Hydrodynamic or non-positive-displacement pumps such as centrifugal or turbine designs are used primarily in the transfer of fluids where the only resistance encoun­tered is that created by the weight of the fluid itself and friction.

Most non-positive-displacement pumps (Figure 3-1) operate by centrifugal force where fluids entering the center of the pump housing are thrown to the outside by a rapidly driven impeller. There is no positive seal between the inlet and outlet ports, and pressure capabilities are a function of rotating speed.

Although they provide smooth continuous flow, the output of a hydrodynamic pump is reduced as resistance is increased. It is possible to completely block off or dead­ head the outlet while the pump is running. For this and other reasons, non-positive­ displacement pumps are seldom used in hydraulic systems.


Hydrostatic or positive-displacement pumps, as their name implies, provide a given amount of fluid for every stroke, revolution, or cycle. Except for leakage, their output is independent of outlet pressure or back pressure from the system. This properly makes them well suited for use in the transmission of power.


All rotary pumps have rotating parts, which trap the fluid at the inlet port and force it, through the discharge port, into the system. Gears, screws, lobes, and vanes are com-


monly used to move the fluid within the pump. Rotary pumps are classified as posi­tive, fixed-displacement type.

Rotary pumps are designed with very small clearances between their rotating and sta­tionary parts to minimize slippage from the discharge side back to the suction side of the pump. They are designed to operate at relatively moderate speeds, normally below I ,800 rpm. Operation at higher speeds can cause erosion and excessive wear.

There are numerous types of rotary pumps and various methods of classification. They may be classified by the shaft position, the type of driver, their manufacturer’s name, or their ser­vice application. However, classification of rotary pumps is generally made according to type of rotating element. A few of the more common types include the following.

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