Hydraulic cylinders are mechanical actuators that get power from pressurized hydraulic fluid. A hydraulic cylinder typically includes a cylinder barrel in which a piston connected to a piston rod moves back and forth. The piston divides the hydraulic cylinder into a first chamber and a second chamber. When the hydraulic pump pushes hydraulic fluid into the first chamber, a valve in the second chamber is open allowing hydraulic fluid to drain from the second chamber into a reservoir as movement of the piston within the hydraulic cylinder increases the volume of the first chamber and correspondingly reduces the volume of the second chamber. Likewise, when the hydraulic pump pushes hydraulic fluid into the second chamber, a valve in the first chamber is open allowing hydraulic fluid to drain from the first chamber into the reservoir as movement of the piston within the hydraulic cylinder increases the volume of the second chamber and correspondingly reduces the volume of the first chamber.
Typically, the hydraulic pump runs at a constant speed to produce hydraulic pressure. If motion is not imminent, the unused pressured hydraulic fluid is returned to the reservoir or stored in an accumulator.