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Fluid Power Systems (ME353)

Fall 2012

Lecture 9

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Directional ControlDevices

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Directional control devices allow a system operator to control the directionof fluid flow in the system –

Starting and stopping of actuators – Control of actuator movement direction

Directional control devices can be grouped in four general classifications – Shut-off or two-way valves – Check valves – Three-way valves – Four-way valves

Symbols for directional control valves

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The primary purpose of shut-off valves is to block fluid flow through ahydraulic system line – Globe valve

– Gate valve – Ball valve – Spool valve – Needle valve

Typical globe valve

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Typical gate valve

Typical ball valve

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Typical spool valve Typical needle valve

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The primary purpose of check valves is to allow free flow inone direction while preventing reverse flowOther functions include: – Bypassing components during the return cycle of the system – Providing flow resistance to maintain a minimum system

pressure required for pilot operations

Typical inline check valve Typical right-angle check valve

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A standard check valve consists of a valve body containing aone-way valve located between inlet and outlet ports – The one-way valve allows fluid flow through the valve in

only one direction – Some designs contain a spring that seats the valve poppet or

ball – In other designs, the poppet is seated only by fluid flow

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Restriction check valves allow free flow in one direction andrestricted flow when flow direction is reversed

This is accomplished via a metering orifice machined into the poppet

Typical restriction check valve

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Pilot-operated check valves can allow reverse flow throughthe valve

– Typically, pilot pressure opens the valve – In some designs, pilot pressure may also hold the valve shutto block flow in both directions

Pilot pressure to open check valve

Pilot pressure to block flow through valve

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Three-way directional control valves provide a means to extendrams and single-acting cylindersThe actuator is returned to its original position by an external force

– System load – Spring built into the actuator

Typical three-way directional control valve

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During extension, the three-way valve connects the actuator inlet line to asystem supply line, allowing fluid to enter and extend the unitDuring retraction, the valve blocks the supply line and connects the actuator

line to a system return line, allowing external force to return the actuator toits original position while directing displaced fluid to the reservoir