Hydraulic Lever Control Spool Valves

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The two primary characteristics for selecting a directional-control valve are the number of fluid ports and the number of directional states, or positions, the valve can achieve. Valve ports provide a passageway for fluid to flow to or from other components. The number of positions refers to the number of distinct flow paths a valve can provide.
 
A 4-port, 3-position spool valve serves as a convenient illustration, (See Picture). One port receives pressurized fluid from the pump, and one routes fluid back to the reservoir. The other two ports are generally referred to as work ports and route fluid to or from the actuator. In this case, one work port routes fluid to or from the rod end of the cylinder, the other routes fluid to or from the cap end.
 
The valve represented in (See Picture) can be shifted to any of three discrete positions. As shown, in the neutral position, all ports are blocked, so no fluid will flow. Shifting the valve to the right routes fluid from the pump to the rod end of the cylinder, causing its piston rod to retract. As the piston rod retracts, fluid from the cylinder's cap end flows to the reservoir. Shifting the valve to the left routes fluid from the pump to the cap end of the cylinder, causing the piston rod to extend. As this occurs, fluid from the rod end of the cylinder flows to the reservoir. Returning the valve spool to the center position again blocks all flow. (In reality, a relief valve would be provided between the pump and directional valve. It is omitted here for simplicity.)

4/3 Spool Valve
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4/3 Spool Valve

Schematic shows simple circuit to control cylinder extension and retraction using a 4-port, 3-position spool valve.

Spool-type valves are widely used because they can be shifted to two, three, or more positions for routing fluid between different combinations of inlet and outlet ports. They are used extensively for directional control of actuators because a single valve can produce extension, retraction, and neutral. 

Hydraulic Lever Spool Valves
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Hydraulic Lever Spool Valves
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Hydraulic Lever Spool Valves
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Hydraulic Lever Spool Valves
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Hydraulic Lever Spool Valves
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