design of a stair climbing wheelchair
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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
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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
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7.4 Roller cage 32
8. Conclusions and proposed future works 33
9. References 34
Appendix 35
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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
An overview of existing stair climbing wheelchairs and idea about different stair climbing
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
come up with a relevant design of the climbing mechanism to meet the aim of this project.
Wheelchair climbing device: Patent 5423563, Franklin J. Wild, Lawrence Mich.
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 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
An overview of existing stair climbing wheelchairs and idea about different stair climbing
s relevant to the subject.
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.
lin J. Wild, Lawrence Mich.
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
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.
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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.
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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.
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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
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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)
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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)
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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
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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
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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
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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
IIST Thiruvananthapuram 2011 Page: 19
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.
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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
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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
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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.
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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
IIST Thiruvananthapuram 2011 Page: 24
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
IIST Thiruvananthapuram 2011 Page: 25
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.
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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
IIST Thiruvananthapuram 2011 Page: 27
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
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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.
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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.
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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.
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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.
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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
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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.
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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
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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
IIST Thiruvananthapuram 2011 Page: 36
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
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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
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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.
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