On a typical passenger vehicle weighing around 3,400 pounds, each pair of front-wheel bearings, as well as the rear-wheel or axle bearings, supports around 850 pounds, depending on the weight balance and driveline configuration. If it’s a 6,000-pound SUV, each bearing might carry about 1,500 pounds. This load is concentrated on the relatively small bearing surfaces, which do not even take into account the dynamic loads produced by cornering.
These dynamic loads that are put on the bearing are called thrust and radial loads and are the forces the bearing must endure when the vehicle is cornering or braking. Another force that cannot easily be measured is the force of impacts from potholes and curbs.
Most serviceable wheel bearings need maintenance every 25,000 to 30,000 miles, or during every brake service. The average life of a sealed wheel bearing and hub assembly is about 85,000 to 100,000 miles, without the opportunity for you to repack the bearings. You may have only one chance during a vehicle’s life to replace these parts. If you miss this opportunity, it may be gone forever.
According to a recent Shop Owner survey, 51 percent of bad wheel bearings are identified and replaced as a result of a customer complaining about noise, 24 percent are found during a brake job and 19 percent are discovered during an alignment.
Why Bearings Fail
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 some 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, or perhaps abnormal tread wear on the front tires. The noise may change when turning or 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 while 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 break down and eventually expel the lubricant. This slippery slope can quickly lead to a catastrophic failure.
When a bearing wears out, it is usually a case of inadequate lubrication, faulty installation or improper adjustment. For the repair to be successful, you must first determine why the previous bearing failed. For sealed hub units, examining the internal bearings and races is impossible, so your investigation must go in another direction.
Interview the customer to find out what kind of roads they typically drive on, as well as what types of loads they carry. If the customer overloads the vehicle, bearing damage could be inevitable.
The most common failure pattern for 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 bearings on the driver side of the vehicle fail first, take an extremely close look at the passenger-side bearings: failure may not be far behind.
Most bearing components are heat-treated to harden the metal. However, this heat-treating can only penetrate so 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 bearing overheats, the hot lubricant breaks down and can cause scoring and even etching of the bearing surfaces.
When a vehicle hits a curb, pothole or other object in its path, the force is transferred to the small surface area of the bearing. The impact may cause damage to the races and the rollers/balls. This damage is called Brinelling.
Brinelling is a material surface failure/defect caused by contact stress that exceeds the material’s hardness limit. It is caused by impact great enough to exceed the material’s hardness. The result is a permanent dent or “Brinell” mark.
Brinell marks may cause the bearing to make noise immediately or it may not. But, as the marks keep rotating, it could be damaging the entire bearing. If the impact is great enough, the pre-load on the bearing can change, leading to more damage and noise.
Water and other corrosive elements can create this condition as well, which leads 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. If a bearing gets really hot, cages and seals could be deformed and lead to bearing lockup.
Seals are critical components for the longevity of a bearing. If contaminants from the outside find their way inside, this could cause a wear pattern called bruising. Never re-use seals, as they can leak and contaminate brake linings or cause premature bearing failure.
Bearings are precision products that require complex manufacturing processes. Inferior bearings that use low-quality steel and have poor heat-treating can wear and spall prematurely. The poor-quality steel may have inclusions of hard or soft metal that can cause a premature failure. In summary, an inexpensive bearing may look the same as a high-quality bearing, but it is what you can’t see that makes a difference between a comeback and a satisfied customer.