Torque Value

Flog it up until it breaks, then back it off half a turn.

Can't remember what stress figures B7 is good for, but usual formula for torque (N.m) is 0.2*F*d/1000 where F = bolt load, N (at torqued condition), d = bolt nominal dia. mm.

In theory that applies to clean dry threads, but in my experience it's OK if they're lubricated with ordinary oil or grease, I believe because the high contact pressure cuts through the grease layer. If chesterton is something fancy you might need to reduce torque to allow for it.

Lubricating the threads of a precision torqued bolt prevents binding from reducing the head compression at that torque.

Seriously, why not ask the bolt manufacturer?

Answer: Because the bolt manufacturer has no clue about the joint. He doesn't know what its unique requirements are. He only knows (or should know!) the characteristics of his bolt.

Asking the bolt manufacturer how much torque to apply to a bolt that you had just purchased from him is like asking an oven Salesperson how hot the oven should be. How is this person to know?? Your intent could be to bake a soufflé or to warm-up a pudding or, to burn-out the accumulated residue. There is no one single temperature setting that you could use; It all depends on the application. The same is true for bolted joints. Thus, anybody who asks a bolt manufacturer how tight his bolt should be or, who refers to those silly yet ubiquitous torque charts published by everybody and their brother's dog is surely missing the boat

Or you could just ask Google:

http://www.portlandbolt.com/technicalinformation/bolt-torque-chart.html

Look at what I found after doing a Google search.

_{^{No, it's not the Google results page.}}

If you really want to measure tightness, you'll need to measure the elongation of the fastener.

Torque will get you in the ball park, but, that's all.

Absolutely! But many times (blind holes) this cannot be achieved.

Poor design.

I don't know of any internal combustion engine that has head bolts that can be measured in situ to determine the tightness.

I know. Just being contrary.

In very big diesel engines there are such possibilities based on elongation measurements with a reference at the bottom accessible via a hole in the bolt center. Such bolts are subject to fatigue and an optimal preloading is compulsory.

I am deeply surprised after so many discussions on the preloading subject how low level are the counsels given to the people who believe in CR4 as a support for doing better their work.

Only to give a feeling if the preload is too low there is a risk of over loading of the bolt and early fracture, if the preloading is too high then the yield limit can be reached in working conditions and the bolt streches and looses its preload so that the joint is not any more tight.

It would be better to give opinions in the fields where one is sure about what he says.

I wrote it several times but it seems that we have alround experts who know all and a bit more so that any question gets an answer delivered good or bad but there is an answer.

It's possible to go into a lot of detail about bolt tightening, and it's been discussed on cr4 before. No doubt there are critical applications where it's necessary to go to some trouble to get the preload right. But I wouldn't mind betting that in the vast majority of cases, using the listed or calculated values and a torque wrench, or just what feels right to somebody with experience, will work just fine.

Do you know for which application the question was asked ? Is it a "critical" or a not critical one ?

As for personal "experience " I could tell a lot of stories which lead to extremelly unpleasant results. Bolting technology is a TRICKY technology and very very risky even for people with experience who thought to feel right.

I wasn't referring to the OP's question, I was making a general point. There must be a huge number of bolts tightened every day around the world, and my guess is only a very few use anything more than the basic kit I mentioned.

To take a specific example, I looked out a book on ductile iron pipework, including flanged joints. As they can be up to 25 bar, you might say it's a fairly critical application. It gives recommended torques, and says to grease the bolts. Also says "The relationship between applied torque and the actual load imparted to the bolts is not precisely predictable, therefore the values given in the charts are of necessity an approximate guide". But it only gives recommended torques, doesn't say what the bolt load should be at those torques, so even if kit were available to measure actual bolt load it wouldn't help.

Incidentally, a range of torques to seal against various pressures is given, and even the highest torque is well below the usual figure recommended for the bolt, even for the lowest grade.

"But many times (blind holes) this cannot be achieved."

Of course this can be achieved in joints with blind holes! In fact, it's all the more important in these applications!

I've seen many sad, sad cases where somebody had screwed a fastener into a blind hole and then stopped when they applied the "correct torque" with a calibrated torque wrench only to be told later (by one of our Techs) that the bolt was, in fact loose. In most of these cases, these bolts had either galled in the holes or, they had bottomed-out against crud (or the bottom of the hole itself if the fastener was slightly too long. Without us having had measured the elongation, these conditions wouldn't have been discovered and thus the joints would have been too loose

Ok, I'll bite. How does one measure bolt elongation while the bolt is in a blind hole?

@ RedFred.

We do it ultrasonically using specialized instrumentation based on time-of-flight methods in pitch-catch mode. What this means is that we fire a burst of ultrasound down the length of a fastener and wait for it to bounce back. When taking into account the effects that material, temperature and stress have on the signal, the traverse time is converted to a length.

Each fastener is measured for its reference length before it's tightened. Then we measure the delta Ls after the initial tightening force and are thereby able to effectively tune the fastener to the proper load. What's really effective about this method is that we can go back to any fastener at any time and compare the existing delta to the desired. Only if it's out of the necessary tolerance band, is a wrench applied to the fastener.

The longest bolts that I had controlled using this technology were, coincidentally, 8m -long (yes, metre!) entablature bolts on a slow speed marine diesel engine.

If you drilled the whole length then you'd have a stud that could be made vacuum compatible.

Vacuum compatible?...I don't follow.

Pitch ?

Application ?

Unbrako has a good engineering guide on their website with lots of valuable info on bolting. Below is a snippet from page 62 about the general formula (as per Codemaster's post #2) used to calculate bolt torque and...what you should keep in mind when applying the formula. I circled the important part!!

Thanks for that. For anybody interested, derivation of torque formula with k = 0.2 is as follows. Working in N and m, tangential force round the bolt = T/(d/2) = 2*T/d. Thread pitch is typically d/8, bolt circumference = pi*d, so mechanical advantage = pi*d/(d/8) = 8*pi. It is assumed that 90% of the applied force is lost in friction etc, so tensile force P produced in the bolt = 0.1*2*8*pi/d = 5*T/d to the approximation we're working to. So T = 0.2*P*d. For d in mm, T in N*m, T = 0.2*P*d/1000.

The 90% figure is said to be based on clean dry threads, but I don't think grease makes much difference in practice. I've been greasing and tightening bolts for about 50 years and haven't broken any yet, except when there were extenuating circumstances .

Oh no, here we go again...

So, Mr OP, you'd like to know how much torque to apply. Well, I'm sure that most people here in this forum can give you a torque "target" but, you have to give us a bit of guidance first in order for us to provide you with even that. The most important question is:

• How tight do you need the bolt to be?
  

Once you've answered that, please tell us how much friction will be encountered during the initial assembly.

And then, how much resistance will be encountered during each future re-assembly?

If you are very confident in your ability to predict the future, it's quite easy to come up with a number of torque values to cover each case (indeed, the calcs have already been provided above). If, on the other hand, your seer skills aren't on par with say, Nostradamus, the calculated torque values could be quite meaningless if the joint requires accurate bolt loads. Ah, but you can address this issue by verifying the effects of the torquing exercise and then always modifying the input torque accordingly on each bolt. This means, of course, that you might have to apply a different torque force on each bolt during each assembly in order to have them exhibit the same "tightness". Otherwise, it's a crap shoot...

"Otherwise, it's a crap shoot..." Yup! 100% agree. For critical applications we use hydraulic tensioners. It removes the variables caused by friction, no stress imparted on the stud due to torque, full preload (75% of yeild) can be achieved accurately.

The draw back...limited applications and they are a pain in the a$$!

Indeed, TerraMan makes a good point about bolt tensioners. However, one must realize that although they do remove the effects of friction out of the equation, there still is a bit of "assumption" required in the process. This is particularly true when one can't perform a 100% tensioning pass and must therefore rely on 50% or less. It means that there will be 'cross talk' or rather "load transfer" of the bolts whenever all of the fasteners can't be tightened simultaneously. Furthermore, there will be minor load loss as the load is transferred from the tensioner bridge to the nut.

All in all, direct tensioning is a clearly a much more accurate way of applying bolt load then torquing but, it's not infallible either!

One must remember that there is a hierarchy in bolting procedures. At the bottom is "beat it 'n bash it". Then comes impact wrenches followed by torquing. Next comes tensioning. The penultimate is tensioning supported by elongation measurement in order to fully verify and thus control the bolt loads. The ultimate is, of course, welding !