Key points in precision bearing assembly

Roller bearing slippage leading to bearing scratches is a common form of damage. Do you know how to prevent roller bearing slippage?

1. Reduce the number of rollers and reduce the roller diameter

The roller is run by the rotating ring. If the strength allows, its weight can be reduced to achieve the purpose of reducing inertial resistance. It becomes very important to prevent and reduce sliding. This measure is relatively simple and only needs to be reduced in number and diameter. In aviation turbine engines, due to the small external load, contact fatigue is no longer the main damage factor of rollers. According to conventional standards, the bearing size margin is still large. It is generally believed that reducing the number of rollers by 50% and reducing the diameter by 25% will still not affect its contact fatigue life. Many engines have adopted this measure. For example, the main bearing of the Spey engine is of this type. Not only are the rollers obviously sparse, but they are also compact in size.

2. Choose a reasonable structure and reduce the weight of the cage

In addition to reducing the weight of the cage as much as possible when the strength allows, its own characteristics should also be considered. For example, its resonance, high-speed rotation resistance, etc. should be avoided.

The cage guidance method is also a very important factor in high-speed bearings. If the imbalance of the cage is properly controlled, changing the cage from external guidance to internal guidance is also very beneficial to reducing slippage. Because of the relative movement between the outer guide, the cage and the guide surface of the bearing, the lubricating oil film forms a viscous resistance, while the opposite is true for the inner guide. The viscous force of the lubricating oil film between the cage and the guide surface becomes a drag force. . In this way, the viscous resistance becomes a force dragging the cage to rotate. The difference between the two cannot be ignored.

3. Oil supply under the ring

The advantage of using under-ring oil supply is that it takes away the heat of the bearing and minimizes the additional losses caused by stirring. Another example is changing the outer ring rib to an inner ring rib, which will undoubtedly reduce the resistance to stirring.

4. Use hollow roller preload bearings

Hollow roller preload bearings can preload all rollers, making all areas slip-free. Due to the wide operating speed and temperature range of turbine engine rotor bearings, care should be taken to select the appropriate preload. Even under very low applied loads, excessive preload will still cause premature roller failure. A common form of failure is bending fatigue. For a typical hollow roller, the ratio of outer diameter to inner diameter is 2, and the maximum hole bending stress should be less than 25,000 pounds/inch; the concentricity of the inner and outer diameter should be maintained at 0.005 mm.

5. Use elliptical bearings

The most common form of anti-slip is the use of elliptical bearings, which generally have two structures: double-lobed and triple-lobed. Taking the double-lobed elliptical bearing as an example, let's talk about its structure and anti-skid mechanism. This kind of bearing generally has an elliptical outer ring outer diameter, and a circular outer ring inner diameter (raceway) and bearing seat. After assembly, the outer ring raceway becomes elliptical, that is, preload is added to the rollers. That is to say, the radial clearance between the roller and the raceway at the short half axis of the elliptical raceway is eliminated (interference is formed), causing the roller to be compressed. Therefore, in addition to the lowermost rollers that are affected by the gravity load of the rotor, the rollers at 90° to the left and right are also each subject to a certain amount of preload, so that the number of rollers bearing the load increases to About 60% of the total number of rollers (the number of rollers in roller bearings generally accounts for about 20% of the total number of rollers). Therefore, the increased drag force helps prevent the rollers and cage from slipping. However, the method of giving ovality will reduce the fatigue life of the bearing, so it is necessary to consider the selection of the optimal ovality during use.