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

6.1.2 - Analysis of Pulley Based Mechanical Advantage
Figure 6.1.9 – Mechanical Advantage by the Use of a Pulley
Using basic physics, a pulley can be used to theoretically cut the input force needed to move an
object in half. When a pulley is not used, the input force is equivalent to the output force.
Likewise, when a pulley is being used as shown in Figure 6.1.9, the tension in the cable around
the pulley is the same. In the system used for this project, the “W” would be the hook post. The
“W/2” on the right side is the grounded cable and the “W/2” on the left side is the cable that
connects to the customer’s back harness.
A disadvantage to the pulley system is the work required still remains the same.
Work = force * distance
Since the force is cut in half, the distance required is twice as much as when the pulley wasn’t
used. This trade off of cutting the input force in half but doubling the necessary cable travel
could be an issue in our finalized design.
6.2 Concept Screening, Development and Selection
Through our interaction with Tim, he stated that he wished for more grip force out of his current
prosthetic, but also cited pain in his back and shoulders after a long days work with his current
prosthetic’s grip force. Realizing that this was a major issue to Tim, we began discussing ways to
address this problem. We very quickly realized that a mechanical advantage system was most
likely the only way that our group would be able to achieve this feat. The system would allow
for reduced stress on his body due to the input force being cut in half, therefore allowing for the
possibility of a higher grip strength. After speaking with Tim and explaining the concept of the
mechanical advantage system, he seemed very eager to see what our design had in store for him,
as well as to test the input force required to open the hook. Therefore, we began focusing on,
and designing a mechanical advantage system to fit within Tim’s forearm.
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