What are the properties of hydraulic fluids

Chapter 2: Physical properties of hydraulic fluid

Figure 2.1. Hydraulic fluid test kit. (Courtesy of Gulf Oil Corp. , Houston, Texas.)

2.2. Fluids: Liquids and Gases

The term fluid refers to both gases and liquids. A liquid is a fluid that, for a given mass, will

have a definite volume independent of the shape of its container. This means that even though a liquid will assume the shape of the container, it will fill only the part of the container whose volume equals the volume of the quantity of the liquid. For example, if water is poured into a vessel and the volume of water is not sufficient to fill the vessel, a free surface will b e formed, as shown in the figure 2.2(a). A free surface is also formed in the case of a body of water, such a lake, exposed to the atmosphere [figure 2.2(b)]. Liquids are considered to be incompressible so that their volume does not change with pressure changes. This is not exactly true, but the change in volume due to pressure change is small that it is ignored for most engineering applications.

Figure 2.2 free surface of a liquid