Compression set is the end result of 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.

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) taken from molded slabs. 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 26), the plates are then forced together using either a calibrated spring or a pre-defined external force. In method B (see Figure 27), 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. Because ASTM D 395 primarily describes the testing of materials to be used in high temperatures, a similar test procedure for materials to be used in low temperatures is outlined in ASTM D 1229.

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 2 shows how several of the most commonly used sealing materials respond to increasing temperatures.

Because the elastomeric lip of a shaft seal is not normally compressed, compression set tests have limited applicability when it comes to choosing a lip material. In relation to shaft seals, compression set tests are most helpful when a seal design calls for a rubber-covered outside diameter (O.D.). This O.D. will need to maintain tight contact with the housing bore after installation, so being able to anticipate the degree to which the rubber may set is important. Of course, compression set tests find their widest use in selecting materials for use as O-rings and other static seals that rely on an optimal degree of compression.

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