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16 September 2018

NEWS & INSIGHTSWhy do my wheel nuts / bolts keep loosening?

We are often asked this question. Loose wheel nuts / bolts can occur for a variety of reasons, including (but not limited to) under-torqueing, over-torqueing, differential thermal contraction, and improper mating surfaces.

Many people use air impact wrenches to fit the wheels to trailers, cars and trucks. Both under-torqueing and over-torqueing are common when air impact wrenches are used because the actual torque that is applied to the stud / bolt depends on so many variables.  A common approach is to purposefully over-torque wheel nuts, in part to combat this variability and in part based on the reasoning that “more is better”.  However, over-torqueing actually reduces (not increases) clamping force in many cases, by stretching the studs or threads beyond their ability to respond – especially when this is done repeatedly.  Over-torqueing can also cause other problems such as cracked, seized, or cross-threaded nuts (which cannot apply the appropriate clamping force), and increases the frequency of stud failure and cracked wheels. In addition, it damages the chamfer around the stud hole in the wheel disc and this causes an improper mating surface.

Differential thermal contraction can occur when wheels are mounted at low temperatures in cold climates.  As the wheel components warm to ambient temperatures, clamping force is lost.  Wheel nuts that experience any rotation during this time will not regain their original torque / clamping force values. The opposite also applied when wheels are fitted at very high temperatures.

Improper mating surfaces include both damaged and contaminated mating surfaces. Proper clamping force cannot be achieved with non-flat mating surfaces such as damaged or bent hubs and wheels, or worn or elongated boltholes (raised metal).  Contaminants such as excess dirt, sand, rust, metal burrs, and paint on mating surfaces can wear away with use, causing a settling effect.  When present on the threads or between a nut and the wheel surface, these contaminants can also change the clamping force / torque relationship, resulting in “false torques” where much of the torque applied is used to overcome friction and is not converted into clamping force.

A loose wheel nut can originate from any of these sources individually, or more probably, from a combination of these sources – which makes the task of eliminating all loose wheel nuts / bolts very difficult indeed.

While in service wheels on vehicles are subjected to a bewildering variety of forces, including vertical forces from the vehicle and its cargo, road vibration and shock forces, cornering forces when the vehicle turns, and rotational forces from the turning of the wheel, especially during acceleration and braking. When a wheel nut loosens these forces are redistributed among the remaining nuts and studs, but preferentially to the nuts and studs adjacent to the loose nut, causing these nuts to back off at reduced wheel force levels.  This loosening process accelerates with each successive nut that loosens, as the total clamping force drops and the stress concentration at the remaining nuts and studs increases.  At this point, these studs can fracture due to fatigue or overstress, accelerating the process further – especially with heavy wheel loads.

When the wheel forces exceed the clamping force of the remaining nuts and studs, the wheel will move relative to the hub which results in side loading and loosening of the remaining nuts, bending fatigue failure of studs, elongated bolt holes and wheel pilots on the hub, fretting fatigue cracks between the bolt holes, and wheel separation if not detected quickly. The rate of this process, and thus the potential for detection prior to wheel separation, depends on the type and magnitude of the wheel forces being experienced.

We recommend that the correct wheels be used with our axles. Also that the toque of wheel nuts / bolts be monitored regularly so that corrective action can be taken before expensive damage occurs. This is even more important in the case of hire trailers, off road and other trailers that are operated by more than one person and where the operating conditions are variable and the loads are not consistent.



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