6 years ago


Distributor's Link Magazine Summer Issue 2012 / VOL 35 / NO.3


140 THE DISTRIBUTOR’S LINK WHEEL STUD FAILURES continued from page 130 These fracture surfaces indicate multiple stress initiation points from different times. The photograph below exhibits ‘chatter’ markings from the 5 o’clock to 11 o’clock position, indicating rotational loosening and multiple stresses. This photo above is an excellent example of multiple stress points and rotation. Therefore, if these fracture markings are present, they were not caused from the last person to perform maintenance on this joint. Retorquing the wheel nuts or stud bolts, or any fastener for that matter, after use for 100 miles or less, will regain the loss of clamp load normally experienced by every joint from relaxation and settling. An impact wrench can cause the threads to be stretched into yield, which causes loss of clamp load, and it causes internal damage to the threads of the nut or bolt. Rust acts as a cushion layer preventing a solid joint. It also indicates the joint was loose enough to allow water or moisture to get inside the joint and cause rust, indicating a loose connection. Oil present will reduce the friction between the mating threads sufficiently enough to cause thread stripping or to cause the stud’s threads to stretch into yield. The majority of wheel failures occur on vehicles with over 100,000 miles. As the mileage climbs, the rate of failures also becomes more frequently found in vehicles with over 125,000 miles, etc. Frequent tire removals will create a loss of clamp load from the threads of the nut being distorted. It is a known fact that a new standard nut onto a bolt will produce 90% friction between the threads and rotating surface, leaving only 10% of torquing energy to stretch the fastener properly. With the conical area of the wheel nut and stud bolt that mates into the conical wheel boss area for centering, the mating surfaces will increase the overall friction from 90% to 94%. Therefore, only 6% torque energy is left to tighten the connection instead of 10%. Furthermore, it is also a known fact that every time a nut is used, the internal threads become slightly distorted so it may carry the intended load. This slight distortion, when under the pressure of tightening, will increase the overall friction slightly. Therefore, 94% now becomes 95% and so on, decreasing the amount of usable torque being applied to the joint. Thus, clamp load is lost on each nut reuse. The stud bolt does not lose clamp load as quickly or as significantly as the nut because the internal threads of the wheel hub are stronger than the nut. Many European cars use stud bolts. Frequent removal and replacements come from tire rotations, new tires, hole repair, balancing, snow tires, brake inspections and replacement, and new struts or shocks. Every time the nut is reused, the clamp load greatly decreases. Add up the number of times in a vehicle’s life the wheels have been removed and replaced. The number is staggering. Road hazards, or pot holes, rail road tracks and offroading will jolt the wheel system enough to begin the process of loosening. More damage is done to the right front tire than the others. Although the left front also receives sudden shocks from engine torque and the heavy front weight from the engine. Automatic Braking Systems (ABS) will also help loosen a wheel and exacerbate a metal fatigue failure. Most systems will sense a spinning wheel and quickly apply the brake to the wheel within 1/4 rotation of the wheel or slightly more. This is an abrupt force that is quickly repeated under the forward momentum of a heavy vehicle. The same can be said with Traction Control, which operates on all four wheels as does a 4X4 or AWD (All Wheel Drive) which senses a spinning wheel and suddenly stops it from turning to equalize the traction of the vehicle. Again, these shocks, or sudden jolts begin or will continue crack propagation to failure. After-market wheels are fine until customers begin using several tire diameters too large or have the wheels stick out. The suspension geometry becomes upset and unstable as suspensions are designed for certain wheel sizes and even tire grade and construction. There is more sprung weight with larger wheels than the suspension was designed to support. Also, some custom wheel nut manufacturers may not use the proper strength of nut to match the strength of the stud. They don’t know on what vehicles their products are being used. Just remember, it isn’t always the last person to use or assemble the fastener that caused the problem. Check all of the signs around the failure site, The fastener will tell you what happened.


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