Tempo estimado de leitura: 9 minuto

A pressure-reducing circuit using a pressure-reducing valve makes a certain cylinder or a certain stroke in the circuit work at a pressure lower than the set pressure of the relief valve. The pressure reducing valve with a one-way valve is for the oil flow through the one-way valve without the pressure reducing element when the piston returns.

1. Issues to be Paid Attention to in The Design of The Decompression Circuit

Leakage Problem of Pilot-operated Pressure Reducing Valve

Pay attention to the situation that which the leakage of the pilot-operated pressure reducing valve is larger than that of other control valves. The leakage of this kind of valve can be as high as 1L/min or more, and the leakage always exists as long as the valve is in working condition. Take this into consideration when selecting hydraulic pump capacity. At the same time, it should also be noted that the minimum adjustment pressure of the pressure-reducing valve should ensure that the difference between the primary pressure and the secondary pressure is 0.31MPa.

The Problem of Secondary Pressure Instability

o damping hole of the main spool of the pressure reducing valve is blocked so that the oil at the outlet of the pressure reducing valve cannot smoothly flow into the cavity between the main valve and the pilot valve, and the pilot valve also reduces the control effect on the secondary pressure of the pressure reducing valve outlet, resulting in The secondary pressure is unstable. 

2. The Problem of Setting The Components of The Decompression Circuit

• As shown in the figure, the pressure of the rear branch of the pressure-reducing valve 3 (cylinder 2) is lower than the oil pressure of the branch of the master cylinder 1, which is called the pressure-reducing circuit. The problems with this circuit are as follows.
• When cylinder 2 stops for a long time, the secondary pressure behind the pressure reducing valve 3 gradually increases. This is because when cylinder 2 stops for a long time, a small amount of oil is discharged through the pilot valve through the valve core gap to keep the valve in a working state. Due to the leakage in the valve, the flow through the pilot valve increases, and the secondary pressure (outlet pressure) of the pressure-reducing valve increases. To prevent this failure, the oil circuit shown by the dotted line in the figure can be added to the pressure reducing circuit, and a safety valve is installed at b to ensure that the outlet pressure of the pressure reducing valve does not exceed its adjustment value.
• The speed adjustment of the hydraulic cylinder in the decompression circuit fails or the speed is unstable. As shown in the figure, this problem occurs when the leakage of pressure-reducing valve 3 (the oil flowing back to the tank from the drain port of the pressure-reducing valve) is large. The solution is to change the throttle valve from the position in the figure to the position behind the pressure-reducing valve in series so that the influence of the leakage of the pressure-reducing valve on the speed of cylinder 2 can be avoided.

3. Unstable Working Pressure of The Decompression Circuit

• In the system shown in the figure, the hydraulic pump is a quantitative pump. The hydraulic cylinders 7 and 8 in the main oil circuit are controlled by the two-position four-way electro-hydraulic reversing valves 5 and 6, respectively. The control oil of the electro-hydraulic reversing valve comes from the main oil circuit and the decompression circuit is connected in parallel with the main oil circuit. After decompression by decompression valve 3, the movement direction of hydraulic cylinder 9 is controlled by the two-position four-way electromagnetic reversing valve. The return oil circuit of the control oil circuit of the electro-hydraulic reversing valve and the external leakage oil circuit of the pressure reducing valve are combined and returned to the fuel tank, and the working pressure of the system is adjusted by the relief valve 2.
• The main oil circuit in the system works normally, but in the decompression circuit, the downstream pressure of the decompression valve fluctuates greatly, so that the working pressure of the hydraulic cylinder 9 cannot be stabilized at the set pressure value of 1MPa.
• In the decompression circuit, the downstream pressure of the decompression valve, that is, the working pressure of the decompression circuit has a large fluctuation, which is a frequent failure phenomenon. The main reasons are as follows.

• The precondition for the pressure reducing valve to stabilize the downstream pressure of the valve at the set value is that the upstream pressure of the pressure reducing valve is higher than the downstream pressure, otherwise the downstream pressure of the pressure reducing valve cannot be stabilized. When the actuator in the main oil circuit of the hydraulic system is indifferent working conditions, the working pressure will change significantly. If the minimum pressure value of this change is lower than the downstream pressure of the pressure-reducing valve, it will have a great impact. Because when the upstream pressure of the pressure reducing valve increases, the downstream pressure of the pressure reducing valve may be increased instantaneously, but it is quickly restored to the set value of the pressure reducing valve after the adjustment of the pressure reducing valve; on the contrary, when the pressure of the pressure reducing valve upstream When lowering, the downstream pressure of the pressure reducing valve should also be reduced instantaneously, but the pressure reducing valve will adjust quickly to increase the downstream pressure to the set value. If the upstream pressure of the pressure reducing valve fluctuates and changes, and its minimum pressure is lower than the set value of the downstream pressure value of the pressure reducing valve, the downstream pressure of the pressure reducing valve will decrease accordingly, and cannot be stabilized at the set pressure value. Therefore, in the working condition of the load change of the main oil circuit actuator, when the minimum working pressure is lower than the downstream pressure of the pressure reducing valve, the design of the circuit should take necessary measures, such as adding a check valve in front of the pressure reducing valve, one-way An accumulator can also be added between the valve and the pressure reducing valve to prevent the upstream pressure of the pressure reducing valve from being lower than the downstream pressure of the pressure reducing valve when the pressure changes.
• The load of the actuator is unstable. In the decompression circuit, the downstream pressure can be stabilized only due to the regulating effect of the decompression valve. On the premise that the actuator has sufficient load, the downstream pressure of the decompression valve must still follow the objective law that the pressure depends on the load. No-load can create pressure, and with low load, pressure is low. If in the pressure reducing circuit, the pressure behind the valve of the pressure reducing valve is set under the load condition at a certain time, but during the working process of the pressure reducing circuit, the load decreases, and the downstream pressure of the pressure reducing valve will decrease until the pressure decreases. zero pressure. When the load increases again, the downstream pressure of the pressure-reducing valve increases accordingly. When the pressure increases with the load to the regulated pressure of the pressure reducing valve, the pressure does not increase with the increase of the load but remains at the adjustment pressure of the pressure reducing valve. on a constant pressure value. Therefore, under variable load conditions, the downstream pressure value of the pressure reducing valve varies, and this variation range can only be lower than the setting value of the pressure reducing valve, but not higher than the setting value.
• There is back pressure in the external drain circuit. The control oil circuit of the pressure reducing valve is an external leakage oil circuit, that is after the control oil pushes open the poppet valve, it returns to the oil tank separately. If there is back pressure on this external leakage oil circuit and the back pressure is changing, it will directly affect the pressure of the pressure oil that pushes the poppet valve, causing pressure changes, resulting in changes in the downstream working pressure of the pressure reducing valve.

• The fault phenomenon in the system shown in the figure, after inspection and analysis, is caused by the change of backpressure in the oil drain circuit outside the pressure-reducing valve.
• It is not difficult to see that during the reversing process of the electro-hydraulic reversing valves 5 and 6 in the system, the oil return flow and pressure of the control oil circuit are changed. The oil in the external leakage oil path of the pressure reducing valve also fluctuates, and the two oil liquids merge to produce unstable backpressure. After debugging, it is found that when the electro-hydraulic reversing valves 5 and 6 act at the same time, the reading of the pressure gauge 10 reaches 1.5MPa. This is because the electro-hydraulic reversing valve has a large instantaneous flow under the action of high-pressure control oil, and when the oil drain pipe is long, high backpressure is generated, and the backpressure increases, making the main valve port of the pressure reducing valve. As the opening degree increases, the partial pressure of the valve port decreases, so the working pressure of the pressure-reducing valve increases.
• To eliminate this fault, the oil leakage pipe of the pressure reducing valve and the control oil pipes of the electro-hydraulic reversing valves 5 and 6 should be separately connected to the oil tank, so that the leakage oil of the pressure reducing valve can flow back stably. The fuel tank will not produce interference and fluctuations, and the downstream pressure will be stable at the set pressure value.
• It can be seen from the above analysis that in the process of system design and installation while understanding the working performance of each component, we should seriously consider whether the components will interfere with each other.