FLUID POWER TECHNOLOGY
RADZI ABDUL RASIH FACULTY OF MECHANICAL ENGINEERING, UITMCPP
radzi_rasih@ppinang.uitm.edu.my
radzi_rasih@ppinang.uitm.edu.my
Chapter 5
Hydraulic Valves
radzi_rasih@ppinang.uitm.edu.my
Objectives
Explain the function of each of the three general types of control valves used in hydraulic systems. Compare the design and operation of direct-acting and pilot-operated pressure control valves.
Objectives
Describe the function of the various types of pressure control valves used in hydraulic systems. Compare the design and operation of twoway, three-way, and four-way directional control valves.
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Primary Control Functions in a Hydraulic System
Control valves allow hydraulic systems to produce the type of motion or level of force needed to complete the functions expected of a hydraulic circuit
Primary Control Functions in a Hydraulic System
A variety of valves can control actuator direction, speed, and force output
Used with permission of CNH America LLC
Primary Control Functions in a Hydraulic System
The three basic types of control valves are:
Pressure control Directional control Flow control
Primary Control Functions in a Hydraulic System
Pressure control valves can:
Protect the system from damage due to excessive pressure Sequence motion Limit pressure in selected sections of a circuit
Primary Control Functions in a Hydraulic System
A system pressure control valve
Primary Control Functions in a Hydraulic System
Directional control valves direct fluid flow to establish and control actuator movement
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Used with permission of CNH America LLC
Primary Control Functions in a Hydraulic System
Flow control valves control the operating speed of actuators They provide a means to vary the rate of fluid flow
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Primary Control Functions in a Hydraulic System
A typical flow control valve
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Basic Structure and Features of Control Valves
Fluid control valves incorporate several internal elements to provide a desired operation The elements allow, direct, meter, or stop the flow of fluid The elements include fixed orifices, needle valves, spools, poppets, and sliding plates
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Basic Structure and Features of Control Valves
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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Basic Structure and Features of Control Valves
Fixed orifices
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Basic Structure and Features of Control Valves
A spool is a cylindrical metal piece fitted into the bore of a valve body The spool is used to block or direct fluid through a valve to produce a desired fluid flow characteristic
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Basic Structure and Features of Control Valves
A typical spool
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Used with permission of CNH America LLC
Basic Structure and Features of Control Valves
Internal and external forces are used to position the various valve elements
Springs and pilot pressure are typical internal forces used to operate valve elements Manual, pilot pressure, and electromagnetic force 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 fluid flow 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 while preventing reverse flow Other functions include:
Bypassing components during the return cycle of the system Providing flow resistance to maintain a minimum system 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 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 25
Directional Control Devices
Restriction check valves allow free flow in one direction and restricted flow when flow direction is reversed This is accomplished via a metering orifice machined 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 and single-acting cylinders The actuator is returned to its original position 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 fluid to enter and extend the unit During retraction, the valve blocks the supply line and connects the actuator line to a system return line, allowing external force to return the actuator to its original position while directing displaced fluid to the reservoir
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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 the actuator Internal structure of the valve allows the ports to be alternately connected when a change in actuator direction 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 threeposition valves This provides several operating options when 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 the direction In three-position valves, a center position is added that provides additional circuit operating 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 Five general activation methods categories: are used to shift Flow actuation the internal Manual operation components of Mechanical operation directional control Pilot operation valves 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 operator presence and are typically found in lesscomplex systems
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Directional Control Devices
Mechanical operation methods include: Rollers Cams Levers Rams Mechanical operation is often used when the opening and 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 physical labor 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
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Courtesy of Eaton Fluid Power Training
Directional Control Devices
Electrical control of hydraulic systems is common in many types of equipment
Simple solenoid devices to shift basic valves Electronic controllers operate proportional solenoid valves to produce extreme accuracy and repeatability
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Directional Control Devices
Typical electrically controlled valve
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The Oilgear Company
Directional Control Devices
Multiple-position directional control valve 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
Simple pressure relief valves depend on heavy internal springs to establish valve operating pressure
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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 a spring This opens a ball or poppet valve, allowing excess 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 a hydraulic circuit
Primary actuator moves as soon as fluid flow is directed to the actuator section of the circuit Sequence valve blocks flow to the secondary actuator until a predetermined pressure is reached, 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 is reversed
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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 actuator movement. It is often used with cylinders
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Pressure Control Devices
Counterbalance valve with integral check valve
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Pressure Control Devices
Counterbalance valves allow the downward movement of loads supported by a cylinder only when the system pump is operating Force generated by system pressure and the force created by the weight of the load are needed to move the load downward
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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 pump operates at near-zero pressure This reduces energy consumption and maintenance costs
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Pressure Control Devices
Pump unloading controls use an unloading valve and a dual pump or accumulator to maintain desired system pressure while dumping unneeded pump output to the reservoir at very low pressure
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Pressure Control Devices
A typical unloading valve
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Pressure Control Devices
Circuit containing an unloading relief valve
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Pressure Control Devices
A typical unloading relief 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 pressure setting of the system relief valve Allows a system to operate using two or more pressures Maximum pressure is determined by the system relief valve 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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Pressure Control Devices
Pressure-reducing valves are normally open valves with an internal pilot and external drain
When the valve is operating at the reduced pressure, there is a continual fluid flow through the drain line Any resistance to fluid flow from the drain line reduces valve operating efficiency
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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, sharpedged 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 viscosity changes Compensated valves automatically adjust for fluid pressure variations to produce a consistent flow rate under varying load and 79 temperature conditions
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 the flow 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 pressurecompensated flow control valve
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Flow Control Devices
Pressure compensator operation
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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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