BEARING MAINTENANCE

Troubleshooting
Ball and roller bearings are designed for longevity. Their life expectancy,

based on metal fatigue, can usually be calculated if general operating

conditions are known. Bearing failures not caused by normal material

fatigue are called premature failures. The causes may range from improper

lubrication to incorrect mounting, to poor condition of shaft housing or

bearing surfaces.

Premature bearing failure can be avoided. That’s the objective of this

chapter: to identify the visible “danger” signs on bearing, shaft and housing

surfaces. These signs of bearing damage include:

• Brinelling

• Contamination

• Fretting

• Peeling

• Spalling

• Misalignment

• Electric Arcing

• Seizing

Brinelling

Brinelling refers to indentations pressed into the bearing race,

so tiny that they are hardly visible to the eye .hese indentations,

although minor, usually precede more serious, deeper cuts and dents that

ultimately result in bearing failure.

Brinelling found on the race causes corresponding dents on the balls or

rollers. Though less visible than those on the race, the brinelling on balls or

rollers is more noticeable in its interference with bearing movement.

Causes

Brinelling on the high part of the race shoulder is often caused by

pressure against the unmounted – rather than the press-fit – race during

installation. Hammering the bearing during installation or removal may

cause brinelling on the shoulder of the race.

Brinelling also is caused by bearing impact during operation.

In a wheel, for example, a bearing set with excessive end play may not

resist the impact and pounding action as the wheel goes over uneven or

rough roads. The rapid short impact pounds the rollers into the races,

causing brinelling and even fracturing.

Prevention

Brinelling caused by incorrect mounting can be prevented by exerting

pressure only on the press-fit race. Eliminating any direct hits with a

hammer on the bearing is another measure worth taking. Brinelling

caused by heavy impact during operation is usually avoided by setting the

bearing to the recommended amount of end play.

Contamination

Contamination of the bearing shows up as scratches, pitting and scoring

along the raceways, with corresponding marks on the ball and roller

surfaces. Unlike brinelling, these small indentations are scattered,

rather than centralized, on the bearing surface.

Causes

When seals or shields are defective it is not hard for abrasive particles,

dirt, or dust to get into the bearing. There can even be dirt or dust in the

lubricant.

Wherever foreign material enters into the bearing by way of contaminated

lubricant, the particles are pressed into the metal surface. Small dents

and pits are formed which roughen the load-carrying surface. Severe

roughness will result in flaking and premature bearing failure.

Contamination can also be caused by improper cleaning of the housing or

shaft, or by using dirty tools and hands during mounting and assembly.

Prevention

Everything coming into contact with the bearing, shaft or housing should

be clean, including your hands, tools and work area. Lubricant should be

stored only in clean containers and covered with a tight lid during storage.

Before mounting a new bearing, be sure the shaft and housing are free

of dirt, dust and moisture. When changing the wheel or installing a new

bearing, use only new, not used, seals.

Fretting

When small metal particles decay and break off of the bearing races, it

is called fretting. Fretting occurs when there is excessive rubbing

between the inner race and the shaft, or the outer race and the housing,

or any surface overstressed under excessive oscillated loads. This excess

friction causes the contact area to wear down or corrode.

Some fretting corrosion is a normal part of the bearing fatigue process.

The condition worsens until the contact surfaces become so weakened that

soon, the bearing fails.

Causes

Fretting frequently is caused by poor shaft or housing fits. Races with too

loose a fit, for example, may rub against the shaft or housing when they

should be stationary. Lubricant levels kept too low also permit excess

friction and corrosion.

Prevention

Be sure shaft and housing fits are correct for the application, so the

bearing cannot move out of place. Micronized graphite and other special

lubricants are sometimes used to relieve the rubbing pressure.

Peeling

Peeling is a light scraping away of the bearing’s surface

The damage is usually just superficial, normally less than .001˝ deep. It

should not affect bearing performance unless conditions exist that promote

greater damage.

Causes

Peeling damage most often is related to improper lubrication. There may

not, for example, be enough lubrication in the bearing (along the inner

diameter of the cup) which can result in peeling on the unprotected

surface. Use of a high viscosity lubricant – one that is unable to flow freely

– also promotes peeling. Misalignment, which may cause peeling at the edge of heavy contact.

Prevention

Check lubrication levels in the bearing to be sure an ample amount

is present at all times. When lubricating a wheel hub, for example, force

grease into and around the races, cage and rollers. Also check the shaft,

housing and bearing during mounting for misalignment.

Spalling

Spalling is an advanced stage of bearing decay. Caused by metal fatigue,

the failure begins as microscopic cracks beneath the bearing surface. These

tiny fractures work their way to the surface, and eventually result in the

flaking away of metal particles. The uneven surfaces caused by

metal flaking away prevent normal bearing operation. Failure is inevitable.

Because of the rough race surface and loose metal chips, there also will be

bearing vibration and noise.      

Causes

Spalling occurs under normal conditions as part of bearing fatigue.

However, it can result from another type of initial bearing damage, such as

brinelling or fretting, that has caused indentations, weakening or abrasion,

on one or both of the races.

Almost any type of handling, installation, mounting or maintenance

procedure done incorrectly can result in spalling. For example, a bearing

mounted on a shaft with excess press-fit causes friction to build up and

wear down the bearing surfaces. Dirt, dust and contamination will abrade

and score the races. Moisture in the housing can settle on and then

corrode the bearing surface, resulting in flaking. Improper lubrication can

fail to relieve friction, leading first to peeling, and then possibly to spalling.

Misalignment prevents even load distribution, and spalling may occur in

the high stress areas. High spots or grooves in the housing also may cause

corresponding grooves in the bearing that lead to spalling. Whether it is

normal or premature, bearing failure from spalling is irreversible.

Prevention

Use proper procedures for handling, assembling and inspecting bearings.

Replace defective seals and shields, so contaminants cannot get into the

bearing. Use only clean lubricants and tools and be sure lubrication levels

are adequate. Check for scratches, nicks and grooves on the shaft and in

the housing. Double check bearing alignment and press-fit.

Misalignment

If the balls or rollers of a bearing are running from one side of the race

to the other side – and not along a straight path – then one race is

misaligned with respect to the other. That is, the two races are not square

with each other. Because of the misalignment, there will be uneven load

distribution on the races and rollers, causing friction and heat to build up

at the points where there is excess pressure and weight.

Misalignment can be identified by a diagonal polishing on the inside of the

stationary ring, while the rotating ring develops flaking across the entire

raceway. The excess friction that builds up will eventually discolor the roller

path and rollers, and destroy the lubricant.

Causes

The shaft may be misaligned in relation to the housing, causing an

overload on the balls or rollers and eventual bearing failure. Misalignment

also may be caused by the housing being cocked and not square with the

shaft. The housing shoulder may become disoriented, forcing the bearing’s

outer race to cock in relation to the inner race. The bearing may have been

installed with too much clearance or press-fit, causing the two races to be

out of line with each other.

Prevention

Misalignment of the shaft and housing should be checked and corrected

before bearing installation. Be sure the shoulders are in line and square.

When press-fitting a bearing, follow the steps outlined in Chapter Seven.

Electric Arcing

Electric currents can damage a bearing. When even a small amount of

voltage passes through a bearing, it will burn a pit into the race at the

point of contact.

Causes

Electric current passing through a bearing, such as during welding on a

vehicle without proper grounding, causes arcing and burning at the point of

contact between the races and rollers (fig. 7). This can range from a single

burn spot, or, as often happens with roller bearings, a series of small burns

between the roller and race along the line of contact. These burns cause

grooves along the affected surface (fig. 8). As the current continues to pass

through the bearing, the contact points change as the bearing turns.

Prevention

Properly ground the vehicle so that it will route electrical current around

the bearings. Inspect the bearing to be sure that any stray currents have

not passed through the races.

Seizing

Seizing is a common form of failure when bearings are first put into

service. When the rolling elements fail to roll, the resulting friction

generates excessive heat very rapidly (fig. 9). Seizing frequently occurs

between the cone back face and the large end of the roller

on tapered roller bearings. The cages are usually either damaged

or destroyed when this occurs.

Causes

Improper or inadequate lubrication can result in a breakdown

of the oil film between the rolling elements and raceways. The resulting

metal to metal contact generates excessive heat which reduces the

hardness of the metal. Localized welding of the rollers or balls to the

raceways will rapidly seize the bearing.

Prevention

Carefully select the proper amount and type of lubricant that will maintain a

film between the rolling elements and raceways. Remember to prelubricate

whenever necessary before installation.

Conclusion

Preventing bearing failure depends on how well you know the bearing, and the

application for which it will be used. This study guide should provide you with

the basics for bearing care. Remember, it takes only a small mistake in handling,

lubrication, installation or maintenance to result in large scale damage – not

only to the bearing, but to the overall area in which it operates.

Whenever there is a bearing, there is a seal – either working in tandem

or close by. To help you become as familiar with seals as you now should be

with bearings, there is a companion self-study program. The SKF Shaft Seals

Self-Study Program covers selection, installation and maintenance of grease

and oil seals, along with the Speedi-Sleeve line of wear sleeves