Hot tags: rolling bearing (2)
Abstract: the factors that should be paid attention to in the application of precision rolling bearings are summarized, such as accuracy, stiffness, load, speed, life, preload, lubrication and sealing, etc; And the application examples are given to give a specific explanation.
Key words: precision rolling bearing; Precision shafting; rigidity; roller
1 Introduction
The correct application of precision rolling bearings in precision shafting involves the characteristics of different types of bearings and many external factors that affect the accuracy of bearings. If not carefully considered, the significance of using precision bearings will be lost. Especially when the spindle rotates at high speed and requires high precision, the related factors of precision rolling bearing are more prominent.
2 Factors
2.1 accuracy
When using precision rolling bearings, only when the form and position tolerance accuracy and surface roughness of the shaft and bearing seat are consistent with the bearing accuracy, can their efficiency be fully exerted. That is to say, the accuracy of mounting components is the main factor affecting the rotation accuracy of bearings. However, too high a requirement for the accuracy of mounting components will cause difficulties in machining. In the application, reasonable selection shall be made based on the recommended value in the mechanical design manual and combined with practical experience. The dimensional accuracy of the bearing should not be ignored. This factor should be taken into consideration when selecting the fit between the mounting parts and the bearing. The selected fit will affect the bearing clearance or the preload of the bearing.
2.2 rigidity
The stiffness of the bearing depends on its type and size. The main judgment criteria are: roller element type (such as spherical or cylindrical), roller number, size and contact angle.
The influence of roller number on stiffness is greater than that of bearing size. The bearing stiffness increased by increasing the number of rollers with the same percentage is greater than that increased by increasing the bearing size. For this reason, precision rolling bearings increase the rigidity by increasing the number of rollers.
The influence of the contact angle on the stiffness is generally that if the contact angle is small, the radial high stiffness is satisfied; If the contact angle is large, the axial high rigidity is satisfied. When arranging bearings, installing two or more bearings at the same position can improve the rigidity. Angular contact ball bearings are most suitable for such use and are often used in pairs.
The bearing stiffness is also affected by the main shaft and the bearing seat, and its fitting tolerance should be carefully selected; The stiffness of the paired bearing set is also affected by the preload.
Bearing stiffness is expressed as the ratio of elastic deformation and recovery degree under load. Because the elastic recovery of rolling bearing is not linear with the load, it is difficult to describe it with a single function; In application, the linear relationship can be used for approximate estimation.
2.3 load
When the precision rolling bearing is used in the precision shaft system of the equipment, the load capacity of the bearing is not the only key factor to be considered in the selection of the bearing, but the design requirements of the shaft determine the inner diameter of the bearing. Other key factors such as bearing stiffness, allowable speed and rotation accuracy are decisive factors.
In the application, when selecting the bearing type and arrangement form, the basic rules are as follows: roller bearings with the same size have greater bearing capacity than ball bearings; Angular contact ball bearings and tapered roller bearings can withstand complex radial and axial loads; Two way angular contact thrust ball bearing can bear two-way load.
2.4 speed
The rolling speed of the bearing can be adjusted under the allowable temperature; Precision rolling bearings have low friction characteristics, thus forming low heat characteristics. Therefore, precision rolling bearings can be used for high-speed operation. The heat generated by the bearing largely depends on the working speed, and is also affected by the bearing type, lubrication mode, preload and working load.
2.5 service life
In application, as a basic requirement, the selected bearing should have a long life. Measuring the basic life of precision bearings mainly includes two parameters: static load capacity and acting load. In addition, the influence of preload on bearing life and the influence of main shaft strength and bearing self factors should be considered.
For high-speed operation, the spindle should have high wear resistance; The centrifugal force generated by the roller elements will also affect the load situation and shorten the service life of the bearing. For low-speed rotation or slow swing, the load on the bearing can be considered as equal static load without any influence of motion characteristics. In this case, the permanent deformation of the roller and the rail due to the load will affect the service life; At the same time, the bearing must withstand the periodic heavy impact load.
2.6 preload
Preload can not only improve the stiffness of the bearing, but also improve its rotational accuracy. Preload bearings and paired bearing sets are generally used in the application of equipment spindles.
For single row angular contact ball bearings and tapered roller bearings, the preload is usually obtained by adjusting the axial clearance between the inner and outer rings; For cylindrical roller bearings, the preload can be obtained through proper installation of the tapered hole of the inner ring; For the two-way angular contact thrust ball bearing, adjust the size of the isolation sleeve, and the preload will be obtained during installation.
Under the action of external force, the load distribution inside the bearing will change. Some roller elements will bear excessive load, and at the same time, other roller elements will be released, and the elastic deformation will change, as shown in Fig. 1.
Fig. 1 elastic deformation curve
Fig. 1 illustrates that the same elastic deformation occurs under the same preload F0; Under the action of external force Fa, bearing A generates δ a is deformed and the deformation of bearing B is reduced; Continue to increase the applied external force to Fa ', and the preload elastic deformation of bearing B is completely released. This is not conducive to the use of the bearing. During the application, attention should be paid to the size of the external force, so as to select the appropriate preload.
On the other hand, the maximum value of bearing preload should be limited. Because the friction will increase after the preload is increased, resulting in temperature rise.
For the paired bearing group, the determination of the preload is complex; Its purpose is to adapt to different requirements such as accuracy, stiffness and speed.
2.7 lubrication
The purpose of lubrication of rolling bearing is to prevent the roller elements from direct contact wear and rust. For precision rolling bearings, this factor deserves more attention.
In order to form a lubricating oil film between the roller elements and the raceway, only a small amount of lubricating oil is required. Grease lubrication is the most simple and universal choice; However, for high-speed spindle bearings, oil lubrication must be used, because in this case, the service life of grease is very short, as shown in Fig. 2.
A: precision angular contact ball bearing
B: matched precision angular contact ball bearing set
C: Double row cylindrical roller bearing; Two way angular contact thrust ball bearing
D: tapered roller bearing
Fig. 2 illustrates that the normal working time of the grease lubrication mode is related to the type, size and speed of the bearing. When using grease lubrication method, pay attention to the selection of grease type, the calculation of the amount of grease loaded into the bearing cavity and the short low-speed test run. In actual operation, the trial run stage should not be neglected so as to make the grease evenly distributed and extrude excessive grease.
Fig. 2 lubrication life analysis
Fig. 3 illustrates the relationship between the amount of lubricating oil and bearing temperature and friction. Area A: insufficient lubricating oil, no effective protective film can be formed between roller element and track, and T and WF will increase rapidly. Area B: the amount of lubricating oil is just right, and T and WF are minimum. Area C: when the amount of lubricating oil is increased, both T and WF will increase, and the increase of T will be greater. Area D: if the oil quantity continues to increase, WF will increase, and T will slowly decrease after reaching the highest point. A balance is formed between heat dissipation and friction. Area E: continue to increase the oil quantity, and T will drop rapidly after point m, because the cooling of oil plays a dominant role. B reflects the oil injection lubrication mode, and D represents the characteristics of oil circulation lubrication.
Fig. 3 Analysis of lubricating oil quantity
When using oil lubrication, attention should be paid to the selection of methods and the determination of oil viscosity and oil quantity. It shall be reasonably selected according to the specific environment and conditions.
2.8 sealing
When using precision rolling bearings, they must be effectively sealed between the inside and the outside; The following factors shall be considered, including environmental conditions, lubrication mode, seal surrounding speed, seal friction and temperature rise caused by seal friction.
For precision rolling bearings, the last two factors are more important. Unacceptable heat of the bearing is likely to be generated due to seal friction. Therefore, non rubber seals are often used in high-speed precision rolling bearings.
The sealing features of the non rubber seal are formed by its end cap structure and can be used in axial, radial or axial and radial combination. This seal is free of friction and wear.
The rubber seal is in contact with the sealing surface. Due to the contact friction, the temperature will rise, and the use of the rubber seal is limited. This form is rarely used for sealing of precision rolling bearings; This form is only suitable for low-speed situations.
The combination of rubber seal and non rubber seal can improve the sealing effect. In practical application, when the critical speed is less than 12 m / s, V-shaped rubber seal can be used. The friction generated by this application is very small, and the installation is simple and the use is economical.
3 Application examples
Fig. 4 shows the spindle structure of an electronic special equipment. The rotation speed is 2 840 r / min, the main shaft end jump is 0.005 mm, and the main shaft diameter jump is 0.005 mm. It is subject to axial and radial loads.
Because of the high requirements for end jump and radial jump of the shaft and the medium speed, the precision angular contact ball bearing is adopted. The bearings are used in pairs and are divided into two groups. One group is axially fixed and the other group is axially outer ring relaxed, so as to eliminate the influence of shaft deformation caused by temperature rise on the normal operation of the bearing. The arrangement is beneficial to improve the stiffness of the shaft system and solve the joint action of complex loads. During assembly, the spacer ring is repaired to obtain the preload. The shaft system adopts a combination of non-contact seal and rubber seal; Grease lubrication is adopted.
Figure 4 spindle structure
1. Lock nut 2. Locating ring 3. Seal (non-contact) 4. Sealing ring 5. Precision angular contact ball bearing 6. Bearing seat 9. Main shaft
4 Concluding remarks
Precision rolling bearings are widely used in electronic professional equipment; The key to the design of precision rolling bearing system is the rational application of precision rolling bearing. In this paper, several aspects that should be paid attention to in the application of precision rolling bearings are summarized, and the viewpoints in this paper are expounded through application examples; Only when the precision rolling bearing is applied correctly, can it exert its excellent performance.
More about VISTA High quality Rolling Bearing:
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