5-2. Directional-Control Valves
Directional-control valves also control flow direction. However, they vary considerably in physical characteristics and operation. The valves may be a-
- Poppet type, in which a piston or ball moves on and off a seat.
- Rotary-spool type, in which a spool rotates about its axis.
- Sliding-spool type, in which a spool slides axially in a bore. In this type, a spool is often classified according to the flow conditions created when it is in the normal or neutral position. A closed-center spool blocks all valve ports from each other when in the normal position. In an open-center spool, all valve ports are open to each other when the spool is in the normal position.
Directional-control valves may also be classified according to the method used to actuate the valve element. A poppet-type valve is usually hydraulically operated. A rotary-spool type may be manually (lever or plunger action), mechanically (cam or trip action), or electrically (solenoid action) operated. A sliding-spool type may be manually, mechanically, electrically, or hydraulically operated, or it may be operated in combination.
Directional-control valves may also be classified according to the number of positions of the valve elements or the total number of flow paths provided in the extreme position. For example, a three-position, four-way valve has two extreme positions and a center or neutral position. In each of the two extreme positions, there are two flow paths, making a total of four flow paths.
Spool valves (see Figure 5-11) are popular on modern hydraulic systems because they-
- Can be precision-ground for fine-oil metering.
- Can be made to handle flows in many directions by adding extra lands and oil ports.
- Stack easily into one compact control package, which is important on mobile systems.
Spool valves, however, require good maintenance. Dirty oil will damage the mating surfaces of the valve lands, causing them to lose their accuracy. Dirt will cause these valves to stick or work erratically. Also, spool valves must be accurately machined and fitted to their bores.
a. Poppet Valve. Figure 5-12 shows a simple poppet valve. It consists primarily of a movable poppet that closes against a valve seat. Pressure from the inlet tends to hold the valve tightly closed. A slight force applied to the poppet stem opens the poppet. The action is similar to the valves of an automobile engine. The poppet stem usually has an O-ring seal to prevent leakage. In some valves, the poppets are held in the seated position by springs. The number of poppets in a valve depends on the purpose of the valve.
b. Sliding-Spool Valve. Figure 5-13 shows a sliding-spool valve. The valve element slides back and forth to block and uncover ports in the housing. Sometimes called a piston type, the sliding-spool valve has a piston of which the inner areas are equal. Pressure from the inlet ports acts equally on both inner piston areas regardless of the position of the spool. Sealing is done by a machine fit between the spool and valve body or sleeve.
c. Check Valves. Check valves are the most commonly used in fluid-powered systems. They allow flow in one direction and prevent flow in the other direction. They may be installed independently in a line, or they may be incorporated as an integral part of a sequence, counterbalance, or pressure-reducing valve. The valve element may be a sleeve, cone, ball, poppet, piston, spool, or disc. Force of the moving fluid opens a check valve; backflow, a spring, or gravity closes the valve. Figures 5-14, 5-15 and 5-16 show various types of check valves.
(1) Standard Type (Figure 5-17). This valve may be a right-angle or an in-line type, depending on the relative location of the ports. Both types operate on the same principle. The valve consists essentially of a poppet or ball 1 held on a seat by the force of spring 2. Flow directed to the inlet port acts against spring 2 to unseat poppet 1 and open the valve for through flow (see Figure 5-17, diagram B, for both valve types). Flow entering the valve through the outlet port combines with spring action to hold poppet 1 on its seat to check reverse flow.
These valves are available with various cracking pressures. Conventional applications usually use the light spring because it ensures reseating the poppet regardless of the valve's mounting position. Heavy spring units are generally used to ensure the availability of at least the minimum pressure required for pilot operations.
(2) Restriction Type (Figure 5-18). This valve has orifice plug 1 in the nose of poppet 2, which makes it different from a conventional, right-angle check valve. Flow directed to the inlet port opens the valve, allowing free flow through the valve. Reverse flow directed in through the outlet port seats poppet 2. Flow is restricted to the amount of oil, which can be altered, to allow a suitable bleed when the poppet is closed. Uses of a restriction check valve can be to control the lowering speed of a down-moving piston and the rate of decompression in large presses.
(3) Pilot-Operated Type (Figure 5-19). In diagram A, the valve has poppet 1 seated on stationary sleeve 2 by spring 3. This valve opens the same as a conventional check valve. Pressure at the inlet ports must be sufficient to overcome the combined forces of any pressure at the outlet port and the light thrust of spring 3 so that poppet 1 raises and allows flow from the inlet ports through the outlet port. In this situation, there is no pressure required at the pilot port.
In diagram B, the valve is closed to prevent reverse flow. Pressure at the outlet port and the thrust of spring 3 hold poppet 1 on its seat to block the flow. In this case, the pilot port has no pressure.
In diagram C, pressure applied at the pilot port is sufficient to overcome the thrust of spring 3. The net forces exerted by pressures at the other ports raise piston 4 to unseat poppet 1 and allow controlled flow from the outlet to the inlet ports. With no pressure at the inlet ports, pilot pressure must exceed 40 percent of that imposed at outlet to open the poppet.
Figure 5-20 shows another pilot-operated check valve. This valve consists of poppet 1 secured to piston 3. Poppet 1 is held against seat 4 by the action of spring 2 on piston 3. In diagram A, the valve is in the free-flow position. Pressure at the inlet port, acting downward against poppet 1, is sufficient to overcome the combined forces of spring 2 against piston 3 and the pressure, if any, at the outlet port. (The pressure at the outlet port is exerted over a greater effective area than that at the inlet because of the poppet stem.) The drain post is open to the tank, and there is no pressure at the pilot port. Diagram B shows the valve in a position to prevent reverse flow, with no pressure at the pilot port and the drain opening to the tank.
Diagram C shows the pilot operation of the valve. When sufficient pressure is applied at the pilot port to overcome the thrust of spring 2 plus the net effect of pressure at the other ports, poppet 1 is unseated to allow reverse flow. Pilot pressure must be equal to about 80 percent of that imposed at the outlet port to open the valve and allow reverse flow.
d. Two-Way Valve. A two-way valve is generally used to control the direction of fluid flow in a hydraulic circuit and is a sliding-spool type. Figure 5-21 shows a two-way, sliding-spool, directional-control valve. As the spool moves back and forth, it either allows or prevents fluid flow through the valve. In either shifted position in a two-way valve, a pressure port is open to one cylinder port, but the opposite cylinder port is not open to a tank. A tank port on this valve is used primarily for draining.
e. Four-Way Valves. 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.
Table 5-1: Classifications of directional-control valves
|Path-of-flow type ||Two way |
|Allows a total of two possible flow paths in two extreme spool positions Allows a total of four possible flow paths in two extreme spool positions |
|Control type ||Manual operated |
Solenoid controlled, pilot operated
|Hand lever is used to shift the spool. Hydraulic pressure is used to shift the spool. Solenoid action is used to shift the spool. 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. 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.) 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 action automatically returns the spool to the center position as soon as the shifter force is released. (Spring-centered is always a three- |
|Spool type ||Open center |
Partially closed center Semi-open center
|These are five of the more common spool types. 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). |
(1) Poppet-Type Valve. Figure 5-23, page 5-16, shows a typical four-way, poppet-type, directional-control valve. It is a manually operated valve and consists of a group of conventional spring-loaded poppets. The poppets are enclosed in a common housing and are interconnected by ducts so as to direct the fluid flow in the desired direction.
The poppets are actuated by cams on the camshaft. They are arranged so that the shaft, which is rotated by its controlling lever, will open the correct poppet combinations to direct the fluid flow through the desired line to the actuating unit. At the same time, fluid will be directed from the opposite line of the actuating unit through the valve and back to the reservoir or exhausted to the atmosphere.
Springs hold the poppets to their seats. A camshaft unseats them to allow fluid flow through the valve. The camshaft is controlled by moving the handle. The valve is operated by moving the handle manually or by connecting the handle, by mechanical linkage, to a control handle. On the camshaft are three O-ring packings to prevent internal and external leakage. The camshaft has two lobes (raised portions). The contour (shape) of these lobes is such that when the shaft is placed in the neutral position, the lobes will not touch any of the poppets.
One cam lobe operates the two pressure poppets; the other lobe operates the two return/exhaust poppets. To stop the rotating camshaft at the exact position, a stop pin is secured to the body and extended through a cutout section of the camshaft flange. This stop pin prevents overtravel by ensuring that the cam lobes stop rotating when the poppets have unseated as high as they can go.
Figure 5-23 shows a working view of a poppet-type, four-way valve. The camshaft rotates by moving the control handle in either direction from neutral. The lobes rotate, unseating one pressure poppet and one return/exhaust poppet. The valve is now in a working position. Pressure fluid, entering the pressure port, travels through the vertical fluid passages in both pressure poppet seats. Since only one pressure poppet is unseated by the cam lobe, the fluid flows past the open poppet to the inside of the poppet seat. It then flows out one working port and to the actuating unit. Return fluid from the actuating unit enters the other working port. It then flows through the diagonal fluid passages, past the unseated return poppet, through the vertical fluid passages, and out the return/exhaust port. By rotating the camshaft in the opposite direction until the stop pin hits, the opposite pressure and return poppets are unseated, and the fluid flow is reversed. This causes the actuating unit to move in the opposite direction.
(2) Sliding-Spool Valve. The four-way, sliding-spool, directional-control valve is simple in operation principle and is probably the most durable and trouble free of all four-way, directional-control valves in current use. Figure 5-24 shows a typical four-way, sliding-spool, directional-control valve. The valve body contains four fluid ports: pressure, return/exhaust, and two working ports (referred to as cylinder ports). A hollow steel sleeve fits into the main bore of the body. Around the outside diameter of the sleeve are O-ring gaskets. These O-rings form a seal between the sleeve and the body.
In Figure 5-24, diagram A, the valve is at the far right in its cylinder. Liquid from the pump flows to the right end of the cylinder port, while liquid from the left end returns to the reservoir. In diagram C, the situation is reverse. The piston is to the far left in its cylinder. Liquid from the pump flows to the left end of the cylinder port, while liquid from the right end returns to the reservoir. In diagram B, the piston is in an intermediate position. Flow through the valve from the pump is shut off, and both ends of the cylinder can drain to the reservoir unless other valves are set to control the flow.
In a closed-center spool valve, a piston is solid, and all passages through a valve are blocked when a piston is centered in its cylinder (see Figure 5-24, diagram B). A closed-center valve is used when a single pump or an accumulator performs more than one operation and where there must be no pressure loss in shifting a stroke direction at a work point.
In an open-center spool valve, the spools on a piston are slotted or channeled so that all passages are open to each other when a piston is centered (see Figure 5-25). In some open-center valves, passages to a cylinder port are blocked when a valve is centered and liquid from a pump is carried through a piston and out the other side of a valve to a reservoir (see Figure 5-26). Liquid must be carried to both ends of a piston of a directional valve to keep it balanced. Instead of discharging into a reservoir when a valve is centered, liquid may be directed to other valves so that a set of operations is performed in sequence.
Open-center valves are used when a work cylinder does not have to be held in position by pressure and where power is used to perform a single operation. These valves also avoid shock to a system when a valve spool is moved from one position to another, since in the intermediate position, pressure is temporarily relieved by liquid passing from a pump directly to the reservoir.
(3). Manually Operated Four-Way Valve. 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.
(4) Pilot-Operated, Four-Way Valve. 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.
(5) Solenoid-Operated, Two- and Four-Way Valves. These valves are used to control the direction of hydraulic flow by electrical means. A spool is shifted by energizing a solenoid that is located at one or both ends of the spool. When a solenoid is energized, it forces a push rod against the end of a spool. A spool shifts away from the solenoid and toward the opposite end of the valve body (see Figure 5-29). In a spring-offset model, a single solenoid shifts a spring-loaded spool. When a solenoid is deenergized, a spring returns a spool to its original position.