The Ultimate Guide to Lubricating Electric Motor Bearings

The task of providing supplemental lubrication to motor bearings is a routine part of their maintenance. But do you know how to do it? You may think it’s straightforward; pick up the oil and pour it in – how complex could it be?

In fact, while it may not be particularly complicated, there are still numerous details to be mindful of when adding oil to motor bearings. Overlooking these details could lead to issues with the motor.

Many tasks appear to be exceedingly simple, like “putting an elephant in the refrigerator,” which is only three steps. However, the details within those steps are not so easy. The analogy of putting an elephant in the refrigerator is apt for maintaining a motor.

Steps for Supplementing Lubrication:

There are also three steps to add supplemental lubrication to motor bearings: take out the grease, add it, and then put the grease away. There are numerous details within these steps:

1. What kind of grease should be taken out?

2. When should it be added?

3. What preparations should be made before adding lubrication?

4. Where should it be added?

5. At what speed should it be added?

6. How much should be added?

7. What checks should be carried out after it’s been added?

Just off the top of my head, there are seven questions. In actual operation, the process flows smoothly and doesn’t need step-by-step guidance, but these details cannot be ignored within the flow of operations.

Details Matter

When to add supplemental lubrication grease is an important decision. This question can be broken down into two sub-questions:

1) What should be the interval between each lubrication replenishment?

2) When is the ideal time to add the lubrication grease for this round?

1. Regarding the Interval for Lubrication Replenishment

This matter should be discussed with motor design engineers.

2. The Ideal Time to Add Lubrication Grease

The primary concern for operators is how and when to lubricate. Here, “when” refers to three general scenarios:

  • During normal operation of the equipment;
  • During low-speed operation or manual cranking;
  • When the equipment is idle.

During normal operation, the motor runs typically at a high speed. If lubrication is added at this time, new grease may not fully blend with the old grease before the system resumes operation. While this is not technically incorrect, it’s not the optimal timing either.

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When the equipment is idle, the new and old grease, though placed in the same bearing chamber, are not evenly mixed after addition. The grease is only mixed when the equipment starts operating. Again, while not technically incorrect, it’s not the optimal timing.

The best time to add lubrication grease is followed by a suitable greasing action. In fact, the state of low-speed operation, or manual cranking, is similar to the process of slow-speed greasing. At this time, the added lubrication grease slowly blends with the old grease under low-speed bearing operation, which helps with the grease mixture and subsequent lubrication. Real-world situations are not always perfect; the equipment may not allow the maintenance personnel to operate at low speed, and manual cranking may not even be possible sometimes. In such cases, the next best option is to grease when the equipment is idle.

If the equipment doesn’t even allow for greasing during idle times, then the only option left is to replenish the lubrication grease during normal operation. As mentioned before, although the latter two timings are not ideal, they are not incorrect either. The difference between an experienced operator and a novice often lies in these minute details.

The Role of Molybdenum Disulfide

Should molybdenum disulfide be added during supplemental lubrication? Let’s address this query up front.

Primarily, molybdenum disulfide is occasionally used in motor bearing lubrication. Its fundamental function is to act as an extreme pressure additive.

Specifically, when grease fails to form an oil film or struggles to do so, molybdenum disulfide safeguards the bearing raceway surface to some extent.

Secondly, molybdenum disulfide is a solid additive that exhibits a certain “lubricating” effect due to its molecular structure.

However, molybdenum disulfide is not a lubricant in itself, and its use alone won’t resolve motor bearing lubrication issues. Likewise, during supplemental lubrication, molybdenum disulfide should not be the only additive used.

Thirdly, if the original grease in the motor contains a certain amount of molybdenum disulfide, it can be added to the supplemental grease during lubrication. An appropriate method involves incorporating molybdenum disulfide into the grease, and adding it concurrently during supplemental lubrication.

This approach ensures a thorough mix of molybdenum disulfide and the supplemental grease prior to lubrication, preventing uneven distribution or inappropriate quantities that could occur when added separately.

Lastly, molybdenum disulfide cannot be added on its own during supplemental lubrication, as it is essentially an additive.

In other words, it should not be a prerequisite for all bearing lubrication choices. I’ve witnessed the misuse of molybdenum disulfide in several motor factories.

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Therefore, it’s crucial to note that incorrect usage of molybdenum disulfide not only fails to aid lubrication but can significantly shorten both the lubrication and bearing lifespan.

Molybdenum disulfide is not suitable for the following situations:

(1) It’s not suitable for high temperature environments. Molybdenum disulfide typically performs well at temperatures up to 80 degrees Celsius. If the temperature of the motor bearings reaches or exceeds 80 degrees Celsius, molybdenum disulfide should not be used.

(2) Molybdenum disulfide should not be used in high-speed applications. At high speeds, the fine molybdenum disulfide particles not only fail to lubricate, but sometimes even act as abrasives, intensifying bearing wear.

(3) Molybdenum disulfide may not be suitable for applications with frequent changes in speed, for reasons similar to the previous point.

(4) Molybdenum disulfide should not be used in environments or with greases that chemically react with it.

In fact, besides molybdenum disulfide, there are many other types of extreme pressure additives available, both in liquid and solid form. For motors, some greases already have extreme pressure additives mixed in, so there’s no need to use molybdenum disulfide.

I would also recommend engineers to prefer greases that come with extreme pressure additives for applications where such lubricants are needed. Manually adding and stirring molybdenum disulfide or other extreme pressure additives can lead to difficulties in controlling the amount added and issues with uneven distribution.

Extreme pressure additives are not indispensably necessary, nor are they necessarily better when used. The actual operating conditions of the motor bearing should be considered.

Supplemental Lubrication for Motor Bearings

The maintenance of motor bearings is a regular or irregular task performed during the operational phase of a motor. Among these tasks, supplemental lubrication for motor bearings is a crucial element.

Supplemental lubrication, as commonly understood, involves oiling the motor bearings, simply put, adding lubricant. This seemingly simple action encompasses many details that require careful attention. Sometimes, the reliability of maintenance work is directly related to some factors considered during the motor design phase, hence it merits the attention of motor engineers.

The tasks of supplemental lubrication for motor bearings involve understanding the following aspects:

1. What is used for supplemental lubrication?

2. How is supplemental lubrication applied?

3. How much should be applied?

4. How should the supplemental lubrication be added?

5. What should be noted after adding the lubricant?

In fact, on many occasions, the author has introduced this topic. Now, let’s systematically discuss the details of supplemental lubrication for motor bearings.

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What should be used for supplementary lubrication?

Not all lubricants can be used for supplementary lubrication!

Are all greases in motors the same?

In short: Not necessarily!

This is because:

Bearings of the same size in motors may not carry the same load;

Bearings of motors with the same center height may not rotate at the same speed;

The reference temperature chosen during the design of bearings with the same speed and center height may not be the same;

The working conditions and duty cycles of the same motors may not be identical.

Any variation in the above factors could lead to a different choice of grease. These are considerations when selecting the type of lubrication for motor bearings. If a motor designer takes these factors into account, then the lubrication of the motor bearings could very well differ.

Of course, from a standpoint of universality and cost, similar types of motors are often “grouped together” so that the choice of grease remains consistent. However, it is essential to verify whether all the greases you are using in your motors are the same.

So, how do you confirm this?

It’s simple, check the nameplate; it’s usually written there!

Can lubricating greases be mixed?

Not necessarily!

Compatibility testing is required to determine if greases can be mixed. Otherwise, different lubricating greases may chemically react when mixed, resulting in a loss of lubrication ability.

Clumping, color change, and viscosity alterations after mixing lubricants could all indicate problems caused by combining different types. Such changes can severely impact the lubricating performance, leading to motor bearing overheating and premature failure.

Therefore, to mix lubricating greases, a compatibility test must be performed first. Only those that pass the test can be mixed; otherwise, it’s not advisable!

Which type of lubricant should be used for supplementary lubrication?

The answer is, of course, a lubricant of the same brand.

What if there isn’t lubricant of the same brand available?

In this case, it’s necessary to verify the new lubricant’s lubricating properties, base oil viscosity, grease thickness, and so on. After these calculations, a compatibility test should be conducted. Only after passing this test can the lubricant be used.

What if there isn’t a specific grade of lubricant available, and it’s possible to calculate the performance of a new grease, but compatibility testing cannot be performed?

The solution is thorough flushing!

Maximize the removal of old grease by flushing it out with new grease, ensuring the bearings are filled with new grease, and the old one is expelled. Then, run the machinery at a slow speed to expel any excess new grease. After steady monitoring for a period, the process can be concluded.

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