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Checking Torque Wrench Calibration


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Beam-type torque wrenches are very imprecise, but much better than nothing. Clicker wrenches should be periodically calibrated, but virtually none of us go to the expense and effort to have that done. But you CAN check them yourself.

 

Checking the accuracy of a torque wrench is not too difficult IF you have at least two wrenches with overlapping ranges. Here's how:

 

 

  • You need a high grade bolt and nut - I prefer grade 8 (six radial lines on the head) where a 7/16-14 bolt would allow you to test up to around 70 ft lbs. Make sure the threads on the bolt go all the way to the head, and put at least one large washer on the bolt.
  • Now clamp the nut in a vise with the top side flush with surface of the vice, then screw the bolt all the way in.
  • Set your first torque wrench to a low setting in the range that is covered by your second wrench. For example, if both wrenches go from 10-100 ft*lbs, I would start at 15 ft*lbs.
  • Carefully tighten the bolt in the vise until the wrench clicks.
  • Now switch to the second wrench, set to the same torque, and see if it clicks BEFORE the bolt turns at all. If so, that is good, but you are only 1/2 done!

What you have just shown is that the first wrench did not tighten the bolt any LESS than the same setting on the second wrench, but it MAY have tightened it more. So to complete the check, you need to loosen the bolt, then repeat the above steps starting with wrench #2. If this test also shows the second wrench clicks before the bolt turns any more, then you can be confident that both wrenches are setting the same torque, so they are almost certainly both correct.

 

Now increase the setting on both wrenches up to something in the middle of the range and re-do the whole test. The only thing you need to be careful about is to make sure the max torque setting you test is within what the bolt grade and size can handle.

 

If the two wrenches you are testing do not both click at the same setting, no matter which one you start with, then at least one of them is bad. To find out which one, you will have to find a third wrench to test. When you have two wrenches that both click the same when you do the entire test above, then you can use either one of those as a baseline to compare other wrenches.

 

Just an FYI to anyone who brashly wants to claim all Harbor Freight torque wrenches are junk - I have a total of 5 torque wrenches here - two from HF (a 1/4" drive clicker and a 3/8" drive clicker), an old beam type wrench in in*lbs, and two very high quality 1/2" drive clickers that were calibrated by a USAF test lab. ALL of those wrenches test the same in every point where the ranges overlap, so the HF wrenches are dead accurate within a reasonable tolerance. The only complaint I have at all against one of the HF wrenches (the 1/4" drive) is that the gradations on the handle are a bit more difficult to use when setting the torque value - I had to compare it to the other wrenches to be sure I was reading it correctly.

 

So if anyone in the North Texas area wants to check their torque wrenches, we can compare them to mine.

Goose

 

BTW - How you store your clicker-type torque wrench is important. If you do not have (or haven't bothered to read) the instructions that came with your wrench, you might be in the habit of just chucking the wrench back in your tool box after that last "click" on the last bolt of the job. But it is important to turn the torque setting all the way down to remove the tension from the internal spring before putting it away. Leaving the spring under tension is the biggest reason why these types of torque wrenches change calibration.

Edited by V7Goose
adding information suggested by "pofarm" (trial member)
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