The inside of a bearing can be a hot place. When a bearing is cooling off, the contracting metal, air, and lubricant, can create a vacuum that is hopefully held by the seals. If the seals are worn and can’t hold the vacuum, the bearing or sealed hub unit will suck in outside air, debris, and water. In parts of the country that use salt on the roads, it is almost as bad as ocean water on wheel bearings.
As these contaminants circulate through the grease and between the races and bearings, the components wear and possibly change their metallurgy.
A driver may notice noise coming from the vicinity of the wheel, maybe some steering wander or looseness in the steering, and abnormal tread wear on the front tire. The noise may change when tuning, become louder, or even disappear at certain speeds. This noise should not be confused with the clicks and pops produced by a worn outer CV joint on a FWD car. A bad outer CV joint usually only makes noise when turning, not when driving straight ahead.
Once a bearing is worn, the wear rate is accelerated by seals that no longer keep out contaminants, and increased heat may breakdown and eventually expel the lubricants. This is a slippery slope that could quickly lead to catastrophic failure.
VISUAL INSPECTION OF WHEEL BEARING
When a wheel bearing wears out, it is usually a case of inadequate lubrication, faulty installation, or improper adjustment. For the repair to be successful, an experienced mechanic must first determine why the previous bearing failed. For sealed hub units, examining the internal bearings and races is impossible.
A good service adviser will interview the customer to find out what kinds of roads the vehicle predominately drives on. Also, the adviser may ask what types of loads the vehicle carries on a regular basis. If a driver overloads the vehicle, bearing damage could be inevitable.
The most common failure pattern for wheel bearings is for those on the passenger side of the vehicle to fail first. The passenger side bearings are exposed to the most standing water in the gutter. If the bearing on the driver side of the vehicle fail first, take an extremely close look at the passenger side bearings, as failure may not be far behind.
HOW HEAT AFFECTS THE COMPONENTS OF A WHEEL BEARING
Most wheel bearing components are heat-treated to harden the metal. But, the heat-treating can also penetrate far into the metal. Once the bearing has worn through this layer, rapid and catastrophic wear occurs to the softer metal below. This type of fatigue failure is called “spalling.” This kind of damage causes the metal to come off in flakes.
If a wheel bearing overheats, the hot lubricant breaks down and can cause scoring and even etching of the bearing surface. Also, water and other corrosive elements can create this condition, which lead to spalling down the road. Burned or oxidized lubricant may leave a dark coating on bearing surfaces. Remember that with tapered roller bearings, excessive pre-load can mimic this same damage pattern. If a wheel bearing gets really hot, cages and seals could be deformed and lead to bearing lock-up. For these reasons, a vehicle owner should seek a professional facility to perform such repairs. Wheel bearing replacement requires experience to both remove the old bearing without damaging other components and install the replacement part in a manner that will provide optimal and lasting performance from the new part. A wheel bearing is not a component that should be purchased with cost being the primary buying decision variable.
Seals are critical components for the longevity of a wheel bearing. If contaminants from the outside find their way inside, this could cause a wear pattern called busing. One must never re-use seals. Used seals can leak and contaminate brake linings or cause premature wheel bearing failure.
Bearings are precision products that require complex manufacturing processes. Inferior bearings that use low-quality steel an have poor heat-treating can wear and fail prematurely.
Also, the poor quality steel may have inclusions of hard or soft metal that can cause a premature failure. In summary, an inexpensive wheel bearing may look the same as a high-quality, more expensive wheel bearing, but it is what you can’t see that makes a difference when having to replace the new part much sooner than the original component lasted.
MAKING ADJUSTMENTS TO WHEEL BEARINGS
Over-tightening adjustable tapered roller wheel bearings is a common error of amateur/un-trained mechanics that can lead to premature failure. Tapered roller bearings on the font of RWD vehicles are never pre-loaded. They’re snug up with no more than 15 to 20 ft-lbs of torque while rotating the wheel to make sure the bearings are seated. Then, the adjustment nut is loosened 1/6 to 1/4 turn, and locked in place with a new cotter pin.
As a rule, end-play should be about 0.001 to 0.005 inches. There should be no play on most FWD cars. Up to 0.010 inch of play in the front wheel bearings may be acceptable on RWD cars and trucks with adjustable bearings. On FWD cars with adjustable tapered roller rear wheel bearings, the bearing adjustment procedure is usually the same as with RWD vehicles (zero pre-load), but some do require a slight pre-load. Ford, for example, says the rear wheel bearings on older Taurus models should be lightly pre-loaded to 24 to 28 in-lbs (2 ft-lbs).
According to automotive aggregate data services, the replacement market for wheel bearings and hub assemblies is estimated to be roughly $120M annually. Larger suppliers include Timken, SKF, BCA, and Centric. However, many wheel bearings that may need to be replaced are not because they are overlooked when other wheel services and repairs are being performed on the vehicle.
Courtesy of hollenshades