Circuitos redutores de pressão de prensa hidráulica

Tempo estimado de leitura: 9 minutos

A pressure-reducing circuit using a pressure-reducing valve (Figure 1-1) allows a cylinder or stroke in the circuit to operate at a pressure lower than the set pressure of the relief valve. A pressure reducing valve with a check valve is designed so that when the piston returns, the oil flow passes through the check valve without passing through the pressure reducing element.

Problem 1: Design of pressure reducing circuits to be note

1. Leakage of pilot-operated pressure reducing valves

It is important to note that the leakage of pilot-operated pressure reducing valves is larger than that of other control valves. The leakage of such valves can be as much as 1 L./ min or more, and the leakage is always present as long as the valve is in working condition. This should be fully taken into account when selecting the capacity of the hydraulic pump. 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.

2. Secondary pressure instability problem

Pressure reducing valve main spool damping hole blockage, so that the pressure reducing valve outlet oil can not smoothly flow between the main valve and the pilot valve captive cavity, the pilot valve also reduces the pressure reducing valve outlet secondary pressure control role, resulting in secondary pressure instability.

Problem 2: Pressure reducing circuit component setting problem

As shown in Figure 1-2, the pressure in the branch circuit (cylinder 2) after the pressure reducing valve 3 is lower than the oil pressure in the branch circuit of the main cylinder 1, called the pressure reducing circuit. There are several problems with this type of circuit as follows.

Firstly, When cylinder 2 is stopped for a long time, the secondary pressure after the pressure reducing valve 3 gradually rises. This is because when cylinder 2 is stopped for a longer period time, a small amount of oil is discharged through the spool gap by the pilot valve to keep the valve in working condition. 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, you can add the oil circuit shown in the dashed line in the pressure reducing circuit, and install a safety valve at b to ensure that the pressure reducing valve outlet pressure does not exceed its adjustment value.

Secondly, The speed adjustment of the hydraulic cylinder in the decompression circuit is out of order or the speed is unstable. As shown in Figure 1-2, this problem occurs when the pressure reducing valve 3 leakage (the oil flowing back to the tank from the pressure reducing valve relief port) is large. The solution is to change the throttle valve from its position in the diagram to one connected in series after the pressure reducing valve so that the effect of the pressure reducing valve leakage on the speed of cylinder 2 can be avoided.

Problem 3: Unstable operating pressure in the pressure reducing circuit

In the system shown in Figure 1-3, the hydraulic pump is a quantitative pump, the hydraulic cylinders 7 and 8 in the main oil circuit is controlled by the two-position four-way electro-hydraulic reversing valve 5 and 6 respectively to control the direction of movement, the control fluid of the electro-hydraulic reversing valve comes from the main oil circuit, the decompression circuit is connected in parallel with the main oil circuit, and after decompression by the decompression valve 3, the two-position four-way electromagnetic reversing valve controls the direction of movement of the hydraulic cylinder 9. The electro-hydraulic reversing valve controls the return of the oil circuit and the discharge circuit of the pressure reducing valve and then returns to the tank. The working pressure of the system is regulated by the relief valve 2.

The main oil circuit in the system works normally, but in the pressure reducing circuit, the downstream pressure of the pressure reducing valve fluctuates greatly, so that the working pressure of the hydraulic cylinder 9 cannot be stabilized at the regulated pressure value of 1MPa.

In the pressure reducing circuit, the downstream pressure of the pressure reducing valve is the working pressure of the pressure reducing circuit, the occurrence of large fluctuations is a frequent failure phenomenon, the main reasons for which are the following aspects.

Firstly, a pressure-reducing valve can make the valve downstream pressure stable on the adjusted value of the prerequisite is a pressure reducing valve upstream pressure to be higher than the downstream pressure, otherwise, pressure reducing valve downstream pressure can not be stable. In the main oil circuit of the hydraulic system. If the minimum pressure of such a change is lower than the downstream pressure of the pressure reducing valve, it will have a greater impact. Because when the upstream pressure of the pressure reducing valve is increased, the downstream pressure of the pressure reducing valve may be increased instantaneously, but the regulating effect of the pressure reducing valve will be quickly restored to the regulating value of the pressure reducing valve: conversely, when the upstream pressure of the pressure reducing valve is reduced, the downstream pressure of the pressure reducing valve will also be reduced instantaneously, but the pressure reducing valve will be quickly adjusted to make the downstream pressure rise to the regulating value. If the upstream pressure of the pressure reducing valve fluctuates and its minimum pressure is lower than the downstream pressure value of the pressure reducing valve, the downstream pressure of the pressure reducing valve should be reduced accordingly and cannot be stabilized at the regulated pressure value. Therefore, in the main oil circuit actuator load changes in the operating conditions, the minimum working pressure is lower than the downstream pressure of the pressure reducing valve, the design of the road should take the necessary measures, such as a check valve before the valve of the pressure reducing valve, check valve and pressure reducing valve can also be added between the accumulator, etc., to prevent the upstream pressure of the pressure reducing valve changes below the downstream pressure of the pressure reducing valve.

Secondly, the load on the actuator is unstable. In a pressure-reducing circuit, the downstream pressure can only be a stable value due to the regulating effect of the pressure-reducing valve. On the premise that the actuator has sufficient load, the downstream pressure of the pressure-reducing valve still has to follow the objective rule that the pressure is determined by the load. Without a load, no pressure can be formed, and with a low load, the pressure is lower. If the pressure reducing circuit, the pressure after the valve of the pressure reducing valve is set under the load condition at a certain moment, but in the working process of the pressure reducing circuit, the load is reduced, the downstream pressure of the pressure reducing valve should be reduced until it drops to zero pressure. When the load increases again, the downstream pressure of the pressure reducing valve increases, and when the pressure increases with the load to the regulating pressure of the pressure reducing valve, the pressure does not increase with the load but remains at the regulating pressure value of the pressure reducing valve. Therefore, under variable load conditions, the downstream pressure of the pressure reducing valve is variable, and the range of this variation can only be lower than the regulating value of the pressure reducing valve, but not higher than the regulating value.

Thirdly, there is back pressure in the external drainage oil circuit. The control oil circuit of the pressure reducing valve is the external drainage oil circuit, i.e. the control oil pushes open the cone valve and returns to the oil tank alone. If there is back pressure on this external drain line and the back pressure is changing, it will directly affect the pressure of the pressure oil pushing the cone valve, causing pressure changes and thus leading to changes in the downstream working pressure of the pressure reducing valve. The fault phenomenon in the system shown in Figure 1-3 is analyzed by inspection and is caused by a change in back pressure in the external drain line of the pressure reducing valve. It is easy to see that the electro-hydraulic reversing valves 5 and 6 in the system in the process of reversing, control oil flow, and pressure of the return oil circuit is changed. The oil in the external drainage circuit of the pressure reducing valve is also fluctuating, and the two streams of oil merge to produce unstable backpressure. After debugging, it was found that when the electro-hydraulic reversing valve 5 and 6 acted at the same time, the reading of pressure gauge 10 reached 1. 5 M Pa. This is because the electro-hydraulic reversing valve is under the action of a high-pressure control fluid, the instantaneous flow is larger, and in the case of a longer drainage pipe, higher backpressure is generated, and the backpressure increases, so that the opening of the main valve opening of the pressure reducing valve increases, and the local pressure at the valve opening decreases, so the pressure reducing valve. The working pressure rises.

To eliminate this fault, the drainage oil pipe of the pressure reducing valve and the control oil pipe of the electro-hydraulic reversing valve 5 and 6 should be connected back to the oil tank separately, so that the drainage oil of the pressure reducing valve can flow back to the oil tank steadily, without interference and fluctuation, and the downstream pressure will be stabilized at the regulated pressure value.

The above analysis shows that in the design and installation of the system while understanding the working performance of each component, careful consideration should be given to whether the components will interact with each other. The components should be carefully considered to see if they will interfere with each other.