A guideline for the design of a four-wheeled walker.pdf

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Assistive Technology: The Official Journal of RESNA
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A Guideline for the Design of a Four-Wheeled Walker
Joshua Finkel M.A.Sc. a , Geoff Fernie Ph.D. and P.Eng. a & William Cleghorn Ph.D. and
P.Eng. b
a Centre for Studies in Aging, Sunnybrook Health Science Centre, Toronto, Ontario, Canada
b Department of Mechanical and Industrial Engineering, University of Toronto, Toronto,
Ontario, Canada
Available online: 22 Oct 2010
To cite this article: Joshua Finkel M.A.Sc., Geoff Fernie Ph.D. and P.Eng. & William Cleghorn Ph.D. and P.Eng. (1997): A
Guideline for the Design of a Four-Wheeled Walker, Assistive Technology: The Official Journal of RESNA, 9:2, 116-129
To link to this article: http://dx.doi.org/10.1080/10400435.1997.10132303
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DESIGN
Asst Technol1997 ;9:116-129
© 1997 RESNA
A Guideline for the Design of a Four-Wheeled
Walker
*J oshua Finkel, M.A.Sc., *Geoff Fernie, Ph.D., P.Eng., and
tWilliam Cleghorn, Ph.D., P.Eng.
*Centre for Studies in Aging, Sunnybrook Health Science Centre, Toronto, Ontario, Canada
t Department ofMecha nical and Industrial Engineering, University of Toronto, Toronto, Ontario, Canada
More people use assistive technology devices to compensate
for mobility impairments than for any other
general type of impairment. Increasing numbers of
people with mobility or balance problems use walkers
with four wheels. Four-wheeled walkers are often outfitted
with seats to make it possible to travel longer
distances with intermediate resting periods. The dangers
ofsitting on a parked walker are well known. Many
physiotherapists tell walker users to park the walker
against a wall to prevent injury in case the user forgets
to apply the brakes or the brakes fail. To design a safer
walker that can be used for sitting, the demands placed
on it must be measured. With these data, three modes
of walker instability must be considered: first, the
brakes may hold but the wheels may slide along the
ground; second, the entire walker may tip over; and
third, the brakes may fail to hold the wheels in place,
and they may begin to roll. Mathematical models can
be constructed to simulate how different walker designs
will perform. By this process, design improvements
can be made for existing walkers, and future
walker designs can also be proposed.
Key Words: Walker-Rollator-Design-Safety.
About 8% of Canadians have limitations in mobility
(Statistics Canada, 1991), and in the United
States, 1.7 million people use walkers. The most
rapidly growing portion of the market is walkers
with four wheels. In general, these walkers have
two fixed wheels at the rear and two casters at the
front. The rear wheels are typically braked by a cable
system, similar to a bicycle.
Addr ess corres pondence and reprint request s to Dr. Geoff Fernie,
Centre for St udies in Aging, Sunnybrook Health Science Centre,
2075 Bayview Avenu e, Toronto (Ontario) M4N 3M5, Canada.
116
After canes, walkers are the second most widely
used mobility aids, yet little research has been
done toward making them safer. Four-wheeled
walkers perform several functions in addition to
increasing balance and stability. They are often
outfitted with seats so that it is possible to travel
longer distances with intermediate resting periods.
Almost all walkers have adjustable handle
heights, and some have adjustable seat heights.
The average seat height is 58 em (23 in.) but it is
useful to be able to raise or lower the height by
about 5 em (2 in.) in each direction to accommodate
taller and shorter adults.
Walkers must be safe for both walking and sitting.
This study focused on the comfort and safety
of walkers when used for sitting. The loads applied
to a walker were determined by having walker
users sit down and rise from a simulated walker
supported on measuring equipment. Mathematical
models were constructed to predict the effect of
variation in design parameters. By this process ,
design improvements can be suggested for existing
walkers and improved designs can be proposed.
BACKGROUND
Modes of Instability
When a walker user intends to sit on the walker,
the brakes are applied and the user turns around
and sits down (Fig. 1). When the user is ready to
walk again, she rises, turns around, unlocks the
brakes, and continues walking. To examine the
performance of a parked walker, three modes ofinstability
should be considered. The first mode ofinstability
occurs when the brakes hold, but the
wheels slide along the ground. The second mode results
when the entire walker tips over. The third

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FIG. 1. Walking with a walker (Fig. Ia), and sitting on a
walker (Fig. lb),
GUIDE LINE FOR WALKER DESIGN
Braked
wheels
Back \ -
FIG. 2. Distribu tion of subject-applied forces.
Front
mode is brake failure, in which the brakes cannot
hold the wheels and they begin to roll.
Sl iding
When sitting and rising from a parked walker,
the user applies a force F to the walker. A normal
(vertical) force F z is transmitted through all four
wheels , and a shear (horizontal) force F y is transmitted
through th e braked wheels only (Fig. 2).
The normal force acting through the braked
wheel s comprise s a portion of the force applied by
the user and a portion of the walker weight. Ifthe
shear force divided by the total normal force at either
of the braked wheels exceeds the coefficient of
friction between the braked wheel and the ground,
sliding will result.
Tipping
With the braked wheels locked, the walker can
tip in two directions, as seen in Figure 3. The first
is counterclockwise about the contact point of the
braked wheels and the ground, caused by a negative
moment M B' and the second is clockwise about
the axle of the casters, caused by a positive moment
M F " If the user force is applied behind or in
front of the wheelbase and creates enough ofa moment
to overcome the weight ofthe walker, tipping
FIG. 3. The two directions of tipping.
117

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FIG. 6. Normal and shear forces appli ed through plate A and B, resp ectively (Figs. 6a and b), and normal force applied
through both casters (Fig. 6c).
to rest. During this adjustment, the subject could
sit on a stool beside the apparatus. Each of the six
seat positions was tested twice, for a total of 12 trials.
The entire test took 1 hour.
During the rest periods, the subjects were asked
two questions: first, whether the walker would be
more comfortable if the seat were higher or lower,
or whether they were comfortable the way it was;
and second, whether they would be more comfortable
with the walker's handles further toward the
back or toward the front.
GUIDELINE FOR WALKER DESIGN
RESULTS AND DISCUSSION
A typical example of the forces recorded while
one subject sat down and then stood up for one trial
is shown in Figure 6. Figures 6a and b show the
normal and shear forces applied at each of the
braked wheels, and Figure 6c shows the normal
force applied at the two casters.
The responses to the questions regarding the
comfort of the handle and the seat are summarized
in Table 1. Varying the handle position for a fixed
119

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ping and sliding modes of instability. These tables
can be applied to walker designs to evaluate performance
or aid in design decision making.
The data imply that most present-day walkers
are potentially dangerous when used as seats. Existing
models may slide along the ground or tip
backward or forward, causing, at best, anxiety,
and at worst, injury. These results suggest that to
create a safer mobility aid, the seat and center of
gravity should be placed closer to the braked
wheels. This will transmit greater normal forces
through the braked wheels, and less sliding will result.
This study, however, was limited to examination
of the static stability of walkers during sitting/standing
transfers, and the compatibility of
our recommendations with safe dynamic perfor-
APPENDIX A
This appendix presents six sets of design tables.
Each set corresponds to one of six configurations
(seat height and handle position) tested. A total of
four tables are included in each set corresponding
to walkers with wheelbases of 40 em and 50 em,
Wheel base " 40 em
-20 -10 0
sIb
10 20 30 40 50
10 + + 19 + + + + +
20 + + + + + + + +
elb 30 + + + + + + + +
40 + + + + + + + +
50 + + + + + + + +
60 + + + + + + + +
70 + + + + + + + +
Wheel base" 50 em
-20 ·10 0
sIb
10 20 30 40 50
10 + + 18 18 + + + +
20 + + 17 19 + + + +
elb 30 + + 19 + + + + +
40 + 19 + + + + + +
50 + + + + + + + +
60 + + + + + + + +
70 + + + + + + + +
Configuration #1
mance during walking requires confirmation. Rising
was considered only when the user was seated
in the walker. Forces applied through the walker
when using it as an aid in rising from another chair
or the end of a bed were not examined.
Acknowledgments: This research was supported
in part by the Ontario Rehabilitation Technology
Consortium, which is funded by the Ministry of
Health of Ontario, Canada.
REFERENCES
Hall , J ., Clarke, A. K., & Harrison, R. (1990). Guidelines for
prescription of walkin g frames. Physiotherapy, 76, 118-1 20.
Statistics Canada. (1991). Health and activi ty limitation survey
(1991). Ottawa: Statistics Canada.
equipped with low friction ( J..L = 0.4) and high friction
(J..L = 0.8) wheels. Each table shows the minimum
safe walker weights (kg) corresponding to
different locations ofthe seat (sib) and ofthe center
of gravity of the walker (c/b).
Wheel base " 40 em
-20 -'0 0
sIb
10 20 30 40 50
10 + + 16 18 + + + +
20 + + 16 19 + + + +
elb 30 + + 17 + + + + +
40 + 18 18 + + + + +
50 + 16 19 + + + + +
60 + 18 + + + + + +
70 + 19 + + + + + +
Wheel bas e " 50 em
-20 -10 0
sIb
10 20 30 40 50
10 + + 18 13 15 18 + +
20 + + 12 13 16 19 + +
elb 30 + + 11 14 16 19 + +
40 + 19 12 15 18 + + +
50 + 16 13 16 19 + + +
60 + 14 14 18 + + + +
70 + 14 16 19 + + + +
GUIDELINE FOR WALKER DESIGN 123

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e/b
e/b
Configuration #2
Wheel base = 40 em Wheel base = 40 em
sib
-20 ·10 0 10 20 30 40 50
10 12 13 14 15 16 18 19 +
20 13 14 15 16 18 19 + +
30 14 15 16 18 19 + + +
40 15 16 18 19 + + + +
50 16 18 19 + + + + +
60 18 19 + + + + + +
70 19 + + + + + + +
Wheel base = 50 em Wheel base =50 em
sib
-20 ·10 0 10 20 30 40 50
10 18 12 13 14 14 16 18 19
20 11 13 14 14 16 17 19 +
30 13 13 14 16 17 19 + +
40 13 14 16 17 19 + + +
50 14 16 17 19 + + + +
60 16 18 19 + + + + +
70 18 19 + + + + + +
Configuration #3
u =0.4 u =0.8
Wheel base =40 em Wheel base =40 em
20 30 40 50 50
11 13 16 + +
11 14 17 + +
e/b 13 15 18 + e/b +
13 16 19 + +
14 18 + + +
16 19 + + +
18 + + + +
Wheel base =50 em Wheel base =50 em
30 40 50
11 14 16
12 14 18
e/b 13 16 19 e/b
14 17 +
15 19 +
16 + +
19 + +
124 ASSISTlVE TECHNOLOGY, VOL. 9, NO.2

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where a is the acceleration of the wheel. All variables
are known except this acceleration. Isolating
for the acceleration,
a = A y - WAz/w/2g. (8)
For uniform acceleration (a = const.) over a small
interval of time t,
For the case ofthe wheel not contacting the ground
(Fz = 0),
Converging on a Design Parameter
(9)
where d, is an incremental distance slid over a
small interval of time. Assuming V o = 0 over this
short time span with constant acceleration,
r, = w/ zg 'a
a = Fylw/2g'
(10)
(11)
(12)
Once acceleration is determined, the distance can
be calculated. The distance that the mass moves
between each data point can then be summed for
the total slide distance. Dangerous sliding can occur
at either of the braked wheels at two points
during the trial, sitting down and then standing
up. The maximum of these four sliding distances
was set to 5 em. This distance is large enough not
to demand extreme walker designs, yet small
enough not to cause stress to the user while sitting.
The above calculations can be used to find the
relationship between any two variables ifthe other
five are known. For example, if the effect of the
seat position on the required walker mass must be
determined, all other variables are held constant,
and an iterative procedure can be performed for
each walker mass to isolate the seat position that
will allow for the desired maximum sliding distance.
Any seat position behind this value will be
a safe design with respect to sliding, while positions
further to the front will cause excessive sliding.
The bisection method was used to converge on
desired values to within 5%.
Tipping
Ifat any time the applied force vector and walker
weight combine for a resultant force outside of
the wheelbase, the wheels will begin to rise offthe
ground. The only opposing force that will help to
maintain the stability of the walker is the walker
weight. As long as the resultant of the combined
walker weight Wand applied force F are contained
within the wheelbase, tipping will be avoided. Five
ofthe design variables affect tipping: seat position,
wheelbase, walker mass, center of gravity, and
configuration. For example, a larger wheel base reduces
the tipping moment caused by the forces applied
outside of the wheelbase by shortening the
distances that they are acting from the wheels. The
position of the center of gravity of the walker will
control how great a stabilizing moment the walker
weight will cause about the wheels. Moment calculations
were performed to estimate design variables
that would allow for a 2-cm rise of either the
braked wheels or the casters.
Brake Failure
Brake failure can occur only when there is
enough normal force acting through the braked
wheels to avoid sliding. If the shear (horizontal)
force applied to either ofthe braked wheels creates
enough of a shear stress at the brake pad to exceed
the grip of the brake on the tire, the wheels will
roll. As in sliding, small movements of the walker
will go unnoticed by the user. Figure B6 shows the
shear force applied by the user through one braked
wheel over an entire trial. The horizontal line represents
the shear that the brake can withstand
without allowing rolling. The portion of the curve
above this line indicates rolling. The maximum
shear force that allows less than 5 em (2 in.) of rolling
must be found to avoid significant brake failure.
Design variables that will affect brake failure
are the seat-handle configuration and the walker
mass. As the walker mass increases, there is a
greater load to accelerate, resulting in less movement.
Calculations similar to the sliding analysis
were then performed to estimate design variables
that would allow for a 5-cm roll of either of the
braked wheels.
GUIDELINE FOR WALKER DESIGN 129