Torque Specs

I couldn't give you a text book answer because I don't recall ever seeing one, but I have a few thoughts which may get me flamed:

Given that the area under the head of a hex head bolt is more or less the same as the area of a nut (talking about metric fixings) then it won't make a great deal of difference whether you torque the nut or the bolt. If on the other hand your nut has a washer under it which is smooth and of a lower friction value, then more of the torque you are applying to it will be converted into bolt pre-load. This may be a good or a bad thing depending on how you have calculated your required torque; some formulae make an allowance for friction which you may invalidate if you introduce low-friction components such as oil, grease, ptfe coating etc.

One other point; we quite happily torque up screws into tapped holes using torque figures calculated in the same way.

The British firm GKN published a great deal of their research information at one time, it is worth reading if you can find a copy.

Drew

I believe what you are asking is why torquing a bolt is less accurate (and reliable) that the "turn of the nut" method of fastening bolted joints.

-Bolted joints require the fastener to "stretch" under loading. This supplies the clamping load.

-If foreign material (debris, lube, etc) gets introduced to the joint the bolt "stretch" will vary from a clean joint even when both are tightened to the the same torque (typically lubed joints experience higher clamp loads at lower torques than dry joints due to a lower coefficient of friction b/w fastener and bolt/threaded hole).

-Hence a more reliable method to determine the clamp load is to measure the relative turn of the nut to the bolt. This is known as the clamping angle.

-This method can also be applied to threaded holes by turning the bolt and measuring the clamping angle.

This really means you can turn which ever end you like as long as the bolt stretch is consistent with the material limits of your selected fastener. And of course the joint experiences enough load to keep it together. Hope this helps!

If it's any interest, GKN give a torque formula based on 10% of applied torque induces tensile load in the bolt (other 90% overcomes friction), and thread pitch = 1/8 x bolt dia.

Torque = 0.2 x bolt load x bolt dia (correct in any consistent set of units).

Bolt load of course varies with the bolt strength grade.

Cheers......Codey

You might have come to this understanding because it somehow "feels" right; but for most intents and purposes it really doesn't make significant difference. Torque the nut if accessible with the torque wrench, the bolt if not. In numerous applications--especially car repairs, torque wrenches are not that frequently used; instead torque angle is used (quicker, more consistencly reliable, easier) by counting turns and knowing thread pitch (of new or very hard reused fasteners). This is expecially useful where there's no way to apply a torque wrench; also it is frequently more accurate than having different workers with different skill levels attempting to measure torque with a wrench. The other points made about lubrication and washers also need to be considered. Generally though, torque-ing against a washer will be practically the same as without--unless the washer is so soft than it pancakes entirely or breaks. So the "why" question, for practical purposes, might be like asking why something really doesn't matter that much. It's the kind of "why" that will probably never produce a satisfying, definitive answer--if such a unique answer actually exists

The nut with flat washer of known material, surface area, and force has a relatively consistant coefficient of friction as compared to an undefined length of bolt sliding inside an undefined hole diameter of an undefined smoothness of an undefined material of an undefined cleanliness.

As earlier pointed out, the nut/washer combination is hardly consistent. The difference between dynamic friction compared to static friction of the nut can hardly be avoided unless as pointed out earlier, one counts the angle of rotation rather than using the torque wrench.

I believe your boss won the argument. In your particular application, try the torque wrench in ten applications as compared to counting the flats of turn and check the variation in your specific application. Possibly it may be insignificant. Probably the difference is significant.

Actually you should be more concerned about the ability and method of locking or retaining the nut from loosening after tightening. The challenge is certainly to be acknowledged and addressed in compliance with application. Do you use locking wire, locking threads, locking washers, what type.

Hi James,

Several posts here have pointed out the real issue. Bolt stretch. Bolts are elastic, and stretch until they reach their yield (where they do not return to their normal length when relaxed). When you tighten the nut, or bolt, the bolt shank (the part between the head and the end) stretches and fights to go back to its original length (like a stretched spring) and you have introduced "clamping forces".

The bolt is stupid, and doesn't know or care how you stretched it to a point below its yield strength. The idea is "How accurate is the method to arrive at what is calculated to be the point below the yield strength". Imagine a small diameter bolt 10 meters long. Turning the bolt head will cause the shank to turn, sure, but this method may provide more parasitic resistance of the 'twisting' itself >>> one twist on the head end may only get you 1/2 twist on the nut end before you release the 'torque' from the head end. Your torque wrench may 'click' before the torque is passed along to the other end and cause the shank threads to creep into the nut the same full turn. So, you get a false torque, and the bolt shank if only stretched 25% of what was intended.

Now imagine the same 10 meter long bolt, and go around to the other end and tighten the nut. The nut is pulling straight away on the shank and stretches the shank with minimum parasitic losses, so it is perhaps more accurate in this silly example. Most of your twisting torque goes through an angular mechanism (pointed out by another poster) and produces linear stretch . . . which is the idea.

In my world of high tech machines, we seldom tighten bolts with torque / turn methods. You can see that is the nut is damaged or lubricated, and with what, you can get wild torque variations leading to the goal of linear stretch. Very inaccurate, but simple.

In my world, if we really want to stretch them (the goal), we just begin by stretching them. We use a hydraulic device to thread onto the bolt threads, pump it up and it pulls the shank under X forces shown on a pressure gauge, and we spin the nut down by hand and release the hydraulic puller. Shazam ! We are stretched.

Go to Riverhawk tool website http://www.riverhawk.com/tools.html and there is some animation of bolt stretchers and also a hydraulic nut with a piston stretcher built into the nut itself. http://www.riverhawk.com/tensionnut.html For critical fasteners we NEVER torque them . . . we stretch them.

Open this fun link below, then go to page 11 of this brochure and you will see a engine connecting rod (taller than a man) with 'nuts' that don't even have flats to turn. Just holes to stick a punch in to snug them before we release the hydraulic puller.

http://www.wartsila.com/Wartsila/global/docs/en/ship_power/media_publications/brochures/product/engines/medium_speed/w64_tr.pdf

And keeping this value of 'stretch' in mind, I use bolt standoffs for critical clamping. See a slide below from a class I teach on equipment mounting and alignment.

So now you know the full story ! George

Great suggestions and explanations!

You do not make mention of it, but I presume that the portion of the bolt under the standoff is mostly un-threaded to increase strength, and that the hole is deeper than the threads of the bolt.

A question: Why do studs hold more than bolts?Is it because of the stretch of the stud increases clamping force? Race engine builders always replace head bolts with studs when possible.

Thanks!

THERES NO PROBLEM WITH THAT, REMEMBER THAT IN SOME CASES, YOU HAVE TO WELD THE NUT, AND ALL THE TORQUE IS APLIED, TO THE BOLT, I, THINK ITS THE SAME.

Once in a factory training class, a student asked what to do if a torque wrench was not available?The instructor replied:"Simply tighten until just before the bolt breaks"

"How do I know when the bolt is going to break?"

"That's why you need a torque wrench"

Another aspect is the nut is cheaper than the bolt normally, better to strip out the nut than break the bolt cost wise.

There is another method that has not been mentioned yet:

If you have a critical application that allows access to both ends of the fixing, e.g. a big-end bolt in an engine, then you can use a micrometer to measure the bolt stretch during the tightening process. This gives a much more accurate bolt loading in a similar way to PetroPower's hydraulic device.

Drew

To answer your question ... It doesn't matter. What everyone here is talking about is the ability to "know for you sure" you have loaded the bolt correctly. All these guys are correct, no question. But to sum it up, they are all giving you different takes on how to measure, check and recheck to assure the correct loading on the bolt. But to answer your question. It doesn't matter.

My guess is that a long time ago in your plant somewhere, someone had an argument with another guy and called an engineer, to settle the fight, told them Nut to Bolt. ( I have to do that all the time, two ways to do it and two guys on the floor are going to go to blows over the right way, so I lay down a law, put it in writting and everyone is happy, but it didn't matter which way I went) ... I ramble too much..

It does matter!

there can be a significant amount of friction between the shaft of the bolt & the various components it runs through.

This is also 1 of the reasons race engine builders use studs, the other is alighment of gaskets [ its easier to apply the gasket sealant & install a head gasket over abunch of studs ]