HYDRAULICS
DOE-HDBK-1018/2-93
Miscellaneous Mechanical Components
HYDRAULICS
Many machines and processes use a fluid for developing a force to move or hold
an object, or to control an action. The term hydraulic refers to a liquid. A
number of fluids can be used for developing the force. In a hydraulic system, oil,
water, or other liquids can be used. Oil is the most common.
EO 1.5
Given the appropriate information, CALCULATE the pressure
or force achieved in a hydraulic piston.
EO 1.6
DESCRIBE the basic operation of a hydraulic system.
Introduction
Although any liquid can be used in a hydraulic system, some liquids have advantages over
others. Oil is a liquid often preferred as the working fluid. Oil helps to lubricate the various
sliding parts, prevents rust, and is readily available. For practical purposes, oil does not change
its volume in the hydraulic system when the pressure is changed.
Pressure and Force
The foundation of modern hydraulic powered systems was established when a scientist named
Blaise Pascal discovered that pressure in a fluid acts equally in all directions. This concept is
known as Pascal's Law. The application of Pascal's Law requires the understanding of the
relationship between force and pressure.
Force may be defined as a push or pull exerted against the total area of a surface. It is
expressed in pounds. Pressure is the amount of force on a unit area of the surface. That is,
pressure is the force acting upon one square inch of a surface.
The relationship between pressure and force is expressed mathematically.
F = P x A
where:
F = force in lbf
P = pressure in lbf/in.2, (psi)
A = area in in.2
ME-05
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