text.skipToContent text.skipToNavigation

Glossary of Terms - Fluid Power

You may search any term or definition that contains a particular word or words. Enter the word(s) in the text box then click the Search button.

Additionally you may click on any letter to display terms starting with that letter. You may click an area from the Glossary Subset either before choosing the letter or after a letter has been chosen to limit the terms to just that subset.

Glossary Subsets


The presence of air in the hydraulic fluid. Excessive aeration causes the fluid to appear milky and also causes components to operate erratically. This is because of compressed air trapped in the fluid.


A container in which fluid is stored under pressure as a source of fluid power. The accumulator also absorbs shocks and smoothes out an operation.


A type of measure having its zero point (base point) as being the complete absence of the entity being measured.

Absolute Pressure (PSIA)

The pressure above zero absolute, the sum of both atmospheric and gage pressure.


American Gear Manufacturers Association. One activity of AGMA is the establishment of standards for gear lubricants.

Annular Area

A ring shaped area that often refers to the effective area of the rod side of a cylinder piston, that is, the piston area minus the rod area.

Amplitude of Sound

The loudness of a sound.


A condition where the center lines of two shafts are in line (coaxial).


A device used to convert hydraulic energy into mechanical energy. A device to convert rotary motion into thrust or linear motion.

Ambient Temperature

The temperature of the surrounding environment, usually air, in which the system components operate.

Atmosphere (ONE)

A pressure measure equal to 14.7 psi.

Atmospheric Pressure

Pressure on all objects in the atmosphere because of the weight of the surrounding air. At sea level, this is about 14.7 pounds per square inch.


The American Society for Testing and Materials, an organization devoted to the promotion of knowledge of engineering materials and the standardization of specifications and methods of testing.

Back Connected

A condition where port connections are on normally unexposed surfaces of hydraulic equipment.

Back Pressure

Usually refers to pressure existing on the discharge side of a load, it adds to the pressure required to move the load.


A device, usually a plate, installed in a reservoir to separate the pump inlet from return lines. The baffle also aids in fluid cooling.


An imaginary line running through the center of a shaft, also referred to as center line.


A unit of pressure based on 105 Newtons per square meter, equal to 100kPa (kilopascals), approx. equal to 14.5 psig.

Beta Rating

The method of comparing filter performance based on efficiency. This is done using the multi-pass test which counts the number of particles of a given size before and after a fluid passes through a filter.

Bernoulli Principle

An increase in velocity causes a decrease in pressure.


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


A device which permits air to move in and out of a component to maintain atmospheric pressure.


British Stantard Parallel Pipe


British thermal unit. The amount of heat required to raise the temperature of one pound of water one degree Fahrenheit.


A secondary passage for fluid flow.


British Standard Pipe Thread

Bypass Filtration

A system of filtration in which only a portion of the total flow passes through a filter at any instant.


The process of comparing an instrument or device against a standard to determine instrument accuracy or to make a correction.


The replaceable element of a fluid filter. The pumping unit from a vane pump, composed of the rotor, ring, vanes and one or both side plates. A removable valve element that fits into a cavity in a hydraulic component, e.g. manifold.

Case Drain Line

A line conducting fluid from a component housing to the reservoir.


A Localized gaseous condition within a liquid stream which occurs where the pressure is reduced to the vapor pressure.

Cellulose Media

A filter material made from plant fibers. Because cellulose is a natural material, its fibers are rough in texture and vary in size and shape. Compared to synthetic media, these characteristics create a higher restriction to the flow of fluids.

Centistokes (cSt)

A unit of kinematic viscosity.
• 1 cSt = 10-2 square centimeters per second (cm²/s).


A compartment within a hydraulic unit. May contain elements to aid in operation or control of a unit.


A temperature scale. 0° C is the freezing point of water, 100°C is the boiling point of water.
• C° = (F° - 32) * (5/9)
• F° = (9/5) * C° + 32


Canadian General Standards Board. An organization which develops standards for products and test methods specifically required in Canada.

Charge (Supercharge)

• To replenish a hydraulic system above atmospheric pressure.
• To fill an accumulator with fluid under pressure (precharge pressure).

Charge Pressure

The pressure at which replenishing fluid is forced into the hydraulic system (above atmospheric pressure)

Check Valve

A valve which permits the flow of a fluid in one direction only.


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

Closed Center

The condition where pump output is not unloaded to tank in the center or neutral operating condition.


• Any path over which electric current may flow, such as motor circuits or lighting circuits.
• The complete path of flow in a hydraulic system including the flow-generating device.

Closed Circuit

A piping arrangement in which pump delivery, after passing through other hydraulic components, bypasses the reservoir and returns directly to pump inlet.


A separator that divides a mixture of two liquids on a particular porous medium.

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.


A single pneumatic or hydraulic unit.


The change in volume of a unit volume of a fluid when it is subjected to a unit change in pressure.


A device which converts mechanical force into pneumatic fluid power.

Compounded Oil

A blend of petroleum oil with small amounts of fatty oils is referred to as compounding. Often used in wet applications to prevent washing-off of the lubrication from the metal surfaces. The fatty materials enable the oil to combine with the water inste

Coefficient of Friction

The number obtained by dividing the friction force resisting motion between two components by the normal force pressing both components together.


A heat exchanger used to remove heat from hydraulic fluid.


The filter media support.


A device used to regulate the function of a unit (see Hydraulic Control, Manual Control, Mechanical Control, and Compensator Control).

Cracking Pressure

The amount of pressure which a pressure actuated valve begins to pass fluid.


A device sometimes built into the ends of a hydraulic cylinder which restricts the flow of fluid at the outlet port, thereby slowing the motion of the piston rod.

Counterbalance Valve

A valve which maintains resistance in one direction but permits free flow in the other. Usually connected to the outlet of a vertical double-acting cylinder to support weight or prevent uncontrolled falling or dropping.


Loads being switched in and out of the line, either manually or automatically.


To close the vent connection of a pressure control valve permitting the valve to function at its adjusted pressure setting.


A device which converts fluid power into linear mechanical force and motion. It usually consists of a movable element such as a piston and piston rod, plunger rod, plunger or ram, operating within a cylindrical bore.


A separator that removes water from the system fluid.


A separator that removes air from the system.


Removing air from a liquid, usually by ultrasonic or vacuum methods.


The volume of fluid discharged by a pump in a given time period, usually expressed in gallons per minute (gpm).

Delta P

The change in pressure between two points in a system, such as in a passageway or between the inlet and outlet of a componet. Also expressed as DP.

Directional Valve

A valve which selectively directs (or prevents) fluid to specific ports.

Double Acting Cylinder

A cylinder in which fluid force can be applied in either direction.


The quantity of fluid which can pass through a pump, hydraulic motor, or cylinder in a single revolution or stroke.

Differential Pressure Indicator

An indicator which signals the difference in pressure between any two points of a system or a component.


A term usually used interchangeably with detergent. An additive which keeps fine particles of insoluble materials in an oil. Particles are not permitted to settle or accumulate.

Design Horsepower (DHP)

A rating achieved by multiplying horsepower by the service factor.


Separate water from oil.


An additive that neutralizes contaminants in oil before they become insoluble.


A port or a line from a hydraulic component which returns fluid independently to a reservoir or vented manifold.

Dropping Point

The temperature at which a grease changes from a semi-solid to a liquid state under test conditions. It may be considered an indication of the high temperature limitation for application purposes.

Duplex Filter

An assembly of two filters with valving that allows use of either or both filters.


The ratio of output to input. Volumetric efficiency of a pump is the actual output flow divided by the theoretical or design flow. The overall efficiency of a hydraulic system is the output power divided by the input power. Efficiency is usually expressed as a percent.

Electro-Hydraulic Servo Valve

A directional type valve which receives a variable or controlled electrical signal. This type of valve controls hydraulic flow.


The porous device that performs the actual process of filtration.


A fluid formed by the suspension of one liquid in another.


The ability or capacity to do work, measured in units of work.

Feedback Signal

The output signal from a feedback element. One example would be a motor used in conjuction with an encoder. The encoder supplies a feedback signal to the motor to precisely control motor speed.


A temperature scale. 32°F is the freezing point of water, 212°F is the boiling point of water.
• C° = (F° - 32) * (5/9)
• F° = (9/5) * C° + 32

Film Strength

A property of a lubricant that helps prevents the scuffing or scoring of metal parts.


The process of separating insoluble particulate matter from a fluid, such as air or liquid, by passing the fluid through a filter medium that will not allow the particulates to pass through it.


? A device whose primary function is the retention of insouluble contaminants from a fluid by a porous media. This keeps the oil free of contamination.
? An electrical device used to suppress electrical noise, or to improve the DC output to a DC motor.


A condition where the pump inlet port 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.


A device which measures flow rate, total flow, or a combination of both.


• A liquid that is specially compounded for use as a power transmitting medium in a hydraulic system.
• Can refer to a liquid or gas.


A process designed to remove contamination from a fluid system.

Four-Way Valve

A directional valve having four flow paths.


The number of times an action occurs in a unit of time. Frequency is the basis of all sound. A pump or motor's basic frequency is equal to its speed in revolutions per second multiplied by the number of pumping chambers.


Any push or pull measured in units of weight. In hydraulics, total force is expressed by the product of pressure and the area of the surface on which the pressure acts.
• F = P x A

Front Connected

Piping connections are on exposed surfaces of hydraulic components.


The resisting force encountered at the common boundary between two bodies when one body moves or tends to move relative to the surface of the other body.

Flow Rate

The volume, mass, or weight of a fluid passing through a conductor per a given unit of time. This is normally expressed in gallons per minute (gpm).

Full Flow

In a filter, the condition where all the fluid must pass through the filter element.

GPM (Gallons Per Minute)

Unit of measure representing volume of liquid flow.

Gauge Pressure (psig)

A pressure scale where atmospheric pressure is ignored, in other words, its zero point is atmospheric pressure (14.7 psia at sea level).

GPH (Gallons Per Hour)

Unit of measure representing volume of liquid flow.


The height of a column of fluid above a set point. Head is often used to indicate gauge pressure. The pressure is equal to the height times the density of the fluid.

Heat Exchanger

A device which transfers heat through a conducting wall from one fluid to another fluid.


The process of heating parts to a high temperature and then quenching them, either in oil, water, air, or another solution.


A form of energy that has the capacity to increase the temperature of a substance. Any energy that is wasted or used to overcome friction is converted to heat. Heat is measured in calories or British Thermal Units (BTUs). One BTU is the amount of heat required to raise one pound of water one degree Fahrenheit.

Horsepower (HP)

The power required to lift 550 pounds one foot in one second or 33,000 pounds one foot in one minute. A horsepower is equal to 746 watts or to 2544 BTU's per hour.

Hydraulic Balance

A condition of equal opposed hydraulic forces acting on a part in a hydraulic component.


Engineering science pertaining to liquid pressure and flow. Also referred to as hydrodynamics.


Greek word for water.

Hydraulic Control

A control which is actuated by hydraulically induced forces.


Engineering science pertaining to liquid pressure and flow. Also referred to as hydraulics.


Engineering science pertaining to the energy of liquids at rest.


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


A function of the mass and shape of an object. The inertia represents the property of an object that resists a change in motion.


Joint Industry Conference


Momentarily moving the motor at a slower speed than normal operating speed. This function is normally available on many drive controls as a separate control button.

Kinetic Energy

Energy that a substance or body has by virtue of its mass and velocity.


International organization for standardization. An organization which establishes internationally recognized standards for products and test methods.

Laminar (Flow)

A condition where the fluid particles move in continuous parallel paths.


An process for refining the surface finish and the geometrical accuracy of a surface. Often a finishing process for many component parts or bearings.


A gain in output force over input force by sacrificing the distance moved. Mechanical advantage or force multiplication.


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.


A tube, pipe or hose which acts as a conductor of hydraulic fluid.

Linear Actuator

A device for converting hydraulic energy into linear motion, i.e., a piston or ram.

Load Sensing

A type of circuit where the actual pressure or power required by the load is transmitted to the control elements. This typically allows the system to deliver only the pressure or power required, usually saving energy.


A mechanical device which may be set to prevent the operation of a push-button or other device.


Power expended without accomplishing useful work.

Lock Washer

A washer with tongue and prongs to hold a lock nut in place.


A fluid conductor which provides multiple connection ports.

Manual Control

• A control actuated by the operator, regardless of the means of actuation.
• A lever or foot pedal control for directional valves.

Manual Override

A means of manually actuating an automatically-controlled device.


To regulate the amount or rate of fluid flow.

Mechanical Control

Any control actuated by linkages, gears, screws, cams or other mechanical elements.


To regulate the amount of fluid flow into an actuator or system.


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

Micron Rating

The size of the particles a filter will remove under normal operating conditions.


A rotary motion device which changes hydraulic energy into mechanical energy; a rotary actuator.

Micron (µm)

• The size of a particle one millionth of a meter.
• The symbol for a micron is µm.


Lack of parallelism between the axis of the rotating member and the stationary member.

Newton Meter (Nm)

Metric unit of measure representing force.


National Pipt Taper (requires pipe sealant)


National Pipe Taper Fuel or Dry Seal Pipe Threads (may not require pipe sealant)

Open Center

A condition where pump delivery circulates freely to tank in the center or neutral position.


A restriction, the length of which is small in respect to its cross-sectional dimensions.


Pounds per square inch absolute.


Not in series but fed from the same sources.


The chemical reaction of elements with oxygen which cause corrosion or deterioration.


A unit of pressure in metric system, usually expressed in kPa (kilopascals).
• 1 kPa = 0.01 bar = 0.145 psi.


A fluid conducting path which lies within or passes through a component.

Pilot Pressure

Auxiliary pressure used to actuate or control hydraulic components.

Pilot Valve

An auxiliary valve to control the operation of another valve.


A cylindrically shaped part with only one diameter that is used to transmit thrust. A ram.


A cylindrically shaped part which fits within a cylinder and transmits or receives motion by means of a connecting rod.


Often shown as p, it is the ratio of circumference of circle to diameter. Numerically can be expressed as 22/7 or 3.14159265 more accurately.


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


An internal or external passage in a component.

Positive Displacement

A characteristic of a pump or motor which has the inlet positively sealed from the outlet so that fluid cannot re-circulate in the pump or motor.

Potential Energy

The energy level change which results when an object is raised to a new height.


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

Pour Point

The lowest rated temperature at which a hydraulic fluid will flow.


Work per unit of time, measured in horsepower (hp) or watts.

PPM (Pars Per Million)

Unit of mearsure representing the number of particles in a given body.

Precharge Pressure

The pressure of compressed gas in an accumulator prior to the admission of liquid.


Force per unit area; usually expressed in psi, bar, or kPa.

Pressure Differential

Same as pressure drop. The change in pressure between two points in a system, usually referring to the inlet and outlet ports of a component. Also expressed as DP (Delta P).

Pressure Drop

The change in pressure between two points in a system, usually referring to the inlet and outlet ports of a component. Also expressed as DP (Delta P).

PSIA (Pounds Per Square Inch Absolute)

See Absolute Pressure

Pressure Override

The difference between the cracking pressure and the full flow pressure of a valve.

Pressure Line

The line carrying the fluid from the pump outlet to the pressurized port of the actuator.

PSIG (Gauge Pressure)

A pressure scale which ignores atmospheric pressure, i.e., its zero point is atmospheric pressure (14.7 psia at sea level).

Pressure Reducing Valve

A valve which limits the maximum pressure at its outlet regardless of the inlet pressure.


A device which converts mechanical force and motion into hydraulic fluid flow.


• A single acting cylinder with a plunger rather than a piston and rod.
• The plunger in a ram type cylinder.


Back and forth straight line motion or oscillation.

Relief Valve

A pressure operated valve which bypasses pump delivery to the reservoir, limiting system pressure to a predetermined maximum value.


To add fluid to maintain a full hydraulic system.


A container for storage of liquid in a fluid power system.


A reduced cross-sectional area in a line or passage which produces a pressure drop.

Reversing Valve

A four-way directional valve used to reverse a double acting cylinder or reversible motor.

Return Line

A line used to carry exhaust fluid from the actuator back to the tank.

Rotary Actuator

• A device for converting hydraulic energy into rotary motion.
• A hydraulic motor.

SAE O-Ring

Refers to a style of threaded connection that employs straight thread to secure the fitting and an o-ring to provide sealing. This style of connection is very poplular for use in hydraulic systems and can be removed and reinstalled numerous times without damaging the sealing ability of the connection. Tapered pipe thread is actually damaged to make a fluid tight seal. Vibration and repeated connection of tapered pipe threaded connections will always resulin in leaks.


The Society of Automotive Engineers

Saybolt Universal Seconds (SUS)

A measure of viscosity, the time required for a given fluid to flow through a standard orifice.

SCFM (Standard Cubic Feet Per Minute)

Unit of measure representing volume of gas flow.


• The order of a series of operations or movements.
• To divert flow to accomplish a subsequent operation or movement.

Sequence Valve

A pressure operated valve which diverts flow to a secondary actuator while holding pressure on the primary actuator at a predetermined minimum value after the primary actuator completes its travel.


In line with the load.

Service Factor

• A safety factor designed and built into some motors which allows the motor, when necessary, to deliver greater than its rated horsepower.
• Also used as a factor when sizing system components, based on application, to achieve design hors

Servo Mechanism

A mechanism subjected to the action of a controlling 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

• A valve which controls the direction and quantity of fluid flow in proportion to an input signal.
• A follow valve.

Single Acting Cylinder

• A cylinder in which hydraulic energy can produce thrust or motion in only one direction.
• Can be spring or gravity returned.


A command or indication of a desired position or velocity.

Single Phase

A circuit that differs in phase by 180 degrees. Single phase circuits have two conductors, one of which may be a neutral, or three conductors, one of which is neutral.


• To cut, split or pare away outer layer of a flexible hose.
• Also refers to a type of cut for belting or other rubber products.


Internal leakage of hydraulic fluid.


A spiral of conducting wire, wound cylindrically so that when an electric current passes through it, its turns are nearly equivalent to a succession of parallel circuits, and it acquires magnetic properties similar to those of a bar magnet.


A term loosely applied to almost any moving cylindrically shaped part of a hydraulic component which moves to direct flow though the component.


A coarse filter for trapping large particles.


• The length of travel of a piston or plunger.
• To change the displacement of a variable displacement pump or motor.


An auxiliary mounting for a hydraulic component providing a means of connecting piping to the component.


A reservoir.


See charge

Suction Line

The hydraulic line connecting the pump inlet port to the reservoir.


A momentary rise of pressure in a circuit.


Saybolt Universal Seconds. A measure of viscosity, the time required for a given fluid to flow through a standard orifice.

Swash Plate

A stationary plate in an axial type piston pump or motor which causes the pistons to reciprocate as the cylinder barrel rotates.


The reservoir or sump.

Thermal Overload Protector

A device which protects motor windings from excessive temperature by opening a set of contacts. This device may reach its’ pre-set trip point as a result of ambient temperature, current, or both.

Three Phase

Three lines of power supplied each 120° apart. Most common in heavy industrial applications.

Time Delay Relay (TDR)

A device that performs a timing function upon being energized or receiving a control signal.


To permit passing of a restricted flow. May control flow rate or create a deliberate pressure drop.

Torque Converter

A rotary fluid coupling that is capable of multiplying torque.


• The amount a given dimension may vary from specifications.
• The range between two limiting sizes as a means of specifying the degree of accuracy.


A rotary or twisting force. The output torque of a motor is usually expressed in pound-inches (lb-in) or Newton-Meters (Nm).


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

Torque Motor

An electromagnetic device consisting of coils and the proper magnetic circuit to provide actuation of a spring restrained armature, either rotary or translatory.

Two-Way Valve

A directional control valve with two flow paths.

Turbulent Flow

A condition where the fluid particles move in random paths rather than in continuous parallel paths.


To release flow to prevent pressure being imposed on the system or a portion of the system.

Unloading Valve

A valve which bypasses flow to tank when a set pressure is maintained on its pilot port.


• The absence of pressure.
• A perfect vacuum is the total absence of pressure.
• A partial vacuum is some condition less than atmospheric pressure.
• Measured in inches of Mercury (in.Hg) with the zero point equal to atm

Vapor Pressure

The pressure at which the liquid and gaseous phases of a fluid are in equilibrium.


• The speed of flow through a hydraulic line expressed in feet per second or meters per second.
• The speed of a rotating component measured in revolutions per minute.


A device which controls fluid flow direction, pressure, or flow rate.


• To permit opening of a pressure control valve by opening its pilot port to atmospheric pressure.
• An air breathing device on a fluid reservoir.

Viscosity Index

A measure of the viscosity-temperature characteristics of a fluid as referred to that of two arbitrary reference fluids.


• The size of a space or chamber in cubic units.
• Loosely applied to the output of a pump in gallons per minute (gpm).


Exerting a force through a definite distance. Work is measured in units of force multiplied by distance; for example, pound-feet or joule (1 J = 1 Nm).