Question

1) What is the difference between the closed center Dc valve and the open center Dc valve

2) what is the function of the Needle valve with By-pass check

Answer 1

1. Closed center DCV

In a closed center DCV(spool type) flow from the pump is blocked when the spool is in neutral condition. Refer the diagram below:

At the center position of spool, there is no flow from the pump. When the spool moves either left or right, oil flows either through left or right port (C1 or C2) and returns through the remaining port, ensuring movement of piston in the desired direction.

Since ports are closed to the cylider, this valve locks the cylinder in current position when the spool is at center position. Hence this DCV is used when locking of piston is required.

Open center valve

In open center DCV oil flow from the pump is maintained when the spool is in center position. In a typical DCV, oil flows through the holes in the spool and holes in valve body back to the tank. The left or right movement of spool results in similar movement in cylinder as in closed center DCV. Refer diagram below:

Open center DCVs are used where it is not necessary to lock the piston in position. This arrangement reduces shock in the system. Shock may happen when the spool moves from one position to other position crossing the center position.

2. Refer the diagram below:

The needle valve parallel to bypass check valve allows a controlled amount of oil flow to the cylinder hence controlling the speed of the piston. However, during opposite motion of the piston the speed of the piston in uncontrolled, which can aganin be controlled if we incorporate similar arrangement before other port of the cylinder.