Fact or Friction? sorry about the play on words

[tommyardin]

I have listened to what people have said on all sorts of different topics and I hope I have thought about it in a fair and open way, and of course have learned from some of them.
I have been convinced by different people’s ideas on different subjects and topics.

But just like everyone else, I tend to think that the way I have always done things is the right way, and I know that each and everyone of us is no different, hence all the heated discussion and debates on here and every other forum that you will ever go on, because we all think we are right.

Heated debate is not a bad thing, because just sometimes it manages to find the truth or the right or wrongs on what is being debated.I put forward to you my thoughts on torque settings (Boring! I hear some say) but if you think that, go put the kettle on and read no further.

But here we go, at the end of this you can say ‘I don’t believe it, or he’s talking like a prat, or ‘what a load of bollocks’ but do not just dismiss stuff because it is not the way you have always done things or thought about things, think about it and weigh up what is being said, if you then decide its not for you then that’s OK, at least you have thought about it.

You know what? there was a time when pretty much everyone thought that the world was flat and that if you sailed far enough you would fall off the edge, and then someone put forward the notion that in fact they though it was round and that it was not possible to fall off the edge, and the majority said ‘What a prick’   

I have heard it said that lubricating nuts and bolts will give your false torque settings and it’s possible to over tighten and strip threads, or shear off bolt and screw heads if lubrication is applied.

Now, if you are winding up a nut and bolt that has lubrication applied, as tight as you think it should go using ordinary spanners relying on just touch or feel, then you may very well shear it off, because the passage of the nut on the bolt will be easy, with little resistance because of lack of friction (metal against metal)

Now If you apply the same lubricant to the same nut and bolt and tighten it up with a torque wrench to the recommended torque setting for that nut and bolt, your torque wrench will break away at the setting that you have set the torque wrench at, it will not over tighten it, or shear it, the problem comes when people feel that the nut and bolt seemed to tighten up very easily, because of lack of frictional/resistance because of the applied lube, so they then lay on the torque wrench passed the breakaway point and tighten it just a bit more, and maybe just one more tweak for good luck, because they don’t believe their torque wrench.

20Nm is 20Nm no matter how smooth or rough the passage of the fastener is, it is still 20 Nm.

In fact you will get a more accurate bite using lube because there is no dry resistance to overcome.       

If for example you had a small nut and bolt with fine thread that was a bit rusty or a dirty thread and it was meant to tightened up to the torque of say 5 Nm. Because of the rust/grunge resistance of the none free running nut you could be applying 1 Nm to over come the resistance of corrosion or grunge in the threads (the non free running nut), then you will not get an accurate reading and the bolt will not be as tight as it should be, because 20% has stolen from your rotational resistance.

The way to get a fixing to the right torque is for it to be as free running as it is possible to get it and use a torque wrench. And don’t doubt your torque wrench; trust it.

Right! where’s that kettle.tommy.

[mtread]

I don’t believe it, you're talking like a prat, and what a load of bollocks.

No, hang on a minute, you're right.....

A dry surface between threads, or worse still a rusty surface gives a resistance to sideways pressure. So some torque is taken up just  moving the nut sideways. What we are after is downwards pressure, and the thread is just the convenient way of providing this. So as loose and free running thread as possible means all of the resistance is being caused by upward back pressure, and that's what you're measuring.

Unless of course the manufacturer set the torque values compensating for dry threads.
That's my take on it anyway 

Mtread - GCE 'O' level Physics 1971

[tommyardin]

 

[tommyardin]

Wet or dry 20Nm is still 20Nm, your torque wrench will still brake away at the setting you have dialled in, a shitty thread will detract from that.
You must of had nuts and bolts like that? You know that make your fingers sore just trying to run the nut up the length of the bolt against the mating surface, and in the end take forever slowly winding it down with a spanner cos it has gone as tight as a duck arse and wont shift.


The directional force on the nut or bolt is inline with the body/length or thread of the bolt, no torque should come into play before the nut/bolt head touches the surface that it is mating with. shit or rust is applying friction and stealing torque.

Torquing up a Nylock nut accurately is almost impossible because of that resistance caused by the nylon insert, although Nylocks are most often used in situations where you and not especially locking down hard onto a surface. IE: A brake or clutch lever pivot point.

[Fazerider]

That has always been my view on torquing up nuts and screws.
The setting is determined by the manufacturer to put a specific tension on the shaft of the bolt/screw. They do so using clean smooth threads and can’t offer figures that make allowance for 18 years of road grime and corrosion.
There is a distinction between dynamic friction which is minimal if you go slowly and static friction which is at its maximum when the nut is stationary. Of course, without the latter, the nut would undo itself which is why something… (copaslip, engine oil or threadlock) needs to be present to lubricate the threads while the fastener is getting tightened. Once the nut stops, the film strength of the fluid is exceeded and it is forced away from the contact points. You now need a similar force in the opposite direction to undo it, the tension of the bolt shaft acting via the “gearing” of the thread pitch is far too small a force to overcome the stiction.
(Obviously, Fazer sprocket nuts are the exception.)

[slappy]

If you apply any sort of lubrication to a bolt it will have an effect on the torque.
I torque bolts up at work to between 600nm and 900nm, these bolts always have lithium grease applied, it is part of the standard operating procedure. All the torque values are calculated exactly depending on size of bolt and the operating pressure of the equipment , the bolt being greased is crucial to the calculation as some of this equipment is running at 20000 psi and we test it at up to 30000psi, an incorrectly torqued bolt can cause total failure and is like bomb going off in the test booth.
Have a look at this link below


http://www.engineeringtoolbox.com/torque-lubrication-effects-d_1693.html

[tommyardin]

Fazerider,
I often wonder if one of the problems with the sprocket nut coming undone could be down to a poorly maintained chain, dry graunchy old links that strains to bend tight to go round the small front sprocket and all this happening at an incredible high speed, whether it could causes an effect a bit like a hammer drill/heavy vibration on the sprocket gradually shaking the the damn thing to bits. One indication of that could be vibration in the front footrests.

Slappy,
Thanks for the link it makes interesting reading and puts a new slant on torque and how to apply it and the effects of different lubes on torqued fasteners.

But does the premise still hold then?

That a torque wrench set to XNm will only apply XNm before the torque wrench breakaway point, no matter if it is lubed or not? Because it seems logical that the force of XNm is XNm whether lube is applied or not, its just that the passage to XNm is easier because the lube overcomes friction, because if it is not then I need to rethink all I have ever understood about applied forces.
Thanks for the contributions guys 

I think its all a bit academic in some ways, because i hear some saying never ever apply Loctite to the front sprocket nut because you you really, really struggle to get it undone again. Then I hear people say they always apply Loctite to all their brake fixings, disk bolts and caliper bolts, one guy said to me that he applies Loctite to the pinch bolt on his front axle, that seems like a nightmare waiting to happen if you get a puncture in the front tyre miles from anywhere, is that Loctited allan screw going to come out?
I am aware that you are hardly likely to snap off a steel bolt in the fork leg (Mudguard Mounts) but you could strip out the softer alloy threads in the leg because of Loctie, in some ways the first situation would be better if the bolt did shear at least you would have the option of drilling out the sheared bolt and cleaning the thread with a tapered and then a plug tap, but the threads being pulled out of the fork leg would be a different proposition, it would mean a Helicoil or a new fork leg.
I know that I will continue to apply a dab of Copper Slip to the nuts and bolts that are removed on a regular basis for maintenance.
I have reached a grand old age now and have never had a bolt come out, or, even loose because of lube applied to threads and when I do need to remove the there is a better chance of not snapping or stripping threads, Steel Sparkers in alloy heads certainly benefit from a dab of copper slip, I have heard many times that guys have sheared off sparkers trying to get them undone.

Sorry for my long dissertation, I know it winds up some on here, but I guess the answer is don't read it.
Cheers guys