Real world tolerance issues in product design

Tolerance stack-ups are a common challenge in mechanical design. For instance, a press fit assembly between a shaft and a bearing housing must account for both parts' dimensional variations. If the interference is too high, the bearing can deform and fail prematurely. Too low, and the shaft may slip under load.

Another example involves injection molded plastic parts. Shrinkage after cooling can lead to dimensional changes that cause fit issues. A snap-fit feature designed with nominal dimensions might become too tight or loose if the material shrinkage is not accurately predicted.

Thermal expansion also introduces tolerance problems. In a metal enclosure for electronics, the coefficient of thermal expansion difference between the aluminum housing and steel screws can cause stress or loosening over temperature cycles. Designers must include appropriate clearances or use compliant materials.

Assembly processes themselves can create tolerance issues. For example, welding can cause distortion that alters part geometry. A weldment that was dimensionally correct as individual pieces may warp, leading to misalignment of bolt holes or sealing surfaces.

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