Originally Published MDDI February 2005

Product Development Insight

An example of complex medical instrumentation with multiple mechanisms is an automatic clip applier, a surgical instrument that occludes vessels and ducts. A magazine of clips is automatically fed into a forming mechanism after each subsequent closure. Four separate mechanisms function simultaneously during the firing stroke. The functional position of the pusher in relation to the escapement window for proper clip advancement is ±0.030 in. However, a traditional tolerance stack-up study revealed that the integrated relationship of the four mechanisms created problems. The calculated tolerance for the escapement window was a function of 11 components with more than 40 critical feature dimensions. Even after applying tight tolerances to all critical dimensions, the traditional tolerance stack-up analysis for the escapement window revealed a range of ±0.153 in. When compared with the functional requirement of ±0.030 in., the design engineers found themselves faced with a dilemma. The DFRR methodology can help engineers find solutions to such problems.

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