EMC & Thermal Management
Fig 4 (above left and above): Box and whisker plot and histogram for nominal dimensions and tolerances
Fig 5 (above left and above): Box and whisker plot and histogram for 1.14mm thick gasket
to manufacturing processes that typically take place in accordance with ISO 2768-fH, it is nowhere near as pronounced. Just to be clear, in both cases the SD of the gaskets and stops is the tolerance divided by 6, giving 6Ó either side of the mean. As previously stated, applying the nominal 10 per cent deflection to a 0.81mm gasket requires a compression stop height of 0.729mm. Running the Monte Carlo analysis for 500 simulations produced the data in Figure 4.
As shown, the majority of the simulations occur within the 5-15 per cent deflection range with around 1.6 per cent slightly above the recommended deflection and approximately 3.4 per cent below the recommended deflection. The higher deflection should not really be an issue in the application, although the same cannot be said for the minimum deflection, which at around 1 per cent could prove problematic. As a result, there is a need to adjust either the compression stop height or its tolerance, or both.
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It turns out that there is no perfect solution in this use case, so slightly reducing the stop height and tightening the tolerance represents the best action possible to minimise over- and under- deflection. With the compression stop in place, a nominal compression stop height of 0.715mm with a tolerance of ±0.03mm will result in a design with a maximum deflection of around 18 per cent and a minimum of 4 per cent. There will still be some occurrences of extremely low compression (of approximately 1 per cent) but these will be few and far between. Even in this scenario, the worst-case analysis gives a deflection range of -9 per cent to +27 per cent. However, revisiting the problem with a 1.14mm thick gasket, a 1mm compression stop and a 0.03mm tolerance, makes it possible (within practical reason) to eliminate all under- deflection and reduce the probability of significant over-compression to a minimum,
even though the worst-case analysis here still gives a range of deflections from -4 per cent to +25 per cent.
Negating the tolerance issue Now, as mentioned at the beginning of the article, the rather large tolerances associated with these flat gaskets result from moulding large sheets of raw material. The tolerance for the 0.81mm thick rubber is ±0.13mm. However, a custom-moulded component of the same size as the connector gasket comes with a ±0.08mm tolerance.
Revisiting the 0.81mm gasket as a moulded component with multiple cavities gives a worst-case analysis range of -4 per cent to +21.5 per cent deflection. In the simulations with the nominal 0.729mm stop and ±0.03mm tolerance, the probability of an under- or over-deflected gasket becomes exceedingly small.
Expertise and advice
While it is perfectly feasible to manufacture a custom-moulded part with compression stops, this can prove expensive and often the volumes do not justify the added cost. The use of standard rubber sheet such as CHO-SEAL is, of course, far more cost-effective, but careful calculations and prediction modelling is necessary to ensure the best-possible outcome. Parker Chomerics has decades of experience in both custom and standard solutions, helping customers identify the best way of achieving the optimal result for their specific applications.
While this article has focused on flat gaskets and compression stops the same Monte Carlo technique can be used for the gaskets in a groove, and more variables can be added if necessary if these are critical design parameters.
https://ph.parker.com/ Components in Electronics June 2025 43
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