The principles of pneumatics are the same as those for hydraulics, but pneumatics transmits power using a gas instead of a liquid. Compressed air is usually used, but nitrogen or other inert gases can be used for special applications. With pneumatics, air is usually pumped into a receiver using a compressor.

The receiver holds a large volume of compressed air to be used by the pneumatic system as needed. Atmospheric air contains airborne dirt, water vapor, and other contaminants, so filters and air dryers are often used in pneumatic systems to keep compressed air clean and dry, which improve reliability and service life of the components and system. Pneumatic systems also use a variety of valves for controlling direction, pressure, and speed of actuators.

Most pneumatic systems operate at pressures of about 100 psi or less. Because of the lower pressure, cylinders and other actuators must be sized larger than their hydraulic counterparts to apply an equivalent force. For example, a hydraulic cylinder with a 2 in. diameter piston (3.14 sq. in. area) and fluid pressure of 1,000 psi can push with 3140 lbs. of force. A pneumatic cylinder using 100 psi air would need a bore of almost 6½ in. (33 sq. in.) to develop the same force.

Even though pneumatic systems usually operate at much lower pressure than hydraulic systems do, pneumatics holds many advantages that make it more suitable for many applications. Because pneumatic pressures are lower, components can be made of thinner and lighter weight materials, such as aluminum and engineered plastics, whereas hydraulic components are generally made of steel and ductile or cast iron.

Hydraulic systems are often considered rigid, whereas pneumatic systems usually offer some cushioning, or “give.” Pneumatic systems are generally simpler because air can be exhausted to the atmosphere, whereas hydraulic fluid usually is routed back to a fluid reservoir.

Pneumatics also holds advantages over electromechanical power transmission methods. Electric motors are often limited by heat generation. Heat generation is usually not a concern with pneumatic motors because the stream of compressed air running through them carries heat from them. Furthermore, because pneumatic components require no electricity, they don’t need the bulky, heavy, and expensive explosion-proof enclosures required by electric motors.

In fact, even without special enclosures, electric motors are substantially larger and heavier than pneumatic motors of equivalent power rating. Plus, if overloaded, pneumatic motors will simply stall and not use any power. Electric motors, on the other hand, can overheat and burn out if overloaded. Moreover, torque, force, and speed control with pneumatics often requires simple pressure- or flow-control valves, as opposed to more expensive and complex electrical drive controls. And as with hydraulics, pneumatic actuators can instantly reverse direction, whereas electromechanical components often rotate with high momentum, which can delay changes in direction.

Another advantage of pneumatics is that it allows using vacuum for picking up and moving objects. Vacuum can be thought of as negative pressure — by removing air (evacuating) from the volume between two parts, atmospheric pressure outside the volume pushes the parts together. For example, attempting to pick up a single sheet of paper or a raw egg presents a challenge with conventional grippers. But with a vacuum pneumatic system, evacuating a suction cup in contact with a sheet of paper or eggshell will cause atmospheric pressure to push the paper or egg against the cup, allowing it to be lifted.

Applications

Factory automation is the largest sector for pneumatics technology, which is widely used for manipulating products in manufacturing, processing, and packaging operations. Pneumatics is also widely used in medical and food processing equipment. Pneumatics is typically thought of as pick-and-place technology, where pneumatic components work in concert to perform the same repetitive operation thousands of times per day.

But pneumatics is much more. Because compressed air can have a cushioning effect, it is often called on to provide a gentler touch than what hydraulics or electromechanical drives can usually provide. In many applications, pneumatics is used more for its ability to provide controlled pressing or squeezing as it is for fast and repetitive motion. Moreover, electronic controls can give pneumatic systems positioning accuracy comparable to that of hydraulic and electromechanical technologies.

Pneumatics is also widely used in chemical plants and refineries to actuate large valves. It’s used on mobile equipment for transmitting power where hydraulics or electromechanical drives are less practical or not as convenient and in on-highway trucking for various vehicle functions. And of course, vacuum is used for lifting and moving work pieces and products. In fact, combining multiple vacuum cups into a single assembly allows lifting large and heavy objects.

Standard electric motors typically rotate at 1,800 or 3,600 revolutions per minute (rpm) — much faster than is practical for most machines. Gasoline and diesel engines also rotate at thousands of rpm when powering equipment. Therefore, some form of power transmission is needed to convert power from the motor or engine to a more useable form — slower speed and, often, linear motion instead of rotary.

Mechanical power transmission methods include gear, chain, belt, and other mechanical drives that convert the high-speed mechanical power from the engine or motor’s output shaft to a slower speed with higher torque (twisting force). Mechanical power transmission components also include ball screws, rack-and-pinion assemblies, chain drives, and other components that convert rotational motion and torque to linear motion and force.

Electrical methods of power transmission regulate electrical power to the motor to control speed and torque. But these methods cannot convert the rotary motion of a motor to linear. When linear output is needed, a linear motor may be used, but, its high cost generally makes a mechanical rotary-to-linear motion device more practical for producing linear motion and force.

In many cases, however, mechanical and electrical methods cannot provide a practical power transmission solution. In these cases, fluid power — whether hydraulic or pneumatic — is used because it can deliver linear and rotary motion with high force and torque within a smaller, lighter package than is possible with other forms of power transmission.

A cement mixer is an example illustrating how different methods of power transmission can be used. Early cement mixers used mechanical drives driven either by the truck’s engine or transmission. A system of gears, chain drives, and drive shafts provided the speed and torque necessary to rotate the heavy drum of concrete, but speed was difficult to control. The rotational speed of the drum depended on the engine speed or transmission speed. As the driver shifted gears, the drum would speed up or slow down, and rarely rotated at the ideal speed. Plus, the complexity and bulk of all the mechanical components were highly maintenance intensive.

An electrical drive could provide good speed control but would require a high-power electric generator, controls, and a motor to drive the drum. Plus, the motor would either be prohibitively large or would require a large gearbox to achieve the low-speed rotation of the mixer drum. Either solution would be much larger and heavier than a hydraulic drive.

Hydraulic drives are the primary choice for cement mixer drives. They use a pump, hydraulic motor, and valves to precisely control speed regardless of the engine or transmission speed. The drum rotates at optimum speed or can be manually controlled. Plus, the components are relatively compact: the pump is tucked away within the framework of the truck, and the hydraulic motor is only a small fraction of the size of a comparable electric motor-gearbox combination.