1. Selection methods and steps
The correct selection of SKF rolling bearings has a very important effect on whether the main engine can obtain good working performance and prolong its service life; whether the enterprise can shorten the maintenance time, reduce the maintenance cost, and improve the operation rate of the machine. Therefore, whether it is a design and manufacturing unit or a maintenance and user unit, it must be highly regarded when selecting SKF rolling bearings. Generally speaking, the steps for selecting a bearing may be summarized as:
(1) According to SKF bearing working conditions (including load direction and load type, speed, lubrication method, coaxiality requirements, positioning or non-positioning, installation and maintenance environment, environmental temperature, etc.), select the basic type of bearing, tolerance level and travel Gap
(2) According to the working conditions, stress conditions and life requirements of SKF bearings, determine the bearing model through calculation, or select the bearing model according to the use requirements, and then check the life;
(3) Check the rated load and limit speed of the selected SKF bearing.
The main considerations for selecting SKF bearings are the limiting speed, required life and load capacity. Other factors help determine the final plan of SKF bearing type, structure, size and tolerance level and clearance requirements.
2. Type selection
All types of SKF rolling bearings have different characteristics and are suitable for different use cases of various machines. When selecting the bearing type,
The following factors should generally be considered. Under normal circumstances: SKF thrust bearings and SKF angular contact bearings are used for thrust loads, SKF ball bearings are usually used for high-speed applications, and SKF roller bearings are used for heavy radial loads. SKF 22309 E bearings online , pls click here :
(1) The space and location of machinery occupied by SKF bearings
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 SKF ball bearings are used for small shafts and SKF roller bearings are used for large shafts. However, when SKF bearings are restricted in the diameter direction of the machine, needle roller bearings, extra-light and ultra-light series ball or roller bearings are selected; when the axial position of the bearing in the machine is restricted, narrow or Ball or roller bearings in the special narrow series.
(2) The size, direction and nature of the load on the SKF bearing
Load is the most important factor in selecting bearings. Roller bearings are used to bear heavier loads, and ball bearings are used to bear light or medium loads. Bearings made of carburized steel or bainite quenched can bear shock and vibration loads.
In terms of the direction of the load, deep groove ball bearings, cylindrical roller bearings or needle bearings can be selected when bearing pure radial loads. When bearing a small pure axial load, a thrust ball bearing can be selected; when bearing a larger pure axial load, a thrust roller bearing can be selected. When the bearing is subjected to combined radial and axial loads, angular contact ball bearings or tapered roller bearings are generally used.
(3) Self-aligning performance of SKF bearings
When the center line of the shaft is different from the center line of the bearing seat, there is an angular error, or the shaft has a relatively large distance between the two supports and the shaft is less rigid and easy to be bent or tilted by force, an adjustment with good centering performance can be selected. Center ball or spherical roller bearings, and outer ball bearings. This type of bearing can maintain normal operation when the shaft is slightly inclined or bent.
The self-aligning performance of SKF bearings is related to the different axial degrees allowed. The greater the value of the different axial degrees, the better the self-aligning performance.
(3) Rigidity of SKF bearings
The rigidity of SKF bearings refers to the force required to produce unit deformation of the bearing. The elastic deformation of rolling bearings is very small, and it is not necessary to consider in most machines, but in some machines, such as machine tool spindles, bearing rigidity is an important factor. Generally, cylindrical and tapered roller bearings should be used. Because these two types of bearings bear the load, their rolling elements and raceways are in point contact, and the rigidity is poor.
In addition, various types of bearings can also be pre-tightened to achieve the purpose of increasing supporting rigidity. Such as angular contact ball bearings and tapered roller bearings, in order to prevent the vibration of the shaft and increase the supporting rigidity, a certain axial force is often applied in advance during installation to make them press against each other. It is particularly pointed out here that the preload amount cannot be too large. When it is too large, it will increase bearing friction and temperature rise, which will affect the service life of the bearing.
(4) Speed of SKF bearing
Each SKF bearing model has its own limit speed, which is determined by physical characteristics such as size, type and structure. The limit speed refers to the maximum working speed of the bearing (usually r/min), which exceeds this limit It will cause the bearing temperature to rise, the lubricant will dry up, and even cause the bearing to seize.
The design and structure of the cage also affect the limit speed of SKF bearings, because the rolling elements and the surface of the cage are in sliding contact. The cage made of relatively expensive, reasonably designed, high-quality and low-friction materials can not only change The rolling elements are separated and help maintain the lubricant film in the sliding contact area. But low-priced cages such as stamped cages usually only keep the rolling elements separated. Therefore, they are prone to accidents and distressing sliding contact, resulting in a lower limit speed.
(5) SKF bearing movement and axial displacement
Normally, a shaft is supported by two SKF bearings separated by a certain distance. In order to adapt to the different degrees of thermal expansion of the shaft and the housing, one bearing should be fixed in the axial direction during installation, and the other bearing should be able to move on the shaft (that is, floating support) to prevent extension or contraction of the shaft The stuck phenomenon caused by. The floating support usually uses cylindrical roller bearings with no ribs on the inner or outer ring (formerly 2000 and 32000) and needle roller bearings. This is mainly because the internal structure of this type of bearing allows proper axial displacement of the shaft and the housing. . At this time, the inner ring and the shaft, the outer ring and the housing hole can adopt a tight fit. When non-separable bearings are used for floating support, such as deep groove ball bearings and spherical roller bearings, the outer ring and the housing hole, or the inner ring and the shaft must be loosely matched during installation, so that the axial direction can be Free swimming.
Tapered roller bearings, spherical roller bearings and deep groove ball bearings are basically locating type. When used as non-locating, they are installed with loose fit. All thrust roller bearings are locating bearings.
(6) Facilitate the installation and removal of bearings
When selecting SKF bearing types, it is necessary to consider whether it is convenient to install and disassemble SKF bearings, especially for the installation and disassembly of large and super large bearings. General 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. Their inner and outer rings can be mounted on the shaft or in the housing hole respectively. In addition, spherical roller bearings with an adapter sleeve, double-row cylindrical roller bearings and self-aligning ball bearings with a tapered bore in the inner diameter are also easier to install and disassemble.
(7) Other requirements
In addition to the above factors, the working environment temperature of SKF bearings, bearing seals and special requirements for friction torque, vibration, and noise should also be considered.
3. Clearance selection
Clearance is an important factor for the normal operation of SKF rolling bearings, which is divided into axial clearance and radial clearance. Choosing proper clearance can make the load reasonably distributed among the SKF bearing rolling elements; it can limit the axial and radial displacement of the shaft (or housing) to ensure the rotation accuracy of the shaft; it can make the bearing work normally at the specified temperature ; Reduce vibration and noise, help to improve the life of the bearing. therefore. When selecting bearings, the appropriate bearing clearance must be selected.
When selecting SKF bearing clearance, the following aspects should be considered:
(1) The working conditions of SKF bearings, such as load, temperature, speed, etc.;
(2) Requirements for the performance of SKF bearings (rotation accuracy, friction torque, vibration, noise);
(3) When the SKF bearing has an interference fit with the shaft and housing hole, the bearing clearance will decrease;
(4) When SKF bearings are working, the temperature difference between the inner and outer rings causes the bearing clearance to decrease;
(5) Due to the different expansion coefficients of the shaft and housing materials, the SKF bearing clearance decreases or increases.
According to experience, the most suitable working clearance of ball bearings is close to zero; roller bearings should maintain a small amount of working clearance. In parts requiring good supporting rigidity, the bearing allows a certain value of preload. It is particularly pointed out here that the so-called working clearance refers to the clearance of the bearing under actual operating conditions. There is also a kind of clearance called original clearance, which refers to the clearance before the bearing is installed. The original clearance is greater than the installation clearance. Our choice of clearance is mainly to choose a suitable working clearance.
4. Form factor
Once the final value of the required rated load is determined, you must consider the external dimensions required for SKF bearing installation, including the installation and fixing of the bearing considered here. And the size of the shaft that matches the bearing with the required load rating. These factors help determine the bearing structure and size, and can also exclude certain bearing types. In some cases, due to space constraints, the choice is eliminated even before the rated load is determined.
For example, if SKF bearings are very narrow, ball bearings are the only type that can work here. Or, if you must use a bearing housing with a very small diameter, you need to install two or more SKF roller bearings on the same shaft to achieve sufficient load ratings.