Valve information

Ways The (oil) ports on a valve. A 3-way valve has 3 ports: pressure (P), tank (T), and cylinder (A). A 4-way valve has 4 ports: pressure (P), tank (T), advance (A) and retract (B). Single-Acting cylinders require at least a 3-way valve, and can, under certain instances, be operated with a 4-way valve. Double-Acting cylinders require a 4-way valve, providing control of the flow to each cylinder port.
Directional Control Valves
3-way valve are used with single-acting cylinders		4-way valve are used with double-acting cylinders

Valves may be either pump mounted or remote mounted
Pump Mounted		Remote Mounted

Advance / Hold / Retract
Valves may be either manually or solenoid operated
Manually Operated		Solenoid Operated

Single-acting cylinder Controlled by a 3-way, 3-position valve.		Double-acting cylinder Controlled by a 4-way, 3-position valve.
Advance The oil flows from the pump pressure port P to the cylinder port A: the cylinder plunger will extend.		Advance The oil flows from the pump pressure port P to the cylinder port A and from cylinder port B to tank T
Hold The oil flows from the pump pressure port P to the tank T. The cylinder port A is closed: the cylinder plunger will maintain its position.		Hold The oil flows from the pump pressure port P to the tank T. The cylinder ports A and B are closed: the cylinder plunger will maintain position.
Retract The oil flows from the pump and cylinder port A to the tank T: the cylinder plunger will retract.		Retract The oil flows from the pump pressure port P to cylinder port B and from cylinder port A to tank T: the cylinder plunger will retract.

Positions The number of control points a valve can provide. A 2-position valve has the ability to control only the advancement or retraction of the cylinder. To be able to control the cylinder with a hold position, the valve requires a 3rd position. Centre Configuration The center position of the valve is the position at which there is no movement required of the hydraulic component, whether it is a tool or a cylinder. Tandem Centre. This configuration provides for little to no movement of the cylinder and the unloading of the pump. This provides for minimum heat build-up. Closed Centre. This configuration is used mostly for independent control of multi-cylinder applications. This configuration again provides for little to no movement of the cylinder, but also dead-heads the pump, isolating it from the circuit. Use of this type of valve may require some means of unloading the pump to prevent heat build-up. There are many more type of valves, such as Open Center and Float Center. These valves are used mostly in complex hydraulic circuits and require other special considerations.
Open centre	float centre