Should wheel fasteners be lubricated?
It seems there is general confusion as to whether wheel fasteners should be lubricated. Many mechanics have been trained never to lubricate wheel fasteners whilst others will habitually apply some form of lubricant, often copper grease, to prevent the fastener from seizing.
In order to shed some light on the issue, we must first have an understanding of how a threaded fastener works. A threaded fastener can be compared to a very strong spring. When we apply a torque to it, we are actually stretching it very slightly. Because the fastener material is elastic in nature, it will try to return to its original size, thus creating pre-load. It is this pre-load that clamps the wheel to the hub surface.
The torque applied to the fastener is critical in ensuring that the correct amount of pre-load is applied. If a threaded fastener is tightened beyond its elastic limit, it will become plastic in nature. This means that any further torque applied to the fastener will permanently stretch and deform it, reducing pre-load and resulting to wheel loss.
If we examine a dry fixing, the amount of pre-load that can be obtained is directly affected by frictional losses in two main areas. In general, 50% of the pre-load will be lost by friction at the nut face and a further 40% at the thread contact. This means we are relying on only 10% of the total torque applied to be converted into pre-load to clamp the wheel onto the vehicle.
If a manufacturer has the specified that a fastener should be torque dry, he will have ensured that correct amount of pre-load is achieved in this condition. If we take it upon ourselves to apply a lubricant to either the nut face (conical shoulder) or the threads, we can dramatically change the coefficient of friction within the torque calculation.
This means that we could theoretically reduce the frictional losses at the nut face to say 20% and the threads to say 10%, resulting in 70% of the torque applied being converted into pre-load. It is clear to see that this increase in pre-load is enough to permanently deform or even shear the fastener and is very dangerous.
Some manufacturers will specify a specific lubricant to be used. In this case, the manufacturer will have based the torque value on a correctly lubricated fastener to achieve the desired pre-load. It is imperative that where a manufacturer states that lubrication be applied, we apply it to the correct areas and use the correct lubricant. Failure to lubricate a fastener designed to be lubricated could result in insufficient pre-load being applied. This could result in the wheel fastener being left in a dangerous, under-torqued condition.
In conclusion, you should always make reference to technical data to find out any special instructions relating to wheel fasteners. This information will tell you whether or not a lubricant should be applied, what lubricant to use and where to use it.