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ADVISE & IDEASWhy do bearings fail?

It is a fact. Sometimes when towing a trailer a wheel falls off. There are two possible scenarios – the first scenario is the loss of the wheel assembly after failure of the bolted joint (due to loss of wheel nuts / bolts or failure of the wheel studs / bolts). The second scenario is loss of the wheel and hub assembly after a bearing has failed. Most commonly, these failures are related to improper or deferred maintenance of the axle involved.

Roller Bearings are subdivided into many different classes. Axles are mainly produced with Taper Roller Bearings.

Taper Roller Bearings are a common type using conical rolling elements guided by ribs on the inner ring. Slight crowning of the periphery of the roller near each end relieves the edge loading. These bearings are capable of dealing with both radial and thrust loads in one direction. The angle of the cone results in the generation of an axial force when a radial load is applied. Consequently these bearings are often mounted in pairs. In this case, the proper internal clearance can be obtained by adjusting the axial distance between the inner rings and outer rings of the two opposed bearings (see the section dealing with bearing adjustment on axles).

Taper Roller Bearings are separable. As a result, the inner and outer rings can be mounted independently.

There are five primary causes of bearing failures. They are: lack of lubrication, overloading the vehicle, installing the axle nut too tight, or installing the axle nut too loose, contamination of the grease due to missing / defective grease caps and seals.

Usually, hub failures are progressive and will produce some evidence of the impending failure. Evidence of a failing hub can include leaking hub seals, tire tread wear anomalies, sounds, smells, smoke, and the trailer may not tow in a straight line behind the tow vehicle (the trailer will be wandering).

The bearing preload is adjusted by tightening the axle nut down against the two taper roller bearings. As the nut is tightened, the hub assembly becomes tight and the play is removed from the system. Once the bearing preload is adjusted, a lock is installed to prevent the axle nut from moving. Although, there are many types of locks for axle nuts, some common ones are a split pin placed through a castled hex nut or a keyed washer placed over a flat hex nut followed by a second nut to hold the washer in place. Burquip uses a split pin to retain the castle nut in the correct position on the threaded part of the axle beam / spindle – we provide two holes set 90° apart in the threaded end specifically for this purpose.

The proper method for adjusting and preloading bearings has 5 steps, but is rarely followed. Here is the method used in the Burquip factory:

  1. Tighten the adjusting nut against the trust washer to seat the bearing components. Always rotate or oscillate the hub while torqueing the adjusting nut to ensure that the rollers are fully seated against the cone large rib. Take care not to make the nut too tight as this may damage the bearing raceways. Do not hit directly on the bearings at all (remember that bearings are very hard and may be brittle under unspecified impact load).
  2. Back off the adjusting nut until it’s loose.
  3. This is where you actually establish end play. Tighten the adjusting nut while rotating the wheel hub assembly. Tap the hub form the front and from the rear to be sure that the bearings are not sized on the shaft – this can cause incorrect preload as the hub will move along the shaft after the preload has been set.
  4. Back off the adjusting nut the appropriate amount So that the hub can be easily rotated by hand – there should not be more than 0.5mm end float (i.e. play between the castle nut and the thrust washer).
  5. Install a slit pin into the nearest hole. Fit a grease cap.

A hub that is adjusted too loose will allow the hub and wheel assembly to oscillate laterally. This will cause bearing wear and will cause a further increase in hub looseness. The loose hub allows excessive movement of the bearing rollers resulting in roller cage wear and uneven race wear or “scalloping”. If not repaired, the bearing play will increase progressively to the point where the hub fails.

A hub that is adjusted too tight will not have a lubricant barrier between the bearing rollers and bearing races. Without a lubricant barrier, the bearing will overheat and fatigue. This condition will lead to full bearing lock up and hub separation if not corrected. This condition will be evidenced by heat discoloration and fatigue spalling on the bearings.

Overloading a hub creates similar conditions as over-tightening the hub. The excessive weight forces the lubricant from between the bearing rollers and bearing races on the loaded side of the bearing causing localized heat and fatigue.

Lack of lubrication also causes metal to metal contact between the bearings and races. This results in excessive heat, bearing and race scoring, and micro-spalling fatigue. This condition, if not corrected, will lead to bearing lock up and hub separation.

The grease caps must always be fitted to the hubs to keep prevent contamination of the grease in the bearings.

The mechanism of failed hub separation can vary by the design of the hub. Burquip hubs have a small outer bearing and axle nut. When the outer bearing fails, the hub can just slip past the axle nut and remaining parts of the outer bearing. In hubs with large outer bearings, separation of the hub occurs when the oscillating hub causes the axle nut to be pulled off the axle like a bottle cap. In other cases, the axle nut is pulled off the axle, ripping the threads out of the axle nut and leaving the threads on the axle relatively undamaged.

Bearing failure is a catastrophe that can be avoided by observing proper maintenance procedures, keeping the correct preload and ensuring that grease caps and seals are in good condition.

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