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Hydraulic FluidPurpose & Properties

(Chapter 2)

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IntroductionFluids 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.

hydraulic fluids

a hydraulic fluid has four primary functions:

• Transmission of power

• Lubrication of moving parts

• Sealing of clearances between mating parts

• Dissipation of heat

essential properties of good hydraulic fluid

• Good lubricity• Ideal viscosity• Chemical and environmental stability• Large bulk modulus• Fire resistance• Good heat transfer capability• Low density• Foam resistance• Non-toxicity• Low volatility.

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Compressibility

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.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.

• Bulk modulus is a measure of compressibility. Higher the bulk modulus, the less compressible or stiffer is the fluid.

WhereV is the original volumeΔP is the change in pressure andΔ V is the change in volume.

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Lubrication

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

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SealingInternal leakage is caused by clearances inside hydraulic components, affecting the efficiency of systems Internal leakage also has the potential to create excess heat

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CoolingAny fluid used in hydraulic machinery absorbs and carries heat away from heat generating components such as cylinders and pumps.Some devices designed to maintain fluid quality and ensure long trouble free operation:

Baffled reservoirsCoolersStrainers

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Fluid PropertiesThe fluids used in hydraulic systems must posses specific desirable characteristicsIt 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 IndexPour PointLubricating AbilityOxidationAdditives and InhibitorsRust and corrosion protectionDemulsibilityFire resistance

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Viscosity

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.

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Viscosity too high (fluid is too thick)

(Problems)

High resistance to flowIncreased energy consumption due to increased friction, increased input torque requirement at the pumpHigh temp. created by power loss to frictionIncreased pressure drops due to increased resistance to flow Slow or sluggish operation/actuationInefficient separation of air from the oil in the reservoirPump cavitations

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Viscosity too low (fluid is too thin)

Increased internal leakageExcess wear, seizure, particularly of pumps, could occur under heavy load because of a breakdown in lubrication film between clearances of moving partsDecreased 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)

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ViscosityCoefficient 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

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Kinematic ViscosityK.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

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SUS ViscositySaybolt 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

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Viscosity IndexViscosity 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

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Viscosity IndexIndicates the extent of viscosity change for a given temperature range; should 95 or greaterL = viscosity in SUS of 0-VI oil at 100°FU = viscosity in SUS of unknown-VI oil at 100°FH = viscosity in SUS of 100-VI oil at 100°F

100

HL

ULVI

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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 & compariso

n chart

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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.

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Rust & Corrosion Protection

Corrosion is a chemical reaction between a metal and a chemical – typically an acidExtremely difficult to keep air and moisture out of hydraulic systemsBoth 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 debrisParticular care: Operating & cleaning equipment to prevent the contamination of the hydraulic system with water or

cleaning solvents

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Rust & Corrosion inhibitorsRust inhibitors typically coat metal parts so natural air & moisture do not interact with the metal to form oxide compoundsCorrosive 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 chemistrySome materials such as alloys containing magnesium, lead and zinc are very oxidize Should be avoided in hydraulic systems

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Pour PointThe 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)

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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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Additives and InhibitorsAn 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