Measured as a percentage increase in the I.D. of an O-ring, stretch results in a reduction and flattening of the seal’s cross-section (width). There are two “types” or “phases” of stretch: installation stretch (as the seal is being placed in the groove) and assembled stretch (once the seal is seated). Because installation stretch is temporary (or even momentary), an O-ring can generally undergo a high degree of it, sometimes as much as 100% or more if the installation is particularly tricky. An O-ring may have to be stretched in unusual ways or at odd angles in order to safely navigate an intricate design.

Assembled stretch, on the other hand, is permanent and must therefore be minimized if the seal itself is to have a long life. But though it should be kept to a minimum, assembled stretch should not be eliminated altogether. A small degree of assembled stretch ensures the seal will fit snugly against the piston (or rod) and not sag and buckle in the groove. Assembled stretch is possible when the I.D. of an O-ring is slightly smaller than the groove diameter. This difference between the diameters ensures the O-ring will be subjected to some degree of stretch during both installation and use.

For most situations, assembled stretch should be at least 1% but not more than 5%. Care must be taken to avoid an assembled stretch greater than 5% so as not to exacerbate stresses and reduce the seal’s overall life expectancy. This is especially true when using a nitrile compound. Fluorocarbons, polyurethanes, chloroprene, and ethylene propylene typically last longer under a higher degree of assembled stretch.

Keep in mind that higher amounts of assembled stretch may need to be compensated for by use of a smaller gland, especially if the stretch exceeds 2% or 3%. Reducing the gland depth helps maintain the required amount of squeeze on the O-ring’s reduced and flattened cross-section (see Table 37).

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