Abstract
Abstract With the increased use of rubber as an engineering material, considerable attention has to be paid to the characteristics of natural and synthetic rubbers in compression. For oil seals, in particular, what is loosely termed permanent set of the rubber must not be so great as to lead to breakdown of efficiency. Up to the present the need for laboratory evaluation of different rubbers for this property has been met by tests which involve measuring the dimensional changes in a test-piece subjected to a constant strain or stress for a certain time and then allowed to recover for a certain time. Although it gives some indication of behavior, this method of test suffers from the great disadvantages that the time factor is arbitrary and the change of dimension is measured, whereas in service the dimension of the rubber is maintained virtually constant by the rigid members of the system. In other words, in the case of a sealing ring on a piston working in a cylinder, the effectiveness of the seal depends, in large measure, on the pressure exerted by the rubber on the cylinder wall. As this pressure decays, so, it is believed, the sealing efficiency decreases.
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