No matter what brand of bearing, lubrication is a necessary condition to ensure its normal operation. Lubrication has an important impact on the fatigue life and friction, wear, and vibration of roller bearings. About 40% of bearing damage is related to poor lubrication. The purpose of bearing lubrication is to form an oil film between the rolling surfaces of the bearing to prevent direct metal-to-metal contact.
The effect of lubrication on rolling bearings is as follows:
1. Reduce friction and wear and extend the life of bearings;
2. Dissipate frictional heat to prevent excessive temperature rise of bearings;
3. Prevent foreign matter from invading and play a sealing role;
4. Prevent metal corrosion.
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Comparison of grease lubrication and oil lubrication
Almost all rolling bearings are lubricated with grease or oil, and some special applications are lubricated with solid lubricants such as platinum disulfide and graphite. Among grease lubrication and oil lubrication, grease lubrication is the most used. About 80% of rolling bearings are grease lubrication. Compared with oil lubrication, grease lubrication has the following advantages:
1. The lubrication device is simple;
2. The grease is not easy to leak, and the sealing structure of the bearing is simple.
3. The bearing is easy to maintain and maintain;
4. Grease has a sealing effect, which can prevent external dust, moisture and other impurities from intruding into the bearing;
5. It is easy to improve the cleanliness of mechanical devices. The disadvantage of grease lubrication is that the bearing friction is large, the heat dissipation is not good, and the allowable speed is relatively low.
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Compared with grease lubrication, oil lubrication has the following advantages:
1. Suitable for high-speed rotation;
2. High temperature stability, suitable for use at high temperatures;
3. Good heat dissipation, oil lubrication can play a cooling role;
4. Easy to remove abrasive particles and debris inside the bearing;
5. It can share a lubricant with the parts around the bearings such as gears to implement common lubrication.
Selection of lubricant
Choosing the right lubricant is a complicated task. In addition to some necessary quantitative calculations (such as oil film lubrication parameters), the following factors should also be considered comprehensively:
1. The working temperature, rotational speed of the bearing and the viscosity of the load lubricant have a great influence on the thickness of the oil film. The lower the viscosity of the lubricating oil, the worse the bearing capacity of the oil film and the easier the oil film to break. In order to form the required oil film thickness, the lubricant must have a certain viscosity at the working temperature. The lubricant viscosity decreases with the increase of temperature. At the same speed, the greater the viscosity and the greater the friction, the higher the temperature, but the lower the working viscosity of the lubricant. Therefore, when the speed is high, select a lubricating oil with a lower viscosity or a grease with a lower base oil viscosity. Bearing load affects the thickness of the oil film. The greater the load, the smaller the thickness of the oil film. Therefore, a lubricant with a lower viscosity should be used for light loads; a higher viscosity should be used for heavy loads, and extreme pressure additives should be added.
2. The running performance requires the use of grease-lubricated bearings. When the friction is particularly small, or the starting friction torque is low at low temperatures, a grease with a similar viscosity composed of synthetic base oil and a large penetration should be selected. For small motor bearings that require low noise, filtered hard grease or base oil grease with high viscosity should be selected to reduce shaft bearing noise.
3. Environmental conditions Bearings working in a humid environment should use grease with good corrosion resistance. When the machinery working in a humid environment needs to be parked for a long time, the bearing surface will form condensate due to cooling after parking. To prevent rust, calcium-based grease that does not absorb water should be used. When the dust around the bearing is large, and a curved seal is used, it is hoped that the grease will also play a sealing role, and grease with a small penetration should be selected. When the bearing medium has a strong chemical action, grease with anti-corrosion additives should be used.
4. Grease life The grease life is not only sprinkled on the performance of the grease itself, but also related to external conditions such as temperature, speed, vibration, load, environment and so on. In disposable sealed bearings, the life of the grease used should be greater than the fatigue life of the dry bearings. In the application of bearings without relubrication replenishment device, the life of the grease used should be consistent with the overhaul of the entire mechanical device.