Does anyone out there have a good documented method for calibrating pneumatic air wrenches?

I'll have to double with my torque person as to whether they are included, but I am thinking they may be included in the ISO procedure we use to do all of our torque wrenches.

Let me throw in a little editorialization (MY OPINION) about dynamic torque (including pneumatic air wrenches). At the site where I run the lab (large, FDA-compliant facility) we do a lot with dynamic and static torque, and it is an important parameter in various uses. After some research on the topic, it appears that dynamic torque does not have NIST traceability.

There will probably be some who disagree with this. Briefly, static torque uses a weight/mass and a length (calibration wheel with weights hanging from them). Dynamic torque has those, plus speed and response time of the transducer used to calibrate the tool. If you review the specs of the various OEMs who make torque calibrators, I don't believe you will find those necessary parameters. We had an issue between two different brands of torque calibrators, and after running numerous experiments, found that non-traceable aspects of their operation (i.e.: speed, response time (bandwidth/risetime)) were not accounted for. If anyone has any expert info on this, I'd appreciate being shown that such traceability does exist.

Bottom line for the person asking the question is to understand that two different brands of calibrators used to calibrate pneumatic air wrenches will not necessarily give the same results. So beware.

If there are any torque experts out there who read this, please direct me to any white papers on this aspect of torque traceability.

My torque person just arrived back in lab. I'd like to slightly amend my posting.

The ISO that we use is ISO 6789. It does NOT include pneumatic air wrenches. After refreshing my memory in a little better detail, most all of what I previously said is true.

Most places do not calibrate dynamic torque because traceability does not exist. The major auto makers (as I understand) do not calibrate dynamic torque (electric or pneumatic), and use them as uncalibrated tools (because of the lack of traceability). Then they use a static torque tool to set accurate torque.

So the answer I think as to whether there is a good procedure to calibrate a pneumatic air wrench is no, because they can't be calibrated (no traceability).

At our facility we "calibrate" electric torque screwdrivers; but it is truly a correlation to a specific manufacturer/model of torque calibrator, using an internal method. This serves mainly to provide consistency in manufacturing, as opposed to traceable accuracy.

Most places do not calibrate dynamic torque because traceability does not exist. The major auto makers (as I understand) do not calibrate dynamic torque (electric or pneumatic), and use them as uncalibrated tools (because of the lack of traceability). Then they use a static torque tool to set accurate torque.

So the answer I think as to whether there is a good procedure to calibrate a pneumatic air wrench is no, because they can't be calibrated (no traceability).

At our facility we "calibrate" electric torque screwdrivers; but it is truly a correlation to a specific manufacturer/model of torque calibrator, using an internal method. This serves mainly to provide consistency in manufacturing, as opposed to traceable accuracy.

I don't understand. Are you saying that something for which traceability doesn't exist shouldn't be calibrated? Or can't be? Your last paragraph seems to indicate that you think that calibration isn't properly referred to as calibration in the absence of traceable standards. ISO 9001-2000, 7.6 makes allowances for such cases:
Where necessary to ensure valid results, measuring equipment shall...be calibrated or verified at specified intervals, or prior to use, against measurement standards traceable to international or national measurement standards; where no such standards exist, the basis used for calibration or verification shall be recorded... [emphasis added]

"Calibration" is nothing more than comparison to a standard. If there is no traceable standard, it's still calibration, and meets the ISO requirements so long as an explicitly-defined standard is used.

Thanks for your feedback Jerry. We currently have torque requirements for connecting terminated wires onto switch boxes, fuse blocks, etc. In this process we are using pneumatic tools. I am trying to establish some type of method to adjust and "calibrate" the wrench. All of these are connected using screwdown type connectors. Can you share with me your method for calibrating your electric screwdrivers so I have something to compare to?

This just happens to be one of my hot buttons.

The problem is the assumption of NIST traceability in the calibration of dynamic torque. The problem is that calibrator response time, and torque driver speed/momentum are not the same between torque drivers/calibrators (different brands, and even different units (same mfr/model)). This is not accounted for in specs of the calibrators or drivers (electric/pneumatic).

So yes, NIST traceability is not necessary for a calibration. In a former job, I spent many years in the early days of the semiconductor industry. In that context, NIST traceability was non-existent for many parameters for many years. In such instances, a "de facto" standard, "golden unit", process sample, or other means was used to provide consistency in the process.

We are in agreement about this philosophical detail.

In the context of dynamic torque (as someone with some background in statistical process control), my concern is that there is an absence of "something" to provide consistency. In an electric torque driver, for example, variations in line voltage may change the effective rotational speed, and thus, the actual force applied to the screw/nut/etc. The same may hold true for a pneumatic driver. On the calibrator side, the response curve varies between transducers (think of it analogously to the risetime/bandwidth of an RF device). Two transducers which read identically at a given static torque will not necessarily equate with each other at a given dynamic torque (I've seen this experimentally and practically).

So although it is fully acceptable to have defacto/consensus standards where traceability does not exist, I am only propagating words of caution regarding the pitfalls in the area of dynamic torque.

The reason I am bothered about calibrations to dynamic torque is not the lack of traceability, but the lack of consistency or repeatability.

The most knowledgeable in this area (as I understand) refrain from calibration of dynamic torque because they are aware of these pitfalls/inconsistencies, and instead rely upon static torque to set critical parameters.

Again, I would sincerely love to hear from a torque expert on this, as I have found it quite bothersome.

PUDDYCAT: Sorry for missing your need for the sake of my "soapbox." We "loosely" use ISO 6789 as our method. We use a variety of Hios and similar electric torque screwdrivers at various settings. We have written an internal procedure. We set each for a given inch/pound setting, and seal at that setting (calibrating ONLY to the given setting, not the full range). We set a maximum calibration interval of 3 months, as we have found this is the longest they can USUALLY be expected to remain in tolerance. We specify a specific mfr/model calibrator. I just about guarantee if you switch brand names of calibrators, you will very likely see a shift. We use a run-down adapter to run the torque driver 5 times clockwise to seat a screw, then 5 time counterclockwise to unseat the screw before calibrating.

Then we do 5 clockwise, and 5 counter clockwise readings. We set tolerance at about 6% of setting (I would have to check to verify).

Let me know any specific questions, and I'll try to assist.