The ball bearing isRolling bearingThe ball ball is installed in the middle of the inner steel ring and the outer steel ring, and can withstand largerload。Also calledBall bearing。
The ball bearing isRolling bearingThe ball ball is installed in the middle of the inner steel ring and the outer steel ring, and can withstand large loads.Also calledBall bearing。
Structure composition
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The ball bearing is mainly composed of four basic elements:ball（ball）、Inner ring（inner ring）、Outer ring(outer ring) andRetainerAlso calledHolder（cage or retainer）。Ball bearings for general industry meetAISI52100, ball and ring are usuallyHigh chromium steelmake,Rockwell hardness(Rockwell C - scale hardness) is about 61-65.The hardness of retainer is lower than that of ball and ring, and its materials are metal (such as:Medium carbon steel, aluminum alloy) or non-metal (such as:Teflon、PTEFPolymer materials).Ball bearing ratioJournalRotation of journal bearingFrictional resistanceTherefore, the temperature generated by friction will be lower at the same speed.
purpose
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The purpose of ball bearing is to determine the relative position of two parts (usually shaft and bearing seat), ensure their free rotation, and transfer the load between them.At high speeds, such as in gyro ball bearings, this use can be extended to include free rotation with little wear in the bearings.In order to achieve this state, two parts of the bearing can be separated by an adhesive fluid film called elastohydrodynamic lubrication film.Denhard (1966) pointed out that the elastohydrodynamic lubrication film can be maintained not only when the bearing bears the load on the shaft, but also when the bearing bears the preload so that the positioning accuracy and stability of the shaft do not exceed 1 microinch or 1 nanoinch[1]。
Ball bearings are used in various machines and equipment with rotating parts.Designers often have to decide whether ball bearings or fluid film bearings should be used in specific applications.The following characteristics make ball bearings more desirable than fluid film bearings in many situations,
1. The starting friction is small and the working friction is appropriate.
2。Capable of withstanding combined radial and axial loads
8. Not sensitive to lubrication interruption
4. No self-excited instability.
5. It is easy to start at low temperature.
In a reasonable range, changing the load, speed and working temperature has little effect on the good performance of ball bearings.
The following characteristics make ball bearings less desirable than fluid film bearings.
1. The finite fatigue life varies greatly
2. The required radial space is large
3. Low damping capacity.
1. The noise level is high·
6. The alignment requirements are strict.
6. High cost
According to the above characteristics,Piston engineFluid film bearings are usually used, whileJet engineAlmost only ball bearings are used.All types of bearings have their unique advantages. In a given application, the most appropriate bearing type should be carefully selected.The British Engineering Science Data Organization (ESDU 1965, 1967) once provided useful guidelines for the selection of bearings.
bearing clearance
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Figure 1
Bearing clearance (internal clearance) refers to the total distance that a bearing ring can move in a certain direction relative to another ring before the bearing is installed with the shaft or bearing box.According to the moving direction, it can be divided into radial clearance and axial clearance, as shown in Figure 1.
The internal clearance of the bearing before installation must be distinguished from the internal clearance (operating clearance) of the bearing when it reaches the operating temperature after installation.The original internal clearance (before installation) is usually larger than the running clearance, which is caused by the difference in the fitting degree involved in the installation and fitting, and the difference in the thermal expansion of the bearing inner and outer rings and related parts, which makes the inner and outer rings expand or shrink.
Bearing internal clearance and specified value
The size of internal clearance (also called clearance) in rolling bearing operation has a great impact on bearing performance such as fatigue life, vibration, noise, temperature rise, etc.
Therefore, it is an important research project to select the internal clearance of the bearing, which determines the structural size of the bearing.
Generally, in order to obtain a stable test value, the specified test load is given to the bearing, and then the clearance is tested.Therefore, the measured clearance value is larger than the theoretical clearance (in the radial clearance, also known as the geometric clearance), that is, an additional amount of elastic deformation caused by the test load (called the test clearance to show the difference).
Generally, the clearance before installation is specified according to the theoretical internal clearance.
Selection of internal clearance
The following points shall be considered when selecting the most suitable clearance according to the service conditions:
(1) The clearance changes due to the fit of the bearing with the shaft and housing.
(2) When the bearing is working, the temperature difference between the inner and outer rings causes the change of clearance.
(3) The material used for the shaft and housing affects the change of bearing clearance due to different expansion coefficients.
For bearings in normal operation, the radial clearance of the basic group shall be preferred.However, for bearings operating under special conditions, such as high temperature, high speed, low noise, low friction and other requirements, the radial clearance of the auxiliary group can be selected.yesPrecision bearing. The bearings used for the machine tool spindle shall have smaller radial clearance.If there are special requirements for bearing clearance, the bearing can meet the needs of customers.
Clearance control
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Clearance control of deep groove ball bearing
in the light ofDeep groove ball bearingIn production, the axial clearance is commonly used to control the radial clearance. The radial clearance is out of tolerance after assembly. By strictly controlling the accuracy of internal and external channels and steel balls, and compressing the range of radial clearance when calculating the axial clearance, the lower limit value remains unchanged, and the upper limit value is compressed by 15%.The axial clearance obtained by this method is used as the benchmark to control the actual radial clearance after sleeve assembly. The case verification shows that this method can meet the process requirements[2]。
Clearance analysis of angular contact ball bearings
aboutAngular contact ball bearingFor example, the clearance also determines its fatigue life. If the clearance is not properly selected, it is very easy to cause early failure of bearings.
Angular contact ball bearings are generally used in pairs, which can be divided into pre clearance and pre load.The machine tool bearing with high rigidity requirements adopts preload configuration, and the fit interference of the shaft is very small (about several microns), so it is unnecessary to consider the influence of the fit interference on the working clearance when pairing.However, for bearings with large interference, the influence of interference on the radial clearance must be considered. At this time, the pre clearance configuration is often used.However, JB/T 10186-2000 only specifies the pre clearance value of 7200B and 7300B bearing series, and does not specify other series. Therefore, theoretical calculation is required to select a suitable clearance range.
Factors affecting the preload of angular contact ball bearings: if the preload of the bearing increases, the stiffness can be improved, but excessive preload will cause abnormal heating of the bearing, leading to early failure of the bearing.In the positioning preload, the preload force depends on the installation conditions of the bearing, including the fit of the bearing, centrifugal effect in operation and temperature rise.
1. Fit of bearing
For machine tool bearings, generally the inner ring is interference fit and the outer ring is clearance fit.The interference fit between the inner ring and the shaft changes its radial dimension, which leads to the increase of preload.
2. Centrifugal effect
When the bearing runs at high speed, the internal channel will expand due to centrifugal effect, which will cause the radial clearance of the bearing to change and increase the preload.
3. Temperature rise
When the bearing is running, due to the combined action of its internal friction, lubricant agitation and other external factors, the bearing temperature will rise and the parts will expand.
(1) Among the bearing parameters, the contact angle has a greater impact on the axial clearance variation.(2) Among the influences of interference fit, centrifugal effect and temperature rise on bearing clearance, interference fit has the greatest influence.(3) In practical application, if the interference fit of the bearing needs to take into account the influence of the interference fit on the bearing clearance, a certain clearance should be reserved to avoid excessive preload and premature failure of the bearing.When the angular contact ball bearing is actually paired, its radial clearance change should be converted into axial clearance change for consideration[3]。
Rolling bearing
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Rolling bearings andsliding bearingCompared with sliding bearings, rolling bearings have the following advantages and disadvantages.
advantage
(1) In general working conditions, the friction coefficient of rolling bearing is small and will not change with the change of friction coefficient, which is relatively stable;Small starting and running torque, small power loss and high efficiency.
(2) The radial clearance of rolling bearing is small and can be eliminated by axial preloading, so the running precision is high.
(3) The axial width of rolling bearings is small. Some bearings bear the combined radial and axial loads at the same time, with compact structure and simple combination.
(4) Rolling bearing is a standardized part with high degree of standardization and can be produced in batches, so the cost is low.
shortcoming
(1) The contact area between rolling element and pipeline of rolling bearing is small, especially for ball bearing, which has poor impact resistance.
(2) Due to the structural characteristics of rolling bearings, large vibration and noise are caused.
(3) The service life of rolling bearing is reduced under the condition of high speed and heavy load.
(4) The inner and outer rings of rolling bearing adopt integral structure, and partial structure is not allowed, which makes it difficult to install the bearing in the middle of the long shaft[4]。
