Pneumatics, a subsection of an area called fluid power, is the use of pressurized air to effect mechanical motion.
Pneumatic power is used in industry, where it is common to have industrial factory unit plumbed for compressed air. It also has applications in, among other things, dentistry, construction, and mining. Pneumatic power users need not worry about hazardous leakages as the fuel is commonly just air, although other compressed gases, such as carbon dioxide, may be used.
A pneumatic actuator converts energy (in the form of compressed air, typically) into motion. The motion can be rotary or linear, depending on the type of actuator. Some types of pneumatic actuators include:
- Tie rod cylinders
- Rotary actuators
- Grippers
- Rodless actuators with magnetic linkage
- Rodless actuators with mechanical linkage
- Pneumatic artificial muscles
- Speciality actuators that combine rotary and linear motion–frequently used for clamping operations
- Vacuum generators
A Pneumatic actuator mainly consists of a piston a cylinder and valves or ports. The piston is covered by a diaphram, which keeps the air in the upper portion of the cylinder, allowing air pressure to force the diaphram downard, moving the piston underneath, which in turn moves the valve stem, which is linked to the internal parts of the valve. Pneumatic actuators only have one spot for a signal input, top or bottom, depending on action requried. Valves require little pressure to operate and usually double or triple the input force. The larger the size of the piston, the larger the output pressure can be. Having a larger piston can also be good if air supply is low, allowing the same forces with less input. These pressures are large enough to crush object in the pipe. On 100 kPa input, you could lift a small car (upwards 10,000 lbs) easily, and this is only a basic, small pneumatic valve. However, the resulting forces required of the stem would be too great and cause the valve stem to fail.
This pressure is transferred to the valve stem, which is hooked up to either the valve plug, butterfly valve etc. Larger forces are required in high pressure or high flow pipelines to allow the valve to overcome these forces, and allow it to move the valves moving parts to control the material flowing inside.
Valves input pressure is the “control signal.” This can come from a variety of measuring devices, and each different pressure is a different set point for a valve. A typical standard signal is 20-100 kPa. For example, a valve could be controlling the pressure in a vessel which has a constant out flow, and a varried in flow. A pressure transmitter will monitor the pressure in the vessel and transmit a signal from 20-100 kPa. 20 kPa means there is no pressure, 100 kPa means there is full range pressure (can be varied by the transmiters calibration points). As the pressure rises in the vessel, the output of the transmitter rises, this increase in pressure is sent to the valve, which causes the valve to stroke downard, and start closing the valve, decreasing flow into the vessel, reducing the pressure in the vessel as excess pressure is evacuated through the out flow. This is called a Direct acting process.
Comparison to hydraulics
Both pneumatics and hydraulics are applications of fluid power. Pneumatics uses air, which is compressible, 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 (psi) (500 to 700 kilopascals). Hydraulics applications commonly use from 1,000 to 5,000 psi (7 to 35 MPa), but specialized applications may exceed 10,000 psi (70 MPa).
Advantages of pneumatics
Clean
- Air is used by a machine & is then exhausted to the atmosphere - no return line necessary.
- Any leaks will be of air (which is much less of a problem than oil leaks in Hydraulics).
Availability
- Air is freely available in the pneumatics
- Most factories are pre-plumbed for compressed air distribution - which makes it very easy to set up a manufacturing process
Simplicity of Design And Control
- Machines are easily designed using standard cylinders & other components. Control is as easy as its simple ON - OFF type control
Reliability
- Pneumatic systems tend to have long operating lives and require very little maintenance.
- Because air is compressable, the equipment is less likely to be damaged by shock. The air in pneumatics absorbs excessive force, whereas the fluid of hydraulics directly transfers force.
Storage
- Compressed Air can be stored, allowing the use of machines when electrical power is lost.
Safety
- Very small fire hazard (compared to Hydraulic Oil)
- 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” pressurised air to release the pressure on the load.