Created on:2021-06-23 15:50

Fault diagnosis and maintenance of hydraulic feed modular machine tool

Fault diagnosis and maintenance of hydraulic feed modular machine tool

Hydraulic feed modular machine tools often have such a phenomenon: the fast feed speed is normal, and the worktable will stop when processing begins. There are many reasons for this phenomenon. The faults of hydraulic system involve many aspects, the main possible reasons are: large cutting force; Internal leakage of hydraulic pump; The internal parts of relief valve are stuck, damaged or spring fatigue; The internal parts of the executive control elements (direction valve, flow valve, etc.) are worn or stuck, and the maintenance and assembly errors are found; Internal leakage caused by wear of internal parts of actuator; Serious oil leakage outside the system; The resistance of the actuator increases because the seal is squeezed into the cylinder wall. During the period of initial failure and accidental failure, there is only one reason mentioned above, and the probability of more than two reasons is very small.

In the hydraulic system of modular machine tool (Fig. If you check one by one from the hydraulic station, the maintenance is very difficult, not only can not find the cause, but also because of blind disassembly of components, resulting in new faults. The following is the method to quickly diagnose the "no work progress" fault in the way of item by item elimination.

(1) High cutting force

The main reasons for this kind of problem are tool wear, lack of sharpness, or uneven material of parts, there are hard spots, especially a lot of hard spots in casting blank. Because many modular machine tools basically use the self feedback variable displacement pump as shown in Figure 1-2 in order to meet the processing requirements of "fast forward, large flow, small pressure, small flow, large pressure". When the cutting force increases, the pressure of hydraulic system increases. The plunger (Part 1) pushes the stator (Part 3) to move to the right under the action of feedback hydraulic oil until the stator and rotor are concentric and there is no flow output, so there is no flow in the oil inlet pipeline and the sliding table stops. This situation can be judged from whether the sound produced by processing is abnormal, and whether the reading of the pressure gauge of the hydraulic system exceeds the rated pressure of the pump can also be observed. The simplest way to solve this problem is to check and replace the cutting tools with good quality, or test the hardness of the blank, and improve the casting quality of the blank.

(2) Internal leakage of hydraulic pump

When the oil temperature is too high, the viscosity of the hydraulic oil decreases, the screw of the end cover of the pump is loose, or the gap between the rotor and the end cover is increased due to wear, which can cause the internal leakage of the hydraulic pump, and the pressure oil can not be output, so there is no work in.

B.jpg

There are many reasons for the rise of oil temperature, the most important one is the invariants of hydraulic pump. When the roller (Part 2) is stuck due to wear, or the oil distribution plate and stator coil are crushed, and the feedback hydraulic oil can not push the stator, the variable can not be realized, resulting in a large amount of pressure oil flowing back to the oil tank through the relief valve, resulting in the loss of hydraulic power, generating a large amount of heat energy and increasing the temperature. The method to judge whether the pump is variable is: first, start the hydraulic pump, slowly increase the pressure of the safety valve without unloading the system, and observe the pressure gauge at the same time. If the pump variable is not good, the system pressure will rise all the time, which may exceed the rated pressure of the pump (6.3MPa). Another cause of oil temperature rise is improper system pressure adjustment. In order to prevent the components from being damaged due to the high pressure of the hydraulic system, the relief valve is set in the system, and its pressure is generally 0.2 ~ 0.3MPa lower than the pump pressure. However, if the relief valve is too much lower than the maximum pressure of the variable displacement pump, a large amount of hydraulic oil will flow back to the oil tank through the relief valve during "work in", making the oil hot.

To sum up, to solve the problem of high oil temperature, we should start from repairing the hydraulic pump and adjusting the system pressure. Can open the pump drain pipe to see if the leakage is very large to judge whether the hydraulic pump has internal leakage. If the above work is done, and the internal leakage is still very large, it is necessary to replace it with high viscosity hydraulic oil, especially in summer.

(3) Leakage of hydraulic cylinder and external pipeline

When the hydraulic cylinder and external pipeline leak, the system pressure can not be established, and the work progress cannot be realized. In particular, the leakage in the hydraulic cylinder is hidden and difficult to find. It can stop the worktable at the last position, open the backward oil return pipe, poke the backward reversing valve with hand to see if there is oil outflow, if there is, it is internal leakage. Then it is to see if the internal resistance of the hydraulic cylinder is very large. The resistance of hydraulic cylinder is generally due to the aging, deformation and softening of the seal ring on the piston or the oil seal on the end cover of the hydraulic cylinder due to the high oil temperature. It is squeezed between the piston and the cylinder wall, the piston rod and the end cover hole, resulting in huge friction and offsetting part of the hydraulic pressure. Therefore, it is unable to overcome the cutting force and the worktable does not work. At this time, you can see the reading of the pressure gauge when fast forward. If the fast forward pressure is large, it means that the resistance in the cylinder is large; You can also disassemble the oil pipe, loosen the pressure plate of the sliding table and pry the sliding table. If you feel it is very heavy, you can judge that the resistance in the cylinder is very large. If this phenomenon has been confirmed, it is necessary to disassemble the hydraulic cylinder for inspection and replace the sealing ring.

(4) Inspection of various pressure valves

If the pressure valve (such as relief valve) can not achieve the rated pressure due to wear of mating pair or fatigue of spring, or the change-over valve is stuck and not in the middle position due to oil pollution, so that the pressure oil directly returns to the oil tank, the system pressure can not be established. These can be seen from the pressure gauge. As shown in the schematic diagram a, the method of blocking can be used to determine which valve is leaking, that is, the inlet and outlet of the valve are blocked to see if the pressure can rise. If not, the valve will leak.

Based on the above fault causes, quick judgment and repair should follow the steps shown in Figure C.

C.jpg

According to the analysis steps of the tree diagram, the general direction of the fault can be judged according to the reading of the pressure gauge, and then the oil pipe can be disassembled to judge the internal leakage of the hydraulic pump, the internal leakage of the hydraulic cylinder, the stiffness of the worktable and other faults, and then the valve fault can be judged by blocking. Because the faults of hydraulic pump and hydraulic cylinder are easy to judge, but the repair time is long, in order to reduce the repair time, they should be checked and eliminated first. If there is any problem, they can be replaced first, and then repaired, which can be used for the next time. This method is especially suitable for the production line, because there are many modular machine tools of the same type in the production line, which is convenient to reserve spare hydraulic parts, and the production rhythm is tight, and the downtime is very limited. Using this method, not only can reduce the downtime, but also can alleviate the contradiction of "more idle time, less busy time" of most enterprise machine maintenance workers.

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