NTRODUCTION_2000_Fluid-Power-Dynamics

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INTRODUCTION 
The study of hydraulics deals with the use and characteristics of liquids and gases. 
Since the beginning of time, humans have used fluids to ease their burdens. Earliest 
recorded history shows that devices such as pumps and waterwheels were used to 
generate useable mechanical power. 
Fluid power encompasses most applications that use liquids or gases to transmit 
power in the form of mechanical work, pressure, and/or volume in a system. This def- 
inition includes all systems that rely on pumps or compressors to transmit specific 
volumes and pressures of liquids or gases within a closed system. The complexity of 
these systems ranges from a simple centrifugal pump used to remove casual water 
from a basement to complex airplane control systems that rely on high-pressure 
hydraulic systems. 
Fluid power systems have been developing rapidly over the past 35 years. Fluid power 
filled a need during World War II for an energy transmission system with muscle, 
which could easily be adapted to automated machinery. Today, fluid power technol- 
ogy is seen in every phase of human activity. Fluid power is found in areas of manu- 
facturing such as metal forming, plastics, basic metals, and material handling. Fluid 
power is evident in transportation as power and control systems of ships, airplanes, 
and automobiles. The environment is another place fluid power is hard at work com- 
pacting waste materials and controlling floodgates of hydroelectric dams. Food pro- 
cessing, construction equipment, and medical technology are a few more areas of 
fluid power involvement. Fluid power applications are only limited by imagination. 
There are alternatives to fluid power systems. Each system, regardless of the type, has 
its own advantages and disadvantages. Each has applications where it is best suited to 
do the job. This is probably the reason you won't find a fluid power wristwatch, or 
hoses carrying fluid power replacing electrical power lines. 
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ADVANTAGES OF FLUID POWER 
If a fluid power system is properly designed and used, it will provide smooth, flexible, 
uniform action without vibration and is unaffected by variation of load. In case of an 
overload, an automatic release of pressure can be guaranteed, so that the system is 
protected against breakdown or excessive strain. Fluid power systems can provide 
widely variable motions in both rotary and line, ar transmission of power, and the need 
for manual control can be minimized. In addition, fluid power systems are economical 
to operate. 
Fluid power includes hydraulic, hydro-pneumatic, and pneumatic systems. Why are 
hydraulics used in some applications, pneumatics in others, or combination systems 
in still others? Both the user and the manufacturer must consider many factors when 
determining which type of system should be used in a specific application. 
In general, pneumatic systems are less expensive to manufacture and operate, but 
there are factors that prohibit their universal application. The compressibility of air, 
like that of any gas, limits the operation of pneumatic systems. For example, a pneu- 
matic cylinder cannot maintain the position of a suspended load without a constant 
supply of air pressure. The load will force the air trapped within the cylinder to com- 
press and allow the suspended load to creep,. This compressibility also limits the 
motion of pneumatic actuators when under load. 
Pneumatic systems can be used for applications that require low to medium pressure 
and only fairly accurate control. Applications that require medium pressure, more 
accurate force transmission, and moderate motion control can use a combination of 
hydraulics and pneumatics, or hydro-pneumatics. Hydraulics systems must be used 
for applications that require high pressure and/or extremely accurate force and motion 
control. 
The flexibility of fluid power, both hydraulic and pneumatic, elements presents a 
number of problems. Since fluids and gases have no shape of their own, they must be 
positively confined throughout the entire system. This is especially true in hydraulics, 
where leakage of hydraulic oil can result in safety or environmental concerns. Special 
consideration must be given to the structural integrity of the parts of a hydraulic sys- 
tem. Strong pipes, tubing, and hoses, as well as strong containers, must be provided. 
Leaks must be prevented. This is a serious problem with the high pressure obtained in 
many hydraulic system applications. 
Fluid Power Systems vs Mechanical Systems 
Fluid power systems have some desirable characteristics when compared with 
mechanical systems" 
A fluid power system is often a simpler means of transmitting energy. There are fewer 
mechanical parts in an ordinary industrial system. Since there are fewer mechanical 
parts, a fluid power system is more efficient and more dependable. In the common 
Introduction ix 
industrial system, there is no need to worry about hundreds of moving parts failing, 
with fluid or gas as the transmission medium. 
With fluid or gas as the transmission medium, various components of a system can be 
located at convenient places on the machine. Fluid power can be transmitted and con- 
trolled quickly and efficiently up, down, and around comers with few controlling ele- 
ments. 
Since fluid power is efficiently transmitted and controlled, it gives freedom in design- 
ing a machine. The need for gear, cam, and lever systems is eliminated. Fluid power 
systems can provide infinitely variable speed, force and direction control with simple, 
reliable elements. 
Fluid Power vs Electrical Systems 
Mechanical force and motion controlled can be more easily controlled using fluid 
power. The simple use of valves and rotary or linear actuators controls speed, direc- 
tion, and force. The simplicity of hydraulic and pneumatic components greatly 
increases their reliability. In addition, smaller components and overall system size are 
typically much smaller than comparable electrical transmission devices. 
SPECIAL PROBLEMS 
The operation of the system involves constant movement of the hydraulic fluid within 
its lines and components. This movement causes friction within the fluid itself and 
against the containing surfaces. Excessive friction can lead to serious losses in effi- 
ciency or damage to system components. Foreign matter must not be allowed to accu- 
mulate in the system, where it will clog small passages or score closely fitted parts. 
Chemical action may cause corrosion. Anyone working with hydraulic systems must 
know how a fluid power system and its components operate, both in terms of the gen- 
eral principles common to all physical mechanisms and in terms of the peculiarities of 
the specific arrangement at hand. 
The word hydraulics is based on the Greek word for water, the first-used form of 
hydraulic power transmission. Initially, hydraulics covered the study of the physical 
behavior of water at rest and in motion. It has been expanded to include the behavior 
of all liquids, although it is primarily limited to the motion or kinetics of liquids. 
HAZARDS 
Any use of a pressurized medium, such as hydraulic fluid, can be dangerous. Hydrau- 
lic systems carry all the hazards of pressurized systems and special hazards related 
directly to the composition of the fluid used. 
When oil is used as a fluid in a high-pressure hydraulic system, the possibility of fire or 
an explosion exists. A severe fire hazard is generated when a break in the high-pressure 
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piping occurs and the oil is vaporized into the atmosphere. Extra precautions against fire 
should be practiced in these areas. 
If oil is pressurized by compressed air, an explosive hazard exists. If high-pressure air 
comes into contact with the oil, it may create a diesel effect, which may result in an 
explosion. A carefully followed preventive maintenance plan is the best precaution 
againstexplosions.