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Importance in Testing
The grips are an important part of the testing process, because they must hold the rubber sample securely, as the shear may take place at one or both ends, near the grips. The grips must not only be rigid, they must be longer than the longest edge of the test piece. Failure to overlap the material can compromise the integrity of the test.
Universal Testing Machines
Many rubber shear tests are designed to determine whether the rubber strength meets industry standards. Often, the device used to conduct the tests is known as a universal testing machine or tensile testing machine. Any grips used must be compatible with the machine in terms of both installation fit and applicable level of tension.
Types of Grips
Rubber is easier to hold in a grip than a harder substance. As a result, a wide variety of grips can be used, from simple manual vise grips to what are known as eccentric roller designs. Manual and pneumatic vise grips are most common, and should have rubber-coated or serrated-insert faces for proper hold on the sample.
Other Requirements
In addition to the proper size and fit, relative to the rubber sample and universal machine, grips used in shear testing should be as easy to use as possible, able to maintain a consistent force and meet all appropriate safety standards. As with the samples themselves, testing equipment needs to be able to withstand any unusual elements, such as high temperatures, required during the testing process.