Hi,
Say I do a nut or bolt up to (for example) 120ft/lbs as checked by a decent torque wrench in calibration, can that same nut/bolt be undone by the same amount of force, or does it take more - and if so, how much more and why?
I would guess more, because when doing it up it must be sliding, but when undoing you have to overcome the coefficient of static friction, which is always more than the coefficient of dynamic friction.
More.
Why? As you tighten, the thread heats and expands. When it cools, it 'nips up' as the metal contracts. When you try to undo, the extra force is needed to compensate for this 'nipping up'.
How much? I recall having to do a calculation at Uni, in the first year some of the courses were common to the Mechanicals. I seem to recall there was a set of formulae to work out the forces on a thread but that was 40+ years ago.
I'd agree with that - but the heating effect mentioned in the previous post is interesting.
I'll add another - corrosion. Even the slightest superficial surface corrosion on the threads or under the head is likely to add something to the break force required to get it undoing.
An exception, in the case of some cylinder head bolts, is the use of bolts torqued beyond their elasic limit.
The idea of those bolts is simply to ensure a more predictable clamping force than can be obtained by measuring the torque applied. A torque wrench doesn't measure the clamping force, at least not directly. Other factors, dirt etc, can cause higher/lower torques to be applied by the wrench to achieve the same clamping force.
The bolts, at least when new, are designed to 'go over' they limit reliably. At that point the clamping force should be as expected.
That isn't ex Uni, it was explained to me by a tank mechanic. (Not the water kind.) He was working on an M1A1 engine.
Corrosion throws a complete unknown into the mix so there's no point speculating. ISTR from somewhere (I could be wildly out here, though) that even under optimal conditions (clean, lubricated threads on both components) the force needed to release a fastener can be 2 to 3 times as much as was used to tighten it in the first place. That sounds like a huge discrepancy and I may be wrong. But I might be right. :-/
They're still very probably going to need less torque to undo than they did to do up.
Assuming everything including friction etc is constant, then less as you'll have the thread angle coupled with tension to assist in the undoing.
If there is any form of stiction, cold welding etc, then who knows.
Otherwise bolts would never be able to rattle loose or require crinkle and/or split washers to prevent a bolt or nut from unwinding.
I'd be inclined to speculate away, having eventually undone a steel pedal spindle from an alloy bicycle crank.
Depends how rusty its got in the meantime
And how much plastic deformation and creep has happened.
I'd imagine it depends on the materials involved and the amount it bites into the substance you are bolting together and the amount of stretch as its done up. Remember watching the crews changing out items on both Hubble and the iss having to up the torque to get some bolts out. Brian
also, car cylinder head bolts (and others I'm sure) are specified as a torque *plus* an angle ... presumably to take the fastening into some sort of inelastic deformation, or ensure that any variability in threads is evened out ?
It is particularly difficult in the vacuum of space as metallic surfaces can slowly cold weld together by diffusion. That tends not to happen on Earth unless you are working on parts inside high vacuum systems.
My instinct is that AOBE tightening you are working against dynamic friction until the torque wrench limits. Undoing you are working against static friction which is always higher until the thing starts to move.
In reality corrosion tends to stick steel nuts and bolts together on Earth which can make penetrating oil essential to get them apart.
But with stretch bolts, you start out with a given torque setting, then turn them to a specific angle (number of degrees) after that.
So if the threads were tight etc in any way, that initial torque setting would still be wrong.
You should always clean any thread on a fixing where the torque is critical.
The big problem is torque wrenches are often well out of calibration. And require skill to use. Watching a tyre place the other day, the fitter pushed on the torque wrench long after it had clicked. Giving it a good jerk after it had.
So it might be in practice that stretch bolts are more likely to be set nearly ideal. But I dunno for sure.
I do know that gorilla mechanics will break anything, though.
I always use a thread sealer on all the bolts which go into ally. Otherwise they are likely to corrode over the years. That thread sealer also locks, so requires a lot more torque to undo - and does so with a 'snap'
I disagree. They are torqued up until they yield, but they will work harden so that when you stop, they are a bit stronger than when they were "new". When you undo them, you still have static friction to overcome, plus any contribution from corrosion or other mechanisms which increase the adhesion between the surfaces.
I picked up some threadlock whilst in the accessory store the other day. Didn't read the label carefully enough. It was a Locktite product (can't recall the formulation number) and buried at the end of the instructions it said, and I paraphrase: "use for permanent joints not requiring future dismantling" so clearly much stronger stuff than I'd really wanted. :-/
Is it actually a threadlock or a locking compound? I know that the locking compounds are good enough to lock the drive wheels onto the axles of a 5" gauge locomotive - without threads, splines or any sort of pinning.
SteveW
Roger hasn't considered the 'stress / strain' curve for, in this case, a bolt being tightened.
The stress (tension/load), which acts along the length and provides the clamping force- doesn't suddenly change in gradient at the elastic limit (Yeild point). It changes, but only slightly. The bolt is then in the plastic range, it won't return to its original length if undone. But the stress either side of the Yield Point doesn't change dramatically. If it did, the idea of deformable bolts would be flawed. If you continue to tighten, then you will reach a peak on the curve, the Ultimate Tensile Strength. Go further, and the stress decrease but, oddly, you will eventually reach the Fracture Point where, unsurprisingly, the bolt snaps.
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