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  MSA Studies for Torque Wrenches

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Author Topic:   MSA Studies for Torque Wrenches
Yochio Ito
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posted 24 February 2000 05:37 AM           Edit/Delete Message   Reply w/Quote
I've been looking for some statistical studies that cover requirements of MSA manual for torque wrenchers. But I haven't been successful. Can anyone shed any light on the subject? Or even indicate some standards which broach this subject?

Thanks in Advance,
Yochio

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Jerry Eldred
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Posts: 136
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posted 24 February 2000 04:28 PM     Click Here to See the Profile for Jerry Eldred   Click Here to Email Jerry Eldred     Edit/Delete Message   Reply w/Quote
I have two qualifiers before I make any comments.

1. My library is packed, so I am taking information from memory.
2. I am not an MSA guru (I expect to be by the end of the year).

My recommendation is to get a copy of the MSA manual from the QS9000 set, if you do not already have one. Internal to my company (a fortune 100 electronics company), we have a spec written on MSA requirements, which is essentially a paraphrase of the above MSA manual.

If you want an R&R study of torque wrenches, you need a torque standard. I suppose it depends on what type torque wrenches you are referring to. If they are the type that each user adjusts to a needed setting at each use, that is one case (I'll call those "variable torque wrenches). If they are fixed "click-over" type that are set to a specific torque setting by a calibration lab, and used for a calibration cycle at that setting, that is another case (I will call those "fixed torque wrenches").

For variable torque wrenches, repeatability I think would be to set at a given setting, and make the repeated measurements. Reproducibility would be to return them to a very low setting, then let multiple users on multiple days and shifts set them for the desired setting and make the redundant measurements.

It seems that what is needed is a torque calibrator. You would need to have multiple operators set them up as above, don't let the operator see the readings on the torque calibrator, then have operator set for desired setting, and turn until they click over multiple times on the calibrator (making sure that they cannot see the readings, as this may bias their operation of the wrench. To keep the study blinded even further, take the torque calibrator out to the production environment without notice, and ask operators with wrenches already set as they would normally use them in production to click-over on the torque calibrator.

For fixed torque wrenches, same as above, only the wrenches are already set to the desired torquing force.

If you don't have a torque calibrator, I am not sure what other instrument could be used, other than something that retains a fixed amount of torque.

The alternative if you do not have access to a torque calibrator, is to get hold of manufacturers documented specs for the torque wrenches. Those specs should be based on statistically derived tolerances. If it improves your comfort level, you could contact the manufacturer, and ask for some of the data. If you use their data and/or specs, that may be useful to determine cpK, etc. of your processes. I think even if you have the above data/specs, there is the added consideration of operator-to-operator variability of torque wrench setting. In other words:

a. If each operator set torque exactly the same (doesn't happen in real life), how much variability is there in the actual application. That would be repeatability.

b. Added to that is, how much difference is there between the actual setpoints operators set the torque wrench to.

Just a few thoughts. Again, I am not a guru. But I hope I have helped more than confused.

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Yochio Ito
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posted 28 February 2000 06:37 AM           Edit/Delete Message   Reply w/Quote
Dear Jerry,
I have in my facility, both cases, fixed and variable torque wrenchers. And I have a torque calibrator too.
But the biggest problem I see, is to get a joint that can reproduce the same conditions for all the measurements. R&R MSA studies requires that only one part (10 samples) be measured by all the operators.
A friend told me that a specific statiscal study for torque was developed in Mexico. Does anybody know something about it?

Thanks,
Yochio

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mgruebe3@visteon.com
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posted 09 March 2000 01:50 PM           Edit/Delete Message   Reply w/Quote
I am going through the same process of doing a gage study using torque wrenches. One of the additional things that I will need to gage is how well the operator measures the breakaway torque, which will not be as easy. I was hoping to find some type of device where you could set the initial torque also but I have not yet found it. Hopefully someone will find your note and have some information. If I find anything I will let you know.

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Jerry Eldred
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posted 09 March 2000 02:06 PM     Click Here to See the Profile for Jerry Eldred   Click Here to Email Jerry Eldred     Edit/Delete Message   Reply w/Quote
As a metrologist, I can envision readily some methods that could be used for measuring what I will call instantaneous torque. What I mean by that, is during the process of an operator applying force to set or release a bolt or nut with a torque wrench, you can divide it into an infinite number of points of time (i.e.: say for example it takes 500 milliseconds from the application of force until the wrench breaks over. Graphically picture the increase of force over time as something similar to a sawtooth wave. If you use a torque calibrator, I wonder if any of them have a proportional DC voltage output. If you use a torque calibrator, or some sort of electrical torque transducer with a voltage output, then input to a DC coupled digital storage oscilloscope, it would be quite easy to capture a time versus DC voltage waveform. Define a point in the waveform as the designated breakaway. For example, measure the highest peak voltage. Record that value at each iteration. Use that for repeatability measurements and for the reproducibility measurements.

I apologize one more time. I am just now in the process of transitioning from another quality position (although I have been in metrology 23 years) back into metrology engineering. So my library is all in boxes, and I am moving offices in a week and a half. So I am somewhat indisposed to be able to provide the level of answers I would like. I do finally have a transfer date. So by months end, I will be in the new position with my library back at hand.

In the meantime, I hope my 'seat-of-the-pants' answers on these topics are of benefit.

After my move, I will be happy to dig in my library for answers on manufacturers of electrical torque transducers.

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Brian Dowsett
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Posts: 35
From:Waterford, Ireland
Registered: Nov 1999

posted 15 March 2000 11:05 AM     Click Here to See the Profile for Brian Dowsett   Click Here to Email Brian Dowsett     Edit/Delete Message   Reply w/Quote
Yochio and friends,
I faced a similar problem to establish measurement system variation of dial type torque wrenches , used to measure breakaway torque.
The main source of variation, to my mind, is the amount of shock loading given by the operator, to get a stationary nut or bolt "moving again". Obviously if the wrench is violently used, the reading is higher than if the same joint is checked gently.
Our approach was to use an old fashioned torque checking device consisting of a weight suspended at a fixed distance from a central point (say 1 pound at 1 foot from the checking point = 1 foot pound torque).
We then got 3 operators to check each of 10 different weights (and therefore torques) three times each.
Although this was not an exact recreation of an actual torqued up joint, i'd argue that it is a fair representation (as the operator had to overcome friction in the bearing surface of the rig, pretty much like measuring breakaway torque in real life.)
As a result of one such test we did introduce (on a critical joint) use of a sophisticated torque wrench which sensed electronically when the bolt/nut was starting to move and registered the reading at that point.
Our efforts were never challenged by accreditation bodies but I must say that the auditors we met were never that interested in the test methods.

Brian

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