The difficulty in lubrication of high-speed bearings is that the lubricant is easy to scatter, and it is not easy to adhere to the rolling surface, or is blocked by the high-speed airflow around the bearing and cannot enter the bearing. The speed of the lubricant entering the bearing is too high, and sometimes the bearing is damaged. For example, the cage has pitting, and desquamation increases friction and wear.
The operating temperature of high-speed bearings is mostly around 200 °C, and tends to be higher. If the cooling is not strong, the temperature will continue to rise and the strength and hardness of the bearing will be greatly detrimental. Once the bearing loses its ability to maintain accurate shape, it will surely The high-speed friction is intensified and the temperature rises rapidly, which further leads to a decrease in material strength and hardness, which causes the bearing to quickly fail.
Because high-speed bearings are sensitive to wear and are prone to wear debris, if they are not removed from the bearing in time, they will cause vicious cycles and cause the bearings to fail quickly.
Therefore, the task of high-speed bearing lubrication must effectively deliver the lubricant to the moving surface in the bearing and remove the heat from the bearing while removing the abrasive particles or dirt from the bearing.
The lubrication methods commonly used in high speed bearings and effective lubricants are:
1. Spray lubrication
The injection lubrication method is to spray the lubricating oil into the bearing at a speed of 10-20 m/s from the nozzle hole close to the bearing, usually into the gap formed by the cage and the inner ring or the outer ring to lubricate and cool. The method of bearing.
For small ball bearings with an inner diameter of 30-35mm, the jet lubrication can reach a dmn value of 3 million, and for larger bearings it is 2.5 million.
Double-hole or multi-hole nozzles can increase the oil supply and increase the efficiency of lubricating oil. Multi-nozzles or nozzles on both end faces can improve the lubrication and cooling effect, but when the dmn value is close to 2 million Due to the effects of centrifugal inertia and windage, the oil fed into the bearing is only 70% or less of the nozzle flow.
2. Ring lubrication
The under-loop lubrication method uses centrifugal inertia to directly spray oil through a plurality of radial small holes formed in the inner ring to the surface of the raceway, and a part of the oil flows axially under the inner ring to cool the inner ring. The oil entering the raceway is divided into two left and right passages, and the abrasive debris falling from the parts such as the cage is washed away.
This kind of lubrication method uses much less oil than injection lubrication, and the power loss caused by the power agitation of the oil is also small, the heating condition of the bearing is also greatly improved, and even the inner ring temperature may be lower than the outer ring. Bearing failure rate.
This lubrication method can also be used to cool the outer ring. When the inner ring and the outer ring are all lubricated, the allowable dmn value of the bearing can be further improved.
This ring lubrication method is now widely used in various ultra-high speed applications. The maximum dmn value that can be achieved is 3 million for large ball bearings with an inner diameter of 120 to 200 mm, 2.4 million for tapered roller bearings with an inner diameter of 120 mm, and 3 million for short cylindrical roller bearings with a combined inner ring.
3. Double lubrication method
In this method, the inner ring hole and the journal of the rolling bearing form a hydrostatic oil film sliding bearing, so that the rotation speed of the rolling bearing is greatly reduced, thereby achieving the purpose of reducing the centrifugal force. Such a double-lubricated bearing can significantly reduce the inner ring of the rolling bearing. The speed, thus allowing the spindle to have a higher rotational speed, corresponds to a dmn value of the bearing that can be further increased.
4. Oil and gas lubrication method
The biggest advantage of the oil and gas lubrication method is that it can realize the dmn value of 800,000 to 1.5 million high speed only by reducing the friction of the bearing without changing the bearing structure. For the bearing with more internal sliding, the advantage of this method is more To be significant.
(1) Principles and characteristics of oil and gas lubrication The principle of oil and gas lubrication is to use a quantitative piston type distributor to discharge a very small amount of oil (for example, 0.01m1) intermittently at an optimum cycle, and the discharged oil is before it reaches the bearing. A continuous flow is formed which is fed into the bearing along with the air.
The characteristics of oil and gas lubrication are as follows:
1) A very small amount of the quantitative oil can be continuously and stably supplied to the bearing, so that the minimum amount of oil required can be controlled like grease. Since the bearing type is different and the amount of lubricating oil is different, the quantitative piston pipe is connected to each set of bearings, so that the oil supply of each bearing can be adjusted.
The dynamic friction loss of this lubrication method is about one tenth of the injection lubrication, and its dynamic friction loss and temperature rise are also smaller than grease lubrication.
2) Since oil and gas lubrication is less than the amount of oil for injection lubrication, and the lubricating oil flows into the bearing with a suitable oil droplet, the atmosphere is less polluted. For example, after the oil-lubricated main shaft is continuously operated in the propylene tank for about 2 hours, the air pollution level in the measuring box is only 0.03 mg/m3.
3) It is difficult for foreign dust or cutting fluid to enter the main shaft. This is due to the large amount of air supplied to the oil (20-30 L/min per bearing), which increases the internal pressure of the main shaft.
(2) Oil quantity and viscosity of oil and gas lubrication Oil and gas lubrication should be supplied to the bearing with the minimum necessary amount of oil, so the choice of oil quantity is crucial.
More angular contact ball bearings have more oil than smaller cylindrical roller bearings. There are formulas for the approximate standard of oil supply.
However, the lower the viscosity of the oil, or the higher the rotational speed, the corresponding increase in the amount of oil required. In fact, the amount of oil should be determined according to the conditions of use such as bearing load and speed.
In terms of the effect of lubricant viscosity on bearing temperature rise, oil and gas lubrication is much less than cyclic oil supply. However, under the same conditions of oil supply, when the viscosity is high, the temperature rise is slightly increased. The usual oil and gas lubrication uses an oil having a viscosity of 10 - 3 cmr2 / s at 40 ° C.
Gas lubrication is a new lubrication method with many characteristics. It is used in bearings with more sliding, such as angular contact bearings, and its dmn value can reach about 1.5 million.
5. High speed bearing lubricant
Lubricants for high-speed bearings should be considered as lubricants for high-temperature bearings—that is, synthetic oils that have good performance at high temperatures, or greases based on such synthetic oils. Recently, the diester-based MIL-L-7808 oil and MIL-L-23699 oil are widely used in jet engine bearings.
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