Fluid Power Glossary

ABSOLUTE PRESSURE: Pressure which does not include the weight of the earth's atmosphere. Absolute pressure is normally used instead of gauge pressure when doing calculations that involve vacuum (so you don't have to use negative numbers). You can calculate absolute pressure by subtracting 14.7 psi from gauge pressure.

ACCUMULATOR: A vessel which uses compressed gas and fluid to store energy. Accumulators are normally cylindrical and use either a piston or diaphragm to separate the fluid and the gas. Two common uses for accumulators are to provide emergency power in case of a power failure and to provide a momentary source of additional flow during a system's cycle.

ACTUATOR: A device which converts the energy in a fluid power system into mechanical force. This would typically be a motor, a cylinder, or a rotary actuator.

AERATION: When air is trapped in hydraulic fluid.

AIR BREATHER: A device with a coarse mesh filter which is normally attached to the reservoir or tank and allows air to pass in and out of the reservoir or tank. A "standard" filtered breather is rated at 40 micron.

ANNULAR AREA: A ring shaped area. On the rod end of a cylinder, you calculate the annular area by subtracting the rod area from the piston area. When you put oil into the rod end of a cylinder, the oil pressure works against the annular area of the cylinder.

ANSI FLANGE: A flange manufactured to meet ANSI standards for size, pressure ratings and temperature ratings.

ATMOSPHERE: A unit of pressure that is equal to approximately 14.7 psi (pounds per square inch).

- B -

BACK PRESSURE: The amount of pressure in the return or downstream side of a fluid power system. The back pressure is typically caused by the friction of the fluid passing through the plumbing and the pressure required to push the fluid through any downstream components (such as return line filters, heat exchangers and diffusers).

BAFFLE: A mechanical separator put in a reservoir, tank, or other device to direct the flow of a fluid. Baffles can be used to reduce turbulence, reduce aeration, and improve heat dissipation.

BALL VALVE: A type of valve which typically uses a ball mounted on a stem to control the flow of fluid. A two-way ball valve has one hole drilled through the ball. When the hole lines up with the inlet and outlet ports of the valve, the valve is open and allows flow. Then the ball is turned so the hole does not line up with the valve ports, the flow path is blocked and the valve is closed.

BAR: A unit of pressure in the metric system. One bar equals 14.5 psig

BETA RATIO: A way to express how well a filter traps particles of a given size. The Beta Ratio for a particular size particle is determined by dividing the number of particles upstream of a filter by the number of particles downstream of the filter. The Beta Ratio of a filter allows you to compare the effectiveness of different filters.

BLADDER: A flexible diaphragm normally used in an accumulator as a barrier between fluids and gases.

BREATHER: A device which typically has a coarse mesh filter and is normally attached to the reservoir or tank. It allows air to pass in and out of the reservoir or tank.

BURST PRESSURE: The pressure at which a component, hose, tube or pipe will burst. Burst pressure is commonly 2.5 to 4 times the rated working pressure.

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CASE DRAIN LINE: A line or passage from an internal cavity of a pump, valve, or other component that carries fluid leakage from the device to the reservoir or tank.

CAVITATION: A condition which may occur in a hydraulic pump when the vacuum on the inlet side of the pump is too high. During the high vacuum condition, small vacuum "bubbles" are created in the fluid. The vacuum "bubbles" rapidly collapse (implode) as the fluid pressure increases when the fluid moves from the inlet side of the pump to the pressure side of the pump. The extreme heat created by the implosion melts the metal in the pump away into vapor. The size of the imploding vacuum "bubble" is so small that the heat transfer to the pump occurs in a very small area, creating "pinholes" in the pump.

CHECK VALVE: A two port valve that allows flow in only one direction.

CLOSED CIRCUIT: A circuit which has the pump output passing through the plumbing and various components and then returning directly to the pump inlet instead of going to a tank or reservoir.

COMPENSATOR CONTROL:A control used on variable volume pumps and motors which changes the component's displacement in response to pressure changes in the system.

COOLER: A component used to transfer heat from a fluid to either air or another fluid. Also called a "heat exchanger."

COUNTERBALANCE VALVE: A valve commonly used to balance the weight of a dead load by creating a back pressure in the system cylinders that is high enough to support the weight.

COUPLING: A device used to attach the rotating shaft of a hydraulic pump or motor to another device.

CRACKING PRESSURE: The pressure at which fluid begins to pass through a pressure operated valve.

CUSHION: An assembly fitted into a hydraulic cylinder that restricts the fluid flow path during the last part of the cylinder stroke. This cushions the cylinder's stop at the end of the stroke.

CYLINDER: A device which converts hydraulic power into linear mechanical force when hydraulic fluid works against the cylinder's piston. Common types of cylinders include double-acting, single-acting, and telescoping.

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DECOMPRESSION: When the pressure in a hydraulic system drops. Normally the change occurs in a controlled amount of time to cause an even release of energy in the system.

DIFFERENTIAL PRESSURE: The difference between the inlet pressure and the outlet pressure.

DIRECTIONAL VALVE: A valve which directs flow or prevents flow through selected ports. A two-way directional control valve has two ports. A three-way directional control valve has three ports, and a four-way directional control valve has four ports.

DISPLACEMENT: The volume of fluid required for one revolution (rotary devices like pumps and motors) or one stroke (linear devices like cylinders). Typically measured in cubic inches.

DITHER: A constant cycling of the voltage of a solenoid or coil. By constantly and quickly cycling the voltage, the device driven by the solenoid or coil is kept in constant motion, which reduces the breakaway friction.

DOWNSTREAM: The portion of the circuit beyond a given device.

DRAIN LINE: A passage (hose, tube or pipe) that provides a low pressure passage from components to the reservoir or tank.

DUROMETER: A measurement of the hardness of a rubber or other synthetic compound.

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EFFICIENCY: The ratio of output to input, normally expressed as a percent. The volumetric efficiency of a pump is calculated by dividing the actual pump output (normally expressed as gpm) by the theoretical output. Overall efficiency of a hydraulic system is calculated by dividing the output power by the input power.

ELECTROHYDRAULIC: Using electrical input to control hydraulic output.

EXHAUST LINE: A line which is open to atmosphere. Exhaust lines are normally used in systems that use pressurized air or gas which may be vented into the atmosphere.

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FILLER CAP: A device typically installed on reservoirs to facilitate filling the reservoir with fluid. Filler caps normally have a fine screen to filter out large dirt particles.

FILTER: A device placed in a hydraulic circuit to remove small particles and silt from the fluid. Filters can be placed in the suction line (suction strainers and suction filters), in a pressure line (pressure filters), or in the return line (return line filters).

FLOW CONTROL VALVE: A valve used to control the amount of flow entering or leaving a segment of the hydraulic circuit. Pressure compensated flow control valves have a constant flow rate regardless of operating pressure, while the flow through non-pressure compensated flow control valves may fluctuate as the pressure changes.

FLOW DIVIDER: A device which separates the fluid flow into two or more paths.

FLOW RATE: The amount of fluid passing through a conductor during a given period of time. Typically measured in gallons per minute (GPM).

FLOW SWITCH: A switch that opens or closes when a preset flow passes over the sensing device.

FLOW METER: A device which measures the volume of fluid.

FLUID: The media in fluid power system used to transfer power. Many systems use a petroleum based oil, but synthetic fluids are also available.

FLUID MOTOR: A device that converts hydraulic power into rotary mechanical power.

FLUID POWER: Using a fluid to transmit energy and do work.

FORCE: The result of a pressurized fluid working against a mechanical surface. In hydraulic systems, force is typically generated when fluid works against the piston of a cylinder, the working surface of a hydraulic motor, or the working surface of a rotary actuator.

FOUR WAY: A valve which has four ports, typically referred to as pressure (the inlet port), tank (the exhaust port), A (one of the working ports), and B (the other working port). Four way valves normally have either two or three positions and can be used to start, stop, and change the direction of an output device.

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GASKET: A type of seal which normally has a flat cross-section.

GATE VALVE: A two-way valve which normally allows full flow when open, thus providing very little resistance to the fluid passing through.

GAUGE: A device used to measure the pressure in a system.

GAUGE PRESSURE: A term used to indicate the pressure uses atmospheric pressure as the 0 psi reference point. It is normally referred to as psig (pounds per square inch, gauge). 100 psig is the equivalent of 114.7 psia (psi absolute).

GLAND: A device used to contain a seal, o-ring or gasket in a given space in order to create a leak-proof seal.

GLOBE VALVE: A two-way valve that typically directs the fluid in a non-straight path.

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HEATER: An electrical device normally used to raise the temperature of the fluid in a reservoir.

HEAT EXCHANGER: A component used to transfer heat from a fluid to either air or another fluid. Also called a "cooler"

HORSEPOWER: A measure of energy used to describe the amount of power in a system. 1 horsepower = 550 foot pounds per minute of work. It is commonly believed that Robert Fulton created the horsepower measurement as a way to measure and compare the amount of power generated by his steam engine.

HOSE: A flexible conductor used to transport fluid in a system.

HYSTERESIS: The difference between the response or a unit or system when the signal or pressure is increasing and the response of that same unit or system when the signal or pressure is decreasing.

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INTAKE LINE: The plumbing conductor connected to the inlet port of a component, normally referring to the hose, tube or pipe connected to the inlet port of a pump.

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LAMINAR FLOW: Flow of fluid going in a relatively straight path through the center of conductor. When the flow is not laminar it is normally turbulent.

LEVEL SWITCH: An electromechanical device that opens or closes a digital switch when the fluid in a given area reaches a certain level.

LINEAR ACTUATOR: A device which converts hydraulic energy into linear motion, typically a cylinder.

LITER: A metric measurement of volume. One liter equals 0.2642 gallons.

LUBRICATOR: A device typically used in a pneumatic system that injects small amounts of oil into the pneumatic system to lubricate the pneumatic components.

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MANIFOLD: A block of material (typically steel or aluminum) which has internal passages to control the direction of flow. Manifolds are often manufactured with ports and mounting surfaces to accommodate directional control and pressure control valves, allowing hydraulic circuits to be constructed with a minimum of plumbing.

METER IN: Regulating the flow of fluid (normally with a flow control valve) as it enters the actuator or system.

METER OUT: Regulating the flow of fluid (normally with a flow control valve) as it leaves the actuator or system. Metering out normally gives better speed control than metering in.

MICRON: A measurement typically used to describe the particle size of contamination in a fluid power system. One micron is 1/1000th of a millimeter or 0.00003937 inches. Many component manufacturers specify a minimum filtration level of 10 microns. A typical white blood cell is 20 microns and a typical red blood cell is 5 micron.

MUFFLER: A pneumatic device which uses a series of baffles to create a complex path for air to follow as it exhausts to atmosphere, thus reducing the noise level of the exhausting air.

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NEEDLE VALVE: A two-way valve that controls the flow of fluid. Needle valves are normally mechanically operated, and turning the control knob clockwise gradually closes off the flow path through the valve while turning the control knob counter-clockwise gradually opens the flow path through the valve.

NIPPLE: A short length of pipe which is typically threaded on both ends.

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O-RING: A seal, normally made from rubber or other synthetic material, which is in the shape of a circle and has a round cross section.

OPEN LOOP CIRCUIT: A circuit which sends the fluid back to a tank or reservoir.

OPERATING PRESSURE: The maximum pressure a fluid power system is expected to reach during normal operation.

ORIFICE: A small opening designed to restrict the fluid flow.

OVERLAP: When a spool valve's valve spool contacts the valve body and prevents fluid flow between two adjacent valve chambers. The valve spool must move the length of the overlap before fluid can flow between the adjacent valve chambers.

OXIDATION: The plating of an oxygen/fluid mixture on to metal surfaces after oxygen has been absorbed into a fluid.

- P -

PACKING: A seal or gasket.

PARTICLE: A piece of contamination found in a fluid.

PARTICLE COUNT: A count of the various pieces of contamination found in a fluid sample of a specified size. Particle counts typically provide results based on the number or particles within various size ranges.

PHOSPHATE ESTER FLUID: A synthetic hydraulic fluid made from an ester base which will normally not support combustion. Care should be taken when selecting components for a phosphate ester system to insure the seals are compatible and the suction characteristics are adequate.

PILOT LINE: A separate connector or line which provides dedicated pilot pressure to components in the circuit.

PILOT PRESSURE: The pressure in the pilot line - typically lower than system pressure.

PILOT VALVE: A valve used to control the operation of another (typically larger) valve or control. In a two-stage or "piggy back" valve, the pilot valve is the controlling valve.

PIPE: A rigid fluid conductor made of metal. Increasing the wall thickness of a pipe does not increase the outside diameter of the pipe.

PISTON, CYLINDER: The portion of the cylinder which hydraulic fluid works against to generate force. Cylinder pistons typically have a cylinder rod attached to one or both sides.

POPPET: A part of certain valves that opens or blocks flow by working against a seat.

PRE-FILL VALVE: A valve which normally has one port connected to the reservoir and a second port connected directly to a hydraulic components (typically a cylinder). During a non-working portion of the machine cycle, the pre-fill valve allows a large volume of oil to pass from the reservoir into the component. When the circuit is doing work, the pre-fill valve closes and keeps the high pressure fluid in the component.

PRESSURE COMPENSATOR: A device on a pressure compensated pump which decreases flow when system pressure reaches a pre-set pressure and increases flow as the system pressure drops.

PRESSURE DIFFERENTIAL: The difference in pressure between any two points in a fluid power system.

PRESSURE DROP: The pressure differential as fluid moves through a connector, component, or circuit.

PRESSURE GAUGE: A device to measure the pressure in a fluid power system. Gauges that display "gauge pressure" have a scale which uses atmospheric pressure as the 0 psi reference point. Gauges which display "absolute pressure" have a scale which uses absolute vacuum as the 0 psi reference point. 114.7 psia = 100 psig.

PRESSURE LINE: A fluid conductor (pipe, tube or hose) which carries high pressure fluid, typically from a pump to the valve(s) or from the valve(s) to the actuator(s).

PRESSURE REDUCING VALVE: A valve which reduces the pressure in a particular portion of the circuit by venting a small amount of fluid back to the reservoir.

PRESSURE SWITCH: An electromechanical device that closes or opens an electrical switch when a preset pressure is reached.

PRESSURE TRANSDUCER: An analog device that generates an electrical signal that changes voltage or current in response to changes in fluid pressure.

PRIME MOVER: The device providing the primary source of input power for a fluid power system. This is typically an electric motor or a diesel engine.

PROOF PRESSURE: The highest pressure that a component or fluid conductor may be subjected to without having that component or conductor yield. Proof pressure is normally 1.5 times working pressure.

PSI: A measurement of pressure - Pounds per Square Inch.

PSIA: A measurement of pressure using absolute vacuum as the 0 psi reference point - Pounds per Square Inch Absolute. 114.7 psia = 100 psig.

PSIG: A measurement of pressure using atmospheric pressure as the 0 psi reference point - Pounds per Square Inch Gauge. 100 psig = 114.7 psia

PUMP: A device which when driven by a prime mover will draw low pressure fluid into an inlet port and push high pressure fluid from an outlet port. A pump can be a hand or air operated linear device, or a rotary device that uses gears, vanes or pistons to move the fluid.

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QUICK DISCONNECT COUPLER: A mechanical device, typically consisting of two mating halves, which allows a fluid conductor to be easily connected or disconnected.

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RAM: A single acting cylinder which extends as fluid enters the inlet port, but must be retracted by either external cylinders or the weight of the load.

RATED FLOW: The maximum flow that a fluid conductor, component, or system should see during normal operation.

RATED PRESSURE: The maximum pressure that a fluid conductor, component or system should see during normal operation. See also "Burst Pressure" and "Proof Pressure."

REGENERATIVE CIRCUIT: A circuit which combines outlet flow from the rod end of a differential area cylinder with the pump flow entering the blind end of the same cylinder. This allows the cylinder to extend rapidly, but reduces the output force to that generated by system pressure operating against the rod area.

RELIEF VALVE: A valve which limits the maximum pressure in the system by dumping fluid from the high pressure line to a lower pressure line when the pressure setting of the relief valve is reached.

REPLENISH: Adding fluid to maintain a full hydraulic system.

RESERVOIR: A container that holds fluid. A reservoir in an industrial fluid power system typically holds three times as much fluid as the pump's capacity (a 60 gallon reservoir for a 20 gpm pump), has a baffle separating the suction and return ports, has a clean-out port and a filtered filler/breather cap.

RETURN LINE: A fluid conductor (hose, pipe or tube) that carries fluid from a component or actuator back to the reservoir. Typically limited to low pressures of no more than 150 psi.

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

ROTARY ACTUATOR: A rotary device that converts hydraulic power to work. Unlike a hydraulic motor, which continues to rotate as long as fluid is routed to the motor, a rotary actuator turns a maximum of 360 degrees and may be limited to 90 degrees or 45 degrees.

- S -

SAE PORT: A standard for various sizes, configurations and pressure ratings of flanges and surface mounting configurations that are used to attach pipes, tubes or hoses to a component or manifold.

SAFETY FACTOR: The ratio of burst pressure to rated pressure, calculated under specific static pressure and temperature conditions.

SCRAPER RING: A ring fitted to the rod of a cylinder designed to mechanically remove particles from the rod and prevent them from entering the cylinder seal area.

SEQUENCE VALVE: A pilot-operated two-way valve that is pilot operated and shifts when the pilot pressure reaches a preset pressure level. Sequence valves are normally used to start a secondary set of operations.

SERVO VALVE: A valve which uses an electrical force motor to control a small stream of hydraulic fluid. The controlled hydraulic fluid is then used to position a large spool. There is normally a mechanical feedback mechanism that transmits the exact spool position to the force motor, creating a closed loop control system.

SHUTTLE VALVE: A valve which has two inlet ports and one outlet port. The two inlet ports share a common ball spool type check valve. When flow reaches either of the inlet ports, the outlet port receives flow from the inlet port with the highest pressure.

SILT: Very fine particles of contamination, normally found in areas of the system with little or no circulation (such as in portions of a reservoir or in the pilot areas of directional valves).

SLUDGE: Partially hardened silt.

SOLENOID: A coil of wire wound around a bobbin. When electricity is applied to the coil of wire, a magnetic field is created which produces linear motion in a solenoid spool that is inside the bobbin.

SPOOL: A cylindrically shaped part of a hydraulic component. The spool moves to direct flow within the hydraulic component.

STRAINER: A collection of wire or fabric meshes that are bonded together and are used to strain large particles from the fluid which passes through the strainer.

SUBPLATE: a metal base which has inlet, outlet, and working ports, designed to hold a specific valve.

SUCTION LINE: A fluid conductor (hose, pipe or tubing) which goes from a reservoir to the inlet port of a pump.

SUPERCHARGE: To raise the pressure of the inlet port of a hydraulic pump to above atmospheric pressure.

SURGE: A temporary increase in pressure that is above the normal expected working pressure.

SYNTHETIC FLUID: A hydraulic fluid made from a synthetic base, normally used because it will not support combustion.

SYSTEM PRESSURE: The maximum pressure expected when the system is operating as designed.

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TANK: Also called a reservoir. A container that holds fluid. A reservoir in an industrial fluid power system typically holds three times as much fluid as the pump's capacity (a 60 gallon reservoir for a 20 gpm pump), has a baffle separating the suction and return ports, has a clean-out port and a filtered filler/breather cap.

THREE WAY VALVE: A valve which has three ports - typically one inlet port and two outlet ports. The inlet port is the pressure port. One of the outlet ports is normally open (flow is allowed through) and the other outlet port is normally closed (no flow allowed).

TIE ROD: A threaded metal rod used to hold two or more components together. Typically used to hold the two end plates of a hydraulic cylinder against the cylinder tube.

TORQUE: A measure of the force applied to a lever arm, normally expressed in foot-pounds or inch-pounds.

TORQUE MOTOR: A hydraulic motor which typically operates at low speed while generating high amounts of torque.

TUBE: A rigid fluid conductor which can be bent during installation. Tubing is generally specified by outside diameter, wall thickness, and material type.

TURBINE: A rotary device that is driven by a moving stream of fluid.

TURBULENT FLOW: Fluid in the pipe, hose or tubing is churning and bouncing off the walls of the fluid conductor.

TWO WAY VALVE: A valve which has two ports - normally an inlet port and an outlet port. Two way valves are used to open or close a flow passage and can be configured as normally closed (no flow through the valve) or normally open (fluid flows through the valve).

- U -

UNLOADING VALVE: A valve which is normally closed and opens when a pilot signal from a separate fluid source reaches the pressure setting on the unloader valve. Unloader valves are often used in "hi-lo" circuits on log splitters.

UPSTREAM: The portion of a circuit before a given component. A pump is normally upstream of the relief valve.

- V -

VACUUM: Pressure less than atmospheric pressure, typically expressed in inches of mercury (Hg).

VACUUM GAUGE: A gauge which has atmospheric pressure as the reference point for 0 psi and will measure the level of pressure below atmospheric pressure.

VALVE: A mechanical device used in a fluid power system to control the flow, speed of flow, or direction of flow of the fluid.

VAPOR PRESSURE: The pressure at which a given fluid will turn into a gas.

VELOCITY: The distance traveled by a fluid in a fluid conductor during a specific period of time. May be expressed in terms such as feet per second, inches per second and meters per second.

VENT VALVE: A valve which may be opened to allow high pressure fluid to exhaust to a lower pressured portion of the circuit or to atmosphere.

VISCOSITY: A measurement of a fluid's resistance to flow based on established standards.

VISCOSITY INDEX: A measure of the viscosity characteristics of a fluid a various temperatures as compared to that of two arbitrary reference fluids.

VOLUME: Length times width times height - normally expressed in cubic inches.

- W -

WAFER VALVE: A two way valve which uses a stack of "wafers" to control the flow of fluid. Normally designed to provide low pressure drop when open.

WATER GLYCOL FLUID: A mix of water and glycol which has enough lubricity to function as a hydraulic fluid and is relatively fire resistant.

WIPER RING: A mechanical seal that surrounds a moving shaft to create a low pressure seal.