Even More Trivia: Loose Wheel Nut Indicators

Today's trivia is all about loose wheel nut indicators.

These seemingly simple devices are placed over the nuts used to secure a wheel to its wheel plate. Clearly they are designed to give a visual indicator to the driver when the nut slips. However the question I wanted to answer was: why do wheel nuts slip?

Wheel Failures

The problem of a loose wheel is clearly significant. Both in terms of control of the vehicle involved and anything else in the path of the wheel.

It was estimated in 1992 by the National Transportation Safety Board (US) that there were between 750 and 1050 incidents per year relating to heavy trucks. The actual figure is expected to be higher, as these are the reported incidents.

Typically when a heavy truck loses a wheel it will continue to travel at the speed of the truck, and might fly high into the air because it is suddenly no longer under load.

Certainly we can from video examples that the wheels have a lot of energy.

Driver was ok in this video

Causes of failure

There are numerous reasons why wheels fail to remain attached to the vehicle. Of these the most common are failures of the wheel fastening. Simply put the wheel nuts that hold the wheel to the wheel plate shake loose.

Another possibility is shearing of the wheel plate lug

Nuts and Bolts

In normal use, a nut-and-bolt joint holds together because the bolt is under a constant tensile stress called the preload. The preload pulls the nut threads against the bolt threads and holds it in place. The nut cannot rotate without overcoming the friction between these surfaces.

If the joint is subjected to vibration, however, the preload increases and decreases with each cycle of movement. If the minimum preload during the vibration cycle is not enough to hold the nut firmly in contact with the bolt and the bearing surface, then the nut is likely to become loose.

The amount of tension we can apply to a nut is based on the hardness of the steel. Harder steels are able to withstand greater tension before they fail. There are various standards which govern the hardness of steel fastenings like nuts and bolts.

Graph showing the behaviour of a bolt under tension including the elastic and plastic regions

Steel

Commercial grade steel for use in joint assembly fastenings are hardened by two different methods depending on the type of steel used:

Low (less than 0.25%) carbon mild steel which is 'case hardened'
Medium (0.25%-0.5%) carbon and alloy steel is 'through hardened'

The carbon content of the steel is significant in how its hardness can be improved. Mild steel with a low carbon content when heated and quenched will not increase in hardness. Only medium and higher carbon steels can improve in this way. This is known as 'through hardened'.

Instead low carbon steels are heated and sprayed with a chemically reactive carbon compound. When the steel is quenched this reactive coating forms a hardened case around the softer steel.

Oil, water and high pressure gas can all be used to rapidly quench hot steel

Conclusion

The problem of a loose nut on a wheel is surprisingly complex.

In order to secure a wheel assembly correctly you need appropriate fastners. Nuts secured against the bolts hold the wheel in place, because they can generate enough friction to prevent the nut from loosening.

They can only generate that preload friction because the steel the nuts are made of is sufficiently hardened to ensure the thread does not deform under load. Too little tension and the nut will easily come loose from vibration. Too much tension and the bolt threads might strip or the entire bolt shear off.

Maintenance schedules are there to check that wheels are fastened correctly on regular intervals. However, loose wheel nut indicators are there to provide a visual inspection method by drivers prior to driving the vehicle.