What are the similarities and differences between sliding bearings and rolling bearings on motors, and how to choose them?

Bearings, as an indispensable and important part of 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.

1. Comparison of rolling bearings and sliding 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 rotate to support the rotating shaft, so the contact part is a point, and 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 situation 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 hydro-static sliding bearing uses air as the lubricant, its speed can reach 100000 r/min.

5. Comparison of power loss

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 hydro-static 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 to 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 hydro-static sliding bearing has higher rotation accuracy.

8. Comparison in other aspects

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.

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