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Hydraulic Fluid Purpose & Properties (Chapter 3) 11-12-02


Hydraulic Fluid Purpose & Properties (Chapter 3) 11-12-02 Lisa Thompson Julia Boyd Linh Vuong Introduction Fluids used in mobile and stationary machinery must be ... – PowerPoint PPT presentation

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Title: Hydraulic Fluid Purpose & Properties (Chapter 3) 11-12-02

Hydraulic FluidPurpose Properties(Chapter
  • Lisa Thompson
  • Julia Boyd
  • Linh Vuong

  • Fluids used in mobile and stationary machinery
    must be effective in the transmission of power
    from the source to provide consistent and
    reliable response, safe operation, and optimum

  • Compressibility
  • Lubrication
  • Sealing
  • Cooling

  • Figure 3-2
  • Ensures responsiveness of actuation or
    stiffness in a hydraulic circuit, even under
    high pressure.
  • With the dynamics of loads in industrial
    machinery, slight decompression or compression
    can occur and affect actuation slightly.
  • Properly maintained hydraulic systems are
    extremely responsive and reliable.
  • Petroleum-based fluids are virtually
    incompressible, for example.
  • 0.4 at 1000 psi and up
  • to 1.1 at 3000 psi operating pressure
  • At a constant operating pressure
  • the oil remains compressed at a given value.

  • All hydraulic systems have components with moving
    parts that have the potential to come in contact
    with each other
  • Components need a lubricant to prevent excess
    wear and the production of excess heat
  • Some lubrication condition
  • Full-film
  • Boundary
  • Enhanced (Additives)
  • Anti-Wear

Full-Film Lubrication
  • This prevent metal to metal contact and the
    protect metal surfaces
  • The lubrication film flows between the metal
    parts while keep the metals completely apart
  • This is the best
  • lubrication condition
  • for a hydraulic
  • system

Boundary Lubrication
  • When high pressure is needed, the clearances
    between the moving components should be reduce to
    limit leakage
  • The lubrication film flows between two metal
    parts, now the metal parts can touch each other
  • Disadvantages
  • Excessive wear
  • Severe heat

Enhanced Lubrication
  • To enhance boundary lubrication conditions
    additives are required such as anti-wear (AW) and
    extreme pressure (EP)
  • Anti-wear additives
  • prevent metal to metal
  • contact
  • Additive is usually a
  • zinc compound

  • Internal leakage is caused by clearances inside
    hydraulic components, affecting the efficiency of
  • Internal leakage also has the potential to create
    excess heat
  • The fluid in the system is relied upon to
    minimize leakage across the clearances to improve
    efficiencies and to reduce the production of
  • The leakage rate is determined by the physical
    size of clearances, pressure drop across the
    clearances, and the operating viscosity of the

  • Any fluid used in hydraulic machinery absorbs and
    carries heat away from heat generating components
    such as cylinders and pumps.
  • Some system designs may not allow sufficient
    transfer of fluid to the reservoir,
  • causing a build up of heat and oxidized fluid in
    an isolated segment of a circuit
  • and resulting in the destruction of the fluid and
  • Provisions should be made in machine design for
    the ability to flush these segments regularly
    to prevent cumulative damage to component and to
    the fluid.
  • Some devices designed to maintain fluid quality
    and ensure long trouble free operation
  • Baffled reservoirs
  • Coolers
  • Strainers

Fluid Properties
  • The fluids used in hydraulic systems must posses
    specific desirable characteristics
  • It is sometimes necessary to compromise some
    properties in favor of others that may be more
    important for a specific application requirement
    not all fluids have all the attributes in equal
  • These properties include
  • Viscosity and Viscosity Index
  • Pour Point
  • Lubricating Ability
  • Oxidation
  • Additives and Inhibitors
  • Rust and corrosion protection
  • Demulsibility
  • Fire resistance

  • Viscosity Measure of the oils resistance to
  • Viscosity affects the fluids ability to be
    pumped, transmitted through the system, carry a
    load and maintain separation between moving

Viscosity too high (fluid is too thick)(Problems)
  • High resistance to flow
  • Increased energy consumption due to increased
    friction, increased input torque requirement at
    the pump
  • High temp. created by power loss to friction
  • Increased pressure drops due to increased
    resistance to flow
  • Slow or sluggish operation/actuation
  • Inefficient separation of air from the oil in the
  • Pump cavitations

Viscosity to low (fluid is too thin)
  • Increased internal leakage
  • Excess wear, seizure, particularly of pumps,
    could occur under heavy load because of a
    breakdown in lubrication film between clearances
    of moving parts
  • Decreased pump effiency due to increased leakage
    possible cylinder blow-by. This could cause
    increased cycle times or slower machine
  • Internal leakage causing an increase in operating
  • Most hydraulic systems run with oil (150 300
    SUS or SSU) with the typical ISO viscosity grade
    (22 68)

  • Coefficient of viscosity, dynamic viscosity,
    absolute viscosity, or simply the viscosity of
    the fluid. (Same)
  • viscosity resistance encountered when moving one
    layer of liquid over another
  • µ t(?y/??) µ (NS)/m2 or Pas
  • Cgs system use centipoise poise/100 0.001Pas
  • Usually given

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Kinematic Viscosity
  • K.V. is the most common way of measuring
    viscosity. It is measured by the amount of time
    needed for a fixed volume of oil to flow through
    a capillary tube.
  • ? µ/? ? m2/s or ft2/sec

SUS Viscosity
  • Saybolt viscosimeter very common method in the
  • Industrial applications, hydraulic oil
    viscosities usually are in the vicinity of
  • 150 SUS _at_ 40 C.
  • General rule viscosity should
  • never go below 45 or above
  • 4000 SUS, regardless
  • of temperature
  • Measure how long it take liquid to
  • flow through the orifice

Viscosity Index
  • Viscosity index is an arbitrary number that
    characterizes the variation of viscosity of a
    fluid with variations of temperature.
  • fluid with a high viscosity index
  • exhibits a small change in
  • viscosity with temp.
  • fluid with a low viscosity index
  • exhibits a large change in
  • viscosity with temperature

Viscosity Temperature
  • Hydraulic oils is directly and sometimes
    adversely affected by changes in temp.
  • 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.

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SAE Viscosity Number. ISO Viscosity Grades
comparison chart
Common industrial fluid power systems require
fluid with viscosities in the range of ISO grades
32, 46, or 68 or the kinematic viscosity ranges
for such fluids.
Rust Corrosion Protection
  • Corrosion is a chemical reaction between a metal
    and a chemical typically an acid
  • Extremely difficult to keep air and moisture out
    of hydraulic systems
  • Both rust corrosion contaminate the system
    increase component wear. Increase internal
    leakage past the affected parts causing high
    temp. Cause components to seize through heat
    closure or running clearances with debris
  • Particular care Operating cleaning
  • equipment to prevent the contamination
  • of the hydraulic system with water or
  • cleaning solvents

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
  • Care must be exercised whenever the hydraulic
    system is exposed to atm. To min. 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 Should
    be avoided in hydraulic systems

Pour Point
  • The pour point is
  • lowest temperature at which an oil is observed to
  • 5F (3C) above the temperature at which the oil
    in a test vessel shows no movement when the
    container is held horizontally for five seconds
  • Test Method ASTM D 97 (American Society for
    Testing Materials)

Lubricating Ability
  • Lubricity is the ability of an oil to lubricate
    hydraulic components with adequate clearance to
    run a substantial lubrication film.
  • Full-film lubrication and boundary lubrication

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  • Oxidation occurs when oxygen attacks the fluid.
  • Accelerated by heat, light, metal catalysts and
    the presence of water, acids, or solid
  • Susceptible oil to oxidation
  • Petroleum and vegetable
  • Operation temperature is very important.
  • Temps lt140ºF, petroleum oxidizes very slow
  • Oxidation double for every 18ºF increased in
    operation above 140ºF

Additives and Inhibitors
  • An additive is a chemical substance added to
    fluid to improve certain properties.
  • An inhibitor is any substance that slows or
    prevents chemical reactions, such as corrosion or
  • Some common additives and inhibitors
  • anti-wear additives, antifoam agent, corrosion
    inhibitor, demulsifier, EP additive, oxidation
    inhibitor, pour point depressant, rust inhibitor

  • 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
  • Use when operating at pressures gt3000psig

Additive and Inhibitor Charts
  • Demulsibility is the ability of a fluid to
    separate out or reject water.
  • Demulsifiers are additives that aid the fluid in
    the rejection of water.
  • Rejected water contained in reservoirs should be
    removed periodically to prevent re-emulsification
    and/or reaction with the fluid chemistry.
  • If not removed, the water in the bottom of the
    reservoir could freeze in cold weather and cause
    a potential for cavitation of the pump on

Fire Resistance
  • Fluid used in hydraulic systems must have fire
    resistant properties.
  • Most fluids can be ignited under the right
  • 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.
  • Flash Point temporary ignition point
  • Fire point the temperature the fluid must
    attain for continuous burning
  • Some fluids may continue to burn after the
    ignition source is removed

Fire Resistance
  • All fluids used in hydraulic systems contain a
    variety of additives to improve or augment the
    performance of the fluid under various
  • The fluid supplier must understand the nature of
    the fluid application.
  • Environment
  • Types of components and their manufactures
    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

Fire Resistant Hydraulic Fluids
  • Designed to resist ignition
  • Provide lubrication
  • Prevent corrosion
  • Applications
  • Steel industries
  • Automotive manufacture
  • Offshore industry
  • On aircraft and ships

Test Methods
  • Pour point ASTM D 97
  • The procedure for cooling oil until it will not
    pour out of vessel
  • Oxidation ASTM D 943
  • A controlled flow of oxygen is bubbled through a
    water, oil, and copper and iron catalysts mixture
    at 95ºC until the acid number reaches 2.0 mg KOH
  • Results are given in hours. For example, a
    hydraulic oil with moderate oxidation resistance
    could be 1,000 hours
  • Anti-wear Four Ball Wear Test
  • puts one rotating ball against three fixed balls
    under specific conditions of pressure,
    temperature, revolutions per minute and duration
  • evaluate the friction and wear control ability
    of liquid lubricants in sliding contact
  • ASTM D 5138 (Coefficient of Friction)

Four Ball Wear Test
Typical Test Methods
  • Chapter 3, questions 2, 4, 8 and 9

  • Vickers Industrial Hydraulics Manual 4Th Edition
  • Applied Fluid Mechanics (5 Edition)
  • Fluid Mechanics for Engineering Technology (3
  • www.uniqema.com
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