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 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 .These 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. 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. 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. 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.