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Ying Cheung13019879

Product Design Technology

Product Design Engineering:“Escape to Freedom” - Bridge

P1

Contents

P2

P3 Introduction

P4 Initial Concept (1/4 scale)

P5 2nd Concept (1/8 scale)

P6 Triangular Profiles Testing

P7 Folding Concept (Primary)

P8 Torsional Impact On Folding Structure

P9 Folding Concept (Secondary)

P10 Design Requirements Of Folding Concept

P11 Stabilising Secondary Fold Mechanism

P12 Fixture To The Balcony

P13 Hinge Dimensions

P14 Hinge Clearance Impact On Overall Deflection

P15 Overall Deflection Caused By Hinges

P16 Angle Of Deflection

P17 Improvement Made After Competition

P18 Change In Material Thickness Impact on Bridge’s Stiffness

P19 Mass Of Bridge

P20 Centre Of Mass

P21 Factor Of Safety

P22 Deflection

P23 Deploy The Bridge To Escape

P24 Summary

Overall Design And Feature Development

Relationship Between Hinges And Deflection

Changes Made To The Bridge After Submission

Final Design Details And Summary

P3

Introduction

To construct a bridge that fits on the balcony and travels forward

6m and then 2m at 90° The bridge must not intersect with the No-Go-Zone. Made of Aluminium 2024-0 Under 2.5m when folded Allow user of 80kg to walk on Deploy under 5 mins

Initial Concept (1/4 scale)

Failure due to; manufacturing error, Torsional stress, open cross section profile

Improvement can be made; Create a closed cross section

profile, to resist torsion and buckling

Buckling occur during the testing of the 1/4 scale card model.

P4

Destructive testing of the first card-board bridge (1/4 scale) and understanding of failure

Side view

Bottom view

Cross section profile

Ribs

2nd Concept (1/8 scale)

Shear failure at the “hook” during the testing of the 1/8 scale card model.

Failure due to; Under-Engineering Stress concentration Corrugation of cardboard

Improvement can be made; Create profile with larger

contact surface area to reduce stress concentration,

Increase “h” value to increase second moment of area to reduce stress.

The cross section of this design is triangular to create a closed profile in order to resist torsion and buckling

P5

After changing the cross-section profile into a triangle, and observing failure through destructive testing

Ribs

Previous profile Triangular profile

Triangular Profiles Testing

P6

Using Solidworks simulation to test and identify which profile is suitable for this torsion scenario.

Cross section

Cross section

Cross section

This configuration has more uniform stress range (1.16e+1 – 8.72e+2 Mpa).

And lower stress at the intersection between the bridge and dog-leg

Profile used in the 2nd concept

Folding Concept (Primary)

However the collapsed bridge is relatively large and balky.

Not meeting the maximum size standard – 2.5m (full scale)

Further development; Create internal folding

structure to reduce overall size. Flat pack idea.

P7

Concept of primary folding mechanism to allow the bridge become portable.

Detach the dog-leg off the bridge

BalconyTop view

Side view

3.3 m

The folding concept is to “roll” up the parts to reduce the size

Torsional Impact On Folding Structure

As the torsional force applied, the support plates collide with the section on it’s left to resist the torsional motion

The centre point of the torsional motion is unknown however the plates are added to location as far away as possible to have greater mechanical advantage against torsion

Torsion will force the folding parts to shift, this will cause misalignment and result large deflection

Support plates are joined to the section on it’s right

P8

Rotation of parts caused by torsion will affect the performance of the bridge.

Folding Concept (Secondary)

Combining both the primary and secondary folding mechanisms together to collapse the bridge down to smaller size – 1.88m

P9

Introducing secondary folding structure to reduce size of the folded bridge in order to meet the brief’s requirement – under 2.5m (full scale)

3.3 m 1.88 m

Using 4-bar-linkage to as secondary to minimise the size of the bridge to flat-pack

Previously without the secondary fold result 3.3m in folded size and unable to meet the maximum limit of 2.5m

Design Requirements Of Folding Concept

Tapered beam able to provide linear axis and flat surfaces for folding

Ideal beam curve

Walking surface with constant width

Twisted surface

It is difficult to manufacture a twisted piece and to make it collapsible

Since the ideal beam curved was replaced the stress concentration will increase. However this was sacrificed so that the structure can be folded

P10

Front view

Identifying requirements needed for the secondary fold to be feasible

Hinges require a linear pivot axis

Hinge axis

Stabilising Secondary Fold Mechanism

P11

The Fold mechanism is required to be stabilised or else the structure will collapse

The 2 possible consequences of not having a stabiliser for the secondary folding structure

A B

Providing both tensional and compressive support to avoid failure in both figure “A” and “B”.

Fr = 400N

θ = 46:(for compression calculation)Fθ = 576N

θ = 44:(for tensional calculation)Fθ = 555N

Fc = Compressive force9.9N

Fc = Tensional force9.9N

The forces are relatively small and neglectable however both the compressive and tensional reinforcements are required to be installed for reassuring that the structure will not collapse during operation.

Fixture To The Balcony

As the bridge is fixed to the balcony, the edge of the balcony is likely to shear the fixture.

To avoid the balcony cutting into and shear the fixture, a bottle-cap-remover design was used in order to have control of the location where the force exert.

With a side effect of this design, it allows clearance to slot on to the balcony.

P12

Failure of the 2nd concept

Identify failure occur on the fixture to the balcony and identify features needed to minimise the chance of failure.

< Torsional stress is greater beam stress on the same profile therefore the fixture will be stronger if it is converted into a beam.

Hinge Dimensions

Shear surfaces = 5

H1 H2 H3

The stress shown in the FEA suggest that the hinges in “H1” are the most likely to fail due to low Fos value

P13

Identifying the dimension of the hinges through trial and error due to complicated scenario – difficulty of applying both shear equation and torsion equation onto the pin of the hinge

Deformation of the pin during operation

Diameter (inner)(mm)

Diameter (outer)(mm)

Number of shear facesMax FEA stress

(Mpa)Fos

H1 50 100 5 37.96 1.9

H2 50 100 5 25.06 2.99

H3 50 100 4 20.47 3.66

Outer diameter

Inner diameterDiameter of the pin

Although the stress operating of the pins are different, however they are under the yield stress by at least 1.9 times therefore it is unlikely to yield.

Hinge Clearance Impact On Overall Deflection

The clearance between the pin and the bracket of the hinge is set to be 0.5 mm

the clearance of the hinges will have impact on the overall deflection

Deformation results caused by hinges;

H1 11.1mm H2 3.74mm H3 2.9mm H4 2.48mm

Total 20.22mm

H1 H2 H3 H4

The closer the hinge to the fixture the more deflection it will cause due to the mechanical advantage

P14

Clearance on both brackets therefore the result deformation x2

Hinge clearance allow parts to shift, the movement will cause the bridge to deflect. Referencing to “clearance deformation” calculation within the process book

Overall Deflection Caused By Hinges

P15

H1 H2 H3H4

The pin with in the hinge will deform during operation and allow parts to shift, the movement will cause the bridge to deflect. Referencing to “Hinge Stress Analysis” calculation within the process book

The sum of the deformation caused by hinges is 32.24mm

By adding all the values together – Total estimate value (without clearance) = 12.02mm

H1

H2

H3

H4

The image above shows how the pin of the hinge is deformed.

The overall deformation values were extracted from Autodesk Simulation Mechanical

- Hinges were made without clearance

Referring to “Hinge Clearance Impact On Overall Deflection” = 20.22mm

3.2 cm is visible deformation however user is still enabled to walk on the bridge therefore no actions need to be taken

Angle Of Deflection

P16

Using Solidworks Section Properties to analyse the polar moment of inertia of the cross-section at every 500mm of the bridge to calculate the deflection angle

length (mm)

polar moment of inertia (J) (m^4)

deflection (rad)

deflection (deg)

0 0.000534286 0 0

500 0.000599333 5.80348E-05 0.000364643

1000 0.000669455 0.000103912 0.000652897

1500 0.000744845 0.000140092 0.000880221

2000 0.000825692 0.000168499 0.001058712

2500 0.000912188 0.000190652 0.001197904

3000 0.001004522 0.000207753 0.001305352

3500 0.001102887 0.000220762 0.001387085

4000 0.001207472 0.000230446 0.001447935

4500 0.001318468 0.000237427 0.001491794

5000 0.001436066 0.000242204 0.001521814

5500 0.001560457 0.000245187 0.001540554

6000 0.001691832 0.000246706 0.001550101

Total = 0.014399012

0

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

0.0016

0.0018

0 1000 2000 3000 4000 5000 6000 7000

De

fle

ctio

n (

de

g)

Length from Balcony (mm)

Angle of deflection was calculated by rearranging the torsion equation

The graph shows the amount of deflection caused by each of the cross sections

0.014° of deflection is very small therefore the user can walk on the without inconvenient.

Improvement Made After Competition

P17

As the bridge was loaded with weight, a small displacement was spotted along the secondary folding structure, the bridge structure was weaken by the displacement.

The secondary fold mechanism was removed from the 2 rear parts, this change stabilise the shear motion to reduce the overall deformation

When the bridge is loaded, 2 forces exerted by the wall (blue arrows), that cause a shear motion. Small changes made near the balcony will have significant change to the overall performance.

5526N

The force – 5526N was calculated in the “Free Body Diagram” within the process book

A thread was used as reinforcement to reduce the displacement of the secondary fold mechanism to reduce the overall deformation of the bridge

No secondary fold

2 secondary folding mechanism needed to be kept so that the bridge don’t exceed the size requirement when folded

Change In Material Thickness Impact on Bridge’s Stiffness

P18

Material Thickness (mm)

End of bridge deformation (mm)

Stress (Mpa)

Mpa/deformation ratio

25 6.109 17.483 2.861843182

3 0.769 3.485 4.531859558

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 5 10 15 20 25 30

Mp

a/d

efo

rmat

ion

ra

tio

Material Thickness (mm)

Changing material thickness to reduce weight of the bridge and identify the impact on the stiffness on the bridge

Bridge with material thickness of 25 mm

Weight: 1670kg

Bridge with material thickness of 3 mm

Weight: 196 kg

The graph shows that by reducing the material thickness the stiffness of the bridge has increase significantly

The extra 22mm on top of the 3mm has become a burden to the bridge

By reducing the material thickness, unnecessary weight was removed so that the bridge it self has become lighter and stiffness has increased

Mass Of Bridge

P19

Calculate mass of the bridge made of Aluminium 2024-0

The volume value obtained from Solidworks Mass Properties

Volume = 70389712.29 mm^30.0703 m^3

Mass = Volume X Density0.0703 X 2780195.7kg

The weight of the bridge is too much compare to how much a single human can carry

The maximum combined pulling force exert by a male is approximately 1019N (556+643)

Therefore minimum of 2 people are required to carry the bridge.

Centre Of Mass

P20

Identifying the centre of mass of the bridge with Solidworks Centre of Mass feature.

The centre of mass is located near to the balcony due to larger amount of material used is greater near the balcony.

Therefore it has greater mechanical advantage compare to having the centre of mass at the half way across the bridge

Height from top of the walk way

Distance from front of the Balcony(2106mm)

Distance from side of the bridge

The centre of gravity is closer to one side of the bridge, this suggest that a lot of material is used to construct the dog-leg

Factor Of Safety

P21

height of plank from floor(mm)

Displacement(mm)

Load applied(kg)

110 0 0

87 23 80 (Still)

82 28 80 (Shock load)

Using shock load (user’s footstep) and maximum stress from FEA to calculate the Factor Of Safety.

Physical demonstration was performed so that the deformation of the beam can be measure to calculate force

Load: 80kg800N

Shock load: 800 X (28/23)973.9N

Max Stress = 3.485 Mpa

The FEA simulation was set up with 800Ntherefore

Stress is proportional to force therefore when shock load is applied the stress will increase by 1.2

Stress = 3.584 X 1.24.363 Mpa

Fos = yield stress / operating stress75 / 4.36317.2

The factor of safety shows that the bridge is over engineered by approximately 17 time. This suggest that the cross section of the bridge can be reduced to minimise the FOS and weight.

X1.2

Aluminium 2024-0: Yield stress = 75 Mpa

Deflection

P22

Identify the deflection of the bridge after reducing the material thickness.

0.769mm of deformation for a single body is very little and hardly noticeable.

referring to the change in stiffness after change in material thickness,

Autodesk simulation on the bridge as one solid body (except the balcony)

Referring to “Hinge Clearance Impact On Overall Deflection” = 20.22mm

Is proportional to 1/J

The sum of the deformation is 21.mm

As one increases the other decreases

Referring to the “Factor of safety” the Fos value has increased by 17.2 therefore the deflection will be reduce by 17.2 times

0.014 x (1/17.2) = 0.000813°

These deflection are hardly noticeable and they do not cause any obstruction of inconvenient to the user during operation

Deploy The Bridge To Escape

P23

Step by step of deploying the bridge and assembling the dog-leg onto the end of the bridge

Time required to deploy the cardboard bridge during the competition was 29s. The estimate time to deploy for full scale bridge to be approximately 10 time longer – 4mins 50 seconds

4mins 50s is under the brief requirement however it is very close to the maximum limit of 5 mins.

*unfold secondary mechanism

*unfold secondary mechanism

Summary

P24

• The bridge as a single body is very stiff with estimate 0.77mm of deformation due to the triangular profile. However the folding mechanisms reduce the stiffness (increase deformation by estimate 32mm) due to both stress concentration and clearance on the hinges

- In order to improve the performance of the bridge, keep the number of hinges and parts to minimal

• A large deformation was detected due to displacement of the secondary folding mechanism therefore it was removed from the 2 rear sections of the bridge to reduce deformation – this change does not exceed the size limit of the folded bridge.

- Changes made to the rear end of the bridge (near the balcony) will have a greater impact compare to changes made to the end of the bridge.

• Shock loading force is required to take into account to calculate maximum load applying onto the bridge.

- Applying realistic interaction between user and the bridge to capture changes in force, in order to calculate Factor Of Safety

• The full scale design was designated to have 25mm of material thickness however the bridge perform much better when the material thickness was changed to 3 mm.

- The material thickness should be considered at early stages so that the factor of safety can be minimised.

Not interact with the no-go zone ✓

Made of Aluminium 2024-0 ✓

Under 2.25m when folded ✓ 1.88m when folded

Allow user of 80kg to walk on ✓ The bridge can hold up to 17 users of 80kg

Deploy under 5 mins ✓ 29 seconds to deploy the cardboard model

The dog-leg has a triangular fixture that fit around the end of the bridge to transfer the moment when the user is standing at the end

Reinforcement to support the secondary folding mechanism (4-bar-linkage) to avoid it collapsing during operation

Secondary folding mechanism (4-bar-linkage) to collapse the parts to minimal size – flat pack

Bottle-Cap-Opener alike design used to convert failure mode of the fixture from “shear” to “beam”

Reinforcement to transfer the torsional motion along the bridge

P13 –P15

P12 P17

P18

P9 –P11