Part 1. Principle construction

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Preface

This report is the result of an examination project for the study programme Utvecklingsteknink at the Department of Mechanical Engineering, Blekinge Institute of Technology. The project is done on commission for U-lift AB, Backaryd Sweden.

We would like to thank U-lifts managing director Tommy Pettersson and the rest of the employees, particularly Edward Janice.

We would also like to thank Jan Anders Månsson for helping us with computer related problems.

Finally we would like to thank Tekn. Dr. Mats Walter for his help with the disposition of the project and the thesis.

Karlskrona, May 2001

Oskar Bengtsson Marcus Hansson

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Abstract

The purpose with this thesis has been to develop a new platform and to make calculations of the lifting device to wheelchair lift VB-400.

The project has been performed in association with U-LIFT AB in Backaryd.

The task formulation were:

1. Develop a suggestion to construction of a new platform made in aluminium for the wheelchair lift VB-400. The platform should be able to be used on the existing lifting-device.

2. Make stress calculations of the lifting device for the wheelchair lift VB-400.

The project with developing the new platform in aluminium has been performed in alignment with Fredy Olsson’s method for integrated product development. The parts of the method that been used are principal- construction and primary-construction.

The new platform turned out to have a weight loss of about 30% even though it has a much bigger area compared to the old platform. The aluminium-section which been used to build the floor gave the platform a new and innovative design. Despite the big reduction of weight the new platform can stand for at least the same amount of stress and torsion that the old one did. It also fulfils the norms of the factor of safety.

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Sammanfattning

Syftet med detta examensarbete har varit att ta fram en ny lyftplatta till rullstolsliften VB-400 och att göra en beräkning av den existerande lyftanordningen till samma lift. Arbetet har gjorts i samarbete U-LIFT AB i Backaryd.

Problemställningen var att:

1. Utveckla ett konstruktionsförslag till en ny lyftplatta av aluminium till rullstolsliften VB-400. Lyftplatta skall kunna användas på existerande lyftanordning.

2. Utföra en hållfasthetsberäkning över lyftchassi och lyftok till rullstolsliften VB-400.

Arbetet med framtagningen av den nya lyftplattan har följt Fredy Olssons metod för integrerad produktutveckling. De delar av metoden som använts är principkonstruktion och primärkonstruktion.

Den ny konstruerade lyftplattan blev mer än 30% lättare än sin föregångare, trots att dess area blev avsevärt mycket större. Aluminiumprofilen som användes som golv ger plattan ett nyare och fräschare intryck om men jämför med det gamla sträckmetall nätet. Trots att plattans vikt har reducerats med nästan en tredjedel så har den fortfarande mycket goda hållfasthets egenskaper och en säkerhetsfaktor som uppfyller befintliga normer.

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Table of contents, examination thesis.

Preface... 1

Abstract... 2

Sammanfattning... 3

Table of contents, examination thesis.... 4

Part 1. Principle construction... 7

Abstract... 8

Sammanfatting... 9

Table of contents... 10

Introduction... 12

Company history... 13

1. Task formulation... 15

2. Product definition... 17

3. Product inquiry... 19

4. Criteria... 21

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5. Generation and evaluation of suggestions... 24

6. Presentation of chosen solution... 41

Part 2. Primary construction... 43

Abstract... 44

Table of contents.... 46

1. Product draft.... 48

2. Selection of components... 50

3. Detail construction.... 61

4. Product specification.... 71

Part 3. Calculation of lifting device.... 73

Abstract... 74

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1. Calculations... 78

References... 131

Appendix... 133

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Part 1. Principle construction

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Abstract

This examination thesis has been executed at the Department of Mechanical Engineering (at Blekinge Institute of Technology) as an assignment from U- lift, Backaryd Sweden.

The principle construction part started with defining the problems and the requests and criteria that was going to function as a foundation for the construction of the new platform.

Several different principle solutions was carefully evaluated against the criteria and requests. After the evaluation only one principle solution remained to be further developed in the primary construction part.

The solution that made it to the primary construction part is based on the aluminium floor section, F 38932 from SAPA, and a stabilising frame. The frames main parts are the outside frame, the stabilising support beams and the lip.

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Sammanfattning

Detta examensarbete har utförts på Institutionen för Maskinteknik vid Blekinge Tekniska högskola på uppdrag av U-lift, Backaryd.

I principkonstruktionen började vi, med hjälp av U-lift, att ta fram de kriterier och önskemål som skulle ligga som grund för framtagningen av en ny lyftplatta.

Ett flertal olika principiella lösningsförslag arbetades fram och viktades noggrant mot kriterierna och önskemålen. Efter viktningen återstod endast en principlösning som sedan vidare utvecklades i

primärkonstruktionsetappen.

Lösningsförslaget som gick vidare till primärkonstruktionen bygger på den färdiga golvprofilen F 38932 från SAPA och en stabiliserande ram. Ramens huvuddelar är ytterram och stabiliserande stödbalkar.

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Introduction

The wheelchair lift VB-400 is a platform helping people dependant of wheelchairs getting in and out of their transportations. The platform is released from its position from a control panel inside the vehicle, which enables it to fall outwards. During this process no outside forces are allowed. When the platform hits the ground it stops with an angle, the wheelchair can roll on and a protective plate, called the roll off protection, is released. The platform goes up and the wheelchair can roll into the car.

The platform is attached to the car with a lifting device, which is driven by a one way working hydraulic.

Today the platform is made out of a frame with a metal-net on top, all made of steel; this is the part that brings discontent.

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Company history

In 1951 Mr and Mrs Urbanusson started production of vehicle accessories.

The company grew and in 1957 new facilities was bought in Backaryd.

In 1958 legislation made a protection window between driver and passenger in taxi vehicles mandatory. This was the introduction of Karosseriverken AB to the owner of commercial passenger vehicles.

Production of school busses for up to 30 children was their main feature during the 1960´s. During the 1970´s production expanded to include heavy trucks and larger busses as well. In the late 1970´s the demand for busses with capability of transporting passengers sitting in their own wheelchair increased. Karosseriverken developed systems for lifting wheelchairs into the vehicle as well as to secure the wheelchair and the passenger in the vehicle. Within a remarkably short period of time, Karosseriverken became market leader in Scandinavia in its niche, a position that is still maintained.

The demand of lightweight wheelchair-lifts could not be covered by outside suppliers. In 1980-1982 Karosseriverken developed a series of new wheelchair lifts for installation on small busses. Introduction was successful.

In 1989 the department “Urbanusliften” was converted to a daughter company with the name U-lift AB. Separate facilities was bought in 1987 for the lift production. For the introduction to the German market the liftprogram was presented at the IAA Motorshow in Frankfurt in 1989.

Today all lifts are tested and approved by the German TüV and lift installation brackets are approved by the major European vehicle manufactures. From 1997 all lifts are complying with CE-regulations.

Approximately 60% of the lift production volume are sold on the export markets. Major markets are for the moment Poland, Spain, Germany, England and the Scandinavian countries.

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The lift TRP 300 for busses and trains was awarded by Swedish NUTEC in 1991 and in 1992 a larger number of lifts were delivered to Spain for the Olympic Games in Barcelona. A wheelchairlift range has been developed for the train manufacturers. Among others, AD Tranz and Deutsche Bahn are major customers.

U-lift is one of five European lift manufactures that is member of the standardisation committee CEN TC 98 currently developing new European standards for lifts to vehicles.

Furthermore, Karosseriverken is taking part in the international development by being members of the ISO committee for transportation of disabled people and the CEN committee for wheelchair thigh down- systems.

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1. Task formulation

[1],[12]

The by U-lift given task consist of developing a proposal of construction for a new platform made out of aluminium for the wheelchair lift VB-400. The platform is meant to be attached to an already existing lifting device.

Today the platform is made out of a stretch metal net welded on to frame of circular pipes, all made of steel. This contributes not only to a high weight but also to the platform changes colour after a short while to a dull more brown shade on account of ware from weather and wind.

Despite the request of weight reduction the platform has to be heavy enough to be able to fall by its own weight, no outside force is to be applied during the fall. It is also of great importance that the platform can be emergency released, in case of accidents etc.

Figure: 1.1 VB-400.

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1.1 Demands

• The platform must be constructed in aluminium.

• The product must have lowest possible weight.

• Low angle in downward position is required.

• Existing roll of protection must be able to fit on the lip of the platform.

1.2 Limitations

• The product must fit the back of the cars it is attended for.

• Wheelchairs do not have any standard dimensions.

• The platform must fit the existing lifting device.

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2. Product definition

[1]

2.1 The product

The product name is platform and its main function is to make it easier to transport a physically disabled person with a wheelchair in and out of a car or a bus.

2.2 Process

The main process of the product is to transport the wheelchair in and out of the vehicle.

Figure: 2.1 Process.

Other existing processes are tilting, rolling protection up/down and to achieve the lowest possible angle in downwards position. The main purpose of the redevelopment of the platform are the discontent with the old

platform, made in steel, which surface after a while changed to a brown colour. This phenomenon will not occur on a platform made of aluminium.

Another problem with the steel platform is the high weight.

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2.3 Surroundings

The product is to be used in outdoors environment and attached back on vehicle often used by persons with wheelchairs.

2.4 Man

Persons affected by the product or using it will mainly be physically disabled persons, nursing staff or chauffeurs.

2.5 Environment

The platform will be constructed in aluminium and will therefore be recycled in an efficient way. During the time in which the product is active it will not contribute whit any pollution at all.

2.6 Economics

The platform shall not cost anymore than 5,000 SKr to manufacture, and if a new tool needs to be constructed in shall cost 50,000 SKr or less to develop.

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3. Product inquiry

[1]

Most existing lifting platforms are made of steel, which make the plates very heavy. But the biggest problem with the steel made lifting platforms is corrosion, which gives the lifting plate a run-down exterior after only a couple of months. The function of the already existing lifting platform is reliable.

A wish from U-lift is that the plate should be as transparent as possible. Not only for safety reasons but also for not making the disabled feel like the are in a prison bus.

U-lift has had good experience in using aluminium in covered lift platforms. The biggest advantage with the aluminium lift platform is that the aluminium does not corrode and that it maintains its good looks. But aluminium does not have the same strength as steel, which makes it necessary to increase the material dimensions. Another disadvantage is the aluminium is that it is more expensive than steel.

Figure: 3.1 Existing lift.

(1) Bridge (2) Beam (3) Drive under

protection (4) Attach point (5) Frame (6) Tilt cylinder (7) Lift cylinder

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Figure 3.1 shows an already existing aluminium lift platform. Common for most of the existing lift platforms is that they are made for bigger applications on busses and trucks. None of the lift plates we found in our inquiry hade a surface with see through capability.

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4. Criteria

[1],[12]

4.1 Function

• The product has to be able to lift a wheel chair and a second person into the car.

4.2 Use

• The product has to fall by its own weight.

• The product shall stand for corrosion, to enable outside use.

• The product shall last for as long time as possible.

• The product has to be stabile during transport.

4.3 Safety

• The product has to have a protective bar, keeping the wheelchair on the platform.

• Surfaces, which not are meant to stand on, should be clearly marked out.

• Ice and snow should be easy to remove.

• The product should not inflict on the driver’s view backwards while in upward position.

• The product has to have devices that prevent slipping.

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4.4 Ergonomics

• The product has to be user friendly and not require any demanding moves from the operator.

• The device that prevents the wheelchair from rolling off should be triggered by a foot movement.

• When the platform is in the ground position it should have the lowest possible angle.

4.5 Aesthetics

• The product should be as discreet as possible.

• The product should not give the impression of prison bars

4.6 Construction and product

• The product should be manufactured with existing aluminium sections.

• The product has to be constructed to fit the existing lifting device.

• The product should have an easy construction.

• The product has to have lowest possible weight.

.

4.7 Manufacturing

• The product is planned to be manufactured in a large scale.

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4.8 Elimination

• It should be clear what parts that can be recycled.

• The recyclable parts should be easy to separate from the other parts.

4.9 Economy

• Total cost of the new product must not be more than the cost of the existing platform.

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5. Generation and evaluation of suggestions

^[1]

5.1 Principles for possible solutions, frame.

Suggestion 1, Rectangular frame.

This frame has a rectangular shape and is made of aluminium u-sections.

The frame needs additional sections to be able to stand for the maximum load and to keep the floor sections from deforming.

Suggestion 2, Conical frame.

This frame has a conical shape and is made of aluminium u-sections. The frame needs additional sections to be able to stand for the maximum load and to keep the floor sections from deforming.

Suggestion 3, Half conical frame.

This frame has a half-conical shape and is made of aluminium u-sections.

The frame needs additional sections to be able to stand for the maximum load and to keep the floor sections from deforming.

Figure: 5.1 Platform shapes.

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5.1.1 Primary evaluation, frame.

5.1.2 Comments

One of the reasons for choosing the half-conical frame is that the existing platform has this shape and has come to be a trademark. Other reasons are that it looks better and the existing roll-off protection device can be used without any modifications.

Since only one of the solutions made it through this evaluation there is no need to make any further evaluations of the frame.

Technical demands

3 Will meet the demand

2 Will proably meet the demand

1 Will hardly meet the demand 0 Will not meet the demand

Economical demands

Nr Solution TG EG Keep

1 "Rectangular" 2 3 No

2 "Conical" 2 3 No

3 "Halfconical" 3 3 Yes

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5.2 Principles for possible solutions, floor sections.

[15],[16]

Floor section 1, SFG 250/30

This floor section from SAPA is made of aluminium. The section surface is totally smooth, without any slip protection. The hole percentage of the section surface is approx. 19%. The section is 250mm wide and has a height of 30mm. SAPA delivers this section in lengths from 3 to 7.5 meters.

This floor section from SAPA is made of aluminium. The section surface is provided with a slip protection. The hole percentage of the section surface is approx. 17%. The section is 315mm wide and has a height of 35mm.

SAPA delivers this section in lengths from 3 to 7.5 meters.

This floor section from SAPA is made of aluminium. The section surface is provided with a slip protection. The hole percentage of the section surface is approx. 16%. The section is 300mm wide and has a height of 35mm.

This floor section from SAPA is made of aluminium. The section surface is provided with a slip protection. The hole percentage of the section surface is approx.18%. The section is 250mm wide and has a height of 40mm.

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Floor section 5, F 38932

This floor section from SAPA is made of aluminium. The section surface is provided with a slip protection. The hole percentage of the section surface is approx. 65%. The section is 345 mm wide and has a height of 25 mm.

Floor section 6, SPG 2464

This floor net is delivered by SPG MetallAB and is made out of aluminium.

The net is provided with a slip protection. The hole percentage of the net is 54%. The net are delivered in sheets that measure 2500 x 1250mm or 1250 x 1250mm. The size of the holes is 61 x 25mm and the material width is 5,8 x 5,8mm.

Floor section 7, SPG 2483

This floor net is delivered by SPG MetallAB and is made out of aluminium.

The net is provided with a slip protection. The hole percentage of the net is 33%. The net are delivered in sheets that measure 2500 x 1000mm. The size of the holes is 61 x 25mm and the material width is 8,5 x 4,8mm.

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5.2.1 Primary evaluation sections.

5.2.2 Comments.

The sections SFG 250/30, SFG 300/35 and SFG 250/40 are rejected because they do not meet enough of the technical demands. For example, they do not let enough light through. Furthermore, section SFG 250/30 does not have any slip protection. Other reasons for rejection is that section SFG 300/35 and SFG 250/40 has to be welded or screwed together.

Technical demands

Economical demands

Nr Solution TG EG Keep

1 SFG 250/30 1 3 No

2 SFG 315/35 2 3 Yes

3 SFG 300/35 1 3 No

4 SFG 250/40 2 3 No

5 F 38932 3 2 Yes

6 SPG 2464 2 3 Yes

7 SPG 2483 3 3 Yes

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5.3 Primary solutions

SFG 315/35

This section is provided by SAPA.

Material: aluminium.

Build width: 300 mm.

Total width: 315 mm.

Height: 35 mm.

Delivery length: 3000 – 7500 mm.

Hole percentage: 17%.

Cost: 210 SEK/m.

Figure 5.2 SFG 315/35.

This section needs to be stiffened to prevent bending. When attaching this section to the frame welding will be required.

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F 38932

This section is provided by SAPA.

Height: 25 mm.

Cost: 337.70 SEK/m.

Figure. 5.3 F 38932.

This section needs to be stiffened to prevent bending. When attaching this section to the frame welding will be required.

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SPG 2464

SPG MetallAB provides this net.

Length: 2500 or 1250 mm.

Height: 25 mm.

Hole size: 61 x 25 mm.

String height: 5,8 mm String width: 5,8 mm.

Cost: 1606 SEK/sheet.

Figure 5.4 SPG 2464.

This net needs to be stiffened to prevent bending. When attaching this net to the frame welding will be required.

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SPG 2483

SPG MetallAB provides this net.

Length: 2500 mm.

Height: 25 mm.

Hole size: 61 x 25 mm.

String height: 8,5 mm.

String width: 4,8 mm.

Cost: 2417 SEK/sheet.

Figure 5.5 SPG 2483.

This net needs to be stiffened to prevent bending. When attaching this net to the frame welding will be required.

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5.4 Middle evaluation 5.4.1 Criteria

Technical criteria ( TG )

Grading scale:

3. Will meet the demand.

2. Will probably meet the demand.

1. Will probably not meet the demand.

0. Will not meet the demand.

Financial criteria ( EG )

Grading scale:

3. Very economical 2. Economical 1. Hardly economical 0. Not economical

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Criteria 1 The product has to be able to lift a wheelchair and a second person into the car.

Criteria 2 The product has to fall by its own weight.

Criteria 3 The product shall stand for corrosion, to enable outside use.

Criteria 4 The product shall last for as long time as possible.

Criteria 5 The product has to be stabile during transport.

Criteria 6 Surfaces, which not are meant to stand on, should be clearly marked out.

Criteria 7 The product has to have devices that prevent slip.

Criteria 8 When the platform is in the ground position it should have the lowest possible angle.

Criteria 9 The product has to be constructed so it fits the existing lifting device.

Criteria 10 The product should have an easy construction.

Criteria 11 The product has to have lowest possible weight.

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Criteria 12 The product is planned to be manufactured in a large scale.

Criteria 13 It should be clear what parts that can be recycled.

Criteria 14 The recyclable parts should be easy to separate from the other parts.

Financial

criteria The total cost off the new product must not be more than the cost off the existing platform.

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5.4.2 Middle evaluation

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5.4.3 Comments

One reason for keeping solution 1 and 2 is that they have better slip protections than solution 3 and 4, they are also stronger and will therefore last for a longer period of time. The most important reason for keeping the chosen solution is that they do not need as much welding as the others do.

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5.5 Pair comparison

Way of action:

A>B put 2 A=B put 1 A<B put 0

> More important than…

= Just as important as…

< Less important than…

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5.5.1 Arrangement of wishes

• The product should be manufactured with existing aluminium sections.

• Ice and snow should be easy to remove.

• The product should be as discreet as possible.

• The product should not inflict on the driver’s view backwards while in upward position.

• The product should not give the impression of prison bars.

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5.6 Final evaluation

5. The suggestion will clearly meet the demand.

4. The suggestion will meet the demand.

3. The suggestion will probably meet the demand.

2. The suggestion will hardly meet the demand.

1. The suggestion will not meet the demand.

5.6.1 Comments

Solution number two, the aluminium section from SAPA, got a higher score on this final evaluation mainly on account of the ability to let light in and the fact that snow and ice do not cause any problems.

(After finishing the final evaluation we received news from SAPA that they are cancelling the production of the aluminium section SFG 315/30.)

Exist. sections Ice and snow Discreet See through Stetic (prison) Total add. Keep

A B C D E

Nr Description k 28% 20% 20% 16% 16% 100%

1 SFG 315/30 u 5 3 3 2 2 15

t 1.4 0.6 0.6 0.32 0.32 3.24 NO

2 F 38932 u 5 4 3 3 2 17

t 1.4 0.8 0.6 0.48 0.32 3.60 YES

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6. Presentation of chosen solution

[1]

The chosen solution is the aluminium section F38932 from SAPA, in combination with the half conical frame.

The platform will let enough light through to minimise the feeling of imprisonment, and at the same time give the driver a clear view out of the rear window. It is robust enough to be able to stand the force from the heaviest wheelchairs on the market. Because of the “click on“ attachments on the sections, the parts that need to be welded are much fewer than those of a regular net. Ice and snow will fall through the holes in the net so it will not get slippery. Most important of all, the platform will keep its colour and shine for a much longer period of time than the old platform did.

Figure: 6.1 Solution F38932.

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6.1 Meeting the criteria

The chosen solution is very easy to put together. It has a high hole percentage which makes it easy for light to shine trough. It also has a very god slip protection on the surface. In total, the chosen solution fulfils almost every criteria although one of the drawbacks might be that it is more expensive than the other solutions.

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Part 2. Primary construction

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Abstract

In the primary construction part a finished concept was developed from the solution that was chosen in the principle construction part. The work in the primary construction part has, as in the principle construction part been based on Fredy Olssons product development method.

The solution that was chosen in the principle construction part was called

“F 38932” and it consists of five main parts, a outside frame, support beams, roll off protection, lip and floor sections.

Figure: The new platform.

The new platform will, when used, be attached to the existing lifting device and function in the same way that today’s platform does.

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Sammanfattning

I primärkonstruktionsetappen arbetades ett färdigt koncept fram från det principförslag som fördes vidare från principkonstruktionen. Arbetet i primärkonstruktionen har liksom i principkonstruktionen grundats på Fredy Olssons produktutvecklingsmetod.

Förslaget som fördes vidare från principkonstruktionen kallades ”F 38932”

och bestod huvudsakligen av en ytterram, stödbalkar, avrullningsskydd, läpp och golvprofiler.

Figur: Den nya lyftplattan.

Den nya lyftplattan kommer vid användning monteras på den befintliga lyftanordningen och fungera på samma sätt som existerande lyftplatta.

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Table of contents.

Abstract... 44

Table of contents.... 46

1. Product draft.... 48

2. Selection of components... 50

2.1 Selection of construction components – Net.... 50 2.1.1 Demands on the construction components... 50 2.1.2 Search of component alternatives.... 52 2.1.3 Judgement of the above mentioned component alternatives.... 54 2.1.4 Elaboration of chosen component.... 56 3. Detail construction.... 61

3.1 Determine of safety factor.... 61 3.2 Detail construction – Frame.... 63 3.2.1 Solution search... 63 3.2.2 Material selection.... 67 3.2.3 Draft technique... 67 4. Product specification... 71

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Introduction

I the previous construction part, the principle construction, a product draft was developed. This product draft was then used as a based for the work in the primary construction.

The primary construction consist mainly of four parts, the product draft, the component selection, the detail construction and the product specification.

The goal with this primary construction has been to develop a product specification for the manufacture of a new platform for the wheel chair lift VB-400.

The work in the primary construction part has in great deal been made with help of the computer program I-DEAS and the calculation program

Mathcad 6.0 Professional Edition.

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1. Product draft.

[2]

The lift platform consist of following main parts:

1. Frame and lip 4. Attach beam

2. Roll of protection 5. Vertical reinforcement 3. Floor sections 6. Horizontal reinforcement

The lift platform will mainly be used on different mini busses and it will be manufactured in a rather large series.

The platform parts are located as shown in figure 1.1.

Figure: 1.1 VB-400 aluminium platform.

3

2 1

5

6

4

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Approximated dimensions.

Total width: approx. 1300 mm.

Total length: approx. 1550 mm.

The platforms adaptation to its constructional environment.

The platform will be built so that the existing lifting device can be used without any modifications.

Finished or unique parts.

Finished parts are the floor sections, the attach beam and the roll of protection. Unique parts are the frame.

Treatment type.

Finished parts Unique parts

Routine Special Routine Special

Frame X

Roll of protec. X

Attach beam X

Floor sections X

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2. Selection of components

[2],[3],[4],[15],[16]

2.1 Selection of construction components – Net.

In this chapter a restricted selection of construction components will be made, the relevant component groups are stretch metal and different floor sections.

Component task.

The construction component will work as the surface that the wheelchair and its operator will stand on.

When using floor sections you will need to connect a couple of sections to obtain the total width of the platform, but if you use stretch metal these are delivered in big enough sheets to cover the hole frame of the platform.

2.1.1 Demands on the construction components.

Process:

• The net should not bend down more then the already existing model.

• The net should be able to lift a point weight of 400 kg at 60 cm from the back edge of the platform.

• The net should be corrosion resistible.

• The net should be made out of existing aluminium sections.

• The net should be as light as possible.

• The net should have an easy construction.

• When recycled it should be easy to separate the recyclable parts.

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Surroundings:

• The net should contribute to that the platform is as discreet as possible.

• The net should be made so that ice and snow is easy to remove.

Man:

• The product in an upward position should not inflict on the driver’s view backwards.

• The net should not give the felling of prison bars.

• The net should be made to prevent slippery.

Financial:

• The total cost of the new platform should not exceed the cost of today’s platform.

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2.1.2 Search of component alternatives.

Stretch metal.

One of U-lifts suppliers is a company called SPG Metal AB. SPG Metal has a big program of different stretch metals, were of two fits our product. The relevant models are 2464Al and 2483Al.

When using these stretch metals a lot a welding will be needed to attach the stretch sheet to the frame. It will also require some reinforcement of the stretch metal in shape of support beams that are attached to the frame.

The two relevant models of stretch metal has a hole size of 61 x 25 mm and weigh 2,2 kg/m³ for the model 2464Al and 1,5 kg/m³ for the model 2483Al.

The hole percentage of the stretch metal is 54% for model 2464Al and 33%

for model 2483Al.

The cost for these two models is 1606 SEK/sheet for 2464Al and 2417 SEK/sheet for 2483Al.

Floor sections.

SAPA (Scandinavian Aluminium Profiles AB) has a wide program with different floor sections, several of them being relevant for our project. The models that fitted our purpose are called SFG 250/30, SFG 315/35, SFG 300/35, SFG 250/40 and F 38932.

SFG 250/30:

This floor section can be bought in lengths between 3000 and 7500 mm, and costs 190 SEK/m. But the minimum order is 275 m.

The section is 250 mm in build width, witch means that five sections has to be connected to each other in order to cover the whole frame.

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SFG 250/40:

The section is 250 mm in width, witch means that five sections has to be connected to each other in order to cover the whole frame.

SFG 300/35:

The section is 300 mm in width, witch means that four sections has to be connected to each other in order to cover the whole frame.

SFG 315/35:

The section is 300 mm in build width, witch means that four sections has to be connected to each other in order to cover the whole frame.

Model SFG 315/35 is about to be cancelled from SAPAs model program.

All these sections have to be reinforced by the frame so that the don’t bend down to much.

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2.1.3 Judgement of the above mentioned component alternatives.

Process criteria’s:

All the solutions, except the stretch metal net 2464Al meets the “bend down” criteria. The solutions that meets this criteria the best is the ones with low hole percentage.

The load criteria of 400 kg at 60 cm from the back edge of the platform is fulfilled by the floor sections but not by the stretch metal nets. The floor section F 38932, that was chosen in the Princ.Const.part, had the best result, mainly because it has built in stiffening bars every ten centimetres. To get a good result we had to apply stiffening bars to the frame that supports the sections.

The criteria for an easy construction is fulfilled by the floor sections but not by the stretch metal nets since they require much more welding to be attached to the frame.

The fact that all the platform parts are made out of aluminium makes the platform easy to recycle and contributes to a low weight. It also means that corrosion isn’t a problem.

Surrounding criteria’s:

All the solutions are rather discreet and will therefore contribute to an discreet platform.

The stretch metal nets and section F38932, chosen in the Princ.Const.part, has an smaller surface for ice and snow to stick on, and there shape makes it easy to simply brush of any of the ice and snow that will stick on the platform. The other solutions has an bigger and more plane surface on witch ice and snow will stick better. On these solutions it will probably take more than a brush to remove ice and snow.

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Man/use criteria’s:

To not inflict with the drivers view backwards the sections should have an hole percentage of at least 50%. Its only two of the solutions that meet this criteria, the stretch metal net 2464Al with 54% holes and the floor section F 38932 with 65% holes.

Since all the solutions are different nets or sections its hard not to get an imprisoned felling. But the stretch metal nets and the section F 38932 don’t give that strong imprisoned felling as the other solutions.

All solutions except SFG 250/30, 2464Al and 2483Al are made to prevent slippery.

Financial criteria’s:

The platform that is used today costs about 3000 SEK in material costs and 5000 SEK in total, and the new platform shouldn’t cost any more.

The relevant cost is the section-cost/platform, but this cost can not be 3000 SEK since the cost for the frame isn’t included. The frame will cost between 700 and 1000 SEK witch makes the maximum cost for the sections approximately 2000 – 2300 SEK.

The cost for the different solutions vary from 1450 to 2400 SEK. This means that almost all the solutions meets this criteria, its only the stretch metal net 2483Al that is to expensive.

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2.1.4 Elaboration of chosen component.

Specification of floor section.

Following floor section has been chosen because it best met the criteria’s, SAPAs floor section F 38932.

Figure 2.1 F 38932.

Height: 25 mm.

Minimum order: 275 m.

Weight: 4 kg/m.

Cost: 337.70 SEK/m.

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Calculation of floor section.

The following calculations are made in I-DEAS FEM application.

The maximum load on a single part of a section is determined to be 125 kg, witch is the load over one wheel of a electric wheelchair with a passenger in it. This load is then multiplied with the safety factor 2.1 (see determining of safety factor, chapter 3.1 primary construction).

The maximum allowed stress for the material is 170 MPa.

To make FEM calculations possible the model had to be simplified, all round corners and radiuses where skipped. Witch led to high stress concentrations in these areas. To get the actual stress the areas with high stress concentrations has to be overlooked.

Load case 1:

In the first load case the load was divided in four points, as shown in figure 2.2.

Results: (See figure 2.3 and 2.4)

The actual stress was 120 MPa witch is less than the maximum allowed material stress of 170 MPa and the displacement in the material was 0.508 mm witch is acceptable.

Figure 2.2 Position of loads.

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Figure 2.3 Stress.

Figure 2.4 Displacement.

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Load case 2:

In the second load case the load was divided in three points, as shown in figure 2.5.

Results: (See figure 2.6 and 2.7.)

The actual stress was 135 MPa witch is les than the maximum allowed material stress of 170 MPa and the displacement in the material was 0.599 mm witch is acceptable.

Figure 2.5 Position of loads.

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Figure 2.6 Stress.

Figure 2.7 Displacement.

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3. Detail construction.

[2],[3],[4],[6],[8],[11],[14],[15]

The detail construction will be made on the frame. But first a safety factor must be determined for the tail lift. This safety factor will be used on all details.

3.1 Determine of safety factor.

In chapter 5.12, page 21 in the mechanical directive a safety factor of 2.1 is found for tail lifts on which a person is permitted to be on. Despite of an appointed safety factor a calculation will be executed to determine if this safety factor is relevant.

Calculation of safety factor.

The safety factor, Γ, is determined through a number of factors:

Strength factors:

γR1 = spread in material parameters. = 1.05 γR2 = spread in calculation methods. = 1.05

γR3 = reliability in relevant mechanical criteria. = 1.0 Load factors:

γS = load factor. = 1.1 Part factors:

γn1 = security demand. = 1.05 γn2 = control. = 1.05

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Additional factors:

ψ = elevation factor = 1.3 η = lift factor = 1.2

Γ = γR1 ⋅ γR2 ⋅ γR3 ⋅ γS ⋅ γn1⋅ γn2 ⋅ ψ ⋅ η

Γ = 1.05 ⋅ 1.05 ⋅ 1.0 ⋅ 1.1 ⋅ 1.05 ⋅ 1.05 ⋅ 1.3 ⋅ 1.2 = 2.09 The calculations showed that a safety factor of 2.1 is accurate.

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3.2 Detail construction – Frame.

Determination.

The frames main purpose is to hold and support the aluminium floor sections and to give stability to the whole construction. The frames surface should be smooth and without any sharp edges to prevent injuries or puncture of a wheelchair wheel.

Limitations of dimensions are that the frame should fit the lifting-device and not inflict on the existing function.

Criteria’s.

The criteria’s on the frame is the same criteria’s that were made for the whole construction in the principle construction, see chapter 4 in the principle construction.

Additional criteria’s:

• The calculated stress can not be higher than the allowed material stress with a safety factor of 2,1.

• A person who stands on the platform should not be able to feel any deformation.

3.2.1 Solution search

When dimensioning the frame notice was taken to the limitations of space of the platform, it has to fit the back of cars. Other things to consider is to give the frame the lowest possible weight and make it absorb the applied force in an efficient and stabile way.

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The frame should not be over 1400 mm wide and no longer than 1600 mm to the tip of the lip.

In order to attach the roll-off-protection the lip must be 200 mm long or more. For the platform to function as it is supposed to the attach beam must be attached 100 mm from the back edge of the frame. To create a model that fulfils these demands a number of suggestions needs to be constructed.

Suggestion 1:

This frame is constructed of U-sections in aluminium welded together. The lip is made of folded aluminium sheets with a hole for the roll-off- protection. The frame got a pair of reinforcements on the sides. This construction is very easy and fast to process.

Figure: 3.1 Suggestion 1.

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Suggestion 2:

This frame is constructed of folded aluminium sheets with a slim reinforce- section in the middle. The lip is made of folded aluminium sheets with a hole for the roll-off-protection.

This suggestion is the lightest of all the frames and does not need any reinforcement on the sides.

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Suggestion 3:

This frame is constructed in the same way as suggestion 2 but with a broader reinforce-section in the middle. This makes this suggestion a little bit heavier than suggestion 2 but still lighter than suggestion 1.

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3.2.2 Material selection.

Since one of the criteria’s from U-lift was that existing aluminium sections should be used the material selection process is skipped.

3.2.3 Draft technique

Process

The main function of the frame is to make the platform more stabile and to keep the components together. In this point of view the construction is correct.

Manufacture

The fact that the components in the frame are made of standard sheet the only processing necessary is folding and attaching. The attaching process constitutes of welding.

Disposal

When the frame is used and needs to be disposed of it will not require any separation of components because all the parts are made of the same material. This construction can be recycled in a very efficient way.

Assessment of solutions

All solutions have been designed to meet the arranged demands. After the FEM-calculations were made on every frame solution 3 proved to be the one with the best results. This was the solution that absorbed the forces in the most efficient way. Solution 1 was very easy to construct but could not stand for larger amount of torsion. See appendix page 160.

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The chosen frame has following components and dimension:

1. Outside frame and lip, made off a 3 mm thick aluminium sheet.

2. Roll-off-protection, already exist.

3. Floor sections, SAPA F 39832 4. Attach beam, already exist.

5. Vertical reinforcement, made off a 3 mm thick aluminium sheet.

6. Horizontal reinforcement, made of VKR 50X50X3 mm.

Total mass of this construction is 19,5 kg

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FEM

An FEM-model is constructed in I-DEAS^TM in order to perform calculation of stresses and the displacements in a specific model caused by a force.

In this case the force is applied, at 60 cm from the rear end of the platform.

The force is the maximum force allowed on the platform multiplied by the factor of safety, (10000N). The rand conditions are applied on the back of the reinforcement beam, the same places were the gloves are attached. See figure 3.3.

The results of these calculations clearly show that solution 3 is the most satisfactory of the three candidates.

As displayed in figure 3.4 the maximum stress in the frame do not exceed the maximum stresses of the material, which is 170 MPa.

Figure 3.5 shows the total displacement of the frame is 5,07 mm and will not be noticed by a person standing on the platform and most important of all is the fact that the force doesn’t cause any remaining deformations.

Figure: 3.3

Rand condtions Translation Rotation

X LOCKED X LOCKED

Y LOCKED Y LOCKED

Z LOCKED Z LOCKED

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Figure: 3.4 Stress.

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4. Product specification.

The new platform should be manufacture in aluminium. This project has been divided in to two different parts. These parts contain choosing aluminium floor sections and to reshape the frame.

Due to the fact that the old platforms shape have come to be a trademark for U-LIFT, the aim was to make the new platform as similar to the old platform as possible.

A big variety of floor-sections from two of Sweden’s largest aluminium- providers were graded and arranged by the ability to meet the criteria.

Calculations were made of the section with the highest grade and it proved to stand for the forces the platform should manage.

Three different types of frames were developed. The first one was made of VKR-sections in aluminium. This solution turned out to be very heavy and weak. The second solution was made of an aluminium sheet, which were folded into a u-shaped section. This solution was not as heavy as the first and more cost efficient but it vas still to fragile.

The final suggestion was basically the same as the second solution but with a bigger reinforcement beam in the middle.

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For more drawings see appendix under drawings.

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Part 3. Calculation of lifting device.

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Abstract

From the calculations that were made in the calculation part recommendations of changes in dimensions were made.

The calculation part mainly consists of three parts. In the first part magnitude and position of the working forces on the lifting device was calculated. These forces was then used in part two, in witch a FEM calculation over the stress and displacement in the lift chassis and lift ok were made. In the third and last part two different weldings were calculated, one in I-DEAS FEM application and the other by hand with help of welding norms.

All calculations were executed on the already existing lifting device for the wheelchair lift VB-400.