HYDRAULICS & PNEUMATICS
Hydraulic Fluids
Presented by: Dr. Abootorabi
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Task for hydraulic fluids
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Performance Characteristics of a Hydraulic
Fluid
 When selecting a fluid, consider its:
 Lubricating power
 Oxidation resistance
 Viscosity
 Ability to separate from water
and dirt
 Viscosity stability
 Ability
to
temperatures
operate
in
cold
 Resistance to foaming
 Fire resistance
The most important distinguishing feature of hydraulic fluids is viscosity.
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Performance Characteristics of a Hydraulic
Fluid
 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.
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Performance Characteristics of a Hydraulic
Fluid
 Viscosity is the internal resistance to flow of a liquid (resistance to
flow).

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

Viscosity may be the most important property of a hydraulic
fluid.

If the viscosity is too high, it may results in:
1. High resistance to flow, causing sluggish operation
2. Increased power consumption due to frictional losses
3. Increased pressure drop through valves and lines
4. High temperature due to friction
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Viscosity
 On the other hand, if the viscosity is too low, it may result in:
1. Increased oil leakage past seals
2. Excessive wear due to breakdown of the oil film between
mating parts
 There are two kinds of viscosity: Absolute viscosity (μ) and
kinematic viscosity (ν).
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Viscosity & Temperature
 Hydraulic oils is directly affected by changes in temperature.
 For this reason, machinery should not be put into high speed or
heavily loaded operation until the system fluid is warmed up to
operating temperatures to provide adequate lubrication.
 Viscosity changes as temperature and pressure of a liquid change.
Warm fluid flows easier than cold fluid.
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Viscosity – temperature diagram
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Viscosity – pressure characteristics
At approx. 350 to 400 bar the viscosity is
generally already double that at 0 bar.
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Viscosity Index
 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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Performance Characteristics of a Hydraulic
Fluid
 Pour point is the ability of a fluid to flow when cold and:
 Important to consider if a hydraulic system is exposed to cold
weather
 Should be 20° Fahrenheit below the coldest-expected ambient
system operating temperature
Pour point is important
in cold weather.
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Performance Characteristics of a Hydraulic
Fluid
 Pour
point
is
3°C
above
the
temperature at which movement can
no longer be detected in a fluid that
has
been
cooled
following
an
established test procedure.
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Performance Characteristics of a Hydraulic
Fluid
 Normal operating temperature range for
hydraulic fluid in the reservoir is typically
110°F to 140°F (43°C to 60°C) .
 Operating the system in this temperature
range will result in an acceptable fluid
service life.
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Performance Characteristics of a Hydraulic
Fluid
 Factors causing system fluid to operate above the recommended
temperature are:
 High ambient temperatures
 Reservoir is too small
 Reservoir inlets and outlets are too close
 System pump has excessive flow capacity
 Higher-than-required relief valve setting
 Slower-than-necessary circuit sequencing
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Performance Characteristics of a Hydraulic
Fluid
 A well-designed reservoir helps maintain proper fluid temperature.
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Performance Characteristics of a Hydraulic
Fluid
 Demulsibility is the ability of a fluid to separate out or reject
water.
 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
periodically to prevent re-emulsification and/or reaction with
the fluid chemistry
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Performance Characteristics of a Hydraulic
Fluid
 Water that enters a system having a water-based fluid will modify
the oil/additive/water ratio.
 The correct ratio is required to maintain proper fluid viscosity and
other critical properties.
 Re-establishing the ratio requires fluid testing and adjustment of the
elements to their proper proportions.
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Performance Characteristics of a Hydraulic
Fluid
 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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Performance Characteristics of a Hydraulic
Fluid
 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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Fire Resistance
 Fluid used in hydraulic systems must have fire resistant properties.
 Most fluids can be ignited under the right conditions
 Fire resistant fluid will not sustain combustion when an ignition
source is removed
 Fire resistant fluid will not allow flame to flash back to the
ignition source
 It is important to analyze the working environment of the specific
application to determine fire hazards.
 Some fluids may continue to burn after the ignition source is
removed
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Types of hydraulic fluid
 1. Hydraulic oils (divided into 3 classes according to DIN 51524 and
51525)
 HL
 HLP
Example:
 HV
 2. Hydraulic fluids with low inflammability
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Types of hydraulic fluid
 1. Hydraulic oils
Most common hydraulic fluid in use consists of petroleum base
blended with additives to produce the desired operating properties.
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Types of hydraulic fluid
2. Hydraulic fluids with low inflammability
 VDMA:
The VDMA (Verband Deutscher Maschinen- und Anlagenbau - German
Engineering Federation) is one of the key association service providers
in Europe and offers the largest engineering industry network in
Europe.
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Hydraulic fluids with low flammability
 Fire-resistant hydraulic fluids will not burn without sustained
exposure to an ignition source.
 Oil-water (soluble oil emulsions)
 Water-oil emulsions
 Water-glycol fluids
 Synthetic fluids
Applications:
– Steel works and rolling mills
– Hard coal mining
– Automotive manufacture
– Die-casting machines
– Offshore industry
– Control units for power
– On aircraft and ships
station turbines
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Hydraulic fluids with low flammability
 Oil-water (soluble oil emulsions) hydraulic fluids are used in
metal forging, extrusion equipment, or other large applications.
 Fluid designated as an oil-in-water emulsion commonly contains
only 1% to 5% oil:
 Extremely fire resistant
 Requires typical additives
 Subject to freezing
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Hydraulic fluids with low flammability
 Water-in-oil emulsion fire-resistant fluids contain approximately 40%
water in an oil base.
 Not to be confused with soluble-oil emulsions or high-water-
content fluids (oil-water emulsions)
 Called inverted emulsions because water is suspended in oil, rather
than oil in water
 Limited anti-wear characteristics
 System operating pressures limited
 Has limited use because of instability and maintenance needs
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Hydraulic fluids with low flammability
 Water-glycol fire-resistant hydraulic fluids usually contain 40% to
50% water with the remainder of a polyglycol.
 Polyglycol is similar to automotive antifreeze
 An additive is added for improving the viscosity
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Hydraulic fluids with low flammability
 All synthetic fluids
provide excellent fire resistance. These are not
water based.
 Phosphate ethers are the most common synthetic hydraulic fluids.
 All synthetic fluids meet the basic requirements of a hydraulic fluid:
– Appropriate viscosity
– Good high-pressure performance
– Good lubrication
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Hydraulic fluids with low flammability
 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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Water???
 Although water is readily available and inexpensive, it is not used
alone:
 Poor lubricant
 Promotes rust and corrosion
 Low viscosity
 Freezes
 Rapidly evaporates at temperatures within the operating range of
many typical hydraulic systems
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Another type of hydraulic fluids
 Biodegradable hydraulic fluid reduce the harmful effects of fluid spills
on soil and waterways.
 Biodegradable fluids are:
 Primarily vegetable-based oils
 Easily broken down by organisms found in nature
Biodegradable
important
fluids
when
are
reducing
environmental impact.
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Selection of fluids
 The fluid supplier must understand the nature of the fluid application:
 Environment
 Types of components and their manufacture’s specifications relative
to fluids
 Duty cycles
 Loads (pressure)
 Storage ability
 Temperature extremes
 Any unusual or special considerations in the operation of the
machinery that could affect the life of the fluid or its performance
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Hydraulic Fluid Additives
 Chemicals are used as additives in hydraulic fluids to increase the
stability and overall performance of the fluid.
 An inhibitor is any substance that slows or prevents chemical
reactions, such as corrosion or oxidation.
 Some common additives and inhibitors:
anti-wear
additives,
antifoam
agent,
corrosion
inhibitor,
demulsifier, extreme pressure (EP) additive, oxidation inhibitor,
pour point depressant, rust inhibitor, Viscosity-index improvers .
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Hydraulic Fluid Additives
 Three types of anti-wear additives:
 Anti-wear (AW)

form a protective film on the metal
surface when exposed to low frictional
heat
 Wear resistant (WR)

protects the rubbing surfaces against
wear, particularly from scuffing
 Extreme pressure (EP)


Either prevent surfaces from coming
into contact with one another or
prevent surfaces from welding to one
another when expose to high frictional
heat
Use when operating at pressures >3000
psi (or 200 bar)
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Hydraulic Fluid Additives
 Demulsifier additives increase the fluid’s surface tension:
 Promote separation of water from petroleum-based fluids
 Any water that enters the system separates more quickly
from the oil
 Antifoaming agents reduce surface tension:
 Allow air bubbles to break down before a sufficient quantity
of foam is formed
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Hydraulic Fluid Additives
 It’s extremely difficult to keep air and moisture out of hydraulic systems.
 Corrosion is a chemical reaction between a metal and a chemical-typically
an acid.
 Rust and corrosion inhibitors protect the metal parts of system
components:
 Rust inhibitors protect ferrous metals
 Corrosion inhibitors protect nonferrous metals
 Rust and corrosion inhibitors either:
 Neutralize acids
 Form a film on metal surfaces
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Rust & Corrosion inhibitors
 Rust inhibitors typically coat metal parts so natural air & moisture do
not interact with the metal to form oxide compounds.
 Corrosive elements are often created through oxidation.
 Care must be exercised whenever the hydraulic system is exposed to
atmosphere to minimum the introduction of incompatible elements
that may react with the fluid chemistry.
 Some materials such as alloys containing magnesium, lead and zinc
are very oxidize and should be avoided in hydraulic systems.
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Rust & Corrosion Protection
 Both rust & corrosion contaminate the system & increase component
wear; increase internal leakage past the affected parts causing high
temp.; and cause components to seize through heat & closure or
running clearances with debris.
Particular
equipment
care:
to
Operate
&
prevent
clean
the
contamination of the hydraulic system
with water or cleaning solvents.
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Oxidation
 Oxidation occurs when oxygen attacks the fluid.
 Accelerated by heat, air entrainment in the fluid, metal catalysts
and the presence of water, acids, or solid contaminants
 Susceptible oil to oxidation:

Petroleum and vegetable
 Operation temperature is very important:

Temps <140ºF (60ºC) , petroleum oxidizes very slow

Oxidation double for every 18ºF increased in operation above
140ºF
 Oxidation-inhibitor additives reduce oxidation of fluids.
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Inhibitor Charts
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Hydraulic Fluid Specifications
 Kinematic viscosity is a very precise measurement for
indicating hydraulic fluid viscosity.
 Test involves measuring the time required for a specific volume
of fluid at a specified temperature to flow through a calibrated,
glass capillary viscometer and then multiplying that value by a
constant established for each instrument.
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Hydraulic Fluid Specifications
 A calibrated, glass capillary
viscometer
is
used
to
determine kinematic viscosity.
 ISO
outlines
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viscosity
grades.
 Grades
are
based
on
kinematic viscosity tests
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Hydraulic Fluid Specifications
 The ball viscometer can
also
be
determine
used
to
kinematic
viscosity.
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Viscosity Classes
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Common viscosities
Common industrial fluid power
systems
require
fluid
with
viscosities in the range of ISO
grades 32, 46, or 68.
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Hydraulic Fluid Specifications
 SAE outlines several petroleum product viscosity grades:
 Based on kinematic viscosity ratings
 Extensively used with automotive products
 Used with hydraulic fluids in the past, but less commonly so today
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SAE Viscosity Classification
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Hydraulic Fluid Specifications
 Flash point is the temperature at
which
the
flammable
fluid
vapors
mixture
form
with
a
air
(temporary ignition point).
 Fire point is the temperature at
which the fluid is vaporizing rapidly
enough to support combustion (the
temperature the fluid must attain for
continuous burning).
Flash and fire points are important
factors for many applications.
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Handling and Maintaining Hydraulic Fluids
 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
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Handling and Maintaining Hydraulic Fluids
 Storing new, unused hydraulic fluids is
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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Handling and Maintaining Hydraulic Fluids
 Reservoir and cylinder rod areas are especially susceptible to foreign
materials entering the system.
 Seal around pipes entering the reservoir
 Filter air that enters the reservoir
 Use piston rod wiper rings or boots to prevent dirt from entering
through the cylinder rod seal
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Handling and Maintaining Hydraulic Fluids
 Cylinder rod boots help keep dirt out of the system on rod
retraction.
A & A Manufacturing Co., Inc., Grotite
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The end.
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