Fewer topics in the fluid power realm garner more conversation than that of the differences between hydraulics and pneumatics. The two types of fluid power motivation are very similar, both in the use of a fluid to transmit mechanical energy and the executions using similar terminology, symbols and components.
Both hydraulic and pneumatic systems require a pump, although compressed air is first stored in receivers/tanks before being transmitted for use. Both systems use valves to control the force and velocity of the actuators, which are also similar to each motive. The real difference between hydraulics and pneumatics is the medium itself.
Contrary to popular belief, liquids are compressible. For hydraulic oil, a rule of thumb is that it will compress 0.5% for every 1000 psi that is exerted. However, for most applications this side of a flight simulator, we can consider it incompressible. If you push a large quantity of oil into a fixed volume, pressure will rise until something gives way, and this will happen very quickly.
The advantage of non-elastic oil being used to transmit mechanical energy is that transient behavior is excellent, and little energy is lost to compression of the fluid. This means as you pull a valve lever, the actuator starts moving immediately with little concern changes in fluid compression. You can consider a column of oil to move energy just like pushing a solid rod.
However, the downside to non-elastic oil is how difficult it is to move the fluid through valves, components and plumbing. Restrictions to flow of a liquid result if much higher resistance and energy loss than compared with pneumatics. The restrictive nature of oil is easily overcome by properly sized components matched to handle the flow of the system.
Another advantage of the high bulk modulus—the resistance to change in volume—of oil is its advantage over pneumatics is power density. Running a hydraulic system to 10,000 psi has little detrimental effect on performance compared to running compressed air to even 300 psi. Air compressors are limited to the amount they can compress air, and even with multi-stage compression, a lot of energy can be lost to heat.
Regardless, compressed air can do things that oil simply cannot. The compressibility of air can actually be used as an advantage, because when compressed air energy is released, it expands with vigor. And because the volume of air able to be moved though pneumatic valves and actuators is so high, especially when pressure drop is intelligently considered, the velocity of pneumatic actuators can be lightning fast.
When properly applied, pneumatically actuated machines are snappy and quick. Pneumatic applications are great for automated processes in the production of relatively light manufacturing and assembly—I say relatively because pneumatic applications can still produce thousands of pounds of force, whereas hydraulic applications can produce thousands of tons.
Additionally, the mathematics in applying hydraulics and pneumatics are someone different. Hydraulics are not concerned with compression ratios, and pneumatics are not concerned with pressure compensation. But for the most part, the difference between hydraulics and pneumatics is the fluid itself. However, you may get different answers if you ask a hydraulic specialist about pneumatics, or if you ask a pneumatic specialist about hydraulics. Feel free to tell me what you think about the differences between the two.
Josh Cosford, Certified Fluid Power Hydraulic Specialist
Scott Industrial Systems