Why things work loose
Nuts and bolts work loose for reasons that few suspect. It is not necessarily caused by vibration – and the commonly used ways of securing them are not that effective. The reason fastenings work loose is this: a thread being spiral, a nut under load pushes up on one side of the thread and pushes down on the other. If any form of repeated sideways movement allows or causes the slightest sideways movement between the threads, that fastening is likely to work loose.
Inter-thread movement may be enabled in various ways. Wheel studs are subject to road shock. Cylinder head studs continually expand and contract, allowing sideways movement. Repeatedly bending plates bolted together causes such movement.
Because the bolt or stud is under tension, even slight but ongoing side movement between the threads not simply allows the tensioned fastening to loosen, that tension actually causes it. Nuts and bolts are literally sprung undone.
If camper trailer suspension lacks shock absorbers, the resultant shock as the wheel is repeatedly slammed onto the road not only enables wheel nuts to loosen. It causes it.
The correct torque
Only a fraction of nut turning force ends up as clamping force. Of the force applied, about 50% is absorbed by friction between the threads. A further 35-40% is absorbed by friction between the nut, or bolt head, and whatever it presses onto.
Even with clean lubricated threads, the turning effort converted into clamping force is typically only 15% but can be as low as 10%. If threads or nut face are dirty, corroded or damaged, friction may be so high that clamping force is almost non-existent.
Bolts and studs must be stretched only by that amount resulting from the manufacturer’s specified torque settings. For high tensile fastenings, that is about 25% of that point where all elasticity is lost.
Many mechanics believe they can correctly tighten high tensile nuts and bolts by ‘feel’. Experiments with highly experienced mechanics (by Vauxhall Bedford Research), however, showed variations of up to 50% in what they believed to be the ‘correct’ tightening torque. Always use a torque wrench for critical applications.
Never let a mechanic or tyre fitter use a ‘rattle gun’. In theory, those guns are adjusted to suit the threads concerned. But some users assume that ‘one setting suits all’. It doesn’t.
If necessary, offer to accept responsibility – and do that final torquing yourself.
Always use a torque wrench for final wheel nut tightening. Pic: source unknown.
Corrosion – a major problem
Corrosion works down exposed threads and locks nuts and bolts together. Corroded nuts will not readily work loose, but there is no way of knowing if adequate, or indeed any, clamping force exists.
Torque checks on corroded fastenings are totally meaningless. Most or all of the spanner’s torque is absorbed by friction.
Corroded nuts and bolts must be replaced. This is a major problem with axle/spring U-bolts clamps – and a known cause of their failing.
Keeping things tight
Where there is no repetitive sideways movement, correctly torqued nuts and bolts stay tight. Where things may move or vibrate, however, some form of secure locking is vital. Traditional methods only partially work: most hardly at all.
Plain washers spread the clamping force, and may enable smoother contacting surfaces, but that’s their only benefit.
Spring washers behave much as plain washers, excepting that, if the nut begins to loosen, the washer’s angled-cut edge digs into the adjacent surfaces, acting as a partial ‘brake’. But if sideways movement is possible, a spring washer will not prevent a fastener eventually working itself loose.
Castellated nuts (where a split pin is inserted through a hole in the stud or bolt) are time-consuming to fit and remove. Also, because pin, bolt-hole and castellation must line up, in so doing, the required torque may not be possible to apply.
A second nut tightened securely above the first is fairly effective, but both may still work loose if there is ongoing sideways movement.
Compounds such as Loctite (now manufactured by Henkel AG) address the primary problems, but do not work remotely the way that many users think – i.e. that they are some form of glue.
These materials perform two main functions. Initially, they act as a lubricant that ensures that most (clean) bolts and nuts behave alike in terms of inter-thread friction. Secondly, the compounds form a plastic sleeve between the threads. This increases inter-thread friction but more importantly it expands to fill the gap between the male and female threads. This inhibits the sideways movement that is the main cause of fastenings working loose.
Wheel studs and nuts should be clean and then torqued to the maker’s settings. If correctly torqued, the nut’s cone-shaped contact area normally precludes sideways movement. No additional locking is required. If the suspension is undamped, however, through lack of (or worn out) shock absorbers, the impact forces on wheel nuts and studs can be massive.
Despite the cone restricting movement, the repeated shock loads cause slight sideways movement between the cone and its housing. This can cause wheel nuts to loosen and studs to be sheered off – even if correctly torqued. Here, the only solution is to address the cause. Fit or replace the shock absorbers.
The lower spring holding plate is already bending and may continue to do so. It needs to be or preferably be of inverted channel section. Pic: source unknown.
It is often argued that correctly torqued bits and pieces need no re-torquing. This is not always so – particularly with fastenings such as U-bolts – where the bottom of the ‘U’ may need time to bed down and conform to the curve of the axle.
If, after a second tightening, the third requires more than (say) a quarter of a turn, have an engineer check whatever is being tightened/ It probable that something is bending and may eventually fail. This is a known issue with some axle U-bolt clamping plates as shown (left).