PROSTHETIC ARM FORCE REDUCER
Dan Cole, Jay Duffy, Greg Harvey, Josh Hlebak, Mike Massey,
Lisa Molitoris, Lou Monnier, Lena Richards
Ohio University, Mechanical Engineering
ABSTRACT
Through connections with the Athens County Bureau of Vocational Rehabilitation, contact was
made with Tim Lang. Mr. Lang is a disabled dairy farmer from Marietta, OH who has been
operating his farm for two years with a full right arm prosthetic.
Mr. Lang provided the team with three needs to make his current prosthetic more appropriate for his
farming applications: 1) Increase the grip strength of the prosthetic, 2) reduce the necessary input
force and therefore physical strain implemented on his body, and 3) make the device easily
maintainable by the customer.
A prosthetic arm force reducer was manufactured by designing a pulley mechanical advantage
system housed within the hollow forearm section of the prosthetic. The system consists of a two
piece module and can easily be slipped into the forearm. The product is intended for anyone with a
full arm body-powered prosthetic who needs to reduce the strain on his/her body in order to
continue in the career or to do their job more effectively. The system reduces the input force
required by the user by 47% from 18 lbs to 9.5 lbs and costs only 9.6% or an additional $575 dollars
of the total prosthetic arm.
BACKGROUND
11.9% of the U.S. population between the ages of 16 - 64 has an employment disability affecting
their ability to work at a job or business. Of those 36 million Americans 100,000 have upper limb
loss. [1] One of the major contributors to such a large number of upper limb loss comes from the
field of agriculture. According to the Bureau of Labor Statistics, in 2006 farmers and ranchers had
the 6th highest fatality rates with 37.1 deaths per 100,000 workers. [2] “More attention should be
given to farm-specific occupational rehabilitation programs, such as AgrAbility, and in the
engineering of prostheses and other assistive technology.” [3]
STATEMENT OF THE PROBLEM
Our focus is on a person with upper extremity loss within the field of agriculture that needs a
specific prosthetic that reduces the amount of input force required to open the hooks and therefore
reduces the physical impact on the user’s body, increase the amount of grip strength, and any
maintenance can easily be done by the customer.
Two problems needed to be solved in order to increase Mr. Lang’s productivity. The first stemmed
from the mechanism used to actuate the split hook. Due to the current designs of nearly all body
powered split-hook prosthetics, the hooks are opened by the user shrugging his/her shoulders.
Many users experience discomfort due to the excessive input force needed to operate the system.
1 See Fig. A in Appendix for a view of the shoulder strap system. To combat this problem, the
decision was made to implement some type of mechanical advantage into the system resulting in a
lower required input force.
Another major inconvenience to Mr. Lang was the amount of grip force provided by his current
split hook. Our customer’s current hook utilizes rubber bands to supply the closing force of the
hook. As a result, the customer has resorted to attaching a large number of rubber bands in order to
achieve the desired grip strength. It was readily apparent how both of these problems worked
against each other; the result of improving one problem made the other problem worse.
RATIONALE
Fabricating a split hook from scratch was deemed beyond the scope of this project; therefore,
through benchmarking the decision was made to use the Otto Bock Model 10A60 vector hook with
our prototype (Fig B of Appendix). This hook features two settings that change the amount of grip
force delivered, thus changing the amount of input force required with a body-powered prosthetic
system. Other considerations involved myoelectric hook actuation or battery driven electrical
prosthetic arms such as the Utah Arm 3 (Fig C of Appendix) but were deemed too costly and
inefficient by our customer.
Our conceptual design (Fig. D & E of Appendix) implements a mechanical advantage system
housed within the prosthetic forearm. A typical mechanical advantage uses a moveable pulley and
a cable to lift an object at a value approximately half the objects weight. Making lifting or pulling
much easier, the ideal case for this system would give the user a 2:1 advantage in the input force
required to open the hooks. The impact of this system to the user would be a drastic reduction of
the input force required for both settings on the two-load hook, and the concept was proven by the
use of a mock-up (Fig F of Appendix). This system would be made almost entirely out of stainless
steel to protect it from the corrosive environment in which it will be used and it will be protected
within the forearm section of the prosthetic. Our conceptual design is also very easy for the customer to clean, make adjustments or provide
maintenance if needed. The moving pulley unit (Fig G of Appendix) is stopped by the mounting
screws on each end of the track. The pulley unit can easily be pulled out by unscrewing two
mounting screws. Also, the input cable, which wraps around the pulley, can be disconnected by
removing a single nut. Finally, the force reducer system is a designed to be a single module that can
easily be slipped into the forearm.
The entire package will allow the customer to achieve a higher grip force while at the same time
reducing the stress on their body as well as making it easy to service and install. Though designed
for use in an agricultural setting, this system could benefit anyone working in a labor intensive
environment.
DESIGN
A typical body-powered prosthetic uses one cable that runs from the back harness, down the arm,
and terminates at the cable post. When the user shrugs his shoulders, enough tension is applied to
the cable to overcome the resistive force of either springs or rubber bands keeping the hooks closed.
2
Our design requires the user to actuate the hooks in the same fashion but when tension is applied to
the input cable (the existing cable connected to the harness) it is also applied at its termination point
on the other side of the pulley. This causes the pulley wheel to rotate moving the entire pulley unit
towards the elbow.
The axle of the pulley unit is grooved and runs between two slotted tracks. The tracks are
manufactured from 304 stainless steel square tubing. The “c track” shape shown is manufactured
by cutting a slot down the middle of one side of the tubing. The output cable is fixed to the eyelet of
the pulley and runs through a protected hole in the prosthetic and connects to cable post. Therefore,
as the pulley is pulled towards the elbow tension is applied to the output cable which intern
overcomes the resistance of the
terminal device and opens the hooks.
This design gives the user a 2:1
mechanical advantage requiring half
as much force to open the hooks as
before.
The selection of the pulley was
dictated primarily by the size of the
prosthetic. Because of the confined
dimensions of the prosthetic, a small
pulley was selected to allow for the
manufacture of the assembly around
it.
Lastly, the rings provide the
connection point for the mechanical
Figure 1: Final Prototype Before Installation
advantage system to the prosthetic
arm. The mechanical advantage system is mounted to the inside of the prosthetic forearm by eight
round headed truss screws which also act as stoppers for the pulley unit and an anchor point for the
input cable. The rings are manufactured from a 304 stainless steel tube; with an outside diameter of
2.5” and a wall thickness of 0.25”. This tubing can then be cut to the correct length for the rings
and machined on a lathe to the correct outside diameter. All parts were ordered from McMasterCarr and a bill of materials can be found in the Appendix as Table 3.
DEVELOPMENT
Customer Interaction
From the very beginning a 360° customer interaction has been very critical to our project’s
development. Our research began by getting in contact with the Athens County Bureau of
Vocational Rehabilitation (BVR). The BVR is a branch of the Ohio Rehabilitation Services
Commission (ORSC) which is a state agency that is annually responsible for vocational
rehabilitation of 55,000 Ohioan’s with disabilities [4]. Our main contact at the BVR was George
Platounaris. George is the rehabilitation vocational supervisor at the BVR with over 30 years of
experience. Upon meeting with him several times George was able to connect us with two
rehabilitation design engineers who work in the field. He also helped aid us in locating a potential
customer Tim Lang.
3
Over the past five months of development our team has met with Tim on five different occasions to
discuss his needs as well as our current designs and features. Much of our design such as the
mechanical advantage system stems from Tim’s needs as well as his insight of dealing with his full
arm body powered prosthetic day in and day out on working on the dairy farm. Tim helped refine
our design as well as get us in contact with prosthetic manufacturers.
Another aspect of our development was with prosthetic manufacturers such as Otto Bock who
graciously donated the 10A60 vector hook and Hosmer who donated a titanium standard hook.
Also, contact with Tim’s prosthetic manufacturer Yankee Bionic donated a prosthetic forearm very
similar to Tim’s current one as well and extra parts, cable, and instructions on assembly and
manufacturing. With contact from prosthetic manufacturers it gave our team a great deal of insight
to our design’s limitations.
Manufacturing
Utilization of a mock-up proved the concept of the mechanical advantage system, but as
manufacture of the initial prototype (Fig H of Appendix) began it was clear that there were several
issues with the design. As such, the production of the prototype was ceased with the decision made
that re-design was necessary in order to ensure a quality product.
Once all of the internal components of the mechanical advantage system were complete, the rings
were placed into the forearm and the four holes were drilled into the forearm and bolted into place
with the rails attached. From this point, the pulley with the axle and bearings attached needed
placed within the system and the cable grounded to the bolt nearest to the elbow of the user. Also,
a tapered hole was drilled into the front (wrist area) of the forearm so that the cable can travel
through the wall of the forearm and attach to the cable post.
The total cost of materials was $48, the manufacturing costs of the tracks $120, the rings $191, the
modified pulley $74, and installation costs $74. The total manufacturing cost was $459.
EVALUATION
Our project successfully met our objectives (See table 1 of Appendix). Tim’s current prosthetic
utilizes 1.75” of cable travel, while our prototype and the Otto Bock vector hook requires 2.125” of
cable travel. Upon testing our prototype the extra cable travel was not an issue and was
compensated by tightening the user’s harness.
Also, through testing the force reducer system on Tim he mentioned that it was much easier to open
and close the hooks and felt a lot more comfortable to use. The system reduces the input force
required by 47% from 18 lbs to 9.5 lbs as measured by a standard fish scale. He also noted that he
was able to obtain a much higher grip strength with our prototype and the Otto Bock vector hook
making carrying 5 gallon bucket of feed much more manageable, and that that the system felt light
and ran smoothly.
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DISCUSSION
Overall, Tim will be able to improve the quality and ease of his workday. Though Tim Lang will
not be using our exact prototype, he will be working with the Bureau of Vocational Rehabilitation
engineers to implement our mechanical advantage system in a new prosthetic forearm that he will
be receiving from Yankee Bionics.
The true value of this design allows Tim a way to more comfortably work in the fields exerting onehalf less input force than he traditionally had to use with his former prosthetic. This will greatly
lessen the discomfort that he experiences from using the prosthetic in his daily work. The system is
easily cleanable, corrosion resistant, simple to operate, and utilizes the standard cable and cable
connectors that both Yankee Bionics and Tim are familiar with already. The mechanical advantage
system being fully contained in the forearm is extremely unique. Currently, no product on the
market like this exists. The system is very compact, yet the materials chosen are quality and easily
machineable and replaceable.
A full prosthetic arm typically costs around $6,000. Our system would only cost an additional $575
or 9.6% of the full prosthetic arm’s cost (see table 2 for an example cost analysis). This is a small
price to pay for the value added with respect to increased comfort and ease of operation. Tim says
that becoming accustomed to living with a prosthetic was extremely difficult at first, but stated,
“The body’s like a chameleon; it will adapt to whatever circumstances.” Tim will be reaping the
rewards of our mechanical advantage system, and we hope many other people will have this same
benefit.
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APPENDIX
Figure A – Image of Harness Straps Over Back
Figure B – Otto Bock Model 10A60 Stainless Steel Vector Hook [5]
Figure C – Utah Arm 3 [6]
6
1
4
3
2
Figure D – Mechanical Advantage System Module
1 = Hoops, 2 = Pulley, 3 = Tracks, 4 = Axle
Anchor Point (X)
Inp
ut C
abl
e
X
Ou
tp
ut
Ca
bl
e
Fig. 3 – Rear View of Assembled Mechanical
Advantage
Table 1 – Cost of Raw Materials
Figure E – Overall System Cable Path
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Output Force
Pulley
Anchor Point
Input Force
Figure F – Mechanical Advantage Mock-Up
Collars Axel Pulley Figure G – Moveable Pulley Unit
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Initial Design
Final Design
Figure H – Initial & Final Conceptual Designs
Table 1 – Project Specifications
Specification Actual Acceptable? Input Force 1/2 Current ~1/2 Current Yes Closing Force As strong or stronger than 7 rubber bands Equivalent to 11 rubber bands at hook’s highest setting Yes Unit Price Less than $700 Total Prototype cost $507 Yes Unit Dimensions Fit into prosthetic forearm Fits into prosthetic forearm Yes Unit Life 10,000 Cycles (3 years) 36,000 Cycles (9 years) Yes Unit Weight Less than 5 lbs 4 lbs Yes Table 2 – Cost Analysis Example
a. Total time to complete operation(s) in hours
b. Labor rate for the operation ($/hr)
c. Labor Cost ($) = a x b
d. Basic overhead factor
e. Equipment factor
f. Special operation/Tolerance factor
g. Labor/Overhead/Equipment Cost ($) = c x (1+d+e+f)
h. Purchased materials/Components cost
Total Cost
3
15
45
1
0.5
0
112.50
7.82
120.32
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Table 3 – Bill of Materials
Part
Use
Price
Quantity
2-1/2" DIA.
Rings
$72.89
1
Round Piping
1/2"x1/2" Square
Tracks
$7.82
5
Piping
Pulley
Pulley
$20.46
10
1/4" DIA. Stock
Axle, Misc.
$5.95
2
3/32" Wire Rope
Cable
$1.34/ft
10
Oval Sleeve for
Cable
$8.43/pack 3 packs
3/32" Rope
Attachments
1" OD Flat
Bolts for
$8.64/pack 3 packs
Washer
Attachment
Truss Head
Attach Rings
$10.64/pack 3 pack
Machine Screw
to Arm
Set Screw
Lock Pulley
$2.84
20
Shaft Collar
in Place on Axle
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REFERENCE
[1] Waldrop, J. and Stern, S.M., “Disability Status: 2000” Census Brief 2000, pp. 2, Washington,
DC, 2003.
[2] U.S. Department of Labor, Bureau of Labor Statistics, Census of Fatal Occupational Injuries,
2006.
[3] Reed, D. B. and Claunch, D.T., “Journal of Agricultural Safety and Health”, Special Issue 1, pp.
129-137, 1998.
[4] Ohio Rehabilitation Services Commission, “A Brief History of Vocational Rehabilitation”,
http://www.rsc.state.oh.us/history/default.aspx, viewed on May 2008.
[5] Otto Bock, “Body-Powered Adult Hooks”,
http://www.ottobockus.com/products/upper_limb_prosthetics/body_powered_hooks_adults.a
sp, viewed on May 2008.
[6] Preferred Orthotics and Prosthetic Services, “Utah Arm 3”,
http://www.preferredoandp.com/innovations.asp, viewed on October 2007.
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