Bearings, as an indispensable and important component in mechanical products, play an important role in supporting the rotating shaft. According to the different friction properties in the bearing, the bearing is divided into rolling friction bearing (referred to as rolling bearing) and sliding friction bearing (referred to as sliding bearing). The two types of bearings have their own characteristics in structure and advantages and disadvantages in performance.
Table of Contents
rolling bearing
Rolling bearing is a precise mechanical element that converts the sliding friction between the running shaft and the shaft seat into rolling friction, thereby reducing friction loss. Rolling bearings are generally composed of four parts: inner ring, outer ring, rolling elements and cage. The function of the inner ring is to cooperate with and rotate with the shaft; the function of the outer ring is to cooperate with the bearing seat and play a supporting role; the rolling elements are evenly distributed between the inner ring and the outer ring by means of the cage, Its shape, size and number directly affect the performance and life of the rolling bearing; the cage can make the rolling elements evenly distributed, prevent the rolling elements from falling off, and guide the rolling elements to rotate for lubrication.
Plain bearing
Sliding bearings are bearings that work under sliding friction. Sliding bearings work smoothly, reliably and without noise. Under liquid lubrication conditions, the sliding surfaces are separated by lubricating oil without direct contact, which can greatly reduce friction loss and surface wear, and the oil film also has a certain ability to absorb vibration. There are many types of sliding bearings, which can be divided into radial (radial) sliding bearings and thrust (axial) sliding bearings according to the direction that can bear the load. Sliding bearing applications are generally under low speed and heavy load conditions, or operating parts where maintenance and lubricating oil are difficult. Domestically, the methods of repairing welding, inserting bushings, and pockmarking are generally used for sliding bearing wear.
Comparison of rolling and plain bearings
1 Comparison of structure and movement mode
The most obvious difference between rolling bearings and plain bearings is the presence or absence of rolling elements.
1) Rolling bearings have rolling elements (balls, cylindrical rollers, tapered rollers, needle rollers), which support the rotating shaft by rotation. The contact part is a point. The more rolling elements, the more contact points.
2) The sliding bearing has no rolling elements, and the rotating shaft is supported by a smooth surface, so the contact part is a surface.
The difference in structure between the two determines that the movement mode of the rolling bearing is rolling, and the movement mode of the sliding bearing is sliding, so the friction form is completely different.
2 Comparison of carrying capacity
Generally speaking, due to the large pressure bearing area of ​​sliding bearings, the bearing capacity of sliding bearings is generally higher than that of rolling bearings, and the ability of rolling bearings to withstand impact loads is not high, but fully liquid lubricated bearings can withstand more large shock loads. When the rotational speed is high, the centrifugal force of the rolling elements in the rolling bearing increases, and its carrying capacity should be reduced (noise is prone to occur at high speed). For hydrodynamic sliding bearings, the load-carrying capacity increases as the rotational speed increases.
3 Comparison of friction coefficient and starting frictional resistance
Under normal working conditions, the friction coefficient of rolling bearings is lower than that of sliding bearings, and the value is relatively stable. The lubrication of sliding bearings is easily affected by external factors such as rotational speed and vibration, and the friction coefficient varies widely.
When starting, since the sliding bearing has not yet formed a stable oil film, the resistance is greater than that of the rolling bearing, but the starting frictional resistance and the working friction coefficient of the hydrostatic sliding bearing are very small.
4 Applicable working speed comparison
Due to the limitation of the centrifugal force of the rolling elements and the temperature rise of the bearing, the rolling bearing cannot rotate too high, and is generally suitable for medium and low speed working conditions. Due to the heating and wear of the bearing, the working speed of the incomplete liquid lubricated bearing should not be too high. The high-speed performance of the completely liquid-lubricated bearing is very good, especially when the hydrostatic sliding bearing uses air as the lubricant, its speed can reach 100000r/min.
5 Power Loss Comparison
Due to the small friction coefficient of rolling bearings, the power loss is generally not large, which is smaller than that of incomplete liquid lubricated bearings, but it will increase sharply when lubrication and installation are improper. The friction power loss of completely liquid lubricated bearings is lower, but for hydrostatic sliding bearings, the total power loss may be higher than that of hydrodynamic sliding bearings due to the power loss of the oil pump.
6 Comparison of service life
Due to the influence of material pitting and fatigue, rolling bearings are generally designed for 5-10 years, or replaced during overhaul. The bearing pads of incomplete liquid lubricated bearings are severely worn and need to be replaced regularly. The life of a fully liquid lubricated bearing is theoretically infinite, but in practice, due to stress cycles, especially for hydrodynamic sliding bearings, the bearing pad material may experience fatigue failure.
7 Comparison of Rotation Accuracy
Due to the small radial clearance of rolling bearings, the rotation accuracy is generally high. Incomplete liquid lubricated bearings are in the state of boundary lubrication or mixed lubrication, and the operation is unstable, the wear is serious, and the precision is low. Completely liquid lubricated bearings have high precision due to the presence of oil film, buffering and vibration absorption. The hydrostatic sliding bearing has higher rotation accuracy.
8 Other comparisons
Rolling bearings use oil, grease or solid lubricant. The dosage is very small, and the dosage is large at high speed. The cleanliness of the oil is required to be high, so it is required to be sealed, but the bearing is easy to replace and generally does not need to repair the journal. For sliding bearings, except for incompletely lubricated bearings, the lubricant is generally liquid or gas, and the amount is large, and the cleanliness of the oil is also required. The bearing bush needs to be replaced frequently, and sometimes the journal is repaired.
Selection of rolling bearings and sliding bearings
Due to the complex and diverse actual working conditions, there is no unified standard for the selection of rolling bearings and sliding bearings. Due to the small friction coefficient, low starting resistance, sensitivity, high efficiency, and standardized, rolling bearings have excellent interchangeability and versatility, and are very convenient to use, lubricate and maintain. widely used. Sliding bearings themselves have some unique advantages, and are generally used in some occasions where it is impossible, inconvenient or without advantages to use rolling bearings, such as the following occasions:
1 The radial space size is limited, or split installation is required
Because of the inner ring, outer ring, rolling body and cage in the structure of rolling bearing, the radial size is large, and the application is limited. When the radial size requirements are strict, needle roller bearings can be selected. When necessary, sliding bearings need to be selected. For bearings that are inconvenient, or cannot be installed from the axial direction, and parts that must be installed separately, split sliding bearings are used.
2 high-precision occasions
When the bearings used have high precision requirements, sliding bearings are generally selected, because the lubricating oil film of the sliding bearings can buffer and absorb vibration. When the precision requirements are extremely high, only hydrostatic sliding bearings can be selected. For precision and high-precision grinding machines, various precision instruments, etc., sliding bearings are widely used.
3 Heavy duty occasions
Rolling bearings, whether they are ball bearings or roller bearings, are prone to heat and fatigue in heavy-load applications. Therefore, when the load is large, sliding bearings are often used, such as rolling mills, steam turbines, aero-engine accessories and mining machinery.
For a kind of machinery and equipment, the application of rolling bearings and sliding bearings has advantages and disadvantages, and should be selected reasonably according to the actual engineering. In the past, large and medium-sized crushers generally used sliding bearings cast with babbitt alloys, because they can withstand large impact loads, and are relatively wear-resistant and stable. Small jaw crushers use rolling bearings, which have high transmission efficiency, are more sensitive, and are easy to maintain. With the improvement of rolling bearing manufacturing technology, most large jaw crushers also use rolling bearings.
