There are two different kinds of hydraulic pump types that are used in hydraulic drive systems; they are hydrostatic and hydrodynamic. The difference is that hydrostatic pumps are positive displacement pumps while hydrodynamic pumps are typically fixed of variable displacement pumps. Displacement simply is referring to the flow through the rotation of the pump. Fixed displacement pumps are when the displacement cannot be adjusted, while the variable displacement pumps allows the displacement to be adjusted since it has a more complicated construction.
A simpler and more economical of the hydraulic pump types is the gear pump, which has external teeth and is of the fixed displacement types. The displacement or volume of these gear pumps will be between 1 and 200 cubic centimeters. Some of these can be noisy compared to the other types, but the newer, more modern ones have gotten considerably quieter and of a higher quality than the older ones. These pumps force the fluids around the gears in order to pressurize the outlet side by meshing the gear teeth together.
Some hydraulics can be fixed or simple adjustable displacement, such as the rotary vane pumps. They are more efficient than gear pumps, but can be used for mid pressures up to 180 bars as well. A simple adjustable pump can be attained by changing the center of the vane body. These types of pumps are constant pressure pumps. That means that the displacement is either increased or decreased until the equilibrium is reached.
Screw pumps are fixed displacement hydraulic pump types, are closed, and they are in a double Archimedes spiral. This simply means that one body uses two screws. These kinds of pumps are used for relatively low pressures of up to 100 bars, and for high flows. These pumps were commonly used on board of ships where there was a constant pressure system throughout the ship. They would probably be used for the control of ball valves, for the steering gear, and also for the help drive systems. These pumps are not very efficient, but they have an advantage of having a low sound level.
Bent axis pumps and axial piston pumps and motors can be either fixed or adjustable displacement, but they exist in two different designs. These types are the most efficient of all of them. Most of the larger displacements are 1 liter per revolution, but if necessary, you can build a 2 liter. The variable displacement are used often in order to carefully adjust the oil flow. Most of these can work with a working pressure of anywhere from 350 to 420 bars.