Solenoid valve are divided into three categories in principle, direct-acting, step-by-step direct-acting, and pilot-operated.
1. Direct acting solenoid valve
There are two types: normally closed and normally open. The normally closed type is in a closed state when the power is off. When the coil is energized, an electromagnetic force is generated, so that the moving iron core overcomes the spring force and the static iron core is attracted to directly open the valve, and the medium is in a path; when the coil is powered off, the electromagnetic force disappears, and the moving iron core The core is reset under the action of the spring force, the valve port is directly closed, and the medium is blocked. The structure is simple, the action is reliable, and it works normally under zero pressure difference and micro-vacuum. Normally open is just the opposite. Such as a solenoid valve with a flow diameter smaller than φ6.
Principle: When the normally closed type is energized, the electromagnetic coil generates electromagnetic force to lift the opening piece from the valve seat, and the valve opens; when the power is off, the electromagnetic force disappears, the spring presses the opening piece on the valve seat, and the valve opens. (Normally open is the opposite of this)
Features: It can work normally in vacuum, negative pressure and zero pressure, but the diameter generally does not exceed 25mm.
2. Step-by-step direct-acting solenoid valve
This kind of valve adopts the one-time opening valve and the second-time opening valve to connect together. The main valve and the pilot valve step by step make the electromagnetic force and pressure difference directly open the main valve port. When the coil is energized, electromagnetic force is generated to make the moving iron core and the static iron core pull together, the pilot valve port is opened and the pilot valve port is set on the main valve port, and the moving iron core and the main valve core are connected together. The pressure of the cavity is unloaded through the pilot valve port, and the main valve core moves upward under the simultaneous action of the pressure difference and the electromagnetic force, opening the main valve medium to flow. When the coil is powered off, the electromagnetic force disappears. At this time, the moving iron core closes the pilot valve hole under the action of its own weight and spring force. At this time, the medium enters the upper cavity of the main spool in the balance hole, which increases the pressure in the upper cavity. The main valve is closed under the action of spring return and pressure, and the medium is cut off. Reasonable structure, reliable action, and reliable operation under zero pressure difference.
Principle: It is a combination of direct action and pilot type. When there is no pressure difference between the inlet and the outlet, after the power is turned on, the electromagnetic force directly lifts the pilot small valve and the main valve closing piece up in turn, and the valve opens. When the inlet and outlet reach the starting pressure difference, after the power is turned on, the electromagnetic force pilots the small valve, the pressure in the lower chamber of the main valve rises, and the pressure in the upper chamber drops, so as to use the pressure difference to push the main valve upwards; when the power is off, the pilot valve uses a spring The force or medium pressure pushes the closing member, moving downward, causing the valve to close.
Features: It can also operate under zero pressure difference or vacuum and high pressure, but the power is large, and it must be installed horizontally.
3. Indirect pilot solenoid valve
This solenoid valve is composed of a pilot valve and a main spool to form a channel combination; the normally closed type is in a closed state when it is not energized. When the coil is energized, the generated magnetic force makes the moving iron core and the static iron core pull together, the pilot valve port is opened, and the medium flows to the outlet. At this time, the pressure in the upper chamber of the main valve core is reduced, which is lower than the pressure on the inlet side, forming a pressure difference to overcome the spring. The resistance will move upwards accordingly to achieve the purpose of opening the main valve port, and the medium will flow. When the coil is powered off, the magnetic force disappears, and the moving iron core resets and closes the pilot port under the action of the spring force. At this time, the medium flows in from the balance hole, the pressure in the upper cavity of the main spool increases, and moves downward under the action of the spring force. Close the main valve port. The normally open principle is just the opposite.
Principle: When the power is turned on, the electromagnetic force opens the pilot hole, the pressure in the upper chamber drops rapidly, and a pressure difference between upper and lower parts is formed around the opening, and the fluid pressure pushes the opening to move upward, and the valve opens; The hole is opened, and the inlet pressure quickly forms a pressure difference around the valve closing member through the bypass hole, and the fluid pressure pushes the opening member to move downward to open the valve.
Features: Small size, low power, high upper limit of fluid pressure range, can be installed arbitrarily (need to be customized) but must meet the fluid pressure differential conditions.
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