Torque Wrench Accuracy Headscratcher

[vtcnc]

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?

 

[RandyM]

I do not think orientation should even enter into the equation, torque is torque. I think the wrench was just out of calibration. What size torque wrench are you using? If you are using a 300 in.lb wrench to accurately torque 20 in.lbs you may want to consider a 50 or a 100 in.lb unit. Also, torque wrenches are to be wound down to 0 when in storage to help preserve calibration.

 

[vtcnc]

I do not think orientation should even enter into the equation, torque is torque. I think the wrench was just out of calibration. What size torque wrench are you using? If you are using a 300 in.lb wrench to accurately torque 20 in.lbs you may want to consider a 50 or a 100 in.lb unit. Also, torque wrenches are to be wound down to 0 when in storage to help preserve calibration.

Agreed on all accounts. We back off the unit accordingly when in storage. I think the unit range is 10-60 in-lbs.

Again, if you were standing there with all of us, the look on our faces was associated with “orientation shouldn’t matter”.

I think we are mistaking this with some other factor that expresses itself this way. It’s definitely weird.


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[Giles]

I have that type torque wrench 3/8 and also an Armstrong dial wrench.
I use the dial most of the time because of weight and length.
I agree your situation is puzzling, could it be that lifting the weight of wrench has an effect?

 

[DHarris]

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!

 

[Technical Ted]

Is it possible that the wrench is being held differently in the two positions when said torque is applied?

Some wrenches are very sensitive to the distance from the axis of the socket/bolt to the handle position where they MUST be held and the "pounds" applied to give accurate readings. An inch pound is measured (as a foot pound is) at a set pressure at a set distance from the rotational axis. If this distance is not maintained accurately, then your readings will be off. So, if the operator is not holding the wrench correctly, they will not be getting accurate readings. Also, a slow, steady, constant pressure must be applied with no jerks or sudden movements as this will throw things off as well.

Just a thought,
Ted

 

[Mitch Alsup]

It is stories like this that make me continue to love my bean TQ wrench. Plus, you can watch the TQ coming up to load, and hold at TQ to see when the bolt/nut actually stops moving at the held TQ value.

Spring TQ wrenches are only accurate if the spring is accurate and 3-5 internal surfaces are properly greased and retain their factory polished surface finishes.

 

[cvairwerks]

Any orings, sealant, gaskets or compressible materials in the assembly that would be clamped by this fastener set?

 

[Holescreek]

There are a lot of things that can go wrong. Stiction, compression/relaxation and possibly stretching, not to mention thread deformation etc. More details are needed as to the materials involved, hardness of the fastener etc.

The very first thing to do is eliminate the wrench as a culprit by trying to duplicate the test using a different wrench. If it also fails start investigating the materials. I've had similar situations over the years with thread strength testing. After verifying the materials are in spec, and that the tapped holes have the correct minor and pitch diameter, I'd section the threaded hole and contrace the thread form looking for deformation after the test. I had an instance one time where the bolt being torqued had an inverted flange. As you're probably aware, bolts that get torqued have an inverted V shape (very slight, like 1 degree) flange that makes contact with the part and the flange compression is what holds torque. I've also received bolts with thread issues.

Sounds like your QC lab has some work to do.

 

[DHarris]

I vtcnc, and welcome to the forum! (forgot to say that in above post!)

A couple of questions for you (please don't take my questions as doubting you and your people know how to do this, but i'm trying to understand what's happening at your work).

1) when you tested the 2 different starting points that you were testing on a meter - is this your calibration meter / torque watch or ?
a) was the "meter" in the same orientation as the bolts under production typical work? e.g. vertical test
2) the you said that you had "A calibration of the torque wrench put us back on track..."
a) does this mean that you now get the same torque readings from the top as from the bottom when using the wrench now?

20 inch pounds is not a great deal of torque - - depending on the size of the bolt! - - so if the bolt is small, could it be that the worker is able to move the torque wrench at a higher velocity when moving down from the top than when lifting the wrench from the bottom? This could result is more or less "relaxation" on the bolt after initial torque value is reached (speed and settling time of the torque wrench).

curious problem!