Torque to apply w/ Prevailing Torque nut. 2

[gt6racer2]

Having consulted the earlier posts (thread31-78159 and others ), I found a general concensus that the running torque of the nut before contact should be added to the desired clamp load torque.
I can understand this approach for a nyloc or such, however, I'm using a nut with defomation ( squashed out of round). As we get some elastic changes to the thread geometry on torquing, does this same rule still apply ?
BTW, I plan to make a study - just wondered if any of you have been here before ...

 

[CoryPad]

Torque + angle control will help reduce preload scatter. You can build in a preload control feature - use a waisted shank rod that yields at a lower force than the threaded region. Or, you can use one of the new ultrasonic fasteners:

http://www.intellifast.de

http://www.innovationplus.com

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[MintJulep]

Ok, so my question stands. You want to optimize performance wrt cost of assembly. Performance is directly related to preload. So there is some range of preload that results in acceptable performance, with lower and upper limits where the performance becomes unacceptable.

So what is that range?

Using a prevailing torque fastener adds another assembly-to-assembly variable, increasing preload scatter. Yes, in the crude analysis you need to account for the prevailing torque, however that torque is not the same for every assembly.

HeviiGuy is correct when he says that it is impossible to verify preload before a joint is tightened. However it is possible to statistically predict the range where most of the assemblies will fall. This requires some initial testing of a sufficiently large sample, and continuing batch testing to ensure that the parameters established remain valid. Torque+angle control might be good enough for your application. Or it might not be. It depends on how much variation in preload you can tolerate, which in-turn depends on how much variation exists in many material and process elements.

 

[HgTX]

Watch with the accusations. There is nothing "misleading" about the AISC link given above. The question in that link was about what RCSC means by a "calibration device capable of indicating bolt tension". The answer was that a Skidmore-Wilhelm thingummy is an example of such a device. For the scope of applications covered by the RCSC, the Skidmore is THE device. It is what the RCSC had in mind, and naming that device is exactly the right answer to that question.

The reference to the "calibrated wrench installation method" is again a reference to someting in the RCSC spec. So watch what you're saying about AISC. And for that matter, watch what you're saying about RCSC. For structural applications, they are pretty much *the* authority.

The calibrated wrench method is not just a matter of achieving a particular torque. It depends on frequent calibration of the torque values to tension values and on careful attention to bolt condition. Within the context of the RCSC specification, it only applies to two (four if you count twist-offs) types/grades of bolt, with a certain expectation of consistency of manufacture and lubrication. I'm not completely sold on it, but it's a far cry from simply pulling a torque value from a chart.

Even if you're looking at changes in length, you can't just check that and walk away. If you haven't kept an eye on your bolt quality, and you have too much torque for the tension you're achieving, then the bolt is a lot closer to its limit state than it's supposed to be. So use of the UT devices doesn't mean torque goes out the window.

Not to mention that UT is not the only way to look at length. Unless you need very precise measurements, turn-of-the-nut is an elongation-based tightening method in wide and successful use. I cannot imagine sending someone up to UT every bolt in a connection with several hundred bolts when witnessing the TOTN process and checking the markings will do the job.

Eng-Tips policies: faq731-376

 

[HeviiGuy]

With all due respect, just because a certain group is an "authority" does not in the least mean that it is correct. Take for example, a certain über-authority which for the longest time had vehemently insisted that the earth was at the center of the universe. Had society continued to slavishly follow this doctrine, where would we be today?

It's a matter of evolution and development. Those organizations who suggest that the amount of force required to turn a nut has a direct correlation to a bolt's tension simply have not yet evolved to the same point where technology has. Not questioning them in the face of obvious disconnect is indeed a case of following a naked emperor.

Regarding the Skidmore-Wilhelm process: This procedure tells us to make sure that we've tightened the joint properly before we've even set wrench to it. As an engineer, how ludicrous do you think that is?

Angle-of-turn method? Same thing. At what point do you mark the nut and begin measuring the angle? Furthermore, what happens if you have a "sprung" joint?

You're right: UT for every every application is overkill. Its' use should be determined based on the potential cost of joint failure, both in tangible and intangible terms. Indeed, I agree with you that it's not necessary for most structural applications (although there are some which cry out for this level of quality control).

Ciao,

HevïGuy

http://www.heviitech.com

 

[CoryPad]

HeviiGuy,

You have made your points regarding variability in fastened joints. The reality is that your viewpoint is not held by all. It would be best to allow the thread to close, considering that the topic has moved beyond its original intent. If you want to create a new thread to discuss this, I am sure you will get plenty of responses.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[HeviiGuy]

Fair enough, Cory. Thanks for pulling me back down to Earth ;-)

Ciao,

HevïGuy

http://www.heviitech.com