Machinery's Handbook, 31st Edition
Fluid Power FLUID POWER Introduction
2749
A fluid is a substance which deforms continuously when subjected to a shear stress. Fluid power systems convert mechanical or potential energy into fluid energy, and usually perform work with the fluid energy. The fluid used can be either a liquid or a gas. When liquids are used, it is a hydraulic system, and when a gas is used it is a pneumatic system. Properties of Liquids and Gases.— One of the main differences between liquids and gases has to do with compressibility. Compressibility is a measure of the relative volume change of a substance with a change in pressure. Liquids are only slightly compressible, and are usually considered to be incompressible for the purposes of fluid power calcula tions. Gases, on the other hand, are very compressible under normal conditions. Specific weight is the weight of a fluid per unit of volume. Mass density of a fluid is the mass contained within a unit of volume. When a fluid is pressurized, its density and spe cific weight increase, and when a fluid is heated, its density and specific weight decrease. Liquids are affected very little by these factors, while gases are very much affected. Spe cific gravity is the ratio between specific weight or density at actual versus standard condi tions. Specific gravity changes with specific weight or density. Standard conditions are taken to be 4°C for water, and 0°C for air. Other fluids may have different standard condi tions. Standard symbols and equations for these properties are shown below. Table 1, Table 2 and Table 3 contain the properties of some common fluids. Fluid Property Symbols and Equations Specific weight: lbf/ft 3 , N/m 3 γ ρ g w V -- = =
m V = --
Density
ρ
slugs/ft 3 , kg/m 3
r r g
lbm/ft 3
lbm = r g c c = 32.2
Gravitational Constant: lbm-ft/lbf-s 2 Specific Gravity: dimensionless
γ γ c -- ρ ρ c --- = =
SG
The control specific weight g c for liquids is water, and for gases is air. The density of dry air at 32 degrees F and atmospheric pressure (29.92 inches of mercury or 14.70 pounds per square inch) is 0.08073 pound per cubic foot. The density of air at any other temperature or pressure is ρ 1.325 B × T = ------------ in which r = density in pounds per cubic foot; B = height of barometric pressure in inches of mercury; T = absolute temperature in degrees Rankine. (When using pounds as a unit care must be exercised to differentiate between pounds mass and pounds force. See Ac- celeration of Gravity g Used in Mechanics Formulas on page 157. The absolute zero from which all temperatures must be counted when dealing with the weight and volume of gases is assumed to be - 459.7 degrees F. Hence, to obtain the abso lute temperature T used in the preceding formula, add the value 459.7 to the temperature observed on a regular Fahrenheit thermometer. In obtaining the value of B , 1 inch of mercury at 32 degrees F may be taken as equal to a pressure of 0.491 pound per square inch.
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