Bolt Torque and Tensioning

Please see this recent discussion:

http://cr4.globalspec.com/thread/31556/Specification-for-Bolt-Torque-Tensioning-for-Flanged-Piping

I am a bit surprised you are the one who promised to provide a spec and you want to discuss the basics of bolting technology?

I do not understand your behaviour, if you offer a spec this assumes that you master the subjects you want to discuss

If you want to discuss it means you are not very sure of your knowledge and then how can you provide a spec?

Confusing

As a matter of fact the basics as termed are not really cleared to anybody in the industry. All the specs have flaw and just wanted to make my spec lil bit better by advises of the forum but..........................

Master doesn't really mean that the learning process has finished my dear friend.

I would not be really honest and would not put this message in " Off Topic"

Apologies!!! :)

Cheers,

Ash Bandy

I am sorry to say that again you are wrong.

If you do not "master" the basics you should not consider that all eng. in the industry share same problem. There at least some with an expert level and know what they speak about.

Specs have not always flaws Specs were made at one time for at that time existing conditions and level of knowledge. As you know knowledge does evolute so that some aspects of specs are not any more so valid as it was normal to consider them at their birth day.

Of course one has to learn ALL his life this the fate of engineers as I many times wrote on this forum.

But I dare say again when for you basics are not clear enough, as it seems to be, you take a great risk in writing a spec for a new engineer who will believe every thing you wrote and may be make a big error. Being a "consultant" is a GREAT responsibility do never forget it !

In general I am concerned in bolting with "special" problems which are NOT in an official spec but very complex and some times even very complicated, never the less I did not propose to write the spec since the input was not sufficient.

Please do not take it the wrong way we are on a free forum and I always write what I think and think what I write.

Thanks for rating me but realyy when I started with comments never thought that I am not really sharing the ideas with technical people. I think its all here quite

Bureaucratic to rate one another individual but surely not the issue being open on the desk.

Your question is more relevant than most people realise.

The two are often taken for granted to be directly related, the higher the torque the higher the tension.

This could be true but in order to hava a reliable relationship between the two it is necessary to have proper lubrication on the nut.

A good quality grease will do the job but a copper or moly type lubricant is better.

If proper lubrication is not used all you measure with the torque wrench is the friction in the screw threads and the mating face of the nut.

The tension in a bolt is of utmost importance especially if temperature differences happen.

A bolt tightened too much in high temperatures may snap when the temperature drops and a bolt tightened at low temperatures might go loose at higher temperatures.

When assembling important items like flanges on high pressure pipelines this may be critical.

On engines the bolts holding the top are usually torqued again after the engine has run at working temperature for some time.

Often you would find manufacturers prescribing that a nut be tightened to a specific torque and then be turned another half turn. This is to compensate for the inaccurate torque reading in the last half turn of the nut as the friction increases rapidly in that part of the operation.

You see that the answer to your question is by far not as simple as it sounds.

In a previous thread I mentioned the importance of lubrication in tightening nuts and some correspondents thought it was crazy as they would prefer a 'dry' nut to prevent it from going loose. The correct way to prevent it from going loose would be to use a spring lock washer or something similar if the nut may be prone to self loosening due to excess vibration.

Lubrication on a nut has other benefits as well like preventing scuffing of the thread, preventing moisture ingress and preventing corrosion of the thread.

we are all working on the thread in #1 (since both threads are on same topic)

A bolt tightened too much in high temperatures may snap when the temperature drops and a bolt tightened at low temperatures might go loose at higher temperatures.

Not true In fact it can be exactly the opposite. All depends on the materials used for the bolt and the parts+gasket. In general the parts + gasket have a higher thermal expansion as the bolt which leads to an over load if temperature after tightening increases and to a loose bolt when temperature after tightening decreases. In some new applications bolt are made from Mg alloys so that they have same dilatation as engine perts and are not either over loaded or underloaded by temp changes.

On engines the bolts holding the top are usually torqued again

This is not always -today- any more necessary. It was necessary because of the head gasket behaviour and its set. The design of bolting assemblies did evolute a lot and settings can be compensated by different means.

Often you would find manufacturers prescribing that a nut be tightened to a specific torque and then be turned another half turn. This is to compensate for the inaccurate torque reading in the last half turn of the nut as the friction increases rapidly in that part of the operation.

The reason is not the one you mention. the torque reading are as accurate as possible depending on the torquing tool. The 30°,90° or 180° are recommended as part of the tightening procedure which I already mentioned "torque-angle" and its goal is to reduce the influence of friction on the pre-load of the bolt. The friction increases after the snug fit proportional to the pre-load which is proportional to the angle it is not more rapid since usually the Coeff of friction are quite constant over a threshold in contact pressure of sliding surfaces.

Where you are right is the importance of lubrication

The correct way to prevent it from going loose would be to use a spring lock washer or something similar if the nut may be prone to self loosening due to excess vibration.

Please do not recommend spring washers they are VERY inefficient. there other safeties against loosening much more efficient as the tests done with Junkers machine show.

Thanks for your comments.

You are right of course but I did not try to give the ultimate solution, I merely indicated some of the important factors to keep in mind.

To provide all the answers on this topic could fill several books.

Interesting

Agree!!!

Thats the reason why we use molykote or nickel based lubricants. Johan, but the question remain unanswered that why the recommended torques generally 50 to 65% of yield strength.

As I had told earlier, does the degree of tightness is soley based on N2-He leak test and detection of leak as per API 521??

Cheers,

Ash Bandy

I assume that the maximum torque specified for an application will depend on the safety factor for that application. To torque close to the yield point would be unwise.

Once the yield point is reached the tension will either remain constant or get less due to necking of the bolt, untill it eventually fails.

For every application one will have to look at the load variation and the criticality of a failure.

You will not specify the same maximum torque on a bolt for a boiler than for the one holding your front gate.

Hello Johan,

You seem to be well-versed in bolting technology. Certainly more than many so-called experts which we sometimes encounter on these forums ;-) However, there is one point which I make for the benefit of those who happen to wander upon this particular thread (without viewing the original)

Torque application and any variation of this method (including torque-and-angle) cannot be relied upon to accurately produce the required bolt stress. It is complete folly to assume that one would encounter the same K factor on the actual application as that which was used in the engineer's load calculation. On critical applications this is a very dangerous assumption to make.

Even if the friction factor of the lubrication is indicated, the condition of the threads, the amount of lubrication, the placement of the lubrication, the condition of the seating surfaces, the angularity of the bolt relative to the seat, gasket compression, flange rotation, bolt load transfer and a host of other factors conspire to prove the assumption totally unreasonable. This analogy usually helps to make the point:

Telling somebody that you know how much bolt load will result from a certain torque input is like describing what your soup tastes like before even putting the spoon into your mouth. Certainly, you might have an idea but, without tasting it, you really don't know what the cook actually did this time. Same thing goes for torqued fasteners: You really don't know what the residual bolt load is without measuring it after all of the nuts have been turned.

Shut up and listen!!!

U and bolt integrity are really naives as far as the subject is concerned. You guys don't have basic idea. What's are the qualifications of you guys, just post it next time I will decide if you guys should be included in my discussions

Though non-human- let me act moderator between you two (rather three)

Please - this is an open forum- we do not question our abilities/ in-abilities.

We (at least I) discuss matters to the best of my knowledge and try to be helpful to others.

I am not the master of my field, and accept it. Some other (lot of others) know more than me on every subjects.

We can argue- some times I win, some times other(s) do.

No bad bloods - only arguments/

Please remember this is an engineering forum and - practicing the science as applied is what makes us engineers - the application may/ may not be through book knowledge -

James-watt was not definitely first locomotive engineer - but he was- and he was not the ultimate.

Technology - as NN? said evolves - and we come to know through papers/ forums like this.

Let us question ourselves what we are doing is it best or what others are saying is best. If we think we are the better one- we can put on the weak points of others view vs the strength of us- need not fight it out (since here we have time to prepare our argument) un-like live presidential election face-to face debates.

Hope you all make up and enlighten us further instead of fighting.

Thanks and have a good time.

If you read the comment I made on this subject you should have the answer you ask for. It is on the other thread.