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

forearm. This gives the user the ability to push the unit into the forearm and obtain a tighter fit
due to more contact area between the hoop surface and forearm. It also increases the amount of
rigidity in the system. Furthermore, the circular shape increases the distance between the tracks,
which allows more room for the pulley’s installation. The wrist hoop is chamfered to increase the
contact area and therefore make a tighter fit and to ensure that the corner of the hoop is not
catching against the side of the forearm (this would have been much more difficult to do with
square tubing). Both hoops will be made of type 304 stainless steel due to its high corrosion
resistance, good weldability and machinability, and 30,000 psi yield strength. The only
disadvantage of using circular hoops was the need for flats milled on their outside surfaces to
ensure proper hole drilling/tapping, and on their inside surfaces to fasten down of the tracks.
Part #2 – Pulley: The entire pulley sub-assembly (the pulley wheel, pulley housing, and the
pulley axle) were purchased as one part. Our choice of pulley has a removable axle, so a
different axle (one which met our diameter requirements) could be used in its place. Another
important feature of the pulley is that it has a rigid eyelet which reduces twisting in the output
cable. The entire pulley assembly is technically a “pulley block”, which means that the wheel is
housed in a steel casing. The casing is an important feature, because it keeps the input cable
from slipping off the groove of the wheel. The pulley wheel is capable of guiding a cable of up to
1/8” diameter which is well above the 3/32” cable being used. The pulley will be somewhat
loosely mounted to the axle and no machining will be required.
Part #3 – Tracks: To make the tracks, ½” x ½” square tubing was chosen. Square tubing lent
itself to track design, requiring minimal milling and drilling operations after the stock had been
cut to length. Another critical issue is that the tracks and hoops are made of the same stainless
steel alloy (type 304), which ensures a better and more uniform weld when the track is welded to
the wrist hoop.
Part #4 – Axle: The choice of a removable-axle pulley block allowed quick removal of the stock
axle without damage to the pulley. The original axle was replaced with a stainless steel one,
which was not only more resistant to environmental effects, but was also turned to the
appropriate diameter to operate within the track openings. The ends of the axle had a slightly
larger OD than the center section, and were separated by grooves. This was done to prevent the
axle from shifting perpendicular to the motion of the pulley.
Other Considerations: The forearm prosthetic will require some machining such as drilling for
all eight of the mounting screws. Also, the output cable will require a drilled slot in the forearm
to reach the hook’s cable post. All of the machining operations on the forearm will be aided by a
template that lays out the location of each machining operation.
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