design of a stair climbing wheelchair

DESIGN OF A WHEELCHAIR CAPABLE OF

CLIMBING STAIRCASES USING ONLY USER

EFFORT

PROJECT REPORT

Submitted by

SUPRATIM NASKAR

FOR THE AWARD OF DEGREE

OF

BACHELOR OF TECHNOLOGY

IN

AEROSPACE ENGINEERING

JUNE 2011

INDIAN INSTITUTE OF SPACE SCIENCE AND

TECHNOLOGY

THIRUVANANTHAPURAM

IIST Thiruvananthapuram 2011 Page: (i)

CERTIFICATE

This is to certify that this project entitled DESIGN OF WHEELCHAIR

CAPABLE OF CLIMBING STAIRCASES USING ONLY USER

EFFORT is a bona fide record of the work done by Supratim Naskar under

our supervision at Indian Institute of Space Science and Technology from 4th

March 2011 to 25th

May 2011, in partial fulfilment of the requirements for the

award of degree of Bachelor of Technology in Aerospace Engineering from

Indian Institute of Space Science and Technology, Thiruvananthapuram.

Dr. K. Kurien Issac

Senior Professor and Head,

Department of Aerospace Engineering

Indian Institute of Space Science and Technology

Place : Thiruvananthapuram

Date : 3rd

June 2011

IIST Thiruvananthapuram 2011 Page: (ii)

ACKNOWLEDGEMENT

The completion of this project and this report owes itself to the invaluable help and support of

Dr. K. Kurien Issac, Senior professor and Head, Department of Aerospace Engineering,

Indian Institute of Space Science and Technology. I would also like to express my gratitude

towards Mr. Thomas Varghese, Manufacturing lab in-charge, Indian Institute of Space

Science and Technology, for his assistance in fabrication and other valuable inputs.

IIST Thiruvananthapuram 2011 Page: (iii)

ABSTRACT

The project aims at designing a wheelchair capable of climbing and descending staircases

using manual effort only. It also aims at fabrication of different components and sub

assemblies that makes realising the project completely. This is a problem that has been

addressed with over the years and hence an extensive literature survey of different patents

and ideas have been done to evaluate the existing solutions.

The mechanism designed in this project is inspired by an idea which has been patented under

the name of G.H. Green. G.H. Green’s wheelchair uses a ingenious mechanism to climb by

making two arms move along a trajectory similar to that of a foot. He uses a very complex

mechanism driven by a motor to accomplish this. One of the important innovations in our

work is to simplify the realisation of foot like motions by decoupling the normal and

tangential motions and using two separate actuators for each motion. The new conceptual

elements that has been proposed to improvise on this idea are mainly to simplify the

mechanism and make it work using human effort only. The conceptual design, preliminary

design, sizing of different components have been done and explained in this report.

Fabrication of some of the components which have actually been made in the manufacturing

lab has also been detailed out in the report along with fabrication drawings. The report also

contains a list of specifications and constraints that we had to keep in mind while designing.

Because of lack of time the entire fabrication and retrofitting of the chair with the mechanism

was not possible, hence suggestions for future works which will help realising this project has

been included as a part of this report. Certain idea of the design of the wheelchair as a whole

to overcome certain shortcomings has also been included.

IIST Thiruvananthapuram 2011 Page: (iv)

LIST OF FIGURES

Figure 1: fully retrofitted chair when mechanism has not been deployed

Figure 2: when mechanism deployed and chair is climbing

Figure 3: intermediate position when the mechanism is in a process of being deployed

Figure 4: shows the force analysis

Figure 5: side view of the fully retrofitted chair

Figure 6: refer appendix

Figure 7: refer appendix

Figure 8: deploying

Figure 9: climbing

Figure 10: retraction

Figure 11: Front view of the wheelchair

Figure 12: Side view of the wheelchair

Figure 13: rear view of the wheelchair

Figure 14: Chair dimensions

Figure 15: Schematic diagram of the assembled plate

Figure 16: Schematic diagram of the rack and pinion

Figure 17: drive of the pinion

Figure 18: support plate is on the ground

Figure 19: inner plate is on the ground

Figure 20: schematic diagram of the mechanism

Figure 21: schematic diagram of the wheelchair when retrofitted

Figure 22: Side view of the wheelchair when retrofitted

Figure 23: raising the wheelchair

Figure 24: sliding up the staircase

Figure 25: returning to initial configuration

Figure 26: front view of the slider assembly

Figure 27: side view of the slider assembly

Figure 28: Movement of the center of gravity along the length of the slider channel

Figure 29: Front view of the slider assembly showing the dimension of the rollers

Figure 30: Intermediate column

Figure 31: Side 1 of inter mediate member

Figure 32: Side 2 of intermediate member

IIST Thiruvananthapuram 2011 Page: (v)

Figure 33: Sideview of the inner plate

Figure 34: Topview of the inner plate

Figure 35: movement of center of gravity along the length of the inner channel

Figure 36: showing the drive for sliding the chair

Figure 40: manufacturing drawing of a rollerFour bar lever arm

Figure 38: Gear arrangement

Figure 39: Column cross section

Figure 40: manufacturing drawing of roller

Figure 41: fabricated roller side view

Figure 42: fabricated roller top view

Figure 43: manufacturing drawing of the channel

Figure 44: Fabricated side view

Figure 45: fabricated front view

Figure 46: fabricated top view

Figure 47: Manufacturing drawing of the plate

Figure 48: Fabricated side view

Figure 49: fabricated top view

Figure 50: fabrication drawing of the cage

IIST Thiruvananthapuram 2011 Page: (vi)

TABLE OF CONTENTS

Acknowledgement.................................................................................................................(ii)

Abstract................................................................................................................................(iii)

List of Figures......................................................................................................................(iv)

Ch. No. Title Page

Chapter 1: Introduction and problem addressed 9

Chapter 2: Literature Survey 10

2.1 Wheelchair climbing device: Ref [1] 10

2.2 Stair climbing wheelchair: Ref [2] 12

Chapter 3: Requirements and Constraints 14

3.1 Functional Requirements 14

3.2 Constraints 14

3.3 Parameter values 14

Chapter 4: Preliminary ideas 16

4.1 Rack and pinion 16

4.2 Decoupling the movement of the wheelchair 17

4.3 Working of mechanism 17

Chapter 5: Preliminary design and sizing 20

5.1 Sliding connection for horizontal movement 20

5.2 Intermediate plate linking the horizontal and vertical movement 22

5.3 The inner plate 22

5.4 The drive for horizontal movement (lever arm 1) 23

5.5 The gear system 24

5.6 The drive for vertical movement (lever arm 2) 25

Chapter 6: Detailed design 26

6.1 Gear tooth design 26

6.2 Shaft design 27

6.3 Column Cross section 27

Chapter 7: Fabrication 29

7.1 Rollers 29

7.2 Slider channel 30

7.3 Connecting plate 31

IIST Thiruvananthapuram 2011 Page: (vii)

7.4 Roller cage 32

8. Conclusions and proposed future works 33

9. References 34

Appendix 35

IIST Thiruvananthapuram 2011 Page: 8

Chapter 1

Introduction and problem addressed

According to data from the National Health Interview Survey on Disability (NHIS-D) in the

year 1994-95, roughly 0.4% of the adult population in the world between the age of 18 and

65 use wheelchairs. Unfortunately in India awareness regarding welfare of people in

wheelchairs is not very high resulting in most buildings not being designed for accessibility

by people using wheel chairs. Hence the problem of making existing buildings accessible to

the given demographic, is an important one.

The aim of this project is to design a wheelchair capable of climbing and descending

staircases, with the user being able to climb without any external help or support. The project

also aims at retrofitting an existing wheelchair and demonstrating the working and basic

design elements of the climbing mechanism.

IIST Thiruvananthapuram

An overview of existing stair climbing wheelchairs and idea about different stair climbing

mechanisms involved were available from a number of patent paper

In this section a few such patented mechanisms are discussed, which are being referred to

come up with a relevant design of the climbing mechanism to meet the aim of this project.

2.1 Wheelchair climbing device: Patent 5423563, Frank

Main feature of this mechanism as can be seen from the figure below is the use of two pairs

of spider wheels. Each pair is retrofitted on each side of the wheelchair. Below is given a side

view of the wheelchair. Another feature

staircase is given by rotating the rear wheel of the wheelchair.

Figure 1: fully retrofitted chair when mechanism has not been deployed

The above figure shows the wheelchair

when the wheelchair is moving on plane ground.

2011

Chapter 2

Literature Survey


mechanisms involved were available from a number of patent papers relevant to the subject.

a few such patented mechanisms are discussed, which are being referred to


Wheelchair climbing device: Patent 5423563, Franklin J. Wild, Lawrence Mich.



view of the wheelchair. Another feature is the force required to climb up or descend down the

staircase is given by rotating the rear wheel of the wheelchair.

: fully retrofitted chair when mechanism has not been deployed

The above figure shows the wheelchair with the retrofitted mechanism in normal position i.e.

when the wheelchair is moving on plane ground.

Page: 9


s relevant to the subject.

a few such patented mechanisms are discussed, which are being referred to


lin J. Wild, Lawrence Mich.



is the force required to climb up or descend down the

: fully retrofitted chair when mechanism has not been deployed

with the retrofitted mechanism in normal position i.e.


Figure 2: when mechanism deployed and chair is climbing

Figure 3: intermediate position when the mechanism is in a process of being deployed

Figure 4: shows the force analysis

Figure 4 shows how the horizontal driving force F1 is generated. As can be seen from the

arrangement in figure 2 the axle of the rear wheel (A), the spider wheels (B and C) are on the

same frame. So the frame is basically the triangle ABC (has been shaded). The rotation of the

axle (shown by a red arrow) provides a force on the belt (shown by a blue arrow). This force

on the belt is transferred to the frame and develops forces F1 and F2 as shown in the figure.


2.1.1 Advantages

This arrangement can be attached to a wheelchair independently on both sides of the rear

wheel. Because of the grooved axle which can be used as a gear, the effort used by the

occupant can be reduced by increasing the gear ratio. Hand driven, no need to fit a motor.

Speed of climbing can thus be controlled easily by the occupant.

2.1.2 Disadvantages

The belt might wear off from friction and also use of hydraulic system is a cumbersome

affair.

2.2 Stair climbing wheelchair – G.H.Green, Patent-3,142,351

The main feature is that the chair basically climbs a set of stair case with the help of ellipsoid

motion of a walking support arm.

Figure 5: side view of the fully retrofitted chair

The walking arm is shown by the part 120 (coloured orange) and the support arm is shown by

the parts 87 and 88 (coloured blue). The climbing happens when force is exerted by the

protruding triangular platforms attached to the walking arm and is shown as part number 121.

These protrusions exerts force on the step of the staircase, this force is produced because the

walking arm is always in ellipsoidal motion with respect to the frame of the chair while the

support arm is steady and is attached to the chair frame. This relative motion between the

walking arm and the support arm takes place with the help of the connector between the

support arm and the walking arm. This connector is coloured as dark brown in the above

figure.


Figure 6: refer appendix Figure 7: refer appendix

In figure 6 the coloured parts show the deployment of the mechanism and the way by

which the support arm is aligned along the slope of the staircase. In figure 7 the coloured

parts shows the engaging of the motor with the driving gear while deploying the climbing

mechanism. (Ref Appendix).

Figure 8: deploying Figure 9: climbing Figure 10: retracting

Figure 8 shows the chair in the process of deploying the mechanism just before hitting the

staircase. Figure 9 shows the chair while climbing the staircase, it shows the configuration of

the chair during one of the power strokes in the mechanism. Figure 10 shows the mechanism

being retracted when the chair has already climbed up and is on level ground.

2.2.1 Advantage

The entire climbing is done through motor power, very less human effort is required.

2.2.2 Disadvantages

A pretty complex gear arrangement to carry out the entire climbing of the chair. The chair

configuration is reversed i.e. small wheel at the rear and large wheel at the front. Hydraulics

is used. The entire arrangement adds a large weight to the system


Chapter 3

Requirements and Constraints

3.1 Functional Requirements

a) The wheelchair must be capable of climbing and descending straight staircases of typical

dimensions using manual effort by the user, without any external help or support.

b) The climbing and descending mechanism must be capable of deployment and retraction

within a short span of time. The deployment and retraction must be done by the user.

c) It must be possible to retrofit a standard wheelchair with the attachment and remove it

when not in use.

d) The wheelchair must be capable of carrying a user up to the weight of 80kg.

e) The wheelchair must be durable and must have a sufficiently large life span. .

f) The attachment must not be aesthetically unpleasant.

3.2 Constraints

a) Staircase width should be at least 1.5 times that of the wheelchair width.

b) Constraints on how much effort an average human can impart will determine whether a

motor is required or not to run the climbing mechanism

c) The wheelchair must be stable during climbing and the user must be adequately secure in

his position.

d) The cost of the attachment must be moderate in order to be competitive in the market.

3.3 Parameter values

Wheelchair and typical dimensions

Figure 11: Front view of the wheelchair Figure 12: side view of the wheelchair Figure 13: rear view of the wheelchair

Weight – 30lbs (approximately 15 Kgs); Load taking capacity – 300lbs ( approx 150kgs)


Figure 14: chair dimensions

Height: 34 inches; Width: 25 inches; Breadth: 33 inches; Main Wheel diameter: 23 inches;

Rim diameter: 19 inches; Support Wheel diameter: 8 inches; Height of seat from ground:

19.5 inches; Width when folded: 10 inches.

Staircase

Slope 26.5° (approximately); Tread width = 2 x Rise height

Human arm strength

The upper arm flexor strength – 6.3 Kg/cm2; The forearm – 4.7 Kg/cm2 (On an average the

strength ranges from 4 Kg/cm2 to 8 Kg/cm2)


Chapter 4

Preliminary ideas

Based on the literature survey and considering every kind of design, the walking arm

mechanism that has been discussed above seemed to be the most simple and intriguing.

The major problem was the complexity of the entire design as discussed in the patent

involving a number of gears. We had to come up with a few ideas to simplify the design and

hence simplify the fabrication of the whole thing. The following are a few ideas.

• Rack and pinion

• Decoupling the movement of the wheelchair

4.1 Rack and pinion

This approach involves a rack and pinion mechanism as discussed below.

Figure 15: schematic diagram of the assembled plate Figure 16: Schematic diagram of the rack and pinion

Figure 13 shows the entire arrangement. Rod 1 is the transmission rod connecting gear wheel

1 and gear wheel 2 (figure 12). So as in figure 14 when gear wheel 2 moves along the rack

(due to rotation of gear wheel 1), plate 1 follows a similar trajectory as gear wheel 2, with

respect to plate 2. The main idea of the mechanism is when gear wheel 2 reaches the lower

part of the rack then plate 1 comes in contact with the ground and as gearwheel 2 moves

along the lower part of the rack plate 2 is lifted up from its initial position and moved forward

with respect to plate 1. This happens because when gear wheel 2 is on the upper part of the

rack the entire weight of the wheelchair is on plate 2 and when gear wheel 2 moves along the

lower part of the rack the entire weight of the wheelchair gets transferred to plate 1, because

then plate 1 is in contact with the ground.

Figure 17: drive of the pinion Figure 18: support plate is on the ground Figure 19: inner plate is on the ground


Figures 17 and 18 shows the climbing mechanism. In figure 17 the weight is supported by

plate 2 when gear wheel 2 is moving on the upper part of the rack and in figure 18 the weight

is supported by plate 1 when gear wheel 2 is moving along the lower part of the rack. Figure

15 shows gear wheel 1 is driven by rotating the rear wheel of the wheel chair. The force

applied by the occupant to rotate the rear wheel is transmitted through the transmission chain

connecting the axle of the rear wheel (gear wheel 0 in figure 14) and gear wheel 1. Thus the

effort of the occupant can be reduced by using the right gear ratio between the gears and the

transmission chain shown in figure 15.

The idea is such that the wheel chair will be rigidly connected to plate 1 at all times during

climbing.

4.2 Decoupling the movement of the wheelchair

The idea is to de-couple the motion of the chair while moving up the staircase into two. The

following figure will illustrate the idea.

Figure 20: schematic diagram of the mechanism

In the above figure part 3 is directly attached to the wheelchair. The cam shown in the above

figure can move part 3 vertically up and down with respect to part 2 (inner slide). Thus the

wheelchair that is directly attached to part 3 can be raised from the ground by this cam action.

Now the inner slide can move to and fro in the horizontal position with respect to part 1

(outer slide). Only vertically up and down movement is allowed between part 3 and part 2

and only horizontal movement is allowed between part 1 and part 2. Thus combining these


movements, the chair can be made to climb up a staircase. The following figure shows the

entire setup of the above idea.

Figure 21: schematic diagram of the wheelchair when retrofitted

In this report we have tried to make an elaborate design of decoupling the movements as

discussed above. The following section will show the preliminary design done for realising

this mechanism.

4.3 Working of the mechanism

The figure below shows how this mechanism is designed to make the wheelchair climb up a

staircase.

Figure 22: Side view of the wheelchair when retrofitted


Step 1

Figure 23: raising the wheelchair

Lever arm 2 is pulled up. This raises the wheelchair from the staircase. The raising happens

as the bearings forces the protrusions on the inner plate to move up. The inner plate can easily

move vertically up along the vertical prismatic joint between the intermediate plate and the

inner plate. The wheelchair is attached to the inner plate directly, hence as the inner plate

moves up the wheelchair also moves up. When the chair has completely moved up lever arm

2 is locked at that position so that the chair is locked in that elevated position. In this

configuration the slider column is on the staircase and is supporting the entire weight of the

system.

Step 2

Figure 24: sliding up the staircase

Now when the wheelchair is locked in the lifted position it can only slide along the slope of

the staircase through the slider column. As lever arm 1 is pushed forward, gear 1 rotates in a

clockwise direction, forcing gear 2 to rotate in an anti-clockwise direction. The link

connecting gear 2 and the slider column is fixed with gear 2 and is pivoted at the center of

gear 2. So as gear 2 rotates anti-clockwise, the link also tries to rotate anti-clockwise, but the


slider column on the other end of the link is supported on the stairs and cannot move. This

provides a reaction force on the shaft of gear 2. Due to this reaction force the chair slides up

along the slider column. For the movement of the chair along a straight line there is a slot at

the slider column end of the link, this will be discussed later. When the chair has moved

along the slope of the staircase lever arm 1 is locked at that position and the chair stays there.

Step 3

Figure 25: returning to initial configuration

We unlock lever arm 2 and push it down, so that the inner plate carrying the chair comes

down on the staircase pushing the slider column to move up along the vertical prismatic joint.

Thus the chair attached to the inner plate now rests on the stairs and the slider column is free

to slide up. We pull lever arm 1 up to the initial position and this makes the slider to move up

the slope of the staircase and the initial configuration is restored as shown in the above figure.


Chapter 5

Preliminary design and sizing

We had to come up with a number of ideas and designs for the components due to which the

horizontal and vertical movements of the chair will happen and also driving this components

to carry the chair up the staircase.

• Sliding connection for the horizontal movement

• Intermediate plate linking the horizontal and vertical movements

• The inner plate

• The drive for the horizontal movement

• The gear system

• The drive for the vertical movement.

5.1 Sliding connection for horizontal movement

The outer plate of the mechanism is a slider. The slider is made with a column sliding inside

another column with the help of rollers. A column referred to as the slide or the inner column

in Figure 25 moves inside the slider channel. This relative movement between the columns

due to the rolling of the rollers between the columns. The following figures show the

assembly of entire assembly

Figure 26: front view of the slider assembly

Figure 27: side view of the slider assembly


The connecting plate as shown in the above figure connects the slide with the intermediate

column. Thus the intermediate plate can move to and fro only horizontally with respect to the

slider column.

The function of the slider column is to act as a ground support forming a support pattern on

the staircase as the wheelchair will slide up on it. The one and only sizing criteria in this case

is stability. The center of gravity of the wheelchair should at all time stay within the support

pattern formed by the slider column. The length between the staircase is 335.4 mm along the

slope. Therefore to keep the center of gravity within the support pattern it should always be at

least 335.4 mm from both ends of the slider column. Therefore ideally keeping the center of

gravity at the center of the slider column the length of it would be 335.4 x 2 mm =670.8 mm

Figure 28: movement of Center of gravity along the length of the slider channel

Considering in one drive of the lever arm the chair can slide up by a distance of 200mm, then

from the following figures the size the slider column is 670.8 + 200 = 870.8 mm. For a safety

margin the length has been taken to be 950 mm

The rollers are sized such that to fit between

the slider column and the slide and can roll

without any friction from the inner wall of

the slider column. The rollers are basically

solid stainless steel cylinders which can

easily take on the entire weight coming on it

with the diameter provided to them. Figure

below shows a roller and its assembly within

the slider.

Figure 29: Front view of the slider assembly showing the

dimension of rollers


5.2 Intermediate plate linking the horizontal and vertical movement

The intermediate column is connected to the horizontal slide, as discussed above, on one face

and on the other face the intermediate column is fixed with a pair of prismatic sliders that can

move up and down vertically. The figures below show both faces of the intermediate column.

Figure 30: intermediate column Figure 31: side 1 of intermediate membe Figure 32: side 2 of intermediate member

The pair of vertically moving sliders on the intermediate column as shown in the above

figures are the connecting joints between the intermediate plate and the inner plate. Thus the

relative movement between the intermediate plate and the inner is restricted to vertically up

and down strokes only.

Hence combination of the relative horizontal movement between the slider column and the

intermediate plate and the relative vertically up and down movement between the

intermediate column and the inner plate provides the entire movement of the wheelchair up

the stair case.

5.3 The inner plate

The inner column is attached directly to the chair on one face and to the prismatic

connections with the intermediate column on the other face. Hence any movement of the

inner column describes the movement of the chair.


Figure 33: side view inner plate Figure 34: top view inner plate

The above figure shows the inner plate and its connection with the intermediate plate.

The main sizing criterion for the inner plate is maintaining the stability of the wheelchair

when the inner plate acts as the ground support and the wheelchair is at rest. During this time

the slider column moves up the staircase changing the center of gravity of the entire system.

The following figures will explain the scenario. To keep the center of gravity within the

support, as discussed in sizing the slider column, the minimum length of the inner plate

should be 670.8 mm, considering the center of gravity is at the center of the inner plate.

Figure 35: movement of center of gravity along the length of the inner channel

From the above figure the length of the inner plate should be 700.8 mm considering that the

movement of the support plate will change the center of gravity by 30mm. For a margin of

safety the inner plate length is taken to be 750mm.

5.4 The drive for horizontal movement (lever arm 1): The lever arm 1 is connected to

gears 1 of the gear arrangement and is pivoted at the axle of the main wheel. The link

between gear 2 and the slider column that transfers the force given by the lever arm to the

slide has a slot at the slider column end. This is to make the chair slide up in a straight line


Figure 36: showing the drive for sliding the chair

The length of the lever arm decides the force required to be exerted by each hand to move the

wheelchair up the slope of the staircase. Taking a limit of 100N force per hand, and the

dimensions of the chair the lever arm has been sized. The link length between the slider

column and the gear arrangement is kept as 220 mm. Weight acting on both the slides inside

the slider columns on both sides when they rests on staircase = �� sin � = 1000 sin 26.5° =

450 �. This is the total weight acting on both sides, hence weight acting on the slide on one

side =��

�= 225 �. Therefore torque acting at the shaft of gear 2 = 225 × .22 = 49.5 ��.

Referring to the gear sizing section below the gears considered for the design have 100 mm

diameter. Hence the tangential force acting at the point of contact =��.�

.��= 990 �. Now

torque acting on the shaft of gear 1 = 990 × 0.05 = 49.5 ��.

Considering a lever arm of length 600 mm pivoted at the shaft of gear 1 to drive the entire

gear arrangement and hence move the slider, therefore the force exerted by one arm at the tip

of this lever arm should produce a torque of 49.5 Nm. Hence force exerted by human arm at

one end of this lever arm is F =��.�

.�= 82.5 �. Therefore the lever arm length is taken to be

600 mm.

5.5 The gear system: The gears are installed so that reversal of movement can be obtained.

That is when we push lever arm forward the chair climbs up. This requires less effort.The

gear system is installed for reversal of movement of the slider with respect to lever arm 1.

This reversal of movement is necessary because pushing lever arm 1 and sliding up the


staircase requires less effort compared to pulling the lever arm and sliding up the staircase.

As we need the gears only for reversal of movement we don’t need a gear ratio other than 1.

The gear diameter is chosen such that the link length between gear 2 and the slider channel is

optimum in way that the force coming on the teeth is not large and the stresses developed can

be easily taken up by the gear teeth. Suppose if the radius is small then due to the torque

coming on the gear axle and small radius the gear teeth will experience a large force and if

the radius of the gear is large enough then the radial load coming on the shaft will be large.

After several iterations we have concluded that the gears should have a pitch radius of 50 mm

5.6 The drive for vertical movement (lever arm 2): A four bar mechanism as shown in the

figure below is used to push up the inner plate with respect to the intermediate plate. As we

rotate the lever arm in anti-clockwise direction the bearings A and B act as a cam and pushes

up the protrusions on the inner plate and due to this the inner plate moves up along the

vertical prismatic joint and hence moves vertically upward with respect to the intermediate

plate.

Figure 37: Four bar lever arm

In the above figure we have used columns at some parts to make the four bar set up as light as

possible.

Lever arm 2 acts as a simple effort arm pivoted at its end and the load is coming in between

the point of action of the effort and the pivot point. As we pull the lever up the chair rises.

Considering the load arm length to be 170 mm and the effort arm length to be 900 mm, hence

effort required = ��.��×��

��= 139 �.

This is the total weight of the wheelchair on both sides and hence on one hand the effort

required is 0.5 x 139N = 69.5 N. This is allowable effort as per human strength is concerned.

Hence the length of lever arm is taken to be 900mm.


Chapter 6

Detailed design

6.1 Gear tooth design

As discussed before the gear ratio

required is 1 because we are installing

this gear arrangement for the reversal

of the relative movement between the

lever arm 1 and the slide inside the

slider column. The pwer transmission

by the gears has been calculated and

the design has been done as follows.

Torque required to be transmitted � ! = 49.5 ��

Assumed rotational speed

�"! = .314 #$%&/&

Hence, power required = × " = 49.5 × .314 = 15.54 (

From a standard power vs module curve, we choose

Module � = 2

Diametral pitch )* = 12

Now 8 )*+ < -$./ 01%2ℎ �14 14.ℎ/&! < 16

)*+

Therefore nominal face width = ��

56= 1 14.ℎ = 25 ��

Considering the diameter of the gears to be 100 mm and considering a standard pressure

angle of 20°,

The tangential force acting at the teeth of the gears (7 =��.�

.��= 990 �

Radial force at the point of contact of the gears (8 = 990 tan 20° = 360.33 �

Teeth depth is taken to be 10mm

Bending stress at the base of each teeth due tangential force is, ; =<=

>

Where, M = moment acting at the base = 990 × 5 = 4950 ��

. = Mean distance = 1.7 ��

@ = Area moment of Inertia =�

��A2� =

�

��× 25 × 3.4� = 81.88 ��

Therefore bending stress ; =��×�.B

��.��= 102 C)$

Figure 38: Gear arrangement


Now allowable bending stress is 250 MPa, hence the gear will not fail.

6.2 Shaft design

The shaft has been attached to the frame of the chair parallel to the main wheel shaft as has

been shown in the figure above. This shaft carries gear 2 of the gear arrangement. The

designing has been done as follows.

Depending on the availability of the shaft we got a hollow cylinder for of 25 mm of thickness

2mm. Therefore the inner diameter of the shaft is 23 mm

Now area moment of inertia is @D = E

��25� − 23�! = 10870.36 ��

The shaft length is considered to be 15mm. Hence maximum moment is acting at the end of

the shaft where it is directly connected to the frame of the chair.

Force coming on the shaft because of which it will bend is 360.33 N

Therefore moment M = 360.33 x 15 = 5404.95 N-mm

Hence stress acting due to bending ; =<=

>=

��.��×��.�

��B�.��= 6.215 C)$

Force producing the torque is 990 N at a distance of 62.5 mm

Hence torque acting T = 990 x 62.5 = 61875 N-mm

Therefore shear force due to torsion is = G=

>=

��B�×��.�

��B�.��= 71.151 C)$

Considering the allowable stress to be 200 MPa, therefore shaft is fail proof.

6.3 Column cross section

The columns that have been used in the mechanism mainly

experience bending stresses when one of the members is

supporting the entire weight of the wheelchair. From the

available sizes and considering the dimensions we can

actually use to easily retrofit with the wheelchair we have

chosen box sections of cross-sectional dimension 62mm x

39mm and thickness of 1.2 mm. Testing these box sections

were done mainly experimentally although analysis of this

columns is shown below.

Weight coming on the column on one side is 500 N

Figure 39: column cross section


Moment acting due to the force when acting at the mid-point of the part of the column when

between two stairs is maximum.

Therefore M = 500 ×��

�= 83750 ��

Area moment of inertia I = 113788 ��

Mean distance c = 31mm

Stress ; = <=

>=

��B��×��

��B��= 22 C)$

Considering allowable bending stress for aluminium is 100 MPa, therefore this is fail proof.


Chapter 7

Fabrication

7.1 Rollers

The rollers are made out of a stainless steel rod. The shape of the rollers is decided such that

wire can be wound around the rollers without interfering smooth rolling. Steel wires are used

to make the roller cage as will be discussed later. The fabrication of these rollers has been

done by cutting them into small pieces of required length and turning them in Lathe. The

cutting of the rod is also done in Lathe using grooving tool of groove width 3mm. Thus for

every small piece cut from the rod some material corresponding to 3mm length was lost.

Hence from a rod of length 600mm we could make 10 rollers of length 34 mm. The

manufacturing drawing of a roller is as follows

Figure 40: manufacturing drawing of a roller

The groove on the roller is made to wind wire and connect to other rollers to make the cage.

The groove length is taken to be 2.5 mm considering that the grooving tool available had a

minimum width of 2.5 mm. The outer diameter after fabrication was less than 8mm, was

precisely 7.75mm; this is because the rod from which the roller are made was 8mm in

diameter and had a rough surface. Hence for a smooth finish to facilitate rolling properly we

had to remove .25mm material by turning the rod. The roller fabricated is shown in the

picture below.


Figure 41: fabricated roller sideview Figure 42: fabricated roller topview

7.2 Slider channel

The slider channel has been fabricated by removing a strip of material from one face of a

62mm x 39mm box section column of thickness 1.2mm. This is done in a milling machine.

The length of the slider channel is 950mm which is larger than the maximum movement of

the bed of the milling machine. Hence the cutting could not be done continuously, we had to

put the column at different positions on the bed and clamp it to cut different lengths. The

manufacturing drawing of the slider column is given below.

Figure 43: manufacturing drawing of the channel

As from the above figure, looking at the sectional view, leaving out 9 mm from above and 14

mm from below the remaining portion i.e. a strip of width 39 mm has been removed. A

picture of the fabricated slider channel is shown below.


Figure 44: fabricated side view Figure 45: fabricated front view Figure 46: fabricated top view

7.3 Connecting plate

The connecting plate is made from a 100mm thick aluminium plate. A portion matching to

the dimensions of the connecting plate was cut from the entire aluminium plate using a

mechanical power saw. Then the cut portion was surface finished at the sides in the milling

machine.

Two holes of required diameter of at least 9.5mm were to be drilled. But the next best size of

the drill bit available had the outer diameter of 10 mm. Hence that has been used and a pair of

10 mm diameter holes was drilled on the connecting plate using the drilling machine. These

holes are required to bolt the connecting plate with the slide and the intermediate column.

The manufacturing drawing of the connecting plate is shown below.

Figure 47: manufacturing drawing of the plate

Bolts of outer diameter 9.2 mm are used to bolt the connecting plate with the slide and the

intermediate column. A picture of the fabricated connecting plate is given below.


Figure 48: fabricated side view Figure 49: fabricated top view

7.4 Roller cage

The cage is made by winding wires in the groove of the rollers. The diagram below shows a

part of the fabricated cage.

Figure 50: fabrication drawing of the cage


Chapter 8

Conclusions and proposed future work

The design of the wheelchair was success fully done to meet the aim of the project. The

realisation of the wheelchair was not possible because of time constraints. All the required

components and sub assemblies could not be fabricated due to time constraints and hence the

assembled wheelchair cannot be demonstrated. The entire assembly of the chair can be done

by fabricating the remaining components and assembling them as per the design.

One major short coming is the retraction and deployment of the mechanism. So far as the

current design goes we have thought of rigidly attaching the wheelchair with the inner plate

of the mechanism by clamping and then driving the mechanism to climb up the stair case or

descend down the staircase. But in practical scenario directly attaching the wheelchair with

the inner plate is not a valid option because that voids the capacity of the wheelchair to move

on plane level ground. Hence one major area of work that has to be done to realise this

project in the future is designing the deployment and retraction of the mechanism. One

conceptual design so far regarding the deployment and retraction is as follows. We can

rigidly connect the entire mechanism to a lever arm that can be pivoted at a convenient point

on the chair frame. By rotating this lever arm we can bring the mechanism in contact of the

staircase and hence deployed. But this idea can be possible if it can be detailed out. Finally it

is necessary to have a full prototype testing and get consent from the members of the

wheelchair using community.


9. References

[1] Patent no. 5,423,563, ‘Wheelchair having apparatus for climbing stairs’ by Franklin J.

Wild, Lawrence Mich, 1995

[2] Patent no.3,142,351, ‘Stair climbing wheelchair’ by G.H.Green, 1964

[3] ‘Machine Elements in Mechanical Design: Fourth Edition’ by Robert L. Mott, Pearson

publications


Appendix

Details of the walking arm Wheelchair as per the patent by

G.H.Green

Figure 6: The deployment of the support arm and the mechanism involved in the ellipsoid

movement of the walking arm.

Figure 51

The ellipsoidal movement of the walking arm takes place due to the combination of a gear

meshing with the inner grooves of a slot (part 139) and the rotation of the cam given by part

133 in the above figure. The gear wheel (coloured yellow) meshes with the grooves on the

inner wall of the slot given by part number 139 (coloured light brown). As the gear wheel

meshes along the inner contour of the slot 139 ( which is ellipsoidal) the walking arm tend to

move accordingly. This produces a to and fro motion of the walking arm and hence the

wheelchair. This happens because the gear wheel is fixed to the frame of the wheelchair. The


movement of the cam i.e. part 133 produces the vertically up and down movement of the

walking arm, due to the shape of the cam. These two movements go hand in hand and the

combined result of this is the ellipsoid movement of the walking arm . Ellipsoid majorly

because of the shape of the cam. The degree of freedom for such a movement of the walking

arm provided by the link with double pin joint given by part 122 (coloured dark brown). This

link actually connects the walking arm with the support arm (coloured light blue) and thus as

the gear wheel meshes with the inner grooves of the slot and the cam rotates, producing the

required movement, the walking arm moves with respect to the support arm and hence frame

of the chair, as the support arm and the chair is rigidly connected at all times.

Figure 7: The alignment of the support arm and the engaging of the motor with the main gear

arrangement.

Figure 52


In the above figure the coloured part shows the mechanism to align the support arm as

discussed in the previous figure. Under normal conditions the support arm is segmented and

retractable. When the lever marked as part 103 (coloured green) is pushed down, to a lower

slot in the triangular plate marked as 113 (coloured violet), then the part 104 (coloured green)

moves down and hence pushes the entire setup below along with the spring (this setup is

shown as 104a and the spring is shown as 116). This spring and rod setup pushes the plate

117(coloured light blue) down. When the lever 103 is pushed down far enough then the plate

117 gets locked by the link 118(coloured light blue). This link 118 is between the lower

retractable part of the support arm given by part 119 and the movable plate 117. Once the

plate 117 is locked with the lower part of the support arm i.e. part 119 then on further pushing

lever 113, part 119 goes down till it gets aligned with the rest of the support arm. In this event

of alignment the large wheel of the wheelchair gets raised from the ground. But in such a

situation when the large wheel is not in contact with the ground and only part 119 is in touch

with the ground the wheel chair becomes unstable, hence we need push down the small

wheels of the wheelchair as well to get in touch with the ground to make the entire

wheelchair stable at this point. The mechanism to

push the small wheels down will be explained later.

In the above figure part 110 shows the motor. Part 109

is a gear wheel that has been attached to one end of

the part 100 (coloured yellow). On the other end of

the part 100 there is a slot 108 in which the link 106

(coloured green) can rotate. The link 106 connect part

100 and part 104. Now when the lever 103 is pushed

down, the gear wheel 109 which is driven by the

motor 110 comes in contact with the entire stair

climbing gear system given by 111. Thus the motor

gets engaged with the stair climbing gear set up and

hence the walking arm starts moving in an ellipsoidal

fashion as explained earlier.

The above figure shows the mechanism to pivot down

the small wheels of the wheelchair to keep in touch with the ground. A hydraulic system is

attached to the frame of the wheelchair along the part 41. The hydraulic extends part 85 and

Figure 53


forces the part 82 to pivot about 83. Thus the wheel 81 comes down and touches the ground.

As the hydraulic extends part 85, part 90 is also pulled down as a result part 89 pushes part 88

to align with the support arm 87.

Figure 54

As previously stated when lever 103 is pushed down sufficiently, the part 100 engages the

motor through the gear attached to 100 with the rest of the climbing mechanism i.e. part 111.

The above figure shows the gear arrangement of part 111. The motor drives gear 109 which

engages with the large gear 126 as shown above. Gear 127 and 126 are mounted on the same

axle 128. Hence the drive from gear 126 is transferred to 127 directly. 127 drives gear 131a

which shares the same axle as 131b. 131b drives gear 132. Now cam 133 and gear 132 shares

the same axle. Thus rotation of gear 132 makes the cam to rotate. As from the figure above,

cam 133 is always in touch with the part 142. Due to the shape of the cam, when the cam

rotates for a certain part of the rotation it pushes down part 142 which in turn lifts up part 141

as can be seen from the figure, and for the remaining part of the rotation the cam does not

push part 142 and allows it to come back to its original position which in turn lowers part

141. This lifting and lowering of part 141 produces the upward and downward movement of

the walking arm. Now gear 127 drives gear 134 which shares the same axle as gear 134a.

This gear 134a drives gear 135 which engages with a small gear 136. Gear 136 drives gear

137 which meshes with the inner grooves of the slot 139.

design of a stair climbing wheelchair

Documents