A tutorial on the Apple Rubber Products Web site (www.applerubber.com) explains how tolerance stack-up factors in the o-ring selection process. According to Apple Rubber, tolerance stack-up is a particularly important selection criterion because tolerances account for a large percentage of the nominal o-ring size.

The graph here 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, according to Apple Rubber.

In an application utilizing a standard size o-ring, Apple Rubber says 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, Apple Rubber says there is little room for such compensation.

For more sealing tutorials from Apple Rubber Products, visit www.applerubber.com/library/tips.cfm.