Which Motor Bearings Require Preloading? And Why?

Engineers at motor factories often encounter situations where preloading force needs to be applied to motor bearings, but is this necessary for all motor bearings? The answer is not exactly.

Another question is, what would happen to the motor bearings if they were not preloaded?

Before discussing preloading, to avoid confusion, let’s clarify that the preloading we are referring to here is axial preloading, not radial.

Let’s discuss which motor bearings require preloading from the perspective of the fundamental configuration of different motor bearings.

Small horizontal motors

For small horizontal motors, a dual deep groove ball bearing structure is usually adopted when there is no significant external axial radial load.

The motor configurations for dual deep groove ball bearings are basically a fixed end + floating end configuration, or a cross-positioning configuration.

For the fixed end + floating end configuration, applying preload will cause the floating end bearing to withstand a certain axial load. This axial load is transmitted to the fixed end through the shaft system, causing the fixed end bearing to also bear a certain axial load. Under the action of the axial load, the axial clearance of the two bearings is compressed to zero.

At this time, when the bearing is operating, the collision between the rolling body and the raceway due to the remaining clearance will be suppressed, thereby reducing the noise of the motor bearings.

For motors without preloading, the above situation will not occur, so the noise of the motor bearings will be larger. However, such noise does not greatly affect the bearing itself, and for industrial motors and fields that do not pay attention to noise, not applying preloading will not necessarily cause damage.

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For the cross-positioning structure with dual deep groove ball bearings, when the motor is running stably, the expansion of the shaft will generate a certain axial preload on the bearings at both ends. This serves to reduce noise as mentioned above. However, due to restrictions on controlling thermal expansion due to temperature, shaft size, etc., manual application of axial preloading is often used.

In the above structures, regardless of which structure applies axial preloading, in addition to reducing bearing noise, it also prevents false Brinelling.

A small motor equipped with a pulley belt

Small motors equipped with belt pulleys may experience substantial radial loads at the shaft ends. Therefore, sometimes a cylindrical roller bearing combined with a deep groove ball bearing structure is employed.

In such a motor bearing system, if an axial pre-load is applied, it can reduce axial clearance and lower noise in the deep groove ball bearing. However, it has virtually no effect on the cylindrical roller bearing.

This is because the cylindrical roller bearings typically used in motors are of the NU or N series, which do not have axial load capacity and can accommodate axial movement within the bearing. So even if the axial pre-load is applied to this bearing, the most it achieves is a slight axial displacement, with minimal impact on the bearing’s operational performance.

For medium-sized horizontal motors

Most medium-sized motors predominantly employ cylindrical roller bearing structures, though sometimes spherical roller bearings are used.

For cylindrical roller bearings, the most common configuration in motors is a two-pillar one-ball bearing setup. This structure operates similarly to the aforementioned one-pillar one-ball setup when a preload is applied to the shaft system. In this bearing configuration, the main operational load-bearing bearing is the cylindrical roller bearing.

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Therefore, the noise reduction effect of applying preload to the deep groove ball bearing, which acts as a positioning bearing, is not very pronounced. Moreover, the permissible noise levels for the bearings in medium-sized motors under normal operating conditions are relatively high, so there’s little benefit in using axial preloading in this type of setup.

As for structures using self-aligning roller bearings, the application of axial preload tends to excessively load one of the rows in the double-row rollers. When the preload is not overly significant, it doesn’t greatly impact bearing noise.

However, when the preload is substantial, it can cause issues like internal bearing heat due to load distribution problems. Therefore, it is also unnecessary to use axial preload in this sort of configuration.

For motors that bear axial loads

When a motor endures a certain axial load, the shaft system, or the locating end within it, is clamped by the axial force. Applying a counteracting axial force at this time can mitigate this effect, while a force in the same direction can enhance the impact of the axial force.

Therefore, one should determine the magnitude and direction of the axial pre-load based on the actual operating conditions; a standardized setting is not advisable.

For vertical motors, their load conditions are similar to motors enduring axial forces, and the specifics should be determined based on the operating circumstances.

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