Fluid Power - (ME353)- Lec2

8/17/2019 Fluid Power - (ME353)- Lec2

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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

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