There is a wide range of available hydrostatic pumps and motors from the market and the purpose of this chapter is to describe the operating principles and features of the most commonly used types. The formulae that are used for determining the performance of pumps and motors are presented and some of the major param­eters that can be used as a basis for comparison are outlined as a background for the selection process. However, because of the wide variety of the types of units that are available it is impossible to generalise on the selection process in any given application.

Commonly machine builders and users have preferences for particular types of pumps and motors that are based on experience with particular applications which are determined by factors such as the system function, its control, servic­ing aspects, environmental features, life expectancy, duty cycle and type of fluid to be used. The designer needs to be aware of the relative performance of the different types and how this knowledge can be utilised in the selection process to suit a particular application.