A number of reasons cause wheel looseness, but topping the list is improper torque. When you torque down a nut to the recommended level, it stays put. But to maintain the proper torque, you have to re-torque.
It's necessary to maintain torque at the recommended torque level for the stud size, so when the vehicle is first brought into the garage, check the torque number. With new wheels, torque should be rechecked after the first 50 to 100 miles, then at regular intervals depending on the application. That could be 10,000 miles; it could be every 2,000 miles.
Over time, the tightness of the clamped surface changes. The clamping force may change due to paint crushing when the wheel is torqued down or it could be caused by vibration. A torque wrench measures resistance to nut rotation. With brand new studs and little thread imperfections, that's all measured as part of the initial torque. As the nut is loosened and retorqued, however, it can have a different value than the initial torque. This is called "seating in." All the clamping force isn't being generated to the components because some of it is being used up in friction between the nut threads and the stud and between the nut and the surface it's going against.
Retorquing at certain intervals applies to all wheels. Even though used wheels may hold the torque better because they've already gone through "seating in," they still must be retorqued to maintain the proper clamping force.
Rust, corrosion and excessive paint play a role in the loss of torque and torque retention.
In routine wheel maintenance, most fleets clean off the rust and corrosion and repaint the wheels. It not only maintains the appearance of the wheels, but also protects wheel performance. If wheels corrode, the rust often pits the surface. Once that occurs, the effective thickness of the part is only the thickness from the bottom of the pit to the other side. If the pits become too deep, the wheel no longer has its designed strength.
Pits develop into cracks. Any small crack or blemish acts like a magnet for stress, which then concentrates itself around that blemish. In wheel refurbishment, after rust has been filed and removed, check the surface for signs of cracks or pitting. If there are none, the wheel can be repainted and put back into service. If corrosion is evident, don't use the wheel. If the corrosion is minor, sand it off. Too deep, however, and the wheel strength is undermined.
Although it is common practice to paint wheels, keep paint off of the threads. When torquing the nuts, the paint will build up resistance to rotation of the nut. That gives a false reading and will cause improper clamping force.
Some fleet technicians torque down the wheels and paint over the top of the nuts. The threads under the nut are protected; but when they are removed, the paint gums up the nut. To avoid later trouble, never paint wheels while they are on the vehicle. If that's not possible, make sure the threads are protected.
Excessive corrosion on the nuts or threads will render torque wrenches inaccurate because of friction. A few drops of oil on the threads will alleviate the problem, if carefully applied. If oil comes into contact with the area where the nut sits, clamping force will be lost. It will slip and slide, and an accurate torque reading will be impossible. This applies primarily to stud-piloted wheels
With hub-piloted wheels, the two-piece flange nuts come lubricated between the hex and the flange. They must not be lubricated between the nut flange and the wheel surface.
Worn out threads also diminish clamping force by preventing transfer of clamping force to the nut. As a result, wheels loosen. The nut mounting area can wallow out or wear, and parts could actually break because the nuts aren't loaded properly. Cleaning threads, putting a drop of oil on them, and keeping them well serviced will help to prevent this from happening.
Don't try to refurbish fasteners in stud-piloted wheels. If worn, they need to be thrown away. Once they are used and no longer have the proper chamfer, they should be scrapped.
At the end of the day, the recommended procedure for correcting wheel looseness is pretty straight forward — maintain proper nut torque, refurbish the wheel to maintain its integrity, and replace worn fasteners with new nuts, studs and clamping plates.