Fluid Power

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2.5 Pneumatics and Hydraulics

The use of air or liquids to provide motion in actuators is known as fluid power. In addition to electric motors and actuators it is one of the most common method of creating motion in automated processes.

2.5.1 Pneumatics

Pneumatic or compressed air is used to move cylinders or actuators and is even used in some switching logic. Air is typically filtered, dried and regulated to a usable pressure and distributed from a compressor to the various devices and actuators where it is required. A variety of quick disconnect devices and fittings have been designed in metric and standard sizes and are in wide use in industry. Pneumatic air supplies are distributed through most industrial facilities and are readily available for a wide range of uses.

Air pressure is usually applied to an additional filter regulator with a gauge for setting pressure at a machine or system. An additional lubricator is sometimes used to apply a minute amount of oil to lubricate the insides of air cylinders. Filter-Regulators (FR) and Filter-Regulator-Lubricators (FRL) are relatively low cost and are regularly used at the entry point to a pneumatic control system. Additional pressure regulators, flow controls and valves are used to control actuators for the desired effect. Control valves are common in 120VAC and 24VDC varieties.

Air pressure in industrial facilities can be as high as several hundred PSI, but typical pneumatic cylinders usually use 60-80 PSI.

2.5.2 Hydraulics

Hydraulic systems, unlike pneumatic or air systems typically have a pump located at each machine and are therefore more self-contained. Hydraulic pressure is used where more force is required than a pneumatic application. Like pneumatic systems, there are a wide variety of hydraulic cylinders and actuators available from manufacturers for each application. Hydraulic systems are much more costly to implement than pneumatic since the systems usually operate at higher pressure and more care must be taken to ensure that fluids do not leak from the system. Various other components such as oil coolers and intensifiers can also add to the cost of hydraulic systems.

Hydraulic systems do not operate as quickly as pneumatic systems, but due to the lower compressibility of oil can be more precisely controlled. Pressures and force obtained from hydraulic systems are typically much higher than those of pneumatic systems and are therefore often used in metal forming applications.

2.5.3 Pneumatic/Hydraulic Comparison
Advantages of pneumatics:

• Cleanliness
• Simplicity of Design And Control
o Machines are easily designed using standard cylinders & other components. Control is as easy as it is simple ON – OFF type control.

• Reliability
o Pneumatic systems tend to have long operating lives and require very little maintenance.
o Because gas is compressible, the equipment is less likely to be damaged by shock. The gas in pneumatics absorbs excessive force, whereas the fluid of hydraulics directly transfers force.
• Storage
o Compressed Gas can be stored, allowing the use of machines when electrical power is lost.
• Safety
o Very few fire hazards (compared to hydraulic oil).
o Machines can be designed to be overload safe.

Advantages of hydraulics:

• Fluid 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.

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