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A pressure control loop is a loop that uses a pressure control valve to control the overall or partial pressure of a system.

los pressure circuit controlled by the pressure valve can be used to realize the control of voltage regulation, decompression, pressure increase, and multi-stage pressure regulation to meet the requirements of the actuator in terms of force and torque. Standard components of the pressure valve include relief valve, pressure reducing valve, sequence valve, and one-way pressure reducing valve, and one-way sequence valve combined with a one-way valve in parallel.

los pressure regulating circuit refers to the working pressure of the control system, so that it does not exceed a pre-adjusted value, or makes the working mechanism have different pressures in each stage of the moving process.

1. Selection of Voltage Regulation Mode

• Voltage Limiting Circuit

In hydraulic circuits, it is best to use a relief valve to limit the maximum pressure. The figure is the circuit of a common pressure processing machine. The low-pressure relief valve 1 is used to keep the piston from falling due to its weight when the cylinder piston rises (not working) and reaches the endpoint. This saves power consumption and avoids the heating phenomenon of oil spilling from the overflow valve.

• Pressure Remote Control Circuit

As shown in the figure, when the electromagnetic three-way valve is demagnetized, the circuit pressure is the set pressure of the main relief valve 10MPa; when the electromagnetic three-way valve is excited. And by changing the surge path of the main valve and the overflow valve a or b for the rumor control through the four-way solenoid valve, the pressure of the main circuit can be converted to 7MPa or 5MPa. The capacity of each valve, except the main valve, all use small flow valves.

• Secondary Pressure Regulating Circuit

In the figure, when the cylinder piston rises and falls and the piston is kept at the highest position, the oil circuit pressure is p,=5MPa (the left high-pressure pump is unloaded). But when the piston reaches the bottom, the load increases, the pressure relay works, and the electromagnetic three-way valve is manipulated to make p1=10MPa, and the high-pressure oil enters the circuit.

• Pressure-regulating Circuit of The Compound Pump

In the design, the capacity of the pump must be adapted to the requirements of the job and to reduce the generation of useless heat when driven at low speeds. The circuit is electrically controlled and can work with various flow rates and oil pressures as required to maintain maximum circuit efficiency, with the advantages of a pressure-compensated variable pump. The control oil circuit of the electro-hydraulic reversing valve in the circuit is led out from the remote control port of the relief valve, which prevents the impact caused by the switching of the main reversing valve.

2. Adjustment of Pressure Parameters

• The Set Pressure of The Relief Valve Is Improper

Improper set pressure of the relief valve causes the movement speed of the hydraulic cylinder to fail to meet the requirements. The circuit of the figure requires smooth movement when lifting and lowering, a large speed adjustment range and the piston can stop at any position. However, during operation, when adjusting the ascending speed of the elevator, the speed does not change in a wide range. Only when the throttle valve opening is adjusted to a very small size, the ascending speed change, which cannot meet the due performance requirements. This is because the pressure of the relief valve is increased. The set pressure of the relief valve should be the sum of the working pressure of the hydraulic pump exactly equal to the load pressure of the hydraulic cylinder and the pressure drop required when the full flow of the pump passes through the throttle valve.

• Improper Pressure Setting Parameters

Improper pressure setting parameters cause the oil temperature of the constant pressure pump oil supply system to be too high. In the hydraulic circuit of the constant pressure pump shown in the figure, due to improper pressure setting parameters, the oil temperature is too high when the system is running. The reason for the above problem is that the system pressure P set by pressure valve 1 is lower than the pressure Pt set by the pressure regulating spring of valve 2, so the pressure is used as T, and the valve is used under the row star. The pressure P is overflowed back to the fuel tank, and all of it is converted into heat, which increases the temperature of the system. Therefore, valve 1 is used as a safety valve, and its pressure is adjusted to 0.5-1MPa higher than the maximum pressure required by the system. The above problems are solved. can be solved.

• Examples of Pressure Parameter Adjustment Failure

In the pressure control circuit of the quantitative pump, as shown in the figure, the hydraulic pump is quantitative, and the neutral function of the three-position four-way reversing valve is a Y type. Therefore, when the hydraulic cylinder stops running, the system is not unloaded, and the pressure oil output by the hydraulic pump is all overflowed into the oil tank by the overflow valve. The relief valve in the system is a YF-type pilot-operated relief valve, and the structure of this relief valve is a three-stage concentric type.

There is a problem: when the reversing valve in the system is placed in the neutral position, and the pressure of the relief valve is adjusted, it is found that when the pressure value is below 10MPa, the relief valve works normally; when the pressure is adjusted to any pressure value higher than 10MPa, the system emits a screeching sound like a flute, and at this time, the pointer of the pressure gauge vibrates violently. After testing, it was found that the noise came from the overflow valve.

Analysis of the problem: In the three-stage coaxial high-pressure relief valve, the main valve core has two sliding fits with the valve body and the valve cover. If the coaxiality of the inner hole after the valve body and the valve cover is assembled exceeds the design requirements, the main valve core cannot move flexibly but sticks to one side of the inner hole to make abnormal movements. When the pressure is adjusted to a certain value, the main spool will inevitably vibrate. This kind of vibration is not the normal vibration of the main spool during the working movement, but the high-frequency vibration caused by the main spool stuck in a certain position (at this time, the main spool is simultaneously subjected to hydraulic clamping force). This high-frequency vibration will inevitably cause strong vibration of the spring, especially the pressure regulating spring, and cause noise resonance.

In addition, since the high-pressure oil does not overflow through the normal overflow port, but overflows back to the fuel tank through the stuck overflow port and the inner drain passage, this high-pressure oil flow will emit high-frequency fluid noise. This vibration and noise are excited under the specific operating conditions of the system, which is why there is no squealing sound when the pressure is below 10MPa.

Solution: The manufacturing precision of the YF type relief valve is relatively high. The coaxiality of the inner and outer circular surfaces of the connecting part of the bonnet and the valve body, and the coaxiality of the outer circular surfaces of the three shoulders of the main valve core should be within the specified range. In addition, the damping hole on the main spool has a damping effect when the main spool vibrates. When the viscosity of the working oil is low or the temperature is too high, the damping effect will be reduced accordingly. The high-temperature rise of the system is also conducive to vibration reduction and noise reduction.

• The Problem of Pressure Parameter Adjustment Failure

1. The pressure cannot be adjusted up. The main reason is that the pressure regulating spring of the relief valve is too soft, wrongly installed, or missing; the main valve damping hole of the pilot relief valve is blocked, and the slide valve overcomes the hydraulic pressure of the upper chamber and the main valve spring under the action of the oil pressure at the lower end. Therefore, the main valve opens the overflow port under lower pressure to overflow; the valve core and valve seat are not tightly closed, and the leakage is serious; the valve core is Burrs or other dirt stuck in the open position.
2. The pressure is too high and cannot be adjusted down. The main reason is that the spool is stuck in the closed position by burrs or dirt, and the main valve cannot be opened; during installation, the oil inlet and outlet of the valve are connected incorrectly, and there is no pressure oil to push the spool to move, so the spool cannot be opened; the pilot valve The front orifice is blocked, so that the main valve cannot be opened.
3. The pressure swing is large. The main reason is that the oil is mixed with air; the valve core and the valve seat are in poor contact; the diameter of the damping hole is too large, the damping effect is weak; resonance occurs; the valve core is not flexible in the valve body.
4. For the above problems, it can be improved in terms of circuit design, component selection, component parameters and system adjustment, pipeline installation, and hydraulic oil use.

3. Problems in The Secondary Pressure Regulating Circuit

• Pressure Shock Problem

In the second pressure regulating circuit shown in the figure, when the 1DT is not energized, the system pressure is regulated by the relief valve 2; when the 1DT is energized, the system pressure is regulated by the relief valve 3, and the pressure of this circuit is switched It is realized by valve 4. When the pressure is switched from p1 to p2, since there is no pressure in the oil circuit between valve 4 and valve 3 before switching, when valve 4 is switched (1DT is energized), the relief valve 2 at the remote control port When the instantaneous pressure drops from p to almost zero and then rises back top, the system naturally produces a large pressure shock.