Hydraulic Pilot-operated check valves

The cylinder in the system in Figure 4.22 should, theoretically, hold position when the control valve is in its centre, off, position. In practice, the cylinder will tend to creep because of leakage in the control valve.

Check valves have excellent sealage in the closed position, but a simple check valve cannot be used in the system in Figure 4.22 because flow is required in both directions. A pilot-operated check is similar to a basic check valve but can be held open permanently by application of an external pilot pressure signal.

There are two basic forms of pilot-operated check valves, shown in Figure 4.23. They operate in a similar manner to basic check valves, but with pilot pressure directly opening the valves. In the 4C valve shown in Figure 4.23a, inlet pressure assists the pilot. The symbol of a pilot-operated check valve is shown in Figure 4.23c.

The cylinder application of Figure 4.22 is redrawn with pilot operated check valves in Figure 4.23d. The pilot lines are connected to the pressure line feeding the other side of the cylinder. For any cylinder movement, one check valve is held open by flow (operating as a normal check valve) and the other is held open by pilot pressure. For no required movement, both check valves are closed and the cylinder is locked in position.

Hydraulic 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.

Hydraulic 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.

Classification Description
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).
Tandem center
Partially closed center
Semi-open center

(1). Poppet-Type Valve.
(2). Sliding-Spool Valve.
(3). Manually Operated Four-Way Valve.
(4). Pilot-Operated, Four-Way Valve.
(5). Solenoid-Operated, Two- and Four-Way Valves.

Hydraulic 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; back flow, 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 inline 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.