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Fluid Power Systems (ME353)
Fall 2012
Lecture 2
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Fluid Power Standards
and Symbols
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Standardization
Standards affect our daily
activities:
– Health and safety issues
– Quality of manufacturedgoods
– Service industry
Standards can be defined in
many ways. In manufacturing, a standard is
usually a set of specifications
that define a procedure or
product
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In general, the standardizing process iscarried on by:
– Trade and business associations
– Scientific and professional societies
– General membership associations
– Testing and certifying groups
– Consortia with a common interest
– Government
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© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.5
International standards-coordinatingorganizations exist to assist the many groups
involved in standards development
– American National Standards Institute (ANSI) is
the primary American coordinating group
– International Organization for Standardization
(ISO) is the international coordinating group
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These four professional organizations act in the areas of:
– Manufacturers and distributors association activities
– Standards development – Professional society functions
– Educational promotion
The four professional organizations in the fluid
power area are:
– National Fluid Power Association (NFPA)
– Fluid Power Distributors Association (FPDA)
– International Fluid Power Society (IFPS)
– Fluid Power Educational Foundation (FPEF)
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Typical standards include information on:
– Scope
– Purpose
– References
– Definitions
– Identification statements
– General rules of the standard
– Appendices
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Fluid Power Symbols and
Circuit Diagrams
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Symbols are used in all parts of the world for
designating components in fluid power circuits
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Knowledge of common symbols is
considered a valuable tool for anyoneworking in the fluid power field
The types of symbols most often
encountered in the fluid power field are: – Graphic
– Pictorial
– Cutaway – Combination
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Graphic symbols are the most common symbol
type
– Relatively simple to draw
– More easily standardized than other types
The intent of a graphic symbol is to
represent the:
– Type
– Functions
– Operation
– External connections
– Does not show the actual
construction of the unit
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Fluid power graphic symbols
consist of basic figures:
– Lines – Circles
– Squares
– Triangles
– Dots
– Arrows
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For example:
– Circles are used to represent components such as pumps,
motors, and pressure gauges – Squares depict valves and conditioning units
Other graphic elements designate the operating medium of the
system, direction of movement, or the source of system energy
For example, a small, equilateral triangle shows the type offluid and the direction of energy flow
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Pictorial symbols consist of
line drawings of the exterior
shapes of fluid power
components
Cutaway symbols are miniature
section drawings of the
components
Pictorial symbols and cutaway symbols are very useful in training and other
applications
Pictorial symbols and cutaway symbols do not adapt well to standardization
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Joining of lines in a circuit is represented by a
graphic symbol, which is a dot
Lines that cross without joining are shown as an
arc crossing without a dot
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Basic graphic symbols for energy conversion devices are the
circle and the rectangle
– Pumps, compressors, and motors are depicted by circles
– Cylinders are represented by rectangles
A capsule is the symbol used to show energy storage devices in both hydraulic and pneumatic systems
– Accumulators are the storage devices found in hydraulic
systems
– Air receivers are the primary pneumatic storage devices
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Basic valve graphic symbols.
One or more box shapes serve as the basic symbol for control
valves
– Each box represents a single valve position.
– Boxes are drawn with contiguous sides when the symboldepicts a multiple-position valve
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Symbols for pressure control valves are shown as normally
open or normally closed – Normally open valve is open at rest, allowing flow through
the valve
– Normally closed valve is closed at rest, blocking flow
through the valve
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Simplified symbol for an adjustable flow control valve shows
an orifice with an arrow drawn diagonally across it.
– Arrow indicates the orifice is adjustable – When appropriate, symbols also indicate pressure and
temperature compensation
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A number of reservoir symbols are
shown in a typical circuit diagram
– Only one reservoir exists inmost systems
– Using multiple reservoir
symbols reduces the
complexity of a diagram byeliminating the return lines
extending from the components
to the reservoir
Lines returning to the
reservoir
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Fluid-conditioning devices include filters, separators, air dryers,
lubricators and heat exchangers
Basic graphic symbol for fluid conditioning devices is a square
shown resting on one corner
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Circuit diagrams provide a variety of information about fluid
power systems for use during system assembly, operation, and
testing. – Schematic diagrams
– Component lists
– Sequence of operation
Circuit diagram schematics must:
– Include all components and connections
– Be logically arranged so it is possible to easily follow the
operating cycle of the system
All system components in a circuit diagram should be identified
with a code number to allow easy identification
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A wide variety of technical information can
be included in a circuit diagram – System prime mover information
– Flow rating of pumps
– Level of filtration – Conductor sizes and weights
– Actuator sizes
– Pressure settings of valves
– Other appropriate information
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The Energy Transmitting Medium
Hydraulic Fluid
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Functions of a Hydraulic
Fluid Transmitting the energy to do the work of the system is the primary function of
liquid in a hydraulic system
The fluid is just as important as any of the hardware components
When selecting a fluid, consider its:
– Lubricating power
– Viscosity
– Viscosity stability
– Ability to operate in cold temperatures – Oxidation resistance
– Ability to separate from water and dirt
– Resistance to foaming
– Fire resistance
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Fluid Properties &Specifications
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1- Density (ρ):
• It is defined as a fluid mass per unit volume.
ρ = mass / volume
In the BG system “ ρ” has units of slugs/ft³and in SI theunits are kg/m³.
The density of a gas is strongly influenced by both pressure and temperature, but for liquids variations in pressure and temperature generally have only a smalleffect on the value of density.
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The change in the density of water with large variations in
temperature.
Density of water as a function of temperature.
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Temperature increases result in expansion of hydraulic fluid,and a corresponding reduction in density.
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• Specific Weight ( ):
g
It is defined as a fluid weight per unit volume. Thus, specific
weight is related to density through the equation
Where g is the gravitational acceleration (9.8 m/s² ).In the BG system has units of lb/ft³ and in SI the units
are N/m³ .
The specific volume, “v” is the volume per unit mass and is
therefore the reciprocal of the density-that is,
v = V/m = 1/ ρ
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Specific Gravity (SG):
It is defined as the ratio of the density of the fluid to the density ofwater at 4 C (39.2 F), and at this temperature the density of water is1.94 slugs/ft³ or 1000 kg/m³ . In equation form ,specific gravity isexpressed as:
SG = ρ / 1000
Since SG is the ratio of densities, then it is dimensionless quantityand its value does not depend on the system of units used.
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Specific gravity and API gravity
provide comparisons between the
weights of a volume of a substance
and an equal volume of distilled water
– Specific gravity can be used with
any material
– API system was developed
primarily for petroleum oils
Distilled water has a specific
gravity of 1.0
Distilled water has an APIgravity of 10.0
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2- Compressibility of Fluids and BulkModulus ( E v ):
• A property that is commonly used to characterize compressibilityis the bulk modulus of elasticity, E v , defined as:
where dp is the differential change in pressure needed to create a differential
change in volume, dV , of a volume V . The negative sign is included since an
increase in pressure will cause a decrease in volume.
The above relation is used with liquids to calculate the volume change due to
pressure change.
For gases the gas low is used
P 1 V 1 / T 1 = P 2 V 2 / T 2
where P and T are the absolute values
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Bulk modulus of hydraulic
fluids increases slightly withpressure, but decreases sharply
with increases in temperature
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3- Viscosity
It is a property describe the resistance of the fluid to the
motion either it is internal resistance between the fluid layers
or between the fluid and the solid boundaries.
For liquids, it is found that the shear stress is proportional tothe rate of shear strain ( Velocity gradient)
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Where the constant of proportionality is called the absoluteviscosity, dynamic viscosity, or simply the viscosity of thefluid
It can be readily deduced that the dimensions of viscosityare ML¯¹T¯¹ ,Thus, in BG units viscosity is given as lb.s/ft² and in SI as N.s/m² or Pa.s .
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• The Kinematic Viscosity
It is defined as the ratio of the absolute viscosity to thefluid density .i.e.,
The dimensions of kinematic viscosity are L²/T and the BG
system are ft²/s and SI are m²/s .Important Note:
Dynamic viscosity is often expressed in the metr ic CGS (centimeter-gram -
second) system with units of dyne. s/cm² . This combination is called a
“ poise ”.
I n the CGS system, kinematic viscosity has units of cm²/s , and this
combination is called a “ stoke ”.
For Dynamic viscosity : 1 pa.s = 10 poise = 1000 cpFor kinematic viscosity : 1 m2/s = 104 St = 106 cSt
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The effect of temperature on viscosity
Viscosity of hydraulic fluid is
relatively unaffected by pressureup to approximately 20.7 to 27.6MPa Changes in viscosity must be accounted for above thesepressures.
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Friction is the resistance to movement between two surfaces in contact
The amount of friction depends on: – Roughness of the surfaces in contact
– Force pushing the surfaces together
Lubrication reduces friction between two surfaces by placing a layer of
liquid between them
A properly selected liquid produces a film that separates the surfaces and
allows them to freely move past each other
A film of hydraulic oil fills irregularities in contact surfaces
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Viscosity is the internal resistance to flow of a liquid
A liquid with the proper viscosity provides a strong film
that: – Greatly reduces friction between the bearing surfaces of
component parts
– Provides a seal between those parts
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Viscosity changes as temperature and pressure of a liquid
change
– Warm fluid flows easier than cold fluid
– Viscosity index is the rate of viscosity change in
relation to temperature change
– The higher the viscosity index number, the lower the
rate of viscosity change
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4- Pour point is the ability of a fluid to flow when cold – Important to consider if a hydraulic system is exposed
to cold weather – Should be 6 °C below the coldest-expected ambient
system operating temperature
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Oxidation rate of a hydraulic fluid is affected by:
– Temperature
– Air entrainment in the fluid
– Contact with metals used in the construction of a system
– Contaminants, such as dirt and water, that enter a system
A variety of ASTM standards provide test specifications to
establish rust-, corrosion-, and oxidation-prevention capabilities
of hydraulic fluids
These factors are critical to the service life of system component
parts and the fluid itself
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Demulsibility and foaming characteristics of hydraulic fluids may be
determined by test procedures detailed in ASTM specifications
Results of these tests indicate the ability of a hydraulic fluid to separate
from water that has entered the system and resist foam formation whenair is introduced through components
Petroleum-based fluids must have the ability to easily separate from
water
– Select a fluid that resists emulsification
– Drain accumulated water from the bottom of the reservoir
Foaming increases fluid oxidation
– Caused by air being drawn into system inlet lines or churned into
reservoir fluid
– Increases air/fluid contact because of bubble surface area
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Fire Hazard:
The possibility of fire exists to some extent in
many hydraulic applications
– Petroleum-based fluids can supply adequate safety
levels in many systems
– Fire-resistant fluids using water or synthetic bases
are required when higher fire protection is needed
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Commonly Used
Hydraulic Fluids
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Although water is readily available and inexpensive, it is not
used alone:
– Poor lubricant – Promotes rust
– Freezes
– Rapidly evaporates at temperatures within the operating
range of many typical hydraulic systems
Most common hydraulic fluid in use consists of petroleumbase blended with additives to produce the desired operating
properties
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Biodegradable hydraulic fluids reduce the harmful effects offluid spills on soil and waterways
Biodegradable fluids are:
– Primarily vegetable-based oils
– Easily broken down by organisms found in nature
Biodegradable fluids are important when reducing
environmental impact
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Fire-resistant hydraulic fluids will not burn withoutsustained exposure to an ignition source
– Water-oil emulsions – Water-glycol fluids
– Synthetic fluids
Water-in-oil emulsion fire-resistant fluids contain
approximately 40% water in an oil base
– Not to be confused with soluble-oil emulsions and high-
water-content fluids
– Called inverted emulsions because water is suspended in oil,
rather than oil in water
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Water-glycol fire-resistant hydraulic fluids usually contain 40%
to 50% water with the remainder a polyglycol
– Polyglycol is similar to automotive antifreeze
– Fluids adversely affect some seal materials and paint
All synthetic fluids provide excellent fire resistance
- Phosphate esters are the most common synthetic hydraulic
fluidsAll synthetic fluids meet the basic requirements of a hydraulic fluid:
-Appropriate viscosity
-Good high-pressure performance
-Good lubrication
Disadvantages of synthetic fluids include:
-Special seal material requirements
-Tendency to dissolve paint
-Environmental toxicity level must be carefully considered before
using in sensitive areas
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Hydraulic Fluid Additives
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Chemicals are used as additives in hydraulic fluids to increase
the stability and overall performance of the fluid
Extreme-pressure and anti-wear agents help prevent metal-to-
metal contact of bearing surfaces to reduce friction and wear
Viscosity-index improvers reduce changes in viscosity as the
fluid changes temperatures
Pour-point depressant allows the fluid to flow freely at lower
temperatures (This is important for fluids used in systems that
are exposed to winter weather)
Oxidation of hydraulic fluids is caused by:
HeatExposure to air
Catalytic effects of metal
Oxidation-inhibitor additives reduce oxidation of fluids
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Demulsifier additives increase the fluid’s surface tension
– Promote separation of water from petroleum-based fluids
– Any water that enters the system separates more quicklyfrom the oil
Antifoaming agents reduce surface tension
– Allow air bubbles to break down before a sufficient quantity
of foam is formed – Foam causes operational problems in the system
Rust and corrosion inhibitors protect the metal parts of system
components
– Rust inhibitors protect ferrous metals – Corrosion inhibitors protect nonferrous metals
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Handling and Maintaining
Hydraulic Fluids
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Proper handling and maintenance of hydraulic fluids
reduces system operating cost
– Extends the service life of fluids – Reduces the amount of maintenance time spent in
cleaning and flushing systems and replacing system
fluid
Storing new, unused hydraulic fluidsis an important consideration
– Store drums in a cool, clean, dry
place
– Place drums on their sides to
reduce chances of contamination
– Carefully clean drum tops before
removing bungs
– Use clean fluid-transfer
equipment
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System operating temperature is a major factor in the service
life of hydraulic fluids
Normal operating temperature of reservoir fluid is typically between 44°C and 60°C.
Factors causing system fluid to operate above the recommended
temperature are:
– High ambient temperatures – Reservoir is too small
– Reservoir inlets and outlets too close
– System pump has excessive flow capacity
– Higher-than-required relief valve setting – Slower-than-necessary circuit sequencing
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A well-designed reservoir helps maintain proper fluid
temperature