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Hydraulic cylinders are used to convert hydraulic power into linear motion. The amount of force a hydraulic cylinder can exert is a function of the piston area and the hydraulic pressure. This is represented by the formula:
Hydraulic Cylinder Basic CircuitF = P x A where
F = force in Newtons (N)
P = pressure in Pascals (Pa)
A = area in square meters (m2)
A basic circuit to drive a hydraulic cylinder is shown in figure 1.
When a double acting hydraulic cylinder is extending the oil pressure acts on the total piston area, however when the cylinder is retracting the area available is the piston less the rod. Therefore for the same pressure the hydraulic cylinder extending force will always be greater than the retracting force
The most common types of hydraulic cylinder construction are welded with threaded front gland, welded with flange and tie-rod (shown in figure 2). Tie-rod cylinders are mostly found in industrial applications and suitable for lower pressures up to 210 bar. For mobile and high pressure applications up to 350 bar welded cylinders are normally used.
A hydraulic cylinder can be double acting or single acting. With a double acting hydraulic cylinder the hydraulic oil drives the cylinder in both directions. A single acting hydraulic cylinder uses hydraulic oil in one direction and an external force (spring, gravity, leverage etc) to the return the cylinder in the opposite direction.The static components of a hydraulic cylinder are the cap, head and barrel sealed using o-rings or welded. The dynamic or moving parts are the piston and rod which are sealed using a combination of elastomeric seals.