Can a met tolerance still result in a non-functioning component?
agosto 19, 2026
The accumulation of tolerances lead to a system that doesn't function properly even when every single component perfectly complies with the design? It's a question that, at least initially, may seem almost contradictory. If we've defined tolerances and every part falls within the established limits, it's natural to expect the assembly to function. We thought so, too. Then we encountered a real-world case that forced us to look at the problem from a different perspective.
Ten years without problems, then something changes
For over ten years, we had been supplying gears for a specific gearbox. The product was well-established. The components were manufactured according to the required specifications, and over the years, no particular critical issues had emerged. No unusual noises, no recurring problems during assembly, nothing that suggested a weakness in the design. Then a new order arrives, and suddenly the first complaints begin: some gearboxes are making unusual noise. The problem is serious enough to require production to be halted. The customer must quickly understand what's happening and, as is common in these cases, begins a thorough analysis of the components. The first hypothesis is almost inevitable: something must be out of tolerance.
When all the checks say that the component is compliant
We then rechecked the details: tooth profiles, pitch errors, concentricity, run -out, center distances, materials, treatments. We searched for the deviation that could explain the gearbox 's anomalous behavior . But that deviation wasn't there. Everything was compliant with the design , and that's where the problem became much more interesting. Finding a component out of tolerance, as unpleasant as it may be, at least allows us to pinpoint a precise direction for analysis. But what do you do when the parts are compliant and the system still doesn't behave as it should? The conformity of a characteristic to its limits is a well-defined concept even in the ISO GPS system. But verifying that each characteristic meets its specification does not, in itself, prove that every possible combination of compliant components guarantees the function of the assembly.
The problem was not in the component, but in the combination
After days of testing, we decided to change our approach: instead of continuing to look for the defect in the single component, we completely redid the coupling calculations, and that's where the problem emerged.
No part was out of tolerance: it was the combination of tolerances that created a critical condition. In that specific job, the housing seats that supported the gears were all very close to one extreme of the allowed tolerance range. At the same time, the gears were close to the opposite extreme relevant for mating. Each component, taken individually, was perfectly acceptable. But when those components were assembled together, their dimensional variations added together in the most unfavorable direction. The result was an extreme configuration. A configuration formally foreseen by the design tolerances, but which in the previous ten years had never occurred critical enough to generate the problem.
What is tolerance accumulation really?
A mechanical assembly is made up of dimensional and geometric features that interact with each other. Each feature has its own allowable range of variation. The problem arises when we look at the system as a whole.
Imagine that a given function depends on five different dimensions. All of them may be perfectly conformal, but their deviations from the nominal value may be compounded. Under normal conditions, some variations can compensate for others. But what happens if, in the same set, all of them are arranged in the most unfavorable direction? This is the so-called worst- case scenario.
This is not just a theoretical concept: there are standardized applications in which the accumulation of tolerances of the different components directly determines the functional play of the assembly and may make it necessary to intervene on the coupling strategy.
A project can be compliant and still have a limit
In our case, this analysis completely changed the question. Initially, we were trying to understand which component had been manufactured incorrectly. The answer was: none. The problem wasn't a manufacturing nonconformity. It was a limitation in the system's tolerance design . This distinction is crucial because it also changes the way we approach the solution. Continuing to increase the number of checks on individual gears wouldn't have eliminated the cause. Repeatedly measuring an already compliant component wouldn't have changed the behavior of the assembly.
Instead, it was necessary to return to the project, analyze the dimensional chains, and understand which combinations were actually compatible with the required function. When we design an assembly, we inevitably work with real components, and these almost never exactly match the nominal value.
Therefore, an important question should not only be: "Does every component meet its tolerance?" We should also ask: "Does the system continue to function when compliant components enter unfavorable dimensional combinations?" This does not necessarily mean designing every system to extremely tight tolerances.
In fact, indiscriminately tightening tolerances can increase costs, waste, and production complexity without creating a commensurate benefit. Instead, it means connecting geometric specifications to the actual function of the product.
Tolerance should not just be a number on the drawing
Above all, this episode taught us a lesson. A tolerance shouldn't be considered solely as the boundary between a compliant and a non-compliant part. It should be related to the function that component must perform within the system. Because the end customer doesn't use a dimension or a tolerance. They use a gearbox, a transmission, or a machine that must function properly. And it's precisely this transition from the compliance of the individual component to the behavior of the assembly that makes tolerance analysis so important.
The conformity of a single component does not automatically guarantee the functional conformity of the entire system. Our case demonstrated this very concretely. For over ten years, the standard dimensional combinations had not revealed any critical issues. Then, a particular project simultaneously pushed several components to the extreme conditions permitted by the design.
For more information on gear design and manufacturing and mechanical transmission solutions, contact GSI Ingranaggi.
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