Understanding Tolerance Stack-Up

Tolerance stack-up is an especially important factor in microminiature O-rings due to the fact that the tolerances account for a large percentage of the nominal O-ring size. In a 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.

The graph above 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 dimensions can vary up to 0.006", while the O-ring can also vary up to 0.006". Therefore, the total tolerance stack-up is 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.

In an application utilizing a standard size O-ring, it is usually possible to compensate for tolerance stack-up in any number of ways including specification of a larger O-ring. In microminiature applications there is little room for such compensation.