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

Failure Mode
Table 7.0.1 - Risk Priority Number Analysis for Split Hook (Environmental)
Potential
Severity
Likelihood of
Occurrence
Probability of
Detecting
Cable Break (at fitting) 7 3 8 168
Corrosion Over Time 2 10 5 100
Threading of Prosthetic
Connection
RPN
6 2 6 72
Environmental Deposits 2 10 1 20
Loss of Movement of
Vector Hook
3 5 1 15
These failure modes were listed in order of importance according to the risk priority number
(RPN) calculated in the far right column. The RPN is the product of potential severity,
likelihood of occurrence, and detect-ability of the particular failure mode, as ranked on a scale
from one to ten.
Since the Otto Bock hook has been left in stock, OEM condition, failures relating to the stresses
present in the hook itself were not included. The mechanical advantage, however, does allow the
user to hypothetically pull twice as hard on the hook at full opening. Although this situation is
not practical, it could lead to failures in the hook, which is not designed to experience twice as
much force as is needed to open it to its maximum width.
Corrosion over time and the hook accumulating environmental deposits are completely
unavoidable considering the working conditions that it will be used under. As such, a stainless
steel hook has been selected and all fabricated parts shall be made of stainless steel.
Additionally, any mud or other element that fills some part of the hook should be able to be
easily removed by our user.
The only other failure mode of the hook of much concern was if the threading of the hook could
be stripped in some way. The prosthetic arm provides the biggest guard against anything
happening to the threading as the hook threads into a bushing that is then inserted into the
prosthetic arm. This ensures that the threading is not constantly under torque but instead the arm
will bear this force.
FMEA was also completed for the mechanical advantage portion of the design in the same
manner as conducted for the environmental effects on the system. The FMEA results are
presented below in table 7.0.2.
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