Tempo di lettura stimato: 11 minuto

In modern machinery, the applied idraulico transmission system is composed of some basic circuits. The so-called basic circuit is a typical oil circuit composed of related hydraulic components to complete specific functions. By mastering their working principle, composition and characteristics, these circuits can be selected correctly and reasonably according to the working performance, requirements and working conditions of the machine to form the required complete sistema idraulico.

Pressure Control Loop Design

The pressure control circuit is a circuit that uses a pressure control valve to control the entire or partial pressure of the system. The pressure circuit controlled by the pressure valve can be used to realize the control of voltage stabilization, decompression, boosting, and multi-stage pressure regulation to meet the requirements of the actuator in terms of force and torque. The pressure valves of standard components include overflow valves, pressure reducing valves, sequence valves, and one-way pressure reducing valves, and one-way sequence valves that are combined in parallel with one-way valves.

Pressure Regulating Circuit

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

Point 1: Selection of Pressure Regulation Method

• Pressure Limiting Circuit

It is best to use an overflow valve to limit the maximum pressure in a hydraulic circuit. The picture 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 own weight when the cylinder piston rises (not working) to the end. In this way, power consumption can be saved, and the heating phenomenon of oil spilled from the overflow valve can be avoided.

• Pressure Remote Control Loop

As shown in the picture, when the solenoid three-way valve is demagnetized, the loop pressure is the set pressure of the main relief valve 10MPa; when the solenoid three-way valve is excited, the four-way solenoid valve is used to change the main valve and the remote control relief The passage of valve a or b can convert the pressure of the main circuit to 7MPa or 5MPa. The capacity of each valve, except for the main valve, is a small flow valve.

• Secondary Pressure Regulating Circuit

In the picture, when the cylinder piston rises and falls and the piston remains at the highest position, the oil pressure is p₁ = 5MPa (the left high-pressure pump unloads). 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 high-pressure oil enters the circuit.

• Pressure Regulating Circuit of Compound Pump

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

Point 2: Pressure Parameter Adjustment

• Improper Set Pressure of Relief Valve

Improper setting pressure of the relief valve causes the hydraulic cylinder movement speed to fail to meet the requirements. The loop requires smooth movement when lifting, a wide range of speed adjustment, and the piston can stop at any position. However, during operation, when adjusting the lifting speed of the elevator, the speed does not change in a large range. Only when the throttle valve opening is adjusted to a very small amount, the lifting speed will change, which fails to meet the due performance requirements. This is because the pressure of the overflow valve has been increased. The set pressure of the relief valve should be the working pressure of the hydraulic pump exactly equal to the sum of 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 oil supply system of the constant pressure pump to be too high. In the hydraulic circuit of the constant pressure pump, as shown in the picture, 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 that the constant pressure pump always works at the maximum displacement, and the excess flow is the pressure P, overflows back to the fuel tank, and is all converted into heat, which increases the temperature of the system. Therefore, use valve 1 as a safety valve and adjust its pressure to be 0.5~1MPa higher than the maximum pressure required by the system. The above problems can be solved.

• Examples of Pressure Parameter Adjustment Failure

In the quantitative pump pressure control circuit shown in the picture, the hydraulic pump is quantitative, and the neutral function of the three-position four-way reversing valve is Y-type. Therefore, when the hydraulic cylinder stops running, the system does not unload, and the pressure oil output by the hydraulic pump is all overflowed back to the oil tank by the overflow valve. The overflow valve in the system is a YF pilot-operated overflow valve, which has a three-stage concentric structure.

There is a problem: when the reversing valve in the system is placed in the neutral position and the pressure of the overflow valve is adjusted, it is found that when the pressure value is below 10MPa, the overflow valve works normally; when the pressure is adjusted to any pressure value higher than 10MPa, The system screamed like a flute. At this time, you can see the pressure gauge pointer vibrating violently. After testing, it was found that the noise came from the overflow valve.

Problem analysis: In the three-stage coaxial high-pressure relief valve, the main spool has two sliding fits with the valve body and bonnet. If the coaxiality  of the inner hole of the valve body and the bonnet after assembly exceeds the design requirements, the main spool can’t move flexibly, but stick to one side of the inner hole to do the abnormal movement. When the pressure is adjusted to a fixed value, it will inevitably arouse the vibration of the main spool. This kind of vibration is not the normal vibration of the main spool in the working movement, but the high-frequency vibration aroused by the main spool stuck in a certain position (at this time because the main spool bears the hydraulic clamping force at the same time). This kind of high-frequency vibration will inevitably cause strong vibration of the spring, especially the pressure regulating spring, and noise resonance will occur. In addition, because the high-pressure oil does not overflow through the normal overflow port, but overflows back to the tank through the jammed overflow port and the internal drain channel, this high-pressure oil flow will emit high-frequency fluid noise. This kind of vibration and noise are excited under the specific operating conditions of the system, which is the reason why there is no squeal when the pressure is lower than 10MPa.

Solution:

The manufacturing precision of the YF relief valve is relatively high. The coaxiality of the inner and outer circular surfaces of the connecting part of the valve cover 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 is vibrating. When the viscosity of the working fluid is low or the temperature is too high, the damping effect will be reduced accordingly. Therefore, choosing the appropriate viscosity oil and controlling the Excessive temperature rise of the system is also conducive to vibration reduction and noise reduction.

• Pressure Parameter Adjustment Failure Problem

1. The pressure cannot be adjusted up. The main reason is that the pressure regulating spring of the overflow valve is too soft, incorrectly installed or missing; the main valve orifice of the pilot-operated overflow valve is blocked, and the spool valve overcomes the hydraulic pressure of the upper cavity and the main valve spring under the action of the oil pressure at the lower end Force, the main valve moves upward, and the pressure regulating spring loses its control of the main valve. Therefore, the main valve opens the overflow port at a 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 valve core is stuck in the closed position by burrs or dirt, and the main valve cannot be opened; during installation, the valve inlet and outlet ports are connected incorrectly, and there is no pressure oil to push the valve core to move, so the valve core cannot be opened; pilot valve The front orifice is blocked, causing the main valve to fail to open.
3. The pressure swing is large. The main reason is that the oil is mixed with air; the valve core is in poor contact with the valve seat; the diameter of the orifice is too large, and the damping effect is weak; resonance occurs; the valve core does not move flexibly in the valve body. For the above-mentioned problems, it is possible to make targeted improvements in the aspects of circuit design, component selection, component parameters and system adjustment, pipeline installation, and use of hydraulic oil.

Point 3: Problems in The Secondary Regulator Circuit

• Pressure Shock Problem

In the second pressure regulating circuit shown in the picture, when the 1DT is not energized, the system pressure is regulated by the overflow valve 2; when the 1DT is energized, the system pressure is regulated by the overflow valve 3. The pressure switching of this circuit is realized by valve 4. When the pressure is switched from force to p2 (p>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) When the instantaneous pressure at the remote port of the overflow valve 2 drops from the force to almost zero and then rises to p2, the system naturally produces a larger pressure shock.

Method of Exclusion:

As shown in the picture, connect valve 4 to the oil outlet of valve 3, that is, the positions of valve 4 and valve 3 are interchanged. Because of this, there is often Full of pressure oil, the system pressure will drop from p to p₂ when valve 4 is switched, and there will be no excessive pressure shock.

• The Problem of Long Boost Time During Pressure Regulation.

In the second pressure regulating circuit shown in the picture, when the remote control pipeline is long, and the system changes from unloading (valve 3 in the neutral position) state to boosting state (valve 3 in the left or right position) At this time because the remote control pipe passes through the oil pool, the pressure oil must be filled with the remote control pipe before the pressure can be increased, so the pressure increase time is long.

Pay attention to This:

Try to shorten the remote control pipeline as much as possible, and add a back pressure valve (or one-way valve) 5 at the oil return point of the remote control pipeline to make it have a certain pressure so that the pressure rise time can be shortened. In the remote pressure regulating circuit, the minimum pressure value of the overflow valve increases, and at the same time, the malfunction of the action lag occurs. The cause of this failure is that the piping from the main relief valve to the remote pilot relief valve is too long (for example, more than 10m), and the pressure loss in the remote control pipe is too large. Therefore, the remote control pipeline generally cannot exceed 5m.