Fault analysis and maintenance of ball screw

Fault analysis and maintenance of ball screw

NC machining center is an efficient automatic machine tool integrating machine, electricity and instrument with high technical content. It integrates the new technical achievements in various fields such as computer technology, automation technology, reverse service drive, precision measurement and precision machinery. It is a new industrial control technology. Jiangsu food vocational and technical college has purchased many processing centers equipped with Siemens 802D SL CNC system, and some faults have occurred in the long-term operation process. This paper briefly introduces some recent faults and solutions

during the working process of the NC machining center, it is found that it cannot return to zero normally. After analysis, it is preliminarily judged that it may be caused by the following reasons:

What are the functional characteristics of the rotating shaft torque durability tester (1) the preload of the ball screw is reduced due to the long working time of the screw rod

(2) the bearings at both ends of the screw rod are worn, resulting in movement between the screw rod and the bearing

(3) the wear of the guide rail leads to the unstable operation of the worktable and affects the machining and zeroing accuracy

after analyzing several causes, the judgment is confirmed to be correct through careful inspection and repeated tests; After research, a plan for one-time repair of the ball screw, guide rail and other hardware of the machining center is formulated

repair of ball screw

the repair of ball screw should first determine what factors determine the working state of ball screw, find out the purpose and amount of pre tension of ball screw and the detection of ball screw clearance, and then repair the ball screw

1 Purpose and pre tension amount of ball screw pre tension

screw pre tension is to apply a certain tension on the length direction of the screw rod with the lock nuts at both ends of the screw rod as required during assembly to adjust the reverse tightness of the positive and negative nuts of the screw rod. The reason is that when the temperature of the screw rod rises during operation, the thermal stress effect will lengthen the screw rod and make the length of the screw rod unstable. The purpose is to effectively eliminate the elastic variable of the component in order to obtain stable motion accuracy. Otherwise, during micro feeding, when the damping force of the sliding saddle or worktable is greater than the elastic torque of the screw rod and nut, the worktable will not move. With the continuous feeding, the cumulative elastic torque of the screw rod and nut exceeds the damping torque of the worktable, and the worktable will produce impulse displacement. Although the total feed rate may not be affected, there is an error between the actual number of steps and the feed rate of each step and the input gate of the controller, which will seriously affect the machining accuracy

too large pre tension will burn out the support bearing, so the pre tension value less than 5 ℃ is generally adopted. However, if the landing diameter of the key project driven by the ball screw exceeds 50ram, it is not suitable for pre tension. If the diameter of the screw is large, it needs a large pre tension, which will lead to the heating and burning of the bearing. It is suggested to take the temperature rise of 3 ℃ as the benchmark of compensation value t (every 1000mm of screw rod, one 0.o2 ~ one 0.03mm)

2 detection of reverse clearance after installation of ball screw

it is used to move the relevant axis (x, y, z) of arterial impulse generator (set the pulse magnification as l × 100 gear, i.e. the motor feeds 0.1mm for each step of change), turn on the dial indicator and observe the movement of the relevant axis. After the unidirectional motion accuracy remains normal, as the forward motion of the starting point, the actual distance of the axis of the machine tool is d = DL = D2 = D3... = 0.1mm for each step of pulse change, indicating that the motor operates well and the positioning accuracy is good. The change of the actual movement displacement of the machine tool can be divided into four stages:

(1) the movement distance of the machine tool DL d = 0.1mm (the slope is greater than l)

(2) d = 0.1mm D2 D3 (slope less than l)

(3) the machine tool mechanism does not actually move, indicating the most standard reverse clearance of the door

(4) the moving distance of the machine tool is equal to the given value of Yu pulse (the slope is equal to 1,), and it will return to the normal movement of the machine tool

the screw rod will deform after being stressed, and the elongation under tension will produce error; Pre tightening is to make the screw rod produce a certain stress in advance, and the deformation will be small after stress, that is, the error during operation is small

errors cannot be completely eliminated, but can be offset by setting parameter compensation; For example, the door pitch error is measured by laser, and then input into the NC system for compensation

after the lead screw is used for a period of time, wear occurs between the lead screw and the nut, including the steel ball, and the preload of the lead screw is weakened. At this time, the use of the lead screw should be tested. Under the condition of no damage, the single nut can replace the steel ball to adjust the preload. For strong nuts, if there is a preload in the middle, the preload or steel ball can be replaced. Others use screws for preloading adjustment, which is more convenient. Just adjust the screws

therefore, two schemes are adopted for this maintenance:

(1) grind the preload plate in the nut (as shown in Figure L) thin, and use the strong nut to reduce the spacing, so as to achieve the preload required by the ball screw (press the lead screw inward)

(2) thicken the pre pressing plate (as shown in Figure L) between the strong nuts, and use the space between the strong nuts to increase so as to achieve the pre tightening required by the ball screw. China's Tariff Commission issued the announcement of the Tariff Commission of the State Council on imposing tariffs on about US $16 billion imported goods originating in the United States (announcement [2018] No. 7 of the Tariff Commission of the State Council) (press the lead screw outward)