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  Measurement, Test and Calibration
  MSA on Torque wrench

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Author Topic:   MSA on Torque wrench
dewie
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Posts: 44
From:BKK
Registered: Sep 1999

posted 04 May 2001 12:14 PM     Click Here to See the Profile for dewie   Click Here to Email dewie     Edit/Delete Message   Reply w/Quote
Somebody helps.......How can we do MSA on torque wrench?

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Jerry Eldred
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Posts: 136
From:
Registered: Dec 1999

posted 04 May 2001 01:17 PM     Click Here to See the Profile for Jerry Eldred   Click Here to Email Jerry Eldred     Edit/Delete Message   Reply w/Quote
MSA's are not my specialty. But my first instinct would be to take a series of readings using a torque calibrator (one of the digital types with a peak indicator). Follow normal MSA methodology, using the same person for a series of readings, then different people and different shifts (as applicable).

It must be understood that the MSA is on the torque wrench. To understand how repeatable the torque wrench is, MSA must be done via a method that verifies what you tighten in ft-lbs or whatever other force unit is used is tightened repeatably to within acceptable variability. This can only be done by measurement of what the torque wrench has done. The only method I am familiar with is via a torque calibrator.

If you don't have a torque calibrator, there needs to be some method of "reading" the breakover point with something of appropriately higher stability than the torque wrench. It is also necessary that either the torque wrench be sealed at a setting (ONLY if it is used that way), or that the wrench be backed down to zero, and re-tightened to the torque setting at each iteration. If the torque wrench is always used at one setting only, and is left in that setting, leave it. If the torque wrench is used at a variety of settings, use a standard methodology for how the wrench is set up for the specific torque of the MSA, and repeat that exact methodology at each iteration.

If you don't have a torque calibrator, perhaps you could rent a calibrated unit for a few weeks for the MSA, contract a calibration subcontractor to do it for you, or pay a calibration subcontractor for time in their lab using their torque calibrator under their supervision.

The one other suggestion would be to see if the manufacturer has any data on R&R for the particular torque wrench (only if you can't derive the data yourself).

Another question I would have if I were in the situation would be to verify as to whether the torque wrench truly did or did not need an MSA. MSA's are normally done on measurements or controls to process that could impact products ability to meet it's claimed specs. The MSA gives a confidence as to whether or not the product meets that spec. I am sure it is conceivable that torque of a screw could have impact on a product spec. But there are also plenty of other potential areas where a calibrated torque wrench is used on a product, but does not truly impact product specs. I truly don't know which category this falls into. But I would certainly ask the question as to whether or not it is a requirement in the particular case.

PLEASE -- don't take that as a recommendation either way. Only to ask the right questions to determine whether or not it is a legitimate requirement. I have seen many cases over the years where companies calibrate everything in sight because that is an apparently simpler path than justifying on an individual basis whether there is a need or not.

I'm not sure how much help this is. Hopefully it will at least give some food for thought.

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Drew H
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Posts: 11
From:Kelowna, B.C., Canada
Registered: Oct 2000

posted 04 May 2001 08:14 PM     Click Here to See the Profile for Drew H   Click Here to Email Drew H     Edit/Delete Message   Reply w/Quote
It is difficult to perform MSA on a torque wrench according to QS standards. Doing GR&R on a "production part" is not possible in the usual sense as repeat applications of the torque to the fastener tends to change the friction characteristics. Crack-on or crack-off methods are somewhat inaccurate due to the frictional nature of the fastener. There was some discussion of this a while ago, but a definative answer was not presented. Anyone have some good ideas/procedures?

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TorqueGuy
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Posts: 10
From:Ontario, Canada
Registered: Jan 2001

posted 18 May 2001 12:59 PM     Click Here to See the Profile for TorqueGuy   Click Here to Email TorqueGuy     Edit/Delete Message   Reply w/Quote
Our company is currenty performing MSA studies for our customers on their torque wrenches. We use three different operators taking 10 readings each on three different days. We use a torque wrench calibrator to accomplish this.

You should perform your MSA's in your plant using your parts and operators, however I don't know how this is possible. You can't get a bolted joint to behave the same way twice especially when you re-torque it or try the breakaway method. The only way to repeat a part is to use a calibrator with a consistant joint rate.

Buying a calibrator for your own MSA's introduces a whole new set of problems from cost to calibration to training, etc.

Some people may say that an MSA performed off-site is invalid but my customers have (so far) done ok with their auditors.

I don't want to put my company name here since I think the cove is for education not advertising but feel free to email me if you would like some more information.

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