Am I incorrect in thinking that the only accurate way of verifying applied torque is to check the tool that delivered it? If you try to use a digital torque wrench to remove a bolt, the reading that you get is breaking torque and it is not the same as applied torque was. Therefore I feel the only way to accurately check the applied torque is to verify the application tool against a calibrated transducer. Input anyone?

You're basically correct in your post. The subject of torque measurement is a complex one. There are a couple of ISO stds relating to torque tools and calibration; ISO 6789:2003 and ISO 1703:1983.

Andy

I agree with that you have said.

That is why, when I worked the in bottle making and then bottle filling biz we only verified removal torque.
We never had a specification for application torque.

That is why, when I worked the in bottle making and then bottle filling biz we only verified removal torque.
We never had a specification for application torque.

When we fill bottles we also do removal torque - which, if you think of a cap on a bottle, is much more important then applied torque.

When we fill bottles we also do removal torque - which, if you think of a cap on a bottle, is much more important then applied torque.

This would work well for the process that you describe. However, applied torque would be more relevant to fastening a head to an engine block, or a brake caliper.

Do you need to collect data of the applied torque, or are you just verifying the torque applied is to specification. If just verifying that the torque meets specification, set your torque wrench to the specification and tighten. If the wrench clicks immediately then it meets the spec. If there is some rotation before the wrench clicks, then it does not meet specification.
We are in the process of doing this to verify wiring connection on electrical contactors.

Phil

Good post, Phil.

I have been watching this one, and haven't posted in that all the previous posts have been excellent.

As already stated, torque is a strange animal, in a class by itself.

Counter-Clockwise torque is usually less than Clockwise torque. Too, given settling/molding factors after something has been tightened, it's difficult to assess the correct application torque after it has gone on.

If you would like more information, maybe you could let us know more details regarding your inquiry.

Am I incorrect in thinking that the only accurate way of verifying applied torque is to check the tool that delivered it? If you try to use a digital torque wrench to remove a bolt, the reading that you get is breaking torque and it is not the same as applied torque was. Therefore I feel the only way to accurately check the applied torque is to verify the application tool against a calibrated transducer. Input anyone?



In my experience in automotive - assembly of front and rear axles, we monitored the application torque in the same way you described, BUT, we would also verify the "Break Away" and "Start Up" torque. This required an electronic torque wrench with force mapping technology. We used a portable datamyte collection system most of the time. Anyway, I am attaching a .jpg of a curve. At point 1, this is the breakaway force. At point 2, it is the start up force. Based on correlation studies of the in-line testing and the torque tests, we found the start up torque was a much better indicator of the application torque. Also, you must test with slow turns to ensure your gage detects the start up force.

With documented correlation studies, you can justify performing the gaging after the application is completed to your customers and auditors, rather than spending the additional costs for in line gaging.

In my experience in automotive - assembly of front and rear axles, we monitored the application torque in the same way you described, BUT, we would also verify the "Break Away" and "Start Up" torque. This required an electronic torque wrench with force mapping technology. We used a portable datamyte collection system most of the time. Anyway, I am attaching a .jpg of a curve. At point 1, this is the breakaway force. At point 2, it is the start up force. Based on correlation studies of the in-line testing and the torque tests, we found the start up torque was a much better indicator of the application torque. Also, you must test with slow turns to ensure your gage detects the start up force.

With documented correlation studies, you can justify performing the gaging after the application is completed to your customers and auditors, rather than spending the additional costs for in line gaging.

Could you define "start up force"? I'm a partial ignoramus on this subject, and it seems to me that what you're talking about makes sense--finding a reliable correlation between application torque and removal torque so that measurement of removal torque is a reliable indicator of application torque, but I don't understand the "start up torque" bit.

I don't have the textbook definition, but in a nutshell, the force you require to start movement on the bolt/lid... is called breakaway because you must overcome the clamping force (like inertia if you are pushing a rock) and begin movement. Once the movement begins, the lowest recordable force (the bottom of the little dip in the curve) is the Start Up force. Once the "rock" or lid or bolt is moving, it initially requires less force to keep it going than it did to start the movement and overcome the clamping or inertial forces. This force will go up quickly as the clamping force goes up because you are actually tightening the bolt/lid.

Generalized:

Once you break the clamping force on the bolt, less energy is needed to keep bolt moving, but as you continue to tighten it, the force curve continues to go up and will surpass the breakaway force as it is tightened. The bottom of the little dip that comes right after the bolt starts moving is the force I found that correlated best with the in-line gages of our process. (over 96% correlation)

Thanks!!

Good post by the way, Katydid. Thanks for the info.

As I'm sure all are aware: Don't forget your basics with torque. Torque measurements can significantly vary by several factors. The speed in which the device is operated, different operators, length of connection, horizontal/vertical/angle measures, erratic motion in the torque device, torque device error, etc. can all add variance in the measurement. The best method of measuring torque is to automate it as much as possible.

Also, the machine, device, etc. has variance in the torque that was used on the application.

If you can control most of those, then you should be able to develop a linear regression estimator, table, correlation, whatever like the previous poster suggested.