Created on:2021-06-29 14:52

Hydraulic fault diagnosis and maintenance of hydraulic press

Hydraulic fault diagnosis and maintenance of hydraulic press (2)

The HPM. 200L press imported from a factory and the domestic 100t four column shaping press all have similar faults: after a period of normal pressing, the oil temperature rises and the system pressure drops sharply. After careful inspection, it is caused by the electromagnetic relief valve.

(1) Structural analysis

The electromagnetic relief valve assembly is mainly composed of pressure valve plug-in, pilot pressure regulating valve and solenoid valve directional valve (some also carry buffer valve), as shown in Figure V (a). This form of electromagnetic relief valve is widely used in the hydraulic system. It mainly controls the loading and unloading of the system. The following is an analysis of its opening process.

① When port a of the pressure valve plug-in is filled with pressure oil, the pressure oil not only acts on the lower chamber of the main valve core directly, but also enters the control chamber C and the oil inlet of the pilot pressure regulating valve 2 through the damping holes R1 and R2. When PA is lower than the set pressure (P1) of the pilot pressure regulating valve, PA = PV = PC, and there is no liquid flow, When the action area SC of the upper cavity (C cavity) is greater than or equal to the action area Sa of the lower cavity (a cavity), plus the action of the spring force F, there is (PCSC + F) "Pasa, PC = PA, SA ≤ SC, and the main valve core is closed reliably.

② When PA continues to increase and PA > P1, the pilot pressure regulating valve opens, and the inlet pressure oil returns to the oil tank through R1 branch. At this time, due to the damping effect of damping hole R1, that is, there is a pressure difference (PA - PV) before and after the damping hole, there is pa > PV, and there is no liquid flow in R2, so there is PV = PC = P1; When the pressure difference (PA - PV) is large enough, the upward hydraulic pressure formed by the pressure difference is greater than the spring force F, (Pasa - PCSC) > F, the main valve core of the pressure valve plug-in moves up, the main valve port opens, and the electromagnetic relief valve begins to overflow. During this opening process, due to the damping effect of R2, there is PC > PV = P1. When the main valve core opening of the pressure valve plug-in is fixed, the pilot pressure regulating valve core, the pilot pressure regulating valve core and the solenoid valve start to overflow When the main valve cores are in the state of pressure balance, there is PV - PC = P1, and the steady-state equation in the state of pressure balance is as follows:

Main spool flow

q=Cd π DY(2/ ρ pA )1/2 (1-1)

Balance equation of main valve core

pASA =p1SC+K(Y0+Y)+CK π DY(sin2 β) pA (1-2)

Flow of pilot pressure regulating valve

q1=QR1=( πφ 4 /128 μ l)(pA-p1) (1-3)

Where CD -- flow coefficient of main valve;

D -- diameter of main valve core;

K -- main valve spring stiffness;

CK -- flow parameters of main valve (CK = cdcy);

CV -- velocity coefficient;

Y0 - main valve pre compression shrinkage;

Y -- opening length of main valve;

β—— Half cone angle of main valve core;

Q -- flow rate of main valve core and valve port;

Q1 -- flow QRI of pilot pressure regulating valve with flow equal to R1;

μ—— Oil viscosity;

l, φ—— Length and diameter of damping hole R1.

This is the whole opening process of the electromagnetic relief valve. Obviously, the set pressure value P1 of the pilot pressure regulating valve determines PA.

(2) Fault analysis

This is the normal opening process of the electromagnetic relief valve. In fact, due to long-term use, the clearance between the valve core and the valve body of the electromagnetic directional valve increases, and the leakage also increases. In particular, the spool type directional control mode adopted by the electromagnetic directional valve has short oil sealing length and small diameter of the valve core, which increases the sensitivity of the leakage of the electromagnetic directional valve to oil pressure and temperature. For the zero opening slide valve, the leakage calculation formula is QX=( π WC3r/32 μ) PV。 It can be seen that the leakage QX of the electromagnetic directional valve is proportional to the third power of the radial clearance CR of the valve core and valve body. It can be seen that the requirement of lubrication for its sealing clearance is quite high. The slight increase of Cr leads to a sharp increase of the leakage. In addition, the temperature rise t also leads to an increase of the leakage, because the viscosity coefficient ‖ is inversely proportional to the temperature rise, The closed solenoid directional valve is regarded as a damping R3, which is inversely proportional to c3r and closely related to oil viscosity μ W is the gradient of valve core area, as shown in Figure V (b).

V.jpg

Obviously, R1 and R3 are connected in series (R1, R2 and R3 are hydraulic resistances), and when the electromagnetic relief valve assembly is normally closed:

R1 = constant, R2 = constant.

Flow rate of R1:

qR1=( πφ 4/128 μ l)(pA-pV) (1-4)

Leakage of electromagnetic directional valve:

qX=( π WC3r/32 μ) pV (1-5)

qR1=qX (1-6)

From the above three formulas and figure V, the following relationship can be deduced.

If R3 increases (CR vs μ Because R1 and R3 are in series, under the same working conditions (PA value is the same), the flow rate QX (qr1) decreases, the pressure difference (PA - PV) decreases, and PV increases. For the whole electromagnetic relief valve assembly, this is exactly what is expected. At this time, the leakage of the solenoid directional valve related to R3 decreases, and the decrease of (PA - PV) makes it difficult to open the main valve core of the pressure valve plug-in. The increase of R3 indicates that the solenoid directional valve is in a normal state. Only when PA rises to the opening of the pilot pressure regulating valve (P1), the solenoid relief valve assembly will open.

If R3 decreases (CR vs μ Because R1 and R3 are in series, under the same working condition (PA value is the same), QX (qr1) increases, pressure difference (PA - PV) increases and PV decreases. For the whole electromagnetic relief valve assembly, it is in an abnormal state. Take the limit state: R3 = 0, then the pilot pressure regulating valve is short circuited and does not work at all, and the electromagnetic relief valve is fully opened. It can be seen that the decrease of R3 leads to the increase of leakage, and the increase of leakage leads to the increase of pressure difference. When the pressure difference (PA - PV) overcomes the spring force, the main valve core opens slightly, and the PA value does not reach the required opening pressure. Therefore, when the decrease of R3 value affects the normal operation of the electromagnetic relief valve, there is 0 < PA ≤ P1. This shows that:

In the slide valve hydraulic control system, because the leakage of the slide valve is very small, when the system is pressurized, due to the compensation effect of the pump, there will be no pressure loss; However, due to the feedback effect of damping R1, the compensation of the pump for the leakage of the slide valve can not be realized normally, which leads to the abnormal opening of the cartridge electromagnetic relief valve.

(3) Case analysis

In fact, in the existing cartridge valve hydraulic system, there are many cases of pressure drop, and the pump source pressure (PP): 0 < PP < P1. After careful inspection, the possibility of pump, cylinder and other directional flow valve failure was ruled out. The following is an example analysis of a 100t four column press, as shown in Figure W.

W.jpg

For convenience, a part of the original hydraulic diagram is intercepted, and the hydraulic system is very simple: when the master cylinder piston descends, CT3, CT2, 4dt, 6DT and 5DT are powered on, and the master cylinder piston descends rapidly; When the travel switch K4 is contacted, 6DT is powered off and 7dt is powered on. The master cylinder piston slowly pressurizes and enters the integral die, and the system outputs full pressure; When the travel switch K4 is contacted, the work is in place, CT3, CT2, 4dt, 7dt and 5DT are powered off, the system pressure relief is delayed until the signal is sent, CT2, CT3 and 3DT are powered on, and the master cylinder piston returns.

However, after running for a period of time (1 ~ 2 h) and oil temperature rising, the system pressure can only reach about 9 MPa, while the required shaping pressure is about 16 MPa, which causes the master cylinder piston to stop halfway and cannot be shaped in place, and the system pressure is obviously insufficient.

It is understood that this device has been the existence of pressure problems, but not before now obvious. In addition, the number of products is small, which can be stopped. Once the number of products increases, the problem will be highlighted. This shows that the fault is a long-term formation, not a sudden failure, so we consider that it may be the problem of system leakage.

First of all, considering that the system pressure is provided by the piston pump, the gear pump only provides flow. Under the condition of no-load operation, the hydraulic pump is detected first, and it is considered that the leakage in the piston pump is serious, because the rise of oil temperature can also lead to similar faults. For this reason, when the solenoid valve CT3 is manually operated, it is found that the maximum pressure of the pump source can only reach about 9Mpa under the condition of no-load operation, while when the oil temperature is cooled and then the solenoid valve CT3 is manually operated, the maximum pressure of the pump source can reach about 32Mpa. Therefore, the possibility that the hydraulic valves controlled by 3DT and 4dt may have faults and tandem cavities is eliminated. Because it is useless to change the pump, the problem of the pump is also eliminated.

Secondly, there may be leakage in the electromagnetic relief valve assembly. After careful inspection, no obvious mechanical failure was found, and the leakage test of the main valve core of the pressure valve was done, and there was no problem.

There may also be gaps in the manifold block. After the whole disassembly, there was no problem at first. Later, after analysis, it was found that there was something wrong with the originally hidden electromagnetic directional valve.

Conclusion: when the clearance between the valve core and the valve body is too large and the temperature rise is obvious, the viscosity of the oil decreases and the micro leakage increases, resulting in the micro opening of the main valve core. If the cause is found, just replace the solenoid directional valve. This also gives us enlightenment: in order to reduce the leakage of the pilot pressure regulating valve, we should try to use the electromagnetic directional valve with small clearance between the valve core and the valve body and long matching section.

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