Created on:2021-07-08 14:14

Fault diagnosis and maintenance of CNC hydraulic plate bending machine (1)

Fault diagnosis and maintenance of CNC hydraulic plate bending machine (1)

The numerical control system of we67k-63 / 2500 numerical control hydraulic plate bending machine is the special numerical control system of da-24e bending machine of delem in Holland; The hydraulic system is the special hydraulic system of Bosch bending machine; The back stop (x-axis) is driven by mt30m4-38 DC servo motor with encoder and semi closed loop control; The slider (Y-axis) is driven by the hydraulic cylinder of the hydraulic system, the grating ruler position detection, and the electro-hydraulic synchronous control controlled by the numerical control system. The machine tool has many faults in use.

(1) Mechanical failure

① Fault phenomenon shortly after the installation and debugging of this machine tool, it was found that the positioning of x-axis (back stop) was inaccurate during bending and positioning, and there was a phenomenon of error accumulation during continuous bending. For example, if the workpiece size error exceeded 0.30mm after four times of continuous bending, it was equivalent to repeated positioning for four times, and the accumulated positioning accuracy error exceeded 0.30mm. It is also found that the positioning accuracy varies irregularly between 0.05 and 0.10 mm.

② Fault analysis and troubleshooting according to the fault analysis, the conventional problems should be: A. there is a gap or rotation in the rear stop (x-axis) drive chain; b. The installation of the farthest point travel switch is loose or aging, and the contact is poor, which causes the positioning accuracy error of each startup. Check the motor and lead screw coupling, and check the axial movement of the lead screw with a dial indicator, all of them are normal; Check the travel switch. It is firmly installed without looseness. After it is replaced with a new switch of the same model, the fault still exists; Check the reverse clearance of the lead screw nut with a dial indicator, and the accumulated error of four repeated positioning is more than 0.30mm.

Is the problem really in the screw nut reverse clearance?

Careful analysis shows that the machine has just been installed and debugged, and the use time is not long; Before the use of the accuracy is completely normal, CNC system parameter settings have not been changed, high-precision ball screw as an important part to ensure the accuracy of CNC machine tools, should not be so fast. Turn the lead screw forward and backward by hand to check the reverse clearance of the lead screw nut. The result error is within the inspection accuracy range of the machine tool, which is completely normal!

The problem should be in the motor part. Open the rear end cover of the motor for inspection, the original fastening screw of the encoder and motor shaft has been loose. When the motor rotates in the forward and reverse directions, the encoder can not correctly reflect the actual position. The more times the motor rotates in the forward and reverse directions, the greater the error between the theoretical position detected and the actual position of the back stop, and the more times the parts are bent, the greater the dimensional error.

③ Enlightenment later reflected the matter to the manufacturer. It turned out that the machine tool had replaced the encoder. The root cause of this failure should be that the top screw was not tightened when the encoder was installed. After transportation and a period of use, it gradually loosened.

(2) Hydraulic system failure

① When the machine starts up normally, press the start button of the hydraulic pump, the slider does not lift up quickly, but stops at the bottom; Moreover, the hydraulic pump and hydraulic pipeline make a "buzzing" sound, which seems to be "boring", while the numerical control system has no alarm. Check the electrical appliances and their connecting lines, and they are normal.

② Fault analysis and troubleshooting according to the fault analysis, the problem should be in the hydraulic system. According to the action specification diagram (Figure m), the causes of the failure are as follows: a. the 4y5 proportional directional valve controlled by the numerical control system in the synchronous control block does not act (Figure n), so that the oil from the hydraulic pump can not enter the lower chamber of the hydraulic cylinder, so the slider can not be lifted; b. The iyi proportional relief valve controlled by the numerical control system in the pump control block has no action and is in the unloading state, so that the oil from the hydraulic pump can not enter the synchronous control block and directly flow back to the oil tank, or the action is not completely in place, and the pressure provided to the hydraulic cylinders on both sides is not enough to lift the slider.

N.jpg

Cause a was quickly eliminated, because the left and right hydraulic cylinders of the machine tool each have the same 4y5 proportional directional valve, the probability of two valves failure at the same time is very small, so the problem is locked in cause B.

There may be foreign matters stuck in the spool of iy1 proportional relief valve, even if the numerical control system sends an action signal and the spool action is not fully in place, the pressure provided to the hydraulic cylinders on both sides is not enough to lift the slider. The stuffy noise of hydraulic pump and hydraulic pipeline should be caused by the vibration of high-pressure oil flowing through the system pipeline at high speed and the operation of hydraulic pump under abnormal load.

After the 1y1 proportional relief valve was removed, the valve core and parts were taken out, and the valve body was soaked in gasoline for three times (the rubber sealing ring was cleaned with kerosene). The last time it was installed, the machine was started up again, the fault was eliminated, and the machine was running normally.

③ It is revealed that although the fault occurred on the valve, the most fundamental reason is the pollution of hydraulic oil. Therefore, the correct use of filter to control pollution, timely replacement of filter element and filter screen in hydraulic system, timely cleaning of oil tank and replacement of correct type of hydraulic oil, usually should strengthen the maintenance of hydraulic equipment, in order to reduce the failure rate of equipment, reduce downtime and maintenance cost, and improve the utilization rate of equipment. Otherwise, it will cause unnecessary losses.

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