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Tuesday, January 31, 2012

Grip it Right

Gripping a specimen correctly is important if you want to acquire accurate test data. To most test operators, this is second nature.
  • Make sure the grips and specimen are well aligned
  • Make sure the grips and grip jaws are suitable for the material being tested and the test loads expected
  • Make sure the grips and grip jaws are clean and undamaged


However, often overlooked is ensuring the specimen is correctly inserted into the jaws, particularly when using wedge-type grips. You should insert the specimen so that it is centered and that it contacts the full length of the grip jaws. Further, the grip jaws themselves, once gripping the specimen, should not protrude below the lower face of the grip. If they do, then you should select a smaller grip for the test. See the illustration below. If a test is performed with a partially clamped specimen or with the grip jaws protruding below the lower grip face, the grip jaws can experience severe twisting loads leading to their failure.

7 comments:

Anonymous said...

I'm a little concerned about this. My grips look (and have always looked since day one of getting my machine (15+ years ago)) like the middle diagram which apparently is wrong. Some help and reassurance would be welcome.

Anonymous said...

Ditto

Anonymous said...

Souldn't there be a product recall in the works

Instron said...

It appears we have caused some confusion with this recent technical tip. So, let’s take a moment to clarify... Paul Martin, Instron Manager of Mechanical R&D, said that on the first image it is not OK if the specimen is inserted less than ½ way into the grip. We recommend at least 2/3 engagement. Regarding the 2nd image, Paul clarified that our grips are designed so that the jaw faces don’t protrude too far out of the grip. Please keep in mind there are always some rare situations. If you have additional comments or questions, let us know by leaving a comment or contacting us directly. Thank you~

Anonymous said...

How far is "too far"? My jaw faces protrude about 8mm out of a total jaw length of about 57mm.
This all seems a bit vague!

Anonymous said...

All our grips look exactly like the middle diagram so this is not that rare!

Instron-PM said...

Yes, I agree. The Techtip is confusing so let me shed some light if I can: My take is that it's definately OK that the faces protrude out of the grip a little bit since almost all wedge grips will do that. I don't have an exact number for how far is too far since it depends on how far the specimen is inserted and how it deforms in the jaw, but in my experience if it sticks out of the grip more than 1/4 of the overall length of the jaw then that is probably too much.
This rarely happens however. The only situation I can think of where it the wedges may be pulled out too far is when testing very compliant materials that are close to the lower limit of the range for the jaw face. Since most grips have some sort of stop to keep the jaws from pulling out too far what would likely happen in that case is that the spcimen will slip before anything bad happens to the grip.
I hope that helps.

Paul Martin