Created on:2022-04-09 09:33

Working principle of several common directional valves

Working principle of several common directional valves

1. Classification of directional valve

Directional valve can be divided into slide valve type, rotary valve type and cone valve type according to structure type and movement mode; According to the installation mode of the valve, it can be divided into pipe type, plate type, flange type, etc; According to the number of main oil circuits connected to the valve body, it can be divided into two-way, three-way, four-way, etc; According to the working position of the valve core in the valve body, it can be divided into two, three, four, etc; The valve core can be divided into electro-hydraulic and electro-hydraulic operation modes; According to the positioning mode of valve core, it can be divided into steel ball positioning and spring return. Among them, slide valve directional valve is widely used in hydraulic system, so this section mainly introduces slide valve directional valve.

2. Working principle of reversing valve

The work of the reversing valve is to use the change of the relative working position between the valve core and the valve body to connect, disconnect or change the direction of oil flow, so as to control the start, stop or reversing of the actuator. The working principle of the directional valve is shown in Figure 17. When there is no hydraulic oil in the two chambers of the hydraulic cylinder, the piston is in the shutdown state. If the spool of the directional valve is moved to the left, the oil ports P and a and B and t on the valve body are connected. At this time, the hydraulic oil enters the left chamber of the hydraulic cylinder through P and a, and the oil in the right chamber returns to the oil tank through B and T, and the piston moves to the right. Conversely, if the valve core is moved to the right, P and B are connected, a and T are connected, and the piston moves to the left.

3. Several common directional valves

(1) Motorized directional valve. Motorized directional valve fork scale travel valve. It must be installed near the hydraulic cylinder, and the valve core is pressed down by the stop or cam installed on the moving parts to shift the valve. Figure 18 shows the structural principle and symbols of the two position four-way motorized directional valve. Motorized directional valve is usually a spring return two position valve. The structure of the reversing cam and the position of the attack angle can be changed simply and reliably, so that the shape of the reversing cam can be changed, and the position of the attack angle can be reduced.

Refer to relevant product samples for specifications and models of motorized directional valve.

(2) Solenoid directional valve. It is a directional valve that uses the suction of electromagnet to manipulate the transposition of valve core. Figure 19 shows the structural schematic diagram and symbols of the three position four-way solenoid directional valve. There is an electromagnet and a centering spring at both ends of the valve, and the valve core is in the middle position under normal conditions. When the electromagnetic iron at the right end is energized and engaged, the armature pushes the valve core to the left end through the push rod, and the directional valve works in the right position; On the contrary, when the electromagnetic iron at the left end is energized and closed, the directional valve will work in the left position.

Figure 20 shows the symbol of the two position four-way solenoid directional valve. Figure 20 (a) shows the spring return type and Figure 20 (H) shows the double electromagnet steel ball positioning type. The valve can still maintain the state when the electromagnet is powered off and has the "memory" function. Therefore, it not only saves energy and prolongs the service life of electromagnets, but also does not cause system failure or accidents due to power interruption. It is commonly used in automatic machinery and automatic lines.

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Electromagnets can be divided into AC and DC according to different power supplies. The AC electromagnet has the advantages of convenient use and large starting force, but the commutation time is short, which takes 0.01 ~ 0.07s. The commutation impact is large, the noise is large, and the commutation frequency is low (about 30 times per minute). Moreover, when the valve core is stuck or cannot be pulled in due to low voltage, it is easy to burn the coil. The commutation time of DC electromagnet is long, it takes 0.1 ~ 0.15s, the commutation impact is small, the commutation frequency is up to 240 times per minute, and the working reliability is high, but DC power supply is required, and the cost is high. In addition, there is also an integral (local rectifier) electromagnet, which is attached with a diode rectifier circuit and an impulse voltage absorption device, which can rectify the connected AC for self-use, so it has the advantages of both.

The technical parameters of electromagnetic directional valve are shown in the table below.

Technical parameters of electromagnetic directional valve

model

通径(mm)

压力(MPa)

流量(L/min)

产单位

联合设计

H 系列

6 10

31.5

1040

油研

联合设计

H 系列

6 10

21.14

730

WE

5 6 10

2131.5

16100

力士乐

 

(3) Hydraulic directional valve. It is a directional valve that uses pressure oil to push the valve core to move. The structural principle and symbols of the hydraulic directional valve are shown in Figure 21. When the control pressure oil is input from the control port K, the compression spring of the valve core moves under the action of the pressure oil to shift the valve. Its working principle is similar to that of solenoid valve.

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(4) Electro hydraulic directional valve. The layout is flexible and easy to realize automation, but the electromagnet suction is limited and it is difficult to switch large flow; The hydraulic directional valve is generally less used alone, and a small directional valve is needed to change the flow direction of control oil. Therefore, the standard components usually combine the solenoid valve and hydraulic valve to form an electro-hydraulic directional valve. The solenoid valve is used to change the flow direction of the pilot valve (called the main valve), which is used to change the flow direction of the pilot valve (called the main valve).

Figure 22 shows the structural principle and symbols of the electro-hydraulic directional valve. Among them, Figure 22 (a) shows the structural diagram of the stroke adjustment mechanism with main valve core at both ends. The working principle can be explained in combination with the graphical symbols of the combined valve with double dotted line box in Fig. 22 (b). Under normal conditions, the pilot valve and main valve are in the middle position, and the control oil circuit and main oil circuit do not feed oil. When the solenoid iron at the left end is energized, the pilot valve works in the left position, the control oil acts on the left chamber K of the main valve from P through the pilot valve, so that the main valve reverses in the left position, and the oil chamber K at the right end of the main valve returns oil to the oil tank through the pilot valve. At this time, the main oil circuit P is connected with B, a and t. On the contrary, when the left electromagnet of the pilot valve is powered off and the right electromagnet is powered on, the oil port of the main oil circuit is replaced. At this time, P is connected with a and B is connected with t to realize commutation. Figure 22 (d) shows a simplified symbol of the electro-hydraulic directional valve. It is often represented by simplified symbols in the circuit.

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The following describes the oil inlet and return modes of the control oil of the electro-hydraulic directional valve and the additional devices of the valve.

1)       Control the oil inlet and return mode of oil. If the pressure oil (i.e. control oil) entering the pilot valve comes from the P chamber of the main valve, the oil inlet mode of this control oil is called internal control, that is, the oil inlet of the solenoid valve is communicated with the P chamber of the main valve. Its advantage is that the oil circuit is simple, but because the working pressure of the pump is usually high, the energy consumption of the control part is large, which is only applicable to the system with few Electro-hydraulic Directional Valves. If the pressure oil entering the pilot solenoid valve is led from the oil circuit outside the P cavity of the main valve, such as a special low-pressure pump or a part of the system, this oil inlet mode of control oil is called external control. If the oil return port of the pilot solenoid valve is connected to the oil tank separately, this oil return mode of control oil is called external oil return. If the oil return port of the pilot valve is connected with the T cavity of the main valve, it is called internal oil return. The advantage of the internal return type is that there is no need to set a single oil return pipeline, but the allowable back pressure of the pilot valve is small, and the back pressure of the main oil return circuit must be less than it before it can be used, while the external return type is not subject to this restriction. It can be seen that the oil inlet and return of the pilot valve can be in four ways: external control external return, external control internal return, internal control external return and internal control internal return. Figure 22 shows the external control and external return type.

2) The optional additional devices of electro-hydraulic directional valve are mainly as follows.

① Reversing time regulator. Also known as damping regulator, it is a superimposed one-way throttle valve, which can be superimposed between the pilot valve and the main valve. Figure 22 (c) shows the symbol of electro-hydraulic directional valve equipped with double damping regulator. After the left solenoid is energized, the control oil reaches the left control chamber of the main valve core through the left check valve, and the return oil from the right control chamber can be returned to the oil tank through the pilot valve through the right throttle valve. By adjusting the opening of the throttle valve, the reversing time of the main valve can be adjusted, so as to eliminate the reversing impact of the actuator.

② Main valve core stroke adjustment mechanism. In Figure 22 (a), adjusting the screws at the two ends of the main valve cover, the movement of the main spool and the opening of each valve port can be adjusted, and the flow rate of the main valve changes accordingly, thus having a coarse speed regulating function on the execution elements.

③ Pre pressure valve. For the Electro-hydraulic Directional Valve with internal control oil supply, if the pump is unloaded in the normal position, in order to overcome the defect that the main valve cannot act due to no control oil pressure after the valve is powered on, a preloading valve (i.e. one-way valve with hard spring) is inserted into the oil inlet of the main valve, so that it still has a certain control oil pressure under the unloading state, which is enough to reverse the main valve core. Figure 23 shows the symbol of an internal control external return Electro-hydraulic Directional Valve with M-type intermediate function and equipped with preloading valve F.

④ Plug in damper. That is, a fixed orifice restrictor is inserted into the oil inlet of the pilot valve when necessary to limit the flow into the pilot valve, so as to control the reversing speed of the main valve.

The technical parameters of electro-hydraulic directional valve are shown in the table below.

Technical parameters of Electro-hydraulic Directional Valve

型号

通径(mm)

压力(MPa)

流量(L/min)

生产单位

联合设计

E

Y

D

16 20 32

50 65 80

21 31.5

751250

 

WEH

16 25 32

31.5

1701100

力士乐

 

(5) Manual change-over valve. The reversing valve core is manually operated by lever. Figure 24 shows the symbol of manual directional valve. According to the different reversing positioning methods, it is divided into steel ball positioning type and spring return type. When the external force of the joystick is cancelled, the former can keep the valve core in the reversing position because the steel ball is stuck in the positioning groove, and the latter will automatically return the valve core to the initial position under the action of spring force.

The manual directional valve has simple structure and reliable action. Some can also artificially control the size of the valve port, so as to control the movement speed of the actuator. However, due to the need for manual operation, it is only suitable for intermittent action and manual control. When in use, the oil leakage of the positioning device or spring cavity must be eliminated, otherwise the resistance will be generated due to the accumulation of oil leakage, which will affect the operation of the valve and even fail to realize the reversing action. For example, the control of oil circuits such as bulldozer, truck crane and forklift is manual reversing.

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(6) Rotary valve directional valve. Through manual or motorized rotation and transposition of the valve core, the directional valve that changes the state of the oil circuit is realized. Figure 25 (a) shows the structure and symbols of the three position four-way O-shaped rotary valve. In the position shown in the figure, P is communicated with a through the ring groove C and the axial groove B on the valve core, and B is communicated with T through the axial groove E and the ring groove a on the valve core. If the handle 2 is turned 90 ° clockwise, P is connected to B through slots C and D, and a is connected to t through slots E and a. If you turn the handle 45 ° to the middle position, all four oil ports will be closed. The rotary valve can be mechanically reversed through the stop paddles 3 and 4. Due to the poor sealing performance of rotary valve, the radial force is not easy to balance and the structural size is limited. It is generally used in occasions with low pressure and small flow. The graphical symbols of rotary valve directional valve are shown in Figure 25 (b) and (c).

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