EngineerBenchChecking clearance after thermal expansion
Two mating parts can grow by different amounts even when both experience the same temperature change. This thermal-gap calculation starts with assembly clearance and subtracts the modeled differential growth. It also applies the entered combined tolerance to show how the operating gap can vary.
Check the direction of growth
Identify which part's growth closes the gap and which opens it. Enter lengths along that direction, not unrelated overall dimensions. The sign convention matters: swapping the two parts can reverse the effect on clearance.
Choosing an assembly gap
The reverse mode estimates the assembly clearance needed to retain a specified minimum operating gap. It uses a common temperature change and constant expansion coefficients. If the parts run at different temperatures or the assembly constrains their movement, use a model that includes those conditions.
Following the clearance sign convention
Write the gap as the difference between two physical boundaries before entering the calculation. Expansion of a surrounding bore can open a diametral clearance, while expansion of the enclosed shaft can close it. Other assemblies may have a different arrangement. The two effective lengths must describe the dimensions that affect the gap, not merely the largest dimensions visible on a drawing.
Suppose differential growth closes a gap by 0.2 mm and the initial gap is 0.5 mm. The nominal operating clearance is then 0.3 mm before applying the entered tolerance allowance. If the combined allowance is 0.1 mm, the lower endpoint is 0.2 mm under that model. Reversing the direction of differential growth changes the conclusion. Evaluate the temperature extremes relevant to the assembly, including cooling if it can govern. The reverse mode can size the starting clearance for a minimum operating requirement, but that requirement and the tolerance allowance must come from the actual function and manufacturing plan.
Formula
Operating gap = assembly gap − differential thermal growth ± tolerance.