ATTACHMENT FOR “AVPOS USING FEA

i

OPTIMISATION OF CEILING ATTACHMENT FOR “AVPOS"

USING FEA

Bachelor Degree Project in Mechanical Engineering C-Level 22.5 ECTS Spring term 2015 Mikael Koskenranta Supervisor: Moyra McDill Examiner: Karl Mauritsson

1

Abstract

In product manufacture and assembly it is important to lower materials usage and assembly time while retaining a safety margin against structural failure. In this project the ceiling attachment of the AVPOS offloading arm is redesigned based on objectives identified by Löfs Specialmaskiner. The original ceiling attachment is analysed using finite element analysis to identify any structural weaknesses to be addressed during redesign. Based on information gained during the FEA three concepts are generated and rated based on their estimated assembly times, material usage, parts count and the cutting length for machining. One concept is selected for further topology optimisation and iterative FEA, where material is removed from the selected concept design while maintaining safety objectives. The optimised design developed during this project is shown to have a lower number of parts, lower usage of material and assembly time while retaining an overall FOS value of 4. In addition to redesigning the ceiling attachment, alternative bearings are also investigated which changes the existing ball bearings to a self-aligning sliding bearing with self-lubricating properties, lowering the machining tolerances as well as lowering the needs for service. This project contributes to lowering material usage and ease of assembly in the product AVPOS that is manufactured by Löfs Specialmaskiner. The redesigned ceiling attachment along with the suggested bearings will likely simplify future manufacturing as well as lower any concerns for damage to people and property as a result of structural failure.

ii

Certification This thesis has been submitted by Mikael Koskenranta to the University of Skövde as a requirement

for the degree of Bachelor of Science in Mechanical Engineering. The undersigned certifies that all

the material in this thesis that is not my own has been properly acknowledged using accepted

referencing practices and, further, that the thesis includes no material for which I have previously

received academic credit.

Mikael Koskenranta

Skövde 2015-05-20

Institutionen för Ingenjörsvetenskap/ Department of Engineering Science

iii

Acknowledgements I would like to use this space to extend many thanks to Löfs Specialmaskiner AB for the opportunity

to perform this final year project in mechanical engineering in collaboration with them as well as the

support they have given.

I would also like to acknowledge with gratitude the contribution of Ida Lindell in developing the

envelope of loading cases.

Furthermore, I would like to express my gratitude to my project advisor Moyra McDill for always

finding time in her busy schedule to share her engineering know-how and for all the invaluable

feedback and support she has given.

iv

Contents Abstract .................................................................................................................................................... i

Certification ..............................................................................................................................................ii

Acknowledgements ................................................................................................................................. iii

Appendices ............................................................................................................................................... v

Figures ..................................................................................................................................................... vi

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

1.1 Background .............................................................................................................................. 2

1.2 Problem Statement ................................................................................................................. 2

1.3 Limitations ............................................................................................................................... 3

1.4 Technology, Society and Environment .................................................................................... 3

1.5 Overview.................................................................................................................................. 3

Approach ......................................................................................................................................... 4

FEA of original design ...................................................................................................................... 4

3.1 FEA method ............................................................................................................................. 4

3.1.1 Geometry ......................................................................................................................... 5

3.1.2 Material properties and yield criterion ........................................................................... 6

3.1.3 Boundary conditions ....................................................................................................... 6

3.1.4 Load cases........................................................................................................................ 6

3.1.5 Solution method .............................................................................................................. 7

3.2 Analysis and key findings ......................................................................................................... 7

Design modification and preliminary concept designs ................................................................... 8

4.1 Concept generation ................................................................................................................. 8

4.2 Design for manufacture and assembly .................................................................................... 9

4.3 Screening of concept designs ................................................................................................ 12

Optimisation of the selected concept design ................................................................................ 13

5.1 Size optimisation ................................................................................................................... 13

5.2 Topology optimisation ........................................................................................................... 13

Finite Element Analysis of the Optimised Concept 2 .................................................................... 15

v

Finding suitable bearings ............................................................................................................... 15

Recommendations for new conceptual design and discussion .................................................... 16

Conclusions .................................................................................................................................... 18

References ................................................................................................................................. 18

Appendices Appendix 1 – Division of Labour

Appendix 2 – Time plan

Appendix 3 – Load cases

Appendix 4 – FEA of original ceiling attachment

Appendix 5 – Assembly times

Appendix 6 – Iterative FEA and optimisation

vi

Figures Figure 1.1 Overview of AVPOS. ............................................................................................................... 1

Figure 2.1. Breakdown of approach. ....................................................................................................... 4

Figure 3.1 Original ceiling attachment. ................................................................................................... 5

Figure 3.2 Simplified ceiling attachment ................................................................................................. 5

Figure 3.3. Typical mesh after refinement. ............................................................................................. 5

Figure 3.4. Boundary conditions. ............................................................................................................ 6

Figure 3.5. Load contact points. (Lindell, 2014) ...................................................................................... 7

Figure 3.6. Hot spots and bending in the ceiling attachment. Load case 5. ........................................... 7

Figure 3.7. High stresses in load case 7. .................................................................................................. 7

Figure 4.1. Points in the original ceiling attachment between which distances may not change. ......... 8

Figure 4.2. Concept 1............................................................................................................................... 8

Figure 4.3. Concept 2............................................................................................................................... 9

Figure 4.4. Concept 3............................................................................................................................... 9

Figure 5.1. Factor of safety of Concept 2 after thickness optimisation. ............................................... 13

Figure 5.2. Result of removing material in the first iteration of topology optimisation. ...................... 14

Figure 5.3. Iso-cutout showing areas with stresses greater than 15 MPa for load case 7.................... 14

Figure 5.1. Final model of Concept 2. .................................................................................................... 15

Figure 6.2. FOS for the final model for load case 7. .............................................................................. 15

Figure 8.1. Close-up of milled slots. ...................................................................................................... 17

Figure 8.2. Completed redesign of Concept 2 mounted on the swing arm. ......................................... 17

Table 3.1. Material properties of Plancast Plus 5083(n) (Löfs Specialmaskiner AB, 2014b). .................. 6

Table 4.1. DFA evaluation of the original ceiling attachment. (Boothroyd, Dewhurst, & Knight, 2002)

............................................................................................................................................................... 10

Table 4.2. Contour lengths and thicknesses. ......................................................................................... 11

Table 4.3. Pugh selection matrix for Concept 1-3. ................................................................................ 12

Table 8.1. Objectives. ............................................................................................................................ 16

Figures and tables for appendices are listed separately in each appendix.

1

Figure 1.1 Overview of

AVPOS.

A – Ceiling attachment

B- Swing arm

C – End effector

Image courtesy of Löfs

Specialmaskiner AB.

Introduction Although the steady improvements in power sources and machining capabilities throughout history

have greatly improved the rate and quality of production, worker health and safety were often

overlooked or entirely absent. It took until the late 1800’s for the creation of laws limiting the use of

child labour in Britain (Venning, 2010). Similarly in Sweden, children under the age of 14 made up

over 5% of the workforce in 1875 and it was not until 1949 that child labour was abolished by law

(Forsström, 2010).

Today there exist several organisations, unions, and laws meant to protect workers from injury and

ill-health. One such organisation is the Swedish Work Environment Authority which has published

regulations relating to workplace ergonomics. One of these regulations states that the employer

must provide a workplace where fatigue and injuries to the body in general, and the back and spine

in particular, due to heavy lifting or repetitive work are minimised, preferably by offloading such

work by the use of technical equipment (Arbetsmiljöverket, 2012).

One way to offload physically exerting operations from a human

operator is to use a powered torque arm or offloading arm. There

are several types of both manual and powered torque arms and

offloading arms being produced today by several manufacturers,

such as Atlas Copco (Atlas Copco, 2015) and Givens Engineering

(Givens Engineering Inc., 2011). The main drawback of most pre-

existing torque arms is their slow and cumbersome operation,

often caused by having long horizontal members, or by having

only a small work area (Löfs Specialmaskiner AB, 2014b).

Löfs Specialmaskiner AB (LS) is a small company which mainly

works with customized machines and mechanical construction in

project form (Löfs Specialmaskiner AB, 2014a). LS is attempting to

solve the problems with existing offloading systems by developing

a pneumatically powered offloading arm, mounting it directly

above the workstation and by using a pneumatic balancing system

to cancel out the forces that would be transmitted to the operator

(Löfs Specialmaskiner AB, 2013).

LS has purpose-designed AVPOS (Avlastnings- och

positioneringsarm), a unique tool which eases manual assembly

operations performed by an operator at a workstation, such as

lifting operations or offloading the torque from a powered torque

wrench from the operator as shown in Figure 1.1. As such AVPOS

is not only intended as a tool for offloading operations such as

tightening bolts, but also has the ability to be used as a lifting tool

for objects up to 20 kg (Löfs Specialmaskiner AB, 2013).

AVPOS is built as a pendulum that is free to swing around two

2

axes, allowing the lower end to be positioned anywhere on a two-dimensional plane. With the

extension of the swing arm this enables a large three-dimensional volume to be used as a work area.

The swing arm (B) of AVPOS is balanced by a pneumatic cylinder at its top which can keep the arm

stationary at any point in its swing, as well as aid in lifting small loads or prevent movement when the

end effector (C) holds a tool such as a torque wrench. The swing arm (B) and pneumatic cylinder are

held up by a ceiling attachment (A).

1.1 Background

Only simple stress calculations have been performed by LS of the component parts of AVPOS to

ensure they can manage intended loads. The ceiling attachment has to date not deformed nor failed

destructively, but the ceiling attachment needs to be further analysed and developed to ensure

required performance in regards to both operator safety and manufacturing costs (Löfs

Specialmaskiner AB, 2014b).

This work is performed to suggest an improved ceiling attachment for the industrial tool AVPOS in

such a way that it is easier and less expensive to manufacture, assemble and is safe to use. These

improvements are expected to come in the form of fewer component parts, fasteners and better

defined safety margins. The bearings used in the ceiling attachment are also to be investigated for

suitability in regards to the expected loads and a suitable replacement suggested.

This bachelor’s degree project is performed in collaboration with Löfs Specialmaskiner AB, in an

effort to lower the manufacturing cost of a mechanical offloading and positioning system through

the simplification of its ceiling attachment.

1.2 Problem Statement

The existing design needs to be analysed to determine any structural limitations to be improved as

well as propose a new design (Löfs Specialmaskiner AB, 2014b) that is:

easier to assemble,

has less than half the number of parts and fasteners,

has a material usage that is lower by at least 20%,

has a factor of safety of 4 in regards to plastic deformation,

made from machined aluminium plates (Löfs Specialmaskiner AB, 2014b),

possible to swap with the current design without further changes to the rest of the system.

Material costs should be kept as low as possible by using thin aluminium plates where waste from

machining can be minimised (Löfs Specialmaskiner AB, 2014b). As the price of aluminium plates is

directly related to the weight, rather than the dimensions, thinner plates are preferred where

possible to minimise materials cost before machining operations.

LS has determined that the greatest possible loadings from misuse are likely to be caused by

the total weight of a person hanging on the arm (as if by falling and grabbing onto the arm),

and a person pushing on the arm at any of the end positions.

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In addition to these cases of misuse, LS requires that the arm must withstand regular usage such as

the force from an attached torque wrench set to 200 Nm. The swing arm is considered to be at most

4 m in length.

1.3 Limitations

Only the ceiling attachment and bearings are to be studied and optimised, other parts of AVPOS as

well as the bolts connecting the ceiling attachment to external points are to be assumed stiff.

Changes to manufacturing methods are to be avoided, i.e. made from machined aluminium plate.

Changes to the rest of AVPOS aside from the ceiling attachment are also to be avoided.

1.4 Technology, Society and Environment

Repetitive movements and fatigue often appear in the work place which over time lead to a lower

work output and injuries (Emery & Côté, 2012). To combat the effects of fatigue and work place

related injuries the Swedish Work Environment Authority recommends that physical work should be

performed in front of the body with relaxed back, arms and shoulders (Arbetsmiljöverket, 2012). To

prevent heavy loading of these areas of the body a tool such as AVPOS can be a good aid. For AVPOS

to be able to be used in the intended manner it needs to be deemed safe and malfunctions should

not cause users any harm. The ceiling attachment is one important part of the system in regards to

safety and according to the Swedish Work Environment Authority the strength of any load bearing

part needs to be ensured (Arbetsmiljöverket, 2006).

Aluminium exposure is currently one of the lesser environmental problems, but production and

refinement of aluminium are linked to the release of fluorides which can cause skeletal fluorosis in

animals as well as cause so-called “aluminium asthma” in workers in the aluminium plants

(Environmental and Health Aspects Related to the Production of Aluminium, 1994). Exposure and

uptake of aluminium is known to cause renal failure and may also be linked to neurodegenerative

disorders, such as Alzheimer’s disease (Kumar & Gill, 2009). Although aluminium is less toxic than

many other metals, lowering any unnecessary use of aluminium is preferable. As such, ensuring the

lowest possible usage of aluminium in the production of the ceiling attachment of AVPOS will lower

the impact on the environment as well as the monetary cost of the starting materials.

1.5 Overview

In this project the main concern is to determine any structural weaknesses in the original ceiling

attachment by FEA (finite element analysis). To perform this FEA the boundary conditions, geometry

and material properties are investigated. (Section 3. FEA of original design).

Following the initial analysis a redesign of the ceiling attachment is performed, in Section 4. Design

modification and preliminary concept designs, by generating concepts which could be interchanged

with the original ceiling attachment and then evaluating these using a Pugh selection-matrix. This

matrix contains the assembly time, initial weight, cutting length and parts count of each concept as

screening criteria.

Optimisation to the weight and material usage is performed in Section 5. Optimisation of the

selected concept design. A final verification of the redesign is made in Section 6,

4

Figure 2.1. Breakdown of approach.

Finite Element Analysis of the Optimised Concept 2.

New bearings are investigated in Section 7, Finding suitable bearings. The optimised design of the

ceiling attachment is proposed in Section 8,

Recommendations for new conceptual design and discussion. Finally, a series of appendices with

more detailed analyses are attached.

Approach As the tool is mounted above the work area where the operator is present, there are safety concerns

regarding the strength of the attachment, in addition to the concerns of lowering manufacturing

costs. To address these concerns a systematic approach is used as outlined in Figure 2.1.

To determine weaknesses in the existing ceiling attachment a FEA is performed. To accomplish that

task any applicable simplifications and assumptions are determined, as well as what loads and

boundary conditions to apply.

Load cases have been previously determined in 2014 (Lindell, 2014) and are presented in Appendix 3.

These load cases are used as a starting point for the FEA. Knowing the expected loads also makes it

possible to search for suitable replacement bearings and gives pointers to new concept designs.

FEA of original design

3.1 FEA method

Modelling and FEA for this project were performed using the software Solidworks1 which has several

modules for analysis and optimisation. Of these modules only the modelling and static FEA were

used.

The steps that were taken to produce meaningful output from the FEA were:

1. Creation of a suitably simplified model.

2. Determination of load cases and boundary conditions.

1 Solidworks 2014 SP 2.0

5

3. Mesh creation, application of loads and boundary conditions.

4. Interpretation of the results.

The full analysis and results are presented in Appendix 4.

3.1.1 Geometry

The original ceiling attachment as shown in Figure 3.1 consists of 12 individual components along

with a large number of fasteners. To simplify meshing and analysis the fasteners and their

corresponding holes were removed, and a 2 mm fillet was applied to inner corners (Figure 3.2).

As the loads were applied in several directions in three-dimensional space it was not possible to

apply symmetry when modelling for FEA, instead a complete solid body was used. The main

drawback of combining the parts into a single solid was that stress concentrations in the boundaries

between components may differ from the actual stresses. This did not pose a problem unless very

high stresses were observed, in which case further analysis could be considered.

Solidworks automatic meshing was used based on an initial tetrahedral element size of 20 mm edge length. During the actual analysis automatic mesh refinement further reduced the elements in some areas. The smallest elements after refinement were on the order of 1 mm. Mesh convergence results are included in Appendix 4. A typical mesh is shown in Figure 3.3.

Figure 3.2 Simplified ceiling attachment

Figure 3.1 Original ceiling attachment.

Figure 3.3. Typical mesh after refinement.

6

3.1.2 Material properties and yield criterion

The material used in the ceiling attachment is an aluminium alloy called Plancast Plus 5083(n) (Löfs

Specialmaskiner, 2014), which is ductile and has a linear elastic region with the properties outlined in

Table 3.1. These properties were used in all FEA. Small deformations were assumed for the analyses,

as such the material behaves as linear elastic. If large deformations would have been observed this

would have been revised.

Table 3.1. Material properties of Plancast Plus 5083(n) (Löfs Specialmaskiner AB, 2014b).

Yield strength: 1.25e+008 N/m2 Poisson's ratio: 0.33

Tensile strength: 2.5e+008 N/m2 Mass density: 2660 kg/m3

Elastic modulus: 6.9e+010 N/m2 Shear modulus: 2.59e+010 N/m2

The ceiling attachment is a 3D structure required to withstand several loading cases in different

directions, which means calculations need to be performed using generalised equations with a

suitably chosen effective stress yield criterion. As the material used in the ceiling attachment is

loaded within its linear elastic region von Mises effective stress was chosen as a suitable yield

criterion (Sundström, 1998).

3.1.3 Boundary conditions

The ceiling attachment is constrained by four fastening holes at the corners. For FEA modelling

purposes these fastening holes were surrounded by a small, raised, puck-shaped features as shown

in Figure 3.4. These were used to apply the constraints. One (A) was constrained from movement in

any direction while the remaining three were prevented from out-of-plane movement (Figure 3.4).

Defining the constraints this way served two purposes; it was possible to apply the constraint to a

smaller area than the entire bottom surface and it allowed some out-of-plane deflection to occur.

3.1.4 Load cases

A total of thirteen important load cases were previously identified (Lindell, 2014), however, under

closer examination several of the cases were found to be similar and it was possible to lower the

number of analyses to perform to eight, as follows:

1. the swing arm hanging straight down with a person’s weight and a load of 20 kg on it,

2. the swing arm hanging straight down with an applied moment of 200 Nm,

Figure 3.4. Boundary conditions.

7

Figure 3.6. Hot spots and bending

in the ceiling attachment. Load

case 5.

Figure 3.5. Load contact points. (Lindell,

2014)

Figure 3.7. High stresses in load case 7.

3. an applied horizontal load in the positive x-direction from a person pushing on the lower

end of the swing arm and a load of 20 kg,

4. an applied moment of 200 Nm with the swing arm in the positive x-direction,

5. an applied horizontal load in the positive y-direction from a person pushing on the lower

end of the swing arm and a load of 20 kg,

6. an applied moment of 200 Nm with the swing

arm in the positive y-direction,

7. an applied horizontal load in the positive x=y-

direction from a person pushing on the lower

end of the swing arm fully in the positive x

and y-direction and a load of 20 kg, and

8. an applied moment of 200 Nm with the swing

arm fully in the positive x and y-direction.

In the FEA the loads, as described fully in Appendix 3,

were applied at the five contact points A through E as

seen in Figure 3.5.

3.1.5 Solution method

The solution method used was FFEPlus with h-adaptive mesh refinement. FFEPlus is an iterative

solver and was chosen mainly as it is much faster than a direct solver, but it has low errors when

used for static analyses (Dassault Systèmes, 2015). The reason h-adaptive was chosen is because

meshing errors were very common when attempting to use a p-adaptive method, and time did not

allow for drawn-out troubleshooting.

3.2 Analysis and key findings

The purpose of the FEA of the original ceiling attachment was to identify areas in which re-design or

modification were necessary. Careful observation of the results (fully presented in Appendix 4)

showed that for most load cases there was no risk of plastic deformation and overall the stresses did

not propagate far from very small hot-spots at inner edges.

Stress hot-spots often appeared where the vertical pieces attach to the mounting plate as can be

seen Figure 3.6, as the upper vertical pieces were not in line with the lower which gave rise to a

8

bending moment in the mounting plate. This could be avoided by aligning any load bearing members.

The most severe load case is number 7, which occurs when an operator tries to push the swing arm

outside its operating area while the arm is carrying a 20 kg payload. In Figure 3.7 large areas of the

horizontal plate at the midsection show high stresses nearing the yield limit of the material as well as

a few hot spots in corners where plastic deformation is likely to occur. The stresses do not propagate

entirely through the large plate so structural collapse is unlikely. By aligning the load bearing

members the bending moment in the large plate could likely be lowered.

Overall the original ceiling attachment could handle normal usage well, but more severe loading

caused hot spots in some areas while other parts were mostly unaffected. With a careful redesign

material usage could be lowered while at the same time prevent similar hot spots from appearing.

Design modification and preliminary concept designs

4.1 Concept generation

Concept generation was performed by keeping in

mind the distances between axles and contact

points which because of the interchangeability

requirement could not be altered from the original

design (Figure 4.1). The approach taken was to

sketch ways to connect these points using plate

material.

Several concepts were produced, but upon closer

examination only three of these were found

suitable for further development. These concepts

were named Concepts 1 to 3 in no particular order.

Concept 1, presented in Figure 4.2, consisted of five

parts cut from aluminium plates, with a horizontal

centre plate like in the original ceiling attachment

and two vertical sides. The sides slot into a notch cut in the centre plate to help transfer the weight

Figure 4.1. Points in the original ceiling

attachment between which distances may not

change.

Figure 4.2. Concept 1.

9



of the swing arm and payload to the attachment points in the ceiling, without loading any connecting

screws between the sides and the centre plate with large shear stresses. In addition to combining the

separate top and bottom vertical pieces from the original ceiling attachment into a single vertical

piece per side, the top axle was rotated ninety degrees in relation to the bottom axle which could

then be connected to the vertical sides with only two slender plates.

Concept 2, shown in Figure 4.3, was also made up of five components but used a different layout

compared to Concept 1 and the original ceiling attachment. In this concept the slender top plates

were extended down to the centre plate in an inverted V configuration. At the centre plate both the

lower and upper vertical plates meet, where they were all joined orthogonal to each other, which

was thought could improve the stability of the overall structure.

In Figure 4.4 it can be seen that Concept 3 is similar to Concept 1 with a few key differences. In this

concept the side plates were extended to provide attachment points instead of placing these on the

centre plate and slots have been cut to offload shear stress from screws between the sides and other

components.

4.2 Design for manufacture and assembly

A common way of lowering material, manufacturing, and assembly costs is to use the method named

as Design for manufacture and assembly (DFMA). DFM is mainly concerned with producing single

components with as few and as simple operations as possible, whereas DFA is interested in designing

components to be easy to assemble with as few parts as possible. Bogue (2012) presents broad

guidelines for applying DFMA when optimising an existing assembly:

10

minimise parts count,

minimise use of custom parts,

use fewer fasteners, preferably of the same size and type,

design for a specific assembly technique,

keep simple manufacturability in mind.

Product assembly time is an important metric that directly impacts the final cost and can be

estimated using tables of assembly times. These tables were developed by Boothroyd and Dewhurst

in the early 1980’s (Boothroyd, Dewhurst, & Knight, 2002) and take into account the size and

symmetry of the parts being assembled, as well as how easy they are to align and if parts needs to be

held in place while screws are inserted.

The original ceiling attachment as well as the three generated concepts were evaluated using the

Boothroyd and Dewhurst assembly time method. To avoid bias in relation to the original ceiling

attachment, screws were incorporated into the concepts. In areas where the load is transferred by

the use of slots where the parts mate only the minimum number of screws was used, whereas other

areas used more screws. As this was an initial estimation of assembly time no calculation of shear

forces in the screws was performed.

According to the Boothroyd and Dewhurst method, assembly of the original ceiling attachment

would take 282 seconds, without taking into account the time it would take to acquire any tools

between operations as this is highly dependent on the specific assembly station layout. In Table 4.1 a

summary of the full evaluation is presented. The detailed evaluation is presented in Appendix 5.

Table 4.1. DFA evaluation of the original ceiling attachment. (Boothroyd, Dewhurst, & Knight, 2002)

No

. o

f it

em

s

RP

Han

dlin

g

cod

e

Han

dlin

g

tim

e T

H

Inse

rtio

n

cod

e

Inse

rtio

n

tim

e TI

Tota

l ti

me

RP

*(T

H+T

I) Comment

1 Top plate 1 0 0 1.13 0 0 1.5 2.63 First part

2 Attachment for top

axle

2 1 0 1.5 0 3 5.2 13.4 Need to hold part in place

3 Screw 6 1 0 1.13 3 1 5.3 38.58 Attaches [2] to [1]

4 Triangular piece 2 3 0 1.95 0 3 5.2 14.3 Need to hold part in place


6 Vertical side part 2 2 0 1.8 0 3 5.2 14 Need to hold part in place


8 Screw 6 1 0 1.13 3 1 5.3 38.58 Attaches [6] to [1]. Part aligned

from previous step.

9 Middle plate 1 2 0 1.8 0 3 5.2 7 Need to hold part in place



from previous step.

--- Sub assembly ---

12 Attachment for

lower axle

1 3 0 1.95 0 0 1.5 3.45 First part in sub assembly

11

13 Triangular piece 1 3 0 1.95 0 3 5.2 7.15 Need to hold part in place.


---

15 Sub assembly 2 3 0 1.95 0 3 5.2 14.3 Need to hold part in place.


Total: 282

A large part of the 282 seconds of this assembly is taken up by applying screws in parts that are

difficult to align. By lowering the number of screws the assembly time could be reduced by up to 205

seconds. Performing the same evaluation for Concepts 1, 2 and 3 yielded assembly times of 137 s, 59

s and 63 s respectively. The full evaluations are presented in Appendix 5.

Another important metric in addition to assembly time is machining or cutting time, which indicates

how long it would take to machine an individual component. Boothroyd, Dewhurst & Knight (2002)

use equation 1 to calculate the cutting time for vertical milling where a tool with a diameter of dt

starts just outside the work piece and makes a cut of length lw through the work piece and stops

outside the work piece again where vf is the feed speed of the tool head.

For longer cuts the contribution from the width of the tool head quickly becomes insignificant

compared to the overall cutting time, and the cutting speed is dependent on the type of machine and

cooling. Therefore the best metric to compare the generated concepts with is the length of the

cutting path. The cutting path length was approximated by summing the length of all internal and

external contours for each component, for the original design and the three concepts. As milling

operations usually have a maximum depth of cut it is sometimes necessary to perform several

progressively deeper passes, to cut the whole way through the work piece, as is needed for the

manufacture of these concept designs. To take this into account when comparing the concepts the

total cutting length was multiplied by the thickness of each piece. Table 4.2 presents the lengths and

thicknesses for each part and concept.

Table 4.2. Contour lengths and thicknesses.

Concept 1 Concept 2 Concept 3 Original

Par

t

Co

nto

ur

len

gth

Thic

kne

ss

No

. par

ts

Par

t

Co

nto

ur

len

gth

Thic

kne

ss

No

. par

ts

Par

t

Co

nto

ur

len

gth

Thic

kne

ss

No

. par

ts

Par

t

Co

nto

ur

len

gth

Thic

kne

ss

No

. par

ts

Middle plate:

2520 20 1 Middle plate:



2846 20 1

Side plate:

1444 20 2 Top vertical plate:

1801 20 2 Side plate:

2662 20 2 Top vertical plate:

1629 20 2

Top plate:

903 20 2 Lower vertical plate:

1122 20 2 Top plate:

823 20 2 Lower vertical plate:

731 20 2

Triangular 246 15 4

𝑡𝑚 =𝑙𝑤+𝑑𝑡

𝑣𝑓 (1)

12

piece:

Top plate: 770 15 1

Upper axle attachment:

284 15 2

Tota

l

: 73

14

Tota

l

: 84

66

Tota

l

: 93

10

Tota

l

: 10

09

3

4.3 Screening of concept designs

To determine how well the new concepts met the criteria presented in the problem description they

had to be quantified and compared in a systematic way. The Pugh concept screening matrix as

described in Ulrich & Eppinger (2008) is a suitable way of comparing concepts that ranks them in

relation to each other. The matrix is made up of a column for each concept to be evaluated and a

row for each metric that is to be compared. One concept is chosen as the starting point for

comparison and all the others are compared relative to this concept, then for each metric listed on

the left hand side of the matrix the others are either rated better or worse in comparison. Finally the

score is tallied and one concept is chosen to proceed with, possibly by combining several strong

points from several concepts.

The concepts were compared by their parts count, materials usage before optimisation (weight), by

estimating their assembly-times and cutting lengths. In Table 4.3 Concept 1 is used as the benchmark

against which the other concepts were compared.

Table 4.3. Pugh selection matrix for Concept 1-3.

Selection criteria Concept 1 (Reference)

Concept 2 Concept 3 Original

Assembly time 0 (138 s) + (59 s) + (63 s) - (282 s)

Parts count 0 (5 pcs) 0 (5 pcs) 0 (5 pcs) - (12 pcs)

Material usage (weight) 0 (14.8 kg) + (10.2 kg) - (18.2kg) - (17.1 kg)

Cutting area: 0 (7314 mm2) - (8466 mm2) - (9310 mm2) - (10093 mm2)

Sum +'s 0 2 1 0

Sum 0's 0 1 0 0

Sum -'s 0 1 2 4

Net score 0 1 -1 -4

Rank 2 1 3 4

As seen in Table 4.3 all three concepts were better in all aspects compared to the original ceiling

attachment, apart from Concept 3 which had a greater weight. Concept 2 was slightly better than the

others in assembly time and materials usage, and was considered for further optimisation.

13

Figure 5.1. Factor of safety of Concept 2 after

thickness optimisation.

Optimisation of the selected concept design Optimising mechanical components and structures has historically been a very laborious task, given

the fact that optimising requires both extensive experience to determine when a design is optimal

and a large amount of calculation.

The optimisation process consists of several steps. Firstly, the design variables, the cost function to

be minimised and any constraints that must be satisfied are identified. Information to describe the

detail to be optimised is collected and the initial design is estimated. Following the initial steps the

iterative optimisation begins; which means analysing the part, checking the constraints, and

determining if the current design is optimal. If the design is not optimal it needs to be changed

according to some optimisation method and analysed again (Arora, 2004).

Structural optimisation methods are commonly divided into three types; size optimisation, shape

optimisation and topology optimisation (Lövgren & Norberg, 2011). Of these three methods size

optimisation and topology optimisation were used for optimising the concept chosen in section 4.3.

5.1 Size optimisation

Size optimisation encompasses determining the optimal size of some dimension in a structure, such

as thickness, to meet the cost function and constraints (Lövgren & Norberg, 2011). This can be

performed either by determining a function which returns the required dimension as a function of

the constraints and loading case or by an iterative process where the dimension is modified and the

system is analysed until an adequate result is obtained.

The thickness optimisation of the constituent parts

of Concept 2 was performed by setting up a FEA

similar to the FEA done on the ceiling attachment

for load case 7, one of the more severe load cases

in terms of the applied loads and resulting stresses.

Three iterations of FEA and manual optimisation of

the constituent parts of Concept 2 were performed

to arrive at final thicknesses of 20 mm for the

middle plate and 15 mm for both the upper

vertical plates and the side plates. Presented in

Figure 5.1 is a factor of safety plot of the ceiling

attachment after size optimisation. Some areas can

be seen to approach a value below 4 which

indicates that it is not possible to make the parts

much thinner. The iterative FEA and optimisation

are presented in Appendix 5, along with the topology optimisation and analysis of the final design.

5.2 Topology optimisation

Topology optimisation consist of creating a design space, applying loads and boundary conditions,

and then determining which areas of the design space actually carry the loads and then removing

material everywhere else (Lövgren & Norberg, 2011). The design space can be an existing part or

14

system from which material is removed to create a lighter structure. Alternatively it can be defined

as a space which is larger than needed while still satisfying any boundary conditions, when a new

optimal shape which might differ significantly from a previous design is to be found. This latter case

often results in organic-looking structures where only needed load paths are present.

Following the size optimisation of Concept 2, a rudimentary topology optimisation was performed to

remove some of the most superfluous material. Load cases 7 and 2 were used to identify load paths

in the model, otherwise the FEA performed was identical to those used for thickness optimisation.

Two iterations of FEA and material removal were performed.

In Figure 5.3, large areas can be seen where stresses are below 15 MPa. These areas were

subsequently cut out to yield the model in Figure 5.2. This model was then analysed and optimised a

second time, where it was found that the upper portion of the side plates was largely unloaded,

which resulted in a shortening of that part.

Figure 5.3. Iso-cutout showing areas with stresses

greater than 15 MPa for load case 7.

Figure 5.2. Result of removing material in

the first iteration of topology optimisation.

15

Figure 6.2. FOS for the final model for load case 7.

Finite Element Analysis of the Optimised Concept 2 To verify the suitability of the optimised version of Concept 2 a final FEA was performed using load

case 7. The final model was prepared by rounding the outer corners of all parts, as well as combining

these into a single solid as for the other analyses. This model is presented in Figure 6.1 and has a final

weight of 6.7 kg.

Figure 6.2 shows that overall the factor of safety (FOS) greatly exceeds a value of 4 and only in some

areas the FOS is in the vicinity of a value of 3, which can be seen as an indication that the amount of

material left in those areas is near the minimum needed while still maintaining an adequate margin

against failure.

Finding suitable bearings The bearings in the ceiling attachment for the AVPOS system fall into the category oscillating

bearings, i.e. the bearings do not complete a full rotation, but rather oscillate back and forth.

Few articles have been published which deal with oscillating bearings, but the conclusion that ball

bearings normally perform less than optimally in an oscillatory configuration has been made (Massi,

et al., 2014).

For long service life in bearings and bushings, where lubrication is difficult to perform or non-

existent, polymer-based sliding surfaces are recommended for their self-lubricating properties as

well as their low friction coefficients (Lawrowski, 2007).

According to Rac & Vencl (2005) determining an optimal design of sliding bearings is difficult and

largely based on empirical results.

As it is difficult to design a bearing and this work is only concerned with finding an existing bearing as

a replacement to the ball bearings currently used, the work will consist of evaluating existing

bearings in the market to find a bearing that complies with the following criteria:

sliding bearing or bushing,

Figure 6.1. Final model of Concept 2.

16

sliding surfaces should be self-lubricating,

low friction.

should also be self-aligning to lower machining tolerances elsewhere in the ceiling

attachment on request of LS.

These criteria point towards some polymer-based bearing, which are currently (2015) commercially

available.

There are several manufacturers who provide suitable bearings to be used in the ceiling attachment,

unfortunately resellers for the Swedish market are less common. One reseller of a suitable bearing

has been located, Igus, which provides the Igubal KGLM spherical bearing that is being marketed as

suitable for heavy loads, oscillatory use and is self-aligning. The maximum radial load of the Igubal

KGLM-20 is 10 kN and maximum allowed axial loads are 5 kN (Igus, 2015). These maximum ratings

are not exceeded during normal use such as lifting or using a torque wrench, however the radial

loading is exceeded when the swing arm is misused by pushing on the swing arm at the maximum

reach of the work area. This risks the integrity of the bearings but does not pose a risk for personnel

beneath the ceiling attachment.

Recommendations for new conceptual design and discussion The intent of this project was to come up with a new design of the existing ceiling attachment that

fulfils the objectives outlined in Table 8.1. These specifications have been met and exceeded.

Table 8.1. Objectives.

Original Concept 2

Material usage less by 20% 17.1 kg 6.7 kg

50% fewer parts 12 pcs 5 pcs

Fewer fastneners 38 pcs 10 pcs

Interchangeable with original - ✓

Easier to assemble 282 s 59 s

FOS of 4 against yielding - ✓

Made from machined aluminium plate - ✓

17

Figure 8.2. Completed redesign of

Concept 2 mounted on the swing arm.

To allow the parts with square corners to be inserted into each other the inner corners had to be

modified to allow regular milling techniques to be used. In Figure 8.1 the rounded reliefs, at the

corners of the slots cut in the side plate, can be seen. These reliefs are necessary as the milling tool

would otherwise leave a corner radius inside the slot, preventing the square tab from being inserted.

This also allows milling of all parts as flat two dimensional structures, as they do not need to be

milled from more than one direction to create tabs that can fit into rounded corners.

By switching the original ceiling attachment with the optimised version of Concept 2 in the assembly

model of the swing arm it was verified to be compatible as seen in Figure 8.2.

As this project dealt mainly with iterative optimisation and FEA as the means of arriving at a

conclusion the main sources of error are the same as for any FEA, namely boundary conditions and

loads, discretisation and the methods used for solving the stiffness matrix.

The boundary conditions for all analyses were applied to raised puck shapes which were not present

in the physical model of the ceiling attachment. This allowed for applying the boundary conditions to

a smaller surface than otherwise would have been possible, as well as letting out-of-plane flexing to

occur. One of the surfaces was fixed while the remaining three were constrained from out-of-plane

movement. This caused the fixed puck-shape to experience torsion and gave rise to stresses which

likely would not be present in the physical object being constrained by bolts to a stiff beam. As these

stresses did not propagate far into the model they could safely be ignored. The largest source of

error in terms of boundary conditions is the fact that the model was treated as a single solid, instead

of individual parts. This gave rise to tensile stresses in some interfaces which would normally be

pulled apart.

The load cases were previously determined in 2014 (Lindell) and were considered as the worst case

scenarios that the ceiling attachment would ever experience, especially in load case 7 which

contained the largest forces. This in conjunction with a factor of safety value of 4 gives a high

confidence that the structure will not yield.

The automatic meshing method used created a fairly coarse tetrahedral mesh, which saves on

Figure 8.1. Close-up of milled slots.

18

computations but could have large errors (Ottosen & Petersson, 1992). As the solution method

automatically refined the size of the elements according to an element-based error estimate this

issue was resolved.

Conclusions By creating a new design which uses less material and is easier to manufacture Löfs Specialmaskiner

can increase profit margins or lower the cost of the system to the customer. With a clearly defined

safety margin users can feel safe in not having the system fail in a dangerous manner and with lower

materials usage the environment is put under less strain.

References Arbetsmiljöverket. (2006, November 28). AFS 2006:6 - Användning av lyftanordningar och

lyftredskap. Sweden.

Arbetsmiljöverket. (2012). AFS 2012:02 - Belastningsergonomi. Stockholm, Sweden.

Arora, J. (2004). Introduction to optimum design. 2-7. San Diego, USA: Elsevier.

Atlas Copco. (2015). Atlas Copco torque arms. Retrieved January 01, 2015, from

http://194.132.104.143/websites/tools/Publications/acc11.nsf/va_TechLang/4511D2D28ED6

10A3C12579530048E714/$File/TorqueArms_US.pdf

Bogue, R. (2012). Design for manufacture and assembly: background, capabilities and applications.

Assembly Automation, 32(2), 112-118.

Boothroyd, G., Dewhurst, P., & Knight, W. (2002). Product Design for Manufacture and Assembly. 99-

141. New York: Marcel Dekker, Inc.

Dassault Systèmes. (2015, May 15). 2014 SOLIDWORKS Help - Analysis Solvers. Retrieved from

http://help.solidworks.com/2014/English/SolidWorks/cworks/c_Analysis_Solvers.htm

Emery, K., & Côté, J. N. (2012, February). Repetitive arm motion-induced fatigue affects shoulder but

not endpoint position sense. Experimental Brain Research, 216(4), 553-564.

Environmental and Health Aspects Related to the Production of Aluminium. (1994). Science of The

Total Environment. Oslo: The National Institute of Occupational Health. doi:10.1016/0048-

9697(95)90270-8

Forsström, A. (2010, 08 08). Utbrett barnarbete i Sverige under 1800-talet - DN.SE. Retrieved January

20, 2015, from Dagens Nyheter: http://www.dn.se/nyheter/vetenskap/utbrett-barnarbete-i-

sverige-under-1800-talet/

Givens Engineering Inc. (2011). Givens Engineering Inc. Retrieved January 1, 2015, from

http://www.giveng.com/TorqueArms.php

Igus. (2015). igus(R) igubal(R)-spherical bearings - KGLM. Retrieved from

http://www.igus.co.uk/wpck/2493/igubal_Gelenklager_KGLM_mm?C=GB&L=en

19

Kumar, V., & Gill, K. D. (2009, November). Aluminium neurotoxicity: neurobehavioural and oxidative

aspects. Archives of Toxicology, 83(11), 965-978. Retrieved March 03, 2015

Lawrowski, Z. (2007). Polymers in the construction of serviceless sliding bearings. Archives of civil and

mechanical engineering, VII, 139-150.

Lindell, I. (2014). Load cases for analysis of AVPOS. Högskolan i Skövde, Department of Engineering

Science.

Löfs Specialmaskiner AB. (2013). Lifting device for use at a manual work station. Swedish patent

application: 1330102-3.

Löfs Specialmaskiner AB. (2014a). Företagspresentation. Tibro, Sweden.

Löfs Specialmaskiner AB. (2014b). Meeting at Löfs Specialmaskiner. Tibro, Sweden.

Lövgren, S., & Norberg, E. (2011). Topology optimization of vehicle body structure for improved ride &

handling. Masters thesis. Sweden: Linköpings Universitet.

Massi, F., Bouscharain, N., Milana, S., Le Jeune, G., Maheo, Y., & Berthier, Y. (2014). Degradation of

high loaded oscillating bearings: Numerical analysis and comparison with experimental

observations. Wear, 317(1-2), 141-152.

Ottosen, N., & Petersson, H. (1992). Introduction to the finite element method. Prentice Hall.

Rac, A., & Vencl, A. (2005). Tribological and design parameters of lubricated sliding bearings.

Tribology in Industry, 27, 12-16.

Sundström, B. (1998). Handbok och formelsamling i Hållfasthetslära.

Ulrich, K. T., & Eppinger, S. D. (2008). Product design and development. 130-133. New York: McGraw-

Hill.

Venning, A. (2010, September 17). Britain's child slaves: New book says their misery helped forge

Britain. Retrieved January 20, 2015, from Mail Online:

http://www.dailymail.co.uk/news/article-1312764/Britains-child-slaves-New-book-says-

misery-helped-forge-Britain.html

Appendix 1 - 1

Appendix 1 - Division of labour All of the work performed in the main body of the thesis Optimisation of ceiling attachment for

AVPOS using FEA as well as the accompanying appendices 1, 2, 4, 5, and 6 has been done by Mikael

Koskenranta.

The contents of Appendix 3, Load cases, were contributed by Ida Lindell in 2014.

Appendix 2 - 1

Appendix 2 - Time plan The initial planning for the project can be seen in Figure 1 at the time of the first presentation for

specification and planning.

Figure 1. Plan at the time for the first presentation.

Overall the project was on track by the time of the Mid-project presentation, but the redesign had

yet to be started and the sections for method had slightly lagged behind.

Figure 2. Plan at the time for Mid-project presentation.

Appendix 2 - 2

Figure 3. Plan as of May 18th, 2015.

The redesign of the ceiling attachment was postponed until the March 24th, and the FEA and

optimisation was pushed back until April 23rd. An original goal of suggesting alternative

manufacturing methods was removed as time did not permit its inclusion. By the time of May 18th all

parts of the project were on time.

Appendix 3 - 1

Appendix 3 – Load cases

The work in this appendix was performed by Ida Lindell in 2014 and was used as a starting point for

finite element analysis. The text below has been copied verbatim with no editorial changes or

corrections.

The loading cases

Table 1 show the average and median weight from a survey 2010-2011 of Swedes of the ages 16 years

and up (Statistiska Centralbyrån, 2013).

Table 1 The average and median weight of Swedes 2010-2011

Average weight (kg) Median weight (kg)

Total 75.2 74

Men 82.9 82

Women 67.4 65

The force applied to the swing arm when a person hangs on it is the mass of the person (82.9 kg)

multiplied by the gravitational acceleration g = 9.82 kg m s2⁄ . 82.9 kg is the average weight of

Swedish men and is here used instead of the average weight of Swedes because of the significant

difference and to be on the safe side in later calculations. However, since it is an average weight an

actual person who hangs on the swing arm could weigh more but it is unlikely that the person weighs

four (4) times more which the safety factor allows for.

How hard a person can push (or lean against) the swing arm, 𝐹𝑎𝑝𝑝𝑙𝑖𝑒𝑑, depend on the friction force,

𝐹𝑓𝑟𝑖𝑐𝑡𝑖𝑜𝑛, under the persons feet. The friction force in turn depends on the mass, 𝑚, of the person,

giving the normal force, 𝑁, from the floor, and the coefficient of friction 𝜇 between the shoes and the

floor (eq. (6)). The coefficient of friction between rubber and dry concrete is 0.6 < 𝜇 < 0.85 (The

Engineering ToolBox, n.d.).

Figure 1 Force diagram

Mechanical equilibrium: The sum of all the external forces and the sum of the moments of all the

external forces about any arbitrary point (A) is zero for a motionless object or an object moving with

constant speed (eqs. (2) and (3)) (Grahn & Jansson, 2010).

Appendix 3 - 2

30

20

AM

F

Equilibrium equations from the force diagram in Figure 1 using eq. (2) divided vertically and

horizontally (eqs. (4) and (5)):

7

6&5,4

6

50:

40:

0

mgF

NF

FF

mgN

maF

applied

friction

appliedfriction

Choosing a high value for the coefficient of friction (𝜇 = 0.85), and thereby obtaining a large force, is

again a choice to be on the safe side in later calculations.

There are five contact points where forces affect

the ceiling attachment due to applied loadings and

the mass of the swing arm and of the cradle. Those

points are shown in Figure 2 and are indicated with

the letters A to E.

The weight of the swing arm is 43.8 kg and that of

the cradle is 4.6 kg, large enough to not be

insignificant, and needs to be accounted for. The

mass centre for the arm is located 675 mm

straight beneath the centre of the cradle when

hanging freely. When the swing arm is at an angle,

the pneumatic cylinder applies a force at the swing

arm and contact point A in Figure 2 to balance it

up. The force at the swing arm is transferred to the

ceiling attachment at points B and C. When AVPOS

is used as a lifting tool, the mass lifted is also

balanced with the pneumatic cylinder and the resulting forces affect the ceiling attachment at the

contact points A, B, and C.

There are thirteen (loading case 1-13) different loading cases shown in Table 2 which are calculated

using mechanical equilibrium (eqs. (2) and (3)). The loading cases are;

1. the swing arm hanging straight down without any applied load,

2. the swing arm hanging straight down with a person’s weight on it,

3. the swing arm hanging straight down with a person’s weight and a load of 20 kg on it,

4. the swing arm hanging straight down with an applied moment of 200 Nm,

5. an applied horizontal load in the positive x-direction from a person pushing on the lower end

of the swing arm,

Figure 2. Contact points where forces affect the ceiling attachment.

Appendix 3 - 3

6. an applied horizontal load in the positive x-direction from a person pushing on the lower end

of the swing arm and a load of 20 kg,

7. an applied moment of 200 Nm with the swing arm in the positive x-direction,

8. an applied horizontal load in the positive y-direction from a person pushing on the lower end

of the swing arm,

9. an applied horizontal load in the positive y-direction from a person pushing on the lower end

of the swing arm and a load of 20 kg,

10. an applied moment of 200 Nm with the swing arm in the positive y-direction,

11. an applied horizontal load in the positive x=y-direction from a person pushing on the lower

end of the swing arm,

12. an applied horizontal load in the positive x=y-direction from a person pushing on the lower

end of the swing arm and a load of 20 kg, and

13. an applied moment of 200 Nm with the swing arm in the positive x=y-direction.

Table 2 Forces that affect the ceiling attachment due to the different loading cases

A (x,y,z) [N] B (x,y,z) [N] C (x,y,z) [N] D (x,y,z) [N] E (x,y,z) [N]

1 0,0,0 0,0,-237.6 0,0,-237.6 0,0,0 0,0,0

2 0,0,0 0,0,-645.2 0,0,-645.2 0,0,0 0,0,0

3 0,0,0 0,0,-743.4 0,0,-743.4 0,0,0 0,0,0

4 0,0,0 0,-617.3,-237.6 0,617.3,-237.6 0,0,0 0,0,0

5 255.5,0,1975.6 0,0,-8977.2 437.3,0,6526.2 0,0,0 0,0,0

6 946.9,0,7320.9 -254.1,0,-11748 0,0,3755.4 0,0,0 0,0,0

7 255.5,0,1975.6 -255.5,-681.1,-1225.5 0,681.1,-1225.5 0,0,0 0,0,0

8 0,254.1,1964.3 0,219.35,-22871 0,219.35,-22871 0,0,21651.2 0,0,21651.2

9 0,945.5,7309.6 0,-126.4,-25641.9 0,-126.4,-25641.9 0,0,21651.2 0,0, 21651.2

10 0,254.1,1964.3 0,-686.5,-1480.7 0,432.4,-958.9 0,0,0 0,0,0

11 234.6,234.6,2012.9 0,127.6,-20376.5 255.3,127.6,-

10317.2 0,0,14102.7 0,0,14102.7

12 871.4,871.4,7476.3 -381.5,-190.8,-

23150.9 0,-190.8,-13202.6 0,0,14102.7 0,0,14102.7

13 234.6,234.6,2012.9 -234.6,-734.6,-1512.4 0,500,-975.8 0,0,0 0,0,0

Appendix 3 - 4

Figure 2 together with Table 2 show the magnitude and direction of the forces which affect the ceiling

attachment due to the different loading cases. A minus sign in Table 2 indicates that the force has a

direction opposite to that of the coordinate system shown in Figure 2.

References

Grahn, R. & Jansson, P., 2010. Mekanik - Statik och dynamik. Denmark: Studentlitteratur AB.

Statistiska Centralbyrån, 2013. Statistiska Centralbyrån. [Online]

Available at: http://www.scb.se/sv_/Hitta-statistik/Statistik-efter-

amne/Levnadsforhallanden/Levnadsforhallanden/Undersokningarna-av-levnadsforhallanden-

ULFSILC/12202/12209/

[Accessed 26 Mars 2014].

The Engineering ToolBox, n.d. The Engineering ToolBox. [Online]

Available at: http://www.engineeringtoolbox.com/friction-coefficients-d_778.html

[Accessed Mars 2014].

Appendix 4 - i

Appendix 4 – FEA of the original ceiling attachment

Contents Appendix 4 – FEA of the original ceiling attachment ............................................................................... i

Finite element analysis of original AVPOS ceiling attachment ........................................................ 1

1.1 Geometry of the ceiling attachment ....................................................................................... 1

1.2 Material properties ................................................................................................................. 2

1.3 Boundary conditions ............................................................................................................... 3

1.4 Results ..................................................................................................................................... 4

1.4.1 Load case 1 ...................................................................................................................... 5

1.4.2 Load case 2 ...................................................................................................................... 6

1.4.3 Load case 3 ...................................................................................................................... 7

1.4.4 Load case 4 ...................................................................................................................... 8

1.4.5 Load case 5 ...................................................................................................................... 9

1.4.6 Load case 6 .................................................................................................................... 10

1.4.7 Load case 7 .................................................................................................................... 11

1.4.8 Load case 8 .................................................................................................................... 12

References ..................................................................................................................................... 13

Appendix 4 - 1

Finite element analysis of original AVPOS ceiling attachment This FEA is performed to identify any structural problems that may exist in the original ceiling

attachment for the AVPOS system. Any shortcomings will then be addressed in the redesign of the

ceiling attachment.

The modelling and FEA is performed using Solidworks 2014 SP 2.0 with the Solidworks Simulation

plugin.

1.1 Geometry of the ceiling attachment

The ceiling attachment consists of 13 components joined together as a solid for the purpose of

analysis. The original ceiling attachment is seen in Figure 1.1; a simplified version for analysis is seen

in Figure 1.2, along with the small 2 mm fillets applied to all sharp inner corners to avoid singularities

in the simulation; and the major dimensions are shown in Figure 1.4.

The entire model was analysed and symmetry planes were avoided as forces in multiple directions

were present in most analyses, which makes symmetry unsuitable.

Solidworks automatic meshing was used based on an initial tetrahedral element size of 20 mm in

width. During the actual analysis automatic mesh refinement further reduced the elements in some

areas. The smallest elements after refinement was on the order of 1 mm.

Presented in Figure 1.3 is a typical convergence plot with the error plotted as a percentage. This

specific plot shows the convergence for load case 7, which is one of the most severe load cases in

terms of the applied loads and resulting stresses.

Figure 1.1. Original ceiling

attachment.

Figure 1.2. Simplified ceiling

attachment.

Figure 1.3. H-adaptive convergence

plot, load case 7.

Appendix 4 - 2

1.2 Material properties


Specialmaskiner AB, 2014b),which is ductile and has a linear elastic region with the properties

outlined in Table 1.1. These properties were used in all FEA. Small deformations were assumed for

the analyses, as such the material behaves as linear elastic. If large deformations would have been

observed this would have been revised.

Table 1.1. Material properties of Plancast Plus 5083(n)

Yield strength: 1.25e+008

N/m2

Poisson's ratio: 0.33

Tensile

strength:

2.5e+008 N/m2 Mass density: 2660 kg/m3

Elastic

modulus:

6.9e+010 N/m2 Shear

modulus:

2.59e+010

N/m2

Figure 1.4. Geometry of the ceiling attachment. All dimensions in mm.

Appendix 4 - 3


Figure 1.6. Load contact points. (Lindell, 2014)

1.3 Boundary conditions

The solid body was constrained at the four raised puck shapes beneath the fastening holes in the

large plate at the ceiling attachments midsection. As shown in Figure 1.5Figure 1.5. Boundary

conditions. One of these was constrained from movement in any direction while the remaining three

were prevented from out-of-plane movement. The raised puck shapes are not present in the physical

ceiling attachment, but were placed in the model to allow the constraints to be placed in a smaller

area than the entirety of the plate as well as to allow some out-of-plane deflection to occur.

Loads were applied at the five contact points A through E as seen in Figure 1.6 and the list of used

load cases is written in Table 1.2. The load cases were determined in 2014 by Ida Lindell and the

method used to determine these is available in Appendix 3.

Table 1.2. Table of load cases.


1 0,0,0 0,0,-743.4 0,0,-743.4 0,0,0 0,0,0

2 0,0,0 0,-617.3,-237.6 0,617.3,-237.6 0,0,0 0,0,0

3 946.9,0,7320.9 -254.1,0,-11748 0,0,3755.4 0,0,0 0,0,0

4 255.5,0,1975.6 -255.5,-681.1,-1225.5 0,681.1,-1225.5 0,0,0 0,0,0

5 0,945.5,7309.6 0,-126.4,-25641.9 0,-126.4,-25641.9 0,0,21651.2 0,0, 21651.2

6 0,254.1,1964.3 0,-686.5,-1480.7 0,432.4,-958.9 0,0,0 0,0,0

7 871.4,871.4,7476.3 -381.5,-190.8,-

23150.9 0,-190.8,-13202.6 0,0,14102.7 0,0,14102.7

8 234.6,234.6,2012.9 -234.6,-734.6,-1512.4 0,500,-975.8 0,0,0 0,0,0

Appendix 4 - 4

1.4 Results

(Page intentionally left blank.)

Appendix 4 - 5

1.4.1 Load case 1

Number of elements: 392962 Maximum stress: 27.9 MPa Deformation scale:1196

Comment: A full load of

approximately 150 kg hanging

straight down.

Minimum factor of safety: 4.5

Appendix 4 - 6

1.4.2 Load case 2

Number of elements: 497430 Maximum stress: 46.9 MPa Deformation scale: 395

Comment: 200 Nm of torque

applied to the swing arm hanging

straight down.


Appendix 4 - 7

1.4.3 Load case 3

Number of elements: 446956 Maximum stress: 298 MPa Deformation scale: 81

Comment: A 20 kg load hanging

from the swing arm with a person

pushing in the x-direction at the

furthest x-position.

The maximum stress of 298 MPa

appears as a single node in a corner

between plates. This corner would

not exist if the entire model was not

a single solid and should not pose a

structural risk.


Appendix 4 - 8

1.4.4 Load case 4


Comment: 200 Nm of torque applied

to the swing arm at the furthest x-

position.


Appendix 4 - 9

1.4.5 Load case 5


Comment: A 20 kg load hanging from

the swing arm with a person pushing

in the y-direction at the furthest y-

position.

The maximum stress of 306 MPa

appears as a single node in a corner

between plates. This corner would

not exist if the entire model was not

a single solid and should not pose a

structural risk.


Appendix 4 - 10

1.4.6 Load case 6

Number of elements: 289836 Maximum stress: 63.4 MPa Deformation scale: 190


to the swing arm at the furthest y-

position.

Minimum factor of safety: 2

Appendix 4 - 11

1.4.7 Load case 7


Comment: A 20 kg load hanging from

the swing arm with a person pushing

in the x and y-direction at the

furthest x and y-positions.

A maximum effective stress of 1509

MPa appears at several corners

which would not be present as a

single solid in the non-simplified

model, this likely does not pose a

structural risk but may lead to plastic

deformation.


Appendix 4 - 12

1.4.8 Load case 8



to the swing arm at the furthest x and

y-position.


Appendix 4 - 13

References Lindell, I. (2014). Load cases for analysis of AVPOS. Högskolan i Skövde.


Appendix 5 - 1

Appendix 5 – Assembly Times In Tables 1, 2 and 3 the handling and insertion times can be read for each evaluation by reading the

handling and insertion codes from the evaluation table and looking up the corresponding row and

column. Tables 4, 5, 6 and 7 show the assembly times for the original design and the three concepts.

Table 1. Handling times for parts that can be manipulated with one hand. (Excerpt from (Boothroyd,

Dewhurst, & Knight, 2002))

No handling difficulties

Thickness > 2 mm

Symmetry (deg) = (alpha+beta) Size > 15 mm

Handling code (Row & Column)

0

sym < 360 0 1.13

360 <= sym < 540 1 1.5

540 <= sym < 720 2 1.8

sym = 720 3 1.95

Table 2. Insertion times for parts that are not immediately secured. (Excerpt from (Boothroyd, Dewhurst, &

Knight, 2002))

Secured by separate operation or part

No holding down required Holding down required

Easy to align Not easy to align

Easy to align Not easy to align

Insertion code

0 1 2 3

No access or vision difficulties

0 1.5 3 2.6 5.2

Obstructed acces or restricted vision

1 3.7 5.2 4.8 7.4

Table 3. Insertion times for screws. (Excerpt from (Boothroyd, Dewhurst, & Knight, 2002))

Easy to align

Not easy to align

Insertion code

0 1

No access or vision difficulties

3 3.6 5.3

Appendix 5 - 2

Assembly time evaluation for the original ceiling attachment

Table 4. Assembly times for the original design.

No

. of

item

s

RP

Han

dlin

g

cod

e

Han

dlin

g

tim

e T

H

Inse

rtio

n

cod

e

Inse

rtio

n

tim

e TI

Tota

l tim

e

RP

*(T

H+T

I) Comment

1 Top plate 1 0 0 1.13 0 0 1.5 2.63 First part

2 Attachment for top

axle

2 1 0 1.5 0 3 5.2 13.4 Need to hold part in place


4 Triangular piece 2 3 0 1.95 0 3 5.2 14.3 Need to hold part in place


6 Vertical side part 2 2 0 1.8 0 3 5.2 14 Need to hold part in place



from previous step.

9 Middle plate 1 2 0 1.8 0 3 5.2 7 Need to hold part in place



from previous step.

--- Sub assembly ---

12 Attachment for

lower axle

1 3 0 1.95 0 0 1.5 3.45 First part in sub assembly

13 Triangular piece 1 3 0 1.95 0 3 5.2 7.15 Need to hold part in place.


---

15 Sub assembly 2 3 0 1.95 0 3 5.2 14.3 Need to hold part in place.


Total: 281.99

Appendix 5 - 3

Assembly time evaluation for concept 1

Table 5. Assembly times for Concept 1.

No

. of

item

s R

P

Han

dlin

g co

de

Han

dlin

g ti

me

TH

Inse

rtio

n c

od

e

Inse

rtio

n t

ime

TI

Tota

l tim

e R

P*

(TH

+TI)

Comment

1 Base plate 1 0 0 1.13 0 0 1.5 2.63 First part

2 Side plate 2 1 0 1.5 0 2 2.6 8.2


4 Top plate 2 1 0 1.5 0 3 5.2 13.4


Total: 137.83

Assembly time evaluation for concept 2 Table 6. Assembly times for Concept 2.

No

. of

item

s R

P

Han

dlin

g co

de

Han

dlin

g ti

me

TH

Inse

rtio

n c

od

e

Inse

rtio

n t

ime

TI

Tota

l tim

e R

P*

(TH

+TI)

Comment

1 Base plate 1 0 0 1.13 0 0 1.5 2.63 First part

2 Side plate 1 1 1 0 1.5 0 2 2.6 4.1


4 Top vertical plate 2 1 0 1.5 0 0 1.5 6


6 Side plate 2 1 1 0 1.5 0 0 1.5 3

7 Screw 4 1 1 1.8 3 0 3.6 21.6 Attaches [6] to [1] and [4]

Total: 58.93

Appendix 5 - 4

Assembly time evaluation for concept 3 Table 7. Assembly times for Concept 3.

No

. of

item

s R

P

Han

dlin

g

cod

e

Han

dlin

g ti

me

TH

Inse

rtio

n

cod

e

Inse

rtio

n

tim

e TI

Tota

l tim

e R

P*

(TH

+TI)

Comment

1 Base plate 1 1 0 1.5 0 0 1.5 3 First part

2 Side plate 1 1 1 0 1.5 0 2 2.6 4.1


4 Top bar 2 1 0 1.5 0 0 1.5 6


6 Side plate 2 1 1 0 1.5 0 0 1.5 3

7 Screw 4 1 1 1.8 3 0 3.6 21.6 Attaches [6] to [1] and [4]

Total: 62.7

References Boothroyd, G., Dewhurst, P., & Knight, W. (2002). Product Design for Manufacture and Assembly. 99-

141. New York: Marcel Dekker, Inc.

Appendix 6 - i

Appendix 6 – Iterative FEA and optimisation of Concept 2

Table of Contents

Contents Appendix 6 – Iterative FEA and optimisation of Concept 2 ..................................................................... i

Table of Contents ..................................................................................................................................... i

Iterative FEA and optimisation of Concept 2 .................................................................................. 1

1.1 Geometry of Concept 2 ........................................................................................................... 1

1.2 Material properties ................................................................................................................. 2

1.3 Boundary conditions ............................................................................................................... 2

1.4 Results ..................................................................................................................................... 4

1.4.1 First iteration of thickness optimisation ......................................................................... 5

1.4.2 Iteration 2 of thickness optimisation .............................................................................. 6

1.4.3 Iteration 3 of thickness optimisation .............................................................................. 7

1.4.4 First iteration of topology optimisation .......................................................................... 8

1.4.5 Iteration 2 of topology optimisation ............................................................................... 9

1.5 FEA of optimised model ........................................................................................................ 10

References ..................................................................................................................................... 11

Appendix 6 - 1

Iterative FEA and optimisation of Concept 2 This iterative FEA and optimisation was performed to first optimise the thickness of the constituent

parts and then to remove any unneeded material.

For thickness optimisation load case 7 was used as that was the load case with the highest observed

stresses in the original ceiling attachment. An initial FEA was performed where all the constituent

parts of Concept 2 had a thickness of 20 mm. Following that the thickness of each component was

decreased in increments of 5 mm until non-negligible areas of the components had a factor of safety

near a value of 4, at which point the thickness optimisation for that component was stopped.

For topology optimisation load case 7 and load case 2 were used to determine the behaviour of

Concept 2 both when it was loaded with large forces as well as when a torque was applied. First an

initial FEA was performed where the middle plate had a thickness of 20 mm and the side and top

plates had a thickness of 15 mm, as determined in the thickness optimisation.

The modelling and FEA is performed using Solidworks 2014 SP 2.0 with the Solidworks Simulation

plugin.

1.1 Geometry of Concept 2

Concept 2 consists of 6 components joined together as a solid for the purpose of analysis. Concept 2

prior to optimisation is presented in Figure 1.2 and a simplified version for analysis is seen in Figure

1.1, along with the small 2 mm fillets applied to all sharp inner corners to avoid singularities in the

simulation; and the major dimensions of the parts are shown in Figure 1.3.

The entire model was analysed and symmetry planes were avoided as forces in multiple directions

were present in the analyses, which makes symmetry unsuitable.

The mesh used for the analysis was a tetrahedral mesh, as other mesh types were not available in

Solidworks simulation for solid bodies. Initially the mesh was set to use elements smaller than 20 mm

in width, but as h-adaptive refinement was used for the analyses the final mesh used elements as

small as 1 mm in some areas.

Figure 1.2. Concept 2 prior to

optimisations.

Figure 1.1. Simplified version

of Concept 2 for FEA.

Appendix 6 - 2


1.2 Material properties


Specialmaskiner, 2014b), which is ductile and has a linear elastic region with the properties outlined

in Table 1.1. These properties were used in all FEA. Small deformations were assumed for the

analyses, as such the material behaves as linear elastic. If large deformations would have been

observed this would have been revised.

Table 1.1. Material properties of Plancast Plus 5083(n)

Yield strength: 1.25e+008

N/m2

Poisson's ratio: 0.33

Tensile

strength:

2.5e+008 N/m2 Mass density: 2660 kg/m3

Elastic

modulus:

6.9e+010 N/m2 Shear

modulus:

2.59e+010

N/m2

1.3 Boundary conditions

The solid body was constrained at the four raised puck shapes beneath the fastening holes in the

middle plate, as shown in Figure 1.4 One of these was constrained from movement in any direction

Figure 1.3. Geometry of Concept 2. All dimensions in mm.

Appendix 6 - 3

Figure 1.5. Load contact points.

while the remaining three were prevented from out-of-plane movement.

The raised puck shapes were placed in the model to allow the constraints to be placed in a smaller

area than the entirety of the plate as well as allow some out-of-plane deflection to occur.

Loads are applied at the five contact points A through E as seen in Figure 1.5 and the list of used load

cases is written in Table 1.2. The load cases were determined in 2014 by Ida Lindell and the method

used to determine these is available in Appendix 3.

Table 1.2. Table of load cases.


1 0,0,0 0,0,-743.4 0,0,-743.4 0,0,0 0,0,0

2 0,0,0 0,-617.3,-237.6 0,617.3,-237.6 0,0,0 0,0,0

3 946.9,0,7320.9 -254.1,0,-11748 0,0,3755.4 0,0,0 0,0,0

4 255.5,0,1975.6 -255.5,-681.1,-1225.5 0,681.1,-1225.5 0,0,0 0,0,0

5 0,945.5,7309.6 0,-126.4,-25641.9 0,-126.4,-25641.9 0,0,21651.2 0,0, 21651.2

6 0,254.1,1964.3 0,-686.5,-1480.7 0,432.4,-958.9 0,0,0 0,0,0

7 871.4,871.4,7476.3 -381.5,-190.8,-

23150.9 0,-190.8,-13202.6 0,0,14102.7 0,0,14102.7

8 234.6,234.6,2012.9 -234.6,-734.6,-1512.4 0,500,-975.8 0,0,0 0,0,0

Appendix 6 - 4

1.4 Results

(Page intentionally left blank.)

Appendix 6 - 5

1.4.1 First iteration of thickness optimisation

Number of elements: 217804 Maximum effective stress: 1583 MPa Deformation scale: 155

Comment: Comment: A 20 kg load hanging from

the swing arm with a person pushing in the x-

direction and y-direction at the furthest x and y-

position.

A few small spots with high stresses appear, but

these are all in corners that are not joined as a

single solid in the physical object.

The middle plate has some areas where the factor of

safety is around 4, as such the middle plate was not

made any thinner in further iterations.


Appendix 6 - 6

1.4.2 Iteration 2 of thickness optimisation


Comment: The thickness of the middle plate

was 20 mm, whereas the side plates and upper

vertical plates were 15 mm in thickness.

As in the first iteration a few small spots of high

stresses appeared in corners that are not

present in the physical object.

Overall the factor of safety greatly exceeded a

value of 4, but in the upper vertical pieces

some areas with a FOS of around 3 was

observed. Further thickness reduction of those

parts were stopped.


Appendix 6 - 7

1.4.3 Iteration 3 of thickness optimisation


Comment: The thickness of the components for this

iteration was 20 mm, 15 mm, and 10 mm for the

middle plate, the upper vertical plates and the side

plates respectively.

In the factor of safety plots large areas can be seen

where the FOS is near a value of 3. These areas were

deemed to be too large for comfort and the thickness

for the side plates was then set to 15 mm instead of the

10 mm in this FEA. With this the thickness optimisation

of the components was completed and topology

optimisation was initiated.


Appendix 6 - 8

Figure 1.7. Areas with stresses above 15 MPa in

load case 7. Figure 1.6. Areas with stresses above 1 MPa

in load case 2.

1.4.4 First iteration of topology optimisation

Figure 1.8. Model after removing parts with low stresses.

Comment: For the first iteration of topology optimisation the model that had been thickness optimised was analysed using load case 2 and 7. Figure 1.6 and

Figure 1.7 show the areas which contain stresses of some significance. By removing parts of the areas which are not present in these two figures, while

taking symmetry into account the model in Figure 1.8 is obtained for the next iteration.

Appendix 6 - 9

Figure 1.10. Areas with stresses above

15 MPa in load case 7.

Figure 1.9. Areas with stresses above 2 MPa

in load case 2.

1.4.5 Iteration 2 of topology optimisation

Figure 1.11. Model after removing parts with low stresses.

Comment: By running the FEA for load case 2 and 7 again with the edited model the stresses in Figure 1.9 an Figure 1.10 were observed. There was still two

large areas that were mostly unaffected by the stresses, the top part of the side plates and the legs of the top plates. The top plate has long and slender legs

from which further removal of material would be difficult, but the top part of the side plates was shortened to give the model in Figure 1.11.

Topology optimisation was stopped at this point as further improvements would be negligible in comparison with the manual effort involved.

Appendix 6 - 10

1.5 FEA of optimised model


Comment: Following the thickness and topology

optimisations a final FEA was performed to verify that that

the ceiling attachment can withstand the most severe load

case, which is load case 7.

As in previous analyses a few single elements exceeded

the yield strength of the material, but as stated these

stresses are between nodes which would not be

connected in the physical object.

Overall the factor of safety greatly exceeds a value of 4

and only some small areas are in the vicinity of a value of

3, which indicates that there is not a large amount of

unneeded material.


Appendix 6 - 11

References Lindell, I. (2014). Load cases for analysis of AVPOS. Högskolan i Skövde, Department of Engineering Science.


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