Probabilistic Tolerance Analysis of Compliant Assemblies 2007-01-0408

Tolerance analysis is essential for estimation of final error of key product characteristic (KPC) in a mechanical assembly caused by errors in production and assembly of individual parts. Many mechanical assemblies, especially in automotive and aerospace industries, include sheet metal parts with a small ratio of thickness to width or length. Such parts are substantially flexible and thus, are prone to large errors due to elastic deformations during assembly production. Phenomena such as deformation due to fixture force, spring back, welding distortion and so on are common to such parts. Unlike tolerance analysis of rigid parts, here, the elastic deformation of the parts plays a significant role in the error, and thus stress analysis of the parts, e.g. using finite element method, is necessary.

In this article, the influence of tolerance between locator and pin hole in the assembly fixture on the KPC error is studied using a probabilistic design approach. In this approach, random positions for parts in the assembly fixture within the given ranges of pin and locator tolerances are generated. An analysis methodology is developed which generates a mechanical model of error emission in compliant parts in single and multi station assembly line, and iteratively calculates the final KPC error using FE analysis. In this way, the sensitivity of final KPC's error to the fixture tolerances is determined. The developed software yields the sensitivity of final KPC's error to tolerance of locators, their location and change of locator's layout in each station of the assembly line. Such sensitivity analysis allows the designer to choose optimum tolerances for high quality and low cost.