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Three functions are performed by directional control valves:
- stop fluid flow
- allow fluid flow, and
- change the direction of fluid flow.
It is common for these three functions to work together.
Valve classification for directional control valves
Various types of direction control valves can be classified as follows:
- Ports counted
- positional data
- from actuation methods
- based on spool type.
A 5/2 directional control valve has five ports and two spool positions, for instance.
Number of Ports
The ports that will be counted must be external ports only. 5/2
Number of Positions
It is possible to have a valve spool take two positions, three positions, or proportional positions, including both normal and working positions…
Actuating Methods
There are manual, spring, electrical, pneumatic, and hydraulic types.
Manually Operated
Using simple levers or paddles, manually operated valves are operated by applying force to the lever or paddle. The valve is sometimes repositioned by using spring force.
To return the spool, some manual valves use either a lever or an external pneumatic or hydraulic signal.
Mechanically Operated
Mechatronically operated valves apply forces by using cams, wheels, rollers, etc., so these valves are subject to wear.
Hydraulically Operated
The hydraulically operated Directional Control Valve operates at much higher pressures than its pneumatic counterpart. Since they must be far more robust, they should be precision machin from stronger and higher quality materials.
Solenoid Operated
In the hydraulics industry, they are widely use. For sliding the spool, these valves use electromechanical solenoids. Electricity provides simple control, so these valves are widely use.
Electrical solenoids are, however, incapable of generating large forces unless they are supplied with large amounts of electrical power. When energized over time, heat generation can threaten the long-term performance of these valves. Most have a limited duty cycle. Therefore, their direct acting use is restricted to low actuating forces.
A low-power solenoid valve is often use to operate a small hydraulic valve (called the pilot) that initiates fluid flow that drives a larger hydraulic valve that requires more force.
Typically, a bi-stable pneumatic valve is a three-port, two-position detented valve. In the event of a loss of power, the valve maintains its position, hence the name bi-stable.
Bi-stability can be achieved by using a mechanical detent and opposing solenoids, or by using a “Magna-latch” magnetic latch with a polarity-sensitive coil. Openings that are positive and closings that are negative. When the coil is actuated, it is held in position magnetically.
Types of Spools
There are two types of spools: sliding and rotary. The sliding spool is cylindrical in cross-section, and the lands and grooves are likewise cylindrical. The sphere-like lands and grooves of a rotary valve are form by drilling through them.
Directional Control Valve
The 2-way valve is the simplest directional control valve. A 2-way valve either stops or allows flow. Water faucets are good examples of 2-way valves. The flow of water can be controlled manually by a faucet.
To operate, a single-acting cylinder requires supply from and exhaust from its port. Three-way valves are require for this. The 3-way valve allows fluid to flow to an actuator in one position and exhausts the fluid in the other. There are some 3-way valves with a third position that blocks flow at all ports.
A double-acting actuator requires a four-way valve. Four-way valves pressurize and exhaust two ports independently. In a 3-position, the 4-way valve stops or allows an actuator to float. The four-way function is a common type of directional control valve for both air and hydraulic circuits. Hydraulic circuits are more likely to have four-way 3-position valves.
Five-way valves are most commonly use in air circuits. The 5-way valve performs the same function as a 4-way valve. The only difference is that it has an additional tank or exhaust port. Suppliers may refer to their 5-way valves as “5-ported 4-ways.”
The exhaust ports on all spool valves are five-port, but the exhaust ports on hydraulic valves internally connect to a common outlet. Considering that oil must return to the tank, it is convenient to connect the dual tank ports to a single return port
Since the atmosphere is the tank for air valves, exhaust piping is usually not significant. By using two exhaust ports, the valve is smaller and less expensive. Dual exhausts can use for speed-control mufflers or as dual-pressure inlets in this configuration as will be explain later.
The following schematics show examples of directional control valves.
Two-way directional control valves
A 2-way directional valve has two ports, which is normally call inlet and outlet. As shown in Figure 8-1, the inlet is “normally close” (NC) when it is block when at rest. The at-rest box or the normal condition is the one with the flow lines leading to and from it.
Valve positions are represent by boxes or enclosures. Figure 1 shows blocked ports, or a closed condition, while the upper box shows a flow path.
Shifting the valve is equivalent to sliding the upper box down to replace the lower box. Flow is from inlet to outlet in the shifted condition. In Figure 1, releasing the palm button allows the valve spring to return to the normal stop flow condition.
By using a two-way valve, you can make a blow-off device or run a fluid motor one way. Even a single acting cylinder cannot be cycle by itself with a 2-way valve.
When the solenoid on this valve is activate, fluid flow is stop.
Different valve operators are available. As shown in Figure 3, a solenoid pilot operator moves the working directional spool with solenoid-controlled pressure from the inlet port. This type of valve is operate by a machine member that moves.
3-way directional control valves
Three working ports are present in a 3-way valve. Ports include an inlet, outlet, and exhaust (or tank). In addition to supplying fluid to an actuator, a 3-way valve also allows fluid to return from it.
When this type of valve is connect to a single-acting, weight- or spring-returned cylinder, it can extend, retract, or stop at any point along its stroke.
For this operation, use a spool-type valve. A diverter valve can also use in this situation. Diverter valves send fluid in two different directions.
