Double Acting A cylinder in which fluid force can be applied in either direction to the moveable element. These cylinders are also classified as differential cylinders because of their unequal exposed areas during extend and retract. The difference in effective area is caused by the area of the rod that reduces the piston area during retraction. Extension is slower than retraction because more fluid is required to fill the piston side of the cylinder. However, more force can be generated on extension because of greater effective area. On retraction, the same amount of pump flow will retract the cylinder faster because of the reduced fluid volume displaced by the rod. Less force, however, can be generated due to less effective area. Single Acting A cylinder in which hydraulic energy can produce thrust or motion in only one direction. Once the pressure is released, the load (can be spring or gravity returned) on the cylinder retracts the rod. The inactive end is sometimes vented to atmosphere through a breather/filter or vented to reservoir below the oil level.