Principal Causes of Electric Motor Bearing Failure

Understanding the Principal Causes of Electric Motor Bearing Failure

Electric motor bearings play a critical role in maintaining proper rotor position and allowing the motor shaft to rotate with minimal friction. In a properly designed and maintained motor, bearings can provide many years of reliable service. However, bearing life can be significantly shortened by mechanical stresses, lubrication problems, contamination, improper installation, environmental conditions, vibration, and electrical currents. Recognizing the principal causes of electric motor bearing failure is the first step toward preventing it.

Because many bearing failures produce similar symptoms in electric motors—such as increased vibration, noise, heat, or lubricant degradation—identifying the actual root cause is important. Simply replacing a failed bearing without determining why it failed can result in repeated failures and unnecessary motor downtime.

Lubrication-Related Bearing Damage

Lubrication is one of the most important factors affecting bearing life. The lubricant creates a microscopic film between the rolling elements and bearing raceways, preventing direct metal-to-metal contact while also helping dissipate heat and protect bearing surfaces from corrosion.

Insufficient lubrication can cause the lubricating film to become too thin to adequately separate the bearing surfaces. As the rolling elements contact the raceways, friction and operating temperature increase. Over time, this can lead to surface wear, discoloration, scoring, smearing, and eventual bearing failure.

Over-lubrication can also create problems. When a bearing housing contains too much grease, the rotating components must continually push through the excess lubricant. This condition, commonly referred to as grease churning, can generate excessive heat. Higher temperatures accelerate lubricant degradation and may ultimately shorten both grease and bearing life.

Lubrication problems may also result from using the wrong grease consistency, incorrect oil viscosity, incompatible grease formulations, contamination of the lubricant, or lubrication intervals that are either too frequent or too infrequent.

Motor speed, bearing size, operating temperature, load, and environmental conditions should all be considered when determining the correct lubricant and lubrication interval.

Contamination-Related Bearing Damage

Bearing components are manufactured to extremely precise tolerances, and even very small contaminants can damage their polished surfaces.

Dust, dirt, metal particles, fibers, moisture, and other foreign materials can enter a bearing through damaged seals, improper storage, contaminated grease fittings, or poor maintenance practices. When these particles pass between the rolling elements and raceways, they can create microscopic dents and scratches.

These imperfections increase vibration and create localized stress concentrations. As the bearing continues operating, the damaged areas can develop into larger fatigue defects, eventually producing spalling or surface material loss.

Moisture contamination is particularly harmful because it can degrade lubricant performance while also promoting corrosion.

Maintaining clean lubrication equipment, properly sealing bearing housings, protecting motors during storage, and following clean bearing installation procedures can significantly reduce contamination-related failures.

Misalignment and Improper Bearing Loading

Motor bearings are designed to operate within specified radial and axial load limits. When a motor or the equipment it drives is improperly aligned, these loads can increase significantly.

Misalignment between a motor and driven equipment may place uneven forces on the shaft and bearings. These forces can cause abnormal loading patterns across the bearing raceways, producing localized wear and elevated operating temperatures.

Several operating conditions can contribute to excessive bearing loads, including:

• Improper motor-to-equipment coupling alignment
• Excessive belt tension
• Improperly positioned pulleys or sheaves
• Excessive axial thrust
• Bent shafts
• Unbalanced rotating components
• Improper motor mounting
• Soft foot conditions

For belt-driven equipment, excessive belt tension is a particularly common problem. Increasing belt tension beyond the amount required to prevent slippage places additional radial load on the motor shaft and bearings.

Proper shaft alignment, belt tension, mounting practices, and load verification are therefore essential components of a motor reliability program.

Vibration and False Brinelling

Bearings can be damaged even when the motor is not operating.

When a stationary motor is exposed to continuous external vibration, small oscillating movements may occur between the bearing rolling elements and raceways. These repetitive movements can gradually wear depressions into the raceways.

This condition is commonly referred to as false brinelling.

Motors stored near large compressors, pumps, crushers, fans, or other vibrating equipment can be particularly susceptible. Motors transported over long distances may also experience bearing damage if the rotor is not properly secured.

When the motor is eventually placed into service, the rolling elements repeatedly pass over the damaged areas. This can create vibration, noise, and accelerated bearing deterioration.

For motors placed into long-term storage, manufacturers may recommend periodically rotating the shaft or using other storage procedures to redistribute bearing contact points and reduce the risk of false brinelling.

Corrosion and Moisture Bearing Damage

Electric motors frequently operate in environments where moisture and corrosive materials are present.

Condensation may develop inside a motor when temperature changes cause humid air to release moisture onto cooler internal surfaces. Motors used outdoors, in washdown areas, wastewater facilities, food-processing operations, cooling towers, and other high-humidity environments may be particularly vulnerable.

Moisture can damage bearings in several ways. It can promote rust formation on bearing raceways and rolling elements, reduce lubricant effectiveness, and introduce abrasive corrosion products into the bearing.

Even small corrosion pits create interruptions in the otherwise smooth bearing surface. Each time a rolling element passes over these damaged areas, localized stresses increase, potentially accelerating fatigue failure.

Proper seals, space heaters, environmental controls, correct lubricant selection, and appropriate storage practices can help protect bearings from moisture-related damage.

Installation and Handling Bearing Damage

Bearings are precision components and can be damaged before a motor ever begins operating if they are improperly handled or installed.

One of the most common installation errors involves applying mounting force through the rolling elements. For example, if force is applied to the outer race when pressing a bearing onto a shaft, the installation load may travel through the balls or rollers to the inner race.

These concentrated forces can create permanent indentations in the raceways. Once the motor begins operating, the rolling elements repeatedly encounter these damaged areas, producing noise, vibration, and accelerated wear.

Other installation problems can include:

• Hammering directly on the bearing
• Incorrect shaft or housing fits
• Installing bearings at an angle
• Contaminating the bearing during installation
• Excessive heating during thermal installation
• Improper retaining-ring or locknut installation
• Damaging seals during assembly

Using the correct bearing mounting tools, induction heaters, presses, and installation procedures helps prevent these problems.

Bearing Fatigue and Normal Service-Life Failure

Even under ideal conditions, bearings do not have an unlimited operating life.

Repeated loading of bearing raceways produces cyclic stresses below the surface of the metal. After a sufficiently large number of operating cycles, microscopic fatigue cracks can begin developing beneath the surface.

Eventually, these cracks may reach the bearing surface and cause small pieces of material to break away. This process is commonly known as spalling.

Bearing life is strongly influenced by operating load. Even relatively modest increases in load can significantly reduce expected bearing life. Conversely, proper loading, lubrication, alignment, and contamination control can allow bearings to achieve or exceed their calculated service life.

Distinguishing normal fatigue from premature failure is important because premature electric motor bearing failure often indicates an underlying problem such as excessive loading, contamination, improper lubrication, or electrical damage.

Bearing Overheating

Excessive temperature is both a cause and a symptom of bearing problems.

High bearing temperatures can reduce lubricant viscosity, accelerate grease oxidation, damage seals, and reduce the hardness and dimensional stability of bearing components. As lubricant performance deteriorates, friction increases further, potentially creating a self-accelerating failure condition.

Common causes of excessive bearing temperature include:

• Over-lubrication
• Insufficient lubrication
• Excessive loading
• Misalignment
• Incorrect bearing internal clearance
• Excessive motor operating temperature
• Contaminated lubricant
• Improper bearing installation

Monitoring bearing temperature trends can provide an early warning of developing problems before catastrophic failure occurs.

Electrical Bearing Damage

Unlike the mechanical and lubrication-related failures described above, some electric motor bearing failures originate from electrical current passing through the bearing.

Voltage can develop between the motor shaft and frame. When the electrical potential becomes high enough to overcome the insulating properties of the lubricating film, current may discharge through the bearing at the point where a rolling element approaches or contacts the raceway.

These discharges can create extremely localized heating and microscopic melting of the bearing surface. The resulting damage may initially appear as very small pits or crater-like marks.

As electrical discharges continue, thousands or millions of microscopic damage points can accumulate. The bearing raceway may eventually develop a dull, frosted appearance or a repetitive washboard-like pattern commonly referred to as fluting.

Electrical bearing damage may be associated with several different electrical phenomena, including circulating shaft currents, magnetic asymmetry, electrostatic charging, and high-frequency common-mode voltages produced by variable frequency drives.

Variable frequency drive applications have made electrical bearing damage an increasingly important reliability issue because the rapidly switching voltages produced by modern drives can create high-frequency shaft voltages and bearing currents that are not normally present when a motor is operated directly from line power.

Once electrically induced bearing damage begins, simply replacing the bearing does not address the source of the problem. If the electrical current path remains unchanged, the replacement bearing may eventually experience the same type of damage.

Diverting that current path around the bearing is what prevents recurrence. The SGS^TM CR Series shaft grounding system provides a low-resistance path for shaft current to discharge safely to the motor frame, protecting the bearings from electrical discharge damage.

Identifying the Root Cause of Electric Motor Bearing Failure

A damaged motor bearing often represents the final result of another problem occurring elsewhere in the motor system.

Lubrication issues, contamination, alignment problems, vibration, excessive loads, improper installation, environmental conditions, and electrical currents can all reduce bearing life. In some cases, several of these conditions may be present simultaneously.

A thorough bearing failure investigation should therefore consider the entire motor and driven-equipment system rather than focusing only on the damaged bearing.

Wear patterns, lubricant condition, raceway appearance, operating temperature, vibration data, shaft alignment, electrical measurements, and operating history can all provide important clues regarding the true cause of failure.

Understanding these different failure mechanisms allows maintenance and reliability personnel to move beyond simply replacing failed bearings and instead correct the conditions responsible for electric motor bearing failure.

In the next article, we will focus specifically on electrically induced motor bearing damage – examining how shaft voltage and bearing currents develop, how electrical discharge damages bearing surfaces, why variable frequency drives can increase the risk, and the methods available to protect motor bearings from electrically induced failure.