As should be clear by now, designing a seal and selecting a material that will function well in its environment are far from simple tasks. Even experienced seal engineers are often met with unusual service requirements that test both their ingenuity and the capabilities of the seal. Whether your sealing needs are simple or complex, the factors to be considered are numerous enough to guarantee that not every seal will be successful in every application.

When O-rings do fail, the cause can generally be traced back to a handful of usual suspects. These include use of an incorrect O-ring size, non-compatibility between the seal and its environment, installation error, and lack of proper lubrication. All of these dangers were discussed in previous sections of this design guide.

A fifth common culprit is improper gland design. Since the correct amount of O-ring squeeze is vital to a seal’s longevity, a design that allows for either not enough compression or too much compression is problematic. A gland that does not provide enough room for the seal to undergo normal expansion is equally troublesome. Since the various elements of any seal design each carry their own tolerances, these potential deviations must be incorporated into the design calculations.

An improper design can often be spotted by calculating the tolerance stack-up (also known as tolerance build-up, see Table 54). Let’s say, for example, that an O-ring cross-section has a tolerance of ± 0.003". This means that the cross-section may actually be either 0.003" larger or 0.003" smaller than the stated measurement, thus creating a 0.006" tolerance range. In the same design, the groove diameter has its own tolerance of ± 0.001" (a 0.002" range), and the bore diameter also has a tolerance of ± 0.001" (again, a 0.002" range). The sum total of these three tolerance ranges (in this case, 0.006" plus 0.002" plus 0.002", or 0.010") is the total tolerance stack-up for this particular seal.

Keep in mind that there is no “magic number” when it comes to tolerance stack-up. While 0.010" might be an acceptable stack-up tolerance for one application, this same total in a different design may result in either too little or too much O-ring compression, and, ultimately, seal failure. It is also important to realize that as the overall size of a seal decreases, the importance of even the slightest tolerances increases. In other words, the smaller the space in question, the less “elbow room” you have.

Many times there is not just a single cause for seal failure. Rather, a combination of factors often act in unison to damage the O-ring and doom the seal. These causes should be looked at individually, however, before any interaction among factors can be fully understood. With that in mind, what follows are separate discussions of the most common causes of O-ring failure.