Torque Wrench Calibration - Difficult time with repeatable measurements

J

jeremiah

#1
Recently, we purchased a digital torque meter to calibrate torque wrenches in-house. I am having a difficult time with repeatable measurements. For an 80 ft/lb torque wrench, I was told to calibrate at +/- 4% tolerance. Ok, but I can't get any torque wrenches in spec! for 20 ft/lbs-readings are above 21 ft/lbs. We are not using a mechanical arm to pull the wrenches-only manually at the moment. Has anyone had a similar problem? Can someone tell me the industry standard or where to purchase one? Thanks:confused:
 
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Marc

Hunkered Down for the Duration with a Mask on...
Staff member
Admin
#2
There are a number of threads on Torque Wrench Calibration and MSA. To get you started, here are a few: Torque Wrench discussion threads.

I'm not a calibration guru so I'll have to let someone else address your specific situation.
 
K

Kevin H

#3
Jeremiah, I'm not a torque wrench expert, but for many similar devices (pressure gauges for example) tolerance is defined as the full scale of the device - so a 0 to 80 lb torque wrench would have a +/- 4% tolerance of 4% * 80 or +/- 3.2 ft/lbs. If that is a correct interpretation/extension then a reading of 21 ft/lbs for 20 ft/lbs would be in tolerance.

If you need to be closer to a true reading of 20 ft/lbs you may may need to have a more discerning measuring tool than a 0-80 ft/lb torque wrench.

I hope the above sparks some good discussion for you from the many experts on the Cove more knowledgable than me.
 

rmf180

Involved In Discussions
#4
jeremiah said:
Recently, we purchased a digital torque meter to calibrate torque wrenches in-house. I am having a difficult time with repeatable measurements. For an 80 ft/lb torque wrench, I was told to calibrate at +/- 4% tolerance. Ok, but I can't get any torque wrenches in spec! for 20 ft/lbs-readings are above 21 ft/lbs. We are not using a mechanical arm to pull the wrenches-only manually at the moment. Has anyone had a similar problem? Can someone tell me the industry standard or where to purchase one? Thanks:confused:
Is +/- 4% good enough for what you are doing? If so, stick with it. If not, you can overcome limitations of your "calibrator" by using multiple trials and using the average value. The inaccuracy of a gage is reduced by the square root of the sample size. If you measure each test point four times, you have effectively cut the error in half. Your reference to using a mechanical arm would definately help your situation if percision is a concern. In most cases, tourque wrenches are not that critical in application so it is not useful to add the cost. Keep in mind that with calibration you are assessing fitness for use and you (the company) defines what that means!
 

Hershal

Metrologist-Auditor
Staff member
Super Moderator
#5
Jeremiah,

What you are expericing could be caused by many things, including the wrenches themselves. If the OOT reading progress in a linear fashion that suggests that th wrench may be adjustable to tolerance, that your calibration device is OOT, or even the mechanical attachment of the wrench to the standard could be loose or off center, or that the pull of the wrench throws it off center.

If the errors are not linear that suggests that the wrench or the standard could be bad.

If the errors are at the edge of acceptability, then your uncertainty study will let you know if you may actually be in.

Of course, I always suggest having an accredited calibration provider perform your cal if at all possible.

rmf180, I am not familiar with what you suggested to reduce jeremiah's error.....I would love to review that method, where is it doumented?

Hope this helps.

Hershal
 

Miner

Forum Moderator
Staff member
Admin
#6
I believe that what he is referring to is taking the average of sample size n reduces the variation by the SQRT of n. Therefore, the average of 4 samples reduces the error by the SQRT of 4, which is 2.
 

Hershal

Metrologist-Auditor
Staff member
Super Moderator
#7
Still, where is this documented? It is not Type A calculation methods.....I am certainly open and willing to review it, but need the documentation that supports this approach.....

Hershal
 
#8
Jeremiah
+/-4% (or +/-6% depending on type) is the norm for torque wrench calibration tolerances, there is a British Standard, but the number escapes me. To eliminate variation, the geometry of the test need to be controlled, i.e., the wrench has to be at 90deg to the calibrator, the load needs to be applied slowly over a period of about 5 seconds, no jerking or sudden movements, (this is important), all the equipment need to be very secure. As a point, the digital wrenches are no more accurate than mechanical indicating items. The method in the British Standard says, "exercise" the wrench at 100% of the scale first, 5 times, and then check at 20%, 60% and 100% of the full scale, take the average of 5 readings at each setting as the result. Repeatability of measurements should be in the order of 0.1ft/lb. You may be able to adjust to "best fit" to meet the tolerance, but this equipment does need regularly checking. One of my customers checks on every issue from the stores. Please PM if you need any more information.
Paul
 

Charles Harvey

Inactive Registered Visitor
#9
You might take a look at these. ASME B107.14M (This standard covers common manually operated torque wrenches and screwdrivers used for control of the tightness of threaded fasteners. It is not intended to describe products infrequently utilized or those designed for special purposes.) and ISO 6789:2003 - Assembly tools for screws and nuts -- Hand torque tools -- Requirements and test methods for design conformance testing, quality conformance testing and recalibration procedure.

Before we had a mechanical loader we had to limit our calibrations to 10%. We added that much error by pulling them by hand.
 

rmf180

Involved In Discussions
#10
rmf180, I am not familiar with what you suggested to reduce jeremiah's error.....I would love to review that method, where is it doumented?

Hershal
Hershal,

I am referring to the practice of reducing variation through multiple readings. This practice is detailed in numerous texts and is used where gaging cannot be improved to adequate level due to cost or technology constraints. This technique can also be used as an interim method while looking for new technology. You can refer to MSA III Chapter V - Section D for details.
 
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