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Automotive Wheel Bearing Failure

 

Why Do Wheel Bearings Fail?

The factors that contribute to wheel bearing failure are often difficult to pinpoint because bearing selection, lubrication and servicing needs vary between makes and models. So does the type of bearing used, the conditions under which it operates and the maintenance procedure employed by the operator.

All the same, this article is based on investigations into failures reported from the field, and the recommendations listed are complementary to those issued by bearing manufacturers and service organisations associated with anti-friction bearings. 

 
Causes of Wheel Bearing Failure

Because of the complexity in design and fine operating tolerances, bearing life is limited by the following factors:

  • Contamination of the lubricant
  • Lack of attention to bearing installation and maintenance
  • Too little or too much lubricant
  • The type of grease used.

Contamination of the lubricant may be due to

  • Foreign matter introduced by careless handling and storage of the bearing prior to installation
  • Foreign matter introduced during repacking by mishandling of the bearing or grease
  • Not thoroughly removing old grease from bearings and hubs
  • Using wet or contaminated air during the drying process
  • Entry of water (particularly with boat trailers) or dirt during service due to faulty or incorrectly fitted seals.

Lack of Attention to Mechanical Details

  • Poor inspection of bearings before refitting, resulting in the re-assembly of damaged bearings (etched, cracked, rusted, dented or in otherwise poor condition)
  • Spinning dry bearings with an air jet
  • Improper seating of bearings so that they do not track correctly
  • Adjustment too tight or too loose
  • Brake shoe clearance not properly adjusted
  • Not greasing immediately after cleaning
  • Use of impact tools including a hammer and punch. Bearing cups or assemblies should be pressed into hubs and housings.

Quantity and Frequency of Lubrication

  • As the moving parts of a bearing come into contact with the grease, a small quantity of oil will be removed from the suspension agent (either Lithium or Bentone base) and adhere to the bearing surface. Therefore, the life of a grease is restricted to the amount of oil which can reasonably be extracted for lubrication purposes. Oil that is picked up is gradually broken down by oxidation or lost by centrifugal force and must be replaced from the reserve held within the grease packing. Not only does regular cleaning and repacking replenish the lubricant supply, removal of old grease cleans the bearing surfaces of dirt, oil depleted grease, abrasives, moisture, free acids and alkalis.
  • Greasing intervals depend upon the conditions under which the bearing operates and should be carried out at periods as recommended by the vehicle or bearing manufacturer.
  • Excessive amounts of lubricant forced into a bearing assembly causes the generation of internal friction and overheating, resulting in grease breakdown, hub seal rupture and premature bearing failure.

The Nature of the Lubricant

  • It should be a rust preventative grease with correct physical characteristics (Lithium and Bentone base greases conform).
  • It must be the correct consistency.
  • It must not deteriorate in storage.
  • It must stay in place.
  • It must not be so hard as to channel and leave bearings dry.
  • It must have a high melting point. The temperature of passenger wheel hubs will rarely exceed 65 C in normal service, but under hard braking and high temperatures, bearing temperatures up to 180 C may be produced. Truck wheels under heavy loads and high speeds may generate bearing temperatures in excess of 126 degrees C.
  • It must be stable in service and resistant to oxidation at the temperatures to be encountered.
  • It should retain its consistency even when contaminated with a little water.


How To Prevent Wheel Bearing Failure

  • Clean bearings properly. Use clean solvent and brushes and dry thoroughly with clean air.
  • Never spin dry bearings; this causes scratching of the polished surface and may result in premature failure.
  • Do not pack the bearing unless it is perfectly dry.
  • Good practice calls for a thorough inspection after cleaning and before reinstallation. Bearings are often hammered or pulled without respect for such precision made assemblies. Re-assembly of a damaged bearing results in short component life.
  • Pack the bearing immediately following cleaning, drying and inspection.
  • Do not use an excessive amount of grease - be sure it is worked into every bearing space, but do not fill the hub with grease.
  • Keep bearings clean, work on a clean bench and do not handle bearings with dirty or moist hands.
  • When re-assembling check the bearing for proper seating.
  • Check adjustments carefully. Most vehicle manufacturers nominate a torsion wrench setting for bearing assemblies. Use a torque wrench and do not exceed the manufacturers recommendation.
  • Check brake shoe clearance, the dragging tip of a brake shoe or pad can create very high wheel temperatures in a short run.
  • Keep grease containers completely covered when not in use to avoid contamination. (Replace the lid immediately sufficient grease has been removed from the container).
  • Do not mix different types of grease. Under no circumstances must lithium based multi-purpose grease be mixed with soda base wheel bearing grease or an adverse chemical reaction will occur.
  • Carefully avoid contact of the packed bearing with dirt, dust, water or dirty hands.
  • Use the correct grade of grease for the job. Do not at any time use chassis grease. If the vehicle is used in very arduous service or under unusual conditions of load and speed, check with the manufacturer for correct grease recommendation.Particular care should be used with wheel bearings on boat trailers. Ensure a suitable grease is used, such as a Marine Grease.The grease should be repacked in boat trailer bearings regularly, after use, particularly ensuring that it is done prior to storing the trailer if it is used only on a seasonal basis. Ensure also that entry of water is minimised by replacing faulty or incorrectly fitted seals.

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