Chapter 5- Hydraulic Valves

8/12/2019 Chapter 5- Hydraulic Valves

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FLUID POWERTECHNOLOGY

RADZI ABDUL RASIHFACULTY OF MECHANICALENGINEERING, UITMCPP

[email protected]

[email protected]


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Chapter 5

Hydraulic Valves

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Objectives Explain the function of each of the three

general types of control valves used inhydraulic systems.

Compare the design and operation ofdirect-acting and pilot-operated pressurecontrol valves.


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Objectives Describe the function of the various types

of pressure control valves used inhydraulic systems.

Compare the design and operation of two-way, three-way, and four-way directionalcontrol valves.


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Primary Control Functions in aHydraulic System

Control valves allow hydraulic systems toproduce the type of motion or level offorce needed to complete the functionsexpected of a hydraulic circuit


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The three basic types of control valvesare:

Pressure control Directional control Flow control



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A system pressure control valve



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Directional controlvalves direct fluid flowto establish and controlactuator movement

Used with permission of CNH America LLC


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Flow control valves control the operatingspeed of actuators

They provide a means to vary the rate offluid flow



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A typical flow control valve



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Basic Structure and Features ofControl Valves

Fluid control valves incorporate severalinternal elements to provide a desiredoperation

The elements allow, direct, meter, or stopthe flow of fluid

The elements include fixed orifices , needlevalves, spools , poppets, and sliding plates


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A fixed orifice is a precision hole either: Machined into the valve body or a component Pressed as a separate part into a valve

passageway Both designs are used to control fluid flow



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Fixed orifices



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A spool is a cylindrical metal piece fittedinto the bore of a valve body

The spool is used to block or direct fluidthrough a valve to produce a desired fluidflow characteristic



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A typical spool

Used with permission of CNH America LLC



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Internal and external forces are used toposition the various valve elements

Springs and pilot pressure are typical internalforces used to operate valve elements

Manual, pilot pressure, and electromagneticforce are common external forces used for

operation


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Directional Control Devices Directional control devices allow a system

operator to control the direction of fluidflow in the system

Starting and stopping of actuators Control of actuator movement direction


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Directional Control Devices Directional control devices can be grouped

in four general classifications Check valves Three-way valves Four-way valves Shuttle valves


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Directional Control Devices Symbols for directional control valves


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Directional Control Devices The primary purpose of check valves is to

allow free flow in one direction whilepreventing reverse flow

Other functions include: Bypassing components during the return cycle of the

system

Providing flow resistance to maintain a minimumsystem pressure required for pilot operations


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Directional Control Devices Typical inline check valve


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Directional Control Devices Typical right-angle check valve


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Directional Control Devices A standard check valve consists of a valve

body containing a one-way valve locatedbetween inlet and outlet ports

The one-way valve allows fluid flow throughthe valve in only one direction

Some designs contain a spring that seats thevalve poppet or ball

In other designs, the poppet is seated only byfluid flow


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Directional Control Devices Restriction check valves allow free flow in

one direction and restricted flow when flowdirection is reversed

This is accomplished via a metering orificemachined into the poppet


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Directional Control Devices Typical restriction check valve


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Directional Control Devices Pilot-operated check valves can allow

reverse flow through the valve Typically, pilot pressure opens the valve In some designs, pilot pressure may also hold

the valve shut to block flow in both directions


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Directional Control Devices Pilot pressure to open check valve


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Directional Control Devices Pilot pressure to block flow through valve


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Directional Control Devices Three-way directional control valves

provide a means to extend rams andsingle-acting cylinders

The actuator is returned to its originalposition by an external force

System load Spring built into the actuator


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Directional Control Devices Typical three-way directional control valve

(retraction)


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Directional Control Devices During extension, the three-way valve connects the

actuator inlet line to a system supply line, allowing fluidto enter and extend the unit

During retraction, the valve blocks the supply line andconnects the actuator line to a system return line,allowing external force to return the actuator to itsoriginal position while directing displaced fluid to thereservoir


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Directional Control Devices Four-way directional control valves provide a means to

power actuators in either direction Valve has four external ports for connection to system

supply line, reservoir, and inlet and outlet of theactuator

Internal structure of the valve allows the ports to bealternately connected when a change in actuatordirection is necessary


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Directional Control Devices Four-way valve powers double-acting

cylinder during extension and retraction


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Directional Control Devices Four-way directional control valves are

typically manufactured as two- or three-position valves

This provides several operating optionswhen designing circuits


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Directional Control Devices Typical two-position, four-way valve


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Directional Control Devices In two-position valves, the first position

operates the actuator in one direction,while the second position reverses thedirection

In three-position valves, a center positionis added that provides additional circuitoperating characteristics


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Directional Control Devices Typical three-position, four-way valve


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Directional Control Devices A number of center position configurations

are available Closed Open Tandem Floating Regenerative


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Directional Control Devices Symbols for four-way valve center position


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Directional Control Devices A number of

activation methodsare used to shiftthe internalcomponents ofdirectional control

valves

Five generalcategories:

Flow actuation Manual operation Mechanical operation Pilot operation Electrical operation


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Directional Control Devices Flow actuation uses internal fluid

movement to actuate the valve No external mechanism or force is used


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Directional Control Devices Manual operation methods include:

Handwheels Levers Push buttons Foot pedals

These devices require constant operatorpresence and are typically found in less-complex systems


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Directional Control Devices Mechanical operation methods include:

Rollers Cams

Levers Rams

Mechanical operation is often used when the openingand closing of the valve must occur at a specific

position in actuator travel


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Directional Control Devices Circuit containing a mechanically actuated

directional control valve


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Directional Control Devices Pilot operation uses system pressure to

activate the valve, rather than physicallabor

This method is effective when: Larger forces are need to shift the valve Remote operation is required because of

safety or tight physical factors


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Directional Control Devices Pilot-operated directional control valve

Courtesy of Eaton Fluid Power Training


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Directional Control Devices Typical electrically controlled valve

The Oilgear Company


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Directional Control Devices

Multiple-position directional controlvalve may be held in a desired position

using springs or detents Springs are located on the ends of the

valve spool to return the valve to its

normal operating position


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Directional Control Devices Symbols for spring-return valves


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Directional Control Devices Detents are locking devices that hold the

spool in a selected position The spool may be held until the operator

manually shifts the valve Increased system pressure at the end of an

operation may automatically shift detent

valves back to the normal position


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Directional Control Devices Typical detent operation


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Directional Control Devices Shuttle Valve


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Pressure Control Devices Pressure control valves may be grouped

into one of five types Simple pressure relief valve Actuator sequence valve Counterbalance valve Unloading valve Pressure reducing valve


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Pressure Control Devices Relief valves are normally closed valves They open when system pressure approaches the set

maximum operating pressure The operation of relief valves can be classified as:

Direct operated Balancing piston (compound)


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Pressure Control Devices Direct-operated relief valves use system

pressure to generate force to compress aspring

This opens a ball or poppet valve , allowingexcess fluid to return to the reservoir


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Pressure Control Devices Sequence valves allow the automatic

sequencing of two or more actuators in ahydraulic circuit

Primary actuator moves as soon as fluid flowis directed to the actuator section of the circuit

Sequence valve blocks flow to the secondary

actuator until a predetermined pressure isreached, then allows fluid flow to the actuator


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Pressure Control Devices A sequence valve is typically fitted with an

integral check valve This allows free flow of fluid around the

valve when the direction of the actuator isreversed


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Pressure Control Devices Sequence valve with integral check valve


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Pressure Control Devices A circuit containing a sequence valve


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Pressure Control Devices Counterbalance valves are used in circuits

to prevent unexpected actuatormovement. It is often used with cylinders


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Pressure Control Devices

Circuit containing a counterbalance valve


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Pressure Control Devices Pump unloading controls hold a desired

operating pressure while the pumpoperates at near-zero pressure

This reduces energy consumption andmaintenance costs


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Pressure Control Devices Pump unloading controls use an unloading

valve and a dual pump or accumulator tomaintain desired system pressure while

dumping unneeded pump output to thereservoir at very low pressure


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Pressure Control Devices A typical unloading valve


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Pressure Control Devices Reduced pressure control allows a portion of a system to

operate at a pressure below the maximum pressuresetting of the system relief valve

Allows a system to operate using two or morepressures

Maximum pressure is determined by the system reliefvalve with the additional pressures lower


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Pressure Control Devices Circuit containing pressure-reducing valve


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Pressure Control Devices Typical pressure-reducing valve


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Flow Control Devices Conceptual operation of a flow control

valve may be traced to a basic orifice


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Flow Control Devices The flow rate through a simple, sharp-

edged orifice depends on: Area of the orifice Pressure difference between the inlet and

outlet sides of the orifice Viscosity of the fluid, which varies with fluid

temperature


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Flow Control Devices Flow control valves may be

noncompensated or compensated The flow rate through noncompensated

valves varies as the load or fluid viscositychanges

Compensated valves automatically adjust forfluid pressure variations to produce aconsistent flow rate under varying load andtemperature conditions


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Flow Control Devices A needle valve is the noncompensated

adjustable flow control device Consists of an orifice fitted with a tapered

needle machined on a threaded stem Turning the threaded stem changes the

effective area of the orifice, which adjusts theflow rate through the valve


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Flow Control Devices Basic adjustable

flow control valve


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Flow Control Devices A basic pressure-

compensated flowcontrol valve


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Flow Control Devices Pressure

compensatoroperation


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Flow Control Devices Pressure compensator operation


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Flow Control Devices Circuit containing restrictor-type,

compensated flow control valve

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Chapter 5- Hydraulic Valves