You may only have one chance during a vehicle’s life to replace a sealed wheel bearing and hub assembly. Miss it, and it may be gone forever.
That’s because the average life of these parts is 85,000 to 100,000 miles, even though most serviceable wheel bearings need maintenance every 25,000 to 30,000 miles or during each brake service.
According to a recent Babcox Research survey, 51% of bad wheel bearings are identified and replaced as a result of a customer complaining about noise; 24% are found during a brake job; and 19% are discovered during an alignment.
Why Bearings Fail
On a typical 3,400-pound car, each pair of front-wheel bearings, as well as the rear-wheel or axle bearings, supports 850 pounds, depending on the weight balance and driveline configuration. On a 6,000-pound SUV, each bearing might carry about 1,500 pounds. These loads do not take into account the dynamic loads produced by cornering.
All of this strain and pressure means the inside of a bearing can be a hot place. And, when a bearing eventually cools 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 where there is 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 their metallurgy could be altered.
A driver may notice noise coming from the vicinity of the wheel, some steering wander or looseness and abnormal tread wear on the front tires. The noise may change when turning or become louder. It may 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 a vehicle is 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 break down and eventually expel the lubricants. This is a slippery slope that could quickly lead to catastrophic failure.
The most common method of testing wheel bearings is to lift the vehicle and grab the wheel at 12 o’clock and 6 o’clock and feel if there is any noticeable play. By grabbing the wheel at these points, any play in the steering system is eliminated. But, with some hub units, the failure tolerance may be so low that a bad bearing is undetectable by this method. In these cases, a dial-runout gauge could be your best friend.
Some vehicles that use sealed hub units have the wheel-speed sensor and tone ring placed between the bearings. If you have an illuminated ABS light and erratic or weak wheel-speed sensor codes, chances are the bearings on one of these types of hub units have too much play and should be replaced. The same is true on vehicles that mount the tone ring to the rotor or outer CV joint.
The wheel-speed sensor acts like a small AC electrical generator. The voltage signal it produces changes in both amplitude and frequency with the speed of the wheel. The faster the wheel goes, the greater the voltage (amplitude) and the quicker the signal changes back and forth (frequency).
Play or wear in the bearings causes the tone ring to move and alter the signal of the wheel-speed sensor. Any tone ring attached to a wheel hub, rotor or CV joint can be influenced by wheel bearing play. Any excessive looseness in the wheel bearing will result in a change in air gap; the sensor itself can also have an impact on the air gap. If the sensor is not tightened properly, it can move away from the tone ring and thus change the air gap. To confirm this condition, it may be necessary to connect the sensor to a voltmeter or oscilloscope and go for a test drive.
To measure a wheel-speed sensor’s output voltage and circuit continuity at the same time, plug a breakout box into the ABS module’s wiring harness and attach the test leads from the meter to the appropriate sensor circuit pins on the breakout box. (The meter’s test leads can also be connected directly to the wheel-speed sensor, but testing the sensor this way won’t show if the signal is getting through to the ABS control module or not.) Spin the wheel by hand and note the sensor’s voltage reading when spinning the wheel.
A good wheel-speed sensor will generally produce an alternating current voltage reading of 50 to 700 mV when the wheel is spun at a speed of about one revolution per second. Refer to a shop manual for the sensor’s exact voltage specifications by the OEM.
Using an oscilloscope is a far more accurate way to diagnosis a bad hub unit with a wheel-speed sensor. If the amount of misalignment in the hub is enough to affect the air gap between the wheel-speed sensor and tone ring, it will produce an undulating wave pattern that changes as the strength of the sensor signal changes with every revolution.
If the scope pattern produced by a wheel-speed sensor is flattened (diminished amplitude) or erratic, it usually indicates a weak signal caused by an excessively wide air gap between the tip of the sensor and its ring or a buildup of metallic debris on the end of the sensor. A weak signal can also be caused by internal resistance in the sensor or its wiring circuit or a loose or corroded wiring harness or connectors. Also, it may be necessary to drive the vehicle under a variety of conditions. You might try driving on a freeway ramp or pulling into a driveway.
When a bearing wears out, it is usually due to inadequate lubrication, faulty installation or improper adjustment. For the repair to be successful, you must first determine why the previous bearing failed.
However, it’s impossible to examine the internal bearings and races in sealed hub units, so you must interview customers. Find out what kind of roads they drive on, and ask about the loads they carry. If a vehicle is regularly overloaded, bearing damage may be inevitable.
It’s most common for bearings on the passenger side of the vehicle to fail first because they are often exposed to standing water in gutters. If the bearings on the driver side of the vehicle fail first, take a close look at the passenger side bearings; failure may not be far behind.
Most bearing components are heat-treated to harden the metal. But, 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,” and it 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. Water and other corrosive elements can also create this condition, which can lead to spalling. 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, leading to bearing lock-up.
Seals are critical 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. Used seals 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. Also, poor-quality steel can have inclusions of hard or soft metal that can result in 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.
Installing wheel bearings is a relatively straightforward job. But, there are several key items to remember when dealing with serviceable or sealed hub units.
On serviceable bearings, the most important service tip is to invest in a good set of seal drivers. Even a slight distortion in a seal created during installation can shorten the life of the bearing inside. Also, do not skimp on the quality of the grease. Many parts suppliers carry wheel bearing-specific grease that does an excellent job of protecting the bearings.
Unit hub assemblies combine bearings, seals, hub and spindle in one pre-assembled unit that simply bolts to the suspension. These are “maintenance free” and non-serviceable units that are pre-set, pre-greased and pre-sealed. Some units may require you to install wheel studs. If at all possible, install new wheel studs.
While it may appear to be easier to use an impact wrench, it is not recommended. OE and bearing manufacturers always recommend using a torque wrench for installation. During removal, an impact wrench can damage the axle nut threads and shock the CV joints. It can also create a false sense of security when adjusting a nut or bolt, which may be under- or over-torqued. This can leave a hub assembly susceptible to failure. Also, in almost all cases, use a new axle nut. Some axle nuts are designed to be used only once and so cannot be adjusted.
Overtightening adjustable tapered roller bearings is a common error that can lead to premature failure. Tapered roller bearings on the front of RWD vehicles are never pre-loaded. They’re snugged up with no more than 15 to 20 ft. lbs. of torque while the wheel is rotated. This is done to ensure 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, endplay should be about 0.001 to 0.005 inches.
There should be no play on most FWD cars, but up to 0.010-inch of play in the front 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.).
The replacement market for wheel bearing and hub assemblies is estimated to be $120 million annually. Don’t miss your chance to tap into this lucrative market. When performing routine service or repair on a vehicle, be sure to check the wheel bearings.