130 THE DISTRIBUTOR’S LINK WHEEL STUD FAILURES continued from page 12 The air supply that drives these beasts operates between 120-90 psi of air pressure which fluctuates with use and with the number of other devices operating off the same air supply. Therefore, any applied ‘torque’ is never even on the same wheel. The output torque is high with the impact wrench. Left unchecked, a common 1/2" drive air impact wrench will spin at 8,000 RPM and have an initial output torque of 325 lb-ft. That is enough for a large truck tire, not a 12 mm stud. Torque sticks have been used for a couple of decades now. No reports of failures have been attributed to their use, but it still is in the hands of the operator and how clean the wheel and studs are. Torque wrenches are also only as good as the operator. Some will initially tighten the nuts with an impact wrench, and then check the torque with a ‘click’ type torque wrench. If the wrench clicks, they move on to the next stud. Unfortunately, that ‘click’ doesn’t indicate if the torque has been exceeded or by how much. The best operational procedure I have ever seen is employed by one chain of stores who uses brake clean to rid the studs of any oil, then uses an Ingersoll Rand torque adjusting air wrench set to 50 lb-ft to initially set the wheel. The nuts or stud bolts are tightened in a crisscross pattern followed by using a torque wrench set to the manufacturer’s specifications. After a few moments, the torque is reapplied to assure full joint compression. They also offer free retorquing after 25 miles. Reasons for Failures Not retorquing the nuts or stud bolts within 100 miles of installation; use of an unregulated impact wrench; rust; oil on the threads; high mileage; too many tire removals and replacement; road hazards; ABS; Traction Control; 4X4; AWD and after-market wheels. However, it is the initial loss of clamp load that begins the fracture mechanism, the aforementioned reasons will accelerate the fracture. Usually, studs will fail from metal fatigue due to loss of clamp load. It is next to impossible for a wheel stud to fail in pure tension because of the stress dynamics of the wheel. The final fracture may be in tension if the front wheels are pushed into a turn or the rear wheels are normally drug into a turn, but only after most of the internal area of the stud has been fractured from metal fatigue. To fail in shear means the entire wheel is completely loose and subjected to a heavy impact. Otherwise, the stud threads will groove into the wheel, the wheel wobbles and elongates the wheel hole. The first wheel hole has been abused prior to the stud failing. There are clockwise indentations which indicate heavy impacting during installation. When the wheel became loose, it elongated the hole. The second wheel boss area exhibits wear and rust. A positive seat cannot be achieved with rust between the joint surfaces as it will relax the load. It only takes 0.001” of thread relaxation for a wheel to lose 30,000 psi of clamp load. Think about that. The above two photographs illustrate the damage prolonged impacting can do to a hexagonal shape on the nuts and stud bolt. Rust not only interferes with clamping the joint together but it also indicates the fracture has been propagating through the stud for a while, not just recently. Many times a fracture may occur within 30 days of the last wheel maintenance and naturally, the last person to touch this is to blame. However the rust proves otherwise. please turn to page 140
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