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Chapter 18

Chapter 18. Controlling a Pneumatic System. Pressure, Direction, and Flow. 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.

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Chapter 18

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  1. Chapter 18 Controlling a Pneumatic System Pressure, Direction, and Flow

  2. 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.

  3. 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.

  4. 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.

  5. 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 Permission granted to reproduce for educational use only.

  6. Primary Control Functionsin a Pneumatic Circuit • Pressure control valves: • Limit maximum system pressure • Regulate the force or torque an actuator generates Permission granted to reproduce for educational use only.

  7. 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 Permission granted to reproduce for educational use only.

  8. 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 Permission granted to reproduce for educational use only.

  9. Basic Control ValveDesign and Structure • Cast valve body Permission granted to reproduce for educational use only.

  10. Basic Control ValveDesign and Structure • Bar stock valve body Permission granted to reproduce for educational use only.

  11. 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 Permission granted to reproduce for educational use only.

  12. Basic Control ValveDesign and Structure • Control elements include: • Fixed orifices • Spools • Pistons • Diaphragms • Precision-machined sliding plates • Needle and poppet valves Permission granted to reproduce for educational use only.

  13. Basic Control ValveDesign and Structure • Fixed orifice Permission granted to reproduce for educational use only.

  14. Basic Control ValveDesign and Structure • Needle valve Permission granted to reproduce for educational use only.

  15. Basic Control ValveDesign and Structure • Spool Permission granted to reproduce for educational use only.

  16. Basic Control ValveDesign and Structure • Diaphragm Permission granted to reproduce for educational use only.

  17. Basic Control ValveDesign and Structure • Precision-machined sliding plate Rotary Directional Control Valve Permission granted to reproduce for educational use only.

  18. Basic Control ValveDesign and Structure • Varying the position of control elements within a control valve determines: • Setting of the valve • Accuracy of system control Permission granted to reproduce for educational use only.

  19. 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 Permission granted to reproduce for educational use only.

  20. Basic Control ValveDesign and Structure • Internal spring force Permission granted to reproduce for educational use only.

  21. Basic Control ValveDesign and Structure • Electromagnetic force (solenoids) Permission granted to reproduce for educational use only.

  22. Pressure Control Methodsand 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 Permission granted to reproduce for educational use only.

  23. Pressure Control Methodsand Devices • The pressure in the distribution lines of a pneumatic system is controlled by: • Bypass • Compressor-capacity control system Permission granted to reproduce for educational use only.

  24. Pressure Control Methodsand 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.

  25. Pressure Control Methodsand 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) Permission granted to reproduce for educational use only.

  26. Pressure Control Methodsand Devices • Start-stop compressor capacity control Permission granted to reproduce for educational use only.

  27. Pressure Control Methodsand 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.

  28. Pressure regulator of an FRL unit Pressure Control Methodsand Devices IMI Norgren, Inc. Permission granted to reproduce for educational use only.

  29. Pressure Control Methodsand 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.

  30. Pressure Control Methodsand 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.

  31. Pressure Control Methodsand 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.

  32. Additional workstation regulator provides lower pressure Pressure Control Methodsand Devices Permission granted to reproduce for educational use only.

  33. Pressure Control Methodsand Devices • Subcircuit is provided with lower pressure Permission granted to reproduce for educational use only.

  34. Pressure Control Methodsand 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.

  35. Pressure Control Methodsand Devices • Air-to-air booster provides higher pressure Permission granted to reproduce for educational use only.

  36. Pressure Control Methodsand 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.

  37. Pressure Control Methodsand Devices • Safety valve on a small compressor Permission granted to reproduce for educational use only.

  38. 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.

  39. 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 Permission granted to reproduce for educational use only.

  40. 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 Permission granted to reproduce for educational use only.

  41. Directional Control Methods and Devices • Check valve allows unrestricted retraction Permission granted to reproduce for educational use only.

  42. Directional Control Methods and Devices • Spring-assisted check valve Permission granted to reproduce for educational use only.

  43. 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.

  44. Operation of a three-way valve Directional Control Methods and Devices Permission granted to reproduce for educational use only.

  45. Directional Control Methods and Devices • Four-way directional control valvesdirect compressed air to power a cylinder during both extension and retraction • They also power air motors during both directions of rotation Permission granted to reproduce for educational use only.

  46. 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.

  47. Directional Control Methods and Devices • Four- and five-port, four-way valves Permission granted to reproduce for educational use only.

  48. 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.

  49. Directional Control Methods and Devices • Packed-spool directional control valve IMI Norgren, Inc. Permission granted to reproduce for educational use only.

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