2- FLDS 385 Chapter 2 Principles of Hydraulics

7/29/2019 2- FLDS 385 Chapter 2 Principles of Hydraulics

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EATON Industrial Hydraulics Manual

Polytechnic

Manufacturing and Automation

Ted Nelson A.Sc.T.Rm T409

403-284-8242

[email protected]

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

COPYRIGHT (2001) EATON CORPORATIONC


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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


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


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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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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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

COPYRIGHT (2001) EATON COPRORATIONC


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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

COPYRIGHT (2001) EATON COPRORATIONC


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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


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


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


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


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


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


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






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






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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Any Questions?

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2- FLDS 385 Chapter 2 Principles of Hydraulics