Breaking down tolerance stack-up. (Brigham Young University's tolerance model)

Tolerance variation in assemblies results from 3 sources: size, form and kinematics. During assembly, tolerances in each part can accumulate to create large residual stack-ups, inducing poor product performance and high cost. Flexible assemblies containing nonrigid parts, produce an additional complexity to tolerance prediction. Several tolerance models account for the stack-up problem in assemblies with rigid components and point to a variety of solutions. One method, however, has been applied early in the design process and calculates stresses in rigid assemblies consisting of flexible parts. The tolerance model developed at Brigham Young University, based on a Direct Linearization Method (DLM), uses a linear approximation of nonlinear assembly functions. The DLM modeler is available in TI/TOL 3D+ from Texas Instruments. This package is integrated with the parametric capabilities of Pro/Engineer from Parametric Technologies Corp. The technique is described.