By Andrew Markel
Editor, Brake & Front End
How much load can a wheel bearing carry?
a typical passenger vehicle weighing around 3,400 pounds, each pair of
front-wheel bearings, as well as the rear-wheel or axle bearings,
support 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. These loads do not even take into account the dynamic loads produced by cornering.
These loads put on the bearing are called thrust and radial
loads. These are the forces the bearing must endure when the vehicle is
cornering or braking. Another force that cannot be measured is the
force of impacts from
potholes and curbs.
Besides reducing friction, what else does wheel bearing grease/lube do?
Lubricants aid in carrying away heat and protecting bearing surfaces from corrosion and reducing friction.
How long can a wheel bearing last? Most
serviceable wheel bearings need maintenance every 25,000 to 30,000
miles, or during every brake service. But, the average life of a sealed
wheel bearing and hub assembly is about 85,000 to 100,000 miles,
without the opportunity for a technician to repack the bearings. You
may only have one chance during a vehicle’s life to replace these
parts. If you miss this opportunity, it may be gone forever.
When are bearings replaced?
to a recent Babcox Research 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.
What is the leading cause of wheel bearing failure?
leading cause of a wheel bearing failing is its seal. 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.
these contaminants circulate through the grease and between the races
and bearings, the components wear. 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.
Can too much grease cause a wheel bearing to fail?
a bearing with too much grease can cause excess churning of the grease
during operation and high temperatures, resulting in overheating and
excess grease purging (leaking). Overheating occurs because the heat
generated cannot dissipate correctly, continually building until damage
occurs. When a bearing overheats and pushes out the grease, the sealing
lip can be damaged and “blown out” in the reverse direction.
Do I need to uncover why the previous bearing failed?
Yes. When a bearing wears out, it is usually a case of inadequate lubrication, faulty installation or improper
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.
the customer to find out what kind of roads they drive on. Also, ask
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.
The lubricant and bearing surfaces have a dark appearance and coating. What happened?
or oxidized lubricant may leave a dark coating on bearing surfaces. If
a bearing overheats, the hot lubricant breaks down and can cause
scoring and even etching of the bearing surfaces. Also, water and other
corrosive elements can create this condition, which leads to spalling
down the road. Remember that with
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
Can wheel bearing grease wear out?
is a precise combination of oil, thickener and additives. Grease acts
like a sponge to retain and release the oil. As a result of time and
temperature conditions, the oil release properties can become depleted.
When this occurs, the grease is worn-out.
A cheaper hub unit has the same appearance as a high-quality, brand-name unit. What is the difference?
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.
some cheaper hub units are using smaller bearing sizes than the OEM
intended. This can lead to premature failure. Unfortunately, the only
way to tell is to destroy the new bearing.
Why can’t a technician use an impact wrench to secure the axle nut?
it may appear to be easier to use an impact wrench, it is not
recommended. OEM 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 can not be adjusted.
What is the right way to adjust a tapered roller bearing?
check the service information for the correct procedure. 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.
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, endplay should be about
0.001 to 0.005 inches.
How does a technician know when to replace and repack wheel bearing grease?
and UV dyes can be added to a grease as assembly aids or to facilitate
inspections. The original color of a grease has little to do with its
wheel bearing grease contains corrosion inhibitors and anti-wear and
water-resistant additives, offering superior protection in challenging
A softened sealing lip surface is the sign of a compatibility problem with the grease and seal.
wrong grease can cause a rubber seal to swell and disintegrate. The
fresh grease is smooth and buttery compared to the water-laden grease,
which is milky white in appearance. As little as one percent water in
grease can have a significant impact on bearing life.