Inspection and maintenance of ball screw
The faults caused by ball screws are diverse and there is no fixed pattern. Some faults are progressive and require a development process, becoming increasingly severe over time of use; Sometimes it is a sudden malfunction, usually without obvious signs, and it occurs suddenly. This type of malfunction is caused by various unfavorable factors and external factors working together. So determining the true cause of the malfunction through correct testing is a prerequisite for rapid and accurate repair.

1) Detection and repair of clearance between ball screw nut pair and support system

When there is a large reverse error, unstable positioning accuracy, or tool marks in the quadrant of the CNC machine tool, the first step is to check whether there is clearance in the screw system. The detection methods include: using a dial gauge with a steel ball placed in the center hole of one end of the lead screw, measuring the axial movement of the lead screw, and using another dial gauge to measure the movement of the worktable. Rotate the screw forward and backward, observe the values reflected on the two dial indicators, and confirm the fault location based on the changes in values.

a) Detection and repair of clearance between screw support bearings

If the pointer of the dial gauge measuring the screw does not swing when the screw rotates in both directions, it indicates that the screw has not moved. If the pointer of the dial gauge swings, it indicates that there is a phenomenon of screw movement. The difference between the maximum and minimum measurement values of the dial gauge is the distance of axial movement of the screw. At this point, we need to check whether the back cap of the supporting bearing is locked, whether the supporting bearing has worn out or failed, and whether the preloaded bearing washer is suitable. If there are no issues with the bearings, simply replace them with preloaded washers. If the bearing is damaged, it needs to be replaced, a preloaded washer needs to be made again, and the back cap needs to be tightened. The axial displacement of the screw mainly depends on the accuracy of the supporting bearing preload washer. The most ideal state for the installation accuracy of the screw rod is no positive or negative clearance, and the supporting bearing also needs to have an interference fit of about 0.02mm. b) Detection and maintenance of clearance generated by ball screw double nut pair

Through testing, if it is confirmed that the fault is not caused by the movement of the screw rod. Then we need to consider whether there is a gap between the screw nut pairs, and the detection method for this situation is basically the same as detecting screw movement. Measure with a dial gauge on the workbench connected to the nut, rotate the screw in both directions, detect the maximum gap between the screw and the nut, and then make adjustments.

As shown in Figure 2, adjust the thickness of gasket 4 to cause axial displacement of the left and right nuts 1 and 2, thereby eliminating the clearance between the ball screw nut pairs and generating preload force. Due to the different structures of screw nut pairs, the adjustment methods are also different, which will not be listed one by one here.

c) Inspection and maintenance of single nut pairs

For a single nut ball screw, the gap between the screw and nut pairs cannot be adjusted. If it is detected that there is a gap between the screw nut pair. Firstly, check whether the threaded arcs of the screw and nut have been worn. If the wear is severe, the entire set of screw and nut must be replaced.

If the wear is minor, a larger diameter ball can be replaced for repair. Firstly, detect the maximum clearance of the screw nut pair, convert it into an increase in ball diameter, and then select the appropriate ball for reassembly. This type of maintenance is quite complex, requires a long time, and demands a high level of technical expertise.

d) The gap caused by the connection between the nut flange and the workbench not being fixed properly

This issue is generally overlooked because the machine tool moves back and forth for a long time, causing the screws that fix the Faraday disc to loosen and create gaps. When checking the clearance between the screw and nut, it is best to eliminate this fault factor first to avoid taking detours during repairs.

e) Repair of faults such as unstable movement and excessive noise of ball screw nut pairs.

The unstable movement and excessive noise of the ball screw nut pair are mostly caused by poor lubrication, but sometimes it may also be due to improper adjustment of the servo motor drive parameters.

2) Poor lubrication of bearings, screw and nut pairs

If the machine tool generates noise and vibration during operation, after checking that there is no problem with the mechanical transmission part, the first thing to consider is poor lubrication. Many machine tools have been running for many years, and the automatic lubrication system of the screw and nut is often blocked and cannot lubricate automatically. Adding high-temperature and high-speed resistant lubricating grease to bearings and nuts can solve the problem. Lubricating grease can ensure the normal operation of bearings and nuts for several years.

3) Servo motor drive problem

Some machine tools generate vibration and crawling during motion, and often the mechanical parts are detected without any problems. No matter how they are adjusted, the vibration and crawling cannot be eliminated. After careful inspection, it was found that the servo motor drive gain parameters are not suitable for actual operating conditions. After adjusting the gain parameters, vibration and crawling can be eliminated.