Motor Bearing Noises: What’s Normal and What’s Not

Motor bearing noise has often been a perplexing issue for many engineers. It’s hard to describe motor bearing noise in words, which poses challenges for technicians when attempting to make a judgement.

However, with extensive on-site experience and a comprehensive understanding and analysis of motor bearings, valuable practical guidelines have been developed. For instance, what type of “noise” is considered “normal noise” for a bearing.

Is There Such a Thing as a “Noiseless” Bearing?

People often ask how to eliminate the noise from a bearing. The answer is that it’s impossible to completely eliminate it because some “noise” will inevitably occur during the bearing’s operation.

This primarily refers to the bearing under normal running conditions, which includes:

Collision of Rolling Elements with the Raceway in the Non-Load Area

As the bearing’s rolling elements move within the raceway, collisions with the raceway, either radially or axially, occur when the rolling elements operate in the non-load area.

This is because the rolling elements, upon exiting the load area, possess a certain linear speed, and due to centrifugal forces, rotate around the center, causing a collision with the raceway and thus creating noise. This collision noise is particularly noticeable in the non-load area when there is remaining clearance.

Collision of Rolling Elements with the Cage

The main function of the cage is to guide the movement of the rolling elements. Collisions between the rolling elements and the cage also contribute to noise. These collisions include radial, circumferential, and possibly axial interactions.

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In terms of motion state, the collisions include those that occur inside the load area when the rolling elements actively push the cage, and those that happen in the non-load area when the cage drives the rolling elements.

Radial collisions between the rolling elements and the cage occur due to centrifugal force. Disturbances can also cause axial collisions between the rolling elements and the cage.

Stirring of Grease by Rolling Elements

During the filling of grease inside the bearings, the operation of the rolling elements involves stirring of the lubricant, which can cause corresponding noise.

Sliding Friction of Rolling Elements Entering and Leaving the Raceway

There exists a certain degree of sliding friction between the rolling elements and the raceway when entering the load zone. A similar degree of sliding friction may occur when leaving the load zone.

Other Internal Movements within the Bearings

Friction at the lip of the bearing seals can also contribute to noise.

In conclusion, it’s clear that these rolling bearings, when operating under normal conditions, inevitably generate some “noise. Hence, the answer to the initial question is: it is impossible to eliminate the inherent “normal noise” of the rolling bearings.

So, What Does the Normal Sound of Motor Bearings Sound Like?

From the preceding analysis, it’s evident that these various states of motion generate noise due to collisions and friction. For a standard, qualified bearing, it is not difficult to see that these noises are closely related to the rotational speed.

For instance, friction as the rolling element enters and exits the load zone, collisions of the rolling element within and outside the load zone with the cage, grease stirring, and lip friction of the seal, all vary with the change in rotational speed.

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When the motor operates at a constant rotational speed, these motions should all be under a stable state. Therefore, the noise excited by the bearing at this time should be steady and uniform.

From this, we can infer that a normal bearing noise should exhibit a fundamental characteristic: stability and uniformity.

The stability and uniformity mentioned here do not mean a continuously unchanging sound. Since many states of motion, such as collisions, occur repetitively, these sounds are those of a stable, small cycle. Of course, this includes some continuous sounds, such as the sound of seal friction.

In actual working conditions, such as when certain interferences are present, the noise may also exhibit a degree of stability and uniformity. However, such noise often does not sound like the frequency that a bearing should have.

Therefore, when judging bearing noise on-site, in addition to stability and uniformity, it typically also needs to be free of abnormal frequencies (aurally).

The above describes what we refer to as normal bearing noise. Some parts of these noises can be reduced through usage methods, but they are difficult to eliminate entirely.

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