Some of these controlling devices are described later in this chapter. Pumps may also be classified according to the specific design used to create the flow of fluid. Practically all hydraulic pumps fall within three design classifications-centrifugal, rotary, and reciprocating. The use of centrifugal pumps in hydraulics is limited and will not be discussed in this text. Gears, screws, lobes, and vanes are commonly used to move the fluid.
Rotary pumps are positive displacement of the fixed displacement type.
Rotary pumps are designed with very small clearances between rotating parts and stationary parts to minimize slippage from the discharge side back to the suction side. They are designed to operate at relatively moderate speeds. Operating at high speeds causes erosion and excessive wear which results in increased clearances. There are numerous types of rotary pumps and various methods of classification. However, classification of rotary pumps is generally made according to the type of rotating element.