hydraulic definitions

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HYDRAULICS: DICTIONARY OF TERMS

A

ABRASION - A wearing, grinding, or rubbing away of material. The products of abrasion will be

introduced into the system as generated particulate contamination.

ABSOLUTE - A measure having as its zero point or base the complete absence of the entity being

measured.

ABSOLUTE FILTRATION RATING -The diameter of the largest hard spherical particle that will pass

through a filter under specified test conditions. This is an indication of the largest opening in the filter

element. It does not indicate the largest particle that will pass through the element, since particles of

greater length than diameter may pass.

ABSOLUTE PRESSURE - The pressure above zero absolute, i.e., the sum of atmospheric and gage

pressure. In vacuum related work it is usually expressed in millimeters of mercury (mm Hg).

ACCUMULATOR - A container which stores fluids under pressure as a source of hydraulic power. It

may also be used as a shock absorber.

ACCUMULATOR HYDROPNEUMATIC - An accumulation in which compressed gas applies pressure

to the stored liquid.

ACTUATOR - A device which converts hydraulic power into mechanical force and motion. (Examples:

hydraulic cylinders and motors.)

AERATION - Air in the hydraulic fluid. Excessive aeration causes the fluid to appear milky and compo-

nents to operate erratically because of the compressibility of the air trapped in the fluid.

AFTERCOOLER - A device which cools a gas usually after it has been compressed.

AIR BLEEDER - A device for removal of air from a hydraulic fluid line.

AIR BREATHER - A device permitting air movement between atmosphere and the component in which

it is installed.

AIR, COMPRESSED (Pressure) - Air at any pressure greater than atmospheric pressure.

AIR, FREE - Air at ambient temperature, pressure, relative humidity and density.

AIR MOTOR - A device which converts compressed gas into mechanical force and motion. It usually

provides rotary mechanical motion.

AIR, STANDARD - Air at a temperature of 68 degrees F, a pressure of 14.7 pounds per square inch

absolute, and a relative humidity of 36% (0.075 pounds per cubic foot) In gas industries the tempera-

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ture of STANDARD AIR is usually given at 60 degrees F.

AMPLIFIER - A device for amplifying the error signal sufficiently to cause actuation of the stroke

control. Several types of servo amplifiers are used at the present time: electronic (DC,AC, phase

sensitive, and magnetic) and mechanical.

AMPLITUDE OF SOUND - The loudness of a sound.

ANALOG - Of or pertaining to the general class of devices whose output varies as a continuous func-

tion of its input.

AND 10050 - A United States Air Force-Navy Aeronautical Design Standard in which a straight thread

port is specified to attach tube fittings to various components. The fitting employs an O ring seal

compressed in a special cavity in the port.

AND DEVICE - A control device which has its output in the logical 1 state if and only if all thecontrol signals assume the logical 1 state.

ANILINE POINT - The lowest temperature at which a liquid is completely miscible with an equal

volume of freshly distilled aniline (ASTM designation D664).

ANNULAR AREA - A ring shaped area - often refers to the net effective area of the rod side of a

cylinder piston, i.e., the piston area minus the cross sectional area of the rod.

AREA - A surface enclosed by a specific boundary.

ASSURANCE LEVEL -The minimum percentage of pressure containing devices of a verified designthat will sustain 10 million applications of its Rated Fatigue Pressure.

ATMOSPHERE (ONE) - A pressure measure equal to 14.7 psi.

ATMOSPHERE, TECHNICAL - A unit of pressure used in Germany and equal to 1 Kp/cm 2 (kilopound

per square centimeter), approximately equal to 14.2 psig.

ATMOSPHERIC PRESSURE - Pressure exerted by the atmosphere at any specific location. (Sea level

pressure is approximately 14.7 pounds per square inch absolute.)

AUTOMATIC COUNT - A particle count obtained by an electromechanical or electronic device asopposed to visual microscopic counting technique.

B

BACK CONNECTED - A condition where pipe connections are on normally unexposed surfaces of

hydraulic equipment. (Gasket mounted units are back connected.)

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BACKGROUND CONTAMINATION - The total of the extraneous particles which are introduced in

the process of obtaining, storing, moving, transferring and analyzing a fluid sample.

BACK PRESSURE - A pressure in series. Usually refers to pressure existing on the discharge side of aload. It adds to the pressure required to move the load.

BACK-UP RING (ANTI-EXTRUSION RING) (JUNKET RING) (BULL RING) - A ring which

bridges a clearance to minimize seal extrusion.

BAFFLE - A device, usually a plate, installed in a reservoir to separate the pump inlet from return lines.

BAR - A unit of pressure based on Newtons per square meter, approximately equal to 14.5 psig. This

unit is not preferred in SI metrics.

BERNOULLIS THEORY - If no work is done on or by a flowing, frictionless liquid, its energy, due topressure and velocity, remains constant at all points along the streamline.

BISTABLE - Of or pertaining to the general class of fluidic devices which maintain either of two posi-

tion operating states in the presence or absence of the setting input.

BLEED - The process by which air is removed from a hydraulic system.

BLEED-OFF - To divert a specific controllable portion of pump delivery directly to reservoir.

BOYLES LAW - The absolute pressure of a fixed mass of gas varies inversely as the volume, provided

the temperature remains constant.

BREATHER - A device which permits air to move in and out of a container or component to maintain

atmospheric pressure.

BRIDGING - A condition of filter element loading in which contaminant spans the space between

adjacent sections of a filter element thus blocking a portion of the useful filtration area.

BULK MODULUS - The measure of resistance to compressibility of a fluid. The reciprocal of the

compressibility of this fluid.

BURST PRESSURE - The pressure which causes rupture. Also, the inside-out differential pressure thatcauses outward structural failure.

BYPASS - A secondary passage for fluid flow.

C

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CARTRIDGE -

1. The replaceable element of a fluid filter.

2. The pumping unit from a vane pump, composed of the rotor, ring, vanes and one or both sideplates.

CASE DRAIN LINE - A line conducting fluid from a component housing to the reservoir.

CAVITATION - Air pockets in the oil circuit (as at the pump inlet). A localized gaseous condition

within a liquid stream which occurs where the pressure is reduced to the vapor pressure.

CELSIUS - A temperature scale. 0 Celsius (or 0 Centigrade) is the freezing point of water (32 degrees

F)

CENTIPOISE - A unit of absolute (dynamic) viscosity.

CENTISTOKE - A unit of kinematic viscosity.

CENTRIFUGE VOLUME - The volume of liquid or solid or both, separated from a volume of liquid

exposed to centrifugal force.

CHAMBER - A compartment within a hydraulic unit, May contain elements to aid in operation or

control of a unit. Examples: spring chamber, drain chamber, etc.

CHANNEL - A fluid passage, the length of which is large with respect to its cross-sectional dimension.

CHARGE (SUPERCHARGE) -

1. To replenish a hydraulic system above atmospheric pressure.

2. To fill an accumulator with fluid under pressure (see precharge pressure).

CHARGE PRESSURE - The pressure at which replenishing fluid is forced into the hydraulic system

(above atmospheric pressure).

CHARLES LAW - The volume of a fixed mass of gas varies directly with absolute temperature, pro-

vided the pressure remains constant.

CHECK VALVE - A valve which permits flow of fluid in one direction only.

CHOKE - A restriction, the length of which is large with respect to its cross-sectional dimension.

CIRCUIT - An arrangement of components interconnected to perform a specific function within a sys-

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CIRCUIT, REGENERATIVE - A circuit in which pressurized fluid discharged from a component is

returned to the system to reduce power input requirements. On single rod end cylinders the discharge

from the rod end is often directed to the bore end to increase rod extension speed.

CIRCUIT, SEQUENCE - A circuit which establishes the order in which two or more functions of a

circuit occur.

CIRCUIT, SERVO - A closed loop circuit which is controlled by some type of feedback; i.e., the output

of the system is sensed or measured and is compared with the input. The actual output and the input

controls the circuit. The system output may be position, velocity, force, pressure, level flow rate, or

temperature, etc.

CLEANLINESS, LEVEL - The analogue of contamination level.

CLEVIS (HINGE) PENDULUM - A U shaped mounting device which Contains a common pin holeat right angle to the axis of symmetry through each arm of the U. A clevis usually connects with an

eye.

CLOSED CENTER CIRCUIT- One in which flow through the system is blocked in neutral and pres-

sure is maintained at the maximum pressure control setting.

CLOSED CENTER SYSTEM - A hydraulic system in which the control valves are closed during

neutral, stopping oil flow. Flow in this system is varied, but pressure remains constant.

CLOSED CENTER VALVE - One in which all ports are blocked in the center or neutral position.

CLOSED LOOP - A system in which the output of one or more elements is compared to some other

signal to provide an actuating signal to control the output of the loop.

COLLAPSE PRESSURE - The outside-iD differential pressure that causes Structural failure.

COMMAND SIGNAL (Or input signal) - An external signal to which the servo must respond.

COMPENSATOR CONTROL -A displacement control for variable pumps and motors which alters

displacement in response to pressure changes in the system as related to its adjusted pressure setting.

COMPONENTS - A single hydraulic unit.

COMPRESSIBILITY - The change in volume of a unit volume of a fluid when it is subjected to a unit

change in pressure.

COMPRESSOR - A device which converts mechanical force and motion into pneumatic fluid power.

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CONDUCTOR - A component whose primary function is to contain and direct fluid.

CONDUIT - Any confining element employed to transfer.

CONTAMINATION LEVEL - A quantitative term specifying the degree of contamination.

CONTAMINANT, ARTIFICIAL - Contaminant of known composition and particle size distribution

which is introduced into fluid systems or fluid systems components for test purposes. The most com-

monly used artificial contaminants include standardized fine air cleaner test dust, standardized coarse

air cleaner test dust, carbonyl iron, glass beads, cotton linters, red iron oxide and black iron oxide.

CONTAMINANT, BUILT-IN - Initial residual contamination in a component, fluid, or system. Typical

built-in contaminants are burrs, chips, flash, dirt, dust, fiber, sand moisture, pipe dope, weld spatter,

paints and solvents, flushing solutions, incompatible fluids and operating fluid impurities.

CONTINUITY GENERATED - Contamination created by the operation of a fluid system or compo-nent. Generated contaminants are products of erosion, fretting, scouring, wear, corrosion, decomposi-

tion, oxidation, and fluid-breakdown. Air bubbles may also be generated under some operating condi-

tions.

CONTINUITY EQUATION - The mass rate of fluid flow into a fixed space is equal to the mass flow

rate out. Hence, the mass flow rate of fluid past all cross sections of a conduit is equal.

CONTROL - A device used to regulate the function of a unit (see Hydraulic Control, Manual Control,

Mechanical Control, and Compensator Control).

CONTROL, SERVO - A control actuated by a feedback system which compares the output with thereference signal and makes corrections the reduce the difference.

COOLER - A heat exchanger which removes heat from a fluid. (See Heat Exchanger.)

CORROSION - The chemical change in the mechanical elements caused by the interaction of fluid or

contaminants, or both. More specifically related to chemical changes in metals. The products of

change may be introduced into the system as generated particulate contamination.

COUNTERBALANCE VALVE -A pressure control valve which maintains back pressure to prevent a

load from falling.

COUNTING CALIBRATION FACTOR - Ratio of the effective filtration area on the test pad membrane

to the area counted, SAE-ARP-598.

CRACKING PRESSURE - The pressure at which a pressure actuated valve begins to pass fluid.

CUSHION - A device sometimes built into the end of a cylinder which restricts outlet flow and thereby

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slows down the Piston.

CUSHION, CYLINDER - A cushion built into a cylinder to restrict flow at the outlet port thereby

arresting the motion of the piston rod.

CYCLE - A single complete operation of a component which begins and ends in a neutral position.

CYLINDER - A device for converting fluid power into linear or circular motion. An actuator. Basic

design types include piston and vane units.

Double-Acting Cylinder- A cylinder in which fluid force can be applied to the movable element in

either direction.

Double-Rod Cylinder- A cylinder with a single piston and a piston rod extending from each end.

Dual-Stroke Cylinder- A cylinder combination which provides two working strokes.

Piston-Type Cylinder- A cylinder which uses a sliding piston in a housing to produce straight move-

ment.

Non-Rotating Cylinder- A cylinder in which relative rotation of the cylinder housing and the piston

and piston rod, plunger or ram, is not recommended.

Plunger (RAM) Cylinder- A cylinder in which the piston has the same cross-sectional area as the

piston rod.

Rotary Cylinder- A cylinder in which fluid force can be applied to the movable element in only onedirection.

Rotating Cylinder- A cylinder in which relative rotation of the cylinder housing and the piston and

piston rod, plunger or ram, is recommended.

Single Rod Cylinder- A cylinder with a piston rod extending from one end.

Single Acting Cylinder- A cylinder in which fluid force can be applied to the movable element in only

one direction.

Tandem Cylinder- Two or more cylinders With interconnected rod and piston assemblies.

Telescoping Cylinder- A cylinder with nested multiple tubular rod segments which provide a long

working stroke in a short retracted envelope.

Vane-Type Cylinder- A cylinder which uses a turning vane in a circular housing to produce rotary

movement.

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D

DAMPING PARAMETER - A measure of the time required as a function of the maximum pressure

excursion of the power supply output to attain essentially steady state operation after an abruptdisturbance. Specifically, it is the transient recovery time divided by the maximum excursion.

DARCYS FORMULA - A formula used to determine the pressure drop due to flow friction through a

conduit.

DEADBAND - The region or band of no response where an error signal will not cause a corresponding

actuation of the controlled variable.

DECAY - A failing pressure.

DECAY RATE - The ratio of pressure decay to time.

DECOMPOSITION - Separation by chemical change into constituent parts, elements, or different

compounds. More specifically related to fluid and seat chemical changes. The materials affected are

primarily organic in nature. The products of change may be introduced into the system as contamina-

tion.

DECOMPRESSION - The slow release of confined fluid to gradually reduce pressure on the fluid.

DECONTAMINATION - The process of removing unwanted material or substance; the reduction of

contamination to an acceptable level.

DELIVERY - The volume of fluid discharged by a pump in a given time, usually expressed in gallons

per minute

DENSITY - A unit of mass per unit volume.

DEPTH (FILTER) - A filter medium which primarily retains contaminant within tortuous passages.

DE-VENT - To close the vent connection of a pressure control valve permitting the valve to function at

its adjusted pressure setting.

DIAGRAM - A drawing which illustrates pertinent characteristics, component positions, sizes, inter-connection, controls and actuation of components and fluid power circuits.

DIAGRAM, COMBINATION - A drawing utilizing a combination of graphical, cutaway and pictorial

symbols showing interconnected lines.

DIAGRAM, CUTAWAY - A drawing showing principle internal parts of all components, controls and

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actuating mechanisms, all interconnecting lines and functions of individual components.

DIAGRAM, GRAPHICAL (SCHEMATIC) - A drawing showing each piece of apparatus including all

interconnecting lines by means of approved ANSI or ISO standard symbols.

DIAGRAM, PICTORIAL - A drawing showing each component in its actual shape according to the

manufacturers installation drawings.

DIFFERENTIAL CURRENT - The algebraic summation of the current in the torque motor; measured

in MA (milliamperes).

DIFFERENTIAL CYLINDER - Any cylinder in which the two opposed piston areas are not equal.

DIGITAL - Of or pertaining to the general class of devices or circuits whose output varies in discrete

steps (i.e., pulses or on-off characteristics).

DIRECTIONAL VALVE - A valve which selectively directs or prevents fluid flow to desired channels.

DIRT CAPACITY (DUST CAPACITY) (CONTAMINANT CAPACITY) - The weight of a specified

artificial contaminant which must be added to the fluid to produce a given differential pressure across

a filter at specified conditions. Used as an indication of relative service life.

DISSOLVED AIR - Air which is dispersed in a fluid to form a mixture.

DISSOLVED WATER - Water which is dispersed in the fluid to form a mixture.

DISPLACEMENT - The volume of oil displaced by one complete stroke or revolution (of a pump,motor, or cylinder).

DITHER - A low amplitude, relatively high frequency periodic electrical signal, sometimes superim-

posed on the servo valve input to improve system resolution. Dither is expressed by the dither fre-

quency (Hz) and the peak-to-peak dither current amplitude (ma).

DOUBLE ACTING CYLINDER - A cylinder in which fluid force can be applied to the movable ele-

ment in either direction.

DRAIN - A passage in, or a line from, a hydraulic component which returns leakage fluid independently

to reservoir or to a vented manifold.

DRIFT - Motion of a cylinder or motor due to internal leakage past components in the hydraulic system.

DUROMETER HARDNESS - An arbitrary indication of hardness of rubber or plastic determined by an

indenter.

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E

EDGE (FILTER) - A filter medium whose passages are formed by the adjacent surfaces of stacked

discs, edgewound ribbons, or single-layer filaments.

EFFICIENCY - The ratio of output to input. Volumetric efficiency of a pump is the actual output in

gpm divided by the theoretical or design output. The overall efficiency of a hydraulic system is the

output power divided by the input power. Efficiency is usually expressed as a percent.

EFFICIENCY (FILTER) The ability, expressed as a percent, of a filter to remove specified artificial

contaminant at a given contaminant concentration under specified test conditions.

EFFLUENT - The fluid leaving a component.

ELECTRICAL QUIESCENT POWER - The power required for differential operation of the servovalve

when the current through each coil is equal and opposite in polarity.

ELECTRO-HYDRAULIC SERVO VALVE - A directional type valve which receives a variable or

controlled electrical signal and which controls or meters hydraulic flow.

ELEMENT (CARTRIDGE) - The porous device which performs the actual process of filtration.

EMULSION - A stabilized mixture of two immiscible components, such as water and oil. It may contain

additives.

EMULSION, OIL IN WATER - A dispersion of oil in a continuous phase of water.

EMULSION, WATER IN OIL - A dispersion of water in a continuous phase of oil.

ENCLOSURE - A rectangle drawn around a graphical component or components to indicate the limits

of an assembly.

ENCLOSURE - A housing for components.

ENERGY - The capacity for doing work. (See Joule.)

ENERGY - Three types of energy are available in modern hydraulics (of the normal hydrostatic pipe).

1. Potential Energy - Pressure energy. The static energy of oil which is standing but is pressurized and

ready to do work. Example: oil in a loaded accumulator.

2.Heat Energy - Friction or resistance to flow. (An energy loss in terms of output.) Example: friction

between moving oil and the confines of lines or passages produces heat energy.

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3. Kinetic Energy - The energy of the moving liquid. Varies with the velocity (speed) of the liquid.

ENTRAINED AIR - A mechanical mixture of bubbles having a tendency to separate from the liquid

phase.

EROSION - The loss of material in mechanical elements caused by the impingement of fluid or fluid

suspended particulate matter, or both. The product of erosion will be introduced into the system as

additional generated particulate contamination.

ERROR (signal) - The signal which is the algebraic summation of an input signal and a feedback signal.

F

FAHRENHEIT - The thermometric scale on which, under standard atmospheric pressure, the boiling

point of water is at 212 degrees above the zero of the scale and the freezing point is 32 degrees above

that of zero.

FEEDBACK (or feedback signal) - The output signal from a feedback element.

FEEDBACK LOOP - Any closed circuit consisting of one or more forward elements and one or more

feedback elements.

FIBER - For the purpose of microscopic particle counting a fiber is a particle whose length is greater

than 100 micrometers but at least ten times greater than its width.

FILLER RING - A ring which fills the recess of a V or U type seal.

FILTERABLE SOLIDS - The solids retained on a membrane for analysis by weight, count, or observa-

tion as it applies to the section on contamination measurement.

FITTING - A connector or closure for fluid power lines and passages.

FITTING, BUSHING - A short, externally threaded connector with a smaller size internal thread.

FITTING, CLOSURE - A cap or a plug.

FITTING, COMPRESSION - A fitting which seals and grips by manual adjustment deformation.

FITTING, CONNECTOR - A fitting for joining a conductor to a component port or to one or more

other conductors.

FITTING, FLARED - A fitting which seals and grips by a performed flare at the end of the tube.

FITTING, FLARELESS - A fitting which seals and grips by some means other than a flare.

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FITTING, REUSABLE HOSE - A hose fitting that can be removed from a hose and reused.

FILTER (OIL) - A device which removes solids from a fluid.

FLANGE - A mounting device usually consisting of a plate or collar.

FLAPPER ACTION - A valve design in which output control pressure is regulated by a pivoted flapper

in relation to one or two orifices.

FLASH POINT - The temperature to which a liquid must be heated under specified conditions of the

test method to give off sufficient vapor to form a mixture that can be ignited momentarily by a speci-

fied flame.

FLIP-FLOP - A digital component or circuit with two stable states and sufficient hysterises so that it has

memory. When the state is changed with a control pulse, a continuous control signal is now neces-sary for it to remain in that state.

FLOODED - A condition in which the pump inlet is charged by placing the reservoir oil level above the

pump inlet port.

FLOW CONTROL VALVE - A valve which controls the rate of oil flow.

FLOW, LAMINAR (STREAMLINED) - A flow situation in which the fluid particles move in parallel

lamina or layers. (See Reynolds Number.)

FLOW METER - A testing device which gauges either flow rate, total flow, or both.

FLOW, METERED - Flow at a controlled rate.

FLOW RATE - The volume of fluid passing a point in a given time.

FLOW, TURBULENT - A flow situation in which the fluid particles move in a random manner (See

Reynolds Number).

FLUID

1. A liquid or gas.

2. A liquid that is specially compounded for use as a power-transmitting medium in a hydraulic sys-

tem.

FLUID, FATTY OIL - A fluid composed of fats derived from animal, marine, or vegetable origin. It

may contain additives.

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FLUID, FIRE RESISTANT (NON-FLAMMABLE)* - A fluid difficult to ignite and which shows little

tendency to propagate flame.

*Depreciated.

FLUID FRICTION - Friction due to the viscosity of fluid.

FLUID, HYDRAULIC - A fluid suitable for use in a hydraulic system.

FLUIDIC - Of or pertaining to devices, systems, assemblies, etc., utilizing fluidic components.

FLUIDICS - Engineering science pertaining to the use of fluid dynamic phenomenon to sense, control,

process information, and/or actuate.

FLUID, PNEUMATIC - A fluid suitable for use in a pneumatic system.

FLUID POWER - Energy transmitted and controlled through use of a pressurized fluid.

FLUID POWER SYSTEM - A system that transmits and controls power through use of a pressurized

fluid within an enclosed circuit.

FOLLOW VALVE - A control valve which ports oil to an actuator so the resulting output motion is

proportional to the input motion to the valve.

FORCE - A push or pull acting upon a body. In a hydraulic cylinder, it is the product of the pressure on

the fluid; multiplied by the effective area of the cylinder piston. It is measured in pounds or tons.

F=PxA

FORCE MOTOR - A type of electromechanical transducer having linear motion used in the input stages

of servovalves.

FOUR-WAY VALVE - A directional valve having four flow paths.

FREE AIR - Any compressible air trapped within a hydraulic system that does not dissolve to form a

part of the system fluid.

FREE WATER - Water droplets or globules in the system fluid that tend to accumulate at the bottom or

top of the system fluid depending on the fluids specific gravity.

FREQUENCY - The number of times an action occurs in a unit of time. Frequency is the basis of all

sound. A pump or motors basic frequency is equal to its speed in revolutions per second multiplied by

the number of pumping chambers.

FRETTING - A type of wear resulting from minute reciprocal sliding motion which produces fine

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particulate contamination without chemical change.

FRETTING CORROSION - Oxidation of fretting wear debris.

FRICTION - The resistance to fluid flow in a hydraulic system (An energy loss in terms of poweroutput.)

FRONT CONNECTED - A condition wherein piping connections are on normally exposed surfaces of

hydraulic components.

FULL FLOW - In a filter, the condition where all the fluid must pass through the filter element or

medium.

G

GAGE - An instrument or device for measuring indication, or comparing a physical characteristic.

GAGE, FLUID LEVEL - A gage which indicates the fluid level.

GAGE MANOMETER - A differential pressure gage in which pressure is indicated by the height of a

liquid column of known density. Pressure is equal to the difference in vertical height between two

connected columns multiplied by the density of the manometer liquid. Some forms of manometers are

U tube, inclined tube, well, and bell types.

GAGE, PRESSURE - A gage which indicates the pressure in the system to which it is connected.

GAGE PRESSURE - A pressure scale which ignores atmospheric pressure. Its zero point is 14.7 psiabsolute.

GAGE, VACUUM - A pressure gage for pressures less than atmospheric.

GEL - A colloidal solution in which the intermingling of internal and external phases results in a system

viscosity greater than the viscosity of the external phase alone, i.e., a jelly-like substance.

GLAND - The cavity of a stuffing box.

GRAVIMETRIC VALUE - The weight of suspended solids per unit volume of fluid. A method employ-

ing membrane filters for this determination is outlined in Society of Automotive Engineers AerospaceRecommended Practices - 785.

H

HAGEN-POISEUILLE LAW - The friction factor of Darcys Formula or the ratio of 64 to Reynolds

Numbers when flow is laminar.

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HEAD - The height of a column or body of fluid above a given point expressed in linear units.

HEAD, STATIC - The height of a column or body of fluid above a given point.

HEAD, STATIC DISCHARGE - The static head from the centerline of the pump to the free discharge

surface.

HEAD, VELOCITY - The equivalent head through which the liquid would have to fall to attain a given

velocity. Mathematically, it is equal to the square of the velocity (in feet) divided by 64.4 feet per

second squared.

HEAT -

1. Added energy that causes substances to rise in temperature, fuse, evaporate, expand, or undergo

any of various other related changes, that flows to a body by contact with or by radiation from bodiesat higher temperatures, and can be produced in a body (as by compression).

2. The energy associated with the random motions of the molecules, atoms, or smaller structural units

of which matter is composed (See Joule).

HEAT EXCHANGER - A Device which transfers heat through a conducting wall from one fluid to

another. (See Cooler [Oil].)

HERTZ - A unit of frequency formerly expressed as cycles per second. 1 Hz=1 CPS.

HORSEPOWER The work produced per unit of time.

HOSE - A flexible line or conduit

HOSE, WIRE BRAIDED - Hose consisting of a flexible material reinforced with woven wire braid.

HYDRAULIC BALANCE - A condition of equal opposed hydraulic forces acting on a part in a hy-

draulic component.

HYDRAULIC CONTROL - A control which is actuated by hydraulically induced forces.

HYDRAULIC HORSEPOWER - Horsepower computed from flow rate and pressure differential.

HYDRAULIC MOTOR - A device which converts hydraulic fluid power into mechanical force and

motion by transfer of flow under pressure. It usually provides rotary mechanical motion.

HYDRAULIC PUMP - A device which converts mechanical force and motion into hydraulic fluid

power by means of producing flow.

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HYDRAULIC PUMP, POSITIVE DISPLACEMENT - A pump that for each cycle or revolution,

positively displaces (usually by mechanical means) a specific amount.

HYDRAULICS - The engineering science of liquid pressure and flow. (In this manual, our main interestis in oil hydraulics as applied to produce work in linear and rotary planes.)

Hydrodynamics - The engineering science of the energy of liquid pressure and flow.

Hydrokinetics - The engineering science pertaining to the energy of liquids in motion.

Hydropneumatics - Pertaining to the combination of hydraulic and pneumatic fluid power.

Hydrostatics - The engineering science of the energy of liquids at rest . (All the systems covered in this

manual operate on the hydrostatic principle.)

I

IMPINGEMENT - The direct impact of fluid flow upon or against a surface.

INCOMPATIBLE FLUIDS - Fluids which when mixed in a system, will have a deleterious effect on that

system, its components or its operation.

INDICATOR - A device which provides external visual evidence of sensed phenomena.

INDICATOR, DIFFERENTIAL PRESSURE - An indicator which signals the difference in pressure

between two points.

INFLUENT - The fluid entering a component.

INTENSIFIER - A device which converts low pressure fluid power to higher pressure fluid power.

INTERCOOLER - A device which cools a gas between the compressive steps of a multiple stage

compressor.

INTERFACE - A point component where a transition is made between power levels, modes of opera-

tion, etc.

JET ACTION - A value design type in which flow effect is controlled by the relative position of a

nozzle and a receiver.

JOINT, ROTARY - A joint connecting lines which rotate in relation to each other.

JOINT, SWIVEL - A joint which permits variable operational positioning of lines.

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JOULE -A unit of work, energy, or heat. lJ(joule)=1 Nm (Newton meter).

JOULE PER SECOND- A unit of power, i.e., the rate of doing work.

K

KELVIN - A temperature scale in the metric system (See Celsius, the preferred system).

KILOGRAM (KG)- A unit of mass. 1 kilogram = 2.2 pounds.

KILOGRAM - METER PER SECOND - See Newton.

KILOPOND - A unit of force used in Germany and equal to I KG(f) (kilogram, force).

KINETIC ENERGY - Energy that a substance or body has by virtue of its mass (weight) and velocity.

L

LAMINAR (FLOW) - A condition where the fluid particles move in continuous parallel paths. Stream-

line flow.

LANTERN RING (SEAL CAGE) - A ring in line with a port in a gland to introduce a lubricant or a

coolant to the packing or stuffing box.

LEVERAGE - A gain in output force over input force by sacrificing the distance moved. Mechanical

advantage or force multiplication.

LIFT - The height of a column or body of fluid below a given point expressed in linear units.

The height a body or column of fluid is raised; for instance, from the reservoir to the pump inlet.

Lift is sometimes used to express a negative pressure or vacuum. The opposite of head.

LINE - A tube, pipe, or hose for conducting a fluid.

LINEAR ACTUATOR - A device for converting hydraulic energy into linear motion -- a cylinder or

ram.

LINEAR SCALE RETICULE - An optical glass having a straight scale marked to permit measurementor estimation of distance or length.

LINE,DRAIN - A line returning drain fluid independently to the reservoir or vented manifold.

LINE, EXHAUST - A line returning power or control fluid back to the reservoir or atmosphere.

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LINE, PILOT - A line which conducts control fluid.

LINE, SUCTION - A supply line at sub-atmospheric pressure to a pump, compressor, or other compo-

nent.

LITER - A unit of volume, 3.785 liters = 1 U.S. gallon.

LITERS PER MINUTE - A unit of volumetric flow rate.

LITER PER REVOLUTION - A unit of volumetric capacity.

LOGIC DEVICES - The general category of components which perform logic functions; for example

AND, NAND OR and NOR. They can permit or inhibit signal transmission with certain combinations

of control signals.

LOGIC STATE - Signal levels in logic devices are characterized by two stable states, the logical 1 (one)state and the logical 0 (zero) state. The designation of the two states is chosen arbitrarily. Commonly

the logical 1 state represents an on signal and the 0 state represents an off signal.

LUBRICATOR - A device which adds controlled or metered amounts of lubricant into a pneumatic

system.

LUG (FOOT) - A mounting device consisting of a block extending past the basic cylinder profile. The

block usually has a tapped or through mounting hole at right angles to the cylinder axis.

M

MANIFOLD - A fluid conductor which provides many ports.

MANIFOLD, VENTED - A manifold which is open to the atmosphere and returns fluid to the reservoir.

MANOMETER - An instrument (as a pressure gage) for measuring the pressure or fluids.

MANUAL CONTROL - A control actuated by the operator, regardless of the means of actuation.

Example; Lever or foot pedal control for directional valves.

MANUAL OVERRIDE - A means of manually actuating an automatically controlled device.

MASS - The property of a body that is a measure of its inertia, that is commonly taken as a measure of

the amount of material it contains and causes it to have weight in a gravitational field, and that along

with length and time constitutes one of the fundamental quantities on which all physical measurements

are based. (See Kilogram.)

MAXIMUM EXCURSION - The maximum pressure deviation from the operating pressure after an

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abrupt disturbance.

MAXIMUM INLET PRESSURE - The maximum rated pressure applied to the inlet port of a device.

MAXIMUM PRESSURE VALVE - See relief valve.

MEAN FILTRATION RATING - A measurement of the average size of the pores of a filter medium.

MECHANICAL CONTROL - Any control actuated by linkages, gears, screws, cams or other mechani-

cal elements.

METER - A unit of length, 1 meter=39.37 inches.

METER - To regulate the amount or rate of fluid flow.

METER, FLOW - A device which indicates either flow rate, total flow, or a combination of both.

METER-IN - To regulate the amount of fluid flow into an actuator system.

METER-OUT - To regulate the flow of the discharge fluid from an actuator or system.

METRIC SYSTEM - A decimal system based on the meter and kilogram, which varies somewhat

between countries (See SI Metric System).

MICRON - One millionth of a meter or about .00004 inch.

MICRON RATING - The size of the particles a filter will remove.

MICROSCOPIC - Particles whose diameter is below the threshold of normal vision, below forty mi-

crometers for most individuals.

MIGRATION - Contaminant released downstream.

MOTOR (Hydraulic) - A device for converting fluid energy into mechanical force and motion - usually

rotary motion. Basic design types include gear, vane, and piston units.

MUFFLER - A device for reducing gas glow noise. Noise is decreased by back pressure control of gas

expansion.

N

NAND DEVICE - A control device which has its output in the logical 0 state if and only if all the

control signals assume the logical 1 state.

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NEUTRALIZATION NUMBER - A measure of the total acidity or basicity of a fluid; this includes

organic and inorganic acids or bases, or a combination thereof. (ASTM Designation D974-64).

NEWT - The standard unit of kinematic viscosity in the English system. It is expressed in square inches

per second.

NEWTON - A unit of force based on the unit of mass, Kg (kilogram) multiplied by the acceleration, M/

S2 (meters per second) which produces KGM/S2, called the Newton.

NEWTON-METER - A unit of torque.

NIPPLE - A short length of pipe or tube.

NOISE, FLUIDIC - RMS of random pressure variations with respect to the operating pressure defined

in terms of a signal-to-noise ratio.

NOMINAL FILTRATION RATING - An arbitrary micrometer value indicated by the filter manufac-

turer. Due to lack of reproducibility, this rating is depreciated.

NOR DEVICE - A control device which has its output in the logical 1 state if and only if all the control

signals assume the logical 0 state.

NOT DEVICE - A control device which has its output in the logical 1 state if and only if the control

signal assumes the logical 0 state. The NOT device is a single input NOR device.

0

OPEN CENTER CIRCUIT - One in which pump delivery flows freely through the system and back to

the reservoir in neutral.

OPEN CENTER VALVE - One in which all ports are interconnected and open to each other in the

center or neutral position.

OPEN CENTER SYSTEM - A hydraulic system in which the control valves are open to continuous oil

flow, even in neutral. Pressure in this system is varied, but flow remains constant.

OPERATING BAND - The range of pressures above and below the operating pressure within which it

is desired to keep the supply output.

OPTICAL DENSITY - A method of expressing degree of contamination of a fluid by removal of

contaminant by filtration and measuring change in optical transmission of the filter disc or fluid, or

both.

OR DEVICE - A control device which has its output in the logical 0 state if and only if all the control

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signals assume the logical 0 state.

ORIFICE -A restricted passage in a hydraulic circuit. Usually a small drilled hole to limit flow or to

create a pressure differential in a circuit.

OUTPUT STATE -The final state of hydraulic amplification used in the servovalve.

OXIDATION - The interaction of air and moisture on the surface of a substance.

P

PACKING - Any material or device which seals by compression. Common types are U-packings, Cup

packings, and O-rings.

PANEL -A Plate or surface for mounting components.

PANELMOUNTING - A panel on which a number of components may be mounted.

PARTICLE - A minute piece of matter with observable length, width, and thickness; usually measured in

micrometers.

PARTICLE COUNT BLANK- An allowance for the determinable background contamination.

PASCAL - Unit of pressure in the metric (SI) system.

PASCALS LAW - A pressure applied to a confined fluid at rest is transmitted with equal intensity

throughout the liquid and that pressure is considered to act at right angles to each surface contactedby the fluid.

PASSAGE - A machined or cored fluid conducting path which lies within or passes through a compo-

nent.

PATCH TEST - Any method of evaluating fluid contamination wherein the sample is passed through a

standardized laboratory filter, and the change in color, reflectivity, etc., of the laboratory filter is

compared with previously established standards.

PERMEABILITY - The relationship of flow per unit area to differential pressure across a filter medium.

PETROLEUM FLUID - A fluid composed of petroleum oil. It may contain additives.

PHASE - A distinct functional operation during a cycle. Some typical sequential phases are: neutral,

rapid advance, feed or pressure stroke, dwell and rapid return.

PILOT LINE - A line conducting pilot fluid to a control device or devices.

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PILOT PRESSURE - Auxiliary pressure used to actuate or control hydraulic components.

PILOT VALVE - An auxiliary valve used to control the operation of another valve. The controlling

stage of a 2-stage valve.

PIPE - A line whose outside diameter is standardized for threading.

PIPE, THREAD - Screw threads for joining pipe.

PIPE THREAD, DRYSEAL - Pipe threads in which sealing is a function of root and crest interference.

PIPE THREAD, TAPERED - Pipe threads in which the pitch diameter follows a helical cone to provide

interference in tightening.

PISTON - A cylindrical part which moves or reciprocates in a cylinder and transmits or receives motionto do work.

PLUNGER - A cylindrically shaped part which has only one diameter and is used to transmit thrust. A

ram.

PNEUMATICS - Engineering science pertaining to gaseous pressure and flow.

POISE - The standard unit of absolute viscosity in the c.g.s. (centimeter-gram-second) system. It is the

ratio of the shearing stress to the shear rate of a fluid and is expressed in dyne seconds per square

centimeter; 1 centipoise equals .01 poise.

POPPET - That part of certain valves which prevents flow when it closes against a seat.

PORE SIZE DISTRIBUTION -The ratio of the number of holes of a given size to the total number of

holes per unit area expressed as a percent and as a function of hole size.

POROSITY (VOID FRACTION) - The ratio of pore volume to total volume to a filter medium ex-

pressed as a percent.

PORT - The open end of a fluid passage. May be within or at the surface of a component.

POSITIVE DISPLACEMENT -A characteristic of a pump or motor which has the inlet positivelysealed from the outlet so that fluid cannot recirculate in the component.

POTENTIOMETER - A control element in the servo system which measures and controls electrical

potential.

POUR POINT - The lowest temperature at which a fluid will flow under specific conditions.

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POWER - Work per unit time. Measured in horsepower (hp) or watts (Joules per second).

POWER BEYOND - An adapting sleeve which opens a passage from once circuit to another. Often

installed in a valve port which is normally plugged.

POWER PACK - An integral power supply unit usually containing a pump, reservoir, relief valve and

directional control.

POWER UNIT, HYDRAULIC - A combination of components to facilitate fluid storage and condition-

ing, and delivery of the fluid under conditions of controlled pressure and flow to the discharge port of

the pump, including maximum pressure controls and sensing devices when applicable. Circuitry

components, although sometimes mounted on the reservoir, are not considered part of the power unit.

PRECIPITATE - Particles separated from a fluid as a result of a chemical or physical change.

PRECIPITATION NUMBER - The number of milliliters of precipitate formed when 10 ml. of lubricat-

ing oil are mixed with 90 ml. of ASTM preparation naptha and centrifuged under prescribed condi-

tions (ASTM Designation D91-61).

PREFERRED NUMBERS - A numerical series derived from a geometric progression, frequently used

to select and standardize sizes for a series of things. It is sometimes referred to as the R series in

favor of its developer, Col. Charles Renard, a French engineer.

PRESSURE - Force of a fluid per unit area, usually expressed in pounds per square inch (psi).

Absolute Pressure - The pressure above absolute, i.e., the sum of atmospheric and gage pressure. Invacuum related work it maybe expressed in millimeters of mercury (mm Hg).

Atmospheric Pressure - Pressure exerted by the atmosphere at any specific location (Sea Level pres-

sure is approximately 14.7 pounds per square inch, absolute.)

Back Pressure - The pressure encountered on the return side of a system.

Breakout Pressure - The minimum pressure which starts moving an actuator.

Cracking Pressure - The pressure at which a relief valve, etc., begins to open and pass fluid.

Differential Pressure - The difference in pressure between any two points in a system or a component.

(Also called a pressure drop.)

Full flow Pressure - The pressure at which a valve is wide open and passes its full flow.

Gage Pressure - Pressure differential above or below atmospheric pressure.

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Operating Pressure - The pressure at which a system is normally operated.

Pilot Pressure - Auxiliary pressure used to actuate or control a component.

Precharge Pressure - The pressure of compressed gas in an accumulator prior to the admission of

liquid.

Proof Pressure - The non-destructive test pressure, in excess of the maximum rated operating pres-

sure, which causes no permanent deformation, external leakage, or other malfunction.

Rated Pressure - The operating pressure which is recommended for a component or a system by the

manufacturer.

Rated Fatigue Pressure - The pressure that a pressure containing component is represented to sustain

10 million times without failure.

Rated Static Pressure - The pressure that a component can withstand without failure.

Static Pressure - The pressure in a fluid at rest. (A form of potential energy.)

Suction Pressure - The absolute pressure of the fluid at the inlet side of the pump.

Surge pressure - The pressure changes caused in a circuit from a rapidly accelerated column of oil.

The surge includes the span of these changes, from high to low.

System pressure - The pressure which overcomes the total resistances in a system. It includes alllosses as well as useful work.

Vapor Pressure - The pressure, at a given fluid temperature, in which the liquid and gaseous phases of

the fluid are in equilibrium.

Working Pressure - The pressure which overcomes the resistance of the working device.

PRESSURE, BURST - The pressure which creates loss of fluid through the component due to the

fracture of the component.

PRESSURE, CHARGE - The pressure at which replenishing fluid is forced into a fluid power system.

PRESSURE, CRACKING - The pressure at which a pressure operated valve begins to pass fluid.

PRESSURE DROP - The difference in pressure between any two points of a system or a component.

PRESSURE LINE - The line carrying the fluid pump outlet to the pressurized port of the actuator.

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PRESSURE OVERRIDE - The difference between the cracking pressure of a valve and the pressure

reached when the valve is passing full flow.

PRESSURE PLATE - A side plate in a vane pump or motor cartridge on the pressure port side.

PRESSURE REDUCING VALVE - A valve which limits the maximum pressure at its outlet regardless

of the inlet pressure.

PRESSURE SWITCH - An electric switch operated by fluid pressure.

PRESSURE VESSEL - A container which holds fluid under pressure.

PROPORTIONIAL FLOW - In a filter, the condition in which part of the flow passes through the filter

element in proportion to pressure.

HOA16./PULSATION - Repeated small fluctuation of pressure within a circuit,

PUMP - A device which converts mechanical force into hydraulic fluid power. Basic design types are

gear, vane, and piston units.

Fixed Displacement Pump - A pump in which the output per cycle cannot be varied.

Variable Displacement Pump - A pump in which the output per cycle can be varied.

Q

QUICK DISCONNECT - A coupling which can quickly join or separate a fluid line without the use of

tools or special devices.

QUIESCENT CURRENT - A direct current that is present in each servovalve coil when using a differ-

ential coil connection, the polarity of the current in the coils being in opposition such that no electrical

control power exists.

R

RATED CURRENT - The specified servovalve input current of either polarity to produce rated flow.

Rated current must be specified for a particular coil connection differential, series, or parallel, anddoes not include null bias current.

RATED FLOW - The maximum flow that the power supply system is capable of maintaining at a spe-

cific operating pressure.

RAM - A single-acting cylinder with a single diameter plunger rather than a piston and rod. The plunger

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in a ram-type cylinder.

RECIPROCATION - Back-and-forth straight line motion or oscillation.

REDUCED PRESSURE RANGE - The adjustment range of the regulator.

REGULATOR, AIR LINE PRESSURE - A regulator which transforms a fluctuating air pressure supply

to provide a constant lower pressure output.

REGENERATIVE CIRCUIT - A circuit in which pressure fluid discharged from a component is re-

turned to the system to reduce flow input requirements. Often used to speed up the action of a cylin-

der by directing discharged oil from the rod end to the piston end.

RELIEF VALVE - A pressure operated valve which by-passes pump delivery to the reservoir, limiting

system pressure to a predetermined maximum value.

REMOTE - A hydraulic function such as a cylinder which is separate from its supply source. Usually

connected to the source by flexible hoses.

RENARD SERIES - See Preferred Numbers.

REPLENISH - To add fluid to maintain a full hydraulic system.

RESERVOIR - A container for keeping a supply of working fluid in a hydraulic system.

RESIDUAL DIRT CAPACITY - The dirt capacity remaining in a service loaded filter element after use,

but before cleaning, measured under the same conditions as the dirt capacity of a new filter element.

RESPONSE TIME - The time required for effective transmission.

RESTRICTION - A reduced cross-sectional area in a line or passage which normally causes a pressure

drop. (Examples: pinched lines or clogged passages, or an orifice designed into a system.)

RESTRICTOR - A device which reduces the cross sectional flow area.

RESTRICTOR, ORIFICE - A restrictor, the length of which is relatively small with respect to its cross-

sectional area. The orifice may be fixed or variable.

RETURN LINE - A line used to carry exhaust fluid from the actuator back to sump.

REVERSING VALVE -A four-way directional valve used to reverse a double-acting cylinder or revers-

ible motor.

REYN - The standard unit of absolute viscosity in the English system. It is expressed in pound-seconds

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per square inch.

REYNOLDS NUMBER - A numerical ratio of the dynamic forces of mass flow to the shear stress due

to viscosity. Flow usually changes from laminar to turbulent between Reynolds numbers 2,000 and

4,000.

RING, O - A ring which has a round cross-section usually used for sealing.

RING, SCRAPER - A ring which removes material by a scraping action.

RING, WIPER - A ring which removes material by a wiping action.

RIPPLE - A periodic variation of the pressure above and below the operating pressure. It is defined as a

percentage of the operating pressure in terms of the maximum peak-to-peak value obtained at the

point of rating.

RISE RATE - The ratio of pressure rise to time.

ROTARY ACTUATOR - A device for converting hydraulic energy into rotary motion -- a hydraulic

motor.

S

SAE PORT - A straight thread port used to attach tube and hose fittings. It employs an O ring com-

pressed in a wedge-shaped cavity. A standard of the Society of Automotive Engineers J514 and

ANSI/B116.1.

SCORING - Scratches in the direction of motion of mechanical parts caused by abrasive contaminants.

SCORING SIZE - A particulate whose dimensions are such that it is capable of entering a working

clearance.

SEAL, DYNAMIC - A sealing device used between parts that have relative motion.

SEAL, LIP - A sealing device which has a flexible sealing projection.

SEAL, PRESSURE ACTIVATED - A sealing device in which sealing action is aided by fluid pressure.

SEPARATOR, ABSORBENT - A separator that retains certain soluble and insoluble contaminants by

molecular adhesion.

SEPARATOR, CENTRIFUGAL - A separator that removes nonmiscible fluid and solid contaminants

that have a different specific gravity than the fluid being purified by accelerating the fluid in a circular

path and using the radial acceleration component to isolate these contaminants.

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SEPARATOR, COALESCING - A separator that divides a mixture or emulsion of two nonmiscible

fluids using the interfacial tension between the two liquids and the difference in wetting of the liquids

on a particular porous medium.

SEPARATOR, MAGNETIC - A separator that uses a magnetic field to attract and hold ferro-magnetic

particles.

SEPARATOR, VACUUM - A separator that utilizes subatmospheric pressure to remove certain gases

and liquids from another liquid because of their difference in vapor pressure.

SEQUENCE-

1. The order of a series of operations or movements.

2. To divert flow to accomplish a subsequent operation or movement.

SEQUENCE VALVE - A pressure operated valve which, at its setting, diverts flow to a secondary line

while holding a predetermined minimum pressure in the primary line.

SERVO MECHANISM (Servo) - A mechanism subjected to the action of a con trolling device which

will operate as if it were directly actuated by the controlling device, but capable of supplying power

output many times that of the controlling device, this power being derived from an external and

independent source.

SERVO VALVE / SERVOVALVE -

1. A valve which modulates output as a function of an input command.

2. A follow valve.

SERVOVALVE, ELECTROHYDRAULIC- A servovalve which is capable of continuously controlling

hydraulic output as a function of an electrical input.

SERVOVALVE, OVERLAP - The lap condition which results in a decreased slope of the normal flow

curve in the null region.

SERVOVALVE, UNDERLAP - The lap condition which results in an increased slope of the normalflow curve in the null region.

SERVOVALVE, NULL - The condition where the servovalve supplies zero control flow at zero load

pressure drop.

SHOCK WAVE - A pressure wave front which moves at a sonic velocity.

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SIGNAL - A command or indication of a desired position or velocity.

SILENCER - A device for reducing gas flow noise. Noise is decreased by tuned resonant control of gas

expansion.

SILT - Fine particulate matter, generally ranging from less than five to sub micro-meters in size.

SILTING - An accumulation of fine particles at a specific location in a fluid subsystem.

SIMETRIC SYSTEM - A modernized and internationally standardized version of the metric system

based on the meter, second, kilogram, ampere, degree Kelvin, and candela.

SINGLE ACTING CYLINDER - A cylinder in which hydraulic energy can produce thrust or motion in

only one direction. (May be mechanically or gravity returned.)

SINTERED - A metallic or non-metallic filter medium processed to cause diffusion bonds at all con-

tacting points.

SLIME - A soft viscous deposit or coating.

SLIP - Internal leakage of hydraulic fluid.

SLOUGHING OFF - The release of contaminant from the upstream side of a filter element to the

upstream side of the filter enclosure.

SLUDGE - Particulate contaminant or a mixture of particulate and liquid contaminant separated from afluid in an unconsolidated state.

SOLENOID - An electro-magnetic device which positions a hydraulic valve.

SPECIFIC GRAVITY, LIQUID - The ratio of the weight of a given volume of liquid to the weight of an

equal volume of water.

SPOOL - A term loosely applied to almost any moving cylindrically shaped part of a hydraulic compo-

nent which moves to direct now through the component.

STARVATION - A lack of oil in vital areas of a system. Often caused by plugged filters, etc.

STRAINER - A coarse filter.

STREAMLINE FLOW - (See laminar flow.)

STROKE

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1. The length of travel of a piston in a cylinder.

2. Sometimes used to denote the changing of the displacement of a variable delivery pump.

STAGE - A hydraulic amplifier used in a servovalve. Servovalves may be single stage, two stage, three

stage, etc.

STANDARD - A document or an object for physical comparison, for defining product characteristics,

products, or processes: prepared by a consensus of a properly constituted group of those substantially

affected and having the qualifications to prepare the standard for voluntary use.

START-UP TIME - The period of time needed to reach a steady state condition within the operating

band starting from a long term off condition.

STEADY STATE PRESSURE REGULATION - A band indicating maximum and minimum pressure ora single curve with maximum deviation indicated in percent of operating pressure, all as a function of

flow.

STOKE - The standard unit of kinematic viscosity in the CGS (centimeter-gram-second) system. It is

expressed in square centimeters per second; 1 centistoke equals .0 1 stoke.

STUFFING BOX - A cavity and closure for a scaling device.

SUB-PLATE (BACK PLATE) - An auxiliary mounting for a hydraulic component providing a means of

connecting piping to the component.

SUCTION LINE - The hydraulic line connecting the pump inlet port to the reservoir or sump.

SUMP - A reservoir.

SUPERCHARGE - (See charge)

SURFACE TENSION - The contractile surface force of a liquid in contact with a fluid by which it tends

to assume a spherical form and to present the least possible surface. It is expressed in pounds per foot

or dynes per centimeter.

SURGE - A momentary rise of pressure in a hydraulic circuit.

SWASH PLATE - A stationary canted plate in an axial type piston pump which causes the pistons to

reciprocate as the cylinder barrel rotates.

SWITCH, FLOAT - An electrical switch which is responsive to liquid level.

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SWITCH, FLOW - An electric switch operated by a fluid flow.

SWITCH, PRESSURE - An electric switch operated by fluid pressure.

SWITCH, PRESSURE DIFFERENTIAL- An electric switch operated by a difference in pressure.

SYMBOL, CUTAWAY - A symbol showing principal internal parts, controls, and actuating mecha-

nisms, interconnecting lines, and functions of a component.

SYMBOL, COMBINATION - A symbol which combines graphical cutaway, and pictorial representa-

tions.

SYMBOL, FLUID POWER - A representation of the characteristics of a fluid power component by

means of lines on a flat surface.

SYMBOL, GRAPHICAL (SCHEMATIC) - A simplified symbol which indicates essential characteristicsapplicable to all similar components.

SYMBOL, PICTORIAL - A symbol showing the actual shape of a component according to the

manufacturers description.

SYNCHRO - A rotary electromagnetic device generally used as an AC feedback signal generator which

indicates position. It can also be used as a reference signal generator.

SYNTHETIC FLUID - Fluid which has been artificially compounded for use in a fluid power system.

SYNTHETIC FLUID, HALOGENATED - A fluid composed of halogenated organic materials. It maycontain additives.

SYNTHETIC FLUID, ORGANIC ESTER - A fluid composed of esters which are compounds of

carbon, hydrogen, and oxygen only. It may contain additives.

SYNTHETIC FLUID, PHOSPHATE ESTER BASE - A fluid which contains a phosphate ester as one

Of the major components.

SYNTHETIC FLUID, POLYGLYCOL - A nonaqueous fluid composed of polyglycol derivatives. It

may contain additives.

SYNTHETIC FLUID, SILICATE ESTER - A fluid composed of organic silicates. It may contain

additives.

SYNTHETIC FLUID, SILICONE - A fluid composed of silicones. It may contain additives.

SYSTEM - One or more series of component parts connected to each other. Often made up of two or

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more circuits.

SYSTEM INTERNATIONALE d UNITES - See SI Metric System.

T

TACHOMETER - (AC) (DC) - A device which generates an AC or DC signal proportional to the speed

at which it is rotated and the polarity of which is dependent on the direction of rotation of the rotor.

TANK - The reservoir or sump; container for the storage of fluid in a fluid power system.

TANK, AIR-OIL - A tank in which pressurized air is used to force oil into the outlet port.

TANK, VACUUM - A tank for storing gas at less than atmospheric pressure.

TEMPERATURE - See Celsius and Centigrade.

THROTTLE - To permit passing of a restricted flow. May control flow rate or create a deliberate

pressure drop.

THERMAL EXPANSION - Expansion of the fluid volume due to heat.

TIE ROD - An axial external cylinder rod which traverses the length of the cylinder. It is prestressed at

assembly to hold the ends of the cylinder against the tubing. Tie rod extensions can be a mounting

device.

TORICELLPS THEOREM - The liquid velocity at an outlet discharging into the free atmosphere isproportional to the square root of the head.

TORQUE - The turning effort of a hydraulic motor or rotary cylinder. Usually given in inch-pounds (in

lbs) or foot-pounds (ft-lbs).

TORQUE CONVERTER - A rotary fluid coupling that is capable of multiplying torque.

TORQUE MOTOR - A type of electromechanical transducer having rotary motion used in the input

stages of servo valves.

TORR - A unit of absolute pressure less than atmospheric pressure, equivalent to 1 mm Hg. It is usedprimarily to describe a vacuum of 25 mm. Hg. absolute or less.

TORTUOSITY - The ratio of the average effective flow path length to minimum theoretical flow path

length (thickness) of a flow medium.

TOTAL STATISTICAL COUNT - The raw count multiplied by a counting calibration factor.

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TRANSDUCER (or feedback transducer) - An element which measures the results at the load and sends

a signal back to the amplifier.

TRANSIENT RECOVERY TIME - The period of time required for ail abrupt change in the powersupply output pressure to dampen out to within the operating band.

TRUNNION - A mounting device consisting of a pair of opposite projecting pivots. The cylindrical

pivot pins are at right angles or normal to the piston rod centerline to permit the cylinder to swing in a

plane.

TUBE - A line whose size is its outside diameter.

TURBULENT FLOW (TURBULENCE) - A condition where the fluid particles move in random paths

rather than in continuous parallel paths.

TURBINE - A rotary device that is actuated by the impact of a moving fluid against blades or vanes.

TWO-WAY VALVE - A directional control valve with two flow paths.

U

UNLOAD - To release flow (usually directly to the reservoir), to prevent pressure being imposed on the

system or portion of the system.

UNLOADING VALVE - A valve which by-passes flow to tank when a set pressure is maintained on its

pilot port.

V

VACUUM - Pressure less than atmospheric pressure. It is usually expressed in inches of mercury (in

Hg) as referred to the existing atmospheric pressure.

VALVE - A device which controls one of the following: 1) pressure of fluid, 2) direction of fluid flow,

or 3) rate of flow.

Bypass Flow Regulator Valve - A valve which regulates the flow to a circuit at a constant volume,

dumping excess oil.

Check Valve - A valve which permits flow in only one direction.

Closed Center Valve - A valve in which inlet and outlet ports are closed in the neutral position, stop-

ping flow from the pump.

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Counterbalance Pressure Control Valve - A pressure control valve which maintains back pressure to

prevent a load from falling.

Decompression Pressure Control Valve - A pressure control valve that controls the rate at which the

contained energy of the compressed fluid is released.

Directional Control Valve - A valve which directs oil through selected passages. (Usually a spool or

rotary valve design.)

Flow Control Valve - A valve which controls the rate of flow (Sometimes called a volume control

valve.)

Flow Divider Valve - A valve which divides the flow from one source into two or more branches.

(Includes priority and proportional types.)

Hydraulic Valve - A valve for controlling liquid flow or pressure.

Load Dividing Pressure Control Valve - A pressure control valve used to proportion pressure be-

tween two pumps in series.

Needle Valve - A valve with an adjustable tapered point which regulates the rate of flow.

Open Center Valve - A valve in which the inlet and outlet ports are open in the neutral position,

allowing a continuous flow of oil from pump.

Pilot Valve - A valve used to operate another valve or control.

Pneumatic Valve - A valve for controlling gas flow or pressure.

Poppet Valve - A valve design in which the seating element autopops open to obtain free flow in one

direction and immediately reseats, when flow reverses.

Prefill Valve - A valve which permits full flow from a tank to a working cylinder during the advance

portion of a cycle, permits the operating pressure to be applied to the cylinder during the working

portion of the cycle, and permits free flow from the cylinder to the tank during the return portion of

the cycle.

Pressure Control Valve - A valve whose primary function is to control pressure. (Includes reliefvalves, pressure reducing or sequencing valves, and unloading valves.)

Pressure Reducing Valve - A pressure control valve which limits outlet pressure.

Pressure Sensing Valve - A device similar to an electrical pressure switch, in which a signal to be

sensed enters a control point, and actuates a mechanism which, at the proper pressure level, causes

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one or more flow passages to change condition. Removal of the signal allows the pressure sensing

valve to reset.

Pressure Sequence Valve - A pressure control valve which directs flow in a preset sequence

Priority Flow Divider Valve - A valve which directs oil to one circuit at a fixed rate and dumps excess

flow into another circuit.

Proportional Flow Divider Valve - A valve which directs oil to all its circuits at all times.

Relief Valve - A valve which limits the pressure in system, usually by releasing excess oil.

Rotary Directional Valve - A valve designed in cylindrical shape. When the valve is turned, it open

and closes drilled passages to direct oil.

Selector Valve - A valve which selects one of two or more circuits in which to direct oil, usuallyoperates manually.

Sequence Valve - A valve whose primary function is to direct flow in a predetermined sequence.

Shutoff Valve - A valve which operates fully open or fully closed.

Shuttle Valve - A connective valve which selects one of two or more circuits because flow or pressure

changes between the circuits.

Spool Directional Valve - A valve designed as a spool which slides in a bore, opening and closing

passages.

Thermal Relief Valve - A valve which limits the pressure in a system caused by heat expansion of oil.

Two-, Three-, Four-, or Six-Way Valve - A valve having 2,3, 4, or 6 ports for direction of oil flow.

Unloading Valve - A valve which allows a pump to operate at a minimum load by dumping the

pumps excess oil at a low pressure.

Volume Control Valve - A valve which controls the rate of flow. Includes flow control valves, flow

divider valves, and bypass flow regulators.

VALVE ACTUATOR - The valve part(s) through which force is applied to move or position

flow-directing elements.

VALVEMOUNTING - The mounting characteristic of a valve.

Base valve mounting - The valve is mounted to plate which has top and/or side ports.

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Line valve mounting - The valve is mounted directly to system valves.

Manifold valve mounting - The valve is mounted to a plate which provides multiple connection ports

for one or more valves.

Sub-plate valve mounting - The valve is mounted to a plate which provides straight through top and/or

bottom ports.

VALVESTACK - A series of control valves in a stack with common end plates and a common oil inlet

and outlet.

VARNISH - Materials generated by the hydraulic fluid due to oxidation, thermal instability or other

reactions. These materials are insoluble in hydraulic fluid and are generally found as brownish deposits

in the work surfaces.

VELOCITY - The distance which a fluid travels per unit time. Usually given as feet per second.

VENT - An air breathing device in a fluid reservoir.

VISCOSITY - The measure of resistance of a fluid to flow.

VISCOSITY, ABSOLUTE - The ratio of the shearing stress to the shear rate of a fluid. It is usually

expressed in centipoise.

VISCOSITY INDEX - A measure of the viscosity temperature characteristics of a fluid as referred to

that of two arbitrary reference fluids.

VISCOSITY, KINEMATIC - The absolute viscosity divided by the density of the fluid. It is usually

expressed in centistokes.

VISCOSITY, SAE NUMBER -The Society of Automotive Engineers arbitrary numbers for classifying

fluids according to their viscosities. The numbers in no way indicate the viscosity index of the fluids.

VISCOSITY, SUS - Saybolt Universal Seconds (SUS) which is the time in seconds for 60 milliliters of

oil to flow through a standard orifice at a given temperature (ASTM Designation D88-56). Often

abbreviated SSU.

VISCOSITY INDEX - A measure of the viscosity temperature characteristics of a fluid as referred to

that of two arbitrary reference fluids. (ASTM Designation D2270-64).

VOLUME - The amount of fluid flow per unit time. Usually given as gallons per minute (gmp).

W

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WATER GLYCOL FLUID - A fluid whose major constituents are water and one or more glycols or

polyglycols.

WATT - A unit of power.

WOBBLE PLATE - A rotating canted plate in an axial type piston pump which pushes the pistons into

their bores as it wobbles.

WORK - Exerting a force through a definite distance. Work is measured in units of force multiplied by

distance; for example, pound-foot.

ABBREVIATIONS

ASAE - American Society of Agricultural Engineers (sets standards for many hydraulic components for

agricultural use).

F - degrees Fahrenheit (of temperature)

ft-lbs - foot-pounds (of torque or turning effort)

gpm - gallons per minute

hp - horsepower

I.D. - inside diameter

O.D. - outside diameter

psi - pounds per square inch (of pressure)

rpm - revolutions per minute

SAE - Society of Automotive Engineers (sets standards for many hydraulic components)

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hydraulic definitions

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