The Gough-Joule effect (see Figure 31) is actually two related phenomena. First, unlike many materials, rubber heats up when stretched quickly. Second, rubber that is held stationary at one end and stretched under a given load will actually retract if localized heat is applied. This is true because the rubber’s stressed macromolecular chains are trying to regain a less-stressful state. The Gough-Joule effect is perhaps most important in rotary seal designs, where excessive installed stretch in conjunction with system heat can cause an O-ring to retract, seizing the rapidly rotating shaft and dooming the design.

With these physical, chemical, and thermal properties in mind, let’s now look more specifically at the relative strengths and weaknesses of the most commonly used O-ring materials.

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