In an axial piston pump, the pumping elements are arranged parallel with the pump drive shaft axis. A cylinder block houses the pumping elements, which consist of a piston and barrel bores machined into the cylinder head. The base of each piston rides against a tilted plate known as a swashplate or wobble plate. The swashplate does not rotate but in some instances the tilt angle can be controlled. Fluid is charged to each pump element as the piston is drawn to the bottom of its travel. As the cylinder head rotates— because the piston follows the tilt of the swashplate— it is driven upward through its stroke, forcing fluid out of the chamber through its outlet.In a fixed-displacement version, the angle of the swashplate cannot be altered. If the angle of the swashplate were fixed (it is in some), the pump would be of the fixed-displacement type. When the swashplate angle is movable and controlled by a servo (slave piston) the distance the pistons are able to move back can be either increased or decreased, so the pump would be classified as variable displacement. The farther the pistons travel, the greater the amount of oil that can be displaced per pump cycle.