Writer: Eric Bearing Limited
After the installation of the SKF rolling bearing, the rolling body and the raceway of the ring are in the proper pre-compacting state, which is called the pre-compacting of the rolling bearing.The purpose of bearing pre-tightening is to improve the rotation accuracy and supporting stiffness in operation, reduce vibration and noise, and reduce the relative sliding of bearing rolling body relative to the inner and outer raceway caused by inertial torque and other factors.Angular contact bearings and shafting with high requirements for rotation accuracy and stiffness are usually pretightened.
Preload is the use of external force applied in the assembly process to give the bearing the appropriate preload.If the preload is too small, there will be clearance when the bearing is working, which will lead to the decrease of supporting stiffness and rotation accuracy, and cause vibration and noise.Too much preload will increase the friction between the rolling body and the inner and outer raceway of the ring. During operation, the temperature will rise too fast, reducing the transmission efficiency and reducing the SKF bearing life.Only by applying proper preload can the axial clearance be eliminated, the reverse error be reduced, the rotation accuracy of the shafting be improved and the vibration and noise be reduced.Give the bearing proper preload, even if the bearing after a certain period of operation, the relative position of the bearing slightly changed, the preload can still make the bearing inner and outer ring position remain stable, so the control of the preload is very important during the installation. Eric Bearing have rich stock for SKF bearings :
By adjusting the size of bushing or gasket, the normal axial preload method can get the proper preload.The proper preload can also be obtained by controlling the starting friction torque.Also can be used to pre-adjust the fixed preload of pairs of double bearing to.After the application of axial preload on the bearing, its relative position will remain unchanged during use.Bearings can also be used to apply the appropriate preload spring, fixed torque wrench.Determination method of preload in assembly:
1) empirical test
The empirical test method is based on the technical workers' ability to feel the torque in practice to judge the bearing preload.During shafting assembly, preload the bearing, rotate the spindle manually, or put the shafting into the low-speed drive test system.The flexibility of spindle rotation can be perceived by the resistance moment of both hands, which is the pre-tightening moment of the bearing on the shaft. Although this method is relatively rough, it is simple to use, and has been adopted by a large number of experienced assembly workers, and can be used as a primary test method of product assembly accuracy.However, this method has low efficiency and high labor intensity, and is not suitable for the occasion with high precision requirements, and the maximum load on the shaft cannot be determined.This method is only suitable for experienced operators.
2) method of measuring rotation moment
Use spring scale or simple dynamometer, in the assembly, the bearing and the rotation axis fixed, and ensure that the axis can rotate, and then put the spring scale or simple dynamometer on the axis.When the reading is in a stable state, it is the final measurement result. By measuring the magnitude of the force, and then multiplying the distance from the tangent line to the axis, it is the value of the bearing pre-tightening torque on the spindle.
Angular contact ball bearings used in pairs are required for friction torque. When the form, internal parameters and lubrication mode of the bearings are certain, the friction torque of the bearings has a certain relationship with the axial load, so the pre-tightening force can be determined by reference to the friction torque of the bearings.Therefore, by calculating the friction torque of paired bearings under preload, the calculated result is compared with the measured value of actual friction torque, so as to judge whether the preload inside the bearing meets the requirements.
3) pre-test and pre-load method
Before the bearing is installed, a test system similar to the actual installation is used to pre-install the bearing. The bearing is installed in the mechanism, and the axial pre-tightening force F required by the conversion is applied to measure the actual bearing distance d.The measured value refers to the dimension value actually required in the assembly of the bearing after the preload is applied, which includes the actual bearing distance and the clearance after the preload is applied to adjust the dimension . SKF 2310 C3 bearings online , pls click here :
In the actual installation process, the matching between the bearing bore and the shaft must be strictly controlled, so that the matching between the bearing inner ring at the shaft neck is not loose, but also can move flexibly along the axis.When the inner and outer rings are required to be assembled, it is most appropriate to push into the shaft neck and bearing seat hole with the thumb of both hands.Then, by adjusting the size of the sleeve and the outer bearing cover, the actual assembly size d can be ensured. If necessary, the adjustment shim of fixed size can be added in the dimension adjustment process to ensure that the value of the bearing pre-tightening force is close to the predicted value.
4) measurement and control method of bearing deformation
Advance measuring bearing friction torque and the relationship between the axial load, the bearing placed in a circle on the body, by formula conversion, can be by gravity or spring pressure control places given a on the bearing inner ring and the pre-tightening force equal to the size of the load, bearing load, the difference in size between the axial now on inner and outer delta K1 and delta K2, numerical measurement using leverage dial gauge.When assembling bearings, the preload of each assembled shaft bearing shall be checked by a special dial meter.If the preload applied is insufficient, gaskets can be added to adjust the bearing clearance size to ensure that the preload is within the fixed range.
5) use torque wrench
In order to eliminate the clearance between the inner and outer rings of the bearing, bearings that rely on nuts for axial fixation can reach the target of bearing pretightening by twisting the inner ring of the bearing in an axial direction.This method is more general method, its structure is simple, applicability is strong, so the preload of the preload bearing is easy to control, the defect is no quantitative data.However, after measuring the final assembly axial dimension and adding the appropriate gasket, the pre-tightening force can be adjusted.In order to improve production efficiency and assembly accuracy, the assembly is usually completed by means of a fixed torque wrench.Fixed torque wrench is the most important feature: can be set torque, and torque adjustable.Fixed torque wrench can be initially and finally tight, its use is to adjust the torque, and then tighten the bolt.In the bearing assembly process, the bearing can be pre-assembled to obtain the tightening torque required for the pre-tightening force and then assembled.
6) sensor method
In the assembly process with high requirements, we can achieve measurement and control with the help of sensors.With the rapid development of modern sensor technology, measurement becomes more and more simple.Torque sensors can be used in the bearing assembly process.The torque sensor is a special strain gauge with strain glue attached to the elastic axis to be measured, and forms a strain bridge. By providing the current to the strain bridge, the electrical signal of the elastic axis subject to torsion can be measured. After the strain signal is amplified, it is converted into a frequency signal proportional to the torsion.The obtained signal can be processed by relevant instruments or computers to display the torque value.Referring to the previous installation method, the fixed shaft rotating bearing seat is measured.The clamping device is used to make the bearing seat rotate together with the measuring machine under the driving device, and the reaction torque value of the clamping device can be measured, that is, the pre-tightening torque value of the bearing.The common driving motor can act as a driving device to provide power.
In this paper, the control method of bearing preload is discussed, which has practical significance for bearing installation.The adjusted spindle can be tested by the test run method until it meets the requirements.