Typically noted in either pounds per square inch (psi) or megaPascals (MPa), tensile strength is the amount of force required to break a rubber specimen. To convert from MPa to psi, simply multiply the MPa figure by 145. For example, 14 MPa converts to 2,030 psi. Converting from psi to MPa is just a matter of dividing the psi number by 145.

Per ASTM D 412, a compound’s tensile strength is generally tested using a molded dumbbell (see Figure 19). The dumbbell is placed in the grips (jaws) of a tensile tester (see Figure 20). The best-known tester is Instron, but tensiTECH and others are also used. When the tester is activated, the dumbbell is pulled steadily at a rate of 20 inches per minute until it breaks. The force being exerted on the sample at the time of rupture is said to be the sample’s tensile strength.

Minimum tensile strength is typically used as both a qualification criterion when specifying a new material and as a control criterion (with a ± 15% production tolerance) when testing batches of mixed material. A shaft seal’s elastomeric lip will undergo a small amount of stretch as the seal is placed on the shaft, but a shaft seal will not be exposed to high degrees of stretch in normal operation. Unfortunately, people often specify tensile strength that is larger than required for a given application.

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