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Chapter 18
Controlling a Pneumatic System
Pressure, Direction, and Flow
Permission granted to reproduce for educational use only.3© Goodheart-Willcox Co., Inc.
Objectives
Explain the function of the three general types of control valves used in pneumatic systems.
Describe the methods used to control air pressure in a pneumatic system.
Compare the design and operation of pneumatic pressure regulator valves.
Describe the design and operation of pneumatic system directional control valves.
Permission granted to reproduce for educational use only.4© Goodheart-Willcox Co., Inc.
Objectives
Name and compare the various types of construction used to assure sealing of the internal passages found in pneumatic directional control valves.
List and compare the methods used to position control spools in pneumatic directional control valves.
Permission granted to reproduce for educational use only.5© Goodheart-Willcox Co., Inc.
Objectives
Describe the structure and operation of flow control valves used to control actuator speed in pneumatic circuits.
Explain how flow control valves are used in pneumatic circuits to control actuator speed.
Permission granted to reproduce for educational use only.6© Goodheart-Willcox Co., Inc.
Primary Control Functionsin a Pneumatic Circuit
Control valves are used to establish the level of force and type of motion needed to operate a machine
These valves control:– Air pressure– Direction of airflow– Rate of airflow
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Primary Control Functionsin a Pneumatic Circuit
Pressure control valves:– Limit maximum system pressure– Regulate the force or torque an actuator generates
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Primary Control Functionsin a Pneumatic Circuit
Directional control valves control airflow direction to establish the direction of actuator movement
Flow control valves regulate the rate of airflow to control actuator operating speed
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Basic Control ValveDesign and Structure
The body of pneumatic system valves may be formed from:– Metal casting– Standard bar stock
Precision bores and passageways are machined into the body– Hold component parts– Allow airflow that controls the operation of the
valve
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Basic Control ValveDesign and Structure
Cast valve body
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Basic Control ValveDesign and Structure
Bar stock valve body
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Basic Control ValveDesign and Structure
The operation of a control valve depends on a series of control elements in the valve
The elements used depend on:– Function of the valve– Operating techniques preferred by the valve
manufacturer
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Basic Control ValveDesign and Structure
Control elements include:– Fixed orifices– Spools– Pistons– Diaphragms– Precision-machined sliding plates– Needle and poppet valves
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Basic Control ValveDesign and Structure
Fixed orifice
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Basic Control ValveDesign and Structure
Needle valve
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Basic Control ValveDesign and Structure
Spool
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Basic Control ValveDesign and Structure
Diaphragm
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Basic Control ValveDesign and Structure
Precision-machined sliding plate
Rotary Directional Control Valve
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Basic Control ValveDesign and Structure
Varying the position of control elements within a control valve determines:– Setting of the valve– Accuracy of system control
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Basic Control ValveDesign and Structure
The force used to position the internal elements of a control valve is provided by:– Internal springs– Internal air pressure– External pilot pressure– Electromagnets– Manually applied force
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Basic Control ValveDesign and Structure
Internal spring force
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Basic Control ValveDesign and Structure
Electromagnetic force (solenoids)
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Pressure Control Methods
and Devices Pressure control is provided in three specific
areas of a pneumatic system:– Air distribution system– Workstation areas– Portions of circuits operating within the workstation
areas
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Pressure Control Methods
and Devices The pressure in the distribution lines of a
pneumatic system is controlled by:– Bypass– Compressor-capacity control system
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Pressure Control Methods
and Devices Bypass pressure control limits maximum
system pressure by bleeding excess air to the atmosphere through a relief-type valve– The compressor in this system operates on a
continuous basis– The compressor continuously functions at the
maximum pressure setting of the system
Permission granted to reproduce for educational use only.26© Goodheart-Willcox Co., Inc.
Pressure Control Methods
and Devices Compressor-capacity control limits maximum
pressure using one of several different control methods– Compressor start-stop– Inlet valve unloading– Compressor speed control– Control of the cross-sectional area of the inlet (in
dynamic compressors)
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Pressure Control Methods
and Devices Start-stop compressor capacity control
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Pressure Control Methods
and Devices Control of air pressure at the workstation is
provided by a pressure regulator It is often a part of the filter/pressure
regulator/lubricator (FRL) unit commonly used at the workstation
Permission granted to reproduce for educational use only.29© Goodheart-Willcox Co., Inc.
Pressure Control Methods
and Devices Pressure regulator of an FRL unit
IMI Norgren, Inc.
Permission granted to reproduce for educational use only.30© Goodheart-Willcox Co., Inc.
Pressure Control Methods
and Devices Pressure regulator is required at the workstation
due to:– Air from the distribution line is at a higher pressure
than desired– Varying distribution line pressure caused by the
compressor-capacity control system
Permission granted to reproduce for educational use only.31© Goodheart-Willcox Co., Inc.
Pressure Control Methods
and Devices A variety of pressure regulator designs are
used to control workstation pressure– Direct operated– Basic, diaphragm chamber– Relieving– Balanced poppet– Pilot operated
Permission granted to reproduce for educational use only.32© Goodheart-Willcox Co., Inc.
Pressure Control Methods
and Devices When sections of a circuit require maximum
air pressures lower than that provided by the FRL unit, additional pressure regulators may be inserted in the circuit to provide the needed lower pressures
Permission granted to reproduce for educational use only.33© Goodheart-Willcox Co., Inc.
Pressure Control Methods
and Devices Additional workstation
regulator provides lower pressure
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Pressure Control Methods
and Devices Subcircuit is provided with lower pressure
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Pressure Control Methods
and Devices When sections of a circuit require a pressure
exceeding that available from the system distribution line, an air-to-air or air-to-oil booster unit can be used to produce the higher pressure
Permission granted to reproduce for educational use only.36© Goodheart-Willcox Co., Inc.
Pressure Control Methods
and Devices Air-to-air booster provides higher pressure
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Pressure Control Methods
and Devices A safety valve prevents excessive air pressure
in distribution lines due to failure of compressor capacity control components
A positive-acting, relief-type valve large enough to exhaust the output capacity of the compressor is used for this application
Permission granted to reproduce for educational use only.38© Goodheart-Willcox Co., Inc.
Pressure Control Methods
and Devices Safety valve on a small compressor
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Directional Control Methods and Devices
Directional control valves can be grouped into four categories:– Shut-off valves– Check valves– Three-way valves– Four-way valves
Permission granted to reproduce for educational use only.40© Goodheart-Willcox Co., Inc.
Directional Control Methods and Devices
Shut-off valves used in pneumatic systems include:– Globe– Gate– Ball– Spool– Needle
These valves are used to allow or block airflow in circuit lines
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Directional Control Methods and Devices
Check valves automatically allow the free flow of air in one direction and block airflow in the opposite direction
They are commonly used to block or allow air to flow around control valves during different phases of circuit operation
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Directional Control Methods and Devices
Check valve allows unrestricted retraction
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Directional Control Methods and Devices
Spring-assisted check valve
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Directional Control Methods and Devices
Three-way directional control valves:– Pressurize a single-acting cylinder during extension– Vent air during retraction to allow the cylinder to be
returned to the initial position by external forces
Permission granted to reproduce for educational use only.45© Goodheart-Willcox Co., Inc.
Directional Control Methods and Devices
Operation of a three-way valve
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Directional Control Methods and Devices
Four-way directional control valves direct compressed air to power a cylinder during both extension and retraction
They also power air motors during both directions of rotation
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Directional Control Methods and Devices
Four-way directional control valves are available in four-port and five-port configurations– A common exhaust is used in the four-port design– The five-port design uses separate exhaust ports for
the extension and retraction positions of the valve
Permission granted to reproduce for educational use only.48© Goodheart-Willcox Co., Inc.
Directional Control Methods and Devices
Four- and five-port, four-way valves
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Directional Control Methods and Devices
In most directional control valves, resilient materials are used to seal the space between the bore and the spool– In packed-spool designs, sealing materials are
attached to the valve spool lands– In packed-bore designs, sealing materials are
attached to the bore in the valve body
Permission granted to reproduce for educational use only.50© Goodheart-Willcox Co., Inc.
Directional Control Methods and Devices
Packed-spool directional control valve
IMI Norgren, Inc.
Permission granted to reproduce for educational use only.51© Goodheart-Willcox Co., Inc.
Directional Control Methods and Devices
Packed-bore directional control valve
Permission granted to reproduce for educational use only.52© Goodheart-Willcox Co., Inc.
Directional Control Methods and Devices
Four-way directional control valves are normally available providing two or three positions of operation– Two-position valves power an actuator in one
direction in the first position and in the opposite direction in the second position
– Three-position valves provide additional operating options using the middle valve position
Permission granted to reproduce for educational use only.53© Goodheart-Willcox Co., Inc.
Directional Control Methods and Devices
Center position provides additional options
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Directional Control Methods and Devices
Three center position configurations are commonly available for pneumatic three-position, five-port valves– Blocked center– Open center– Pressure center
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Directional Control Methods and Devices
Blocked-center configuration
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Directional Control Methods and Devices
Open-center configuration
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Directional Control Methods and Devices
Pressure-center configuration
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Flow Control Methodsand Devices
The most common methods of controlling pressurized airflow in pneumatic circuits are:– Fixed-size orifices– Needle valves
Permission granted to reproduce for educational use only.59© Goodheart-Willcox Co., Inc.
Flow Control Methodsand Devices
Needle valve with integral check valve
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Flow Control Methodsand Devices
Preferred method of flow control in pneumatic circuits is the meter-out design– Involves the placement of the flow control valve in
the outlet line of the actuator– Favored due to the compressibility of air
Permission granted to reproduce for educational use only.61© Goodheart-Willcox Co., Inc.
Flow Control Methodsand Devices
Meter-out and meter-in circuits
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Flow Control Methodsand Devices
Meter-out circuits with four- and five-port valves
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Special-Purpose Control Valves and
Other Devices Quick exhaust valves allow exhaust air from a
cylinder to be vented directly to the atmosphere without passing through the directional control valve
This reduces backpressure in the exhaust line, increasing the efficiency of circuit operation
Permission granted to reproduce for educational use only.64© Goodheart-Willcox Co., Inc.
Special-Purpose Control Valves and
Other Devices Circuit with a quick-exhaust valve
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Special-Purpose Control Valves and
Other Devices Structure of a quick-exhaust valve
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Special-Purpose Control Valves and
Other Devices Shuttle valves automatically select the higher
pressure of two sources Allows a circuit to always be connected to the
highest pressure
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Special-Purpose Control Valves and
Other Devices Structure of a shuttle valve
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Special-Purpose Control Valves and
Other Devices Timing-volume reservoirs are used to time the
shifting of pilot-controlled pneumatic valves The rate of airflow and the volume of the
reservoir determine the time it takes to shift the piloted valve
Permission granted to reproduce for educational use only.69© Goodheart-Willcox Co., Inc.
Special-Purpose Control Valves and
Other Devices Circuit with timing-
volume reservoir
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Special-Purpose Control Valves and
Other Devices Exhaust mufflers:
– Reduce the level of air exhaust noise– Prevent the entrance of dirt and liquids into the
exhaust ports of directional control valves and actuators
Permission granted to reproduce for educational use only.71© Goodheart-Willcox Co., Inc.
Special-Purpose Control Valves and
Other Devices Typical exhaust mufflers
(Norgren, Inc.)
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Selecting and Sizing Pneumatic Valves
A great deal of technical information is available about pneumatic components from manufacturer catalogs– Pressure ranges– Flow rates– Physical dimensions– Graphs illustrating performance under various
conditions
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Selecting and Sizing Pneumatic Valves
Physical dimensions
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Selecting and Sizing Pneumatic Valves
Graphs illustrate performance
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Selecting and Sizing Pneumatic Valves
Several approaches are available to determine the size of the control valves needed in a specific pneumatic circuit
Procedures, formulas, and online calculators are available from manufacturers that can be used to select an appropriate valve
Permission granted to reproduce for educational use only.76© Goodheart-Willcox Co., Inc.
Selecting and Sizing Pneumatic Valves
Typical data required for valve selection:– Cylinder piston area– Cylinder stroke– System compression– System pressure drop– Cylinder stroke time
Permission granted to reproduce for educational use only.77© Goodheart-Willcox Co., Inc.
Review Question
List the three factors control valves regulate to produce a desired performance in a pneumatic circuit.
1) Air pressure2) Direction of airflow3) Rate of airflow
Permission granted to reproduce for educational use only.78© Goodheart-Willcox Co., Inc.
Review Question
What is the most common method used to control the maximum operating pressure in the distribution line of a pneumatic system?
A compressor-capacity control system.
Permission granted to reproduce for educational use only.79© Goodheart-Willcox Co., Inc.
Review Question
List five designs of air pressure regulators.
Direct operated; basic, diaphragm chamber; relieving; balanced poppet; and pilot operated.
Permission granted to reproduce for educational use only.80© Goodheart-Willcox Co., Inc.
Review Question
Name the two basic functions of the center position of a three-position, four-way, directional control valve.
1) Stop actuator movement 2) Provide additional operating characteristics.
Permission granted to reproduce for educational use only.81© Goodheart-Willcox Co., Inc.
Review Question
List five primary types of information available about pneumatic components found in manufacturer catalogs.
1) General valve description2) Pressure, flow, and temperature operating ranges3) Drawings providing dimensions and fitting sizes4) Schematic symbols for valve variations5) Graphs showing performance at various pressures
and flows.
Permission granted to reproduce for educational use only.82© Goodheart-Willcox Co., Inc.
Review Question
List four sources of internal and external force that are used to operate the control elements in pneumatic control valves.
1) Springs2) Air pressure3) Manual force4) Electromagnets.
Permission granted to reproduce for educational use only.83© Goodheart-Willcox Co., Inc.
Review Question
The two most common basic flow control devices used in a pneumatic system are fixed-sized orifices and _____ valves.
needle
Permission granted to reproduce for educational use only.84© Goodheart-Willcox Co., Inc.
Review Question
The _____ flow-control circuit is the preferred method to use for controlling the operating speed of cylinders in pneumatic circuits.
meter-out
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