There are many types of high-speed electric spindle Plain bearing, but the two pairs of angular contact ball bearings with "O" type arrangement of front and rear bearings are typical. Since the rear bearing is also an angular contact ball bearing, a ball sleeve is generally provided to allow the rear bearing to move axially along the housing, so that the main shaft can be freely expanded rearward after being heated. In general, angular contact ball bearings need to be preloaded in the axial direction to work properly. The higher the preload, the higher the stiffness of the bearing but the greater the temperature rise. The simpler method is to select an optimal fixed preload value according to the speed range of the electric spindle and the load to be withstood; a better method is that the preload can be adjusted with the change of the spindle speed at high speed. The preload is reduced and the preload is increased at low speeds.