Tolerance Stack UpIn any sealing application, the tolerances of ALL the parts in contact with the O-ring must be considered in order to create an effective seal. The combination of these tolerances is the tolerance stack-up. Illustration 5.5 shows a situation where the O-ring cross section tolerance is ±0.003", the groove diameter tolerance is ±0.002", and the bore diameter tolerance is ±0.001". In this example the metal and O-ring dimensions can vary up to 0.012". If the nominal O-ring size is 0.030", it is easy to see that the tolerance stack-up is nearly half the size of the O-ring. This can result in too much or too little compression which can cause the O-ring to fail. Illustration 5.6 shows the results of stack-up tolerance. View A illustrates how the nominal dimensions would interact to provide an effective seal, whereas View B illustrates insufficient O-ring compression due to component tolerances. This scenario, while it applies to applications of all sizes, must be addressed as component size decreases because seemingly insignificant tolerances have a direct impact on the life of the sealing system. |
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