ABSTRACT
A fluid is defined as a substance that cannot sustain a shearing stress. A fluid can be liquid or gaseous. The science of fluid power is concerned with the utilization of pressurized liquid or gas to transmit power, but we will be dealing exclusively with hydraulic fluids (i.e., liquids). Many textbooks have units of measurement in the U.S. Customary system based upon the former British (or Imperial) system. The use of the more recently defined S.I. system is becoming more common in U.S. industry and for this reason practicing engineers will have to be familiar with both U.S. basic and S.I. systems of units (see Table 2.1). In many fluid power applications the inch is used as the basic unit of length. Later in this section the use of these units will be demonstrated (see Table 2.2) and some of the special problems that may occur with the use of mass in the U.S. Customary form of units will be discussed. Knowledge of the individual characteristics of hydraulic fluids is essential and this section deals with their fundamental properties.
