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EATON Industrial Hydraulics Manual
Polytechnic
Manufacturing and Automation
Ted Nelson A.Sc.T.Rm T409
403-284-8242
FLDS 385
Principles of Hydraulics
Chapter 2
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Section1. Introduction to Hydraulics
Objectives:
1.3 Examine the principles of hydraulic systems.
1.5 Draw a simple circuit using appropriate schematic representation.
1.6 Build a simple hydraulic circuit.
1.7 Discuss and follow safety practices current in the hydraulic industry.
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Principles of Hydraulics
This Chapter is divided into 3 sections
1) Principles of Pressure
2) Principles of Flow
3) Hydraulic Graphic Symbols
The first two sections further develop the fundamentals of power inthe hydraulic circuit
The last section will deal with the classes and functions of lines andcomponents
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Principles of Pressure
The term hydraulics is derived from a Greek word for water
Therefore: the science of hydraulics also includes any device operatedby water
A water wheel or turbine, is a hydraulic device
flour
flour
flour
flour flour
The moving water hitting the water wheel turnskinetic energy into useful work
Figure 2-1 Hydrodynamic device uses kinetic energy rather than pressure
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Principles of Pressure
Hydraulic Devices defined:
Hydrodynamic Device: A hydraulic device which uses the impact or kinetic energy in the
liquid to transmit power
Hydrostatic Device:
A hydraulic device which is operated by a force applied to a confinedliquid
Pressure is the force applied over an exposed area and is expressed asforce per unit area
(lbs/in2
= psi, Pa, 1 bar = 100 kPa = 0.1 mPa)
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Pressure
How Pressure is Created:
Pressure results from a resistance to fluid flow
Pressure also results from a force that tries to make the fluid flow
A mechanical pump induces flow
Or it could be from the weight of the fluid or load
In a body of water, pressure also increases with depth
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Pressure
How Pressure is Created:
An Italian scientist named Torricelli proved that flow out of a hole inthe bottom of a tank was fastest when the tank was full, and the flowrate decreased as the water level lowered
In other words, as the head of water above the opening lessens, sodoes the pressure
Torricelli could only express the pressure at the bottom of the tank asfeet of head or height in feet of the column of water
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Pressure
How Pressure is Created:
Today, with pound per square inch (psi) as a unit pressure, we canexpress pressure anywhere in any liquid or gas in more convenientterms
All that is required is knowing how much a cubic foot of the fluidweighs
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Pressure
How is Pressure head Created:
Below shows a head of one foot of water is equivalent to 0.433 psi,a five-foot head of water equals 2.17 psi and a ten-foot head of waterequals 4.33 psi
A one-foot head of oil is ~0.4 psi
The terms head and pressure
are sometimes used interchangeably10 ft
4.33 psi
2.165 psi
0.433 psi
1. A foot-square section of water 10 fthigh contains 10 cu ft of water. Ifeach cu ft weighs 62.4 lbs
2. then the total weight is 624 lbs. Thisweight is divided over 144 sq in. Thisgives us a pressure of 4.33 psi at thebottom of the 10 ft column of water
Figure 2-2 Pressure head comes from weight of the fluid
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Atmospheric Pressure
Created by the weight of the air in our atmosphere
~0.5 psi per 1000 feet of elevation
At sea level, a column of air with 1in2 cross-section at the full heightof the atmosphere weighs 14.7 lbs (therefore a pressure of 14.7 psia)
At higher altitudes, atmospheric pressure is less than 14.7psia, due toless weight in the column
Below sea level the pressure
is more than 14.7 psia
Calgary is ~3000 ft above
sea level so our atmosphericpressure is ~13.2 psia
Area = 1 in2
1. A column of air one square inch in cross-sectionand as high as the atmosphere
2. weighs 14.7 pounds at sea level.Atmospheric pressure is therefore14.7 psia
psia0
5
10
15 202530
35
40
Figure 2-3 Atmospheric pressure is a head of air
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Atmospheric Pressure
Vacuums:
Any condition where pressure is less than atmospheric pressure iscalled a vacuum or partial vacuum
A perfect vacuum is the complete absence of pressure or zero psia(zero bar absolute, zero kPa absolute)
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Atmospheric Pressure
Mercury Barometer:
Atmospheric pressure is also measured in inches of mercury (in. Hg) Torricelli discovered that an inverted tube full of mercury will only
fall a certain distance in a pan full of mercury
Mercury
29.92inches
AtmosphericPressure
Vacuum
1. Atmosphericpressure here
2.supports a columnof mercury this high
Figure 2-4 The mercury barometer measures atmospheric pressure
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Atmospheric Pressure
Mercury Barometer:
He reasoned that atmospheric pressure on the surface of the mercuryin the pan was supporting the weight of the column of mercury with aperfect vacuum at the top of the tube
At sea level, the column is
29.92 in. Hg
(rounded to 30 in. Hg)
This is another equivalent of
the pressure of one atmosphere
Mercury
29.92inches
AtmosphericPressure
Vacuum
1. Atmosphericpressure here
2.supports a columnof mercury this high
Figure 2-4 The mercury barometer measures atmospheric pressure
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Atmospheric Pressure
Measuring Vacuum:
Vacuum can be expressed as psia or psi (in negative units) as well asin inches of mercury
Most vacuum gauges are calibrated in inches of mercury
A perfect vacuum which will support a column of mercury 29.92inches high, and is stated as 29.92 in. Hg
Zero vacuum (atmospheric pressure) reads zero on a vacuum gauge
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Summary
Pressure and Vacuum Scales:
0 psig is equal to 14.7 psia
Which is equal to 0 in. Hg or 29.92 in. Hg absolute
Atmospheric Pressure ----(59 F @ Sea Level)
GaugePressure
Scale(psig)
AbsolutePressure
Scale(psia)
Vacuum
Vacuum
Absolute Pressure
0 PSIG
0 PSIA
+14.7 PSIA
Perfect Vacuum ----------(Absolute Zero Pressure)
-5
+5
0
0
+5
-10
+10
-15
+15
+20
-14.7 PSIG -29.92 inchHg
Absolute Pressure
+29.92 inchHg Absolute
0 inchHg Abs
0 inchHg
o
Figure 2-5 Gauge and absolute pressure comparison
COPYRIGHT (2001) EATON CORPORATIONC
AbsolutePressure
Scale(In Hg Abs)
+10
+20
+30
+40
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Summary
Pressures and Vacuums:
1 atmosphere is a pressure unit equal to 14.7 psi (1.01 bar, 101 kPa)
psia (pounds per square inch absolute) is a scale that starts at a perfectvacuum
psi or psig (pounds per square inch gauge) is calibrated in the sameunits as psia but ignores atmospheric pressure
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Summary
Pressures and Vacuums:
To convert psia to psig:
Gauge Pressure + 14.7 = Absolute Pressure Absolute Pressure 14.7 = Gauge Pressure
Atmospheric pressure on the barometer scale is 29.92 in. Hg
Compared to the psia scale 1psi = 2 in. Hg (approximately)
1 in. Hg = 0.5 psi (approximately)
An atmosphere is approx equal to 34 ft of water or 37 ft of oil
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Principles of Flow
Flow in the hydraulic system gives the actuator its motion
Pressure gives the actuator its force
Flow is created by the pump
Pressure is created by a restriction
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Principles of Flow
How Flow is Measured:
Flow of a fluid is measured by two ways:
Velocity is the average speed of the fluids particles past a given point Flow Rate is a measure of the volume of fluid passing a point in a given time
Below, with a constant flow rate of one gallon per minute, the velocitywill either increase or decrease when the cross-section of the pipe
changes in sizepsigpsig
00
100100
200200
300300 400400 500500
600600
700700
800800
psig0
100
200
300400 500
600
700
800
Figure 2-6 Flow is volume per Unit of time; velocity is distance per unit of timeCOPYRIGHT (2001) EATON CORPORATIONC
f
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Principles of Flow
How Flow is Measured:
Velocity:
Imperial
Measured in feet per second (fps), feet per minute (fpm) or inches persecond (ips)
Metric Measured in meters per second (m/s), meters per minute (m/m), or
centimeters per second (cm/s)
P i i l f Fl
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Principles of Flow
How Flow is Measured:
Flow Rate:
Imperial
Large volumes are measured in gallons per minute (GPM)
Small volumes are measured in cubic inches per minute (in3/min)
Metric
Large volumes are measured in liters per minute (l/m)
Small volumes are measured in cubic centimeters per minute(cm3/min)
P i i l f Fl
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Principles of Flow
Flow Rate and Speed:
The speed of a hydraulic actuator always depends on its size and the
rate of flow into it
1 GPM = 231 in3/minute
GPM = in3/minute231
in3/minute = GPM x 231
P i i l f Fl
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Principles of Flow
Flow and Pressure Drop:
Whenever a liquid is flowing, there must be a condition of unbalanced
force to cause motion
Therefore: when a fluid flows through a constant-diameter pipe, thepressure will always be slightly lower downstream
This difference in pressure or pressure drop is required to overcomefriction in the line
P i i l f Fl
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Principles of Flow
Flow and Pressure Drop:
The pressure drops below are due to friction
Succeeding pressure drops (from maximum pressure to zero pressure)are shown in differences in head in the succeeding vertical pipes
1. Pressure is maximumat this point due to thedepth of the fluidcolumn
4. Pressure is zero herebecause the fluid isunrestricted at thispoint
2. Friction in the pipe causes the pressureto drop from maximum to zero
3. The Succeedingly lower fluidlevels in these pipes is ameasure of pressure at thepoints down stream from thesource
Figure 2-7 Friction in pipes results in a pressure drop
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P i i l f Fl
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Principles of Flow
Fluid Seeks a Level:
When there is no pressure difference on a liquid, it stays level
The liquid is subject to atmospheric pressure at all points so the fluidis the same level at all points
P i i l f Fl
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Principles of Flow
Fluid Seeks a Level:
If the pressure changes at one point the liquid levels at the other points
rise only until their weight is sufficient to makeup the difference inpressure
The difference in height (head) in the case of oil is 1 foot per 0.4 psi
1. If the pressure isincreased here
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Principles of Flo
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Principles of Flow
Bernoullis Principle:
Hydraulic fluid in a working system contains energy in two forms:
Kinetic energy by virtue of the fluids weight and velocity Potential energy in the form of pressure
Bernoulli demonstrated that in a system with a constant flow rate,
energy is transformed from one form to the other each time the pipecross-section size changes
Bernoullis principle states: If the flow rate is constant, the sums ofthe kinetic energy and the pressure energy at various points in a
system must be constant
Principles of Flow
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Principles of Flow
Bernoullis Principle:
As the cross-sectional area of a flow path increases, the velocity
(kinetic energy) of the fluid decreases Therefore, if the kinetic energy decreases, there is an increase in
pressure energy
psig psig0 0
50 50
100 100
200 200300 300400 400
500 500
600 600700 700
psig0
50
100
200300
400
500
600
700
1. In the small section of pipe velocity is
maximum and pressure is 300 psi. Whenfluid reaches the large section of pipevelocity of the fluid decreases and thepressure goes up. As the fluid leaves thelarger section it speeds up and thepressure drops back to 300 psi
Principles of Flow
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Principles of Flow
Venturi Effect:
Air flowing through the carburetor barrel is reduced in pressure as it
passes through the reduced cross-section of the throat The decrease in pressure permits gasoline to flow, vaporize and mix
with the air stream
1. Volume of air isdetermined bythe butterfly valve
2. At the venturi throatthe air speeds up andthe pressure drops
3. The pressure is higher in the fuel bowlthan in the venturi throat, this pressure
difference pushes the fuel into themoving air stream
Principles of Flow
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Principles of Flow
Bernoullis Principle:
Effects of friction and velocity changes on the pressure in a line
As the pipe diameter increases, the velocity of the fluid slows andallows the pressure to increase in this section of the pipe
2.However when the pipe diameter is increasedthe velocity of the fluid slows, this reduces thepressure drop allowing pressure to rise in thelarger section of pipe
Figure 2-13 Friction and velocity effect pressureCOPYRIGHT (2001) EATON CORPORATIONC
1.Friction reduces thehead at succeedingpoints
Hydraulic Symbols
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Hydraulic Symbols
Hydraulic circuits and components are depicted in various ways indrawings
Depending on what is needed to be conveyed the symbols may be:
A pictorial representation of the components exteriors
A cutaway showing internal construction
A graphic diagram which shows function
The graphic diagram is most commonly used in industry
Hydraulic Symbols
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Hydraulic Symbols
Symbols are the shorthand of the circuit diagrams using simplegeometric forms to show functions and interconnections of lines and
components
The complete set of Basic Hydraulic Symbols are located on page 547in Appendix B of the EATONS Industrial Hydraulics Manual
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Hydraulic Symbols
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Hydraulic Symbols
Working Lines (solid lines):
Line #1 is the pump inlet, Line #2 is a return line and Line #3 is a
pressure line
1. The pump inletis a workingline, so it is asolid line
2. Return linesare workinglines, so theyare solid lines
3. The pressure line isa working line, so itis a solid line
4. Pilot lines operate valves orother controls, they are longdashed lines. They operate withlow flows only
5. Short dashed lines aredrain lines. They drainleakage oil from pumps,valves, and motors
Hydraulic Symbols
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Hydraulic Symbols
Pilot or Sensing Lines (long dashes):
Line #4 is a pilot line, which operates valves or other components
1. The pump inletis a workingline, so it is asolid line
2. Return linesare workinglines, so theyare solid lines
3. The pressure line isa working line, so itis a solid line
4. Pilot lines operate valves orother controls, they are longdashed lines. They operate withlow flows only
5. Short dashed lines aredrain lines. They drainleakage oil from pumps,valves, and motors
Hydraulic Symbols
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Hydraulic Symbols
Drain Lines (short dashes):
Line #5 is a drain line, which drains leakage oil from pumps, valves,
and motors back to the reservoir It may be less confusing to draw more than one reservoir
1. The pump inletis a workingline, so it is asolid line
2. Return linesare workinglines, so theyare solid lines
3. The pressure line isa working line, so itis a solid line
4. Pilot lines operate valves orother controls, they are longdashed lines. They operate withlow flows only
5. Short dashed lines aredrain lines. They drainleakage oil from pumps,valves, and motors
Hydraulic Symbols
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Hydraulic Symbols
Rotating Components:
A circle is the basic symbol for rotating components
Energy triangles are placed in the symbols to show them as an energysource (pump) or energy receiver (motor)
1. The fluid energy triangle points outshowing the pump as a source of flow
3. The triangle pointing in showsthe motor receiving energy
Pump Motor
Hydraulic Symbols
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Hydraulic Symbols
Rotating Components:
A unidirectional component symbol is drawn with only one triangle
A reversible (bi-directional) component is drawn with two triangles1. The fluid energy triangle points out
showing the pump as a source of flow
2. Two fluid energy triangles show the pump
to be bi-directional, meaning flow mayswitch between ports
3. The triangle pointing in showsthe motor receiving energy
4. Two triangles show the motor
directional, the motor is revers
Pump
Bi-directionalPump
Motor
Bi-directionalMotor
Hydraulic Symbols
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Hydraulic Symbols
Cylinders:
A cylinder is drawn as a rectangle with a piston, piston rod and port
connection(s) A single acting cylinder is drawn with an open end at the rod end and
with only a cap end port connection
Single-acting
Cylinder
Piston
Port connection
Piston rod
Hydraulic Symbols
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Hydraulic Symbols
Cylinders:
A double acting cylinder is drawn with a closed end at the rod end and
with two port connections
Double actingCylinder
Port connection
Port connection
Hydraulic Symbols
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Hydraulic Symbols
Valves:
The basic symbol for a valve is a square (called an envelope)
Arrows are added to show flow paths and the direction of flow
Infinite Positioning Valves:
Pressure & flow control valves are infinite position valves
They can have many positions between fully open and fully closed
depending on the volume of fluid passing through them Drawn as a single square, and can be N/O or N/C
Hydraulic Symbols
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Hydraulic Symbols
Directional Valves:
Directional valves are finite positioning valves
The basic symbol contains an individual envelope (square) for eachposition it can be shifted to
The three position valve shown below is called a bang bang typebecause it goes from one extreme to the other very quickly
Hydraulic Symbols
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Hydraulic Symbols
Infinite Positioning Directional Control Valves:
Proportional and Servo valves, are drawn with two or more envelopes
(squares) to show the directions of flow They also have two parallel lines drawn outside the envelopes to show
infinite positioning capability
These are very high end valves, and very costly
Hydraulic Symbols
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Hydraulic Symbols
Reservoir:
A reservoir is drawn as a rectangle with an open top if it is vented and
with a closed top if it is pressurized Lines are drawn to the bottom of the reservoir symbol when the lines
terminate below the fluid level of the tank (return lines)
Lines are drawn to the top of the reservoir symbol when the linesterminate above the fluid level of the tank (drain lines)
P
T
A
B
Directional Valve
Motor
Relief Valve
Pump
Reservoir
Reservoir
Reservoir
There is typically only one reservoir in a system though the symbol is redrawn for simplicity sake.
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