Torque Calibration Accuracy Criteria

Douglas E. Purdy

Quite Involved in Discussions
#1
Yes, I have done a search and I believe that my question is a bit different than the other threads.

I believe that for the past couple of years our criteria for calibrations have been handled primarily by the Outside Caliabration Sources. In an attempt to better understand what our accuracy requirements are I went to change the accuracy requirement to what the OEM (Mitutoyo) had stated for a Datatroq (range 25 - 250 in.lb.) "+/- 1% plus 5 increments from 10% to 20% of range; and +/- 1% from 20% to full scale." The Outside Calibration Source had been performing the calibration per the ASME B 107.14M, +/- 4%.

Well the torque wrench did not meet the 1% criteria, but does meet the 4% criteria. My question is should I have continued at the 4% because that is what the Outside Calibration Source normally uses for calibration? I do not know what accuracy is required.

An evaluation of the torque results showed a mean of 22 Newton Meters, while the specification is 12 Newton Meters minimum. We learned from the calibration that at 103.2000 standard the wrench read 100.0000, 2.168 in.lbs. below the 1% accuracy. Because our torque results are so high above the specification we are reasoning that the Out-Of-Tolerance (1%) did not allow nonconforming product to be submitted.

Any help or recommendations would be appreciated.

Thanks,
Doug
 
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BradM

Staff member
Admin
#2
Hello Douglas!

I know you are an old hat around here, so I hate to ask the obvious. But, what is your requirement?

Saying, for this application, if your process requirement is substantially broader than the mfg./industry spec. why not accept it under the broader tolerance? You can always ask for adjustment against the tighter tolerance, but you don't have a calibration failure as long as it's within your process requirements.

Did it just fail this one time, and could it "possibly" be something unique with this one event? How has your history been on this instrument?
 

Douglas E. Purdy

Quite Involved in Discussions
#3
Brad,

I thought I was clear in my post, that I do not know what the required accuracy is needed for the torque wrench. I went to go with the Mitutoyo published accuracy and the torque wrench does not meet that. The questions being raised right now at my employer are: why not go with the 4% accuracy since that is what the Outside Calibration Source has been performing per the ASME B107.14; and why did we give the Mitutoyo accuracy in the PO?

I was hoping that the metrology gurus would give some feeback on the ASME as opposed to the Mitutoyo. If we were to go with the 4% accuracy should we be raising the internal torque requirement as well? If the torque results were to approximate the 12 Newton Meters, then our product would be nonconforming.

Thanks,
Doug
 

BradM

Staff member
Admin
#4
I'm not really a guru of anything; nor do I have any expertise as far as comparing the two specifications between Mitutoyo and ASME specifications.

Please be patient, in that I may not be able to understand your responses. I understand that you are not sure what specification you perceive the torque wrench should comply with. My query is about what specification you and your employer can live with.

I have found a huge disparity in how organizations calculate and come about with their stated accuracy. I have had a few flatly tell me they really had no idea. Saying that, sometimes it can be impossible to calculate an instrument to the mfg. specification; you will be having it calibrated every other week!:) Even then, it will only pass every other week!:tg:

Instead of getting frustrated with the circumstances or trash an instrument, I have established tolerances (it sounds in your case like 4%) that comply with my process requirements.

I have heard different philosophies on calibration, most which are thoroughly valid. One of which is calibration is about verifying the instrument performs as the mfg. states. At the end of the day, though, I would feel confident about defending an instrument against my process requirements. So, if I were in your shoes, I would justify grabbing the broadest tolerance possible. Why they are different has many factors which contribute to them being different, and unless you can get someone at Mitutoyo to give you detailed explanation, you may not can justify it.

Maybe some of those Covers more well versed than I can give you some better info. Sorry if I'm not much help.
 

Jerry Eldred

Forum Moderator
Super Moderator
#5
Let me "muddy the waters" a little further. I am not a torque expert, but I am a torque "expert" (i.e.: last one to work on it). We do a lot of torque in my current lab (mostly electric torque - another entire topic; butand a lesser quantity of static).

There is also anISO procedure - ISO 6789 - which specifies tolerances by torque wrench type (rather than by mfr/model). Although I'm not familiar with the ASME procedure you mentioned, I'm assuming it is similar to the ISO method (maybe a national equivalent).

My perception has been that procedures such as the ISO and ASME are considered the more widely accepted method/specification for calibration of torque wrenches.

The other issue in using the manufacturer's specs on the torque driver is what method to use.

One thought might be to do a "Woodshed R & R" study (that is, informal study) on the performance of your torque driver. Run 50 or 100 readings and see what kind of spread you get (variability). 1% seems awfully tight. My wild, uneducated guess would be that the 2 Sigma spread might really 'tax' a 1% limit.

Just a few thoughts.
 
P

Phil Fields

#6
Maybe I am missing what you are trying to achieve, but my question is what is it you are applying the force to and what specification does it need to meet? What effect does the 1% / 4% have on the inspection/assembly process?

Phil
 

Douglas E. Purdy

Quite Involved in Discussions
#7
Let me modify my question. Are the accuracies called out in the ASME B 107.14 for the various torque instruments considered to be acceptable as an industry standard for torque requirements of weld nuts on metal stampings [Torque Out minimum of 12 Newton Meters]?

Thanks,
Doug
 
S

Southern Girl

#8
:bigwave:

I just felt the need to respond to this thread. You speak of accuracies, what are you torqueing and how much, what is the specification for the part or the tool? If the calibration lab is certified, they must use an industry standard or have their own process validated during certification, for calibration procedures. The customer, which is you, specifies the tolerances, either from a standard or of your own making, to which the gages are judged against. If the gage manufacturer's specification is +/- 1%, there is no reason that you can not specify +/- 4% (CW and/or CCW) as your tolerance. The one thing you should not do is change the tolerance to less than that of the manufacturer. There is no way that the gage can be made "more" accurate!

What you have to be careful of is that the gage is accurate enough to be used to measure whatever it is you are using it for. In other words, use the tolerance of +/- 4% if you want to, but if your spec is 100 in-lbs, then the gage tolerance at that range will be +/-4 in-lbs of reading. If you can live with that broad of a potential range then there is no problem.

I have used/specified torque wrenches for over 20 years and the ASME standard is the most prevalent standard referenced by torque manufactures. The majority of wrenches I have purchased for use by the production lines have an accuracy of +/-3 % or 4% of reading in the CW direction at 20-100% of full scale. These have served me well in aerospace, automotive, injection molding and machining production. I would be more afraid of the torque drivers than the torque wrenches in an assembly process. Now you have to include torque angle in the equation and it can really get crazy.

I guess my only question to you is why do you purchase a torque wrench with a 1% accuracy if 4% is acceptable?

I have rambled enough, but I hope this is as clear as mud! :D
 
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