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As an essential part of mechanical products, bearing plays an important role in supporting rotating shaft. According to the different friction properties in bearings, bearings are divided into rolling friction bearings (hereinafter referred to as rolling bearings) and sliding friction bearings (hereinafter referred to as sliding bearings). The two types of bearings have their own characteristics in structure and advantages and disadvantages in performance. When selecting, they should be comprehensively considered in combination with practice.
The obvious difference between rolling bearing and sliding bearing is whether there is a rolling element. Rolling bearings have rolling elements (ball, cylindrical roller, tapered roller and needle roller), which support the rotating shaft by its rotation. Therefore, the contact part is a point. The more rolling elements, the more contact points. The sliding bearing has no rolling element and supports the rotating shaft by a smooth surface, so the contact part is a surface. The difference between the two structures determines that the motion mode of rolling bearing is rolling, and the motion mode of sliding bearing is sliding, so the friction situation is completely different. Why do most modern mining machinery choose rolling bearings instead of sliding bearings?
Generally speaking, due to the large bearing area of sliding bearing, its bearing capacity is generally higher than that of rolling bearing, and the ability of rolling bearing to bear impact load is not high. However, fully liquid lubricated bearing can bear large impact load because the lubricating oil film plays the role of buffer and vibration absorption. When the speed is high, the centrifugal force of the rolling element in the rolling bearing increases, and its bearing capacity should be reduced (noise is easy to occur at high speed). For hydrodynamic sliding bearing, its bearing capacity increases with the increase of rotating speed.
Under general working conditions, the friction coefficient of rolling bearing is lower than that of sliding bearing, and the value is stable. The lubrication of sliding bearing is easily affected by external factors such as speed and vibration, and the variation range of friction coefficient is large. When starting, because the sliding bearing has not formed a stable oil film, the resistance is greater than that of the rolling bearing, but the starting friction resistance and working friction coefficient of the hydrostatic sliding bearing are very small.
Due to the limitation of the centrifugal force of the rolling element and the temperature rise of the bearing, the rotating speed of the rolling bearing can not be too high. It is generally suitable for the working state of medium and low speed. The working speed of the incompletely liquid lubricated bearing cannot be too high due to the heating and wear of the bearing. The high-speed performance of fully liquid lubricated bearing is very good, especially when the hydrostatic sliding bearing uses air as lubricant, its speed can reach 100000 R / min.
Because the friction coefficient of rolling bearing is small, its power loss is generally small, which is less than that of incomplete liquid lubricated bearing, but it will increase sharply when it is not lubricated and installed properly. The friction power loss of fully liquid lubricated bearing is low, but for hydrostatic sliding bearing, due to the power loss of oil pump, the total power loss may be higher than that of hydrodynamic sliding bearing.
Due to the influence of material pitting and fatigue, the general design life of rolling bearing is 5 ~ 10 years, or it can be replaced during overhaul. The bearing bush of incomplete liquid lubricated bearing is seriously worn and needs to be replaced regularly. The life of fully liquid lubricated bearings is theoretically infinite. In fact, due to stress cycle, especially dynamic pressure sliding bearings, the bearing bush materials may be fatigue damaged.
Due to the small radial clearance, the rotation accuracy of rolling bearing is generally high. The incompletely liquid lubricated bearing is in the state of boundary lubrication or mixed lubrication, the operation is unstable, the wear is serious and the accuracy is low. Fully liquid lubricated bearing has high precision due to the existence of oil film, cushioning and vibration absorption. Hydrostatic sliding bearing has higher rotation accuracy.
The rolling bearing uses oil, grease or solid lubricant in a small amount, which is large at high speed and requires high oil cleanliness. Therefore, it requires sealing, but the bearing is easy to replace and generally does not need to repair the journal. For sliding bearings, in addition to incomplete liquid lubrication bearings, the lubricant is generally liquid or gas, with large amount and high requirements for oil quality and cleanliness. The bearing bush needs to be replaced frequently, and sometimes the journal is repaired.
Due to the complex and diverse actual working conditions, there is no unified standard for the selection of rolling bearings and sliding bearings. Rolling bearings are widely used in general machines because of their small friction coefficient, small starting resistance, sensitivity, high efficiency, standardization, excellent interchangeability and universality, and convenient use, lubrication and maintenance. Sliding bearing itself has some unique advantages. It is generally used in some occasions where it is impossible, inconvenient or has no advantages to use rolling bearing, such as the following occasions:
Due to the large radial size of rolling bearing with inner ring, outer ring, rolling element and cage, its application is limited. When the radial dimension requirements are strict, needle roller bearings can be selected, and sliding bearings need to be selected if necessary. For bearings that are inconvenient or cannot be installed axially, and parts that must be installed separately, split sliding bearings shall be selected.
When the bearings used have high precision requirements, sliding bearings are generally selected, because the lubricating oil film of sliding bearings can buffer and absorb vibration. When the precision requirements are very high, only hydrostatic sliding bearings can be selected. For precision and high-precision grinding machines and various precision instruments, sliding bearings are widely used.
Rolling bearings, whether ball bearings or roller bearings, are prone to heating and fatigue in heavy-duty occasions. Therefore, when the load is large, sliding bearings are mostly used, such as rolling mill, steam turbine, aeroengine accessories and mining machinery.
For example, sliding bearings can also be reasonably selected for occasions such as extremely high working speed, extremely large impact and vibration, and working in water or corrosive media.
For a kind of machinery and equipment, the application of rolling bearing and sliding bearing has its own advantages and disadvantages, which should be reasonably selected in combination with engineering practice. In the past, large and medium-sized crushers generally used sliding bearings cast with Babbitt alloy, because they can bear large impact load, wear-resistant and stable. The small jaw crusher uses multi-purpose rolling bearings, which has high transmission efficiency, more sensitivity and convenient maintenance. With the improvement of rolling bearing manufacturing technology, rolling bearings are mostly used in large jaw fracture.