Directional control valves are specified generally by the number of ports or ways (lines attached to the symbol’s box) and the number of positions (boxes or envelopes in the symbol) they have. Other information about them includes whether they are normally closed (not passing fluid), normally open (passing fluid), how they are operated (solenoid, manual, or spring) and other features such as manual overrides, drain ports, pilot ports, etc.
Some general rules for drawing symbols are:

  • only draw flow lines to one box of the symbol
  • always see that flow paths and direction of flow in each box is compatible
  • on 4-way hydraulic valves, pipe the A port to the cap end of the cylinder and the B port to the rod end
  • draw all symbols in their at-rest position. Show valves that are held actuated by a machine member in their shifted condition, and
  • provide information such as pressure settings, flow rates, orifice sizes, horsepower and rpm where applicable.

(According to this method of specifying, check valves and pre-fill valves would be 2-way valves because they have two ports. However, because these valves are basically single function and have infinitely variable flow paths, their symbols and terminology do not follow general directional control valve rules.)

Figure 10-10 shows the symbol for a 2-way directional control valve and how it could function in a circuit. Notice the symbol has two boxes (or envelopes) to indicate two positions. Each position is a flow path. The box with flow lines coming to it is the normal or at-rest position of the valve. The normal or at-rest position is usually at the spring end of a spring-return valve as seen in the figure.

The circuit at rest in Figure 10-10 illustrates how a schematic drawing shows the component symbols for the system builder or troubleshooter. Valves, actuators, flow paths and line connections are all shown according to the ANSI or ISO graphic symbols that were explained in Chapter 4. To understand how the circuit operates, a person must be able to read the symbols and know how they represent a piece of hardware. The valve in this circuit is 2-way, 2-position, direct solenoid-operated, spring return, normally closed. The diagrams to the right of the circuit at rest show how the directional control valve shifts to its second position and ports fluid to the cylinder. In the real world, this is done in a person’s imagination . . . and can be confusing when several valves are working simultaneously. In the diagram it is easy to see that with the solenoid energized, the normally open box moves in line with the input flow and sends fluid to the cylinder. The arrow in the normally open box shows flow from inlet to cylinder port, causing the piston to extend. If the solenoid is de-energized, the spring returns the valve to the circuit at rest condition and the cylinder stops in its last position.

Two-way valves cannot have more than two positions because they can only stop or allow fluid flow. It is easy to see that a 2-way directional control valve will not operate a single-acting cylinder. These valves are only good for operations that require an on-off supply. As shown in the bottom half of Figure 10-10, two 2-way valves are needed to control a single-acting cylinder. A double-acting cylinder needs four 2-way valves to control it. There are both normally closed and normally open valves in these circuits.

Figure 10-11 shows how 3-way valves can replace 2-way valves and make a machine simpler. This circuit at rest has a cylinder powered by a 3-way, 2-position, solenoid pilot-operated, spring-return, normally closed directional control valve. Because this valve has a flow path from the pressure port to the cylinder port and from the cylinder port to atmosphere, it can control a single-acting cylinder. The diagrams to the right show that when the solenoid is energized, the cylinder extends under power. The next schematic diagram shows the cylinder retracting from external forces with the solenoid de-energized.

Two 3-way valves are needed to power a double-acting cylinder as shown in Figure 10-11. The double-acting palm button activates this circuit. The valve on the cap end is normally closed and the valve on the head end is normally open. This is a simple anti-tie down circuit, but is not OSHA safe because one palm button can be depressed before the second one and the cylinder will move. OSHA requires that both buttons be operated concurrently to make the cylinder extend. It does meet the anti-tie down requirement because the cylinder will not retract until both palm buttons are released.

The double-acting inching circuit in Figure 10-11 uses two 3-way, 3-position, spring-centered valves to make it possible to stop the cylinder at any point in its stroke. A 3-way valve can have a third position to perform another function. The pictured center condition has all ports blocked, which stops flow at all ports. This is the center condition normally found on a 3-way valve.

Note: pneumatic inching circuits cannot stop and hold a load consistently. Any change in speed, load, or pressure can produce a different stopping position. About plus or minus one inch would be the best position accuracy an air cylinder would achieve, unless it is moving very slowly. Air leaks in the plumbing or valves also interfere with trying to stop and hold an intermediate position. Leaks may let one end of the cylinder bleed off and allow air from the opposite end to expand and move the cylinder out of position.