Directional control valve

Thread Content

Direction control valves are components used to control the flow direction of compressed air in pipes, as well as to turn the airflow on and off. They represent the most widely used type of valve in pneumatic systems. Based on the direction in which the airflow acts within the valve, direction control valves can be divided into two categories: unidirectional direction control valves and reversible direction control valves. Direction control valves that allow air flow to move in only one direction are known as unidirectional direction control valves, such as check valves, shuttle valves, and dual-pressure valves. A direction control valve that can change the direction of airflow is called a reversing direction control valve, or simply a reversing valve. Direction control valves are used in hydraulic systems to control the direction of fluid flow. Classification: Check valves, directional control valves (check valves include ordinary check valves and fluid-controlled check valves). Directional control valves can be classified into manual, mechanical, solenoid-operated, hydraulic, electro-hydraulic, etc., according to the manner in which the valve spool is actuated. ) I. Unidirectional directional control valves 1. Check valves (1) A check valve is a valve that allows airflow to occur in only one direction; airflow in the opposite direction is not permitted. Check valves are often combined with throttle valves to control the speed of the actuator. Composition: Valve body, valve core, spring, etc. Function: Allows fluid flow in only one direction; flow in the opposite direction is blocked. Working principle: Allows flow in the forward direction and blocks it in the reverse direction. Applications: It is commonly installed at the outlet of pumps, to prevent pressure surges from affecting the proper operation of the pump, as well as to stop the system’s oil from flowing back through the pump to the tank when the pump is not in use. It is used to separate the oil circuits in order to prevent interference between high and low pressures. Other valves include check throttle valves, check pressure relief valves, check sequence valves, etc., which allow the oil to flow in one direction through the check valve and in another direction through the throttle valve, etc. Installed on the return oil circuit of the actuator to provide a certain back pressure for the return oil. The check valve used as a backpressure valve should be equipped with a spring of higher stiffness, with a forward opening pressure of (0.3–0.5) MPa. . Fluid-controlled check valve: A fluid-controlled check valve is a valve that allows reverse flow by utilizing control of fluid pressure. This type of valve plays a significant role in the hydraulic support equipment of coal mine machinery. The difference between a hydraulic control check valve and a regular check valve is the presence of an additional control oil circuit K. When this control oil circuit is not connected to the pressure oil, the hydraulic control check valve functions like a regular check valve: pressure oil flows only from the inlet to the outlet, and cannot flow in the reverse direction. When control pressure is applied to the control oil circuit, the piston rod moves to the right under the action of the pressure oil, pushing open the check valve and thus connecting the inlet and outlet ports. If the oil outlet is larger than the oil inlet, the oil can flow in the reverse direction. Composition: Ordinary check valve + small piston cylinder, with internal and external leakage types. Working principle: a. In the absence of control oil, it functions like an ordinary check valve. b. When control oil is present, flow is possible in both directions. Applications: (1) Maintaining pressure. Sliding valve directional control valves all exhibit leakage due to gaps, allowing pressure to be maintained only for a short period of time. When pressure retention is required, a hydraulic check valve can be added to the oil circuit; thanks to the tight sealing property of the cone valve, the oil circuit can maintain pressure for an extended period of time. (2) “Support” of the hydraulic cylinder. In vertical hydraulic cylinders, due to leaks in the spool and pipes, as well as the weight of the piston and piston rod, it may cause the piston and piston rod to slide downward. Connecting a hydraulic check valve to the oil circuit in the lower chamber of the hydraulic cylinder prevents moving parts such as the piston and slider from sliding downward. (3) Achieve locking of the hydraulic cylinder. When the directional control valve is in the neutral position, the two hydraulic check valves close, thereby sealing off the fluid in the two chambers of the hydraulic cylinder; as a result, the piston cannot move due to external forces. (4) High-flow oil drainage. The effective working areas of the two chambers of the hydraulic cylinder differ significantly. When the piston retracts, the oil discharge volume from the right chamber of the hydraulic cylinder increases sharply; in this case, if a spool valve with a low flow rate is used, throttling will occur, limiting the speed at which the piston retracts ； If a pilot-operated check valve is added, when the piston of the hydraulic cylinder retracts, the control pressure oil opens the pilot-operated check valve, allowing the oil in the right chamber to be discharged smoothly. (5) To serve as an oil filling valve. During the high-speed descent of the piston in a vertical hydraulic cylinder, the combined effects of high-pressure oil and its own weight cause it to descend rapidly, resulting in cavitation and negative pressure. Therefore, an oil replenishment device must be added. The hydraulic check valve is used as a filling valve to perform the oil replenishment function. (6) Assemble into a directional control valve. When designing hydraulic circuits, hydraulic check valves can sometimes be combined to function as directional control valves. For example: by connecting two hydraulic check valves and one check valve in parallel (with the check valve in the middle), it functions as the switching circuit of a three-position three-way directional control valve. It should be noted that when the control pressure oil port is not in use, it should be connected back to the tank; otherwise, it will be difficult for the control piston to return to its original position, and the check valve will not be able to stop the flow of fluid in the reverse direction. II. Directional Valve A directional valve is a flow control valve that has two or more flow patterns and two or more oil inlets. They are valves that enable the establishment, interruption, and reversal of hydraulic fluid flow, as well as pressure relief and sequential action control. They can be divided into manual directional control valves, electromagnetic directional control valves, electro-hydraulic directional control valves, etc. Also known as a Chris valve, it is a type of valve featuring multi-directionally adjustable channels that allow the flow direction of the fluid to be changed as needed. During operation, the drive shaft is rotated by a driving mechanism outside the valve, which in turn moves the rocker arm and activates the valve plate; as a result, the working fluid flows from the left inlet to the lower outlet of the valve at one time, and from the right inlet to the same lower outlet at another time, thereby achieving a periodic change in the flow direction. This type of changeover valve is widely used in petroleum and chemical manufacturing, and is most commonly employed in ammonia synthesis gas generation systems. Furthermore, the directional control valve can also be designed with a disc-type structure, which is commonly used in applications with lower flow rates. During operation, simply turn the handwheel to change the flow direction of the working fluid via the valve disc. Function: A directional control valve is a valve that uses the relative movement of a spool within the valve body’s bore to connect or disconnect the oil passages, thereby changing the direction of the oil flow. Classification of directional control valves: Based on their structural design, they can be divided into spool-type, rotary valve-type, and ball valve-type. Based on the number of main oil passages connected to the valve body, they can be classified as: two-way, three-way, four-way, etc. Based on the working position of the valve core within the valve body, they can be classified as two-position, three-position, four-position, etc. Based on the manner in which the operating spool moves, they can be classified as: manual, mechanical, electromagnetic, hydraulic, electro-hydraulic, etc. 1. Manual directional valve: A manual directional valve is a direction-control valve operated by a manual lever; it serves to change the direction of fluid flow and to turn the flow on or off in hydraulic systems. 2. Motorized directional valve: Also known as a stroke valve, the motorized directional valve is used primarily to control the stroke of mechanical moving parts. It operates by using stops or cams mounted on the worktable to force the valve spool to move, thereby controlling the direction of the fluid flow. 3. Electromagnetic directional control valve: It uses the attraction when the electromagnet is powered on and the release when the power is cut off to directly push the valve spool in order to control the flow direction of the liquid. It is a signal conversion element between the electrical system and the hydraulic system. An electromagnetic directional control valve uses the magnetic force of an electromagnet to move the valve spool and thereby change the valve’s operating position. It is widely used because it can control signal transmission to button switches, limit switches, position switches, and pressure relays; it is simple to operate and facilitates automation. The function of the electromagnetic directional control valve is to change the direction of the control fluid flow, thereby altering the position of the hydraulic directional control valve. Since the hydraulic thrust for operating the hydraulic directional control valve can be very high, the main valve can be designed to be large, allowing a high flow rate to pass through. In this way, a smaller electromagnet can be used to control a larger flow of liquid. The working principle of an electromagnetic directional control valve is to use an electromagnet to control the different positions of the spool, thereby changing the flow direction of the fluid. By changing the relative working position of the spool within the valve body, the various oil ports of the valve body can be connected or disconnected, thereby controlling the direction change or start/stop of hydraulic actuators such as hydraulic rods and hydraulic motors. When the electromagnet is de-energized, the spool is held in the neutral or initial position by a spring (except for pulse valves). Pushing the fault check button can move the spool valve core. 4. Other valves are classified according to the operating position of the valve. The operating position of a valve is referred to as a \"position,\" and a valve that has several possible operating positions is called a \"multi-position\" valve. Commonly used are “two-position” valves and “three-position” valves. The position of the valve when no control signal is applied or when it is not being operated is called the zero position. Valves are classified by the number of their interfaces; the interfaces of a valve (including the exhaust port) are referred to as “ports”. A valve’s interfaces include the inlet, outlet, and exhaust port, but not the control port. Common valves include two-way, three-way, four-way, and five-way valves. (1) Two-position two-way directional control valve. Principle: It operates on a principle that combines direct actuation and pilot operation. When there is no pressure difference between the inlet and outlet, upon energization, the electromagnetic force directly lifts the pilot valve and the closing element of the main valve upward in sequence, thereby opening the valve. When the pressure difference between the inlet and outlet reaches the starting value, upon power application, the electromagnetic force acts on the pilot valve; as a result, the pressure in the lower chamber of the main valve rises while the pressure in the upper chamber drops, and this pressure difference is used to push the main valve upward ； In the event of a power failure, the pilot valve uses spring force or medium pressure to move the closing element downward, thereby closing the valve. (2) Two-position four-way directional control valve: A two-position four-way directional control valve is an integrated control device that uses a DC motor to drive the valve spool, thereby opening or closing the oil supply lines or changing the direction of oil supply ； It remains quite reliable even under harsh working conditions (such as low temperatures or highly viscous greases). This valve is suitable for use in dry and thin oil centralized lubrication systems with a nominal pressure of 40 MPa or less, as well as in the main and branch pipelines of hydraulic systems. It can also be used in three configurations: two-position four-way, two-position three-way, and two-position two-way. (3) 3-position 4-way directional control valve: This type of valve has three operating positions and four ports (usually two inlet ports and two outlet ports), denoted as P, T, A, and B respectively. P is the oil inlet port, T is the oil return port; A and B are connected to the upper and lower chambers of the actuator. In its neutral position, the valve remains in the middle position. The structure of the valve is shown in the schematic diagram. Based on the switching method, it can be classified into three types: electromagnetic, electro-hydraulic, and manual. 　The working principle is as follows: (4) Three-position four-way solenoid directional valve. The three-position four-way solenoid directional valve is a new product developed specifically for G-series forklifts, and it has been granted a **patent**. It is an essential component for electro-hydraulic directional control in various types of forklifts. To ensure quality, the factory test standards for these solenoid valves strictly adhere to international standards: under harsh conditions of an oil temperature of 130 degrees and a rated voltage reduced by 15%, the valves still meet all performance requirements. Disadvantages: The three-way four-position solenoid directional control valve has three disadvantages: it is large in size, has poor vibration resistance and water resistance, which limits the environments in which it can be used. The new three-position four-way solenoid directional control valve features significant improvements in terms of structural design, process design, and material selection. Its volume is 1/3 smaller than that of traditional solenoid valves, and it boasts excellent shock and water resistance. (5) Three-position five-way directional valve: “Three-position” means that this directional valve has three positions: left, center, and right. Five ports mean there are a total of five interfaces, in other words, this solenoid valve has five oil ports that connect to the system pipelines. Its working principle is as follows: