Variation of dimensions within assemblies can unexpectedly displace parts from their intended location and therefore degrade the assembly’s performance. This paper presents a design tool based on the principles of kinematics to analyze nonlinear tolerance stackup, increase the reliability of the assembly without decreasing tolerances and where necessary, judiciously allocate tolerances such that the critical parts fit relative to each other with the specified precision.
Through an analytical sensitivity analysis, the procedure outlined in this paper alters the orientations of the parts to yield an assembly with the highest positional reliability. If the desired level of reliability is not met by the minimum sensitivity approach, a tolerance allocation method incorporating the above sensitivity analysis and the cost of manufacturing each dimension as a function of tolerance is applied. In addition, this approach allows the individual tolerances within the assembly to assume any distribution. The method shown by this paper allows the design engineer to consider manufacturing effects and provides an analytical basis to evaluate design function.