The cutaway view in Figure 10-25 represents a solenoid pilot-operated valve with an all-ports-open center condition. These larger, high-flow valves operate in the same way and perform the same functions as the direct solenoid-operated valves discussed earlier. (Refer to Figure’s. 10-18 through 10-20 for typical spring-centered circuits.)
If you are looking for more details, kindly visit electromagnetic directional valve wholesale.
The main difference in the circuits for solenoid pilot-operated valves is that the models with a pump-to-tank center condition need some method to maintain a continuous minimum pilot pressure to the pilot-control valve. Figure 10-25 shows the standard methods for maintaining such minimum pilot pressure.
These valves may also need a way to provide a free path for pilot oil to flow to tank (port Y) under some conditions. Adding or removing plugs – or installing an orificed plug -- can stop or allow flow to the internal-drain port or from the internal pilot-supply port.
The cutaway in Figure 10-25 shows springs and centering washers at both ends holding the working spool in its center position. The centering washers prevent a stronger spring from pushing the spool past center in case one spring is weaker. The spool in the cutaway view and the complete symbol represent an all-ports-open center condition. Other spools and their simplified symbols are shown below the cutaway.
The solenoid-control valve is a double-solenoid, spring-centered model with a float-center spool. All 3-position, solenoid pilot-operated, directional control valves must have a float-center spool to work properly. Any other spool center condition (except one with T blocked, and A, B, and P connected) will either not let the working spool center or will generate excessive heat. Always make sure the solenoid-control valve is 3 position and has a float-center spool when the pilot-operated directional valve is 3 position.
When the a solenoid of the solenoid-control valve is energized, it sends pilot oil to the right end of the working spool. This shifts the spool to the left. Pump flow at the P port is directed to the A port, and flow from the B port goes to tank through the T port. It is easy to follow flow paths on the complete symbol to see how any manufacturer’s valve functions, even when its construction is different. Energizing the b solenoid shifts the working spool to the right, producing the opposite flow condition and reversing flow to the actuator.
With the valve in Figure 10-25 at rest, it is obvious pressure would be low. When the valve and lines are sized for low pressure drop, pressure would probably be below 50 psi. This pressure would not generate enough force to shift the working spool against the centering springs, so no fluid can flow to or from the actuator. To overcome low or no pilot pressure the following options are available:
Option 1: Use a 75-psi backpressure check valve out of the T port, use an orificed plug in the internal pilot-supply port, and drain the pilot-control valve externally through the Y (external-drain) port. When the pump runs, the backpressure check valve maintains at least 75-psi pilot pressure, so the working spool can shift when signaled. When the circuit operates, pilot pressure may go higher, but it never drops below 75 psi. With an open-center valve, this option can make the cylinder regenerate if the cylinder has low resistance and/or an oversized rod. If this situation is suspect, use Option 2.
Option 2: Install the 75-psi backpressure check valve in the pump line and route a pilot signal from upstream of it to the X (external pilot-supply) port. This makes it possible to internally drain the pilot-control valve. An external drain could still be used, but would not be required. This option keeps pressure off a cylinder that could regenerate with the open-center valve in its center position. It also eliminates the possibility of regeneration when a tandem-center valve moves through an open crossover.
Option 3: Supply the X port with 75- to 3000-psi pilot pressure from another source. The external supply does not need to provide high flow. A constant pressure might be desirable to keep spool shifting and cycle time consistent.
Check the supplier’s catalog information to see what pilot pressure should be for a specific valve size and operating pressure. These figures change for different spools and are directly affected by tank line backpressure when using internal drains.
Contact us to discuss your requirements of hydraulic control check valve supplier. Our experienced sales team can help you identify the options that best suit your needs.
Option 4: Most suppliers offer an internal backpressure check valve option that operates almost the same as Option 1. The difference is that there is no pressure in the T tank line so an internal drain may be used. The main difficulty with this option is that this valve is not standard. Delivery probably is long and could affect machine downtime if the valve needs to be replaced.
The solenoid pilot-operated valve in Figure 10-26 shows some other options that manufacturers offer for special needs. All of these options may not be available from all suppliers, so check with the distributor before specifying a brand.
Directional control valves perform only three functions:
These three functions usually operate in combination.
The simplest directional control valve is the 2-way valve. A 2-way valve stops flow or allows flow. A water faucet is a good example of a 2-way valve. A water faucet allows flow or stops flow by manual control.
A single-acting cylinder needs supply to and exhaust from its port to operate. This requires a 3-way valve. A 3-way valve allows fluid flow to an actuator in one position and exhausts the fluid from it in the other position. Some 3-way valves have a third position that blocks flow at all ports.
A double-acting actuator requires a 4-way valve. A 4-way valve pressurizes and exhausts two ports interdependently. A 3-position, 4-way valve stops an actuator or allows it to float. The 4-way function is a common type of directional control valve for both air and hydraulic circuits. A 3-position, 4-way valve is more common in hydraulic circuits.
The 5-way valve is found most frequently in air circuits. A 5-way valve performs the same function as a 4-way valve. The only difference is an extra tank or exhaust port. (Some suppliers call their 5-way valves, “5-ported 4-ways.") All spool valves are five ported, but hydraulic valves have internally connected exhaust ports going to a common outlet. Because oil must return to tank, it is convenient to connect the dual tank ports to a single return port. For air valves, atmosphere is the tank, so exhaust piping is usually unimportant. Using two exhaust ports makes the valve smaller and less expensive. As will be explained later, dual exhausts used for speed-control mufflers or as dual-pressure inlets make this configuration versatile.
Following are schematic symbols for commonly used directional control valves.
2-way directional control valves
A 2-way directional valve has two ports normally called inlet and outlet. When the inlet is blocked in the at-rest condition, as shown in Figure 8-1, it is referred to as "normally closed" (NC). The at-rest box or the normal condition is the one with the flow lines going to and from it.
The boxes or enclosures represent the valve’s positions. In Figure 8-1, the active box shows blocked ports, or a closed condition, while the upper box shows a flow path. When an operator shifts the valve, it is the same as sliding the upper box down to take the place of the lower box. In the shifted condition there is flow from inlet to outlet. Releasing the palm button in Figure 8-1 allows the valve spring to return to the normal stop flow condition. A 2-way valve makes a blow-off device or runs a fluid motor in one direction. By itself, a 2-way valve cannot cycle even a single acting cylinder.
The company is the world’s best 3-Way Hydraulic Valve supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.