Impact wrenches are fine for running wheel nuts up onto the studs, but a torque wrenches must be used to set the nuts. Retorqueing is highly recommended, but few, apparently, get around to it.

Impact wrenches are fine for running wheel nuts up onto the studs, but a torque wrenches must be used to set the nuts. Retorqueing is highly recommended, but few, apparently, get around to it.

Investigations into hundreds of wheel separation incidents reveals that whether it was loosened wheel fasteners or failed bearings due to improper installation or loss of lubricant, better maintenance could have prevented all but a few of the incidents.

In 643 incidents reported between 2000 and 2003, loose wheel fasteners were to blame 65% of the time while wheel bearings were the cause in 26% of the cases. The remainder were a mix of axle and/or suspension structural failures and other causes.

One Canadian report revealed that 83% of the incident reports noted that repairs or maintenance had been performed on the suspect wheels just a short time prior to the separation.

Wheel-end failures result from over- or under-tightened bearings or lack of lubrication. Under-tightening (excessive endplay) can cause the wheel to wobble on the spindle, damaging the seal, which can lead to a loss of lubricant and eventual failure of the bearings. Over-tightening (excessive preload) can damage the bearing causing overheating, seal failure and lubricant loss.

Where fasteners are to blame, several factors can cause nuts to work loose. Excess paint, rust, scale or dirt between mating areas of wheel end components will lead to low clamping force. Failure to follow the manufacturer’s instructions for specified torque, the correct tightening sequence, and routine in-service torque checks have all be cited as causes for loss of clamping force on wheel nuts.

Improper use of impact wrenches and non-calibrated tools have resulted in wheel studs being stretched beyond their yield point. Fractured or worn out nuts with deformed threads have also been linked to loss of sufficient clamping force at the nut flange.

You might think that trailers would fare worse than tractors, since they are typically in the shop less frequently. But the percentage split between tractor and trailer incidents is about even, with trailers representing about 6% more than trucks.

Read more about how to prevent wheel-offs in the December issue of HDT.

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