By Josh Cosford
Most people don’t consider that air is also a fluid and rests within the boundaries of fluid power motion control. Hydraulics and pneumatics are similar in principle—merely different in execution. Hydraulic actuators have high power density; that is, they produce a lot of force from a small package. However, hydraulic fluid is hard to move around, and tends to create a lot of friction as it does so. It is also nearly incompressible, so it maintains its energy density fairly consistently.
Pneumatics takes advantages of the low friction, compressible nature of air. A lot of air can be moved rapidly, and although compressibility needs to be factored in, it can provide an advantage with unmatched response and cycle times. Most pneumatic applications are low inertia, so they tend to accelerate and decelerate quickly.
Nearly every hydraulic application can be equaled with pneumatics, only with less force output. Many mobile applications can take advantage of pneumatics, because of the availability of compressed air on heavy-duty trucks. Air motors are used for both vibrators and liquid pump drives for salter/spreader trucks, for example. Cylinders can be used for paint nozzle positioning on line painting vehicles, or also for tarp control on dump trucks.
The valving used to control pneumatics differs from hydraulics in that no “return-to-tank” plumbing is required—instead air is merely exhausted to atmosphere. Air still requires control over speed with needle valves, flow controls or quick exhaust valves. Pressure is controlled with regulators, and often sub-circuits run at lower pressure than the primary air supply.
It is important to consider the conditioning of air, just as with hydraulic fluid. The trusty FRL (filter, regulator and lubricator) is a piece of standard equipment for any reliable pneumatic machine. Filters are required to keep particle contamination from wearing valves and actuators, and most filters come with water-removing filter bowls to keep air dry, which is critical in preventing internal corrosion.
The regulator, as discussed above, controls the pressure downstream of the FRL, to both protect actuators and limit force and velocity. The speed of pneumatic actuators is more affected by pressure than hydraulic applications are, as small changes in pressure result in higher potential to flow the fluid.
Lubricators are required to complete the trio to introduce oil into the pneumatic circuit, reducing friction, corrosion and lengthening the life span of seals. Although some actuators are designated as oil-free, most mobile pneumatic applications benefit from lubrication, especially because of the demanding conditions. Filters, regulators and lubricators are often modular, available in separate pieces, allowing you to combine only the functions required, such as if there is a primary lubricator and only a filter/regulator is required.