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Bearings are very important components in modern mechanical equipment. Their main function is to support the rotating body of the machinery and reduce the friction coefficient during its movement. As long as the rotating equipment is inseparable from bearings, bearings are closely related to our lives. From bicycles, cars, high-speed trains, and airplanes, bearings play an important role.

Bearings are generally divided into two categories, namely rolling bearings and sliding bearings.

Rolling bearing is a kind of precise mechanical element that changes the sliding friction between the main shaft and the shaft seat into rolling friction to reduce friction loss. Rolling bearings are generally composed of four parts: inner ring, outer ring, rolling element, and cage. The function of the inner ring is to match and rotate with the shaft; The outer ring is used to cooperate with the bearing seat and play a supporting role; Rolling elements are evenly distributed between the inner and outer rings using a cage, and their shape, size, and quantity directly affect the performance and lifespan of rolling bearings; The cage can evenly distribute the rolling element and guide it to rotate for lubrication.

Sliding bearing is a bearing that works under sliding friction. Under the condition of liquid lubrication, the sliding surface is separated by lubricating oil without direct contact, which can also greatly reduce friction loss and surface wear. The oil film also has a certain vibration absorption capacity. The application of sliding bearing is generally under the condition of high speed and light load.

Compared with sliding bearings, the friction force of rolling bearings is much smaller than that of sliding bearings. Rolling bearings are easy to maintain and have reliable quality, so their applications are more extensive than sliding bearings.

When selecting bearings, it is important to consider several important parameters: bearing load, bearing speed, centering requirements, allowable space, bearing installation and disassembly, tolerance level, and price. Bearings mainly play a supporting and reducing friction role in mechanical equipment, so the accuracy and noise of bearings are directly related to the stable operation and service life of mechanical equipment.

Generally speaking, the steps for selecting bearings include:

1. Select the basic type, tolerance level, and clearance of bearings based on their working conditions, including load direction and type, speed, lubrication method, coaxiality requirements, positioning or non positioning, installation and maintenance environment, environmental temperature, etc;

2. Determine the bearing model through calculation based on the working conditions, stress conditions, and service life requirements of the bearing, or select the bearing model according to the usage requirements, and then verify the service life;

3. Verify the rated load and ultimate speed of the selected bearing.

The main considerations for selecting bearings are ultimate speed and load capacity, while other factors can help determine bearing type, structure, size, tolerance level, and clearance requirements.

7 factors to consider when selecting bearings:

Various types of rolling bearings have different characteristics and are suitable for various mechanical applications. In general, thrust bearings and angular contact bearings are selected when bearing thrust loads; Ball bearings are commonly used in high-speed applications; When bearing heavy radial loads, roller bearings are selected.

1. The space and position of the machinery occupied by the bearings

In mechanical design, the size of the shaft is usually determined first, and then rolling bearings are selected based on the size of the shaft. Usually, ball bearings are used for small shafts and roller bearings are used for large shafts. However, when the bearing is limited in the diameter direction of the machine, needle roller bearings, ultra light and ultra light series ball or roller bearings are selected; When the axial position of the bearing in the machine is limited, narrow or ultra narrow series ball or roller bearings can be selected.

2. The magnitude, direction, and nature of the load on the bearing

Load is the most important factor in selecting bearings. Roller bearings are used to bear heavier loads, while ball bearings are used to bear lighter or medium loads. Bearings made of carburized steel or quenched with bainite can bear shock and vibration loads.

In the direction of load action, when bearing pure radial load, Deep Groove Ball Bearing, cylindrical roller bearing or needle roller bearing can be selected. When bearing small pure axial loads, thrust ball bearings can be selected; When bearing large pure axial loads, thrust roller bearings can be selected. When bearings are subjected to combined radial and axial loads, angular contact ball bearings or tapered roller bearings are generally chosen.

3. Centering performance of bearings

When the centerline of the shaft is different from the centerline of the bearing seat, there is an angle error, or the rigidity of the shaft is small due to the large spacing between the two supports of the shaft, which is prone to bending or tilting under force, self-aligning ball or self-aligning roller bearings with good self-aligning performance, as well as outer ball bearings, can be selected. This type of bearing can maintain normal operation when the shaft is slightly tilted or bent.

The performance of bearing centering is related to the different allowable axial degrees. The larger the values of different axial degrees, the better the centering performance.

4. Rigidity of bearings

The rigidity of a bearing refers to the amount of force required for the bearing to produce unit deformation. The elastic deformation of rolling bearings is very small and can be ignored in most machinery. However, in some machinery, such as machine tool spindles, bearing rigidity is an important factor, and cylindrical and tapered roller bearings should generally be selected. Because these two types of bearings are in point contact with the raceway when subjected to load, their rigidity is poor.

In addition, various types of bearings can also achieve the goal of increasing support rigidity through pre tightening. For example, angular contact ball bearings and tapered roller bearings, in order to prevent shaft vibration and increase support rigidity, a certain axial force is often applied in advance during installation to compress each other. However, the preload should not be too large, otherwise it will increase the friction and temperature rise of the bearing, affecting its service life.

5. Speed of bearings

Each bearing model has its own limit speed, which is determined by physical characteristics such as size, type, and structure. Exceeding this limit can cause the bearing temperature to rise, the lubricant to dry up, and even the bearing to become stuck. Practice has proven that working below 90% of the limit speed is relatively good.

The limit speed of grease lubricated bearings is lower than that of oil lubricated bearings, and the oil supply method of bearings has an impact on the achievable limit speed. It must be noted that for grease lubricated bearings, the limit speed is generally only 80% of the limit speed when using a high-quality repeated circulation oil system, but for oil mist lubrication systems, the limit speed is generally 50% higher than the same basic lubrication system. The design and structure of the cage also affect the ultimate speed of the bearing.

6. Bearing movement and axial displacement

Usually, a shaft is supported by two bearings at a certain distance apart. In order to adapt to the varying degrees of thermal expansion of the shaft and casing, one bearing should be fixed in the axial direction during installation, and the other bearing should be movable on the shaft (i.e., movable support) to prevent jamming caused by shaft elongation or contraction.

7. Easy installation and disassembly of bearings

When selecting bearing types, it is also necessary to consider whether installation and disassembly are convenient, especially for large and extra large bearings.

Bearings are important components in mechanical equipment. Reasonable selection, correct operation and maintenance can effectively improve the performance and service life of bearings and equipment.