Why Do Fasteners Loosen?
With winter fast approaching many of us are lining up to replace our summer tires with winter tires. Tires are mounted on rims which are then secured to a vehicle’s wheel hub with lug nuts. These lug nuts are threaded onto the wheel hub’s studs. Tightened lug nuts clamp the wheel assembly to the wheel hub forming a bolted joint. Lug nuts are generally conically shaped on the surface that contacts the wheel. This conical profile positions the tapered portion of the nut during tightening into contact with the rim, which centers the stud inside the hole and prevents the wheel studs from directly contacting the rim.
Proper lug nut torque is necessary to ensure that the wheel is properly mounted to the vehicle. Vehicle manufacturers typically specify the type of lug nuts that should be used to secure the rim, the tightening pattern that should be used, and the amount of torque that should be applied. Have you ever wondered why you are told to go back to the mechanic for a retorque after a tire change? The retorque is an important safety step that many people skip at their peril.
For a bolted joint to function properly it must maintain a constant clamp load. One drawback with bolted fasteners is that they can self-loosen under vibration. Self loosening has been studied and was documented in the research paper written by Junker Gerhard in 1973, “Criteria for self loosening of fasteners under vibration”.
It has been determined that transverse vibrations are more severe than dynamic axial loads for bolt self loosening. This is the big problem for lugs nuts because they are subjected to repeated transverse movements during vehicle operation. The force from the transverse movement can be a greater than the frictional force created by the clamp resulting in the self-loosening of the lug nuts.
When a lug nut self-loosens the remaining lug nuts are require to bear a greater portion of the wheel’s clamping load. This can introduce more vibration into the system which can accelerate the selfloosening of the other fasteners and can result in the overloading of the remaining studs. If this issue is not corrected immediately, the overloaded studs can fail catastrophically, allowing the wheel to separate from the vehicle. We have observed this exact phenomenon occur a number of times. Figure 1 below displays a wheel hub where 4 of the lug nuts were determined to have self loosened.
Loose bolts can result in major mechanical failures and personal injury. Our staff at Caskanette Udall Consulting Engineers specialize is both of these areas. Keep you and your family safe on the roads this holiday season. Get your lug nuts retorqued.
