Hydraulic Fluid Selection 371

absolute viscosity

A fluid's resistance to flow in response to an external force. Absolute viscosity is expressed in centipoises (cP) or millipascal-seconds (mPa-s).

acidity

The ability of a substance to neutralize bases. Acidity in hydraulic fluid affects its performance.

actuators

An active fluid system component that converts fluid energy into mechanical power. The actuator is at the end of a hydraulic circuit.

additive package

A combination of substances added to hydraulic fluid in order to enhance properties. An additive package makes up about 1% of hydraulic fluid.

additives

A substance added to another substance or material to improve its chemical and physical properties. Additives are used in hydraulic fluid to inhibit rust and oxidation.

alcohols

An organic compound that reacts with organic acids. Alcohols react with animal or vegetable fats to form a chemical base for synthetic fire-resistant fluids.

animal fats

An oil derived from animals. Animal fats in synthetic fire-resistant fluids provide lubrication.

anti-foaming agents

An additive used in hydraulic fluid that causes small air pockets to rapidly dissipate. Anti-foaming agents force small air bubbles to form a larger air bubble that rises to the surface of the fluid.

antiwear additives

A substance added to hydraulic fluid to prevent friction between components. Antiwear additives deposit a protective barrier on metal surfaces.

auto-ignition

The lowest temperature at which a fluid or substance will ignite spontaneously without exposure to a flame. The auto-ignition temperature of some synthetic fire-resistant fluids is very high.

biodegradable

Capable of being broken down and naturally absorbed into the ecosystem. Biodegradable materials degrade into simple stable compounds that are not harmful to the environment.

boundary lubrication

A form of lubrication characterized by a thin film of fluid between components. Boundary lubrication may develop when operating speeds and pressures are high.

cadmium

A soft metal often used as a corrosion-resistant plating on steel. Cadmium is not compatible with water glycol fire-resistant fluids.

capillary tube viscometer

A thick glass tube used to measure a fluid's kinematic viscosity. A capillary tube viscometer allows fluid to flow in a narrow space without facing opposition from an external force.

centipoises

cP. A unit used to express the absolute viscosity of fluid. Centipoise units express the amount of force necessary to move a layer of fluid over another layer.

centistokes

cSt. A unit used to measure the kinematic viscosity of fluid. Centistoke units are used to measure viscosity when gravity is the only external force acting on fluid.

clearances

The small space between moving parts that allows for movement and lubrication. Clearances are necessary for the successful operation of many fluid system components.

confined spaces

An area that has limited means of entry or exit and is not designed for continuous occupancy. Confined spaces include tanks and pipelines.

continuous phase

The liquid in an emulsion within which fluid particles are dispersed. The continuous phase is the external liquid in an emulsion.

corrosion

The wearing away or deterioration of metal due to a chemical reaction with another substance. Corrosion can occur after the formation of rust.

corrosion inhibitors

An additive used in hydraulic fluid to protect metal components from experiencing different types of corrosion. A corrosion inhibitor's effectiveness depends on the quality of the fluid to which it is added.

crude oil

Unrefined petroleum. Crude oil must be refined in order to be usable in products, such as gasoline.

cSt

Centistokes. A unit used to measure the kinematic viscosity of fluid. cSt units are used to measure viscosity when gravity is the only external force acting on fluid.

demulsibility

A fluid's ability to resist forming a stable mixture with another substance. Demulsibility in hydraulic fluid means that the fluid is able to release water.

demulsifiers

An additive used in hydraulic fluid to prevent the mixture of water and hydraulic fluid. Demulsifiers cause water and hydraulic fluid to separate.

density

The mass of a material per unit volume. Density describes the "compactness" of a material.

ecotoxicity

The ability of a chemical or physical agent to have a negative effect on the environment. Ecotoxicity ratings for environmentally acceptable fluids are measured in concentrations of the fluid that may be toxic to the environment.

efflux

The process of a substance flowing out of a container. Efflux time of fluid flowing out of a Saybolt viscometer is measured in Saybolt Universal Seconds.

emulsion

A mixture of two substances, such as oil and water, that are not normally able to be mixed. An emulsion may be water-based or oil-based.

entrained

Trapped and suspended in a substance. Entrained air can form a layer of foam on the surface of hydraulic fluid.

environmentally acceptable fluids

EAF. Industrial hydraulic fluid that meets biodegradability and toxicity standards. Environmentally acceptable fluids are classified as such by various organizations.

extreme pressure additives

A substance added to hydraulic fluid that reduces wear of parts exposed to high pressure. Extreme pressure additives perform well over a wide temperature range.

filtration

The process of removing material and liquid contaminants from hydraulic fluid. Filtration is necessary in order to maintain hydraulic fluid properties.

fire resistance

The ability to withstand flame and maintain temperature for a specified amount of time. Fire-resistant hydraulic fluid may have a water base or synthetic base.

fire-resistant fluid

FRF. A hydraulic fluid that is difficult to ignite and will not sustain a flame. Fire-resistant fluids have a water base or synthetic base.

fittings

A fluid system component that connects, redirects, extends, or terminates the run of a fluid conductor. Fittings in a system need to be compatible with the hydraulic fluid.

flammable

The tendency to ignite or burn when exposed to an open flame or other ignition source. The flammability of petroleum-based oil can present a fire hazard in hydraulic systems used near ignition sources.

fluid layers

A region within a fluid that forms due to shear stress. Fluid layers create friction as they slide over one another.

friction

A force that resists motion between two objects that are in contact with each other. Friction generates heat and causes increased wear.

full-film lubrication

A form of lubrication characterized by a thick film of fluid between components. Full-film lubrication provides a layer of fluid that is thicker than a component's surface imperfections.

glycol

A synthetic chemical in the same family as antifreeze. Glycol is used in water glycol fire-resistant fluids.

heavy crude oil

Liquid petroleum with a high viscosity and a high density. Heavy crude oil is thick and tends to have a higher resistance to flow.

HFB

Invert emulsion. A type of fire-resistant hydraulic fluid consisting of tiny drops of water dispersed in a continuous oil phase. HFB contains about 40% water and 60% oil.

HFC

Water glycol. A water-based fire-resistant hydraulic fluid that contains 35-45% water, synthetic chemicals, and a water-soluble thickener. HFC provides good wear resistance.

HFD

Synthetic fire-resistant fluid. A type of fire-resistant hydraulic fluid with a chemical base. HFD fluid tolerates higher operating temperatures and pressures.

high-water fluid

HFA. A fire-resistant hydraulic fluid consisting of 90-95% water and 5-10% oil. High-water fluids are an emulsion of oil droplets dispersed in water.

hydraulic fluid

A thick liquid that transfers power in a hydraulic system. Hydraulic fluid is often petroleum-based oil, synthetic, or water-based.

ignition sources

Any process or event capable of causing a fire or explosion. Ignition sources include an open flame, hot metal, or sparks.

inlet strainers

A coarse filter installed on a hydraulic pump inlet to filter fluid entering the pump. Strainers must be compatible with the hydraulic fluid in the system.

International Organization for Standardization

ISO. An international organization that establishes documented standards, rules, and guidelines to ensure that products, processes, and services are fit for their purpose. The International Organization for Standardization helped to establish the ISO viscosity grade.

invert emulsions

HFB. A type of fire-resistant hydraulic fluid consisting of tiny droplets of water dispersed in a continuous oil phase. Invert emulsions contain about 40% water and 60% oil.

iron

A silver-white metallic element. Iron is susceptible to rust.

ISO

International Organization for Standardization. An organization that establishes documented standards, rules, and guidelines to ensure that products, processes, and services are fit for their purpose. ISO helped to establish the ISO viscosity grade.

ISO viscosity grade

ISO VG. A standard system used to classify a fluid's viscosity at 104°F and 212°F. The ISO viscosity grade assigns a whole number to indicate viscosity.

kinematic viscosity

A fluid's resistance to flow when there is no external force acting on it. Kinematic viscosity is expressed in centistokes (cSt) or meters squared per second (m²/s).

light crude oil

Liquid petroleum that has a low density and a low viscosity. Light crude oil flows freely at room temperature.

lubricates

To use a substance to reduce the friction between moving parts. A substance that lubricates reduces heat and wear between moving parts.

lubricity

The ability of a fluid to reduce friction and provide lubrication. High lubricity in hydraulic fluid may be achieved by using additives.

mechanical efficiency

A hydraulic system rating that describes the amount of energy loss in a hydraulic system or individual component. Mechanical efficiency can decrease due to friction between mechanical and fluid components.

millipascal-seconds

mPa-s. An SI unit used to express the absolute viscosity of fluid. Millipascal-second units are used to express the amount of force necessary to move a layer of fluid over another layer.

natural esters

A plant-based oil used as a base for environmentally acceptable fluid. Natural esters, or vegetable oils, are highly biodegradable.

neoprene

A lightweight synthetic material, similar to rubber, that is often used to make seals in a hydraulic system. Neoprene is not compatible with some synthetic FRF.

nitrile

A rubber-like material used to make seals in a hydraulic system. Nitrile is not compatible with some synthetic FRF.

nontoxic

A substance that is less harmful to the environment than other substances. Nontoxic hydraulic fluids include polyglycols, natural esters, and synthetic esters.

oxidation

A material's chemical reaction with oxygen. Oxidation causes rust to form on metal surfaces and can cause fluid to deteriorate.

oxidation inhibitors

An additive used in hydraulic fluid to prevent oxygen from reacting with hydraulic fluid. Oxidation inhibitors act as a preservative.

oxidation stability

The ability to resist the damaging effects that occur when hydraulic fluid and oxygen react. Oxidation stability in hydraulic fluid results in a fluid that can withstand higher operating temperatures.

petroleum-based oil

A thick hydraulic fluid developed from refined crude oil. Petroleum-based oils provide good lubrication and are compatible with most seal materials in a hydraulic system.

phosphate ester

An organic chemical compound used as a base in synthetic fire-resistant fluid. Phosphate ester can be used in hydraulic systems operating in aircraft.

polyglycols

A chemical compound used as a base for environmentally acceptable hydraulic fluids. Polyglycols offer high lubricity and can tolerate low operating temperatures.

polyol ester

A chemical compound derived from animal or vegetable fats. Polyol esters provide thermal stability at high temperatures.

pressure drops

The difference in pressure between any two points of a system or a component. A pressure drop occurs when force acts on hydraulic fluid as it flows through a hydraulic system or component.

refining

The process of purifying crude oil to make it usable. Refining crude oil changes its properties.

rust

A reddish-brown substance caused by a metal's reaction with moisture and oxygen. Rust is a type of corrosion.

rust inhibitors

An additive used in hydraulic fluid to protect metal components from rust. Rust inhibitors mix with hydraulic fluid and form a protective film on metal components.

Saybolt Universal Seconds

SUS. The amount of time it takes 60 milliliters of fluid to flow through a Saybolt viscometer when there is no external force acting on it. Saybolt Universal Seconds are often used to express the kinematic viscosity of fluid.

Saybolt viscometer

An instrument used to measure the kinematic viscosity of hydraulic oil. A Saybolt viscometer consists of a capillary tube surrounded by a heated liquid that controls the temperature of the hydraulic oil.

seal

A flexible device in a hydraulic system that prevents external fluid leakage between components and blocks contaminants from entering the system. A seal must be compatible with the hydraulic fluid it comes into contact with.

sludge

A type of liquid contamination that can form in the reservoir when hydraulic fluid and water mix. Sludge is gummy liquid matter that interferes with the oil-cooling process.

spindle

A rotating component of a machine tool. The spindle on a rotational viscometer is immersed in fluid and generates torque as it rotates.

surface asperities

Microscopic peaks found on all surfaces. Contact between surface asperities can cause friction.

synthetic esters

An organic chemical compound used as a base for environmentally acceptable fluids. Synthetic esters have high thermal stability but tend to break down in the presence of water.

synthetics

HFD. A type of fire-resistant hydraulic fluid with a chemical base. Synthetics tolerate higher operating temperatures and pressures than water-based hydraulic fluids.

thermal stability

The ability of a material or substance to resist changes in physical shape or size as its temperature changes. The thermal stability of a hydraulic fluid allows it to resist oxidation and contamination.

total acid number

TAN. The measurement of acidity in a petroleum product. The total acid number indicates the fluid's ability to neutralize acids.

turbulent flow

A type of fluid flow in which fluid particles move roughly and erratically within fluid layers. Turbulent flow can cause entrained air in hydraulic fluid.

valves

An active fluid system component that regulates fluid as it travels from the pump to the actuator. Valves regulate fluid flow rate, direction, and pressure.

vegetable fats

An oil derived from plants. Vegetable fats are included in some synthetic fire-resistant fluids.

vegetable oils

A plant-based oil used as a base for environmentally acceptable fluid. Vegetable oils, or natural esters, are highly biodegradable.

viscometer

An instrument that senses the torque required to rotate a spindle at a constant speed while immersed in fluid. A viscometer measures the absolute viscosity of fluid.

viscosity

A fluid's resistance to flow at a given temperature. The viscosity of a fluid decreases as its temperature increases.

viscosity modifiers

A group of polymer molecules used in hydraulic fluid as additives to stabilize viscosity. Viscosity modifiers are sensitive to temperatures.

volumetric efficiency

The amount of leakage in a hydraulic system or individual component. Volumetric efficiency depends in part on a fluid's kinematic viscosity.

water glycol

HFC. A water-based fire-resistant hydraulic fluid that contains 35-45% water, synthetic chemicals, and a water-soluble thickener. Water glycol fluids provide good wear resistance.

water-soluble

Able to dissolve in water. Water-soluble thickeners provide improved viscosity in water glycol fluids.

zinc

A metal that resists corrosion and has a relatively low melting point. Zinc is often used in fittings for hydraulic systems.