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Motor Bearing Lubrication: 6 Factors That Affect Performance

Motor Bearing Lubrication: 6 Factors That Affect Performance

Given the importance of bearings in the normal functioning of electric motors, it is no wonder that insufficient maintenance and even basic lubrication can make them responsible for 67% of all motor failures, according to the findings of the U.S. Department of Energy. These failures may require an electric motor rewind to fix or, worse, a complete replacement. More importantly, an article by Reliable Plant states that up to 80% of cases of motor bearing failure are due to lubrication failure. Given how much a little bit of grease is responsible for, it pays to learn the factors that can affect their performance in electric motors.

1. Working temperature and environment

Many factors in a motor’s working environment can affect its lubrication, namely its ambient temperature and upkeep.

Under higher temperatures, lubricants naturally experience faster deterioration and, consequently, a rapid loss in their effectiveness. This breakdown occurs because the lubricant’s oxidation rate accelerates, causing it to evaporate faster. As such, it is recommended to increase the frequency of inspections of the motor bearings and apply additional lubricant as necessary in higher-temperature environments.

Colder environments also have an adverse effect on bearings as their oil and grease become too viscous. When lubricants have a higher viscosity, they cannot form the all-important film that separates the rolling elements from other contact surfaces.

Lastly, the environment’s cleanliness or dirtiness can also affect a lubricant’s condition, with the latter almost always leading to lubricants getting contaminated by dust, dirt, and other debris.

2. Operating hours

Critical motors that need to operate continuously will naturally require more frequent lubrication than motors that need only run periodically. Monitoring how long the motor has been operating is the first step to establishing a relubrication schedule, which is not always straightforward; other factors to consider in your decision include the recommendations from your motor’s original manufacturer and grease supplier.

In addition, gearing manufacturers generally suggest relubricating based on the type of grease the motor uses and its operating conditions. For instance, annual relubrication will generally suffice for light to medium-duty motors running continuously. Lastly, it is highly advisable to reduce the relubrication interval by half for every 10℃ above the nominally recommended temperatures.

3. Bearing type and size

The size and type of a bearing often dictate how long a lubricant will last and how much of it is required. Size is simple enough; the bigger the bearing is, the more lubricant it will need to perform optimally during its lifetime. Meanwhile, the grease used in shielded and sealed bearings generally lasts longer than those used in open ball, thrust, and roller bearings.

4. RPM

Motors operating at high rotations per minute will break down their lubricants faster than those running at lower speeds because higher speed applies more heat to the grease and thus breaks it down faster.

5. Vibration

Strong vibrations may cause motor lubricants to flow freely into the path of the rolling bearing element, where it gets worked and heated excessively, reducing lifespan. Such excessive vibration may also indicate other motor problems, so it is highly recommended to conduct a vibration analysis to determine the source of the vibration.

6. Lubricant type and viscosity

Bearings require oil or grease for lubrication, and using the wrong type can negatively impact its operation. Grease is a semi-solid lubricant more commonly used for bearings operating in most linear guide and drive applications because of its longer lifespan, better adherence to bearing surfaces, and is less likely to run off or be ejected from rotating parts compared to oil.

What separates grease from one another is its raw materials, consisting of a base oil, a thickener, and additives. The base oil will either be synthetic or mineral. Synthetic oils are preferred for extreme temperature applications, while mineral oils generally suffice for most electric motor-bearing applications.

Moreover, it is vital to choose the appropriate grease viscosity for the expected speed and load of the application at operating temperatures. Mineral oils used for motor grease typically have a viscosity range of 500-600 SUS at around 37°C. For a more specific recommendation, it is best to consult your electric motor’s OEM.


Lubrication plays a small yet critical role in the optimal functioning of motor bearings and the motor as a whole, as grease incompatibility can lead to motor failure. Given that many things can affect a motor’s performance, ranging from the working environment down to grease compatibility, it makes sense to make relubrication a high priority in your maintenance regimen. Otherwise, unexpected failure may occur and necessitate speedy electric motor overhauling or rewinding that cuts into your budget and production hours.