A Brief Analysis of the Median Function of Directional Control Valves

A Brief Analysis of the Median Function of Directional Control Valves

Abstract: This paper introduces the median function, structural features and functional characteristics of various types of directional control valves, and illustrates the importance of the median function in the selection of directional control valves with examples.

The directional control valve operates by means of the relative motion between the spool valve and the valve body, enabling the various oil circuits connected to the valve body to connect, cut off and reverse the hydraulic oil flow. The median function of a directional control valve refers to the connection form of each oil port in the valve when the spool in the valve is in the middle position or the original position, which reflects the control function of the directional control valve. The use of different forms of spool valves will directly affect the working condition of the actuator. Therefore, when designing the hydraulic system of construction machinery, it is necessary to select a directional control valve with an appropriate neutral function based on the working characteristics of the machinery. The median functions include O-type, H-type, X-type, M-type, Y-type, P-type, J-type, C-type, K-type, and many other forms.

I. The O-shaped symbol is

Here, P represents the oil inlet, T represents the oil return port, and A and B represent the working oil ports. Structural features: When in the middle position, all oil ports are fully enclosed and the oil does not flow. Functional features: 1. The oil inlet and return ports of the working device are both closed. The working mechanism can be fixed at any position and remain stationary. Even if subjected to external force, the working mechanism cannot be moved or rotated. Therefore, it cannot be used in mechanisms with hand cranks. 2. The transition from stop to start is relatively smooth because the return oil chamber of the working mechanism is filled with oil, which can act as a buffer. When the pressure oil pushes the working mechanism to start moving, due to the influence of oil resistance, its speed will not be too fast. During braking, the movement inertia causes a relatively large hydraulic shock. 3. The oil pump cannot be unloaded. 4. High accuracy of reversing position.

Ii. The H-shaped symbol is

Structural features: When in the neutral position, all oil ports are fully open and there is no oil pressure in the system. Functional features: 1. The oil inlet P, return oil port T and working oil ports A and B are all connected, making the working mechanism in a floating state. It can move under the action of external force and can be used in mechanisms with hand cranks. 2. The hydraulic pump can be unloaded. 3. There is an impact from stopping to starting. When the working mechanism stops, the oil in the return oil chamber has already flowed back to the oil tank, and there is no oil to act as a buffer. When braking, the oil ports are interconnected, so the braking is smoother than that of the O-type. 4. For single-rod double-acting cylinders, due to the unequal effective action areas on both sides of the piston, this type of spool valve cannot fully ensure that the piston remains in a stationary state.

Iii. The M-shaped symbol is

Structural features: When in the neutral position, the working oil ports A and B are closed, while the oil inlet port P and the return oil port T are directly connected. Functional features: 1. As the working oil ports A and B are closed, the working mechanism can remain stationary. 2. The hydraulic pump can be unloaded. 3. It cannot be used in mechanisms equipped with hand-cranked devices. 4. The transition from stop to start is relatively smooth. 5. The hydraulic shock caused by the movement inertia during braking is relatively large. 6. It can be used in hydraulic circuits where the oil pump is unloading and the hydraulic cylinder is locked.

Four. The Y-shaped symbol is

Structural features: When in the neutral position, the oil inlet P is closed, and the working oil ports A and B are connected to the return oil port T. Functional features: 1. As the working oil ports A and B are connected to the return oil port T, the working mechanism is in a floating state and can move under the action of external forces. It can be used in mechanisms with hand cranks. 2. There is some shock from stop to start, and the shock and braking performance from rest to start are between Type 0 and Type H. 3. The oil pump cannot be unloaded.

V. The P-type symbol is

Structural features: When in the neutral position, the return oil port T is closed, and the inlet oil port P is connected to the working oil ports A and B. Functional features: 1. For double-rod double-acting cylinders with equal diameters, the hydraulic pressure on both ends of the piston is balanced with each other, and the working mechanism can remain stationary. It can also be used in mechanisms equipped with hand-cranked devices. However, for single-rod or double-rod double-acting cylinders of different diameters, the working mechanism cannot be in a stationary state to form a differential circuit. 2. The transition from stop to start is relatively smooth. During braking, both chambers of the cylinder are filled with pressure oil, thus ensuring smooth braking. 3. The oil pump cannot be unloaded. 4. The change in reversing position is smaller than that of the H-type, and it is widely used.

Vi. The X-shaped symbol is

Structural features: When in the middle position, the A, B, and P oil ports are all connected to the T return oil port. Functional features: 1. Each oil port is connected to the return oil port T and is in a semi-open state. Due to the existence of the throttle port, the P oil port still maintains a certain pressure. 2. At the moment when the spslide valve moves to the center position, the P, A, B and T oil ports are half-open and connected. This can prevent the reversing shock caused by the sudden blocking of the pressure oil port P during the reversing process. The oil pump cannot be unloaded. 4. The reversing performance lies between Type 0 and Type H.

Vii. The U-shaped symbol is

Structural features: The working oil ports A and B are connected, while the oil inlet port P and the return oil port T are closed. Functional features: 1. As the working oil ports A and B are connected, the working device is in a floating state and can move under the action of external force. It can be used in mechanisms with hand-cranking devices. 2. The transition from stop to start is relatively smooth. 3. It is also relatively smooth when braking. 4. The oil pump cannot be unloaded.

Viii. The K-shaped symbol is

Structural features: At the median position, the oil inlet P is connected to the working oil port A and the return oil port T, while the other working oil port B is closed. Functional features: 1. The oil pump can be unloaded. 2. The performance is different when reversing in two directions.

Ix. The J-shaped symbol is

Structural features: The oil inlet P and the working oil port A are enclosed, while the other working oil port B is connected to the return oil port T. Functional features: 1. The oil pump cannot be unloaded. 2. The performance is different when reversing in two directions.

X. The symbol for type C is

Structural features: The oil inlet P is connected to the working oil port A, while the other working oil port B is connected to the return oil port T. Functional features: The oil pump cannot be unloaded. The transition from stop to start is relatively smooth, but there is a significant shock during braking.

Example analysis

1. The neutral function of the spool valve is utilized to design an unloading circuit to achieve energy conservation. When the three-position directional control valve with a neutral function of H, K or M type is in the neutral position, the oil output by the pump is directly returned to the oil tank, forming an unloading circuit. This enables the pump to operate under no-load or very low output power conditions, thereby achieving energy conservation, as shown in Figure 1. This method is relatively simple, but it is not suitable for systems where one pump drives two or more actuating elements.

2. The neutral function of the spool valve can be designed as a braking circuit or a locking circuit. To stop a moving working mechanism at any required position and prevent it from moving due to external influences after stopping, a braking circuit can be adopted. The simplest method is to use a directional control valve for braking. For example, if the spool valve can be an M-type or O-type directional control valve, when it returns to the neutral position, its inlet and return oil circuits can be cut off, causing the actuator to stop moving quickly. As shown in Figure 2 The braking oil circuit of the hydraulic cylinder of the loading boom is composed of an M-type neutral function directional control valve. When the boom lifts the bucket to the highest position and lowers it to the lowest flat position, it can automatically limit and brake. The return limit valves in the figure (i.e., M-type and H-type four-position four-way directional control valves) are positioned by steel balls. When the bucket moves to the limit point and touches the switch, the two-position solenoid valve changes direction and is connected with compressed air. When the positioning steel ball is pressed back into the groove, the return limit valve will return to the center position under the action of the spring, cutting off the oil inlet and return oil circuits of the boom cylinder. Thus, the boom along with the bucket is limited-braked together.

3. Floating is achieved by using H-type and Y-type reversing. For instance, when the slewing mechanism of a hydraulic crane rotates under load, if the braking is too sudden, the inertial force will cause a significant hydraulic shock. Therefore, directional control valves with H-type or Y-type slide valve functions are often used, as shown in Figure 3. When the directional control valve returns to the middle position, the slewing motor is in a floating state, and then the foot brake is used to make it stop rotating smoothly. The hydraulic cylinder of the loading boom shown in Figure 2. When the M-type and H-type four-way directional control valves are in the H position, that is, the floating position, at this time, materials can be lifted down or the ground can be leveled. The bucket can rise and fall according to the height of the ground, thus achieving floating. In addition, this kind of circuit can still smoothly lower the bucket when the system suddenly stops working. The Y-type slide valve function reversing valve is adopted on the reversing valve of the walking motor of the crawler excavator. It can keep the walking motor in a floating state during the excavation operation and not bear the braking load.

The selection of the middle position function of the directional control valve slide valve has a very important impact on the reversing speed of the open vibration hydraulic system of the road roller and the vibration working performance of the road roller. By using the H-type three-position four-way directional control valve, when the spool valve is in the middle position, the four oil ports P, T, A and B are connected and form a connected path. Due to the rotational inertia of the vibrator, the vibrating wheel will generate residual vibration, which will cause indentations on the surface of the compacted pavement. However, for the vibratory roller used to compact the subgrade, this has little impact on the compaction effect of the base layer; instead, it reduces the hydraulic impact force of the system. In Figure 4, in a single-frequency double-amplitude vibration open hydraulic system. For vibratory rollers used for compacting road surfaces, it is required that when the vibration needs to be stopped or the amplitude changed during the compaction operation, the vibrator can quickly stop rotating within 1.5 to 1.7 seconds to prevent momentary residual vibration from causing indentations on the compacted surface and affecting the compaction quality. The M-type three-position four-way directional control valve is often used. When the spslide valve is in the middle position, the two working oil ports A and B are cut off, which can generate a large back pressure, causing the motor to overcome the inertial torque of the exciter and stop rotating rapidly. This avoids the formation of indentations on the road surface, but it will cause a very high instantaneous pressure peak in the motor circuit, increasing the damage rate of the motor and other related components. Therefore, two relief valves are usually installed at the A and B oil ports of the directional control valve to protect the system.

In conclusion, when selecting a directional control valve, it is essential to choose the appropriate neutral function based on the working characteristics of the working mechanism.