Torque Wrench Calibrated with Conditions

#1
Here is another torque wrench question. I have just done a cal. check on a 80 ft.lb. non-ratcheting wrench. I am following the guidelines found in ASME B107.14-1994 and testing at 20%,60%, and 100% of span. This particular wrench was out by up to 6 lb.s at the 20% reading, but was well within the acceptable range at 60% and 100%. This wrench is used only in its upper ranges, if I label this wrench clearly as being off at the 20% range can I continue to use it?
 
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Jerry Eldred

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#2
Depending on how your calibration system documentation is worded. In our system, when a calibration is done to less than full specifications, we call it a "LIMITED USE" with a different labelling. Our labels are printed by a Zebra printer. We then apply a diagonal red marker line with permanent ink to alert user of the limitation. So long as the limitation is documented so that the wrench will not be erroneously used at a setting where it is out of spec, it is generally acceptable. Theoretically you can calibrate anything to whatever spec you want (I know I'm going to catch some flak (sp?) for that remark. So I'd better explain. And please read on to the end of my remarks, as that is where I render my opinion.

For example, a timer has a mfr spec of +/-.005%. I use it in an application for ONLY a 60 second timing with a process limit of +/-5 seconds. This is good, because I have a higher confidence that my timer will perform the way I want it to. This also gives me the advantage that I can calibrate it in place with a good handheld digital stopwatch (uncertainty of around +/- 0.3 seconds; including operator variability). If I calibrated it to it's full specs, it would add no value to my process and be more costly to maintain, as I would have to do it in a lab, and perhaps invest in one of those mechanical simultaneous start/stop gizmos.

Another example is an oscilloscope in a fixed test station used ONLY to check a peak to peak voltage of a 1 KHz squarewave (whose risetime I don't care about). We happen to have a 300 MHz scope laying around the store room when we set up the test bay, and so it is used.

These are not BEST practices, as whenever you calibrate anything at less than it's full specs, you potentially incur a risk that the test equipment could be erroneously used in an application that it wasn't properly calibrated for.

I also make it a practice that I preferrably do not calibrate a LIMITED USE simply because I don't want to repair a problem with the instrument. If an oscilloscope is used for general applications and someone blew up channel 2, I try to make it a policy not to LIMITED USE it as "CHANNEL 2 NOT CALIBRATED", as this is just a way to save the cost of repairing it. I try to make LIMITED USE apply only to dedicated uses of test equipment where the full specification is not needed.

So in the case of the torque wrench, my gut reaction after all of this is that I recommend you repair it. Reason being, if it has gone out of tolerance at the low end of the range, and you only use it at the high end of range, without historical data for that limitation, you don't fully know what is happening to that wrench. Could be that there is some mechanical fatigue on the spring or washers inside it, or other deteriorating factors that place your measurements at risk. And that is the reason I try not to use an out of tolerance condition as a reason for a limited use; it is risky. I won't associate a LIMITED USE with a reliability problem with a measuring instrument, but with a limited application.

I hope that is of some help.

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D

D.Hutti

#3
In this case I would highly suggest that you calibrate/repair the wrench to mfr specs. A torque wrench is too common a tool that could easily be use for an application that it is not certified for. It is human nature to just say "no one will know after all it is just a torque wrench".

The Air Force did an extensive study on the effects of out of tolerance torque werenches, or I should say the mis-treatment of them, they were able link aircraft loss due to them.

Like the response before me, should you decide to use the tool in the condition that it is in, #1 your calibration system should clearly specify how you are handling this condition and #2 you need to take precautions to clearly identify the tool and mark it's limitations on the tool.

Dismas Hutti
Technician
Exelon PowerLabs
 
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