Here’s a statistic you should know: mechanical failure, rather than electrical failure, is the cause of 80% of failures in electric motors.  Unplanned failures cost facilities countless dollars each year due to the resulting loss of productivity, overtime repairs, and other equipment that may be damaged because of the failure. In addition, equipment failure can also cause a safety issue to plant personnel.

Most of these expensive mechanical failures involving electric motors can be traced back to rolling element bearing failure and bearing defects. Understanding the causes behind premature bearing failures can lead to more informed decisions that will prolong the life of the equipment under your responsibility.

Bearing Failure Due to Shaft Voltages

In an electric motor, shaft currents can develop and cause various problems for the bearings. If the shaft currents pass to electrical ground through the motor bearings, then an electrical arc between the shaft and the bearing occurs. The arc leads to extremely high temperatures at the point where it meets the bearing and can cause very problematic surface damage such as fluting, pitting, and frosting. This surface damage invariably leads to premature bearing failure.

Bearing Failure Due to Lubrication Issues

Lubricants serve three primary purposes in a rolling element bearing: reduced friction, protection of surfaces that come in contact with each other, and, to a lesser or greater degree, the conduction of heat away from critical surfaces.

When a bearing failure is tied to lubrication issues, there are three likely culprits:

  • The wrong lubricant was used
  • Either too little or too much lubricant was used
  • A wrong lubrication interval was followed

Incorrect lubrication invariably leads to accelerated wear of the bearings and increases the probability that a mechanical failure will occur. Abrasive wear will often manifest itself between the rollers, races, and cages and lead to serious surface damage that compromises the performance and useful life of the bearings.

Bearing Failure Due to Contamination

Bearings in the electric motors & powertrains can be contaminated with a variety of problematic substances, and that includes not just abrasive particles such as dust, sand, fine metal from grinding, and carbides from gears but also moisture. The presence of moisture can lead to corrosion and compromise the ability of lubricants to do their job. Abrasive particles will damage key surfaces in the bearing, such as where the balls or rollers contact the races. Keep in mind that the rolling elements are transmitting extremely high, concentrated forces, and the presence of a contaminant will scar the surface and reduce the life of the bearing.

Contamination often occurs when the seals responsible for protecting the bearings either wear out or experience damage. Damaged seals will lead to costly contamination, which is why seals need to be replaced on a regular basis. However, contamination can also be introduced through improper cleaning.

Bearing Failure Due to Other Causes

We have talked about the most common causes of bearing failure in electric motors, but there are other potential sources of failure. These include…

  • Improper handling
  • Incorrect mounting and installation
  • Improper bearing load
  • Poor fits

Symptoms of Bearing Failure

The most common symptoms of bearing failure are excessive noise, heat, vibration levels, friction, and/or shaft movement. Any time a motor or any electromechanical system exhibits these signs then bearing failure has either occurred or is imminent. Each of these symptoms can point to a different type of issue or bearing defect cause.

Vibration Analysis of Bearing Defects

As mentioned, vibration can be a sign of bearing failure; however, it can be far more than a sign: vibration analysis can pinpoint the presence of bearing defects that lead to failure. Most of the causes of bearing failure discussed here manifest as surface damage to the bearings. When a rolling element comes in contact with another element surface, the result is an impact force that is transmitted throughout the bearing. These impact forces, or bearing defect amplitudes, can be detected, isolated, and pinpointed through good vibration analysis.

Extending Bearing Life

Bearings will not last forever, but there are certain keys to make them last as long as possible. Proper selection of bearings is the starting point, and that includes loads, speed, lubrication, and the operating environment for the bearings. Installation is another key aspect of extending bearing life and includes mounting, static misalignment, adjustments, and preload as well as a stable housing support, housing itself and shaft deflection. Once the bearing has been installed, lubrication and minimizing contamination are key to its performance and life cycle. And if shaft currents become a problem, there are highly effective grounding rings that can prevent the bearings from being damaged.


Rolling element bearings factor into many forms of mechanical failures experienced by electric motors. Understanding how bearings fail and how those failures can be prevented will help you make wise decisions regarding repair, replacement, and maintenance of the bearings that are integral to your facility’s electric motors.

At Hi-Speed Industrial Service, we have multiple technology and tools to detect, diagnose, and pinpoint issues with bearings before they are ever removed from your motors. Our team of experienced technicians can help troubleshoot, repair, and replace the bearings in your electromechanical equipment. In addition, we offer predictive maintenance services that include vibration analysis, infrared, ultrasonics, motion amplification and others. Contact us today to find out what HI-Speed can offer you!