Compression set is the end result of a progressive stress relaxation, which is the steady decline in sealing force that results when an elastomer is compressed over a period of time. In terms of the life of a seal, stress relaxation is like dying, whereas compression set is like death. The effects of compression set on an O-ring’s cross-section can be seen in Figure 24.

Though it is very difficult to accurately quantify stress relaxation, compression set is easy to measure. ASTM D 395 details compression set testing for rubber that will be compressed in air or liquid media. Two methods are described (“A” for constant force; “B” for constant deflection), but the basic methodology is substantially the same. Testing generally involves use of cylindrical disk compression set test buttons (0.49" thick by 1.14" diameter). In lieu of buttons, die-cut plied (stacked) samples (0.070" thick by 1.14" diameter) may be substituted. The buttons or plied samples are placed between steel plates. In method A (see Figure 25), the plates are then forced together using either a calibrated spring or a pre-defined external force. In method B (see Figure 26), a bolt-tightened device and steel spacers are used. Either way, compression (normally 25% of original thickness) is held for a given time (e.g. 22 hours) at a specific temperature (e.g. 100° C), these last two variables based on anticipated service conditions.

After removal from the compression device and a 30-minute cooling period, the specimens are measured using a dial micrometer. Compression set can then be calculated as either a percentage of original specimen thickness or as a percentage of original deflection.

Though a high degree of compression set is to be avoided, other service variables (such as inadvertent fluid swell or the intentional application of greater squeeze) may compensate. Seals are most likely to fail when there is both high compression set and shrinkage. Table 5 shows how several of the most commonly used materials respond to increasing temperatures. For more information on compression set, see Diagnosing O-Ring Failure.

PHYSICAL PROPERTIES MAIN PAGE