Directional valve operation and principle
The directional valve uses the relative movement of the spool to the valve body to make the oil path switch on, off or change the direction of oil flow, so as to realize the start, stop or change the direction of movement of the hydraulic actuator and its driving mechanism.
According to the movement mode of the valve core relative to the valve body: the slide valve and the rotary valve according to the operation mode: manual, motorized, electromagnetic, hydraulic and electro-hydraulic, etc. The position in the valve body when working according to the valve core: two positions And three others. The number of channels controlled by the directional valve is different: two-way, three-way, four-way and five-way.
1. Working principle Figure 4-3a shows the working principle diagram of the spool valve. When the spool moves to the right by a certain distance, the pressure oil output by the hydraulic pump is transferred from the P port of the valve to the A port. The oil in the left chamber of the hydraulic cylinder and the right chamber of the hydraulic cylinder flow back to the tank through port B, and the piston of the hydraulic cylinder moves to the right; otherwise, if the spool moves to a certain distance to the left, the flow of fluid reverses and the piston moves to the left. Figure 4-3b is its graphical symbol.
2. Structure of directional valve
1) Manual directional valve
Use a manual lever to change the position of the spool to achieve commutation. There are two types of spring automatic reset (a) and spring steel ball (b) positioning.
2) Motorized directional valve
Motorized directional valve, also known as stroke valve, is mainly used to control the stroke of mechanical moving parts. With the help of the iron or cam installed on the work table, the valve core is forced to move, thereby controlling the direction of liquid flow.
3) Solenoid directional valve
Use the electromagnet's energization to pull in and out of power to directly push the spool to control the direction of liquid flow. It is a signal conversion element between the electrical system and the hydraulic system.
Figure 4-9a shows the structure of a two-position three-way AC solenoid valve. In the position shown in the figure, the ports P and A are connected, and the port B is disconnected; when the electromagnet is energized, the push rod 1 pushes the spool 2 to the right. At this time, the ports P and A are disconnected, and the B connected. When the solenoid is de-energized and released, the spring 3 pushes the spool to reset. Figure 4-9b is its graphical symbol.
4) Hydraulic directional valve
A directional valve that changes the position of the spool using pressure oil from the control oil circuit. The spool is moved by the pressure difference of the oil in the sealed chamber at both ends. As shown in the figure, when the pressure oil enters the right cavity of the spool from K2, K1 is connected to the return oil, and the spool moves to the left, so that P and B are connected, and A and T are connected; when K1 is connected to the pressure oil, K2 is connected When returning oil, the spool moves to the right, making P and A communicate, and B and T communicate; when both K1 and K2 return oil, the spool returns to the middle position.
5) Electro-hydraulic directional valve
It is composed of electromagnetic slide valve and hydraulic slide valve. The solenoid valve acts as a pilot and can change the direction of the control fluid flow, thereby changing the position of the spool of the hydraulic slide valve. Used in large and medium hydraulic equipment.
3. Performance and characteristics of directional valve
1) Neutral function of slide valve
The three-position four-way and three-position five-way reversing spool valves of various operation modes, when the spool is in the middle position, the connection of each oil port is called the neutral position function of the reversing valve. Commonly used are "O" type, "H" type, "P" type, K "type," M "type, etc.
When analyzing and selecting the neutral function of the three-position directional valve, usually consider:
(1) When the system holding pressure P port is blocked, the system holds pressure, and the hydraulic pump is used in the multi-cylinder system.
(2) The system unloading P port communicates with the T port smoothly, and the system unloads. (HKXM type)
(3) Steady commutation and accuracy A and B are blocked. The commutation process is prone to impact. The commutation is not stable, but the accuracy is high; both A and B are connected to the T port, the commutation is stable, but the accuracy is low.
(4) When the starting smoothness valve is in the neutral position, a cavity of the hydraulic cylinder is connected to the oil tank. When starting, there is not enough oil to buffer, and the starting is not smooth.
(5) The hydraulic cylinder floats and stops at any position
2) Hydraulic power of spool
According to the law of momentum of liquid flow, the hydraulic force acting on the spool when the oil passes through the reversing valve can be classified into steady-state hydraulic force and transient hydraulic force.
(1) Steady-state hydrodynamic force: After the spool has been moved and the opening is fixed, the force acting on the spool due to the change in momentum when the fluid flows through the valve port has a tendency to close the valve port, which is related to the valve flow rate.
(2) Transient hydrodynamic force: During the movement of the spool, the force of the valve chamber fluid flow on the spool due to acceleration or deceleration is related to the movement speed.
3) Hydraulic clamping phenomenon
Reasons for clamping: dirt enters the gap; the temperature rises and the valve core expands; but the main reason is the effect of the radial imbalance force caused by the change of the geometry and concentricity of the spool valve pair, which mainly includes:
a There is no geometric shape error between the valve core and the valve body, and the axis lines are parallel but not coincident
b The valve core has an inverted cone due to processing errors, and the axis lines are parallel but do not coincide
c There are some protrusions on the surface of the spool
Measures to reduce radial unbalance force:
1) Improve manufacturing and assembly accuracy
2) An annular pressure equalizing groove is formed on the valve core
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