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

equire the most engineering resources for implementation, a 2:1 force reduction was deemed
appropriate, and the corresponding grip force increase would be chosen so as not to negate the
beneficial effects of the pulley. As mentioned in section 7.0, the 2:1 reduction was validated
using a simple fish scale, which indeed registered half the input force to open the hooks when
used with our mechanical advantage system. Additionally, the Otto Bock hook also has less
opening distance than the one the customer currently uses. This presents a clear trade off: if the
customer chooses to use our product, the size of objects he will be able to grasp will be more
limited. On the other hand, the smaller opening will mean less cable travel than he is used to.
His current prosthetic would have to experience 3.5 inches of cable movement with our device,
but the Otto Bock (with less opening) capacity, only requires ~2.75 inches.
Choosing a grip force that corresponded appropriately to our 2:1 mechanical advantage proved
challenging. Unlike the mechanical advantage system, where many options were available for
force reduction, the only feasible way to increase grip force was to outfit the Otto Bock with
higher stiffness springs. This was problematic because many of the springs whose dimensions
were appropriate for our application could not deflect to the necessary distance. Furthermore,
there was much variation in the stiffnesses of the dimensionally-applicable springs; this made
choosing one with an appropriate amount of grip force difficult. Fortunately, one customer
meeting led to a qualitative test of the user’s current grip force versus the Otto Bock’s. This
simple test showed that there was a marked increase in grip force by using the Otto Bock. This
was most likely due to the fact that the “pinch point” of the customer’s hook was much further
from the rubber band than the Otto Bock’s pinch point is from its springs (see Figure 6.1.2)
When the customer was finally outfitted with our prosthetic, we were confident that an increase
in the ease of use and grip force was present. Even at the Otto Bock’s higher grip force setting,
the customer commented that the force required to open the hook was less than his current
prosthetic (it would be much better to have some actual numbers to validate these claims).
8.0 Conclusion
Our project successfully met our objectives. Mr. Tim Lang, the dairy farmer from Marietta for
whom this system was designed, is extremely satisfied with our product. 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 forearm used in our prototype was
donated by Yankee Bionics, and it is the exact size Tim needs, yet it was not equipped to be
fitted with a rotational chuck system. This system allows Tim to rotate his hook 180° to be able
to drive his stick shift tractor and quickly change to operating his milking machines.
Since the forearm we received did not have the capability of utilizing a rotating chuck system,
our team decided that we should not proceed with finding a way to rotate our hook. The 180°
rotation that Tim needed was not feasible for our system, so we thought the slight rotation we
would achieve would not add value to the overall design. Tim’s next forearm will have the
proper rotating chuck system which he will need to fully rotate his hook for everyday operations.
The BVR engineers should easily be able to move the mechanical advantage system to Tim’s
new forearm. Bolting the holes on the forearm and securing the bolts will be the only major
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