Doesn't it make you madder than a cat in a dog pound when you go to make a simple tire change and find a broken stud in the process?
This is especially true if you're doing this on the side of a busy road...at 1:00 in the morning! It's probably at this point that you stand back, scratch your head and ponder why wheel studs fail.
To understand this scourge of the tire technician, it is important to understand how wheel fasteners work. As we all know, in every wheel system there are studs and either one or two lug nuts on each stud holding the wheels onto the vehicle.
But how do these systems work? To explain it simply, think of the stud as a spring. When the stud is correctly installed in the hub and the nut is tightened on the stud, the stud stretches a little bit like a spring and is put into tension. This tension holds the wheel(s) on, keeps them tight and is called clamping force.
Applying torque to the fasteners attains clamping force. Torque is a measure of the twisting force applied to the nuts and is calculated by multiplying weight times the length of a lever. (For example, a 200-lb. man applying all of his weight to a two-foot breaker bar creates approximately 400 ft.-lbs. of torque.)
You can't measure clamping force outside of a laboratory, but you can measure torque. That's why you hear people talking about it and saying you ought to measure it. Since you control the amount of torque applied to a fastener, you ultimately control the amount of clamping force applied to the wheel components. However, the correct torque does not guarantee that the correct amount of clamping force has been achieved.
The primary reason for this is friction. Anything that adds friction to the fastener system reduces clamp force. Hub-piloted wheel systems require that two to three drops of 30-weight oil be placed between the nut flange and the nut as well as on the end of the stud. This lubricates the system and ensures the fasteners achieve the correct clamp load (65,000 lbs.).
Dry nuts and studs will fall short of the optimum clamp load even though they are torqued to the specified 450-500 ft.-lbs. If you add rust, dirt and debris to the threads of the nuts and studs, you'll get even less clamp load despite the fact that you torqued the system to the proper range. This is why with hub-piloted wheels it is so important to wire brush the threads of the nuts and studs, clean the mounting faces and apply oil to the studs and between the nut and nut flange.
In the stud-piloted system, no oil or lubricant of any kind is to be used (this includes anti-seize compounds). This system is a dry system. If lubricant is used, the clamp force on the system is increased significantly and can easily reach the yield point of the stud.
The yield point is when the stud is stretched so far that it loses its tension. Remember the spring? If you stretch it too far it won't spring back. This is what happens to the stud. A yielded stud is worse than a broken stud because the stud has failed-but you can't tell just by looking at it. At least a broken stud gets your attention.
Stud-piloted wheels should be torqued to 450-500 ft.-lbs. dry; demountable rims require 200-260 ft.-lbs. dry. Of course, just as in the hub-piloted system, rust, dirt and debris on the threads and mounting faces will also interfere with attaining the correct clamp load, so get out the wire brush and make sure these components are clean as well. Always aim for the middle of the torque range when tightening fasteners. This will help offset any performance variation of the torque wrench.
Now you're probably sitting there thinking, ``Who has time to use a torque wrench? I just use my trusty impact wrench and I get them wheels on tight every time.''
Well, I'm sure you do! A brand new impact wrench will torque up to 1,400 ft.-lbs. in 5 seconds with 90 pounds of air pressure running through it! This is guaranteed to yield and break studs. However, a well-used, hardly-ever-been-maintained impact wrench will be lucky to unscrew the lid on a pickle jar! Where does your trusty impact wrench fall?
Pneumatic tools should be calibrated at regular intervals to ensure they are performing up to par. They have mechanical parts that wear out, need to be lubricated daily and checked for performance monthly.
You can check the performance of your impact wrench by tightening a nut with it using 90 psi of operating pressure and letting the wrench impact for about 2 seconds. Then use a torque wrench to check the torque. If the wrench is not performing up to par, that is, tightening the nut to 450-500 ft.-lbs., send it out to be rebuilt. Make sure the air lines to your wrench have a filter, regulator and oiler on them so the air is as dry as possible and the tool is lubricated as it works. Don't forget to drain the air tanks frequently-especially in humid weather. Moisture will tear up your trusty impact wrench in no time.
Now, back to that broken stud you're faced with. Why did it break? On stud-piloted wheel systems, the most common reason studs break is low torque. When the wheel system is loose, the wheels ride on the studs and break them. Overtorquing the studs and yielding them will also result in loose wheels that break studs for the same reason.
Excessive corrosion will weaken studs too and can cause them to break. That's why really rusty studs should be replaced before they break, in addition to the fact that the rust on the studs adds friction.
Mismatching nuts and studs of stud-piloted and hub-piloted systems will also cause studs to break. These parts are not interchangeable. Don't try to use a hub-piloted nut with a stud-piloted wheel or a stud-piloted nut with a hub-piloted wheel.
Damaged stud holes in the hub will allow the studs to wobble and will result in broken studs, too, but this is not as common as the previous causes.
So now that you know what may have caused the stud to break, what are you going to do? If you are faced with only one broken stud, the bad news is you have to replace it along with the ones on either side of it. That's a total of three studs that have to be replaced.
The odds are that if there was enough force on the first stud to break it, there was sufficient force on the adjacent studs to weaken them as well so they should be replaced. The really bad news is if you are looking at two broken studs, you've got to replace them all.
When you do this, make sure you have the correct replacement studs. And don't use cheap, unrecognizable studs. Most wheel studs will have a marking on the head of the stud indicating the material grade. Additional markings such as part number and manufacturer I.D. can be located on the head of the wheel stud, too.
If there is no marking on the studs, they probably are off-shore products that do not meet quality guidelines for these fasteners. Don't use them. Check your inventory and either return the unidentifiable studs to your supplier or throw them out. If you use them, they will be a source of constant headaches for you and your customers. Purchase studs and lug nuts made by reputable manufacturers that are clearly identified.
Studs should be replaced using a hydraulic press to press them into the stud holes. If hammers are used to pound them in, the stud head can be bent and that will prevent the stud from getting full contact with the hub. This will result in loss of clamping force due to minimal contact with the hub. Or the stud may not be square with the hub. This condition will end up wallowing the stud hole and also will result in loss of torque/clamp load.
To prevent studs from failing on wheel systems you service in the future, always ensure the correct amount of torque is applied at every wheel installation by using a torque wrench. Make sure that wheel and rim mating surfaces as well as the threads of the studs and nuts are clean. Use only 30-weight oil on hub-piloted wheel fasteners. Do not use anti-seize compounds at all.
And replace worn fasteners before they fail. This will eliminate a host of problems in the future.