Roller bearing skidding is a common form of damage. Do you know what roller bearing skidding is? Why does the roller bearing slip? What measures can be taken to prevent roller bearings from slipping? For these series of questions, ERIC BEARING LTD will share relevant content based on the knowledge of bearings.
What is roller bearing skidding?
When the bearing is working, the rollers should perform pure rolling motion on the inner and outer ring raceways. If the drive roller cage is required to have enough drag force, it can overcome its resistance. When the roller drag force is not enough to overcome the resistance, at this time, in addition to rolling, there will be some sliding. Compared with the running speed difference of the inner ring (or outer ring), it is slipping.
What are the factors that cause roller bearings to slip and scratch?
Roller bearing skidding is a necessary condition for bearing scratches, but slipping does not necessarily cause surface scratches.
Some factors that cause roller bearings to slip and scratch are, such as shaft vibration and alternating loads, which produce a force that can tear the lubricating oil film and cause the part of the working surface (roller and inner ring raceway working surface are common) There is a scratch on the area. SKF 6312-2Z bearings online , pls click here :
Therefore, it is conceivable that the slipping injury is the result of material transfer due to the interaction of the uneven metal surface after the lubricant film is destroyed. Some materials are "stained" to form pits; while the more stained parts protrude above the original surface. In addition, some parts of the working surface are scratched, and some parts are not scratched, which is closely related to the alternating load and the degree of damage to the sleeve. In severe cases, excessive frictional heat is generated between the roller and the inner ring, which causes the inner ring to expand, reducing the radial clearance, and causing the bearing to jam.
What measures can be taken to prevent roller bearings from slipping? That is anti-slip measures
1. Reduce the number of rollers and reduce the diameter of the rollers
The roller is moved by the rotating ring. When the strength allows, reduce its weight to achieve the purpose of reducing inertial resistance. It becomes very important to prevent and reduce slippage. Using this measure is relatively simple, only need to reduce its number and diameter can be achieved. In aero-turbine engines, due to the small external load, contact fatigue is no longer the main damage factor of the roller. According to the conventional standard, the bearing size tolerance is still large. It is generally believed that the number of rollers is reduced by 50%, and after the diameter is reduced by 25%, its contact fatigue life is still not affected. Many engines have adopted this measure. For example, the main bearing of Spey engine is of this type, not only the rollers are obviously sparse, but also the size is small.
2. Choose a reasonable structure and reduce the weight of the cage
In addition to reducing its weight when the strength permits, the cage should also consider its own characteristics, for example, its resonance and high-speed rotation resistance 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, the cage is changed from outer guidance to inner guidance, which is also very beneficial for reducing slippage. Because of the external guidance, the relative movement between the cage and the guide surface of the bearing, the oil film forms a viscous resistance, while the internal guidance is the opposite. The viscous force of the oil film between the cage and the guide surface becomes a drag force . In this way, the viscous resistance becomes the force that drags 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 loss caused by stirring. Another example is to change the outer ring belt rib to the inner ring belt rib, which undoubtedly can also reduce the resistance of mixing
4. Using hollow roller preload bearing
Hollow roller preload bearing. It can apply preload to all rollers so that all areas have no slippage. Due to the wide operating speed and temperature range of turbine engine rotor bearings, care should be taken to select an appropriate preload. Even under a very low applied load, an excessive preload will still cause the roller to fail prematurely. The common form of failure is bending fatigue. For a typical hollow roller, the ratio of the outer diameter to the 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 diameters should be kept at 0.005 mm.
5. Using oval bearings
The use of elliptical bearings is the most common form of anti-skid, and generally has two-lobe and three-lobe structures. Take the double-lobed elliptical bearing as an example to talk about its structure and anti-skid mechanism. This kind of bearing generally has an elliptical outer ring outer diameter, and both the outer ring inner diameter (raceway) and the bearing seat are circular. After assembly, the outer ring raceway becomes an ellipse, that is, the roller is preloaded. That is to say, the radial clearance between the roller and the raceway at the short semi-axis of the elliptical raceway is eliminated (interference is formed), and the roller is compressed. Therefore, in addition to the lowermost rollers subjected to the gravity load of the rotor, the rollers at 90° to the left and right are also subjected 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 (roller bearings generally bear about 20% of the total number of rollers under load). Therefore, increasing the drag force helps prevent the roller and cage from slipping. However, the method of giving the ovality will reduce the fatigue life of the bearing, so it is necessary to consider the selection of the optimal ovality in use.