Determining types of bearing damage

An important part of bearing repair is properly diagnosing the root of the problem.

This story explores the most commonly identified causes of damage for anti-friction bearings, including cylindrical, spherical, tapered and ball designs, taking you a step closer to solving your bearing maintenance issues.

Wear, abrasive contamination

Foreign material (i.e., sand, fine metal) in the bearing can cause excessive abrasive wear. In tapered bearings, the roller ends and cone rib will wear to a greater degree than the races. This wear causes increased end play or internal clearance, which can reduce fatigue life and create misalignment in the bearing.

Wear, pitting and bruising

Hard particles (i.e., metal chips, dirt) rolling through the bearing may cause pitting and bruising of the rolling elements and races. These particles can travel within the lubricant, through the bearing and eventually bruise (dent) the surfaces. Raised metal around the dents acts as surface-stress risers to cause premature spalling and reduce bearing life.

Etching

Etching, or corrosion, is among the most serious problems anti-friction bearings encounter. The high degree of surface finish on races and rolling elements makes them more susceptible to corrosion damage from moisture and water.

Etching often results from condensate collecting in the bearing housing from temperature changes. Moisture can get in through damaged, worn or inadequate seals. Improperly washing and drying bearings can also cause considerable damage.

When preparing bearings for storage, wash and dry the bearings, then coat them with oil or another preservative and wrap them in protective paper. Always store bearings, new or used, in a dry area, and keep them in their original packaging to reduce the risk of static corrosion.

Inadequate lubrication

It’s important that the right lubricant amount, type, grade, supply system, viscosity and additives be properly engineered for each bearing system. Selection should be based on history, loading, speeds, sealing systems, service conditions and expected life. Without proper consideration of these factors, bearing and application performance may be underwhelming.
The following section outlines the progressive levels of bearing damage caused by inadequate lubrication:

Level 1: Discoloration
◾ Metal-to-metal contact results in excessive bearing temperature.
◾ High temperatures result in discoloration of the races and roller.
◾ In mild cases, the discoloration is from the lubricant staining the bearing surfaces. In severe cases, the metal is discolored from high heat.

Level 2: Scoring and peeling
◾ Insufficient or complete lack of lubricant.
◾ Selecting the wrong lubricant or lubrication type.
◾ Temperature changes.
◾ Sudden changes in running conditions.

Level 3: Excessive roller end heat
◾ Inadequate lubricant film results in localized high temperatures and scoring at the large ends of the rollers.

Level 4: Total bearing lockup
◾ High-localized heat produces metal flow in bearings, altering the original geometry and the bearings’ material.

This results in skewing of the rollers, destruction of the cage, metal transfer and complete seizure of the bearing.

Fatigue spalling

Spalling is the pitting or flaking away of bearing material. This primarily occurs on the races and rolling elements. The many types of primary damage referenced throughout this guide may eventually deteriorate into a secondary spalling damage mode. Three distinct modes are classified:

1. Geometric stress concentration (GSC) spalling. The causes include misalignment, deflection or edge loading that initiates high stress at localized regions of the bearing. GSC occurs at the extreme edges of the race/roller paths, or it can also be the result of shaft or housing machining errors.

2. Point surface origin (PSO) spalling. Very high and localized stress generates this type of damage. The spalling is typically from nicks, dents, debris, etching and hard-particle contamination in the bearing. It’s the most common type of spalling damage and often appears as arrowhead-shaped spalls, propagating in the direction of rotation.

3. Inclusion origin spalling. This damage, in the form of elliptically shaped spalls, occurs when there’s bearing material fatigue at localized areas of sub-surface, non-metallic inclusions following millions of load cycles. Due to improvements in bearing steel cleanliness in recent decades, encountering this type of spalling is unlikely.

Excessive preload or overload

Excessive preload can generate a large amount of heat and cause damage similar in appearance to inadequate lubrication. Often, the two causes may be confused. So it is important to check the bearing thoroughly to determine the root problem.

A lubricant that’s suitable for normal operation may be unsuitable for a heavily preloaded bearing, as it may not have the film strength to carry the higher loads.

Excessive end play

Excessive end play results in a very small load zone and excessive looseness between the rollers and races outside the load zone. This causes the rollers to unseat, leading to skidding and skewing as the rollers move into and out of the load zone. This movement creates scalloping in the cup race and can also cause cage wear.

Misalignment

Misalignment will shorten bearing life depending on the degree of misalignment. To achieve longer life, the seats and shoulders supporting the bearing must be within the specified limits set by the manufacturer. If the misalignment exceeds those limits, the load on the bearing won’t be distributed along the rolling elements and races as intended.

Typical causes of misalignment include:
◾ Inaccurate machining or wear of housings or shafts.
◾ Deflection from high loads.
◾ Out-of-square backing shoulders on shafts or housings.

Handling and installation damage

Care must be taken when handling and assembling bearings so the rolling elements, race surfaces and edges aren’t damaged. Deep gouges in the race surface or battered and distorted rolling elements will make metal rise around damaged areas. High stresses will occur as the rolling elements go over these surfaces, creating premature, localized spalling.

Damaged bearing cages or retainers

Careless handling and using improper tools during installation may cause cage or retainer damage. Cages and retainers are usually made of mild steel, bronze or brass and can be easily damaged. In some applications, environmental and operating conditions can cause fractured cages or retainers. If this occurs, contact a service engineer.

High spots and fitting practices

Careless handling or damage caused when driving outer races out of housings or wheel hubs can create burrs or high spots in the outer race seats.

If a tool gouges the housing seat surface, it will leave raised areas around the gouge. If these high spots are not scraped or ground down before reinstalling the outer race, they will transfer through the outer race and cause corresponding high spots in the outer race’s inside diameter. Stresses increase when the rolling elements contact this high area, which can shorten service life.

Improper fit in housings or shafts

Always follow the manufacturer’s recommended bearing fit to ensure bearings perform properly. Generally, the bearing race – where the rotating load exists – is applied with a press or tight fit. An example is a wheel hub, where the outer race should be applied with a press fit.

The races on a stationary axle would normally be applied with a light or loose fit. Where the shaft rotates, the inner race should normally be applied with a press fit, and the outer race may be applied with a split fit or even a loose fit.

Brinell and impact damage

Brinelling from improper bearing assembly and disassembly happens when a force is applied against the unmounted race. When mounting a bearing on a shaft with a tight fit, pushing the outer race will exert an excessive thrust load and bring the rolling elements into sharp contact with the race, causing brinelling.

Extremely heavy impact loads can also result in brinelling of the bearing races, or even fracture the races and rolling elements.

False brinelling

False brinelling is fretting wear caused by slight axial rolling-element movements while the bearing is stationary. Vibration can make the rolling element slide back and forth across the race, wearing a groove into it.

Roller bearings also exhibit false brinelling when used in positions that encounter very small reversing angular oscillation – less than one complete rotation of the rolling element.
To distinguish false brinelling from true, examine the depression or wear area. False brinelling will wear away the surface texture, whereas the original texture will remain in the depression of a true brinell.

Burns from electric current

Arcing occurs when an electric current that passes through a bearing is broken at the contact surfaces between the races and rolling elements. Each time the current is broken while passing between the ball or roller and race, it produces a pit on both parts. Eventually, fluting develops.

Causes of arcing include static electricity from charged belts or processes that use calender rolls, faulty wiring, improper grounding, welding, inadequate or defective insulation, loose rotor windings on an electric motor and short circuits.

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Information for this article provided by The Timken Co.