How do they come up with the correct torque number anyway?

[junkhound]

was going to reply the other thread with this statement but rhe thread closed?

anyway, 40 years ago had to design a ground fitting for nuclear missile silo that not only handled over 300,000 amp 'fault' currents but also had a 60 dB attenuation of up to 100 megaHz signals. 3" dia flange, 3/4" stud. So critical that If there were chatter machining marks on the mating surfaces the 100 MHz requirement could not be met. Needed to have near mirror perfect mating surfaces.

Anyway, fault current testing was conducted and minimum clamping force determined by imbedded strain gauges, but very expensive or difficult to field implement the strain gauges. Found that a simple sample test of the torque to force could be done by placing 3 pieces of 1" length solid solder between the surfaces. Torque ( well over 100 ft-#) and measure the thickness of the solder. Installation procedure was to perform the solder test ever dozen or so installs to verify that installers were maintaining consistent lube and torque calibration.

 

[Sberry]

List several conductor sizes and turn it till it mushes and spreads and splays the wire in the lug with round or wedged shape screw with one torque fits all. Probably another process with flat end screws and screws with clamps like some meter bases.

 

[gar]

230720-1305 EDT

Experience.

One is concerned with compressive force on the joint. But in only a few cases is it possible to directly measure this on a production basis. The next best approach is to measure torque. So it becomes very important to control the factors that relate torque to tension.

The end goal is to tighten the joint to a level that for most applications is sufficient to prevent loosening of the joint. Thus, it is experimentally determined.

The criteria and end result will be a function of the materials used. Ferrous vs non-ferrous makes a big difference in criteria.

Automotive applications in ferrous based materials will stress the joint materials well into their elastic region. Usually this will keep the joint from loosing. But in some cases it is necessary to add something like Locktight.

In nonferrous materials and in the elastic region repetitive cycling may cause over a large number of cycles the elastic curve to shift.

Thus, field experience defines where to set torque specifications..

.

 

[rambojoe]

First and foremost, if you ask the question it is good to first understand the dictionary definition...then all the other good things.
I'd post the def. but i always have to resize images...

 

[drcampbell]

... In nonferrous materials and in the elastic region repetitive cycling may cause over a large number of cycles the elastic curve to shift. ...

Learned that one the hard way, when first installing a stud-mounted diode. (age 16-ish) It looked a lot like a quarter-inch mild-steel bolt, and I torqued it accordingly. I suddenly learned that the stud was tin-plated copper. I don't recall whether the datasheet mentioned that or published a torque specification.

.... Found that a simple sample test of the torque to force could be done by placing 3 pieces of 1" length solid solder between the surfaces. Torque ( well over 100 ft-#) and measure the thickness of the solder. Installation procedure was to perform the solder test every dozen or so installs to verify that installers were maintaining consistent lube and torque calibration.

Sounds vaguely familiar: https://www.plastigaugeusa.com/
I once specified Plastigauge in a procedure to measure "optical" tolerances in a digital x-ray camera. They were unreachable when it was assembled, and didn't exist when it was unassembled. (yes, I am also an engine geek)

 

[Sberry]

Some minor things become instinctual afte3r a while if a guy can be disciplined. I am not a blaze at times installing a service mainly cause I will do something twice if I have to make sure it doesnt looked plumbed by Dr. Seuss.
But I have a last lap and snug it all just a little before I put the cover on so all the wiggle, bending and shaking and pounding has been done and really make a great effort at the expense of some time to make sure big conductors are well bent and well nested without strain on them.
But I bet lotta really good highly experienced masters have a real good feel for this. Its the ones at the extremes are a problem. It aint the guys that do it right, do it by feel. Do it with a wrench which in the end would probably also feel right.
I am engine mechanic, one of my buds, know a few just like sparkies, he says,,, farmer tight. Its the little extra on a hardened machine bolt on a bulldozer but he knows not every one should be like that.