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Pneumatics / hydraulics
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Virtual Machine Shop Forum  |  Machine Tool Technology  |  Questions, answers, ideas  |  Topic: Pneumatics / hydraulics « previous next »
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Author Topic: Pneumatics / hydraulics  (Read 186 times)
Geo.
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« on: November 29, 2016, 11:54:54 AM »

Comparison to hydraulics
Both pneumatics and hydraulics are applications of fluid power. Pneumatics uses an easily compressible gas such as air or a suitable pure gas—while hydraulics uses relatively incompressible liquid media such as oil. Most industrial pneumatic applications use pressures of about 80 to 100 pounds per square inch (550 to 690 kpa) Hydraulics applications commonly use from 1,000 to 5,000 psi (6.9 to 34.5 MPa), but specialized applications may exceed 10,000 psi (69 MPa).

Advantages of pneumatics
•   Simplicity of design and control—Machines are easily designed using standard cylinders and other components, and operate via simple on-off control.
•   Reliability—Pneumatic systems generally have long operating lives and require little maintenance. Because gas is compressible, equipment is less subject to shock damage. Gas absorbs excessive force, whereas fluid in hydraulics directly transfers force. Compressed gas can be stored, so machines still run for a while if electrical power is lost.
•   Safety — There is a very low chance of fire compared to hydraulic oil. Newer machines are usually over load safe.
Advantages of hydraulics
•   Liquid does not absorb any of the supplied energy.
•   Capable of moving much higher loads and providing much higher forces due to the incompressibility.
•   The hydraulic working fluid is basically incompressible, leading to a minimum of spring action. When hydraulic fluid flow is stopped, the slightest motion of the load releases the pressure on the load; there is no need to "bleed off" pressurized air to release the pressure on the load.
•   Highly responsive compared to pneumatics.
•   Supply more power than pneumatics.
•   Can also do many purposes at one time: lubrication, cooling and power transmission.

Pneumatic logic systems (sometimes called air logic control) are sometimes used for controlling industrial processes, consisting of primary logic units like:
•   And Units
•   Or Units
•   'Relay or Booster' Units
•   Latching Units
•   'Timer' Units
•   Sorteberg relay
•   Fluidics amplifiers with no moving parts other than the air itself
Pneumatic logic is a reliable and functional control method for industrial processes. In recent years, these systems have largely been replaced by electronic control systems in new installations because of the smaller size, lower cost, greater precision, and more powerful features of digital controls. Pneumatic devices are still used where upgrade cost, or safety factors dominate.

Components
The circuit comprises the following components:
•   Active components
o   Compressor
•   Transmission lines
o   Air tank
o   Pneumatic hoses
o   Open atmosphere (for returning the spent gas to the compressor)
o   Valves
•   Passive components
o   Pneumatic cylinders
o   Service Unit
•   FRL - Filter Regulator and Lubricator
Pneumatic cylinder
In general, based on the application, a pneumatic single acting cylinder, where there is a single port in the cylinder and were cylinder extension is done by compressed air and retraction by means of open coiled spring. In double acting cylinders two ports both extend and retract by means of compressed air.
 
Single Acting Cylinder
 
Double Acting Cylinder
Direction control valve (DCV)
The direction control valve is used to control the direction of flow of compressed air. Usually classified into normally open (NO)and normally closed (NC)valves. The normally open valves will permit flow from inlet port of valve to outlet port normally the flow will be cut by changing the position of the valve. The normally closed valves will not permit flow from inlet port of valve to outlet port normally the flow will be permitted only by changing the position of the valve. In general valves are designated as 2/2 DCV, 3/2DCV, 5/2 DCV,5/3 DCV etc. In which the first numerical indicates number of ports and second numerical indicates number of positions. To change the position, the valves are generally actuated by:
•   Pedal Operated
•   Push button operated
•   Spring operated
•   Solenoid operated
•   By using Pneumatic source itself etc.
The other auxiliary valves are
Two pressure valve (And Valve)
Generally two valve actuators (push buttons) are used when both the push buttons are pressed at a time the air flow takes place if either any one is pressed at a time air flow will not take place in valve outlet. Generally used in mechanical press and machine tools to ensure operator's both the hands are outside the machine or press during operation.
OR Valve
Generally two valve actuators (push buttons) are used when either one push button is pressed the air flow takes place. This is also called as shuttle valve.
Check valve
The check valve allows air flow in one direction, it is also called as non return valve.
Quick exhaust valve
The valve construction is OR valve with exhaust port,ensures quick return of cylinder therefore cycle time reduces
Flow control valve
The combination throttle valve connected to check valve is called one way flow control valve, while air passes from one direction to other the check valve will not allow the air flow (As the check valve allows flow only in one direction) while through the restricted way of throttle compressed air flow takes place. While the air comes out from other way both the ways of throttle as well as the check valve opens to pass the compressed air therefore the piston moment in one direction can be controlled.
Time delay valve
The combination of 3/2 direction control valve, reservoir and flow control valve is time delay valve. This valve is used to delay the actuation of cylinder after pressing the push button or pedal etc.
Pressure relief valve
The pressure relief valve is used to maintain the system set pressure, in case if the system set pressure increases the pressure relief valve gets opens and exhaust the compressed air to atmosphere

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