By Josh Cosford, Contributing Editor
“Fluid power” is the all-encompassing term to describe how we achieve work through pressurized matter pushed through enclosed circuits. In short, fluid power is hydraulics or pneumatics, sometimes simultaneously (hydropneumatic accumulators, for example). Inherently, we know they’re different because one uses oil and the other air, but what else is different between them?
As far as hydraulic and pneumatic actuators are concerned, there is little difference between the force applied to the piston of a cylinder or the gears/vanes/pistons of a motor. With a cylinder, for example, so long as the opposing end may flow oil or air with little restriction, applying pressurized oil or gas creates a force which extends the cylinder.
With a hydraulic cylinder, however, should the pump stall mid-stroke, the cylinder will also stop moving. This is because hydraulic actuators require continuous flow from the pump (excluding accumulators). You must think of hydraulics as similar to pushing a column of fluid like it is solid. When you stop pushing on that column, you stop pushing on the cylinder.
Pneumatic cylinders, on the other hand, are all about pressure differential. So long as there is a pressure difference between the piston end of a cylinder and its rod end, it will stroke. Pneumatic actuators are much more sensitive to flow-related back pressure, which is why pressure differential is commonly used to describe the force potential of air cylinders. Also, in general, pneumatics are much more dynamic and less “hydrostatic.” Air-powered actuators rely on compressed air, and returning that air to the atmosphere (through work) provides the force potential required to do work.
Once a cylinder is in action for either method, how they behave varies widely. Air quite easily compresses, while hydraulic oil resists compression with vigor. It takes only 14.7 psi to compress an air volume to half its size (remember that atmospheric air is already at 14.7 psi at sea level). However, hydraulic oil may take 150,000 psi to reduce its volume to half.
Should you take an air cylinder with plugged ports and load mass upon its upward-facing rod, the volume in the cap side will compress as you increase that mass. Should you double the mass, the volume will decrease by half. However, minimal compression will occur if you take the same cylinder filled with hydraulic oil and load mass upon the rod end. In fact, should you continue to load mass upon the hydraulic cylinder, it will only compress around 0.4% for every 1,000 psi of load-induced pressure. Even while you increase the mass on the hydraulic cylinder, it will move very little.
In the above example, suddenly removing the mass will cause two very different effects on the unloaded cylinders. First, the hydraulic cylinder will appear to move very little, like it was just a solid rod propping up an object. The pneumatic cylinder will spring up as compressed air expands to stroke the unloaded rod rapidly. Depending on the bore, rod size and loaded pressure, it will oscillate for a short time before coming to a rest.
Air just as quickly expands as it does compresses, which on a pneumatic cylinder could cause linear oscillations. So pronounced is the effect that sometimes hydraulic oil is combined with air to stabilize the actuator. Air-over-hydraulic systems employ compressed air inputs to charge oil-controlled circuits. The air applies the force upon the oil, which in turn constitutes the control and force medium. You essentially create a very low-pressure hydraulic circuit requiring no power unit yet having the controllability of hydraulics.
Air may continue to power hydraulics, even at high pressure. The hydropneumatic accumulator uses high-pressure nitrogen charged on one end of the accumulator, which is separated from the hydraulic fluid with a piston, bladder, or diaphragm. For example, a 3,000 psi hydraulic system may be pre-charged with nitrogen to around 1,800 psi, providing it with about 1,200 psi range of output differential with which to work. As the pressure drops below maximum, the accumulator sends extra fluid into the circuit for supplemental flow, leakage control, or backup energy.
Filed Under: Engineering Basics, Fluid Power Basics