw holes are positioned to resist
momentum both induced by the load case.
Figure 3-17. Rail with linear bearings.
38
3.11.5 Side brackets
The side brackets have a similar design to the present design. The attachment points to the upper step
and to the battery box are carried over so the design at these areas will be the same. Changes have
been made to the lower parts of the side brackets in order to be able to attach the sliding rails, see
Figure 3-18.
Figure 3-18. Side brackets.
3.11.6 Lower bracket
The task of the lower bracket is to hold together the side plates as well as suspending the pneumatic
cylinder. Result is to be viewed in Figure 3-19.
Figure 3-19. Lower bracket.
3.11.7 Pneumatic cylinder
A pneumatic cylinder is chosen for the extraction and retraction of the lower step plate. Since an air
pressure system exist of the truck ant the technique is used previous for the boarding step this is a
reliable choice.
To achieve the desired stair effect the lower step needs to slide 190 mm. This requires a stroke length
of the pneumatic cylinder of 190 mm as well.
39
A single acting cylinder with spring return is chosen to make sure that the step always retracts, even if
the pneumatic system loses pressure. By integrating the spring in the cylinder to total length increases
but it is still preferable before having them next to each other to avoid drawer effect.
Spring and cylinder calculations are shown in Appendix I.
3.11.8 Quality
3.11.8.1 FMEA – Failure Modes and Effects Analysis
By conducting an FMEA, the risks associated with the design can be detected and handled. Since all
the failures of the design can lead into not fulfilling the standards and regulations or even injury, all
Risk Priority Numbers becomes relatively high fast. After further assessment on what solutions that
can lower the values, another weighing of the risks are made.
Advantages [4]:
o Enables prioritizing in an early stage to remedy severe and difficult to detect design errors.
o Strengthens the cross functional work of the product development.
By doing the FMEA, the fail causes of today’s solution mentioned in Chapter 3.1.3 Fail causes, have
been reduced or even removed through the design.
o No fail will be caused by corrosion.
o Cylinder will be hidden and therefore protected from fail causes linked to its earlier
unprotected position.
The result of the FMEA is to be viewed in Appendix J.
3.11.8.2 FTA - Fault Tree Analysis
In Figure 3-20 below, a Fault Tree Analysis have been made regarding the failure of the boarding step
structure. It has been divided into three subsystems, the pneumatic system, the sliding system and the
suspension system marked with boxes. These three systems can fail causing different functions to fail,
marked with a triangle. The cause of the function fail is due to a component fail, marked with a circle.
The root cause of the component fails are marked with a diamond. The dependencies between these
breakdowns are shown with gates.
40
Figure 3-20. Fault Tree Analysis of the boarding step and its subsystems. Box refers to a system,
triangle to a function, circle to a component and diamond to a root cause.
By conduction the FTA analysis, it becomes clear that no of the subsystems have any dependencies of
the failure of the system as a whole. It applies for the function- and component fails. This means that if
any of the underlying events occur the boarding step will fail. However, the consequences will be
different and of different severity, stated in the FMEA.
3.11.9 Design for Safety and Reliability
The functional safety and reliability of the system can be increased by eliminating multifunctional
components in series. To have few functions and one part for each function are favored.
Another way of increasing the reliability is to introduce parallel components that can assume the
function, called redundancy. It can either be active where both components works at the same time or
passive, where the parallel function is activated first the main function fails. [4] This is met due to the
fact that the lower step always can be stepped on, regardless if it have been extracted or not.
The failure rate λ of the design cannot be determined, actual testing needs to be performed.
3.11.10 Geometry assurance
The lower step rests on the rails and therefore do not have large demands on tolerance. However, there
is a demand on surface finish of the sliding areas.
In order to avoid a drawer effect the pneumatic cylinder needs to be positioned in such way that the
motion is linear. The distance between rails and step, where there is a risk of drawer effect is relatively
large.
Over all, the new concept should be more insensitive to geometrical variations than the old one.
41
3.11.11 DFA – Design For Assembly
An investigation has been performed with the principles for Design for Assembly.
3.11.11.1 Sliding material
For the step to be able to slide in and out on the rails without causing severe wear, some sort of sliding
material will have to be assembled. The sliding material can be assembled either on the step plate or
on the rail. The two different methods will affect the assembling, manufacturing and service of the
plastic and related parts. The two different methods are evaluated in Table 3-7. Advantages and
disadvantages of sliding material configuration below.
Table 3-7. Advantages and disadvantages of sliding material configuration
Sliding material on rail Sliding material on step plate
Advantages Disadvantages Advantages Disadvantages
The design of the
rail will probably not
have to be altered
More difficult to service Easier to service Material is exposed
when step is unfolded
More difficult to service Easier to service The step need to be
altered
The comparison of these two shows that from a manufacturing and assembly point of view the sliding
material should be positioned on the rail. However, the robustness of the solution will be reduced. As
stated in the product specification, the assembling is a wish but the robustness is a demand and
therefore the choice falls on the later method.
To ease the assembly and the manufacturing of the sliding material, it can be sprayed directly on to the
rail. This possibility is dependent on type of material chosen for the application.
3.11.11.2 Steps
In the current boarding step, the lower step consists of two separate steps. In the new design, these
have been combined into one single step, having to be assembled. This will ease the assembling of the
lower step, desired in the product specification.
3.11.11.3 Fasteners
In order to simplify or even just enable the assembling of the design, care must be taken regarding
positioning of fasteners. The structure is preassembled at a subcontractor and then mounted on the
trucks at the production line. The step is mounted at a late stage of the production and therefore the
fastening of the boarding step onto the carrier bracket must be done from inside the brackets4.
This will require that the screw nut is fastened on the outside of the bracket, this is fulfilled with weld
nuts positioned on the outside of the left hand side bracket, see Figure 3-21. For the right hand side
bracket, the weld nut is instead positioned on the battery box bracket due to the way of assembly.
The type of screws used in the design is carried over from the current design. This will simplify the
stocks at line. The amount of screws needed in the pre assembly and at line have been reduced from 26
to about 15. The reduction in fasteners has been done for parts, assembled in the pre assembly and will
therefore ease the assembling.
4 Production, Johan Z Karlsson.
42
Figure 3-21. Left hand side suspension bracket with weld nuts.
The amount of bolts have been reduced by half, this will mean less time assembling which leads to
lower costs.
3.11.11.4 Brackets
The three brackets could be manufactured as one solid part. From both an assembling and
manufacturing point of view, the tolerances will be harder to reach. It is hard to obtain narrow
tolerances when doing bending of a sheet metal. Since all the holes need to match in order for the steps
and other associated components to be positioned correct it is favored to have the bracket suspension
divided into several parts.
3.11.11.5 Rails
In order to ease the manufacturing of the rails, they have been designed to be able to be profile casted.
Post processing in the form of drilling holes needs to be done but otherwise the production is the same.
The rails have been made specific to each side regarding positioning of holes in order to make them as
strong as possible when subdued to moment forces. This is disadvantageous in a manufacturing point
of view but is necessary for the design.
The hole pattern of the rails have been positioned so that it is impossible to assemble the in the wrong
way. This will ease the production by eliminating misassembling problems.
3.11.11.6 DFA2 analysis
A DFA2 analysis has been conducted for both the present design and the new design in order to get
comparable results. The result can be viewed in Figure 3-22 and Figure 3-23. The result indicates that
the new design is likely to be more easily assembled.
43
Figure 3-22. FTA2 analysis conducted for the new design.
Figure 3-23. FTA2 analysis conducted for the present design.
Nu
mb
er o
f d
etai
ls
Do
es th
e d
etai
l n
eed
to
be
mo
unte
d?
Ho
okin
g
Fo
rm
Wei
gh
t
Len
gth
Mo
untin
g m
otio
ns
Acc
ess
Fittin
g
To
lera
nce
s
Ho
ld m
ou
nte
d d
etai
ls
Fas
ten
ing
Co
ntr
ol/A
dju
stm
ent
Su
m
Article/Activity
3 3 1 3 3 3 3 6 6 6 1 9 6 53 Assemble upper step with spacers and side brackets
3 3 1 3 3 3 3 1 6 3 3 6 6 44 Assemble side brackets with lower bracket and rails
3 3 1 9 9 9 3 1 1 6 6 9 3 63 Assemble linear bearings on rails
6 6 3 6 6 6 3 6 3 3 6 9 6 69 Mount pneumatic cylinder on lower bracket
9 9 9 9 3 3 6 6 9 9 9 9 6 96 Mount lower step
6 6 6 6 3 3 6 3 3 3 3 6 6 60 Connect pneumatic cylinder and lower step
385 Total sum
M 0,59 Ease of assembly
DFA2 analysis - New designN
um
ber
of
det
ails
Do
es the
det
ail n
eed
to
be
mo
unte
d?
Ho
okin
g
Fo
rm
Wei
gh
t
Len
gth
Mo
untin
g m
otio
ns
Acc
ess
Fittin
g
To
lera
nce
s
Ho
ld m
ou
nte
d d
etai
ls
Fas
ten
ing
Con
tro
l/A
dju
stm
ent
Su
m
Article/Activity
3 3 1 3 3 3 3 6 6 6 1 9 6 53 Mount upper step with spacers and side brackets
6 3 1 3 3 3 3 6 1 1 3 6 6 45 Assemble outer step and lever assembly
1 3 1 3 3 3 3 3 1 1 3 3 3 31 Assemble inner step with lever assembly and spacers
3 3 1 3 1 3 6 6 3 3 1 6 3 42 Assemble lower step with side plates
3 3 3 6 6 6 1 1 6 3 6 6 3 53 Connect pneumatic cylinder and lower step
6 3 1 6 6 6 6 1 1 3 3 6 1 49 Connect pneumatic cylinder and side plate
3 3 3 9 9 9 6 9 6 6 6 6 6 81 Assemble pneumatic cylinder assembly
354 Total sum
M 0,47 Ease of assembly
DFA2 analysis - Present design
44
3.12 Verification and FEM The verification of the design will be performed in Catia GAS, Generative Assembly Structural,
which is an FEA program and comes as an extension of Catia V5 CAD program.
The Von Mises stress levels and the deflection is evaluated, under number of load conditions, later
stated. Further evaluation is done theoretically.
3.12.1 Simplifications
In order to get satisfactory results, while still keeping the computation time at a minimum the sheet
metal parts are modeled as surfaces. This will reduce the number of mesh elements and therefore the
computation time will decrease. This will also ensure a good stress distribution without having to do
parabolic interpolation, which is also time saving [26]. Below, Figure 3-24 show results from using 3D
mesh elements with linear interpolation for the sheet metal component. As can be seen, this will cause
for a patchy stress distribution and insufficient results.
Figure 3-24. Simulation performed with only 3D parts.
The geometry of the lower boarding step have been simplified, the slip prevention teeth have been
removed in order to reduce computation time. The teeth will not affect the stiffness of the construction
in a significant way. The stress levels are likely to increase on the bottom side of the step, due to the
notch action it produces. This has earlier been investigated and verified to pass the criteria. [27]
The bolts are excluded and replaced with rigid beam spiders, no preload is used. The screw is therefore
not evaluated, a theoretical bolt evaluation is done instead. Due to the rigid spiders, the stresses around
the screw holes are not entirely accurate and will therefore not be thoroughly analyzed.
45
A preload require contact conditions for the surfaces included and will make for a heavier model.
Using a preload will relieve stresses in the clamped material and will therefore improve the stress
distribution. The conservancy of the no preload condition will however ensure reliable results.
The assembly is modeled without contact except between the linear bearings and the lower step.
Due to shortcomings in the FEM program, the plasticity behavior of the materials cannot be described.
In fatigue computations, the model often experiences plasticity in the early cycles, which will cause a
strain hardening effect. This means that in the following cycles the plastic strain induced by each cycle,
ΔPEEQ, will decrease as can be seen in Figure 3-25. It indicates that the plasticity will probably stop
after a few cycles. It cannot be evaluated for a linear material model but needs to be taken into
consideration. [10]
For a linear material model this can be regulated with an increased acceptance value, there are
however no standards for this at Scania. The recommendation is set to an increase of 20 %5.
Figure 3-25. Load cycles together with a schematic plot of the plastic equivalent strain. (10)
3.12.2 Model
Model verification Appendix K show a comparison between the simulation model in Abaqus used by
the simulation section on the simulation model in Catia GAS.
3.12.3 Materials
In order to describe the material models in Catia GAS, standards regarding ingoing materials are used.
Variable list in Appendix A. [17] [18] [24]
The values are to be viewed in Table 3-8.
5 Simulation section.
46
Table 3-8. Material specification of components
Material Part
Young
modulus
[GPa]
Poisson’s
ratio
Density
[kg/m3]
Yield
strength
[MPa]
Tensile
strength
[MPa]
Domex 35
Levers
207 0.3 7880 355 430-550 Spacers
Side brackets
Domex 42
Levers
207 0.3 7880 420 480-620 Spacers
Side brackets
Sand casted
Aluminum EN-AC-
43100 ST6
Lower step plate 72.4 0.33 2 685 180 220
Aluminum EN-AC-
44300 DF Upper step plate 71 0.33 2 657 130 240
Wrought aluminum
6063 T6 Rails 71 0.34 2700 170 215
UHMW PE Linear bearings 0.72 0.3 930 17 -
Table 3-8 together with factors below and Equations (19) to (23) are used to estimate the acceptance
criterion for the parts being analyzed[10]. Variable list in Appendix A.
( )
Domex 35:
( )
(19)
Domex 40:
( )
(20)
Wrought aluminum 6063 T6:
( )
(21)
Die casted aluminum 44300:
( )
(22)
Sand casted aluminum 43100:
( )
(23)
47
3.12.4 Mesh and mesh elements
The choice of mesh type available in Catia GAS is limited. Octree tetrahedron mesh is used for 3D
parts and Octree triangle mesh is used for the 2D surfaces. [26]
Either the interpolation of the mesh is done linear or parabolic, for a tetrahedron, 4 nodes or 10 nodes
respectively, as seen in Figure 3-26. By using parabolic interpolation, a more accurate result is
obtained, due to its ability to pick up complex geometry. It however comes with the disadvantage of
longer computation times. [26]
Figure 3-26.Tetrahedron mesh with linear and parabolic interpolation respectively.
3.12.4.1 Mesh sag
Absolute sag is a measure on how much a mesh is allowed to deviate from a nonlinear curvature,
shown in Figure 3-27. According to guidelines, the absolute sag is recommended to be about 10 % of
the global mesh size [26].
This means that together with the mesh size, the local sag regulates how large the elements will be.
Figure 3-27. A schematic picture of the mesh sag definition.
3.12.4.2 Convergence analysis
A convergence analysis is made in order to get reliable results regarding the stresses and deflection.
The convergence analysis is evaluated at a fatigue load case, when stepping at the left hand side of the
lower step, which is the worst-case loading point.
For the same model and load case, an evaluation regarding maximum Von Mises stress and maximum
deflection is made with decreasing mesh size. At first, the global mesh is altered, when approaching a
converging result the areas of high stresses is altered with a decreasing local mesh size until a good
convergence is reached.
48
The results can be viewed in Figure 3-28 and Figure 3-29, whereby a local mish size of 2 mm and a
local mesh size of 0.8 mm at radiuses of the brackets and rails.
Figure 3-28. Convergence analysis of the Von Mises Stress.
Figure 3-29 Convergence analysis of the displacement.
3,3
3,35
3,4
3,45
3,5
3,55
3,6
3,65
3,7
0 1 2 3 4 5 6
Max
. def
lect
ion
[m
m]
Mesh size [mm
Global mesh
Local mesh
380
390
400
410
420
430
440
0 1 2 3 4 5 6
Max
. V.M
str
ess
[MP
a]
Mesh size [mm]
Global mesh
Local mesh
49
Figure 3-30 shows the percentage change in maximum V.M stress and deflection to show the stability
of the system. A change less than a few percent are desired. As expected, the V.M stress is more
sensitive to the mesh size and therefore requires finer mesh size. [26]
Figure 3-30. Stability analysis.
The convergence study results in mesh a shown in Figure 3-31 and Figure 3-32 with mesh sizes:
Global mesh size: 2 mm
Local mesh size for radii: 0.8 mm
Absolute sag: 0.2 mm
The upper step have a coarser mesh size of 5 mm due to its little contribution to stress and deflection
of the rest of the system and the fact that it is not evaluated and verified.
-2
-1
0
1
2
3
4
5
6
0 1 2 3 4 5 6
Δ [
%]
Mesh size [mm]
V.M Stress
Deflection
50
Figure 3-31. Mesh shown for the boarding step unit.
Figure 3-32. Mesh shown in detail for lower step plate and side bracket.
51
3.12.5 Boundary conditions
The boundary conditions and the placement of loads can be viewed in Figure 3-33.
Figure 3-33. Boundary conditions for the boarding step unit.
3.12.5.1 Restrains
The holes joining the boarding step side brackets to the battery box are retrained using a Clamp
condition, see Figure 3-33. For chosen surfaces or line geometries, all nodes are to be blocked in the
subsequent analysis. [28]
An edge of the back of the step plate is restrained in the width & depth direction. This is to avoid time
costly contact connections in order to retain the step in its position. The deflection of the system will
not be affected since the step is free to rotate in all directions and move in the height direction.
3.12.5.2 Fasteners
All fasteners are excluded and replaced with Rigid Connections, connecting the components at the
screw holes. This creates a link, connecting the two bodies which are stiffened and fastened together at
their common boundary, and will behave as if their interface was infinitely rigid. The connection is
established through rigid spiders connecting all the nodes in to a central node, also called “null-length
bar”. A set of relations and constraints are generated, linking the nodes degree of freedom and the
central node degree of freedom. [26] This is illustrated in Figure 3-34.
52
Figure 3-34. Schematic picture of a rigid spider connection.
If the hole is threaded or countersunk, the surface is chosen and when it is not, the edges are. These to
get a, coupling close to reality, see Figure 3-35.
Figure 3-35. Schematic descriptions of the connection surfaces and edges for screw holes shown in
cross section.
The rigid spider connection, contains one master node and number of slave nodes, see Figure 3-36.
Spider element.. The degrees of freedom of the master node are linked to the degrees of freedom of the
slave nodes. [26]
Figure 3-36. Spider element.
53
3.12.5.3 Contact connections
The lower step is connected to the sliding material through contact connections at their common
boundaries, see Figure 3-37. Parts where contact connections are used. This is done frictionless, since
GAS cannot handle nonlinearities, which is the case when using friction contacts. [26]
Figure 3-37. Parts where contact connections are used.
3.12.5.4 Fastened connection
The linear bearings made of plastic are fastened on the rails with glue, positioned as in Figure 3-38. To
handle this in GAS, without including the glue the bearings are approximated to be connected as if
they were a single body. The Fastened Connection relations take into account the elastic deformability
of the interfaces. [26]
Figure 3-38. Parts where fastened connections are used.
3.12.6 Load condition
The acceleration loads are not used in the verification of the design, they are not dimensioning for the
design and are therefore excluded6. The general step load of kg provided from Guidelines is used.
[10]
3.12.6.1 Static load case
Due to the linear material models, the magnitude of the plastic deformation and if it reaches the
ultimate strain of the material cannot be analyzed. Instead an evaluation of the stresses and if they
reach yielding will be done. This at a loading case of N placed in the worst-case area. A
comparison between todays design and the new concept is done to get a comparative result.
Figure 3-40 show the static load induced stresses in the suspension brackets. They reach the yielding
point of the material, Table 3-8, with stresses up to . They therefore need to be
evaluated more thorough using nonlinear material models. Compared to the result of the present
design, available in Figure 3-39, where there are stresses reaching , which is
well above yielding and tensile strength. This tells that the new design should be able to cope better
than the current one.
6 Verification section.
54
Figure 3-40. Static load case, positioned at worst-case position, on the left hand side of the
lower step.
In Figure 3-41 and Figure 3-42, the Von Mises stress is scaled against the yielding stress of the
specific material, available in Table 3-8. It also pass the tensile strain acceptance criteria, the stresses
will not reach the ultimate strength of the material.
Through the analyze, the stresses induced on the linear bearings is reaching 38 MPa which is above
the yielding point of the UHMW PE material. In order to ensure the function of the plastic, real testing
needs to be performed.
This shows that the rails will not likely reach yielding. The maximum stress levels are located around
the screw holes, which are rigidly fastened, and these stresses are thereby not entirely accurate.
55
Figure 3-41. V.M stress distribution of rail scaled to yield stress of material, .
Figure 3-42. V.M stress distribution of rail scaled to yield stress of material, .
Figure 3-43 shows the worst case Von Mises stress of the lower step. The Von Mises stress is scaled
against the yielding stress of the specific material, available in Table 3-8.
The geometry of the lower boarding step have been simplified, the slip prevention teeth have been
removed in order to reduce computation time. The teeth will not affect the stiffness of the construction
in a significant way. The stress levels are likely to increase on the bottom side of the step, due to the
notch action it produces. This has earlier been investigated and verified to pass the criteria. [27]
This indicates that the lower step have passed the acceptance criteria. Due to simplifications of the
geometry, mentioned in Chapter 3.12.1 Simplifications, where the slip prevention teeth have been
excluded, the stress levels might be higher. The teeth are located on the upper side of the step the
stress levels are likely to increase on the bottom side of the step, due to the teeth and the notch action it
produces together with a crack opening force. [27]
56
Figure 3-43. V.M stress distribution of lower step plate scaled to yield stress of material, .
3.12.6.1 Thermal stresses and strength
The construction, with the lower step sitting on the linear bearings and rail instead of being
constrained, makes for a solution not exposed to thermal stresses. Due to this, this will not be
evaluated.
The material choice done in another master thesis of Cederlöf [3] ensures that the material have a
service temperature in the range of the material span present for the solution.
3.12.6.2 Fatigue load case
Early simulations show that for steel with grade 35, the fatigue life of the brackets is insufficient. The
choice is made to increase the steel grade to 40, which will secure the fatigue life. As earlier
mentioned, yielding in the earlier stages of the loading cycle may result in a deformation hardening
effect, that will increase the fatigue strength of the component. This is only possible to be investigated
using non-linear material model and doing cyclic loading simulations. In order to reduce the steel
grade, and thereby also the cost, these analyze needs to be made.
Figure 3-44 V.M Stress for fatigue load case, in the middle of the lower step.to Figure 3-49
shows the Von Mises stress in the brackets for the different fatigue load cases. The color scaling
corresponds to the compensated pulsating amplitude stress.
57
Figure 3-44 V.M Stress for fatigue load case, in the middle of the lower step.
Figure 3-45 V.M Stress for fatigue load case, in the middle of the lower step.
58
Figure 3-46. V.M Stress for fatigue load case, on the right side of the lower step.
Figure 3-47. V.M Stress for fatigue load case, on the right side of the lower step.
59
Figure 3-48. V.M Stress for fatigue load case, on the left side of the lower step.
Figure 3-49. V.M Stress for fatigue load case, on the left side of the lower step.
The results indicate that the brackets, suspending the boarding step are not subjected to loads
exceeding the fatigue limit.
Figure 3-50 shows the Von Mises stress in the bracket for a worst-case fatigue load case. The color
scaling corresponds to the compensated pulsating amplitude stress.
60
Figure 3-50. V.M Stress for fatigue load case, on the right side of the lower step.
The results indicate that the rails are not subjected to loads exceeding the fatigue limit.
3.12.6.1 Positioning of load
It has been decided that only the lower step is being evaluated. Due to carry over, the upper step and
its suspension will have the same design. No further evaluations beyond the one already made have to
be done. The upper step passes the acceptance criteria regarding stress and stiffness. [11]
3.12.7 Structural stiffness
In order to get the structural stiffness of the whole system from the frame member out to the boarding
step the deflection from the frame member out to the suspension of the boarding step is produced from
Abaqus, see Appendix A.
The structural stiffness of the boarding step itself is investigated in Catia GAS, available in Table 3-10
and is conducted according to Equation (7).
Table 3-9. Calculated structural stiffness and the respective deformation of the boarding step.
Loading point
Structural
stiffness and deformations
Lower left Lower
middle
Lower right Upper
middle
[N/mm] 0.46 0.36 0.35 -
[mm] 2.19 2.74 2.86 -
[N/mm] 0.32 0.30 0.38 -
[mm] 3.10 3.28 2.65 -
[N/mm] 0.28 0.33 0.47 -
[mm] 3.62 3.04 2.11 -
The deflection of the frame member to attachment points are then added, with the results shown in
Table 3-10. It is compared to the results of today, a measure marked in green means that the stiffness
has improved and a measure marked in red means that it has decreased.
61
Table 3-10. Summation of the structural stiffness and the respective deformation
Loading point
Structural
stiffness and deformations
Lower left Lower middle Lower right Upper
middle
[N/mm] 0.083 0.085 0.075 -
[mm] 12.44 11.69 13.41 -
[N/mm] 0.068 0.083 0.082 -
[mm] 14.75 11.99 12.16 -
Points show the difference in percentage from the Abaqus results and the Catia GAS ones
Table 3-11. Difference in deformation for the boarding steps from Catia GAS to Abaqus with different
loading points
Loading point
Structural
stiffness and deformations
Lower left Lower
middle
Lower right Upper
middle
[%] -22% -28% -23% -
[%] -19% -26% -23% -
3.12.8 Bolts and bolt pretension
3.12.8.1 Flange screw M10
The M10 flange screws, used to join the upper step together with the side brackets have properties
according to below. [15] [29]
Together with the variables above and Equation (8) to (15) the Load-Deformation diagram can be
conducted for the M10 Flange screw, see Figure 3-51.
62
Figure 3-51. Force-Displacement diagram for M10 flange screw.
From the diagram, the force taken up by the screw, and the force taken up by the clamped material,
is derived to be as follows.
Using Equations (24) to (25), the stress amplitude is verified to be less than the maximum allowed
stress amplitude for pulsating loads, stated in Table 2-2.
(24)
(25)
(26)
3.12.8.2 Countersunk head screw M10
The M10 countersunk head screws, used to join the rails together with the side brackets have
properties according to below [15] . Due to its countersunk head, the preload is reduced and can be
approximated to be 70 % of the standard preload. [30] The tension diameter of screw , is
approximated to be the mean value of the countersunk diameters.
63
Together with the variables above and Equation (8) to (15) the Load-Deformation diagram can be
conducted for the M10 countersunk head screw, see Figure 3-52.
Figure 3-52. Force-Displacement diagram for M10 countersunk screw.
64
From the diagram, the force taken up by the screw, and the force taken up by the clamped material,
is derived to be as follows.
Using Equations (24) to (26) stated before, the stress amplitude is verified to be less than the
maximum allowed stress amplitude for pulsating loads, stated in Table 2-2.
3.13 Prototype The prototype of the new concept is shown in Figure 3-53 and Figure 3-54.
Figure 3-53. Step unit with extracted (left) and retracted (right) bottom step.
65
Figure 3-54. The prototype of the new concept mounted on a truck.
The sliding mechanism was tested and was satisfactory. The lower step slides easily in the UHMW PE
bearings. A test of climbing the step showed that that the coefficient of friction was good enough to
keep the step in place. Over all, the step felt stable and stiff, little deflection when stepping on it and a
large comfortable stepping area, see Figure 3-55.
66
Figure 3-55. Egress test of the prototype of the new concept.
67
4 Discussion
Following chapter discusses the final concept, the goal fulfillment and the methods and processes
leading up to the final design and prototype
4.1 Final concept The concept new concept is considered to be more reliable than the current one. The new design has
overall improved the dependability of the function. Even if the fold out mechanism would fail, the step
can be used, though without a stair effect. The relatively low amount of parts should make for a
boarding step more insensitive and therefore more robust.
4.2 Fulfillment of goal The goal of the project was to improve the entry- and exit function of the Crew Cab.
The main requirements were:
o Durable to stand on
o Dependable
o Robust
o Easily mounted
o Fulfill European standards
o Fit in the interface
With simulations and validation is has been confirmed that the new design should be durable to stand
on.
The new design has overall improved the dependability of the function. Even if the fold out
mechanism would fail, the step can be used, without a stair effect. The low amount of parts indicates
that the new concept could be more robust.
If the new fold out mechanism of the boarding step is more robust than the current one cannot be
validated without further testing in different environments
The robustness of a technical system can be defined as a system with low downtime, failure rate,
variability and insensitivity to continuous changing external environment.
The assembling of the boarding step has been improved. With fewer parts and fewer operations, the
assembling should be made easier.
The step fulfills all European standards.
The new concept has been design in such way that little or no alternations of the interface and
interfacing products need to be made. The upper step has been carried over. The Scania module
system has been considered in the product development.
4.3 Methods and processes Product specification
A large amount of time was dedicated to find requirements in order to conduct a thorough product
specification. This paid off since the detailed and clear product specification worked as a good support
in the whole development process.
68
Concept generation and selection
The relative decision matrix was used in the concept selection process in order to make a decision
based on facts. The process will however always be subjective in some way. If it would have been
performed, again by someone else the outcome may have been different.
FTA and FMEA
The FTA analysis, done to identify the relationship between failures on a system level and their
underlying root causes and their dependencies is a useful tool in complex systems. The FTA analysis
shows that fail of the boarding system will occur, independent of one or more system fails. It applies
to the current step. For the present design, this will mean that the step will not be able to function at all.
For the new concept, the boarding step will be able to be used, though without a stair effect.
4.4 Simulations Simulation model
To verify the model used in the simulations, the simulation results done in Catia GAS was compared
to simulations made in Abaqus by the simulation section. Some differences regarding deflection and
maximum stress levels were revealed, see Table 4-1.Difference in deformation for the boarding steps
from Abaqus to Catia GAS with different loading points. This show that they do not entirely conform
to each other. What model that is closest to reality cannot be determined.
The simulations in Abaqus have been done, with coarse mesh, no convergence analysis and with
simplified connection properties. These setting make for a stiffened solution. However, the result is
the opposite.
The reason for this may lie in other settings made and that the simulation programs work in different
ways. In the Abaqus simulations, the material models used was nonlinear and could have an effect on
the results. The Abaqus simulations were done with parabolic interpolation, describing the model in a
better way.
In the ways possible, the Catia GAS model have been produced to satisfactory, with a converged mesh
and with optimized parts and connections. A large amount of time has been dedicated to optimize the
model, which is of extra importance when using a simple program.
Table 4-1.Difference in deformation for the boarding steps from Abaqus to Catia GAS with different
loading points. Same as Table 3-11.
Loading point
Structural
stiffness and deformations
Lower
left
Lower
middle
Lower right Upper middle
[%] -22% -28% -23% -
[%] -19% -26% -23% -
Verification plan
A thorough verification plan, with a larger extent than have been done previous on the boarding step
unit have been planned and performed.
Simplifications
Excluding the bolts out of the simulations, and replacing them with rigid spiders may have led to
stiffening of the design. A rigid spider connection does not take into account the deformation of the
joint due to screws and neither the deformation of the nodes connected.
69
Due to the comparative analysis, where it was done in the simulations that was compared this is an
approximation is considered good enough but might be an approximation causing deviation from the
real values. For higher accuracy, this needs to be considered.
Static load case
According to the results of the static load case, the bracket suspension reached local stresses well over
yielding. After an analysis on the current solution has been made it became clear that it applies for
parts of the suspension system of the current boarding step unit, see , here with stresses even higher.
This indicates that the new step is better than the old one, however a more thorough analysis needs to
be made, using non-linear material models.
Figure 4-1. V.M stress distribution of current design for current design (left) and new concept (right).
The acceptance criteria state that the boarding step shall withstand the weight of two users without
heavily deforming. 2 mm suggested as an acceptance criteria cannot be validated due to the linear
material models that are used in Catia GAS. To ensure this, further evaluations in another FEM
program needs to be performed.
Fatigue
The fatigue evaluation of the boarding step resulted in an increased steel grade of the suspension
brackets. This leads to higher material costs but will ensure the fatigue life. As mentioned, yielding in
the earlier stages of the loading cycle may result in a deformation hardening effect that could increase
the local fatigue strength. To draw this conclusion, further investigations using cycled loads and a non-
linear material model needs to be done.
Deflection and structural stiffness
The deflection and stiffness have been improved in the new design for five out of six load cases. This
achievement is pleasing since the deflection is of great significance in the experienced impression of
the product. A wobbly solution will not feel robust, even if it may be so.
Bolts
The analysis of the screw joints of the design shows that the fatigue properties of the screws are
satisfactory. The type of bolts, was carried over from the current design in order to maintain stocks at
line and to save time in the product development. A more thorough analysis regarding bolts can result
in different types of joints.
70
4.5 Prototype The prototype should serve as a verification of the fold out mechanism. Due to long delivery, time of
the pneumatic cylinder the extraction and retraction cannot fully be tested. Therefore, the sliding
mechanism was tested only by pushing and pulling the step in and out by hand. The sliding
mechanism was tested and was satisfactory. The lower step slides easily in the UHMW PE bearings. A
test of climbing the step showed that that the coefficient of friction was good enough to keep the step
in place. Over all, the boarding step unit felt stable and stiff, probably more than current design. Little
deflection when stepping it and a large comfortable stepping area.
5 Future work Activities outside the time scope of this project that are recommended for future work in order to
optimize the design are listed below:
o Functional tests in different weather conditions needs to be done to assure the robustness and
make improvements if necessary.
o Tribological tests needs to be done on the sliding material to verify the tribological behavior.
o The lower step plate needs to be finally designed by the styling department.
o The plastic cover needs to be modified by the styling department to fit the new components.
o Choice of pneumatic cylinder needs to be further investigated to optimize function and size.
o Suspension of the cylinder needs to be designed, this might lead to modifications of the lower
bracket.
o Cost calculations needs to be performed on the new concept.
o A more thorough simulation process, using non-linear material models and cycled fatigue
loads.
o Weight optimization of the design, by lighten the boarding step environmental benefits like a
lower fuel consumption is achieved
71
6 Conclusions The goal of the project was to improve the entry- and exit function of the Crew Cab. The
improvements are stated below:
o Durable to stand on
With simulations and validations is has been confirmed that the new design should be durable
to stand on.
o Robust and dependable
The new design has overall improved the dependability of the function. Even if the fold out
mechanism would fail, the step can be used, without a stair effect. The low amount of parts
indicates that the new concept could be more robust.
o Easily mounted
The assembling of the new concept has been improved. Fewer parts and fewer fasteners are
o In line with the module system
The new concept has been design in such way that little or no alternations of the interface and
interfacing products needs to be made. The upper step has been carried over. The Scania
module system has therefore been considered in the product development.
72
7 Acknowledgements I would like to thank my mentor at Scania CV AB, Bertil Hagqvist, for making it possible to perform
this thesis work. Thanks to Sebastian Sjödell for providing with simulation information from Abaquas
as well as guidance and support about FE solvers. I would also like to thank Henrik Bruce for the help
with understanding the software Catia GAS.
A special thanks to my fellow thesis worker Magdalena Cederlöf for good collaboration and support in
this thesis work.
Thanks to my supervisors at Karlstad University, JanErik Odhe and Henrik Jackman for all your
guidance and support.
73
8 Bibliography 1. Standards, SiS - Swedish Institute for. Underhåll - Terminologi. 2010. SS-EN 13306.
2. BusinessDictionary. [Online] [Citat: den 25 05 2017.]
http://www.businessdictionary.com/definition/robust.html.
3. Cederlöf, Magdalena. Retractable boarding step for Scania Crew Cab. Södertälje : u.n., 2017.
4. Johannesson, Hans, Persson, Jan-Gunnar och Dennis, Pettersson. Produktutveckling - effektiva
metoder för konstruktion och design. 1st. Stockholm : Liber, 2004.
5. Renu, Rahul, o.a. A Knowledge based FMEA to Support Identification and Management of Vehicle
Flexible Component Issues. Science Direct. den 11 February 2016, ss. 157-162.
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Vol. 43, ss. 3-12.
7. Scania CV AB. Riskhantering i projekt. [Online] den 22 04 2016. [Citat: den 27 02 2017.]
8. An overview of fault tree analysis and its application in model based dependability analysis. Sohag,
Kabir. 2017, Expert Systems With Applications, Vol. 77, ss. 114-135.
9. Johannesson, Hans, Persson, Jan-Gunnar and Pettersson, Dennis. Produktutveckling Effektiva
metoder för konstruktion och design. 2:nd. Poland : Liber AB, 2013.
10. Scania CV AB. Best practice for Step Loads. Södertälje : u.n., den 03 02 2017.
11. Scania CV AB. FE-Analys av Insteg till CrewCab. [Online] 2016. [Citat: den 27 02 2017.]
12. Scania CV AB. Coordinate system - Reference Coordinate System (RCS). Södertälje : u.n., 2016.
1. STD4463.
13. Scania CV AB. User needs and ergonomic requirements for NCG. Södertälje : Scania CV AB,
2013.
14. Scania CV AB. Hanteringslaster. Södertälje : Scania CV AB, 2013.
15. Scania CV AB. Standard tightening torque - Normal. Södertälje : Scania CV AB, 2014.
16. Colly Components. Handbok om skruvförband. [Online] 09 2013. [Citat: den 10 05 2017.]
http://www.collycomponents.se/wp-content/uploads/2013/09/HANDBOK_skruvfo%CC%88rband.pdf.
17. Scania CV AB. Cast aluminum - Summary. Södertälje : u.n., 2016. 5. STD4279.
18. Scania CV AB. Propert classes - Flat products of steel. Propert classes - Flat products of steel.
Södertälje : u.n., 2016. 17. STD755.
19. SiS - Swedish Standards Institute. Firefighting and Rescue Service Vehicles. u.o. : SiS - Swedish
Standards Institute, 2013. 1. SS-EN 1846-2:2009+A1:2013.
20. Scania CV AB. Standard- Masses and Dimensions. Södertälje : u.n., 2013.
21. EU. Vehicle Access and Maneuverability. 2012. 130/2012.
22. Scania CV AB. External Projections ECE R 61. Södertälje : u.n., 2013. 1. ECE R 61.
23. Scania CV AB. SES – Scania Ergonomic Standard for Design, Ergonomic Load Evaluation
Manual. Södertälje : u.n., 2015. 3. STD4323.
24. Scania CV AB. Wrought aluminum - Summary. Södertälje : u.n., 2017. 4. STD4290.
74
25. Scania CV AB. Tightening torque - Normal. Södertälje : u.n., 2014. 32. STD3637.
26. Scania CV AB. Catia V5 GAS Grundkurs. Södertälje : u.n., 2010.
27. Scania CV AB. Fatigue evaluation of NCG CS Upper Side Step. Södertälje : Scania, 2015.
28. Dassault Sysemémes. Generative Part Stress Analysis. [Online] 1994. [Citat: den 05 04 2017.]
http://www.catia.com.pl/tutorial/z2/generative_part_stress_analysis.pdf.
29. Scania CV AB. Hexagon screws with flange . Södertälje : u.n., 2016. 4. STD4435.
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31. Scania CV AB. Scania Inline. [Online] 2016. [Citat: den 17 02 2017.]
https://inline.scania.com/scripts/cgiip.exe/WService=inline/cm/pub/showdoc.p?docfolderid=135283&
docname=index.
32. Scania CV AB. Hexalobular socket countersunk. Södertälje : Scania, 2016. 15. STD3891.
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Heinemann, 2005.
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1
Appendix A: variable definition
Loads:
: Gravitational force
Step loads:
tep load
eaction forces in
omentum in point
orce on one wheel
orce on one strut
orce degrees from
omentum in point
niformed distri uted force
q niformed distri uted force at distance
ength of the loc ing pin in concept
istance from in the direction
adius of the loc ing pin in concept
istance from point to the loc ing pin in concept
hear stress
hear force
Friction:
: Friction force
:Tilted step
: Horizontal part of tilted load
: Vertical part of tilted load
: Normal load
: Coefficient of friction
: Tilted step angle
2
Fatigue:
: Stress amplitude with regard to pulsating load
: Stress amplitude with regard to alternating load
( ): Reduction factor to take into account for pulsating load
: Reduction factor to take into account for fatigue
Stress range with regard to pulsating load
Bolts and bolt pretension:
: Preload
: Reduced preload if countersunk screw
: Minimum allowed preload in order for the structure not to be full unloaded
: External load
: Screw clamping length
: Unthreaded length of screw
: Threaded length of screw
: Screw elongation
: Clamped material contraction
: Young’s modulus of screw
: Clamped material clamping length
: First clamped material length
: Second clamped material length
: Screw stiffness
: Clamped material stiffness’s
: Stress absorbing area in screw
: Stress absorbing area in clamped material
: Clamped material contraction
: Clearance hole diameter
: Tension diameter of screw
: Outer diameter of screw
: Inner diameter of screw
: Young’s moduli of clamped materials
3
: Requisite preload force in order to cope with radial force
: Radial force
: Coefficient of friction between parting planes with lowest COF
: Force taken up by screw
: Force taken up by clamped material
: Stress amplitude in screw
: Fatigue strength of screw for pulsating load
Materials:
: Yield strength
: Tensile strength
: Young’s modulus
Poisson’s ratio
: Density
Deflection:
: Structural stiffness for load positioned at right hand side of step
: Structural stiffness for load positioned at left hand side of step
: Structural stiffness for load positioned in the middle of step
: Deflection for load positioned at right hand side of step
: Deflection for load positioned at left hand side of step
: Deflection for load positioned in the middle of step
1
Appendix B: Deflection, Frame member to boarding step unit
In order to determine the deflection from the frame member out to the boarding step attachment in the
battery box, Equation 1 is used. An analysis of only the boarding step is performed in Abaqus and
used to subtract the deflection of the boarding step, , from the deflection of the whole
assembly, .
The results are shown in Figure 1 and Table 1 to 3.
Figure 1. V.M stress distribution to the left and displacement to the right for fatigue load applied to
the left hand side in Abaqus
Table 1 shows the structural stiffness and deformation for the whole assembly conducted in Abaqus.
Table 1. Calculated structural stiffness and the respective deformation for the boarding steps with
different loading points performed in Abaqus
Loading point
Structural
stiffness and deformations
Lower left Lower
middle
Lower right Upper
middle
krh [N/mm] 0.32 0.22 0.22 -
urh [mm] 3.15 4.55 4.55 -
klh [N/mm] 0.22 0.22 0.32 -
ulh [mm] 4.55 4.55 3.15
(1)
2
Table 2 shows the structural stiffness and deformation for only the boarding step unit conducted in
Abaqus.
Table 2. Calculated structural stiffness and the respective deformation for the boarding steps with
different loading points performed in Abaqus
Loading point
Structural
stiffness and deformations
Lower left Lower
middle
Lower right Upper
middle
krh [N/mm] 0.075
0.075
0.066
0.147
urh [mm] 13.4 13.3 15.1 6.8
klh [N/mm] 0.066
0.074
0.076
0.149
ulh [mm] 15.1 13.5 13.2 6.7
Table 2 shows the resulting structural stiffness and deformation for the frame member to the boarding
step unit conducted through Equation 1.
Table 3. Final calculated structural stiffness and the respective deformation from chassis to the
battery box in Abaqus
Loading point
Structural
stiffness and deformations
Lower left Lower middle Lower right Upper
middle
krh [N/mm] 0.098 0.111 0.094 -
urh [mm] 10.25 8.95 10.55 -
klh [N/mm] 0.094 0.111 0.010 -
ulh [mm] 10.55 8.95 10.05 -
1
Appendix C: QFD matrix
Column #
Product Characteristics
("Functional requirements
or "Hows")
Customer Requirement
("Whats")
Fulfil safety standard EN1846
Fulfil ECE R61 External Projections
Fulfil Commission Regulation (EU) No 130/2012
Fulfil vehicle width demands
High safety
Always possible to get in/out
No black listed materials
No grey listed materials
Robustness
Ergonomics
Withstand envrionment
High quality
Time to function
Maintenence free
Simple
Modularized
Low weigth
Fit in the current space
Easy to assemble
Easy to perform service
Low cost
Apperance
Target or Limit Value
Weighted rating
+ + ++
+
+ +
+++
-
+
+
+- +
++
+-
+ +
+
+
+
+
+
+
+
+ +
-
7 8 9 10 11 121 2 3 4 5 6 25 26 27 28 29 3019 20 21 22 23 2413 14 15 16 17 18
9 9 9 9 9 9
Easy
access
ed
sp
are
part
s
Po
ssib
le t
o b
e w
ron
gly
ass
em
ble
d
Nu
mb
er
of
stan
dard
part
s
Th
ird
han
d f
un
cti
on
Fati
qu
e
Defl
ecti
on
Tem
pera
ture
resi
stan
ce
So
un
d
Sh
ap
e
Sy
mm
etr
y L
/R
"Fo
ld o
ut"
mech
an
ism
Mate
rial
Co
rro
sio
n r
esi
stan
ce
Matc
h c
ab
in
terf
ace
Carr
y t
he l
oad
of
a p
ers
on
Ste
p w
idth
Ste
p a
ngle
, α1, α2
An
ti-s
lip
su
rface
Ris
k o
f p
inch
ing
Pla
n B
if
the m
ain
fu
ncti
on
fail
s
To
tal
size
Carr
y o
ver
Nu
mb
er
of
co
mp
on
en
ts
Man
ufa
ctu
rin
g m
eth
od
Matc
h i
nte
rface o
f en
tran
ce
1 9 39 9
9 9 9 9 9
9
3
9
9 1 1 1 1 1
9 93
9 31 91 3 9 11 9 9 9 9
39 9 3 3
9 3
9 3
9 9 9 9
9 399 9 99 9
99 139 9 9
9 9 9
9 9 99 93 3 33
9 9
1 19 3 9
1 1 1 1 1
91 99
3 11
11 1 13 9 9 33 3 11
9 9 9 9 9 9
3 1 19 1
11
9 3
9 9 99
9 9 99 91 3
3 1 99 3 99 1 99
1 9 9 13 9 3
N Y
Sh
arp
ed
ges
Ho
rizo
nta
l d
ista
nce c
1,
c2
Heig
ht
of
firs
t st
ep
fro
m g
rou
nd
lev
el,
d
Heig
ht
betw
een
ste
ps,
b1
, b
2
Dep
th o
f fo
ot
space,
a1
, a2
c1≤150 m
m, c2≥150 m
m
d≤550 m
m
b1≤400 m
m, b2≤450 m
m
a1,a
2 ≥
150 m
m
≥300 m
m
α1, α2 ≤
65°
Scan
ia s
tan
dard
s
Fst
atic
N
Ffa
tig
ue
N,
C c
ycle
s
k N
/mm
1
3
→←→ ←
3 3 1
Y Y Scan
ia s
tan
dard
s
→
3
106 172 49 153 78 290151 166 151 151 166 156 237 212
←← Y
81 59 26 20 72 7256 75 23 79 38 83
Tm
in -
Tm
ax
Co
nsi
sten
t
Y Y ←
150 35 56 39
→→
1
Appendix D: Comparison with others
Volvo Atego
Figure 1. Volvo Atego.
Advantages Disadvantages
+ Always possible to get in and out - Different step angle when open and closed
+ Same solution on right and left ride - Dangerous if step folds in when used
+ Same stepping plates gives less articles - Difficult to fold in manually
+ “Clean” step plates - Whole step needs to be removed during service
+ Aligned with the door opening - Does not fit with the interface
2
Mercedes Atego
Figure 2. Mercedes Atego.
Advantages Disadvantages
+ Always possible to get in and out - Takes up relatively large space when unfolded
+ Rapid unfolding - If cab and step have different suspension,
problems with motions can occur
+ Fits with the interface - Risk off slipping, smooth step plates
+ Relatively large stepping area
- Needs change in door interface to be
implemented
+ Little risk of dirty steps
3
Mercedes Atego
Figure 3. Mercedes Atego.
Advantages Disadvantages
+ Same solution on right and left ride - Not possible to get in and out if function fails
+ Underlying components are exposed when
open, easy service
- Not aligned with the door opening
+ Fits with the interface
+ Little risk of dirty steps
4
Scania Rosenbauer
Figure 4. Scania Rosenbauer.
Advantages Disadvantages
+ Wide steps - Not possible to get in and out if function fails
+ Takes up relatively little space - Sharp edges, risk of injury
+ Fits with the interface - Risk off slipping, smooth step plates
+ Relatively large stepping area
- Needs change in interface to be implemented
+ Little risk of dirty steps Closed compartment can give high temperatures
+ Good stair effect
1
Appendix E: Concepts
Concept 13, Single sliding step
The upper step is kept the same as in the current design and
the lower step slides out and in when the door is opened and
closed respectively.
Concept 14, Hammock
The lower step is suspended to the upper step by struts and
swings out when the door is opened.
Concept 19, Sliding stair
The boarding step is designed as a stair, fully retracted while
driving and extracted when the door opens. A new battery
box design is required in order to fit the step.
2
Concept 23, Rotating step
The upper step is kept the same as in the current design. Two
lower steps are attached to each other with four joined struts.
When the door is closed, the upper one of the lower steps
rests on top of the lower one. When the door opens, the step
rotates out until they are in a parallel position.
Concept 28, Swing out
The quadrant shaped steps rotates out when the door is
opened, either mechanically or with pneumatics/hydraulics.
The lower step is larger than the upper one to create a stair
effect.
Concept 31, Stairs inside cab
The stair will be fixed and the upper step will be positioned in
the can floor inside the cab.
1
Appendix F: Concept development
Definition of variables in Appendix
All variables used in the force calculations in Appendix E are listed below.
tep load
eaction forces in
omentum in point
orce on one wheel
orce on one strut
orce degrees from
omentum in point
niformed distri uted force
q niformed distri uted force at distance
ength of the loc ing pin in concept
istance from in the direction
adius of the loc ing pin in concept
istance from point to the loc ing pin in concept
hear stress
hear force
2
Concept 13 a, Fold/Slide out
This concept consists of one single step plate equipped with two wheels, rails and levers in order to
transfer motion. The lower plate, visible in Figure 1 will lock further motion when the step plate is
fully folded and unfolded.
Layout
Figure 1. First 3D concept of sliding step, consisting of rails, wheels, a bottom plate and levers.
Weight
Table 1 shows an estimation of the weight, used later in the comparison of the concepts.
Table 1. Estimated weight of concept 13 a
Component Weight [kg]
Upper step plate 2.85
Lower step board 5.7
Support board 2.5
Lever incl. stop 2.3
U-profiles 2.12
Wheels 0.75
Cylinder -
Total 16.22
Calculations
Equation (1) to (3) together with Figure 2, shown below, will estimate the loads induced in the design.
This may cause high local stresses but they are estimated to be controllable. Through investigations,
providers of wheel and rail solutions that can cope with current loads and conditions are found.
Figure 2. Load case of concept 13 a.
3
The reaction forces and are calculated from a force balance and a momentum balance with
Equation (1) and (2).
(1)
( ) (2)
The force acting on one wheel is equal to divided by two, Equation (3).
(3)
Rollco (1) offers wheels and U-rails who can handle these forces.
Schneeberger (2) offers linear bearings on profiled guide ways that also could be used in this
application.
Properties relative to product specification
In Table 2, the requirements of the solution are stated and the fulfillments of these discussed. This is
used later in the comparison of the concepts.
Table 2. Advantages and disadvantages of concept 13 a, relative to product specification
4
+ -
Robustness Risk of contamination in wheels
Several parts, both rails and levers
Motion in several directions
Several dependent components
Safety Plan B exists Risk of pinching
Ergonomics
Withstand environment Hidden cylinder Risk of contamination in wheels
Plan B Plan B exists
Quality Only one lower step plate Sound when meeting the support
Rotational motion
Time to function Rotation
Maintenance Wheel greased for lifetime Sliding bearings
Simplicity Several dependent components
Modularized Same solution R/L Several parts
Weight
Cost Costly with rail and wheel
Total size Takes up space inwards
Supporting plate is needed
Easy to assemble Rails
Easy to perform service Hidden cylinder
Disassembled to service wheels
Appearance Only one lower step plate
5
Concept 23 b, Fold out
This concept, just as the first one, consists of one single step plate but is equipped with four levers in
order to transfer motion. The lower plate, visible in Figure 3 will also here lock further motion when
the step plate is fully folded and unfolded.
Layout
Figure 3. First 3D concept of, consisting of levers, and a bottom plate.
Weight
Table 3. shows an estimation of the weight, used later in the comparison of the concepts.
Table 3. Estimated weight of concept 23 b.
Component Weight [kg]
Upper step plate 2.85
Lower step board 5.7
Support board 2.5
Lever incl. stop 2.3
Lever 1.56
Cylinder -
Total 14.91
Calculations
Equation (5) to (9) together with Figure 4 and 5, shown below, will estimate the loads induced in the
design. This may cause high local stresses but they are estimated to be controllable. Through
investigations, providers of wheel and rail solutions that can cope with current loads and conditions
are found.
Figure 4. Load case of concept 23 b.
6
The reaction forces and are calculated from a force balance and a momentum balance with
Equation (5) and (6).
(5)
( ) (7)
The force acting on one strut is equal to divided by two, Equation (8).
(8)
The force in Figure 5 is calculated with Equation (9).
Figure 5. Load case of lever in concept 23 b.
(9)
Properties relative to product specification
In Table 4, the requirements of the solution are stated and the fulfillments of these discussed. This is
used later in the comparison of the concepts.
+ -
7
Table 4. Advantages and disadvantages of concept 23 b, relative to product specification
Robustness Supporting plate both when unfolded
an folded
Several moving parts
Safety Plan B exists Risk of pinching
Ergonomics
Withstand environment Insensitive fold out mechanism
Plan B Plan B exists
Quality Sound when meeting the
support
Time to function
Maintenance No need Sliding bearings
Simplicity Few parts
Modularized Same solution R/L
Few unique parts
Weight
Cost
Total size Takes up space inwards
Easy to assemble Narrow tolerances
Easy to perform service
Appearance Only one lower step plate Rotational motion
8
Concept 23 a, Fold out concept
Next concept uses two separate step plates connected with four levers where only one of the two are
moved in order to create a stair effect. The levers are also used to lock further motion of the outer step
plate when fully unfolded. The concept can be viewed in Figure 6.
Layout
Figure 6. First 3D concept of, consisting of a bottom plate and levers.
Weight
Table 5 shows an estimation of the weight, used later in the comparison of the concepts.
Table 5. Estimated weight of fold out concept, 23 a
Component Weight [kg]
Upper step plate 2.85
Lower step board 5.7
Levers 1.6
Cylinder -
Total 10.15
Calculations
In order to do the estimations the whole system is divided into three sub systems shown in Figure 8.
Equation (10) to (16) together with Figure 7 and Figure 8, shown below, will estimate the loads
induced in the design.
Figure 7. Concept layout with positioning of load.
9
Figure 8. Load case of concept 23 a.
1:
Reaction forces , and in the step plate are calculated in Equation (10) to (12).
(10)
(11)
(12)
2:
Reaction forces and in lever 2 are calculated in Equation (13).
(13)
3:
10
Reaction forces , and , in lever 3 are calculated in Equation (14) to (16).
(14)
(15)
(16)
Properties relative to product specification
In Table 6, the requirements of the solution are stated and the fulfillments of these discussed. This is
used later in the comparison of the concepts.
Table 6. Advantages and disadvantages of concept 23 a, relative to product specification
+ - Robustness Several moving parts
Suspended in only two levers Safety Plan B exists Risk of pinching
Ergonomics Different step height when folded and
unfolded Withstand environment Insensitive fold out
mechanism
Plan B Plan B exists
Quality Two lower step plates
Time to function Long folding distance
Maintenance No need Sliding bearings
Simplicity Different step plates
Modularized Same solution R/L
Weight
Cost Extensive machining on step plates
Total size
Easy to assemble Hidden lever suspension
Easy to perform service
Appearance
11
Concept 13 b, Slide out
This concept consists of one single stepping plate equipped with two rails and four wheels and levers
transfer motions and locks it when fully rolled out. The concept can be viewed in Figure 9.
Layout
Figure 9. First 3D concept of, consisting of a bottom plate, lavers, rails and wheels.
Weight
Table 7 shows an estimation of the weight, used later in the comparison of the concepts.
Table 7. Estimated weight of concept with wheels, 13 b
Component Weight [kg]
Upper step plate 2.85
Lower step board 5.7
Levers 0.4
U-profiles 1.59
Wheels 1.5
Cylinder -
Total 12.04
Calculations
Equation (17) to (19) together with Figure 10, shown below, will estimate the loads induced in the
design.
12
Figure 10. Load case of concept 13 b.
The reaction forces and are calculated from a force balance and a momentum balance with
Equation (17) and (18).
(17)
( ) (18)
The force acting on one wheel is equal to divided by two, Equation (19).
(19)
Rollco (1) offers wheels and U-rails that can handle these forces.
Schneeberger (2) offers linear bearings on profiled guide ways that also could be used in this
application.
Properties relative to product specification
In Table 8, the requirements of the solution are stated and the fulfillments of these discussed. This is
used later in the comparison of the concepts.
13
Table 8. Advantages and disadvantages of concept 23 b, relative to product specification
+ -
Robustness Risk of contamination in wheels
Risk of drawer effect
Safety Plan B exists Not locked if not entirely out
Ergonomics
Withstand environment Risk of contamination in wheels
Plan B Plan B exists
Quality Linear motion
Clean
Only one lower step
plate
Regresses when locking
Time to function
Maintenance Wheel greased for
lifetime
Sliding bearings
Simplicity Suspended in rails Several levers
Modularized Same solution R/L
Relatively few
components
Weight
Cost Costly with rail and wheel
Total size Takes up space inwards
Easy to assemble
Easy to perform service Hidden cylinder
Disassembled to service wheels
Appearance Linear motion
Only one lower step
14
Concept 13 c, Slide out
This concept consists of one single stepping plate equipped with two rails with linear bearings in order
for the step plate to slide in and out, see Figure 11.
Layout
Figure 11. First 3D concept of concept 13c,, consisting of a bottom plate, laver and linear bearings.
Weight
Table 9 shows an estimation of the weight, used later in the comparison of the concepts.
Table 9. Estimated weight of concept with linear bearings, 13 c
Component Weight [kg]
Upper step plate 2.85
Lower step plate 5.7
Levers 0.4
Rails 2.38
Wheels 2.8
Total 13.63
Calculations
Same as for concept 13 b.
Properties relative to product specification
Same as for concept 13 b.
15
Concept 25, Rotating concept
This concept consists of two separate steps in different depths that can be rotated in order to get a stair
effect. When out, the rotation is locked with a stop block. When in, the rotation is locked with a pin,
controlled by a pneumatic valve. The concept can be viewed in Figure 12.
Layout
Figure 12. First conceptual design of concept 25, consisting of two bottom plates, two stops and a
lever.
The locking mechanism can be performed in two different ways, either to lock the rotation of the
deeper step or to lock the rotation of the shorter step.
Calculations
Equation (20) to (22) together with Figure 13 shown below, will estimate the loads induced in the
design, locked by the short step.
Figure 13. Load case of concept 25 unfolded, locked by step of smaller depth.
16
The reaction forces , and are calculated from a force balance and a momentum balance with
Equation (20) to (22).
(20)
(21)
(22)
Equation (23) to (24) together with Figure 14 shown below, will estimate the loads induced in the
design, locked by the long step.
Figure 14. Load case of concept 25 unfolded, locked by step of larger depth.
The reaction forces and are calculated from a force balance and a momentum balance with
Equation (23) and (24).
(23)
(24)
17
When folded in, the step is locked with a pin in a disc, in order to make an analysis of the dimension
of the pin and disc, it is dimensioned against the shear strength of the material.
Equation (25) to (29) together with Figure 15 and Figure 16 shown below, will estimate the
dimensions of the pin and disc.
Figure 15. Load case of concept 25 folded, locked by pin.
The momentum around the point “O” is calculated by Equation (25). The momentum MO is used to
calculate the uniform load on the locking pin, see Figure 16.
(25)
Figure 16. Load case of pin, used to lock rotation of step.
18
Shear stress in a circular cross section is calculated in Equation (26), formula from (3).
( )
( ) (26)
The shear force ( ) is calculated from a force balance from the section in Figure 16, Equation (27).
( )
(27)
( )
The maximum shear force, , is obtained when , Equation (28).
(28)
Equation (28) is inserted in Equation (26) to obtain the maximum shear stress, .
(29)
Properties relative to product specification
In Table 10, the requirements of the solution are stated and the fulfillments of these discussed. This is
used later in the comparison of the concepts.
Table 10. Advantages and disadvantages of concept 25, relative to product specification
19
+ -
Robustness
Safety Plan B exists Risk of pinching
Locking tolerances
Ergonomics
Withstand environment
Plan B Plan B exists
Quality Does not give an quality
impression
Time to function
Maintenance Bearings
Valve
Simplicity Many different components
needed
Modularized Identical solution L/R
Weight
Cost Valve
Double step plates
Total size
Easy to assemble
Easy to perform service
Appearance Does not give an quality
impression
20
Concept 27, Rotating concept
This concept is a development of the present design. The foldable is step placed further back in order
to create more space, for the user to be able to step on regardless if it is in or out, showed in Figure 17.
Layout
Figure 17. First conceptual design of concept 27, consisting of a bottom plate and levers.
Weight
Table 11 shows an estimation of the weight, used later in the comparison of the concepts.
Table 11. Estimated weight of concept 27
Component Estimated weight [kg]
Upper step plate 2.85
Lower step unit 8.55
Lever incl. stop 3.1
Total 14.5
Properties relative to product specification
In Figure 12, the requirements of the solution are stated and the fulfillments of these discussed. This is
used later in the comparison of the concepts.
21
Table 12. Advantages and disadvantages of concept 27, relative to product specification
+ -
Robustness Unprotected cylinder
The current solution the hinge
sticks
Safety Risk of pinching
Ergonomics
Withstand environment Unprotected cylinder
Plan B Plan B exists
Quality
Time to function
Maintenance Bearings
Simplicity Different step plates
Modularized Identical solution L/R
Existing components can be used
Weight
Cost
Total size
Easy to assemble
Easy to perform service
Appearance Does not look good in retracted
position
22
Concept 28, Swing out concept
This concept consists of two quadrant shaped steps in different depths that can be rotated in order to
get a stair effect. The opening and closing can be done either mechanically or by
pneumatics/hydraulics. The concept can be viewed in Figure 18.
Layout
Figure 18. First conceptual design of concept 28, consisting of a two plates and a rotation axis.
Weight
Table 13 shows an estimation of the weight, used later in the comparison of the concepts.
Table 13. Estimated weight of concept 28
Component Estimated weight [kg]
Upper step plate 2.85
Lower step unit 11
Total 13.85
Calculations
Due to the complex geometry of the concept, a force balance cannot be performed. Instead, a simple
FEM simulation on the suspension system is done in order to verify the stresses acting on the design.
The result can be seen in Figure 19.
23
Figure 19. FEM simulation of supporting structure.
Properties relative to product specification
In Table 14, the requirements of the solution are stated and the fulfillments of these discussed. This is
used later in the comparison of the concepts.
Table 14. Advantages and disadvantages of concept 28, relative to product specification
+ -
Robustness Robust solution if the rotation
occurs mechanically
Safety
Ergonomics Different shape of upper and
lower step if the upper step is
fixed
Withstand environment
Plan B Plan B may exists
Quality
Time to function Fast if the steps are integrated in
the door
Maintenance Bearings
Simplicity A mechanical solution could be
simple
Modularized Symmetry L/R Not identical solution L/R
Weight
Cost
Total size Takes up a lot of space under the
cab
Easy to assemble
Easy to perform service
Appearance Different upper and lower steps
Different front and rear steps
24
References Appendix F
1. Rollco AB. U-Rail. [Online] [Citat: den 15 May 2017.] http://www.rollco.eu/products/u-rail/.
2. Schneeberger. Linear- and profiled guideways. [Online] [Citat: den 15 May 2017.]
https://www.schneeberger.com/en/products/linear-bearings-and-profiled-guideways/.
3. Sundström, Bengt. Handbok och formelsamling i Hållfasthetslära. Stockholm : Instant Book AB,
2013.
1
Appendix G: Concept scoring
After the concept development has been performed, a relative decision matrix is being done, the
results shown in Table 15.
Table 15. Relative decision matrix- selection 2
27 (ref) 13 a 13 b 23 b 25 28 a 28 b
Robustness 5 - - - - + 0
Safety 5 0 - 0 - - -
Ergonomics 4 0 0 0 0 + -
Withstand environment 4 - - + - + 0
Plan B if function fails 3 0 0 0 0 - 0
Quality 3 0 + 0 0 0 -
Time to function 3 0 0 0 0 + 0
Maintenence 3 0 0 0 - 0 0
Simplicity 2 - - 0 - 0 0
Modularized 2 0 + + 0 - -
Weigth 2 - - - - - -
Cost 2 - - + - - -
Total size 2 - - - 0 - -
Easy to assemble 2 0 0 0 - 0 0
Easy to perform service 2 0 0 0 0 0 0
Apperance 1 + + + 0 + -
1 6 9 0 17 0
27 17 27 20 12 24
17 22 9 25 16 21
0 -16 -16 0 -25 1 -21
2 4 4 2 7 1 6
Yes No Yes Yes No Yes No
D
A
T
U
M
ConceptsRequirement Weight
Sum +
Sum 0
Sum -
Total value
Ranking
Proceed with concept
2
In the next round, the concepts eliminated are removed and a change in the choice of reference is made.
Results in Table 16.
Table 16. Relative decision matrix – 3
23 b (ref) 13 b 27 28 a 13 c
Robustness 5 0 + + +
Safety 5 0 0 - 0
Ergonomics 4 0 0 + 0
Withstand environment 4 - - + 0
Plan B if function fails 3 0 0 - 0
Quality 3 + 0 - +
Time to function 3 0 0 + 0
Maintenence 3 0 0 0 0
Simplicity 2 - 0 - +
Modularized 2 0 - - 0
Weigth 2 0 + - +
Cost 2 - - - +
Total size 2 0 + - 0
Easy to assemble 2 0 0 - 0
Easy to perform service 2 0 0 0 0
Apperance 1 + - + +
4 9 17 15
33 27 5 30
8 9 23 0
0 -4 0 -6 15
2 4 2 5 1
Yes No Yes No Yes
Concepts
Total value
Ranking
Proceed with concept
Requirement Weight
D
A
T
U
M
Sum +
Sum 0
Sum -
3
In the final selection, Table 17, it becomes clear that concept 13 c wins with good margins. This
means that it will be the final concept to proceed with.
Table 17. Relative decision matrix – final selection
13 c (ref) 27 23 b
Robustness 5 - -
Safety 5 0 0
Ergonomics 4 0 0
Withstand environment 4 0 0
Plan B if function fails 3 0 0
Quality 3 - -
Time to function 3 0 0
Maintenence 3 0 0
Simplicity 2 - -
Modularized 2 0 0
Weigth 2 - -
Cost 2 0 -
Total size 2 + 0
Easy to assemble 2 0 0
Easy to perform service 2 0 0
Apperance 1 - -
2 0
30 30
13 15
0 -11 -15
1 2 2
Yes No No
Concepts
D
A
T
U
M
Sum +
Sum 0
Sum -
Total value
Ranking
Proceed with concept
Requirement Weight
1
Appendix H: Friction Force Calculations
Figure 1 shows the load case of the boarding step in its outer position with an angular step load,
reaction forces from the rails and friction force between the step plate and the sliding material.
Figure 1. Load case with step load, reaction forces from the rails and friction forces.
Fx and Fy are the x- and y-components of the step load FStep and are derived in Equation (1) and (2).
(1)
(2)
The reaction forces RA and RB are derived from a force balance in the y-direction and a moment
balance around B, Equation (3)-(6).
(3)
(4)
(5)
(6)
Equation (7) shows the equation for friction force, Ff, where FN is the normal force and μ is the
coefficient of friction. The friction force Ff for this particular load case is derived in Equation (8).
(7)
( ) (8)
2
Equation (1), (2), (4), (6) and (8) were used to calculate the forces at different step loads and step
angles. Six different angles from 0° to 45° and 32 different step loads from 550 N to N were
used. This represents persons of different weight stepping on the boarding step with different angles.
Figure 2 shows the difference between friction force and force in the x-direction, with a friction
coefficient of 0.3. All values are positive and increases with increased step load. This proves that the
friction force is enough to keep the step from sliding when loaded and no other locking mechanism are
needed with this coefficient of friction.
Figure 2. Difference between friction force and force in the x-direction depending on step load and
step angle.The coefficient of friction is 0.3. The friction force are higher than the force in the x-
direction at all step angles and increases with step load.
Figure 3 shows the difference between friction force and force in the x-direction, with a friction
coefficient of 0.25. With this friction coefficient the friction force are lower than the force in the x-
direction at a step angle of 45°. The difference are 14 N at low step loads and up to 53 N at higher step
load, this loads could be hold by the pneumatic cylinder.
Figure 3 shows the difference between friction force and force in the x-direction, with a friction
coefficient of 0.2. With this friction coefficient the friction force are lower than the force in the x-
direction at a step angle of 40° and 45°. The difference at 45° is 89 N at low loads and 339 N at higher
loads.
0
500
1000
1500
2000
2500
3000
550 750 950 1150 1350 1550 1750 1950
Ff-
Fx [
N]
Step load [N]
Difference between friction force and force in the x-direction, μ=0.3
0° 10° 20° 30° 40° 45°
3
Figure 3. Difference between friction force and force in the x-direction depending on step load and
step angle. The coefficient of friction is 0.25.
Figure 4. Difference between friction force and force in the x-direction depending on step load and
step angle. The coefficient of friction is 0.2.
-500
0
500
1000
1500
2000
2500
550 750 950 1150 1350 1550 1750 1950
Ff-
Fx [
N]
Step load [N]
Difference between friction force and force in the x-direction, μ=0.25
0° 10° 20° 30° 40° 45°
-500
0
500
1000
1500
2000
550 750 950 1150 1350 1550 1750 1950
Ff-
Fx [
N]
Step load [N]
Difference between friction force and force in the x-direction, μ=0.2
0° 10° 20° 30° 40° 45°
1
Appendix I: Spring and cylinder calculations
Figure 1 show a spring in equilibrium, with a pre tension and, compressed. Lengths and forces used in
the following equations are defined in the figure.
Figure 1. a) Spring in equilibrium, b) Spring with pre tension in retracted position and c) Compressed
spring in outer position.
To be able to retracted the step completely and keep it in place while driving the pre tension force, F1,
needs to exceeds the friction force, Ff, calculated with Equation (1).
(1)
The spring constant, k, needed to achieve the pre tension force, F1, for a given length, x1, was
calculated with Equation (2).
(2)
where x1 is the difference between the spring length in equilibrium and the spring length in pre tension.
The total compression length of the spring, x2, was calculated with Equation (3).
(3)
where s is the stroke length.
The force needed to compress the spring the total length, x2, with a given spring constant, k, was
calculated with Equation (4).
(4)
The cylinder needs to push out the spring and hold for any step load in the x-direction exceeding the
friction force, see appendix 3. The total cylinder force is calculated with Equation (5).
( ) (5)
2
The piston diameter, d, is calculated with Equation (6).
√
(6)
where p is the cylinder pressure.
Figure 2 shows how the spring constant and the piston diameter should be dimensioned for different
pre tension lengths.
Figure 2. Spring constant and piston diameter depending on pre tension length.
0,0
5,0
10,0
15,0
20,0
25,0
30,0
35,0
40,0
45,0
50,0
0,00
0,50
1,00
1,50
2,00
2,50
3,00
3,50
4,00
4,50
0 10 20 30 40 50 60
Pis
ton d
iam
eter
, d
[m
m]
Sp
ring c
onst
ant,
k [
N/m
m]
Pre tension length, x1 [mm]
Spring constant and piston diameter
Spring constant Piston diameter
1
Appendix J: FMEA
No
Function
Com
ponent
Failu
re m
ode
Failu
re e
ffect
Failu
re c
ause
OS
DR
PN
Solu
tion
Adju
sted
OS
DR
PN
AR
etr
action
A1
Cylin
der
Ste
p w
ill n
ot
retr
act
Legis
lation o
f tr
uck w
idth
are
not
fulf
illed
Spri
ng is
dis
functionin
g1
10
330
-1
10
330
A2
Low
er
step
Ste
p g
ets
stu
ck
Legis
lation o
f tr
uck w
idth
are
not
fulf
illed
Dra
wer
eff
ect, w
edge e
ffect
310
390
Ste
eri
ng c
ylin
der
or
rail,
Posi
tion t
he p
neum
atic
cylin
der
in t
he c
entr
e o
f th
e
low
er
step, U
se low
fric
tion m
ate
rial on t
he
sides
of
the r
ails
210
360
A3
Slid
ing m
ate
rial
Ste
p g
ets
stu
ck
Legis
lation o
f tr
uck w
idth
are
not
fulf
illed
Conta
min
ation c
ause
s th
e
step t
o g
et
stuck
710
3210
Ente
rely
clo
sed s
lidin
g
syst
em
or
an v
ery
open
syst
em
, m
echanic
al dir
t
reje
cto
r5
10
3150
A4
Slid
ing m
ate
rial
Ste
p g
ets
stu
ck
Legis
lation o
f tr
uck w
idth
are
not
fulf
illed
Pla
stic
defo
rmation in p
last
ic
cause
s st
ep t
o g
et
stuck
310
390
Choose
mate
rial w
ith h
igh
hard
ness
and y
ield
str
ength
210
360
BE
xtr
action
B1
Cylin
der
Ste
p w
ill n
ot
extr
act
Reduced f
unction, no
poss
ibili
ty o
ff e
gre
ss f
acin
g
forw
ard
Pneum
atic s
yst
em
lose
s
pre
ssure
310
260
-3
10
260
B2
Low
er
step
Ste
p g
ets
stu
ck
Reduced f
unction, no
poss
ibili
ty o
ff e
gre
ss f
acin
g
forw
ard
Dra
wer
eff
ect
410
280
Ste
eri
ng c
ylin
der
or
rail,
Posi
tion t
he p
neum
atic
cylin
der
in t
he c
entr
e o
f th
e
low
er
step, U
se low
fric
tion m
ate
rial on t
he
sides
of
the r
ails
310
260
B3
Slid
ing m
ate
rial
Ste
p g
ets
stu
ck
Legis
lation o
f tr
uck w
idth
are
not
fulf
illed
Conta
min
ation c
ause
s th
e
step t
o g
et
stuck
710
2140
Ente
rely
clo
sed s
lidin
g
syst
em
or
an v
ery
open
syst
em
, m
echanic
al dir
t
reje
cto
r5
10
2100
B4
Slid
ing m
ate
rial
Ste
p g
ets
stu
ck
Reduced f
unction, no
poss
ibili
ty o
ff e
gre
ss f
acin
g
forw
ard
Pla
stic
defo
rmation in p
last
ic
cause
s st
ep t
o g
et
stuck
310
260
Choose
mate
rial w
ith h
igh
hard
ness
and y
ield
str
ength
210
240
CS
teppin
g
C1
Slid
ing m
ate
rial
Fri
ction w
ill n
ot
keep t
he
step in o
utf
old
ed p
osi
tion
Inju
ryC
onta
min
ation
410
9360
Ente
rely
clo
sed s
lidin
g
syst
em
or
an v
ery
open
syst
em
, m
echanic
al dir
t
reje
cto
r3
10
9270
C2
Slid
ing m
ate
rial
Fri
ction w
ill n
ot
keep t
he
step in o
utf
old
ed p
osi
tion
Inju
ry
Change in o
pera
tion
conditio
n (
tem
pera
ture
)
cause
s change in m
ate
rial
pro
pert
ies
310
10
300
Choose
a m
ate
rial re
sist
ant
again
st t
em
pera
ture
changes
210
10
200
C3
Slid
ing m
ate
rial
Fri
ction w
ill n
ot
keep t
he
step in o
utf
old
ed p
osi
tion
Inju
ry
Wear
cause
s a c
hange in t
he
fric
tion c
oeff
icie
nt
210
10
200
Choose
mate
rial w
ith low
wear
rate
110
10
100
C4
Low
er
step
Ste
p is
not
entire
ly s
lip
resi
stant
Inju
ryC
onta
min
ation
110
550
-1
10
550
C5
Low
er
step
Ste
p is
not
entire
ly s
lip
resi
stant
Inju
ryN
ot
enough p
rofi
le1
10
550
-1
10
550
C6
Low
er
step
Ste
ppin
g d
one a
t to
o h
igh
of
an a
ngle
Inju
ry
Fri
ction n
ot
hig
h e
nough t
o
keep s
tep in p
lace
410
10
400
Use
a c
ylin
der
that
can
carr
y e
xtr
a load
110
10
100
1
Appendix K: FEM Model verification
To verify the model used in the simulations, the simulation results done in Catia GAS was compared
to simulations made in Abaqus by the simulation section, Figure 1 and 2.
Mesh type: 3D Octree tetrahedron mesh
Mesh sizes used:
Global mesh size: 2 mm
Local mesh size for radii: 0.8 mm
Absolute sag: 0.2 mm
The upper step have a coarser mesh size of 5 mm due to its little contribution to stress and deflection
of the rest of the system and the fact that it is not evaluated and verified.
Mesh type: 2D Quadratic mesh
Figure 1. V.M Stress for fatigue load case, 𝐹𝐹𝑎𝑡𝑖𝑔𝑢𝑒, on the left side of the lower step in Catia
GAS.
2
Figure 2. Displacement (left) and V.M Stress (right) for the fatigue load case, on the left side
of the lower step in Abaqus.
Results
The results of the analysis can be seen in Table 1 and 2.
Table 1. Calculated structural stiffness and the respective deformation for the boarding step with
different loading points in Catia GAS
Loading point
Structural
stiffness and deformations
Lower left Lower
middle
Lower right Upper
middle
[N/mm] 0.41 0.30 0.29 -
: [mm] 2.46 3.29 3.50 -
[N/mm] 0.27 0.30 0.41 -
[mm] 3.69 3.35 2.44 -
Table 2. Calculated structural stiffness and the respective deformation for the boarding steps with
different loading points performed in Abaqus
Loading point
Structural
stiffness and deformations
Lower left Lower
middle
Lower right Upper
middle
[N/mm] 0.32 0.22 0.22 -
: [mm] 3.15 4.55 4.55 -
[N/mm] 0.22 0.22 0.32 -
[mm] 4.55 4.55 3.15 -
This shows that the two models do not entirely conform to each other. The result is probably due to
different settings and building of model.
This is however not a large concern since the new design is verified against the old one, both done in
Catia GAS. The deflections are in the same order of magnitude, which is most important-
Table 3 show the difference in percentage from the Abaqus results and the Catia GAS ones
Table 3. Difference in deformation for the boarding steps from Catia GAS to Abaqus with different
loading points
Loading point
Deformations
Lower
left
Lower
middle
Lower right Upper middle
[%] -22% -28% -23% -
[%] -19% -26% -23% -
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