A month or two ago, our almost new Suzuki Jimny was disabled by a bolt working loose in its gearbox - stranding my wife on an outback road with a gearbox full of neutrals. As this example showed, nuts and bolts do still work loose, yet, surprisingly, the causes are not widely understood nor covered in most engineering courses. The reasons why are not necessarily what one might expect. They do not for example necessarily shake loose directly because of vibration.
Little Clamping

Very little of a spanner’s turning force (torque) results in actual clamping. About 50% of the applied torque is absorbed by friction between the threads, and a further 35%-40% by friction between the nut and whatever the nut presses onto. So, even with clean, lubricated threads, only 10%-15% of the turning effort is converted into clamping. If the threads are dirty, corroded or damaged, or the nut-face is rough or corroded, clamping forces may be negligible.

Corrosion - a Major Problem

Corrosion binds threads - locking nuts and bolts together. Once corroded, they will not readily work loose, but there’s no way of knowing what (if any) clamping force still exists. Checking with a torque wrench is meaningless. If the nut moves at all, most (or all) of the applied torque will be absorbed by friction. Worse, corroded threads are impossible to restore - the nuts/bolts/studs must be replaced.

Working Loose

Because threads are spiral, nuts under load push up on one side of the thread, and down on the other. Repeated applied sideways movements reverse the direction of these upward and downward forces and INEVITABLY cause the nut or bolt to unwind. (Test this with a matchbox on a sloping surface. Pushing the matchbox along the length of the surface will cause it to slip sideways.) Any force that causes threads to move sideways will cause nuts to work loose. Vibration may well initiate this, but only if it causes sideways movement. Sideways movement can be caused in various ways. Cylinder head studs, repeatedly heated and cooled undergo differential expansion and contraction - allowing sideways movement. Repeatedly bending plates bolted together will inevitably cause the fastenings to work loose.

Correct Tightening Essential

Correctly tightened bolts or studs are stretched by a pre-determined amount. For a high-tensile example, this is typically 25% of the point at which they are irreversibly elongated (or break!). The required tightness can only be gauged by using a torque wrench - no matter how experienced, it is all but impossible to gauge by ‘feel’. Unfortunately many mechanics would as soon use a torque wrench as wear a ballet costume to work!

Keeping Them Tight

Where there’s no movement, correctly tightened nuts and bolts will remain tight. However where things can move, a locking mechanism should be used, especially if loosening can cause inconvenience, damage or an accident. Locking methods include castellated nuts and pins, spring washers, self-locking nuts, nuts locked together, and chemical compounds. Castellated nuts (a pin passes through a hole in the bolt and slots on the nut) are clumsy to fit and remove, and because the pin, bolt-hole and castellations (slots) must line up, the exactly required torque cannot always be applied. Plain washers provide smooth contacting surfaces, reducing inter-material friction (but that’s their only benefit). A spring washer’s angled cut digs into adjacent surfaces, acting vaguely as a ‘brake’, but if there’s a lot of sideways movement a spring washer may not prevent total loosening. Locking nuts have inserts that introduce static friction. They are effective in many applications, but not where there’s repeated sideways movement. A second nut tightened securely against the first is reasonably effective, but both can still work loose if there is sideways movement.

Locking Compounds

Locking compounds address loosening at source. Firstly they include a specialised lubricant that reduces inter thread friction, thus assisting the intended clamping forces to be achieved when the specified torque is applied. Primarily though, once set, locking compounds form a plastic sleeve between gaps in the threads, hindering undoing; but more importantly preventing the sideways movement that is the basic problem of things working loose. One problem with locking compounds is disturbing the locked threads (e.g. by re-checking the torque setting) destroys the locking action. And it often is necessary to recheck - particularly with U-bolts where the ‘U’ of the bolt takes time to ‘bed down’ against the curve of the axle. One solution is to assemble and torque the assemblies (have clean but dry threads). After 500 km or so apply a ‘self-wicking’ compound such as Loctite 290 (which works down already tightened threads), and quickly re-torque. (Nuts working loose are almost invariably the initial cause of that outback scourge - the broken U-bolt).

Wheel Studs

Correctly torqued wheel nuts or bolts do not require additional locking. Their cone-shaped contact areas preclude sideways thread movement. Wheel nuts should ideally be cleaned, lightly lubricated, and torqued to the vehicle manufacturer’s specifications. If you don’t have a torque wrench, tighten firmly with the spanner supplied. Do not use an extension, nor jump up and down on the spanner. If a wheel is changed by a service station, insist, absolutely insist, that the nuts not be tightened by a rattle gun.