Pneumatic cylinders use compressed air to create linear force. Pneumatic pressure acts upon the piston of the cylinder at a level equal to that pressure times the area of the piston.
Pneumatic systems rarely provide much more than a hundred pounds per square inch of pressure to the piston. The nature of compressed air allows for quick acceleration and rapid velocity, especially if backpressure is low.
Hydraulic cylinders are used instead of pneumatic cylinders in for two major reasons. The first reason to choose hydraulic over pneumatic is force. Hydraulic cylinders are capable of anywhere from 1,500 to 10,000 psi, which can be ten to 100 times the force of a pneumatic cylinder. As an example, a 2-in. bore pneumatic cylinder will produce 314 lbs of force at 100 psi, but a 2-in. bore hydraulic cylinder will produce 9,420 lbs at 3,000 psi.
Hydraulic cylinders are also built stronger, to handle their extra force capability. An NFPA hydraulic cylinder will have a larger standard rod diameter, thicker barrel wall, thicker cap and head, and thicker piston, compared to a pneumatic cylinder. This is not to say an NFPA pneumatic cylinder isn’t strong, because they sometimes only differ from a medium duty hydraulic cylinder by employing an aluminum rather than steel barrel, and the medium duty hydraulic cylinder is rated for 1500 psi.
Hydraulic cylinders are used instead of pneumatic cylinders also because of their superior controllability. Because air is compressible, load-induced pressure changes will consistent velocity difficult. Accurate positioning is also difficult due to this compressibility, as the chamber of air in the cylinder can act like a spring.
Hydraulic fluid is nearly incompressible, so it can be accurately metered and the cylinder will move at the rate of fluid flow regardless load induced pressure, as long as the valves controlling the cylinder are sophisticated enough. Accurate positioning within thousandths of an inch is also possible, especially with the use of electronics and linear position transducers. Also, hydraulic cylinders have less internal leakage, and are capable of load holding when the machine is shut down, something very difficult for pneumatics