CHAPTER 10 HYDRAULIC SYSTEMS

As a CM1, you will be responsible for the maintenance, repair, and troubleshooting of hydraulic systems. You must be able to analyze the malfunctions of these systems and supervise your personnel in the required corrective action. To be able to do this, you must thoroughly understand the basic system, the operational principles, and the components of the system.

NOTE:

Before you continue with this chapter, you should review the appropriate chapters of the CM 3&2, NAVEDTRA 10645-G1.

The first part of this chapter briefly covers some of the basic principles associated with hydraulics, followed by coverage of various system components. The purpose of this information is to give you an analytical understanding of the interrelationships of principles and components in an operating system. When you understand the operation of a system, it is much easier to analyze a malfunction.

BASIC PRINCIPLES OF HYDRAULICS AND PNEUMATICS

In automotive and construction equipment, the terms hydraulic or pneumatic describe a method of transmitting power from one place to another through the use of a liquid or a gas. Several kinds of gases are used in the various hydraulic systems; however, certain physical laws or principles apply to all liquids and gases. As a CM, you should be aware of this. You should also be familiar with the following terms as they are associated with hydraulic and pneumatic systems.

HYDRAULICS is that branch of science that deals with the study and use of liquids, as related to the mechanical aspects of physics.

PNEUMATICS is that branch of science that deals with the study and use of air and other gases, as related to the mechanical aspects of physics.

FORCE is the push or pull on an object. In hydraulics and pneumatics, force is usually expressed in pounds.

PRESSURE is the amount of force distributed over each unit on the area of an object. In hydraulics/pneumatics, pressure is expressed in pounds per square inch (psi).

A FLUID is defined as any substance made up of small particles or molecules that have the ability to flow or move easily (conforms to the outline of its container); this includes both liquid and gas. The terms liquids and fluids are often used interchangeably; however, fluids have a much broader meaning. All liquids are fluids, but not all fluids are liquid; fluids can be liquid, but they can also be air and other gases that are not liquid. In support equipment, hydraulics mean liquid and pneumatics mean air or other gases.

INCOMPRESSIBILITY AND EXPANSION OF LIQUIDS

For all practical purposes, fluids are incom- pressible. Under extremely high pressures, the volume of a fluid can be decreased somewhat, though the decrease is so slight that it is considered to be negligible except by design engineers.

Liquids expand and contract because of temperature changes. When liquid in a closed container is subjected to high temperatures, it expands; this exerts a pressure on the walls of the container; therefore, it is necessary that pressure-relief mechanisms and expansion chambers be incorporated into hydraulic systems. Without these precautionary measures, the expanding fluid might exert enough pressure to rupture the system.

COMPRESSIBILITY AND EXPANSION OF GASES

A gas is a substance in which the molecules are separated by relatively large spaces. The two major differences between liquids and gases are their compressibility and expansion. While liquids are incompressible, gases are highly compressible because of these large spaces between the molecules.

Gases, like liquids, expand and contract because of temperature change; but unlike liquids, a gas expands to till completely any closed container in which it is contained; a liquid tills the container only to the extent of its normal volume.