Check Your Wheels Check The People Who Fit Your Wheels

NickB wrote:well i probably massively exceeded the recommended torque for the studs when i wound them through.I will find out if i have on Saturday when i tow trailer for 1st time in 6months.

Tightening Wheels

The fastener industry and automobile industry emphasise that wheel nuts and studs must never be even remotely tightened by rattle guns (impact wrenches). The final tightening must only be done by using a high quality torque wrench of known accuracy - and to the vehicle manufacturer's specified amount only. They must then be rechecked and tightened further if necessary after 50-100 km, and again after 1000 km. But any number of tyre fitters 'know better' and use only rattle guns.

Be aware that rattle guns are very prone to overtightening - and that is counter-productive. This is because excess tension may stretch the stud to the point where its diameter is reduced whilst under such high over-tension - and thus increasing interthread spacing. In far from unknown extreme cases it may result in the stud eventually cracking and/or sheering off.

If a lubricant is used (and most authorities recommend against it) the tightening torque must be reduced by about 20%. Anti-seize materials must never be used for any but totally static applications. This is because one of their intended roles is to ease undoing.

Follow this sequence:

1. Clean the threads thoroughly and ensure the nuts spin freely, by hand alone, along the stud's full threaded length. Discard any that do not, and never use a nut or stud that is or has been corroded. The studs and nuts need to be totally clean, dry, and unlubricated.

2. Locate the wheel on the studs and finger-tighten using a diagonal sequence. Give the wheel a few wriggles to allow it to locate correctly.

3. Tighten the wheel nuts progressively and working diagonally.

4. Use a torque wrench for the final tightening. Torque only to the vehicle maker’s specifications. Do not exceed specified tightness. The only too common belief that more is better has no foundation.

5. Recheck after 50 km, and after a further 100 km. If further retightening is needed there is a very real possibility that whatever is being clamped is under-engineered and is progressively bending. (This occurred with the U-bolt axle clamping plates on early OKAs (including our own). If so this needs to be addressed as a matter of urgency by an engineer.

If really concerned about loosening apply Loctite 290 after the final bedding down.

Insist on that torque wrench being used. Many mechanics wrongly believe they can ‘feel’ correct tension. They cannot. Personally observed research showed that over half of 300 (tested) experienced mechanics failed to gauge wheel nut tightness by plus or minus 30%. Some were out by up to 50%. Hardly any could repeat settings reliably within plus/minus 15%.

Insist beforehand that that you need the final tightening to be to the vehicle maker’s specifications and by a calibrated torque wrench. Never allow a ‘rattle gun’ to be used for final tightening as there is a very real risk of over serious overtightening. As noted in the main text, this increases the stretch, and may narrow the thread diameter, thus increasing the thread gap.

Ideally, have your own high quality torque wrench and insist on doing the final tightening yourself (I always do this).
This article was originally written and published around 2004. It is updated here (in mid 2012).

The need for shock absorbers on trailers is now more widely recognised - but many makers still believe their products are immune to the more fundamental Laws of Physics. And mechanics seriously overestimate their ability to tighten, correctly and consistently, by 'feel'.

And never, ever, use an anti-seize product for other than the static applications for which it is intended.

Collyn Rivers

Why Things Work Loose

The threaded section of screwed fastenings is by necessity spiral. One side of the tightened nut pulls up on one side of the corresponding stud’s threads; the other side pushes down on the other side of that thread. The tightening force normally maintains these faces in tight contact and that, plus considerable inter-thread friction, prevents sideways movement between the nut and the stud (but clearance must exist to enable the nut to be able to turn).

As the nut is tightened, the stud or bolt stretches very slightly because it is in tension. Even when correctly tightened however, repetitive side loads not only can but do cause the thread within the nut to move very slightly from side to side. The thread being spiral and marginally stretched, such sideways movement momentarily reduces the frictional contact between the stud and the nut: this minutely and momentarily relieves tension in the stretched stud. The vectored forces thus act as a ratcheting mechanism that, in some circumstances, not only enables, but actually causes the nut to undo.

The above is not conjecture – you can actually see it happening.

Hold a large diameter and clean dry threaded stud and nut vertically downwards and you'll find the nut (restrained by interthread friction) stays where it is. But shake the assembly vigorously from side to side and the nut will unwind itself under the relatively minor downward force of gravity alone. A similar effect happens with a nut and stud under tension – excepting that the brief but repeated loss of tension more or less replicates what gravity does in that experiment. If repeated sideways movement exists, nuts will inevitably ‘ratchet themselves’ loose unless somehow constrained. It is not that they may do undo. They will undo.

Spring washers do less than helping - by adding elasticity they may make matters worse.

Nyloc nuts add friction and marginally limit sideways movement but in the more extreme cases are likely to work loose.

Castellated nuts and split pins prevent nut movement but accurate re-torquing is then not usually feasible unless new holes are drilled each time.

Double nuts help because the upper nut is virtually free of loosening forces.

Left-hand thread nuts (on left hand wheels) were used at one time as anything they strike whilst the wheel is revolving tends to turn them anti-clockwise and thus tightens rather than loosen them - as it would were they to be right hand thread. They do not however restrain loosening (as above). They dropped out of use when the main causes of fastening working loose became understood, but a few truck makers still use them (particularly with mining and quarry heavy vehicles - where direct impact is likely).

Of the above, only the Nyloc nut (partially) addresses the initiating cause. There are better solutions, either of which is effective. Used together (although my experience indicates doubling up is rarely required) it is probable that wheels falling off can be virtually eliminated.

A very effective approach however is by precluding or at least limiting sideways interthread movement at source. The male and female conical interfaces on a typical vehicle wheel are often seen as doing so, but are not - it does however reduce the effect of the impact forces involved (and is covered below).

Filling the Gap

The first approach is to eliminate the gap between the threads. If there’s no gap, there is no side movement and hence little likelihood of nuts undoing. This is how Loctite (and similar products) work. They do not ‘glue’ things together – but by filling the gaps between threads they preclude the sideways movement that causes the problem. Most such products are not however suitable for wheel nuts because wheels ‘bed down’ and need re-torquing at about 50 km, and ideally after a further 100 km. Any subsequent enforced movement destroys the effect of the product.

The specialised Loctite 290 product (only) is designed for things known to bed down and thus need retightening (e.g., road wheel and U-bolt nuts). It is a self-wicking fluid that works its way even between horizontal threads. It is applied only after a second or third re-torquing - and then only if that final retorquing required only a few per cent more torque.

As the product effectively precludes nut unwinding, further torque checks are (claimed to be) unnecessary. It must be reapplied after wheel changes. (It is used also on roller coasters etc to prevent loosening of fastenings that were they to do so, would result in a high risk of consequent catastrophic failure)