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Hydraulic Fluid Purpose & Properties (Chapter 3) 11-12-02. Lisa Thompson Julia Boyd Linh Vuong. Introduction.

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  • 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 efficiency.
  • 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
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


boundary lubrication
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
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


  • Additive is usually a

zinc compound

  • Internal leakage is caused by clearances inside hydraulic components, affecting the efficiency of systems
  • 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 heat.
  • The leakage rate is determined by the physical size of clearances, pressure drop across the clearances, and the operating viscosity of the fluid.
  • 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 components
  • 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
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 strength.
  • 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 oil’s resistance to flow.
  • Viscosity affects the fluid’s ability to be pumped, transmitted through the system, carry a load and maintain separation between moving surfaces.
viscosity too high fluid is too thick problems
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 reservoir
  • Pump cavitations
viscosity to low fluid is too thin
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 operation.
  • Internal leakage causing an increase in operating temperatures.
  • 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 = τ(Δy/Δυ) μ = (N*S)/m2 or Pa*s
  • Cgs system: use centipoise = poise/100 =0.001Pa*s
  • Usually given
kinematic viscosity
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.
  • ν = μ/ρ ν = m2/s or ft2/sec
sus viscosity
SUS Viscosity
  • Saybolt viscosimeter: very common method in the USA.
  • Industrial applications, hydraulic oil viscosities usually are in the vicinity of

150 SUS @ 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
  • 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
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.

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
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 & 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 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
Pour Point
  • The pour point is:
    • lowest temperature at which an oil is observed to flow
    • 5°F (3°C) 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
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
  • Oxidation occurs when oxygen attacks the fluid.
    • Accelerated by heat, light, 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, petroleum oxidizes very slow
      • Oxidation double for every 18ºF increased in operation above 140ºF
additives and inhibitors
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 oxidation.
  • Some common additives and inhibitors:
    • anti-wear additives, antifoam agent, corrosion inhibitor, demulsifier, EP additive, oxidation inhibitor, pour point depressant, rust inhibitor
anti wear
  • 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 >3000psig
  • 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 startup.
fire resistance
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.
    • 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 resistance35
Fire Resistance
  • All fluids used in hydraulic systems contain a variety of additives to improve or augment the performance of the fluid under various conditions.
  • 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
fire resistant hydraulic fluids
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
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)
  • 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 Edition)
  • www.uniqema.com