Austin Bonnett
Austin Bonnett Engineering LLC

The purpose of this article is to provide enough rolling element bearing fundamentals so those who are responsible for the application, operation, maintenance and repair of electric motors can take the necessary steps to minimize premature bearing failures and enhance the possibility of bearings lasting until the "end of life" predictions, which is normally referred to as L10 bearing life.

l. Bearing Fundamentals
The five most common rolling element bearings used in industrial electrical motors can be grouped as follows:

1. Deep Groove Ball Bearings
2. Angular Contact Ball Bearings
3. Thrust Ball Bearings
4. Spherical Roller Thrust Bearing
5. Cylindrical Roller Bearings
   (See Figure 1)

Each of these types of bearings has its own unique characteristics, and it is critical when replacing a bearing system that the new bearings are replaced “in kind" unless it is determined that the bearing was misapplied. Substitution may cause serious application issues that could lead to premature bearing failures.

The required lubrication systems for each of these different types of bearings may vary. Some may be oil lubricated and others grease lubricated. Some bearings may be sealed and require no additional lubrication. Others may be shielded or open and require external grease to sustain life. And others may be force lubricated with oil supplied from an external source. In the case of large vertical motors, the lubricating oil may be cooled by an external source of air or water.

II. Causes of Bearing Failures
Bearings are designed and applied so as not to fail before a predetermined number of hours of expected life, which is referred to as L10 life. For example, in an application with an L10 life of one year, 10% of the bearings may fail due to fatigue, and half the bearings are expected to fail by the five-year mark.

As most service centers will confirm, the most common cause for removing a motor from service deals with problems with the bearing system of the motor. If facts were known, many of the bearing issues are repeat problems because there was no accurate "Root Cause Failure Analysis " conducted to determine the cause of failure. And, unfortunately, a significant number of these failures resulted in damage to other critical parts of the motor such as the rotor, stator, shaft or end bracket.

The 10 most common causes of premature bearing failures can be randomly grouped into the following categories.

• Improper lubricant: too much, too little, wrong type.
• Contamination of the lubricant: water, dirt, other foreign materials.
• Shaft vibration: axial or radial that exceeds industry standards (NEMA or IEC).
• Excessive operating temperature of the bearing and/or lubricant, either from internal or external sources.
• Misalignment of the motor shaft to the driven equipment: usually for direct coupled applications.
• Excessive bearing loading: axial or radial or misapplication of bearing.
• Loss of fits between bearing and shaft, or bearing and end bell or fits that are too tight.
• Failure of the shaft of the driven equipment that imposes any of the above conditions on the motor bearings.
• Excessive bearing currents: usually caused by an external source such as variable frequency drives or "standing wave form” issues.
• Incorrect mounting, handling, or transportation of bearings.

III. Recommended Practices
The recommended practices can be broken down into the following five groups: installation and start up, routine operation, routine maintenance, motor repairs and recommissioning.

For each of these groups there exists a series of steps that should be followed, which include:

• Verification that the mounting and connection of the motor to the driven equipment is correct.
• Measure the noise, vibration and bearing external temperatures.
• Verify functionality of thermocouples or other bearing temperature sensing devices and vibration monitoring equipment.
• Benchmark this data for future reference.

IV. Root Cause Failure Analysis (RCFA)
EASA has an excellent manual on Root Cause Failure Analysis (RCFA) that can serve as a guide for conducting a RCFA of the bearing system, which is beyond the scope of this article.

When a RCFA is justified, the service center should retain the failed bearing and a sample of the lubricant for analysis. The end bell and shaft information should also be documented. This is critical so that the actual bearing fits are recorded.

In some cases, it may be necessary to contact the bearing manufacturer and seek additional counsel. In most cases, consideration of the motor application (e.g., a belted application often requires a roller bearing) will suffice.

V. Conclusion
A significant amount of all premature bearing failures could be eliminated or reduced simply by understanding and applying the above principles for rolling element bearings regardless of the application, that is motors, pumps, fans or blowers.

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Categories: Technical topics, Bearings, Ball bearings, Roller bearings, Troubleshooting & failure analysis

Tags: Bearings Rolling element bearing

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