1. According to the working conditions of bearings (including load direction and load type, rotating speed, lubrication mode, coaxiality requirements, positioning or non-positioning, installation and maintenance environment, ambient temperature, etc.), select the basic type, tolerance level and clearance of bearings;
2. According to the working conditions, stress and service life requirements of the bearing, determine the bearing model through calculation, or select the bearing model according to the service requirements, and then check the service life;
3. Check the rated load and limit speed of the selected bearing.
the main factors to consider in selecting bearings are the limit speed, required life and load capacity, while other factors are helpful to determine the bearing type, structure, size, tolerance level and the final scheme of clearance calculation.
1. Type selection
Various types of rolling bearings have different characteristics and are suitable for different applications of various machines. When selecting the bearing type,
the following factors should generally be considered. Generally speaking, thrust bearing and angular contact bearing are selected for bearing thrust load, ball bearing is usually used for high-speed applications, and roller bearing is selected for bearing heavy radial load. In short, the selection personnel should choose the appropriate type from different manufacturers and numerous bearing products. Mechanical space and position occupied by bearing
In mechanical design, the size of shaft is generally determined first, and then rolling bearing is selected according to the size of 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 bearing, ultra-light and ultra-light series ball or roller bearing are selected; When the axial position of the bearing is limited, narrow or ultra-narrow series of ball or roller bearings can be selected. The magnitude, direction and nature of the load on the bearing
Load is the most important factor in selecting the bearing. Roller bearings are used to bear heavy loads, ball bearings are used to bear light or medium loads, and bearings made of carburized steel or quenched by bainite can bear impact and vibration loads.
in the direction of load, deep groove ball bearing, cylindrical roller bearing or needle roller bearing can be selected when bearing pure radial load. When bearing small pure axial load, thrust ball bearing can be selected; When bearing large pure axial load, thrust roller bearing can be selected. When the bearing bears the combined radial and axial loads, angular contact ball bearings or tapered roller bearings are generally selected. Self-aligning performance of bearings
When the center line of important official is different from the center line of the bearing seat, there is an angle error, or the shaft is less rigid due to the large distance between two bearings of the shaft, and it is easy to bend or tilt under stress, self-aligning ball or self-aligning roller bearing with good self-aligning performance and external ball bearing can be selected. This kind of bearing can keep normal operation under the condition that the shaft is slightly inclined or bent.
the self-aligning performance of a bearing is related to its allowable misalignment, and the greater the misalignment value, the better the self-aligning performance. See table 11 for allowable misalignment of various bearings. Stiffness of bearings < P > Stiffness of bearings refers to the force required for unit deformation of bearings. The elastic deformation of rolling bearings is very small, which can be ignored in most machines, but in some machines, such as the spindle of machine tools, 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 they are under load, their rigidity is poor.
in addition, all kinds of bearings can be preloaded to increase the supporting rigidity. 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 make them press each other. It is especially pointed out here that the pre-tightening amount should not be too large. If it is too large, it will increase bearing friction and temperature rise, which will affect bearing service life. Rotational speed of bearing
Each bearing model has its own limit rotational speed, which is determined by physical characteristics such as size, type and structure. The limit rotational speed refers to the highest working rotational speed of the bearing (usually r∕min). Exceeding this limit will lead to the bearing temperature rising, lubricant drying up and even the bearing jamming.
the speed range required in the application is helpful to decide what type of bearing to use. Figure 12 shows the typical speed range of most common bearings. D is the bearing size, which usually refers to the pitch diameter of the bearing. When selecting the bearing, use the average of the inner diameter and outer diameter of the bearing in mm.
Multiply the pitch diameter d by the shaft rotation speed (in r/min) to get a limit speed factor (DN), which is very important when selecting the bearing type and size. Most bearing manufacturers' catalogues provide the limit speed value of their products. Practice has proved that it is better to work under the condition of 9% of the limit speed.
the limit speed of grease-lubricated bearings is lower than that of oil-lubricated bearings, and the oil supply mode of bearings has an influence on the limit speed that can be achieved. Table 12 provides the limit speed correction coefficient (k) of several bearings in lubrication form. It must be noted that for grease lubricated bearings, the limit speed is generally only 8% of the limit speed when the bearing adopts a high-quality circulating oil system, but for oil mist lubrication system, the limit speed is generally 5% higher than that of the same basic lubrication system.
the design and structure of the cage also affect the limit speed of the bearing, because the rolling elements are in sliding contact with the surface of the cage. Using an expensive and reasonably designed cage made of high-quality and low-friction materials can not only separate the rolling elements, but also help to maintain the lubricating oil film in the sliding contact area. However, low-cost cages, such as stamping cages, usually only keep the rolling elements separated. Therefore, they are prone to accidents and disturbing sliding contact, which leads to lower limit speed.
generally speaking, deep groove ball bearing, angular contact bearing and cylindrical roller bearing should be selected in the working situation with high rotating speed; Tapered roller bearings can be used in low speed working occasions. The limit speed of tapered roller bearings is generally about 65% of deep groove ball bearings, 7% of cylindrical roller bearings and 6% of angular contact ball bearings. The limit speed of thrust ball bearing is low, so it can only be used in low speed situations.
for the same kind of bearing, the smaller the size, the higher the allowable speed. When selecting bearings, attention should be paid to making the actual speed lower than the limit speed. Bearing movement and axial displacement
Usually, a shaft is supported by two bearings at a certain distance. In order to adapt to the influence of different degrees of thermal expansion of the shaft and the shell, one bearing should be fixed in the axial direction and the other bearing should be allowed to swim on the shaft (that is, floating support) during installation, so as to prevent the jamming phenomenon caused by the extension or contraction of the shaft. Traveling bearings usually use cylindrical roller bearings (type 2, type 32) and needle bearings with inner ring or outer ring without flange, which is mainly because the internal structure of such bearings allows proper axial displacement between the shaft and the housing. At this time, the inner ring and the shaft, and the outer ring and the housing hole can be tightly matched. When non-separable bearings are used as the floating support, such as deep groove ball bearings and self-aligning roller bearings, it is necessary to allow loose fit between the outer ring and the shell hole or between the inner ring and the shaft during installation, so that they can freely swim in the axial direction.
Figure 13 shows several cylindrical roller bearing structures with and without positioning
Tapered roller bearing, self-aligning roller bearing and deep groove ball bearing are basically positioning type, and when they are used as non-positioning, they are installed loosely. All thrust roller bearings are positioning bearings. Convenient installation and disassembly of bearings
When selecting bearing types, we must also consider whether it is convenient to install and disassemble bearings, especially for large and extra-large bearings. Generally, angular contact ball bearings, tapered roller bearings, cylindrical roller bearings and needle roller bearings with separable outer rings are more convenient to install and disassemble, and their inner rings and outer rings can be installed on the shaft or in the housing hole respectively. In addition, self-aligning roller bearings, double-row cylindrical roller bearings and self-aligning ball bearings with tapered holes in inner diameter and tight sleeves are also easier to install and disassemble. Other requirements
In addition to the above factors, the working environment temperature of the bearing, bearing seal and special requirements for friction torque, vibration and noise should also be considered.