The feed system of the CNC machine tool needs to obtain higher transmission rigidity. In addition to strengthening the rigidity of the ball screw nut, the correct installation of the ball screw and the structural rigidity of its support are also factors that cannot be ignored, the nut seat, and the end of the screw. The inaccuracy of the bearings and their machining and their excessive deformation after being stressed will affect the transmission stiffness of the feed system
Therefore, a good fit must be maintained between the hole of the nut seat and the nut, and the verticality of the hole to the end surface should be ensured. Appropriate ribs should be added to the nut seat, and the contact between the nut seat and the machine tool should be increased. Area to improve the local stiffness and contact stiffness of the nut seat. Incorrect installation of the ball thread and insufficient rigidity of the supporting structure will also greatly reduce the service life of the ball screw.
In order to improve the axial rigidity of the support, it is also very important to choose an appropriate rolling bearing. At present, two combination methods are mainly used in China. One is to use a combination of a radial bearing and a tapered bearing. Although the structure is simple, the axial rigidity is insufficient. The other is the combined use of thrust bearings or angular contact bearings and radial bearings. Its axial rigidity has been improved, but the friction resistance and heat generation of the bearing have been increased, and the structural size of the bearing bracket has been increased.
A special ball screw bearing has appeared abroad. This is a special angular contact ball bearing that can withstand a large axial force. Compared with ordinary angular contact ball bearings, the contact angle has increased to 60., increasing the ball The number and correspondingly reduce the diameter of the ball. The axial rigidity of the new structure of the bearing is more than twice that of the general bearing, and it is extremely convenient to use.The products are sold in pairs, and the thickness of the inner and outer rings is already selected at the factory. As long as the inner ring and the outer ring are pressed tightly with nuts and end caps during assembly, the pre-tightening force adjusted at the factory can be obtained.
In terms of the support configuration, a cantilever single support structure can be used for a short lead screw with a small stroke. When the ball screw is longer, in order to prevent the influence of the elongation of the screw caused by the thermal change, it is hoped that one section of the bearing can bear both the axial force and the radial force, while the bearing at the other end only bears the radial force, and can make a small amount of Floating axially. As CNC machine tools often have to work continuously for a long time, special attention should be paid to the influence of frictional heat. At present, there is also a structure in which both ends are fixed with thrust bearings. A butterfly spring and an adjusting nut are installed at one end of it. This can not only apply pre-tightening force to the ball screw, but also compensate for the heat of the screw. Maintain a nearly constant preload after deformation.
When the ball screw is used for high-speed and large inertia transmission that is vertically lifted and rotated or placed horizontally, because the ball screw is not self-locking, when the external power disappears, the executive part can continue to move under the action of gravity and inertial force, so it is usually It needs to be locked in an unpowered state, and its locking device can be composed of overrunning clutches and friction clutches.