How to choose bearings correctly? When choosing bearings, several important parameters should be based on: bearing load, bearing speed, aligning requirements, allowable space, bearing installation and disassembly, tolerance class and price. Bearings in machinery mainly play a role in supporting and reducing friction, so the accuracy of bearings, noise and so on are directly related to the use and life of machinery.
In general, the steps to select bearings may be summarized as:
1. Select the basic type, tolerance level and clearance of bearings according to the working conditions of bearings, including load direction and type, speed, lubrication mode, coaxiality requirement, positioning or non-positioning, installation and maintenance environment, ambient temperature, etc.;
2. Determine the bearing type by calculation according to the working conditions, stress conditions and life requirements of the bearing, or select the bearing type according to the use requirements, and then check the life;
3. Check the rated load and limit speed of the selected bearing.
The main considerations in selecting bearings are the ultimate speed and load capacity, while other factors help to determine the final scheme of bearing type, structure, size and tolerance class and clearance requirements.
There are seven factors to consider in choosing bearings
Various kinds of rolling bearings have different characteristics and are suitable for different use of various machines. In general, thrust bearing and angular contact bearing are used when bearing thrust load. Ball bearings are usually used for high-speed applications, and roller bearings are selected when bearing heavy radial loads.
1. the space and position of the machine occupied by the bearing
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, the small shaft chooses ball bearings, and the large shaft chooses roller bearings. However, when the bearing is limited in the diameter direction of the machine, needle roller bearings, 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 extra-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 bear heavy loads, ball bearings are used to bear light or medium loads, carburized steel or bainite quenched bearings can bear impact and vibration loads.
In terms of the direction of action of the load, deep groove ball bearings, cylindrical roller bearings or needle roller bearings can be selected when the pure radial load is borne. When bearing small pure axial load, thrust ball bearing can be selected; When bearing larger pure axial load, thrust roller bearing can be selected. When bearings bear radial and axial combined loads, angular contact ball bearings or tapered roller bearings are generally selected.
3. Self-aligning performance of bearings
When the center line of the shaft is different from the center line of the bearing seat, there is an Angle error, or because the distance between the two supports of the shaft is large and the rigidity of the shaft is small, and the force is easy to bend or tilt, the self-aligning ball or self-aligning roller bearing with good self-aligning performance can be selected, as well as the outer ball bearing. This kind of bearing can maintain normal work under the condition that the shaft is slightly tilted or bent.
The self-aligning performance of bearings is related to the allowable different axiality. The greater the value of different axiality, the better the self-aligning performance.
4. Rigidity of bearings
The rigidity of the bearing refers to the size of the force required to produce unit deformation of the bearing. The elastic deformation of rolling bearings is very small, and it can not be considered in most machinery, but in some machinery, such as machine tool spindle, bearing rigidity is an important factor, and cylindrical and tapered roller bearings should be generally selected. Because these two kinds of bearings in the load, the rolling body and the raceway belong to point contact, rigidity is poor.
In addition, all kinds of bearings can also be pretightened to achieve the purpose of increasing the bearing rigidity. Such as angular contact ball bearings and tapered roller bearings, in order to prevent the vibration of the shaft, increase the bearing rigidity, often in the installation of a certain axial force is applied in advance, so that they compress each other. It is specially pointed out here that the preload should not be too large. If it is too large, it will increase the friction of the bearing, increase the temperature rise, and affect the service life of the bearing.
5. Speed of bearing
Each bearing type has its own limit speed, which is determined by physical characteristics such as size, type and structure. Limit speed refers to the maximum working speed of the bearing (usually r/min), exceeding this limit will cause the temperature of the bearing to rise, the lubricant to dry up, or even make the bearing stuck.
The speed range required for the application helps determine the type of bearing to be used. Most bearing manufacturers’ product catalogs provide the limit speed value of their products. Practice has proved that it is better to work under the condition of 90% lower than the limit speed.
The limit speed of the grease lubricated bearing is lower than that of the oil lubricated bearing, and the oil supply mode of the bearing has an effect on the limit speed of the bearing. 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 lubricated 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, because the rolling body and the cage surface is sliding contact, with a more expensive, reasonable design, with high quality and low friction material made of the cage, not only can be separated from the rolling body, but also help to maintain the lubricating oil film in the sliding contact area.
But inexpensive cages, such as stamping cages, usually only keep the rollers separate. As a result, they are prone to accidents and annoying sliding contacts, resulting in lower limit RPM.
Generally speaking, deep groove ball bearings, angular contact bearings and cylindrical roller bearings should be selected at higher speed. Tapered roller bearings can be selected in low speed working occasions. The limit speed of tapered roller bearings is generally about 65% of deep groove ball bearings, 70% of cylindrical roller bearings and 60% of angular contact ball bearings. Thrust ball bearing limit speed is low, can only be used in low speed occasions.
For the same type of bearing, the smaller the size, the higher the allowable speed. When choosing bearings, attention should be paid to making the actual speed lower than the limit speed.
6. Bearing swimming and axial displacement
Normally, a shaft is supported by two bearings separated by a certain distance. In order to adapt to the influence of different degrees of thermal expansion of the shaft and the shell, the installation should be fixed in the axial direction of one bearing, the other bearing can be moved on the shaft (namely moving support), in order to prevent the phenomenon of sticking due to the elongation or contraction of the shaft.
The cylindrical roller bearings with inner ring or outer ring without flanges (original 2000, 32000) and needle roller bearings are usually used for swimming support, which is mainly because the internal structure of such bearings allows proper axial displacement between the shaft and the shell. At this time, the inner ring and the shaft, the outer ring and the shell hole can be tightly matched. When non-detachable bearings are used as swimming support, such as deep groove ball bearings and self-aligning roller bearings, the outer ring and casing holes must be allowed in the installation, or the inner ring and shaft must be loosely matched, so that the axial direction can be free swimming.
Tapered roller bearings, self-aligning roller bearings and deep groove ball bearings basically belong to the positioning type. When used for non-positioning, they are installed with loose fit. All thrust roller bearings belong to positioning type bearings.
7. Easy to install and disassemble bearings When choosing bearing types, it is also necessary to consider whether the installation and disassembly of bearings is convenient, especially for the installation and disassembly of large and extra-large bearings. General outer ring separable angular contact ball bearings, tapered roller bearings, cylindrical roller bearings and needle bearings, easy to install and disassemble, their inner and outer rings can be mounted on the shaft or shell hole respectively. In addition, the inner diameter with tapered hole, self-aligning roller bearings with tight set sleeve, double row cylindrical roller bearings and self-aligning ball bearings are also easier to install and disassemble.