Created on:2021-07-01 09:21

Hydraulic fault diagnosis and maintenance of CNC machine tools

Hydraulic fault diagnosis and maintenance of CNC machine tools

(1) Overview

A MJ-50 CNC lathe is a horizontal lathe with two coordinate continuous control. The clamping and loosening of the chuck, the high and low pressure conversion of the clamping force of the chuck, the loosening and clamping of the rotary tool rest, the forward and reverse rotation of the tool head of the tool rest, and the extension and withdrawal of the tailstock sleeve are all driven by the hydraulic system. The hydraulic system uses variable vane pump to supply oil, and the action of electromagnet of each solenoid valve is controlled by PLC of numerical control system. Figure e is the schematic diagram of its hydraulic system.

E.jpg

The main driving objects of the system are chuck, rotary turret, tailstock sleeve, hydrostatic guide rail, hydraulic fork, variable speed hydraulic cylinder, hydraulic balance of headstock, hydraulic driving manipulator and tool loosening hydraulic cylinder of spindle, etc. The faults caused by the hydraulic system of CNC machine tools are as follows: (1) large cutting vibration of spindle components, damage of gears and bearings, no speed change of spindle, no rotation of spindle, heating of spindle, and gear pushing not in place during hydraulic speed change; ② Poor lubrication of screw nut; ③ The results show that the turret turret has no lifting action, the turret rotation speed is slow or does not rotate, the turret cutter repeated positioning accuracy is poor, the cutter cannot be clamped, and the manipulator tool changing speed is too fast; ④ The tailstock top is not tight or moving; ⑤ Poor lubrication of guide rail.

The faults of hydraulic system seriously affect the normal work of CNC machine tools. For example, the spindle components of CNC machine tools are the main components that affect the machining accuracy of machine tools. Its rotary accuracy affects the machining accuracy of workpieces. The power and rotary speed affect the machining efficiency. Automatic speed change, accurate stop and tool change affect the degree of automation of machine tools. Therefore, it is particularly important to find the hydraulic fault efficiently.

(2) Fault detection sequencing

Generally, people use on-site diagnosis technology to diagnose the hydraulic fault of CNC machine tools. The on-site maintenance personnel use general inspection tools or ask, see, listen, touch, smell and other sensory organs to diagnose the fault of CNC machine tools. At present, field diagnosis technology is widely used, but it needs rich practical experience. In order to find out the cause of hydraulic fault efficiently, a reasonable fault detection sequence must be set. There are two principles to determine the order of fault detection: one is to sort according to the possibility of fault causes; The second is to sort the components according to the difficulty of disassembly and assembly.

① In the process of fault analysis, the most probable fault suspect points should be checked first, and then the more probable fault suspect points should be checked after elimination. The basis for determining the probable cause of the fault is as follows.

a. The components with obvious characteristic information related to the fault cause are more likely to fail. As shown in Figure e, if the movement of chuck, tool rest and tailstock sleeve of MJ-50 CNC lathe is abnormal during operation, we should first suspect whether the common parts of each branch oil circuit - hydraulic oil, filter, hydraulic pump and common pipeline are unobstructed.

b. In the same case, first check the components with long service time, high load rate, proved to be poor quality and prone to failure, and components sensitive to hydraulic oil pollution. It can also determine the possibility of fault causes according to relevant statistical conclusions, that is, when a fault occurs, according to the statistical conclusions accumulated over the years, check the fault points with high probability value first.

② According to the difficulty degree of disassembly and observation, in the face of various possible causes of hydraulic failure, when the possibility of various failure causes is not clear, the detection order should be set according to the difficulty degree of disassembly and observation of hydraulic components, that is, first check the components and environmental factors (such as oil, electrical system, cooling water, etc.) that are easier to observe and test or disassemble, Check the components that are difficult to disassemble, especially the components with large volume and mass; First check the external factors, and then check the internal components; First check the relatively simple components, and then check the components with complex structure and function and unclear condition. For each component, check the valve first, then the pump, and finally the hydraulic cylinder and motor.

(3) Diagnosis examples

The fault symptom of a MJ-50 CNC lathe is that the tailstock sleeve stops working suddenly. How to detect this fault?

First read the system diagram. According to the hydraulic system shown in Figure e, the working principle of tailstock sleeve is as follows.

① When the electromagnet 5ya is powered off and the electromagnet 6ya is powered on, the sleeve extends out. At this time, the oil inlet path is: oil tank → filter → pump → one-way valve → pressure reducing valve 8 → valve 5 (left position) → rod less chamber of hydraulic cylinder; The oil return circuit is: rod cavity of hydraulic cylinder → one-way speed regulating valve 11 → valve 5 (left position) → oil tank.

When the sleeve is extended, the working preload is adjusted by the pressure reducing valve 8, which is displayed by the pressure gauge 13, and the extension speed is controlled by the speed regulating valve 11.

② When 5ya is powered on and 6ya is powered off, the sleeve returns. At this time, the oil inlet is: oil tank → filter → pump → one-way valve → pressure reducing valve 8 → valve 5 (right position) → one-way speed regulating valve 11 → rod chamber of hydraulic cylinder; The oil return circuit is; Hydraulic cylinder rodless chamber → valve 5 (right position) → oil tank.

Then contact the working principle of tailstock to analyze the causes of failure: insufficient pressure, leakage, hydraulic pump does not supply oil or insufficient flow, hydraulic cylinder piston galling, wear or seal ring damage, hydraulic valve broken or stuck, etc.

In view of the above fault reasons, according to the difficulty of disassembly and observation of hydraulic components, the detection sequence is set as follows.

① Check the movement of chuck and rotary turret, and press the movement button of chuck and rotary turret: if the movement is normal, it can eliminate the fault reasons such as reverse installation of hydraulic pump, reverse rotation of hydraulic pump, opposite eccentric direction of stator, too low pump speed to make the blade unable to throw out, blade stuck in rotor slot, too high oil viscosity to make the blade move inflexibly. Next, check the steps of (2) to (10); Otherwise, go to step 2 to check the oil tank. If there is no problem, check whether the filter and oil suction pipe are blocked, whether the oil viscosity is too high, and whether the pump is improperly adjusted or damaged.

② Check whether there is leakage in the system pipes, joints and components.

③ Check the oil level of the oil tank to see if it is above the minimum oil level, whether the oil suction pipe and filter are exposed, and whether the oil return pipe is higher than the oil level to allow air to enter the oil tank.

④ Manually operate the directional control valve 5 (the solenoid valve is pushed by the manual buttons at both ends of the electromagnet). If the valve core can not be pushed, it means that the directional valve is faulty. If the direction valve can change direction and the hydraulic cylinder acts, it means that the electrical circuit of the solenoid valve is faulty. If the hydraulic cylinder still can't work, check step 5.

⑤ Manually operate the one-way speed control valve 11 to increase the opening of the one-way speed control valve. If the hydraulic cylinder acts, it means that the one-way speed control valve 11 is blocked; Otherwise, adjust the knob of the one-way speed regulating valve to the loosest position, and then check step 6.

⑥ Adjust the pressure reducing valve 8. If the hydraulic cylinder acts, it means that the pressure reducing valve is blocked or improperly adjusted; Otherwise, adjust the pressure reducing valve knob to the loosest position, and then carry out the next inspection.

⑦ Check that the pressure direction valve 5 of the pump station is in the middle position, and check whether the reading of the pressure gauge 14 at the pump outlet is adjusted to the set value. If not, do the following checks: A. whether the pressure gauge switch is open, whether the pressure gauge is damaged; b. Check whether the pressure regulating spring of hydraulic pump 15 is too loose; c. Whether the oil absorption filter is partially blocked and the capacity is insufficient; d. Whether the oil suction pipe is partially blocked; e. If the pump is damaged, if there is serious internal leakage, increase the pump pressure. If the movement speed of the hydraulic cylinder meets the working requirements, the fault is eliminated. If the speed cannot meet the requirements, the hydraulic pump needs to be repaired; If the hydraulic cylinder still can't move after the pump pressure is increased, the next step is to check.

⑧ Switch directional control valve 5 to the right and check the reading of pressure gauge 13. If the reading is not close to that of main pressure gauge 14, the right pipeline and one-way speed regulating valve 11 are blocked; If it's close, it means it's not blocked.

⑨ After the above work is finished, the fault is still not eliminated, so it may be that the hydraulic cylinder is out of order. First of all, don't rush to dismantle the hydraulic cylinder. Open the directional valve to the left or right position. After starting the hydraulic pump for a period of time, carefully touch the whole cylinder wall to see if there is local heat. If the piston seal is damaged, there will be oil leakage from the high pressure chamber to the low pressure chamber. When the oil flows through the narrow gap, the hydraulic energy will be converted into heat energy; If there are no local hot spots, proceed to the next step.

⑩ Disassemble the pipe joint T1 at the other end of the hydraulic cylinder and connect it to a three-way pipe joint. The other ends of the three-way pipe are respectively connected with a pressure gauge and a stop valve. The directional valve 5 reverses to the left position and reads the reading of the pressure gauge. Similarly, if the reading is not close to that of the main pressure gauge 14, the pipeline is blocked; If it's close, it means it's not blocked. If the line is not blocked, proceed to the next step.

⑥ Disassemble and inspect the hydraulic cylinder.

(4) Summary

The occurrence of hydraulic transmission failure also has certain conditions and rules. In the work, strictly following the requirements of the operation manual can greatly reduce the failure rate. When a fault occurs, it is not difficult to read the system diagram first, and then find out the cause and remove the fault according to the reasonable order of fault detection.

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