directional control valves

DIRECTIONAL CONTROL VALVES

1. What are Directional Control Valves?

• As the name suggests, the function of a directional control valve (DCV) is to control the direction of flow in a pneumatic circuit.

• The DCVs are used to start, stop and regulate the direction of air flow and to help in the distribution of air in the desired line.

• Types: As discussed in Section 6.6.2, the DC valves can be classified in many different ways.

1. Based on the construction:

(i) Poppet (or seat) valves,

(ii), Sliding spool valves, and 

(iii) Rotary spool valves.

2. Based on the number of ports present: 

(i) Two-way valves,

(ii) Three-way valves, and

(iii) Four-way valves.

3. Based on the method of actuation:

(i) Mechanically actuated valves,

(ii) Electrically (solenoid) actuated valves, and

(iii) Pneumatically actuated valves.

2. Two-Way Valves

Basically two-way valve is an on-off type valve. This 2/2 way has two ports (a supply port and an exhaust port) and two positions (open and closed).

1. Construction and Operation

• The construction and operation of an air-piloted (i.e., air-operated), two-way pneumatic valve is illustrated in Fig.12.1.

• As shown in Fig.12.1, 2/2 way valve is available to operate either normally open or normally closed conditions.

• A normally open two-way valve (Fig.12.1(a)) permits flow in its normal or in its rest position and blocks flow when actuated.

• The normally closed valve (Fig.12.1(b)) block flow in its normal position and permits flow when actuated.

• These valves have long life and can be used to handle dry and lubricated air.

3. Three-Way Valves

The three-way type valves have three ports—an inlet, an exhaust, and a cylinder port.

1. Construction and Operation

The construction and operation of a typical poppet-type three-way two-position (i.e.,.3/2) pneumatic valve is illustrated in Fig.12.2(a). Fig.12.2(b) shows the graphic symbol of the 3/2 way valve.

As could be seen from Fig.12.2(a) that one flow port is connected to either of the other two ports. This valve may also be used to pressurise one port and exhaust the other port. Thus these valves can be used as a pilot relay to operate the other valves.

4. Four-Way Valves

The four way type valves have four ports--an inlet, an exhaust, and two cylinder ports. 

1. Construction and Operation

The construction and operation of a typical valve-seat type four-way two-position (i.e., 4/2) pneumatic valve is illustrated in Fig.12.3(a). Fig.12.3(b) shows the graphic symbol of the 4/2 way valve.

As shown in Fig.12.3, the inlet port P connects to cylinder ports A and B to exhaust port R. When the valve elements are actuated by means of the push button, they are unseated and port P connects to cylinder ports B and A to exhaust port R.

Note 

Generally two-way DC valves are used as on-off type valves; three-way DC valves are used to control single-acting linear actuators; and four-way DC valves are used to control double-acting actuators.