Hydraulics and Pneumatics Vocabulary

Absolute pressure

Pressure measured from zero absolute (a perfect vacuum); it equals gauge pressure plus atmospheric pressure, usually written as psia.

Similar definitions: psia

Example: "A tire gauge reading of 30 psig at sea level corresponds to an          of about 44.7 psia."

Accumulator

A container that stores hydraulic fluid under pressure as a source of fluid power; it can also absorb hydraulic shock. Common types are piston, bladder, and diaphragm.

Example: "The          kept enough stored oil under pressure to cycle the brakes after the engine stalled."

Actuator

A device that converts fluid power into mechanical force and motion, such as a cylinder or motor.

Similar definitions: output device

Example: "A hydraulic cylinder is the most common linear          on a loader."

Additive

A chemical added to a fluid to impart new properties or enhance those that already exist.

Similar definitions: inhibitor, improver

Example: "An anti-wear          helps protect pump surfaces from metal-to-metal contact."

Aeration

Air trapped as dispersed bubbles in hydraulic fluid; it makes the oil look milky and causes erratic operation, noise, and pump erosion.

Similar definitions: entrained air

Example: "Milky, foamy oil in the reservoir is a classic sign of          caused by a suction-side air leak."

Affected employee

Under OSHA lockout/tagout rules, an employee who operates or works in the area of equipment being serviced under lockout or tagout but does not apply the locks.

Example: "Each          must be notified before a machine is locked out for repair."

Aftercooler

A heat exchanger that cools compressed air after it leaves the compressor, causing much of its water vapor to condense for removal.

Example: "The          dropped the air temperature so that water could drain out before reaching the dryer."

Air brake

A vehicle friction brake system in which compressed air acting on a piston or diaphragm applies the brakes; used on heavy trucks and trailers.

Similar definitions: compressed-air brake system

Example: "The semi-truck's          system uses an engine-driven compressor to fill its air tanks."

Air breather

A device on a reservoir that lets air move in and out as the fluid level changes while filtering out contaminants.

Similar definitions: breather cap, filler breather

Example: "A clogged          can create a vacuum in the tank and starve the pump."

Air compressor

A device that converts mechanical force and motion into pneumatic fluid power by raising the pressure of air.

Similar definitions: compressor

Example: "The shop's          supplies air at 120 psi to the impact wrenches."

Air dryer

A device that removes water vapor from compressed air to lower its dew point; common types are refrigerated, desiccant, and deliquescent.

Similar definitions: compressed air dryer

Example: "An          prevents water from condensing in air lines during cold weather."

Air motor

A device that converts pneumatic fluid power into mechanical torque and motion, usually rotary motion.

Similar definitions: pneumatic motor

Example: "The hoist used an          because sparks from an electric motor were not allowed in that area."

Air receiver

A storage tank for compressed air that stores capacity and dampens pressure pulsations from the compressor.

Similar definitions: receiver tank, air tank

Example: "Condensate collects at the bottom of the          and must be drained regularly."

Annular area

The ring-shaped net area on the rod side of a cylinder piston, equal to the piston area minus the rod area.

Similar definitions: rod-side area, effective rod-end area

Example: "Because the          is smaller than the full piston area, a cylinder retracts faster but with less force."

ANSI

American National Standards Institute; the organization that coordinates U.S. voluntary consensus standards, including standard crane and hoist hand signals.

Example: "The crew used          hand signals to direct the crane operator."

Atmospheric pressure

The pressure exerted by the weight of the atmosphere at a location, about 14.7 psi (1.01 bar) at sea level.

Similar definitions: barometric pressure

Example: "A pump relies on          to push oil from the reservoir into its inlet."

Authorized employee

A person who locks out or tags out machines or equipment in order to perform servicing or maintenance on them.

Example: "Only the          who applied the lock may remove it."

Automatic drain

A valve that automatically discharges condensate from receivers, filters, dryers, and drip legs in a compressed air system.

Similar definitions: auto drain, drain trap

Example: "An          on the receiver tank kept water from building up while the shop was closed."

Axial piston pump

A piston pump with several pistons arranged parallel to and around a common drive axis, usually driven by a swash plate.

Similar definitions: inline piston pump

Example: "Many tractors use a variable-displacement          for their load-sensing hydraulics."

Back pressure

The pressure on the downstream or return side of a component or system.

Similar definitions: return-side pressure

Example: "A restricted return filter raised the          and slowed the cylinder."

Back-up ring

A plastic or fabric ring installed next to an O-ring to keep it from being extruded into the clearance gap under high pressure.

Similar definitions: anti-extrusion ring

Example: "At 5,000 psi the designer added a          beside the O-ring to prevent extrusion."

Baffle

A plate inside a reservoir that separates return oil from the pump suction so the fluid can release air, cool, and settle contaminants.

Example: "The          forces returning oil to travel the length of the tank before it reaches the suction strainer."

Bar

A metric unit of pressure equal to 100 kilopascals, or about 14.5 psi.

Example: "The European schematic listed the relief setting as 200         , or about 2,900 psi."

Bend radius

The radius of a bent hose measured to the innermost surface; bending tighter than the minimum rating shortens hose life.

Similar definitions: minimum bend radius

Example: "The hose failed early because it was routed tighter than its minimum         ."

Bent-axis pump

A piston pump in which the cylinder block is angled to the drive shaft, so the pistons reciprocate as the block rotates.

Similar definitions: bent-axis piston pump

Example: "Changing the block angle of a          changes its displacement."

Bernoulli's principle

For a flowing frictionless liquid, the total energy from pressure and velocity stays constant, so pressure drops where velocity increases.

Similar definitions: Bernoulli's law

Example: "         explains why pressure is lower in the narrow throat of a venturi."

Beta ratio

A filter efficiency rating: the number of particles larger than a given size upstream of the filter divided by the number downstream.

Similar definitions: filtration ratio

Example: "A filter with a          of 200 at 10 microns lets only 1 of every 200 such particles pass."

Bladder accumulator

An accumulator in which the liquid and the precharge gas are separated by an elastic bladder.

Similar definitions: hydropneumatic accumulator

Example: "The          was precharged with dry nitrogen, never oxygen."

Bleed-off circuit

A speed control circuit that diverts a controlled portion of pump flow directly to the reservoir instead of to the actuator.

Similar definitions: bleed-off

Example: "In a         , the flow control valve sends excess flow to tank, so the pump only builds the pressure the load needs."

Bleeding

Removing trapped air from a hydraulic or brake system, usually by opening a bleeder valve while fluid is pushed through.

Similar definitions: purging

Example: "After replacing the caliper, the technician performed          until no bubbles came from the line."

Bore

The internal diameter of a cylinder tube; it determines the piston area.

Similar definitions: cylinder bore

Example: "Doubling the          of a cylinder quadruples its piston area and force."

Bourdon tube gauge

A pressure gauge whose sensing element is a curved tube that tends to straighten as internal pressure rises, moving the pointer.

Similar definitions: Bourdon gauge

Example: "The panel used a glycerin-filled          to read pump pressure."

Boyle's law

The absolute pressure of a fixed mass of gas varies inversely with its volume when temperature stays constant.

Similar definitions: P1V1 = P2V2

Example: "         explains why compressing air to half its volume doubles its absolute pressure."

Brake caliper

A hydraulic component that houses the brake pads and clamps them against the rotor of a disc brake.

Similar definitions: caliper

Example: "Fluid pressure pushed the piston in the          against the pads."

Brake fade

A loss of braking power, usually caused by excessive heat from repeated or prolonged brake applications.

Example: "Riding the brakes down a long hill can cause         ."

Brake fluid

The hydraulic fluid used in vehicle brake systems, usually glycol-ether based and rated by DOT number; it absorbs moisture over time.

Similar definitions: DOT fluid

Example: "Old          that has absorbed water can boil and leave the pedal soft."

BSPP

British Standard Pipe Parallel, a straight 55-degree thread (ISO 228) that seals with a bonded washer or O-ring rather than on the threads; it is not interchangeable with NPT.

Similar definitions: British Standard Pipe Parallel

Example: "The imported pump had          ports, so the technician needed adapters to connect U.S. hoses."

Bulk modulus

A measure of a fluid's resistance to compression; it is the reciprocal of compressibility.

Similar definitions: stiffness

Example: "Oil has a high         , so a hydraulic circuit feels stiffer than a pneumatic one."

Burst pressure

The pressure that causes failure of a component and loss of fluid through its envelope.

Example: "Hydraulic hose is typically rated with a          four times its working pressure."

Bypass valve

A valve that opens a secondary path around a component, such as a filter, when pressure drop across it becomes too high.

Similar definitions: filter bypass

Example: "When the element clogged, the          opened and unfiltered oil flowed to the system."

Cap end

The end of a cylinder closed by the cap with no rod passing through it; fluid there acts on the full piston area.

Similar definitions: blind end, cap side

Example: "Pressurizing the          extends the cylinder with maximum force."

Carrying cost

The total cost of holding inventory, including storage, insurance, taxes, shrinkage, and the money tied up in stock.

Similar definitions: holding cost

Example: "The dealer cut its          by stocking fewer slow-moving hose sizes."

Cartridge valve

A valve whose working parts are contained in a cylindrical body that screws or slips into a standard cavity in a manifold block.

Similar definitions: screw-in valve, slip-in valve

Example: "The manifold used a screw-in          for the relief function to save space."

Case drain

A line from the housing of a pump or motor that returns internal leakage oil to the reservoir at low pressure.

Similar definitions: case drain line

Example: "Excessive flow from the          indicated that the piston pump was worn."

Cavitation

The formation and violent collapse of vapor bubbles when pressure in a liquid drops to its vapor pressure, usually at a starved pump inlet; it causes noise and pitting.

Similar definitions: pump starvation

Example: "A clogged suction strainer caused          that made the pump whine and pitted its parts."

Charge pump

A small pump that supplies makeup oil to the low-pressure side of a closed-loop circuit, such as a hydrostatic transmission, and maintains charge pressure.

Example: "If the          fails in a hydrostatic drive, the main pump is starved and the machine loses power."

Charles' law

The volume of a fixed mass of gas varies directly with its absolute temperature when pressure stays constant.

Similar definitions: V1/T1 = V2/T2

Example: "         explains why a sealed air volume expands as it heats up."

Check valve

A valve that allows flow in one direction and blocks flow in the opposite direction, usually with a spring-loaded ball or poppet.

Similar definitions: one-way valve, non-return valve

Example: "A          at the pump outlet keeps the load from forcing oil back through the pump."

Class A fire

A fire involving ordinary combustible materials such as paper, wood, cloth, and some rubber and plastics.

Example: "A burning pile of rags and cardboard is a         ."

Class B fire

A fire involving flammable or combustible liquids, flammable gases, greases, and similar materials.

Example: "A hydraulic oil spray fire is a         , so a water extinguisher should not be used."

Class C fire

A fire involving energized electrical equipment, which requires a nonconductive extinguishing agent.

Example: "A burning solenoid coil that is still powered is a         ."

Class D fire

A fire involving combustible metals such as magnesium, titanium, sodium, and lithium.

Example: "Burning magnesium shavings are a          that requires a special dry-powder agent."

Clevis

A U-shaped mount with a cross pin hole, used to attach a cylinder's rod end or cap end to a machine so it can pivot.

Similar definitions: clevis mount

Example: "The cylinder pivots on a          pin as the boom raises."

Closed-center system

A hydraulic system in which the control valves block pump flow in neutral, so the pump works against a load and pressure is maintained.

Similar definitions: closed-center circuit

Example: "Most modern tractors use a          with a variable-displacement pump that destrokes in neutral."

Closed-loop circuit

A circuit in which return oil goes directly back to the pump inlet instead of the reservoir, as in a hydrostatic transmission.

Similar definitions: closed circuit

Example: "A hydrostatic drive is a          because oil flows from the motor straight back to the pump."

Coalescing filter

A compressed air filter that causes fine oil and water aerosols to merge into droplets that can be drained away.

Example: "A          upstream of the paint booth removes oil mist from the air."

Code 61 flange

An SAE four-bolt split flange connection that seals with an O-ring on a flat face; Code 61 is the standard-pressure series and Code 62 is the heavier high-pressure series.

Similar definitions: SAE J518 flange, four-bolt flange

Example: "The large pump outlet used a         , while the 6,000 psi line required a Code 62."

Compressibility

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

Example: "Air's high          is why pneumatic cylinders are springy and hard to stop precisely."

Condensate

Liquid water (often with oil) that condenses out of compressed air as it cools.

Example: "Automatic drains remove          from the receiver tank and filters."

Conductor

Any line that carries fluid between components, including pipe, tube, and hose.

Similar definitions: fluid line

Example: "Hose is the most flexible type of          in a fluid power system."

Confined space

A space large enough for a worker to enter, with limited means of entry or exit, and not designed for continuous occupancy, such as a tank, silo, or pit.

Example: "Cleaning the inside of a large hydraulic reservoir counts as entering a         ."

Contaminant

Any unwanted material or substance that adversely affects a fluid power system, including solid particles, water, and air.

Similar definitions: contamination

Example: "Dirt is the most damaging          in hydraulic oil."

Counterbalance valve

A normally closed pressure control valve that creates back pressure to hold a suspended load and keep it from running away or falling.

Similar definitions: holding valve

Example: "The          keeps the loader boom from dropping when the control valve is centered."

Cracking pressure

The pressure at which a pressure-operated valve, such as a relief or check valve, first begins to pass fluid.

Similar definitions: opening pressure

Example: "The relief valve's          was 2,300 psi, but it reached full flow at 2,500 psi."

Crimping

Attaching a hose fitting by radially compressing its ferrule onto the hose with a set of dies.

Example: "The shop used a          machine and the manufacturer's chart to make the new hose assembly."

Cushion

A device built into a cylinder that restricts outlet flow near the end of the stroke to decelerate the piston smoothly.

Similar definitions: cylinder cushion

Example: "Without a         , the piston slammed into the end cap every cycle."

Cv

The flow coefficient of a valve, a number that expresses its flow capacity; a higher Cv means the valve passes more flow for a given pressure drop.

Similar definitions: flow coefficient

Example: "The engineer chose a valve with a larger          so the cylinder could cycle faster."

Cylinder

A device that converts fluid power into linear mechanical force and motion, usually with a piston and rod moving inside a barrel.

Similar definitions: linear actuator, ram

Example: "The dump bed is raised by a telescoping         ."

Dash size

Industry shorthand for hose and tube size in sixteenths of an inch: on tube it gives the outside diameter and on hose the inside diameter.

Similar definitions: dash number

Example: "A -08          means 8/16, or 1/2 inch."

Dead-heading

Blocking pump flow so it has nowhere to go, causing pressure to build until a relief valve opens or a component fails.

Similar definitions: dead head

Example: "Holding a cylinder against its stop is          the pump, which wastes power as heat."

Deliquescent dryer

A compressed air dryer that uses a hygroscopic material that absorbs moisture and dissolves, so the material must be replaced periodically.

Example: "The remote pump station used a          because it needs no electricity."

Demulsibility

The ability of an oil to separate from water rather than form an emulsion.

Similar definitions: water separability

Example: "Good          allows free water to settle to the bottom of the reservoir for draining."

Desiccant dryer

A compressed air dryer that adsorbs water vapor onto a porous drying material and regenerates it, reaching very low dew points such as -40 °F.

Similar definitions: regenerative dryer, adsorption dryer

Example: "Air lines exposed to freezing weather need a          instead of a refrigerated dryer."

Detent

A mechanical device that holds a directional valve spool in a selected position until it is moved again.

Similar definitions: detent position

Example: "The          held the valve in the float position after the operator let go of the lever."

Dew point

The temperature at which water vapor in a gas begins to condense; it must be referenced to a stated pressure.

Similar definitions: condensation point

Example: "If the air line's temperature falls below the         , water forms inside the pipe."

Directional control valve

A valve whose primary function is to direct or prevent flow through selected passages, such as to extend, retract, or hold an actuator.

Similar definitions: DCV, spool valve

Example: "Moving the          lever sends oil to either end of the cylinder."

Displacement

The volume of fluid moved per revolution of a pump or motor, or per stroke of a cylinder, often in cubic inches per revolution.

Similar definitions: volumetric displacement

Example: "A pump with a          of 2 cubic inches per revolution moves more oil each turn than a 1-cubic-inch pump."

Dissolved water

Water held in solution at a molecular level in oil, which cannot be seen and is not removed by draining.

Example: "Oil can look clear and still contain         ."

Double-acting cylinder

A cylinder in which fluid force can be applied to the piston in either direction, so it is powered both to extend and to retract.

Example: "A          has a port at each end for powered extension and retraction."

Double-rod cylinder

A cylinder with a single piston and a rod extending from each end, giving equal area and equal speed in both directions.

Similar definitions: through-rod cylinder

Example: "The steering axle uses a          so the wheels turn at the same rate left and right."

Drift

Slow, unwanted movement of a cylinder or motor caused by internal leakage past seals or valve spools.

Similar definitions: creep

Example: "Worn piston seals caused the raised boom to          downward overnight."

Durometer

A measure of the hardness of rubber or elastomer seals, read on a numerical scale such as Shore A.

Similar definitions: Shore hardness

Example: "A 90-         O-ring resists extrusion better than a 70 one."

Dynamic seal

A sealing device used between parts that move relative to each other, such as a rod or piston seal.

Example: "A rod seal is a          because the rod slides through it."

Efficiency

The ratio of output to input; in pumps and motors it is divided into volumetric, mechanical, and overall efficiency.

Example: "A worn pump's volumetric          drops because more oil leaks internally."

Elastomer

A natural or synthetic rubber-like material that can stretch greatly and return to its original shape, used for seals and hose.

Similar definitions: rubber compound

Example: "Nitrile is a common          for seals in petroleum hydraulic oil."

Emulsion

A milky or cloudy mixture of two liquids that do not dissolve in each other, such as water and oil.

Example: "Water contamination turned the hydraulic oil into a cloudy         ."

Energy isolating device

A mechanical device that physically prevents the transmission or release of energy, such as a disconnect switch, line valve, or block.

Example: "A push button is not an         ; a manual disconnect switch is."

EPA

The U.S. Environmental Protection Agency, which regulates the disposal of used oil, fluids, and other materials that can pollute air, water, and land.

Example: "Used hydraulic oil must be collected and recycled according to          rules."

Ergonomics

The practice of designing tasks, workstations, and tools to fit the worker's body to reduce strain and injury.

Similar definitions: human factors

Example: "Setting the workbench at elbow height is a basic          improvement."

External gear pump

A positive-displacement pump in which two meshing external gears carry fluid around the outside of the gears from inlet to outlet.

Similar definitions: gear-on-gear pump

Example: "The log splitter uses a two-stage         ."

FIFO

First In, First Out; an inventory method in which the oldest stock is used or sold first.

Similar definitions: first in, first out

Example: "The parts room uses          so hydraulic oil does not sit on the shelf past its shelf life."

Filter

A device that removes insoluble contaminants from a liquid or gas by passing it through a porous medium.

Example: "The return-line          should be changed at the interval in the operator's manual."

Fire-resistant fluid

A hydraulic fluid that is difficult to ignite and shows little tendency to spread flame, such as water-glycol or phosphate ester.

Example: "Equipment near furnaces uses a          to reduce the danger from a burst hose."

Fitting

A connector or closure for fluid power lines and passages, such as an elbow, tee, adapter, or union.

Similar definitions: connector, adapter

Example: "The technician matched the thread type before installing the replacement         ."

Fixed-displacement pump

A pump in which the volume displaced per revolution cannot be changed; flow varies only with drive speed.

Example: "A gear pump is a         , so its output changes only when the engine speed changes."

Flash point

The lowest temperature at which a liquid gives off enough vapor to ignite momentarily when a flame is applied.

Example: "A fluid with a higher          is less likely to ignite from a hot surface."

Float position

A directional valve position that connects both work ports to tank so the actuator moves freely with external force.

Similar definitions: float spool

Example: "Putting the loader valve in          lets the bucket follow the ground."

Flow

The movement of fluid caused by a pressure difference.

Example: "Pumps create         ; resistance to it creates pressure."

Flow control valve

A valve whose primary function is to control flow rate, and therefore actuator speed, often a needle valve with a bypass check.

Similar definitions: speed control valve

Example: "Turning the          knob slowed the cylinder's extension."

Flow divider

A valve that divides flow from a single source into two or more branches, often in a fixed ratio regardless of load.

Similar definitions: flow dividing valve

Example: "A          kept the two lift cylinders moving together."