Prosthetic Arm Force Reducer Team 1 – Halliday's ... - Ohio University

Results Summary
• Maximum stress = 3154 psi (This value was very localized near the end of the
loading area, and may not be an accurate depiction. These areas are shown in red
in the top screenshot. Much more of the stress occurred in the green region,
which indicated ~2000 psi).
• Maximum displacement = 0.00035 in (The displacement results are shown in the
second and third screenshots).
These results instilled confidence in the design, because of the minimal stresses and
displacements involved.
The unit life was calculated based on the cable being the limiting factor. The cable breaking is
the most likely failure to occur within the specified life (3 years). The cable used is 3/32”
diameter type 304 Stainless Steel. It is constructed of 7x9 strand cord with a plain coating.
Based on the cable breaking strength of 920 lbs. (as specified by McMaster.com), the stress to
failure would be 133,277 psi. Drawing an S-N Curve and calculating the relationship between
stress, S, and unit life, N, yields the relationship, S = 229,723N (-0.0851) . When S equals the
breaking stress, the life of the cable is 13,675 cycles.
7.0.8 Customer
Much of the mechanical advantage design is based on or around customer input. Two significant
parties have contributed a great deal input that is reflected in our system’s design. The first is
from a direct link to a potential customer named Tim Lang. Tim is a young dairy farmer who is
missing his right arm from the shoulder down and needs to reduce the amount of strain on his
back and shoulders due to his body powered full arm prosthetic.
Our first time visiting Tim greatly shifted our project from modifying the terminal device to have
a moveable jaw and a compliant hook to allow for a wider grip to looking further down the arm
and implementing a mechanical advantage system. Tim was very satisfied with his opening
width (approximately 5 inches) and did not see a need to increase the width any further. Much of
our work up to our first visit with Tim had been conjectures of what a typical arm prosthetic user
might desire but by physically seeing him use his prosthetic many dilemmas with our old design
became apparent.
The second time our team visited Tim we came prepared with our refined mock-up
demonstrating the concept of the mechanical advantage system. Upon seeing this Tim suggested
using rounded head bolts so that the arm would not snag on anything. He also mentioned that one
to two extra pounds in his forearm unit would not be a problem. Another piece of feedback that
was critical was Tim agreeing that the trade off of reducing the input force needed to open the
hooks in half was worth doubling the cable travel and therefore exaggerating his shoulder shrug.
With that feedback it confirmed that a mechanical advantage system was not only needed but
wanted by the customer. Tim also led us to his prosthetic manufacturer Yankee Bionic which
happened to have a forearm piece with very similar dimensions to Tim’s forearm.
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