This type of valve is used to control the flow direction by using a pilot pressure. Figure 5-28, shows two units in which the spool is shifted by applying the pilot pressure at either end of the spool. In the spring-offset model, the spool is held in its normal offset position by spring thrust and shifted to its other position by applying pilot pressure to the free end of the spool. Removing pilot pressure shifts the spool back to its normal offset position. A detent does not hold this valve, so pilot pressure should be maintained as long as the valve is in the shifted position.
This valve is used to control the flow direction manually. A spool is shifted by operating a hand lever (Figure 5-27). In a spring offset model, a spool is normally in an extreme out position and is shifted to an extreme in position by moving a lever toward a valve. Spring action automatically returns both spool and lever to the normal out position when a lever is released. In a two-position, no-spring model, a spool is shifted back to its original position. (Figure 5-27 does not show this valve.) In a three-position no-spring model, a detent (a devise which locks the movement) retains a spool in any one of the three selected positions after lever force is released. In a three-position, spring-centered model, a lever is used to shift a spool to either extreme position away from the center. Spring action automatically returns a spool to the center position when a lever is released.
Four-way, directional-control valves are used to control the direction of fluid flow in a hydraulic circuit, which controls the direction of movement of a work cylinder or the rotation of a fluid motor. These valves are usually the sliding-spool type. A typical four-way, directional-control valve has four ports:
• One pressure port is connected to a pressure line.
• One return or exhaust port is connected to a reservoir.
• Two working ports are connected, by lines, to an actuating unit.
Four-way valves consist of a rectangular cast body, a sliding spool, and a way to position a spool. A spool is precision fitted to a bore through the longitudinal axis of a valve’s body. The lands of a spool divide this bore into a series of separate chambers. Ports in a valve’s body lead into a chamber so that a spool’s position determines which ports are open to each other and which ones are sealed off from each other. Ports that are sealed off from each other in one position may be interconnected in another position. Spool positioning is accomplished manually, mechanically, electrically, or hydraulically or by combing any of the four.
Figure 5-22 shows how the spool position determines the possible flow conditions in the circuit. The four ports are marked P, T, A, and B: P is connected to the flow source; T to the tank; and A and B to the respective ports of the work cylinder, hydraulic motor, or some other valve in the circuit. In diagram A, the spool is in such a position that port P is open to port A, and port B is open to port T. Ports A and B are connected to the ports of the cylinder, flow through port P, and cause the piston of the cylinder to move to the right. Return flow from the cylinder passes through ports B and T. In diagram B, port P is open to port B, and the piston moves to the left. Return flow from the cylinder passes through ports A and T.
Table 5-1, lists some of the classifications of directional-control valves. These valves could be identified according to the—
• Number of spool positions.
• Number of flow paths in the extreme positions.
• Flow pattern in the center or crossover position.
• Method of shifting a spool.
• Method of providing spool return.
|Path-of-flow type||Two way||Allows a total of two possible flow paths in two
extreme spool positions
|Four way||Allows a total of four possible flow paths in two
extreme spool positions
|Control type||Manual operated||Hand lever is used to shift the spool.|
|Pilot operated||Hydraulic pressure is used to shift the spool.|
|Solenoid operated||Solenoid action is used to shift the spool.|
|Solenoid controlled, pilot operated||Solenoid action is used to shift the integral pilot
spool, which directs the pilot flow to shift the main spool.
|Position type||Two position||Spool has two extreme positions of dwell.|
|Three position||Spool has two extreme positions plus one intermediate or center position.|
|Spring type||Spring offset||Spring action automatically returns the spool to the normal offset position as soon as shifter force is released. (Spring offset is always a two-way valve.)|
|No spring||Spool is not spring-loaded; it is moved only by shifter force, and it remains where it is shifted (may be two- or three-position type, but three-position type uses detent).|
|Spring centered||Spring action automatically returns the spool to the center position as soon as the shifter force is released. (Spring-centered is always a three position valve.)|
|Spool type||Open center||These are five of the more common spool types.|
|Closed center||They refer to the flow pattern allowed when the spool is in the center position (three-position valves) or in the cross-over position (two-position valves).|
|Partially closed center|