Calibration of Torque Wrenches

[rsm7400]

We recently had our AS9100 audit and something came out of it that we didn’t expect. Torque wrenches not calibrated. Now granted, we only use them to tighten machining fixtures on certain CNC jobs but we were still told they needed to be calibrated.

I have no idea what sizes of torque wrenches we have in the plant, but I need to get started somewhere.
1. Does anyone know of any NIST traceable torque standards to test the wrenches against?
2. What is an acceptable tolerance for torque wrenches?
3. If it is out of tolerance can our in-house calibration team repair one (like we do with mics or calipers for instance)?

Any information or incite will be greatly appreciated!

 

[MintJulep]

1. Purchase a torque wrench calibrator. Have it calibrated by the manufacturer annually, with certs. That establishes NIST traceability.
2. To do it right you really should work that in with an overall fastener preload control strategy for the facility. Or just go for +/- 5%.
3. Yes.

 

[ricklts]

I have never figured out how to calibrate a torquing device. You can measure output in a torque value and measure the input load/pressure, but unless there is an adjustment knob on the device, all you are really doing is recording the output vs input for that tool, in that orientation, with that input device/pump, on that day.

Having said that, and dependent on the type of tooling-manual clicker/beam/dial type, hydraulic or pneumatic types, or even the planetary gear powered torque multipliers, are all generally 'calibrated' against something like a Skidmoore unit or the newer electronic versions. You might want to ask you tooling supplier about re-certification of your tools to meet your audit requirements.

Rick

 

[MintJulep]

For micrometer setting type wrenches you are comparing the actual torque when the wrench "clicks" to the setting. Calibration is done either by adding or subtracting shims below the spring, or by moving the handle relative to the screw that compresses the spring.

For dial indicating types you are comparing the reading of the wrench's dial to the reading on the calibrator. Calibration varies by manufacturer, but generally involves turning an eccentric, moving a pivot in a slot, or carefully bending a tiny part.

For beam types you are also comparing the reading to the calibrator. There is really no way to adjust the calibration, but then again there really isn't much that can go out of calibration either.

 

[Screwman1]

Send them out to an outside service for calibration, unless you have a bunch of them. We have a deadweight calibrator that we use to check our cells in-house, but it takes a lot of time to do a full calibration and then you have to calibrate the calibrator to a master cell. If they are electronic wrenches they will either pass or they will fail, there isn't any adjustment for the ones that I am familiar with. The electronic wrenches tend to stay in calibration unless they have been overloaded and then they don't return to zero in most cases.

 

[berkshire]

I jus asked a friend who has just been through this ordeal and got back essentially the same answer as SCREWMAN1 send them out.

Here is what my friend said , "" Oh yes, those damned torque wrenches! xxxxx finally listened to me and got ours done, even got some new ones that read correctly!

You need to have the wrenches calibrated/tested through a reputable calibration Lab. xxxxx uses DZ Calibrate from El Cajon.
Torque wrenches should be within 1 increment +/- or less for accuracy.

He-he, it is such a fun adventure going through the audit process. We officially have our certificate now.""

This is from a person who has just spent 9 months getting a company up to speed on AS9100

B.E.

 

[rsm7400]

Thanks for the responses! After rounding up all the torque wrenches in the building and seeing the range of torques we use, it was determined it was just going to be too darn expensive to buy that many standards. We already have ~970 tools to calibrate each year and didnt want to add more to the list. We are going to be sending them to one of the companies that we use for calibration on some of other equipment. The ROI on the torque standards was going to be 6-7 years, Not worth it to us.