Chapter 7 Pneumatic and Hydraulic Systems. Pneumatic Systems

Transcription

1 Chapter 7 Pneumatic and Hydraulic Systems 1 Pneumatic Systems Pneumatic systems are designed to move loads by controlling pressurized air in distribution lines and pistons with mechanical or electronic valves. Air under pressure possesses energy which can be released to do useful work. Examples of pneumatic systems: dentist s drill, pneumatic road drill, automated production systems.

2 Components of a Pneumatic System 2 compressor motor reservoir distribution lines air treatment cylinders and valves Compressor is the power source of a pneumatic system. It is usually driven by a motor or an internal combustion engine. The compressed air is first stored in a strong metal tank called reservoir. Before entering the cylinders and valves, the compressed air has to pass through the air treatment devices, including air filter to remove dust and moisture, pressure regulator to adjust pressure, and lubricator to spray lubrication oil.

3 Air Filter -- to remove dust and moisture 3 air filter condensed water water release valve

4 Pressure Regulator -- to adjust pressure adjust screw ventilation hole spring diaphragm 4 valve

5 Lubricator -- to spray lubrication oil 5 siphon tube lubrication oil

6 Pneumatic Actuator -- Cylinder 6 Cylinder is the actuator in the pneumatic system. When compressed air flows into a cylinder, energy stored in the air will release, transferring into kinetic energy to do work. compressed air exhaust

7 Example 1. Calculating the force produced by a cylinder 7 The input air pressure is 0.5 MPa, which means the air would exert a force of 0.5N on each square millimeters. If the area of the piston is 300mm 2, then the total force produced by the cylinder will be: force = pressure piston area = 0.5 N/mm 2 300mm 2 = 150 N mm compressed 壓縮氣體 air 0.5MPa

8 Pneumatic Motor Piston Type and Vane Type 8 piston rod output shaft piston inlet outlet

9 Example 2. Pneumatic Drill 9 control lever air inlet air duct disk valve piston air flow air outlet anvil air flow spring blade (a) (b)

10 Pressure Control Valve Relieve Valve 10 Relief valve, also known as safety valve, is used to maintain the desired pressure. adjust screw spring ball valve

11 Pressure Control Valve Reduce Valve adjust screw 11 spring diaphragm 減壓閥表示符號

12 Directional Control Valve Two Port Valve (2/2) 12 IN IN OUT OUT Directional control valves are commonly described by an x/y designation, where x is the number of ports and y is the number of positions. 2/2 valve: 2 ports, 2 positions. The two port valve is similar to the single pole single throw switch in electric circuits.

13 Directional Control Valve Three Port Valve (3/2) 13 The three port valve is similar to the single pole double throw switch in electric circuits. 3 出氣口 exhaust 氣壓源 1 air supply spring 彈簧 1

14 Example 3. Pneumatic Punching Machine (I) 14 single 氣壓缸 acting cylinder 方向控制閥 three port valve (3/2) 沖壓模 punching mold

15 Example 3. Pneumatic Punching Machine (II) (a) (b)

16 Directional Control Valve One Way Valve 16 The one way valve allows air flow from only one direction. It is similar to the diode in electric circuits. ball 球閥 valve 止回閥表示符號

17 Directional Control Valve Shuttle Valve 17 A shuttle valve has three ports and contains a small rubber piston which is free to move between port 1A and 1B within the valve. If air enters the valve through port 1 A or 1B, the piston is pushed to the other side and air can only escape through port 2. 2 閥 valve 2 1A 1B 1A 1B 梭動閥氣壓表示符號

18 Example 4: Dual Control Pneumatic Punching Machine 18 A D a 3 1 B 2 C b 3 1 2

19 Flow Control Valve the Flow Regulator Air can pass through the regulator in either direction. If air enters from left, the ball valve is pushed open and air can flow through the valve unrestricted. If air enters from right, the ball valve is closed so that air can only pass through the regulator. The flow of air can be controlled by turning a finger screw. IN ball 球閥 valve spring 彈簧 finger 調整螺栓 screw needle 針形閥 valve OUT 19

20 Control of Double Acting Cylinders (I) 20 five 五口二位置方向閥 port valve (5/2) 流量控制閥 flow control valve

21 Control of Double Acting Cylinders (II) Unlike a single acting cylinder, a double acting cylinder does not contain a return spring. Movements in both directions are powered by compressed air. The flow control valve makes the downward movement of piston 2 slower than that of piston 1. However, both pistons move upward at the same speed

22 Air Operated Valves 22 In the valves described so far, the spool which controls the flow of air is moved mechanically, by a button or lever. In order to be automated, direction control valves in the pneumatic systems have to be controlled by air pressure or electrical signals. In air operated valves, the spool is moved by air pressure

23 Example 5: Application of Air Operated Valves 安全區域 safe region

24 Pneumatic Solenoid Valves 24 The spool position is moved by an electrical solenoid, and can controlled electronically. +V +V electrical solenoid 電磁線圈 spool 閥 (a) (b)

25 Hydraulics 25 The working fluid in a hydraulic system is incompressible. Thus a hydraulic system can move large loads. 50N 400N Pascal s Law 100mm 2 800mm 2 A B

26 Hydraulic Systems 26 Pneumatic systems are open systems, always processing new air, and air is simply exhausted to the atmosphere. Hydraulic systems are closed systems, always recirculating the same oil. cylinder oil pump motor hydraulic valves oil reservoir

27 Example 6. Hydraulic Jack 27 Only a small force is required by the operator to raise the heavy load. The large piston can be stopped at any point because the oil cannot be compressed. handle small piston large piston one-way valve valve release screw

28 Hydraulic Actuators 28 In a hydraulic system, the actuators transferring hydraulic energy into mechanical motion are hydraulic cylinders and hydraulic motors. There are 3 types of hydraulic motors : gear pump, vane pump and axial piston pump. low pressure oil high pressure oil

29 Example 7. Hydraulic Brakes 29 brake fluid disc master cylinder brake pedal brake pads brake fluid piston disc caliper wheel cylinder

30 Example 8. Hydraulic Control loop 30 filter control valve cylinder oil reservoir relief valve pump

31 Comparison between Pneumatic and Hydraulic Systems 31 Advantages Pneumatic System Air is easily available Fast response Air is non-flammable Continuous variable transmission Hydraulic System High output force Accurate hydraulic pressure No corrosion Continuous variable transmission

32 Comparison between Pneumatic and Hydraulic Systems 32 Disadvantages Pneumatic System Output force is limited Compressibility of air Corrosion may occur Pipe length is limited Hydraulic System Fluid might leak out Fluid will degrade due to heat Fluid flow speed is limited Pipes are complicated Working fluid is often flammable. Electrical Linear Actuator