Too Much Tolerance?
Despite being a well-known issue, tolerance stack up remains a costly failure mode for many manufacturers.
Alpha supplies gears made to a ± tolerance. Beta supplies a mating component for the Alpha gear which was also manufactured to a ± tolerance.
Let’s start with a hypothetical where Alpha supplies gears made to a ± tolerance. Beta supplies a mating component for the Alpha gear, which was also manufactured to a ± tolerance.
Beta supplies a mating component for the Alpha gear, which is also manufactured to a ± tolerance.
Periodic quality audits showed that both the Alpha and Beta gear components were within specification. Despite this, gear assemblies were failing at a higher than typical rate. The result was out of control warranty costs.
Both the Alpha and Beta components were mounted to gear rods, and each gear rod was connected to a bushing assembly. Examination of the failed units confirmed that they had been manufactured within their respective ± specifications. As a result of close examination of a larger number of failed gear assemblies, the client determined that measurements from both components clustered around the respective high ends of the ± specifications.
The dimension of the opening to both bushings was at the high end of the ± tolerance. This meant that the inner diameter of the openings was as large as it could be and still be within specification. Measurements for the diameter of the gear rods where they connected to the bushings were clustered at the very low end of the ± spec. This meant that the gear rods were as small as they could be and still fall within specification. This combination of large opening and small gear rod allowed some “play” in the fit and resulted in the gear rods being slightly off level.
The Gear rod bushing ends were within specification at the low end of ± tolerance. The gear rod mating gear ends were within specification at the upper end of ± tolerance. And the bushing opening internal diameter was within specification at the upper end of ± tolerance. The effect of this combination of specifications was a gear assembly that functioned, but with tighter than desirable tolerances and no room for error. The result was friction, heat, and eventually failure.
When it results in failure, this additive effect is sometimes referred to as tolerance stack failure.
Avoiding tolerance stack is done in design by calculating how the tolerances of mating or connecting parts add up in aggregate. Ultimate manufacturing tolerances should not be set until this has been done.
Despite compliance with design, stacked tolerances resulted in failure. And because the individual components were built to spec, the suppliers have no liability and the manufacturer is stuck with responsibility for the failures.
Manufacturing risk management includes the idea of avoiding unnecessary failures like this. By unnecessary, we generally avoidable by means of a rigorous requirements engineering process with DFMEA and PFMEA. We can help you avoid tolerance stacking issues with some simple process improvement.
We help you develop systematic processes to assure that your products comply with regulatory, customer and “due care” requirements. Where there is a problem, there is a solution.