Instron 300DX, Руководство по эксплуатации

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Mount fixtures
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Grip jaw selection
A specimen’s size, shape and hardness must be considered when selecting the correct set of grip jaws to
test that specimen. Improper selection of grip jaws can allow specimen slippage. Specimen slippage will
cause excessive and premature wear of the grip jaw’s teeth and further reduce the grip jaw’s gripping
ability.
In general, there are three criteria to consider when selecting a set of grip jaws:
•
Face profile: The grip jaw’s gripping face is made with a certain profile to fit the cross section of the
specimen. The two most common face profiles are flat and vee. Flat-face grip jaws are used to test
rectangular, or flat, specimens. Vee-face grip jaws are used to test round specimens, including rebar
specimens. There are also grip jaws that have a semi-circular profile for testing cable-type
specimens.
•
Tooth profile: The surface of the gripping face is cut with a specific tooth profile to provide gripping
ability. The tooth profile is defined by the cut of the teeth and the spacing of the teeth. The tooth cut
is typically diamond or horizontal (also known as serrated). The tooth cut may affect the gripping
effectiveness on some specimen types. The spacing of the teeth can be reported in either a pitch
measurement (distance between rows of teeth) or the number of teeth per inch (TPI). A finer tooth
(i.e. smaller pitch or larger TPI) typically works best for softer materials and smaller specimens. A
coarse tooth (i.e. larger pitch or smaller TPI) typically works best for harder materials and larger
specimens.
•
Specimen range: Grip jaws are typically designed to test a specific range of specimen sizes.
However, certain types of grip jaws, like those for cable-type specimens, are designed for a single size
of specimen. It is very important that the size of specimen to be tested falls within the specimen
range of the grip jaws. If the specimen is larger than the maximum or smaller than the minimum
designed specimen range of the grip jaws, do not use those grip jaws. Also, once the specimen is
installed in the grip jaws, the jaws should not extend out of the grip body. This includes the position
of the grip jaws at the end of the test. If the grip jaws do extend more than 7 mm (0.25 in) at the end
of the test, this would indicate the wrong size jaws are being used.
Instron offers a wide selection of grip jaws. For further information about the sizes and types of jaws
available, see
Appendix B . You can also contact your regional Instron office or check our web site at
www.instron.com for assistance.
Specimen guidelines
It is important to obey proper guidelines when preparing and testing a specimen:
•
Do not attempt to test specimens that are of incorrect length:
• For typical flat grip jaws and vee grip jaws, the specimen must be long enough to allow it to be
gripped by at least 80 percent of the gripping face of each grip jaw. When less than 80 percent
of the grip jaw’s gripping face is used to clamp the specimen, the grip jaw will tip and cause
damage to the grip pocket and internal grip components. Often the specimen may slip and
cause damage to the face of the grip jaws.
• For cable jaw inserts, the specimen must be long enough to allow it to be gripped by 100 percent
of the gripping face of each grip jaw.
•
Set the jaws firmly in contact with the specimen but do not hammer on the grip jaws or any other
component of the grip assemblies to tighten the grip jaws.
•
Watch for signs of specimen slippage and stop the test if slippage occurs.
•
Be sure that the tooth pattern of the grip jaws is sufficient for the specimen’s material type.