Check and maintenance of the ball screw

There are many kinds of faults caused by ball screw, and there is no fixed mode. Some faults are gradual faults, which need a development process, and become more and more serious with the increase of service time; Sometimes it is a sudden fault, which generally occurs suddenly without obvious signs. This fault is caused by the joint action of various adverse factors and the outside world. Therefore, to determine the real fault cause through correct detection is the premise of rapid and accurate maintenance.

1) Inspection and repair of clearance between ball screw nut pair and support system
When the NC machine tool has large reverse error, unstable positioning accuracy and knife marks across the quadrant, it is necessary to check whether the lead screw system has clearance. The detection methods are as follows: put the dial indicator and steel ball in the central hole at one end of the lead screw to measure the axial movement of the lead screw, and another dial indicator to measure the movement of the worktable. Turn the lead screw forward and backward, observe the values reflected on the two dial indicators, and confirm the fault position according to the changes of values.
a) Inspection and repair of bearing clearance of lead screw support
If the pointer of the dial indicator for measuring the lead screw does not swing when the lead screw rotates forward and backward, it indicates that the lead screw does not move. If the dial indicator pointer swings, it indicates that the lead screw has movement. The difference between the maximum and minimum measured values of the dial indicator is the axial movement distance of the lead screw. At this time, we need to check whether the back cap of the support bearing is locked, whether the support bearing has been worn and failed, and whether the preload bearing washer is appropriate. If there is no problem with the bearing, just refit the preloaded washer. If the bearing is damaged, replace the bearing, refit the preload washer, and then back the back cap tightly. The axial movement of the lead screw mainly depends on the accuracy of the preloaded washer of the support bearing. The most ideal state for the installation accuracy of the lead screw is that there is no positive and negative clearance, and the support bearing should have an interference of about 0.02mm.
b) Inspection and maintenance of clearance produced by ball screw double nut pair
Through inspection, if it is confirmed that the fault is not caused by the movement of the lead screw. It is necessary to consider whether there is a gap between the lead screw and nut pair. The detection method in this case is basically the same as that for detecting the movement of the lead screw. Use a dial indicator to measure the working table connected with the nut, rotate the lead screw in the forward and reverse directions, detect the maximum clearance between the lead screw and the nut, and then adjust it.

The method is to adjust the thickness of gasket 4 to make the left and right nuts 1 and 2 produce axial displacement, so as to eliminate the clearance of ball screw nut pair and produce preload. Due to the different structures of screw nut pairs, the adjustment methods are also different, which are not listed one by one here.

c) Inspection and maintenance of single nut pair
For single nut ball screw, the clearance between screw and nut pair cannot be adjusted. If the clearance of lead screw nut pair is detected. First, check whether the thread arc of the lead screw and nut has been worn. If the wear is serious, the full set of lead screw nuts must be replaced.
If the wear is slight, the ball with larger diameter can be replaced for repair. Firstly, the maximum clearance of the lead screw nut pair is detected and converted into the increase of the ball diameter, and then the appropriate ball is selected for reassembly. Such maintenance is complex, takes a long time and requires a high technical level.

d) The gap caused by the unfixed connection between the nut flange and the workbench
This problem is generally easy to be ignored. Due to the long-term reciprocating motion of the machine tool, the screw fixing the farad plate is loose, resulting in clearance. When checking the clearance of the lead screw nut, the fault factor shall be eliminated first to avoid detours in repair.
e) Maintenance of faults such as unstable movement and excessive noise of ball screw nut pair.
The unstable movement and excessive noise of ball screw nut pair are mostly caused by poor lubrication, but sometimes it may also be caused by the failure to adjust the driving parameters of servo motor.

2) Poor lubrication of bearing, lead screw and nut pair
If the machine tool generates noise and vibration during operation, after detecting that there is no problem in the mechanical transmission part, we should first consider the problem of poor lubrication. After many years of operation, the automatic lubrication system of lead screw and nut is often blocked and cannot be lubricated automatically. The problem can be solved by adding high temperature and high speed resistant grease to bearings and nuts. The grease can ensure the normal operation of bearings and nuts for several years.

3) Servo motor drive problem
Some machine tools produce vibration and creep in motion, and there is often no problem in detecting the mechanical part. No matter how to adjust, the vibration and creep can not be eliminated. After careful inspection, it is found that the servo motor drive gain parameters are not suitable for the actual operation. After adjusting the gain parameters, vibration and creep can be eliminated.