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The measurement method and function of the preload of the machine tool spindle bearing

by:JNSN     2021-12-27
The main purpose of the preload of the machine tool spindle bearing: Generally, the rolling bearing should maintain the proper internal clearance in the running state, and the angular contact ball bearing will be installed with an appropriate negative clearance (axial load) in advance. This is called [preload]. Although the preload has various purposes and functions, the wrong amount of the preload will cause an increase in friction torque, temperature rise, abnormal noise, low life, etc., so you must be extremely careful when applying the preload. The effects of preload are listed as follows: ● Due to the external force, the axial displacement of the bearing is reduced, which can increase the rigidity of the shaft ● As the rigidity of the shaft increases, it can prevent vibration, reduce noise, and improve high speed ● Reduce corrosion caused by external vibration Possibility ● Smooth rotation of the rolling elements ● It can reduce the influence of centrifugal force rotation torque on the rolling elements and control the heating rotation torque. The steel ball of angular contact ball bearing rotates on the revolving shaft (axis) at the same time as the rotating shaft rotates. At this time, due to the angle between the rotation axis and the revolution axis, the steel ball will generate a moment to rotate around the two different axes. This torque is called [Rotational Torque (] Figure 5.1) The size of the rotating torque is directly proportional to the rotation angular velocity and the revolution angular velocity. The rotating torque can be ignored during low-speed rotation, and the heat generated by the slippage of the rotating motion during high-speed rotation cannot be ignored. In order to control the slip caused by rotating motion, the friction force between the steel ball and the raceway surface (u003d rolling element load × friction coefficient) must be maintained. In this sense, a minimum preload amount is sometimes specified. Preload method The preload method of assembling the bearing can be roughly divided into two types: positioning preload and constant pressure preload. The respective legends and features are shown in Table 5.1 (P15). Cylindrical roller bearings with a tapered bore inner diameter are sometimes used with radial preload (negative radial clearance). However, it must be noted that if the radial preload is too large, the bearing life will drop sharply (Figure 5.2). 5-3 Preload measurement a. Use axial load. If the preload is completed by a spring (constant pressure preload), the amount of preload Determine with spring displacement. If the preload is completed with a tightening nut (positioning preload), the amount of preload is determined by the relationship between the clamping torque of the nut and the clamping force. However, the relationship between the clamping torque and the clamping force of the nut varies greatly depending on the accuracy and roughness of the screw part, so attention must be paid. b Use the axial displacement preload amount to determine the relationship between the axial load acting on the bearing and the axial displacement amount. c Use the starting friction torque measurement method of the bearing to make a graph of the relationship between the bearing unit load and the starting torque in advance. But pay attention to the different types of bearings, lubrication conditions, etc. ● Figure 5.2 Cylindrical roller bearing (NN3020) radial clearance and service life An example of the bearing set (Figure 5.3) is preloaded (positioned), and the axial load Tw is applied. The load distribution of the two sets of bearings is calculated graphically based on the axial displacement. The process is described as follows: a. Draw the T-u0026#1048647;a curve of bearing A. b Take the preload Tp on the T axis and terminate at the intersection point P with the curve of bearing A, and draw the T-u0026#1048647;a curve of bearing B through point P. c Connect the two curves with a length equivalent to the external load Tw. d The loads Ta and Tb equivalent to this point become the load of a single bearing under the external load Tw. e The displacement of the bearing is obtained by using the displacement of the bearing Bu0026#1048647;w. The displacement of bearing B will be obtained by subtracting the displacement corresponding to Tp from the displacement corresponding to Tb, because if the bearing is preloaded, the displacement of the two bearings becomes a constant within the range of zero under the external load (Figure 5.3) O-O' is a constant). In other words, bearing B is displaced due to an external load, and bearing A will also have a corresponding amount of slack. If the external load increases and the preload disappears, the load Tb on bearing B will be equal to the external load Tw, and bearing A The load becomes zero. The magnitude of the external load causes the disappearance of the preload, which is represented by Tpo in Figure 5.3.
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