2-17
Preload the Load String
Product Support: www.instron.com
Preload the Load String
This procedure eliminates backlash and deflections within the load string which can degrade
the integrity of test results, especially when testing at high loads. The procedure involves
preloading the entire load string and then hand-tightening all the locknuts on all the grips
and couplings. Even when using self-aligning couplings on the upper grip, it is good practice
to preload the lower grip.
You will need a rigid specimen that is strong enough to sustain the preload value without
breaking. This means a specimen that can sustain a load that is:
• 10% above the expected test load, or
• the maximum load rating of the weakest component of the load string (grips or load cell)
whichever is less. For example, if your grips are rated at 1kN, the load cell at 2kN and your
expected test load is 500N then you should preload to at least 550N but not more than 1kN.
Before inserting the specimen ensure that:
The grips and couplings are installed but the locknuts are not tightened.
Crosshead travel limits are set (refer to “Set the crosshead limit stops” on page 3-
10).
The value of load in the live display is near zero. If it is not, balance the load (refer to
“Balance a transducer” on page 3-12).
The load limits are set in the software to a value that matches the maximum load
capacity of the weakest component in the load string.
To preload the load string:
1. Install the strong specimen.
2. Increase the load on the load string to the chosen preload value.
3. Hand tighten all the locknuts on the grips and any intermediate couplings.
4. Reduce the load to zero.
5. Remove the specimen.
The load string is now preloaded and all the locknuts are tight and should not move during
subsequent testing. The system is now ready to test.
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