MTS Series 647 Hydraulic Wedge Grips - Manual (Page 90)

7. Extend the actuator until the specimen is positioned in the upper grip allowing a space of at least 3 mm
(0.12 in) but not greater than 6 mm (0.25 in) between the top of the specimen and the inner top edges of
the wedges. Lock the upper grip.
8.
Caution:
Specimen slippage during the test can damage the grips.
Specimen slippage typically occurs when:
• Insufficient grip pressure is used to securely grip the specimen.
• Wedges with worn (rounded or flattened) serrations are used.
• A specimen with a hardness of Rc 52 or greater is being tested.
In most instances, when testing specimens with a hardness less than Rc 52, increase hydraulic
grip supply pressure to prevent slippage. Do not exceed the maximum pressure rating of the
grip. However, increasing the pressure setting to hold round specimens can reduce the life
of the vee-notched wedges.
Adjust the grip pressure to the maximum level as determined in “Determine the Gripping Pressure.”
9. If specimen slippage occurs which exceeds the specifications in Step 7 and Step 8, reinstall the specimen
and increase the grip pressure. Note the caution above.
If a series of similar specimens will be used and specimen dimensions permit side insertion, it is not
necessary to repeat this entire procedure for each specimen. When loading a similar specimen, release
the upper and lower grips and remove the old specimen. Then insert the new specimen as described in
Step 6 and clamp the specimen.
Remove a Specimen
Prerequisites for specimen removal:
• After high-temperature testing, open the door slightly to allow the heated air to escape. Do not stand or
put unprotected hands in the path of the escaping air when opening the door. Wait for the air to cool.
• Open the door the rest of the way.
• Allow the specimen to reach a tolerable temperature before attempting to remove it, or use appropriate
tools, such as tongs, to remove the specimen.
90 Series 647 Hydraulic Wedge Grips Reference Manual
Operation