With neat sketch discuss the functional operations of 4/3 D.C. control valve of hydraulic system. Also give symbolic representations.

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 The 4/3 Direction Control valve has four ports marked as P, A, B, T and three distinct positions. Where P is power port, T is return port and A&B are actuator ports.


                                                                 (Spool Position: 1)


                                                              (Spool Position: 2)

Three position four way (3/4) valves are used in double acting cylinders to perform advance hold and return operation to the piston. These types of valves have three  switching positions. They have a variety of possible flow path configurations but have identical flow path configuration. When the centered path is actuated port A and B are connected  with both the ports P and T respectively . In this case valve is not active because all the ports are open to each other. The fluid flows to the tank at atmospheric pressure. In this position work cannot be done by any part of the system. This configuration helps to prevent heat build up. 

When left end (port B) is actuated the port P is connected with ports B and T is connected with port A as shown in figure-1. Similarly when the right end is actuated the port P is connected to A and working Port B is connected to port T as shown in 2. The three position valves are used when the actuator is needed to stop or hold at some intermediate position. It can also be used when the multiple circuits or functions are accomplished from one hydraulic power source.

Spool position - 1: For this spool position, the fluid can flow from port P to port B and return from port A to port T

Spool position - 2: This is the neutral position here all the ports are blocked by the spool

Spool position - 3: For this spool position, the reverse flow is taking place from port P to Port A and return from Port B to port T.

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