Torque Wrench Accuracy Headscratcher

[DHarris]

+1 to what Holescreek said!!!!

 

[Holescreek]

Sorry, I just re-read your post and you've already figured out it was the wrench. So my next question is: was your calibration equipment designed to be wall mounted?

 

[projectnut]

O.k., I'll try to concisely summarize this problem...I'm looking for some insight and expertise on how torque wrenches work and maintain their accuracy:

Case in point: Adjustable, manual, ratcheting torque wrench - typically used at the 20 in-lb setting. The claimed accuracy of the torque wrench is +/-10%, so one could reasonably expect a range of 18-22 in-lb in practice. The torque wrench is stored in a tool drawer and is on a calibration schedule in an ISO 9001:2015 manufacturing facility. In other words, assume no extreme abuse or misuse of the instrument.

Problem/Headscratcher: We torque a series of bolts to 20 in-lbs while the torque wrench is oriented in the vertical plane. The operator normally starts the wrench handle in the "down" position, or pointing towards the floor and swings the handle up to torque the bolt to the setting of 20 in-lbs. Here is the headscratcher: the operator was interrupted after reaching torque on a bolt. When returning to the work, she placed the wrench on the bolt with the handle in the "up" position, or pointing up towards the ceiling. Without remembering that she had already torqued the bolt, she then commenced work and torqued the bolt to 20 in-lbs, suggesting that it hadn't been torqued previously.

This accident was observed by the manufacturing engineer and was curious about how this could happen. He tool the wrench to the QC lab where we have a calibrated torque meter on the wall. Sure enough, the wrench behavior was replicated on the meter. This time however, the wrench was under torquing by 5 in-lbs when starting in the "down" position and over torquing when starting in the "up" position - both outside of the claimed instrument tolerance. A calibration of the torque wrench put us back on track...

Questions:

• The uninformed assumption here is that torque wrenches should work consistently in any orientation? Is this a sound assumption?
• Assuming that it is not a sound assumption, how would orientation affect the torque setting and knock it out of calibration?

Again, we are taking a close look at handling, storage, etc., to rule all of that out - but the confounding part of this problem is that the torque appeared to change when orientation of the handle changed?

You might need to invest in a better quality torque wrench. The one you are using does not meet ANSI standards. Snap On wrenches are certified to be within 4% when used between 20% and 80% of it's capacity. Here's a quote from their website: "The ANSI Standard allows 4% of reading error between 20% and 100% of wrench capacity. Below 20% ANSI allows an error in reading equal to 0.8% of wrench capacity."

Here's a link to the brochure on their website:
https://www1.snapon.com/V2/Groups/cmsINDUKau/PDF/PDFUK/Torque_Brochure.pdf

 

[Ray C]

I know the OP ultimately determined the wrench was faulty but, I just want to toss this out there for those who might not be aware...

Torque specs should always be stated if the threads are dry or lubricated. Makes a fairly big difference; often times, dry threads require 10-15% more torque vs lubricated. The sad thing is that I've read many torque specs that didn't specify dry or wet and am always in a quandary about which approach to take.

Ray

 

[4GSR]

There are other torque wrenches out there besides Snap-on that are just as good if not better and hold up to calibrations, too. If in doubt, check it against a beam type torque wrench. Real easy to calibrate a beam type torque wrench, too.
We have a couple of Tekton torque wrenches. So far so good. Stay away from the HF torque wrenches, tried them!

 

[pstemari]

Back to the original issue, I think stiction is your issue. If you partially torque a fastener, and then start again, it's possible to have the wrench click before the fastener starts to move. The problem is that static friction is usually about 3x higher than sliding friction, all other factors being equal.

If that happens, the correct thing to do is back off the fastener and try again. You'll only get the correct tension on the fastener if it's turning at the point the wrench clicks.

Wrench at 12 o'clock vs 3 o'clock vs 6 o'clock ought not matter. If it does and you are using correct technique, then something is seriously wrong with the wrench and you should send it out for repair.

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[vtcnc]

Sorry, I just re-read your post and you've already figured out it was the wrench. So my next question is: was your calibration equipment designed to be wall mounted?

That was one of my first questions. Waiting to hear back from he manufacturer on that one!


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[vtcnc]

If the only problem you had was "additional torque" required after stopping the process and then returning later to find you had to add more torque - I would say you experienced some "relaxing/stretching" of the bolt after initial torque - - but different torque VALUES from wrench orientation - I've never seen that! (and yes, I ran the quality inspection / control unit of a company where we did our own calibrations too). Please keep us in the loop - this is an interesting problem I would like to know more about.

thanks for sharing this!

We have a ton of experience with relaxation - we are tensioning blades in food cutting cartridges. We often find blades are relaxed after a night of tension.

This problem is as you say, observing values, almost within minutes of changing orientations. I’m going to go a little more hands on tomorrow based on the feed back everyone has provided here and update tomorrow!


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[Mitch Alsup]

Torque specs should always be stated if the threads are dry or lubricated. Makes a fairly big difference; oftentimes, dry threads require 10-15% more torque vs lubricated. The sad thing is that I've read many torque specs that didn't specify dry or wet and am always in a quandary about which approach to take.

And then there are TQ specs (almost always dry) where one TQs to a stated value and then rotates the bolt/nut another number of degrees (say 120). This lets the TQ wrench get a low friction <accurate> representation of the tension in the threads, and then KNOWING the strength of the threaded member, tighten it to within 95% of its tensile strength. TQ wrenches are not generally accurate enough to get this close to the edge of ultimate strength.

The head bolts in my Ferrari are so tightened. First to 120 lb-ft, and then another 120 degrees.