Do you know the principle of directional control valve?
Directional control valve is a component used to control the flow direction and air flow interruption of compressed air in the pipeline. It is the most widely used type of valve in pneumatic systems. According to the direction of the airflow in the valve, directional control valves can be divided into two categories: one-way directional control valves and reversing directional control valves. Directional control valves that only allow airflow to flow in one direction are called one-way directional control valves, such as one-way valves, shuttle valves, and dual-pressure valves. Directional control valves that can change the direction of airflow are called reversing directional control valves, or reversing valves for short.
I. Reversing directional control valves
1. Classification
(1) Classification by valve control method: air pressure control, electromagnetic control, manual control, and mechanical control.
(2) Classification by valve working position: The working position of the valve is called a "position", and valves with several switching working positions are called "position" valves. For example, "two-position" valves and "three-position" valves. The position of the valve when no control signal is applied or it is operated is called the zero position.
(3) Classification by the number of valve interfaces: The valve interface (including the exhaust port) is called "through", and the valve interface includes the inlet, outlet and exhaust port, but does not include the control port. Common valves include two-way, three-way, four-way and five-way.
According to the switching position and the number of interfaces of the valve, the name of the valve can be called, such as two-position two-way valve, three-position five-way valve, etc.
(4) Classification by valve core structure Common valve core structures are cut-off type and sliding column type.
(5) Classification by control number: It can be divided into single control type and double control type. Single control type means that one working position of the valve is obtained by the control signal, and the other working position is obtained by other forces when the control signal disappears (called reset mode). For example, reset by spring force is called spring reset; reset by air pressure is called air pressure reset; reset by spring force and air pressure is called mixed reset. Mixed reset can reduce the diameter of the valve core reset piston. The greater the reset force, the more reliable the valve switching and the more stable the operation. Double control means that the valve has two control signals. For two-position valves, double control is used. When one control signal disappears and the other control signal is not added, the original valve position can be maintained unchanged. The valve is said to have a memory function. For three-position valves, each control signal controls one valve position. When both control signals do not exist, the valve core is placed in the middle position by spring force and (or) air pressure.
(6) Classification by valve installation connection method: pipe connection, plate connection, flange connection and integrated connection, etc.
2. Several typical reversing directional valves
(1) Air pressure control reversing valve Air pressure control reversing valve relies on an external air pressure signal to switch the main valve and control the reversal or opening and closing of the circuit. The external air pressure is called control pressure. Air pressure control is suitable for flammable, explosive, humid and dusty places, and the operation is safe and reliable. According to the working principle, air-controlled reversing valves can be divided into three types: pressure control, pressure relief control and differential pressure control. Pressure control is a control method that applies a gradually increasing pressure value to the valve opening and closing parts to make the valve change direction; pressure relief control is a control method that applies a gradually decreasing pressure value to the valve opening and closing parts to make the valve change direction; differential pressure control is a control method that uses the pressure difference generated by the control air pressure acting on the different areas at both ends of the valve core to make the valve change direction.
2) Solenoid-controlled reversing valve The solenoid reversing valve is the most important component in pneumatic control components. According to the action mode, there are direct-acting elastic seals; according to the power supply used, there are DC solenoid reversing valves and AC solenoid reversing valves.
1. Direct-acting solenoid reversing valve The direct-acting solenoid reversing valve uses electromagnetic force to directly push the valve core to change direction. According to the number of operating coils, there are single coils and double coils, which can be divided into single solenoid control and double solenoid control. The characteristics of the direct-acting solenoid valve are simple and compact structure and high reversing frequency, but when used for AC electromagnets, if the valve stem is stuck, there is a possibility of burning the coil. The reversing stroke of the valve stem is controlled by the electromagnet suction stroke, so it is only suitable for small valves.
2 Pilot-operated solenoid reversing valve
Pilot-operated solenoid valves are composed of small direct-acting solenoid valves and large pneumatic reversing valves. (3) Mechanically controlled reversing valves A valve that switches the valve core by mechanical external force is called a mechanically controlled valve. It uses the mechanical movement of an actuator or other mechanism to operate the valve stem with the help of a cam, roller, lever or bumper on the valve to drive the valve to change direction. Mechanically controlled valves cannot be used as blocks or stoppers. (4) Human-controlled reversing valves A valve that switches the valve core by hand or foot is called a human-controlled reversing valve. Manual valves and mechanically controlled valves are often used to generate air signals for system control, but their operating frequency cannot be too high.
II. One-way direction control
(1) One-way valve :A one-way valve is a valve that allows air to flow in one direction only and not in the opposite direction. A one-way valve is often combined with a throttle valve to control the speed of the actuator.
(2) Shuttle valve: A shuttle valve is equivalent to a valve composed of two one-way valves, and its function is equivalent to the "OR" gate logic function.
(3) Double-pressure valve The function of the double-pressure valve is equivalent to the logic function of the "AND" gate.
(4) Quick exhaust valve When the cylinder or pressure vessel needs to be exhausted in a short time, a quick exhaust valve is added between the reversing valve and the cylinder. In this way, the gas in the cylinder no longer passes through the reversing valve but directly passes through the quick exhaust valve to speed up the movement of the cylinder. Especially when the reversing valve is far away from the cylinder, a quick exhaust valve is set close to the cylinder so that the gas in the cylinder can be quickly discharged into the atmosphere.