While there are many varieties of gripping technologies (wedge, screw, hydraulic, pneumatic, etc.), they all can be classified as proportional or non-proportional according to the way in which the clamping force is exerted on the specimen.
Proportional Clamping Force – The clamping force exerted on the specimen is proportional to the tensile load applied to the specimen.
Non-Proportional Clamping Force – The clamping force on the specimen is independent of the tensile load applied to the specimen.
Proportional Clamping Force – The clamping force exerted on the specimen is proportional to the tensile load applied to the specimen.
Non-Proportional Clamping Force – The clamping force on the specimen is independent of the tensile load applied to the specimen.
2 comments:
Hi Dave,
What are the pro's and con's of using the two different types of clamping force? Are there typical applications where proportional or non-proportional are preferred?
Regards,
Tim
Hi Tim,
Thanks for your question.
The use of either method is very much dependent on the application and the specimen type. One of the areas of focus is around jaw breaks, where the specimen fails near of in, or near, the jaws resulting in reduced maximum load or strain in some cases. The same materials may be more sensitive to this type of failure depending on its form, think sheet steel vs. fine wires. Another concern is slippage, you want enough clamping form to hold your sample during the test, but not too much which could damage it before you begin testing.
There is no golden rule as to when each should be used, both are often just as effective when combined with features such as specimen protect and appropriate jaw faces. There are exceptions to this with some materials…
Typically wedge grips are proportional and side acting grips are non-proportional. One of the nice features of hydraulically actuated moving body wedge grips is that you can set a high initial clamping force, making them virtually non-proportional for the initial phase of the test, but as the load goes above the hydraulic force they become proportional. This allows a great deal of flexibility in the setup.
Thanks,
Dave
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