Analysis and Design of Bearing Systems in Controlled Passive Energy Management (CPEM) Prosthetic Feet
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The overall goal of this study is to advance the availability of prosthetic devices for developing and post conflict regions by establishing a framework for their design based on those used for Class II medical devices. The Niagara Foot Model 2 was used as an example case. This device has been refined within an international consortium of engineers, prosthetists, materials scientists and non-governmental organizations over the past decade. It is a unique Controlled Passive Energy Management (CPEM) foot that can be modified to suit the needs of a particular user. Its success has prompted interest in the development of other devices for this market. Quality Function Deployment (QFD) and Failure Mode and Effects Analysis (FMEA) were used to assess the current design. A systematic literature review was used to generate 29 User-centred Functional Requirements, which were compared to 47 engineering variables extracted from the design archives. In addition, 33 potential failure modes and their mechanisms were assessed based on a Failure Mode and Effects Analysis. Features controlling energy management in the CPEM System had the greatest impact on User-centred Functional Requirements. The potential for top plate fracture at the connection to the pylon had the highest risk rating. These observations prompted the analysis and design of the bearing system for controlled passive energy management in the current design. The analysis showed a bearing contact area, sliding speed and contact force that produced a PV value 25% above the recommended limit for Hytrel® 8238 which the horn portion of the CPEM System is comprised of. A modification to the contact geometry was proposed that would reduce the PV value by 63%, and would move the contact point forward 1.6mm. Based on a simplified static analysis, this would further reduce the peak moment in the top plate by 2%, therefore the risk to top plate failure would not be increased.