MSA - Torque Wrenches - Statistical studies that cover requirements of MSA

[Yochio Ito]

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

 

[Jerry Eldred]

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]

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

 

[[email protected]]

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.

 

[Jerry Eldred]

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]

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

 

[Charles]

MSA - torque wrenches - again !!

Greetings to all from the UK! (first post)

I have read through the previous posts on MSA studies on torque wrenches and nut runners.

I work for a luxury vehicle OEM based in the UK. We are busy implementing TS16949. Obviously there are assembly operations that are Safety Critical in terms of torque, so torque wrenches are on the llist for MSA's - hence this post.

1. I propose to follow a MSA procedure for "click" type wrenches as follows: - 3 trials at one setting, then re-set the wrench (i.e. back to zero, then up to the set torque), repeat 10 times over 3 appraisers. We will use a torque calibrator with the readout hidden from the appraisers to try to reproduce the "real-life" conditions. I also hope to pick up the "click" torque as well as any peak over-torque applied.

Does anyone have any comments / advice to share on the above method?

2. Do you think that I should use the average / range method for analysis? (there were posts suggesting that ANOVA would be a better method?)

3. I have not yet got the 3rd ed. of the QS9000 MSA manual (I am using 2nd ed. as a guide), TS does not seem to be prescriptive in what to method to follow - any comments?

4. I have also had a comment that we should also do an "E.M.P." study, which is a new one on me - any comments?

Thanks in advance, I regularly use the Cove forums to keep current - great help !

Charlie

 

[Atul Khandekar]

Hello Charles and (a belated) welcome to the Cove Forums!

Let me take a shot at your questions and initiate a discussion:

1. The metrics 'feels right' to me. The simulated settings for the 10 jobs of course will have to be within the nominal torque T+/- tolerance. The only problem with the study IMO is that you propose to take consecutive 3 trials for one appraiser for ecah job. I think you should try to randomize the sequence as much as possible.

2. The proposed scheme is replicable, and hence Range-Average method should work. In case of non-replicable study, Nested ANOVA would be suggested. You must however consider the parts x appraiser interaction. Generally there is a tendancy on by operators to 'jerk' the wrench for higher torque specifications. So they tend to use different methods at low torque and at high torque. If this is the case, you should consider interaction effect and use Crossed ANOVA calculations. Also when working with force gages & torque wrenches you are likely to have issues of interaction. IMO, you should also conduct bias and linearity tests.

3. I believe it make good sense to use the latest guidlines published by AIAG.

4. EMP: This is a new one on me. Anyone knows what EMP test is?

Hope this helps a bit.
-Atul.

 

[Brian Dowsett]

Charles,

A question.

Are the torque wrenches being used to apply the final torque to the joint?

Or are they used as an overcheck?

If the former, I'd suggest that they are not "measuring devices"
and don't need an MSA but need a good calibration method.

If the latter, the method used in process must be to apply the wrench to a joint that's already torqued to it's final torque and make sure the wrench clicks without moving the joint.

In this case I'd say your gauge study would be an attribute type,
I.E. does the wrench "pass" the assembly as OK, yes or no.

Cheers

Brian

 

[Atul Khandekar]

This is an interesting observation by Brian.

I found one reference at:
http://Elsmar.com/level2/cal_q1.html which says,

Lets say you have certain screws you torque - if the instrument used to install the screw 'automatically' sets torque, the instrument has to be in the system (even tho it is not a gage per se). If someone installs the screw and then at some point tightens to a torque spec using a torque wrench, the torque wrench has to be in the system. Both cases would require R&R (production equipment performing a repetitive single measurement).QS 9000 is the stickler here and it actually points to the AIAG Measurement Systems Analysis manual which you must follow.

Any other thoughts?