According to incomplete statistics, 16% of motor bearing failures are related to improper installation. In the general motor bearing installation process, we emphasize a series of operating environments, operating procedures, and operating methods, but in fact there are still many front-line production personnel who adopt improper installation methods for bearings.

For deep groove ball bearings, which are commonly used in motors, the most typical of these improper operations is "knocking" on the bearings. There are generally two modes of tapping:

• First, the impact load of knocking does not pass through the bearing rolling elements;

• The second is through the bearing rolling elements.

As long as the installation load, or installation knock, does not pass through the rolling elements of the bearing within a certain limit, the damage to the bearing is controllable. Control the knocking force well to avoid serious damage to the bearings.

However, if the installation force is installed by knocking the rolling elements, it will cause damage to the bearing. In other words, it is a potential important cause of bearing damage. Seriously, plastic deformation is left on the surface of the bearing raceway, and the bearing makes noise when it is running. Fortunately, problems like this are easy to spot. Then, the inner raceway of another type of bearing has slight deformation, but the deformation will not cause macroscopic signs when the bearing is running. As the bearing continues to operate further, the bearing raceway begins to experience fatigue failure at the initial damage site, and the noise of the bearing can be heard.

At this time, the motor has been running under this working condition for a period of time. If the maintenance personnel notice the noise and replace it and repair it in time, the consequences are relatively controllable; but if the maintenance personnel ignore the noise information of the bearing (especially in some places with very loud environmental noise) working conditions), the bearings can easily eventually fail, burn out or even cause greater losses.

In this case, bearing failure will appear as shown below:

This failure mode seems to belong to subsurface fatigue (subsurface fatigue), but its failure traces have inherent characteristics. The biggest point is that the fatigue points are equally spaced, and the spacing is equal to the spacing between the bearing rolling elements.

With these characteristics, it can be more confidently judged that the knocking during installation caused the failure of the bearing raceway. Because the fatigue points at equal rolling element spacing on the raceway surface indicate that initial fatigue damage occurred at the equal roller spacing. And if fatigue occurs when the bearing is running, because the rolling elements make circular motion on the raceway, there is no way to stop and cause damage to the equal roller spacing.

Therefore, this kind of damage can only be caused by the further development of the initial damage caused by strong external forces when the bearing is stationary.

After the motor is installed, it is unlikely to bear a huge radial load when it is not running, so it is most likely that the damage was caused by knocking during installation. (Different from damage under vibration conditions, the distance between failure points in this case is not necessarily equal to the distance between rollers, which will be discussed in another article.)

Another commonly used bearing in electric motors is the cylindrical roller bearing. The most common damage to cylindrical roller bearings during installation is axial strain between the bearing raceways and rolling elements.

The picture below shows the strain marks on the raceway caused by a typical cylindrical roller bearing installation:

Shown in the picture is a NU type cylindrical roller bearing inner ring. There are some axial marks on the surface of the inner ring. The typical characteristics of these marks are:

- Distribution along the axial direction of the bearing;

- Failure traces show signs of axial stretching (if observed with a microscope, the direction of the knife marks on the metal surface will change)

- In many cases, it appears in a triangular shape with one end larger and one smaller.

This failure trace is sometimes equal to the roller spacing, and sometimes shows a similar equal distribution. Especially after multiple installations, it is very likely that there will not be complete equal roller spacing traces.

However, where a failure trace is equal to the roller spacing, other similar traces can often be found.

The formation of such marks is related to the installation process. Usually when installing a cylindrical roller bearing, the inner ring of the bearing is heat-fitted onto the shaft, the outer ring is installed on the bearing end cover, and then the end cover with the bearing outer ring is placed on the bearing inner ring, facing towards the bearing inner ring. push. This "pushing" process often causes strain on the bearing raceway, resulting in marks similar to those in the picture. In the early stages of such traces, there will be noise from the motor bearings. Later, as the failure progresses, more complications will occur.

Whether it is installation damage to a ball bearing or a cylindrical roller bearing, if you look at the frequency domain vibration of the bearing when the motor is running, you will find the defective frequency of the bearing ring. When the bearing is disassembled, you will find signs of failure similar to the picture. However, the severity of the traces varies depending on the degree of failure.

If an electrical engineer encounters a disassembled bearing with the above picture characteristics, he or she can find a complete solution to the problem during the bearing installation process.