Seven factors to consider when choosing bearings

1. The bearing occupies the space and position of the machine

In mechanical design, the size of the shaft is generally determined first, and then the rolling bearing is selected according to 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, the needle roller bearing, extra-light and ultra-light series of ball or roller bearings are selected; When the bearing is limited in the axial position of the machine, narrow or ultra-narrow series of ball or roller bearings can be selected.

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

Load is the most important factor in the selection of bearings. Roller bearings are used to withstand heavy loads, ball bearings are used to withstand light or moderate loads, carburized steel or bainite-hardened bearings can withstand shock and vibration loads.

In terms of the direction of load, when bearing pure radial load, deep groove ball bearings, cylindrical roller bearings or needle roller bearings can be selected. When bearing small pure axial load, the thrust ball bearing can be selected; When bearing large pure axial load, the thrust roller bearing can be selected. When bearings bear combined radial and axial loads, angular contact ball bearings or tapered roller bearings are generally selected.

3. Bearing aligning performance

When the center line of the shaft is different from the center line of the bearing seat, there is an Angle error, or the rigidity of the shaft is small due to the large distance between the two supports of the shaft, and it is easy to bend or tilt by force, the aligning ball or aligning roller bearing with good aligning performance can be selected, as well as the outer ball bearing. Such bearings can maintain normal operation when the shaft is slightly inclined or bent.

The aligning performance of the bearing is related to the allowable non-coaxiality, and the larger the non-coaxiality value, the better the aligning performance.

4. Bearing rigidity

The rigidity of the bearing refers to the force required for the bearing to produce unit deformation. The elastic deformation of rolling bearings is very small, in most machinery can not be considered, but in some machinery, such as machine tool spindle, bearing rigidity is an important factor, generally should choose cylindrical and tapered roller bearings. Because these two types of bearings are under load, their rolling body and raceway belong to point contact, and the rigidity is poor.

In addition, all kinds of bearings can also be pre-tightened to achieve the purpose of increasing the stiffness of the support. Such as angular contact ball bearings and tapered roller bearings, in order to prevent the vibration of the shaft and increase the rigidity of the support, a certain axial force is often applied in advance during installation to make them press each other. However, the amount of preload should not be too large, otherwise it will increase the bearing friction, increase the temperature rise, and affect the service life of the bearing.

5. Bearing speed

Each bearing model has its own limit speed, which is determined by physical characteristics such as size, type and structure, exceeding this limit will cause the bearing temperature to rise, the lubricant to dry out, and even the bearing to jam. Practice has proved that it is better to work under the condition of less than 90% of the limit speed.

The limit speed of grease lubricated bearing is lower than that of oil lubricated bearing, and the oil supply mode of bearing has influence on the limit speed that can be reached. It must be noted that for grease lubricated bearings, the limit speed is generally only 80% of the limit speed when the bearing uses 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 limit speed of the bearing.

6. Bearing swimming and axial displacement

Normally, a shaft is supported by two bearings spaced a certain distance apart. In order to adapt to the influence of different degrees of thermal expansion of the shaft and the housing, one bearing should be fixed in the axial direction when installing, and the other bearing can be moved on the shaft (that is, the floating support) to prevent the phenomenon of sticking caused by the extension or contraction of the shaft.

7. Easy to install and disassemble bearings

When selecting the bearing type, it is also necessary to consider whether the installation and disassembly are convenient, especially the installation and disassembly of large and extra-large bearings is particularly important.