Wheel bearing
From Wikipedia, the free encyclopedia
Wheel bearings perform two very important jobs in a vehicle’s Suspension. They allow the wheels to rotate with minimal friction, and they support the vehicle’s weight. To do both jobs, the bearings must be in near-perfect condition. The seals must also be leak-free, too, to keep the lubricant inside the bearings and contaminants out.
In a typical 3,400-lb. sedan, each pair of front-wheel bearings, as well as the rear-wheel or axle bearings, support 850 pounds depending on the front-to-rear weight distribution of the car. In a large 6,000-lb. SUV, each bearing carries about 1,500 pounds.
Though most wheel bearings are engineered to last 150,000 miles or more, the constant load takes a toll on the bearings, the grease and the seals. A wheel bearing may fail prematurely as a result of misadjustment (in the case of serviceable bearings), contamination or loss of grease. The seals are the most vulnerable part of the wheel bearing assembly, and once a seal starts to leak, the bearings are in trouble. A worn or damaged grease seal can allow grease to leak out of the bearings, and dirt and water to enter the bearing cavity. Unless somebody discovers and corrects the problem almost immediately, the bearings are doomed.
With sealed bearing assemblies, the seals cannot be replaced separately. If a seal has failed, the entire hub assembly must be replaced. But on older vehicles that do have serviceable wheel bearings, a bad seal can be replaced with a new one to extend the life of the bearings.
Water is bad on wheel bearings because it causes rust and contaminates the grease. Most passenger car and light truck wheel bearing seals are not designed to keep out water which is under pressure against the seal (as is the case when the hub is submerged). Consequently, any vehicle (especially boat trailers) that has been driven through hub-deep water or caught in a flood should have its wheel bearings cleaned, inspected and repacked with fresh grease. This obviously is not possible with sealed wheel bearing assemblies, so all you can do is check for play or noise — and replace the unit if it has suffered damage.
Contents |
[edit] Wheel bearing noise
The first symptom of wheel bearing trouble is usually noise. A rumbling, growling, chirping or cyclic noise of any kind from the vicinity of the wheels is a good indication that there may be a problem and the bearings should be inspected without delay. If ignored, a noisy wheel bearing may suddenly fail — with potentially deadly consequences if the vehicle were to lose a wheel.
Wheel bearing noise is usually proportional to vehicle speed, and does not change when accelerating, coasting or decelerating (which distinguishes it from differential, transmission or U-joint noise). The noise may change when turning, or become louder or even disappear at certain speeds. But it shouldn’t 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.
According to a recent Babcox Research survey, 51% of bad wheel bearings are identified and replaced as a result of a driver complaining about noise, 24% are found during a brake job, and 19% are discovered during an alignment. (Carley 2003)
Noisy or rough bearings need to be removed, cleaned and inspected under a bright light. Look closely for hairline cracks. If the bearing rollers, balls or races are worn, pitted, cracked or feel rough when rotated as an assembly, replacement is required. Heat discoloration indicates lack of lubrication or over-tightening.
The bearing hub bore should also be inspected for damage and proper bearing fit. If the bore is damaged or does not hold the race snugly, install a new hub, rotor or drum. Also, pay close attention to the condition of the front spindles. Check them for straightness, wear, damage or cracks. Problems here would indicate a replacement steering knuckle is possibly required.
[edit] Bearing play
Another problem that sometimes occurs with wheel bearings is play. Too much play can allow steering wander (which may be mistaken for worn steering components or the need for an alignment).
To check a wheel bearing, grasp the tire at the 12 and 6 o’clock positions and attempt to rock the tire. As a rule, there should be zero play on most FWD cars, but up to .010 inch of play in the front bearings may be acceptable on RWD applications that have adjustable bearings. Also rotate the tire by hand. Any roughness or noise from the bearings would also tell you the bearings are worn or damaged and need to be replaced.
If one wheel bearing has failed, pay close attention to all of the other hubs on the vehicle, especially if the vehicle has a lot of miles on it. Chances are some of the other bearings may also be nearing the end of their journey.
If there’s play in a sealed bearing, replacement is necessary. But if the bearings are the adjustable variety, a simple adjustment might be all that’s needed. Even so, "loose" wheel bearings should not be adjusted until they have been removed, cleaned and inspected. Bearings do not loosen up under normal use. Looseness usually means the bearings are worn, the hub nut may have backed off (a broken cotter pin or retainer), or the bearings were not adjusted properly in the first place.
[edit] ABS light on
On vehicles equipped with anti-lock brakes and sealed hub assemblies with an integral ABS sensor or tone ring, the ABS warning light will come on if a wheel-speed sensor is reading erratically or the signal is lost. The ABS system will set a fault code that corresponds to the sensor location (left front, right front, right rear or left rear) and disable the ABS system until the fault is fixed. On these vehicles, the only way to get the ABS light to go out is to replace the hub assembly (unless the problem is outside the unit — a wiring fault or loose connector).
There is no way to disassemble and repair a sealed hub assembly. If the internal ABS sensor has failed, or if the external ABS tone ring on the hub is damaged or badly corroded, In many cases, the whole unit must be replaced. However, in Subaru vehicles, the tone ring is screwed to the front hub with five hex screws. It is possible to pull out (or press out) the front hub assembly and unscrew the damaged tone ring and replace with a new tone ring; then the whole front hub/tone ring unit can be re-installed onto the steering knuckle.
[edit] Preventive maintenance & service
Wheel bearings that are not factory sealed require periodic maintenance. But many are neglected and never receive any maintenance whatsoever, except maybe when the brakes are relined. Even so, the adjustable variety should be cleaned, inspected and greased about every 30,000 miles or according to the vehicle manufacturer’s recommendations.
In the case of trailer wheel bearings, even more frequent maintenance is required if the bearings have been in water. With a boat trailer, annual cleaning, inspection and regreasing is highly recommended (usually at the end of the season).
When wheel bearings are serviced, the old grease should first be removed and the bearings cleaned and inspected before being repacked with fresh grease. This will remove any contaminants from the hub that may cause problems later on, and eliminate any risk of incompatibility between the old and new grease.
Bearings must be dry before they’re repacked with grease. Also, they should not be spin dried with an air gun because doing so may damage them. A lint-free paper towel should be used to dry the bearings as cotton rags can leave behind fibers that may cause trouble later on.
Use a quality, high temperature wheel bearing grease (a #2 NLGI lithium-based grease, for example) or a synthetic wheel-bearing grease. When the grease is added — about three heaping tablespoonfuls is usually enough for most passenger car and light truck hubs. To allow room for expansion, the hub should not be packed solid with grease.
[edit] Bearing installation
When serviceable wheel bearings are replaced or repacked with grease, always use new grease seals. Prying out the old seals usually bends and distorts them, even if they’re brand new. Since this may cause leaks they should not be reused.
Bearing adjustment is very important. 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 preloaded. They’re snugged up with no more than 15 to 20 ft·lbf. 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 .001 to .005 inches. Always use whatever adjustment procedure the vehicle manufacturer recommends.
On FWD cars with adjustable tapered roller rear wheel bearings, the bearing adjustment procedure is usually the same as with RWD vehicles (zero preload), but some do require a slight preload. Ford, for example, says the rear wheel bearings on a Taurus should be lightly preloaded to 24 to 28 in.lbs. (2 ft·lbf.).
A new cotter pin should be used to lock the hub nut in place. If the hub nut is a locking nut with no cotter pin, recommend a new nut.
[edit] Replacing sealed bearings
The basic procedure for replacing a front-wheel-drive hub with sealed wheel bearings goes something like this:
- Loosen the axle hub nut with the wheel on the ground. Use an extension handle on a breaker bar rather than an impact wrench so you don’t shock the CV joints.
- Raise the vehicle and remove the wheel.
- Remove the caliper from the knuckle and remove the rotor.
- Remove the axle hub nut.
- The hub/bearing assembly can now be unbolted and pulled out of the knuckle for replacement. If the bearing assembly is rusted in place, you can use a slide hammer to pop it free or a pry bar placed between the knuckle and bearing hub flange (if possible).
On the inside of some bearing assemblies, there is a flinger to help keep dirt and other contaminants out of the bearings. Be careful not to bend or damage this during installation. Also, make sure the mating surfaces in the knuckle are clean and free of nicks before installing the new bearing.
If the vehicle uses a prevailing torque hub nut, don’t reuse the old hub nut on the axle. Install a new one and torque to specifications.
[edit] Press fit bearings
Press-fit wheel bearings that are inside a front-wheel-drive knuckle are more of a challenge to replace. You can remove the knuckle and use a hydraulic ram to push the bearings out of the knuckle, or you can use special wheel bearing service tools that can do the job with the knuckle in place. The latter can be a real timesaver because you don’t have to pull the knuckle. A puller kit can cut the job to about 20 minutes per wheel, and depending on the application, it can also eliminate the risk of damaging the lower ball joint or tie rod end if these parts don’t have to be disconnected.
Another way to install press-fit wheel bearings without using either a puller or hydraulic press incorporates the characteristic of metal to shrink or expand with heat or cold. Once the old bearing is removed (with a hammer), put a bag of dry ice on top of the new bearing making it colder and shrinking slightly. Take a propane torch and lightly heat the knuckle (where the bearing is to be placed). If successful, you should be able to lightly tap the contracted, cold new bearing into the expanded, heated knuckle. Use the old bearing to do this so that the impact of the hammer is to the old bearing, then to the outer race of the new bearing.
Applications that typically require removing the knuckle or using a wheel bearing puller kit include older Japanese cars such as 1986-91 Toyota Camrys, 1986-94 Toyota Celicas, 1987-94 Toyota Corollas, 1992-94 Lexus ES300s, 1985-94 Nissans, 1988-94 Mazdas and 1986-94 Hondas and Acuras. Others include 1973-94 Audis (except 100LS), 1985-88 Chevy Novas, most 1979-90 Chrysler FWD cars, 1995 Chrysler Neons, 1991-94 Saturns, 1975-94 VWs, plus various Ford FWD cars.
The hardest ones to replace are typically the front wheel bearings on older Japanese FWD cars with captured rotors. Besides having to pull the hub to remove the rotor, the bearings may have spacers or shims inside the knuckle housing that must be measured and sized to get the proper fit when new bearings are installed.
The basic service procedure for press-fit bearings goes like this:
- Loosen the hub nut with the wheel on the ground.
- Raise the car and remove the wheel.
- Remove the hub nut and discard it (use a new one for installation).
- Remove the brake caliper from the knuckle.
- If the rotor is the captured variety behind the hub, use a hub puller to remove the hub and rotor. Otherwise, pull off the rotor.
- If the knuckle has to be removed, mark the position of the cam bolts on the struts (if used) to maintain wheel alignment. Then disconnect the tie rod end and lower ball joint from the knuckle. Remove the knuckle and use a hydraulic ram to carefully press out the old bearings. Note the position of any spacers or shims that are used, and measure the thickness as required to maintain the specified clearances. Clean the knuckle cavity and inspect for any damage. Then install new grease seals, bearings, spacers and/or shims as needed.
- If you have a wheel bearing hub puller kit that can extract the bearings without having to remove the knuckle, mount the puller and push or pull the bearings out of the knuckle using the appropriate adapters. When you install the new bearings, use a new grease seal and be sure you position any flingers, spacers or shims that are required in the right position to maintain proper clearances.
[edit] References
Carley, Larry. Wheel Bearing Hub Service. Retrieved on 2006-11-01.
