The main purpose of NACHI machine tool spindle bearing preload: Usually, rolling bearings should maintain proper internal clearance during operation, NACHI angular contact ball bearings will be pre-added with proper negative clearance (axial load) for installation. This is called [preload]. Although the preload has various purposes and functions, the wrong amount of preload can lead to an increase in friction torque, temperature rise, abnormal noise, shortened life, etc., so extreme care must be taken when applying preload. The effects of preload are listed as follows: ● The axial displacement of the bearing is reduced due to external force, which can improve the rigidity of the shaft ● With the increase of the rigidity of the shaft, it can prevent vibration, reduce noise, and improve high-speed performance ● Reduce corrosion caused by external vibration Possibilities ● Smooth rotation of rolling elements ● It can reduce the influence of centrifugal force rotating moment on rolling elements and control the heating rotating moment. The steel ball of the angular contact ball bearing rotates on the revolution axis (axis) at the same time as the rotation axis rotates. At this time, due to the angle between the axis of rotation and the axis of revolution, the steel ball will generate a moment to rotate around the different axis of the two axes. This torque is called [rotation torque (] Figure 5.1) The magnitude of the rotation torque is proportional to the rotation angular velocity and the revolution angular velocity. When rotating at low speed, the rotational torque can be neglected, but when rotating at high speed, the heat generated by the slippage of the rotating motion cannot be ignored. In order to control the slip caused by the rotating motion, the friction force between the steel ball and the raceway surface (u003drolling element load friction coefficient) must be maintained. In this sense, minimum preload amounts are sometimes specified. Preload method The preload method of assembling bearings 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). NACHI cylindrical roller bearings with a tapered bore bore 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 be drastically reduced (Fig. 5.2) 5-3 Measurement of preload a If the preload is done with a spring (constant pressure preload), the amount of preload Determined by spring displacement. If the preload is done by tightening the nut (positioning preload), the amount of preload is determined by the relationship between the clamping torque of the nut and the clamping force. However, attention must be paid to the relationship between the clamping torque and clamping force of the nut, which varies greatly depending on the precision and roughness of the screw portion. b Using the amount of axial displacement preload is determined by the relationship between the axial load acting on the bearing and the amount of axial displacement. c Use the starting friction torque of the bearing. This measurement method requires the relationship between the single bearing load and the starting torque to be prepared in advance. But pay attention to the difference in the type of bearing, lubrication state, etc. ● Fig. 5.2 Example of radial clearance and life of cylindrical roller bearing (NN3020) The bearing set (Fig. 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, terminate at the intersection point P with the curve of the bearing A, and draw the T-u0026#1048647;a curve of the bearing B through the point P. c Connect the two curves with a length corresponding to the external load Tw. d The equivalent loads Ta and Tb at this point become the loads of a single bearing under the external load Tw. e The displacement of the bearing is obtained using the displacement u0026#1048647;w of the bearing B. 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 NACHI bearings becomes constant within the range where the preload is not misaligned to zero under external load (Figure 5.3 O-O' in is a constant). In other words, bearing B is displaced due to the external load, and bearing A will also experience 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 the load on bearing A will be equal to Tw. load becomes zero. The magnitude of the external load causes the disappearance of the preload to be represented by Tpo in Figure 5.3.