Stud and Nut Tension and Controlled Bolting
Let’s look at the stresses that affect a wheel and its studs. We want to know how wheels are attached and how to extend the life of those wheels and their studs.
It is relatively simple to bolt an object that is never going to move. You determine what pressure (tension) to apply to the object, bolt it to that tension, and you’re finished. The object remains in place. Bolting wheels, however, is more complicated. Wheels move – they start, turn and stop. They support different loads and travel at various speeds. As a truck runs down the road, and a wheel rotates, the load (stress) put on each stud changes dramatically. The studs experience different loads (cornering, tractive, normal), sometimes individually and sometimes collectively. If one of these loads applies pressure to a stud that is greater than the amount of tension torqued on the stud, there will be a momentary stretching of the bolt. Any stretch of the bolt, past where it has been tensioned to, will cause a brief relaxation of that bolt. And the result of any relaxation is a loss of pre-load. The majority of stud problems are caused by a reduction or loss of pre-load.
Proper installation is essential to preserve the life and utility of a wheel. This requires controlled bolting, with a superior torque tool. We know that a stud will continue to function as designed as long as the forces acting on it do not exceed the tension applied to it. This is where controlled bolting becomes so important. If all the studs have the same amount of tension, they can react the same throughout the cycles of the wheel. The desired tension should be around 70% of the capacity of the stud. You want it to be in that range because there is a built-in safety factor (Note: With our understanding of torque and tension we know that the friction factor can vary so much that we must have a safety factor, or we will lose studs constantly).
“Proper installation is essential to preserve the life and utility of a wheel.”
For example, let’s assume that 1000 ft./lbs. of torque will give a fastener the correct tension. If all the studs are tensioned the same, and the external forces do not exceed that tension, then there will not be any loss of pre-load. If there are studs that do not achieve the desired tension, there will be problems. If there is an outside stress that exceeds the tension on studs # 1, 4, 5 and 6 they will lose the pre-load. When they have lost their pre-load, the four remaining studs are required to pick up the slack. Nothing is working as it was designed. Use of a torque tool and an understanding of tension will help to prevent this problem.
Unfortunately, theory often differs from reality. To theorize, you must assume some factors as constant. But in the field, anything can go wrong. We stated that when you achieve and maintain the correct tension, the fastener will hold. This is true, but maintaining the tension is not as easy as it sounds. In the field, retorquing is required, and sometimes more than once. Loss of tension has a snowball effect, if one stud loses its tension, the others around will assume the pressure of that stud. Usually, this causes those studs to lose their tension as well. This continues until you have loose wheels and the potential for further damage. On rims, this loss of tension can lead to damage of the gutter and taper on the hub face, as well as valve stem shear through rim rotation. These problems can be expensive to repair, but can be avoided through retorquing. Retorquing helps ensure the correct tension at all times, and allows fastened joints to “settle”.
Here is an example of how Fording River Coal achieves desired fastener tensions. Nick Sutton explained the lengths they go to. We have included his steps here to demonstrate how critical it is to arrive at the correct tension. This process is not required in all applications, but the steps indicate how to arrive at a correct tension and then maintain it.
- Torque wheels in the tire shop and then drive to the location where the truck will be used. This drive is usually between 15 to 40 minutes.
- Retorque on location.
- The truck then hauls two loads.
- Retorque a second time. At this point, if the tension has been maintained, the truck will run for twelve hours. If the tension has not been maintained, repeat, starting with Step 2.
This process is usually much quicker on trucks that use a wheel instead of a rim. Generally, when using a wheel, none of the steps will need to be repeated. If retorques are required, Mr. Sutton concludes that there must have been a problem at installation, or on one of the retorques. The studs may have been over torqued and then yielded during one of the steps.
Mr. Sutton found the installation of a rim generally requires that some of the steps be repeated. This can be explained quite simply. When installing a wheel, there are five components: hub, stud, wheel disc, nut and washer. A rim installation has seven components: hub, stud, rim, spacer band, rim, clamp, and nut. With the increased number of components, the margin for error grows. The fastener may achieve the desired tension, but if one of those seven components shifts during a load, the pre-load on that fastener will be lost. One fastener losing its pre-load has the snowball effect that we spoke of earlier. This is why there may be a greater number of retorques required when rims are used instead of wheels.
The process described may seem like extra effort, but this is what Fording River Coal has found to be necessary to keep their trucks running. Plus, it saves money by reducing damage and down time.