Max and Recommended Thread Torques

VGA (very good answer)

Nick,

GA from me too. However, would it not be simpler and more accurate to email the fastener manufacturer and ask him? Only he knows the metallurgy of the screw, and ultimately it's yield point.

Stu.

Asking the fastener manufacturer for a torque spec is not a good idea for two reasons:

1 - the bolt is only one part of the equation. The necessary bolt load depends on the unique characteristics of the entire joint. Any bolt supplier who readily attempts to provide you with a load spec without anyalyzing the entire system doesn't understand his stuff!

2 - "torque" is not an indication of bolt load. One bolt torqued to x ft-lb (or Nm) with a calibrated tool, even, may be too loose while another bolt torqued to the same "torque" may be too tight. "Torque" is only a measurement of the resistance to force as one tries to turn a nut. It is NOT an indication of a bolt's "tightness".

Analyze your joint. Determine the proper bolt load and only then, take a wild a** guess and pull a K factor out of the air to come up with a torque value. If it's a critical application, make sure that you've got some means of verifying that the proper load has been applied (regardless of what the "torque" input was.

Hevii Guy

You are right, however, the OP was for equations to determine what I understood to be the yeild points for the relatively small fasteners in question, so as to determine their safe tension. I didn't regaard that it was the place of this forum to lecture on the intricacies of fastener technology, ie, the calculation of thread shear, stretch and the whole nine yards.

Sorry! I just tought I'd give the OP the quickest way to get the most salient info.

It didn't appear to me that he needed to know all of the 'other stuff'.

Equations were given and were good, to me anyway.

I've always used manufacturers torque specs to determine the yield points of fasteners - he's the only one who knows ( there are a few generic fastener torque charts available) . Particularly when fastener stretch is to be taken account of. Much of today's machinery, particularly engines, reqire fasteners to be tensioned well into elastic yield, but, well before the elastic limit is reached, and of course, the fasteners are usually to be used only once.

Yes, I know its to achieve the required joint characteristics and integrity. And not enough folks know that it is essential to correctly lube the threads and heads in any high stress tensioning situation.

I take note of your point 2. You are on the right track, however the limiting factor is always the fasteners ability not to yield under stress, and a fastener made to a particular metallurgical standard has a finite stress limit, no matter who has made it.

And I wouldn't trust anyone but the maker to tell me what that limit is.

Cheers

Stu.

You are totally right the supplier can give the values for the material the screw was made off (Sy) and of course with a great accuracy.

But as you see in what I wrote the torque limit depends on TORQUE & FORCE combined result which must be within elastic limits (if you do not want to use a trans-elastic pre-loading as more and more in engines). The supplier can give the limit Sy but not at which torque your bolt, where it will be used, should be tightened to avoid failure. This was the reason I gave the equations. The user can ask his supplier for the Sy value and with some measurements or estimations define the friction conditions for his application and the result is much more on the safe side.

I was confronted several times with situations where a torque was used without any problem for years and out of a sudden the same bolts tightened with same tool and at same torque were either broken or overtightened. This happens when a "recipe" is used. I wanted to give to the question an answer, such, that one can avoid such surprises.

It is what we all want :obtain a safe assembly, or not?.

I did not went into the computation of the assembly since it was not asked for, I tried to answer the question the best way I considered.

Thanks for the replies, Nick Name I'll have to read your comment over a few times but I'll understand it, if not I'll ask some specific questions when they come to me haha.

This "proper" or "recommended" torque is this what everyone is refering to as the seating torque or is that different than what we're discussing?

I think I should contact the supplier about the properties because there are a lot of mixed information for some of the materials used, most are 303 SS and some 17-4 PH.

You are welcome