Glossary of Fluid Power Terms

Click on a letter to see terms beginning with that letter:


“AND” Device:  A control device which has its output in the logical 1 state if and only if all the control signals assume the logical state of 1.

Acetal:  A tough, stable engineering thermoplastic with a high modulus of elasticity, high strength, good rigidity, dimensional stability, resistance to moisture, solvents and other chemicals.

Active Device:  A device that has an input(s) which controls a power supply to the device.

Actuator:  An integral part of a device that transmits force causing the device to act in the intended manner.

Air:  A gas mixture consisting of nitrogen, oxygen, argon, carbon dioxide, hydrogen and small quantities of neon, helium and other gases. (see fluid)

Air Motor:  A device which converts pneumatic fluid power into mechanical torque and motion.  It usually provides rotary mechanical motion.

Ambient:  The normal condition of air, defined by temperature and humidity, surrounding a device.

Automatic Drain:  A mechanical float used to open a drain to expel excess or unwanted fluids from a device.


Balanced Valve:  A term used to denote a valve whose movable member is exposed to equal pressure on both sides to minimize the effects of changing primary pressure.

Boyle’s Law:  The absolute pressure of a fixed mass of gas varies inversely as the volume, provided the temperature remains constant. (Original pressure x original volume = final pressure x final volume)

Break-Away:  Force necessary to inaugurate sliding, expressed in same terms as friction. An excessive break-away value indicates the development of adhesion requiring higher force.


CFM:  Cubic Feet per Minute, a volumetric flow measurement at a specific pressure.

Charles’ Law:  The volume of a fixed mass of gas varies directly with absolute temperature, provided the pressure remains constant. (Original temperature x final volume = final temperature x original volume) or (Original temperature x final pressure = final temperature x original pressure)

Check Valve:  A device to prevent flow in one direction and allows full flow in the reverse direction. (See Pilot Operated Check Valve).

Coalescing Filter:  A filter using a specific element for the removal of oil aerosol particles.

Cold Flow:  The tendency for plastic materials to change their dimensions over time under applied forces.

Compressed Air:  Air at any pressure greater than atmospheric.

Compression Set:  The amount by which a rubber specimen fails to return to original shape after release of the compressive load.

Compressor:  A device which converts mechanical force and motion into pneumatic fluid power.

Condensation:  The process of changing a vapor into a liquid condensate by the extraction of heat or chilling.

Conductor:  A component whose primary function is to contain and direct fluid, (tube, pipe, hose).

Contaminant:  Any material of substance which is unwanted or adversely affects the fluid power system and/or components.

Creep:  1. The progressive relaxation of a given rubber or plastic material while it is under stress.  This relaxation eventually results in permanent deformation or “set”. 2. In regulators, the slow increase of outlet pressure indicative of internal leakage.

Cushion:  A device normally found in the end caps of pneumatic cylinders to absorb the shock of the piston.

Cv:  A dimensionless number expressing the flow capability or conductance of a fixed orifice or pneumatic device.

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

Cylinder Cap:  A cylinder end closure which completely covers the bore area.

Cylinder Capacity, Extending:  Volume required for one full extension of a cylinder.

Cylinder, Double Acting:  A cylinder in which fluid force is applied to the moveable element in either direction.

Cylinder, Single Acting:  A cylinder in which the fluid force is applied to the movable element in only one direction.


Delta-P Indicator:  Pressure drop indicator.

Detented (maintained):  The design intention of a device to maintain the state of its last actuation after the operator force is removed.

Dew Point:  The temperature at which vapors in a gas condense. For practical purposes it must be referred to a stated pressure.

Dried Air:  Air with moisture content lower than the maximum allowed for a given application, usually below the dew point.

Dump Valve:  See Quick Exhaust Valve.

Durometer:  1. An instrument for measuring the hardness of rubber.  Measures the resistance to the penetration of an indenter point into the surface of rubber.  2. Numerical scale of rubber hardness.



Filter:  A device whose primary function is the removal by porous media of insoluble contaminants from a liquid or a gas.

Fitting:  Pneumatic fittings are connectors or closures for fluid power lines and passages.

Flip Flop:  A digital component or circuit with two stable states, sufficient hysteresis or detent so that it has “memory”.  Its state is changed with a control pulse of sufficient length to insure a change.

Flow Rate:  The volume, mass or weight of a fluid passing through any conductor per unit of time.

Fluid:  A class of materials whose molecules flow over one another and tend to conform to the outline of its container.  Typically gases and liquids are classified as fluids.

Fluid Logic:  A branch of fluid power associated with digital signal sensing and information processing, using components with or without moving parts.

Fluid Power:  Energy transmitted and controlled through use of pressurized fluid.

Fluid, Pneumatic:  A fluid or gas suitable for use in a pneumatic system, usually air.

Free Air:  Air at ambient temperature, pressure, relative humidity and density.


Gauge Damper, (snubber):  A device employing a fixed or variable restrictor inserted in the pipeline to a pressure gauge to prevent damage to the gauge mechanism caused by rapid fluctuations of fluid pressure.






Lubricator:  A device which adds controlled or metered amounts of lubricants into a fluid power system.


Manifold Block:  Pneumatic manifolds are conductors which provide multiple connection ports.

Maximum Inlet Pressure:  The maximum rated gauge pressure applied to the inlet port of a regulator or valve.

Memory:  Tendency of a material or a device to return to its original shape or position after an external force is removed.

Metered-In:  A speed control circuit in which the control is achieved by regulating the flow to the actuator.

Metered-Out:  A speed control circuit in which the control is achieved by regulating the flow from the actuator.

Momentary:  The design intention of a device to return to the normal unactuated state after the external force is removed.

Muffler:  A device for reducing gas flow noise. Noise is decreased by back pressure control of gas expansion.


Non-Return Valve:  See Check Valve.

Non-Threaded:  A term applied to exhaust ports without internal threads to prevent connection.

Normally Closed, (electrical):  The state of the output or switch is ON (passing current) with no external influence.

Normally Closed, (fluid power):  Often referred to as normally not passing the state of the output or valve is OFF with no external influence.

Normally Open, (electrical):  The state of the output or switch is OFF (not passing current) with no external influence.

Normally Open, (fluid power):  Often referred to as normally passing the state of the output or valve is ON with no external influence.

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.

Operator:  A device that attaches to another assembly and applies force to the actuator of that assembly allowing it to act in the intended manner.

OR Device:  A control device which has its output in the logical 0 state if and only if all the control signals assume the logical 0 state.

O-Ring:  A ring which has a round cross-section usually made of elastomeric material and used to seal.


Packing:  A sealing device consisting of bulk deformable material of one or more mating deformable elements, reshaped by manually adjustable compression to obtain and maintain effectiveness.  It usually uses axial compression to obtain radial sealing.

Pascal’s Law:  A pressure applied to a confined fluid at rest is transmitted with equal intensity throughout the fluid.

Passive Device:  A device that does not have a dedicated supply source and operates solely on the input(s) alone.

Permanent Set:  The deformation remaining after a specimen has been stressed in tension for a definite period and released for a defined period.

Pilot Operated Check Valve:  A devise that acts like a regular check valve, but will only allow reverse flow when an external pilot signal is applied.

Pneumatics:  Engineering science pertaining to gaseous pressure and flow.

Poppet:  A component of a valve that seals or opens an internal passage across its full area to allow or prevent flow.

Port:  A terminal point of a passage in a component to which conductors can be connected.

Pressure:  Force per unit area, usually expressed in pounds per square inch (bar).

Pressure Gauge:  A device which indicates the pressure in the system to which it is connected calibrated to specific scales.

Pressure, Absolute:  The total pressure measured from absolute zero (i.e. from an absolute vacuum).

Pressure, Atmospheric:  Pressure exerted by the atmosphere at any specific location. (Sea level pressure is approximately 14.7 pounds per square inch absolute. 1 bar = 14.7 psi).

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

Pressure, Break-A-Way, (breakout):  The minimum pressure which initiates movement.

Pressure, Burst:  The pressure which causes failure of and consequential loss of fluid through the product envelope.

Pressure, Cracking:  The pressure at which a pressure-operated valve begins to pass fluid.

Pressure, Differential, (pressure drop):  The difference in pressure between any two points of a system or a component.

Pressure, Proof:  The non-destructive test pressure, in excess of the maximum rated operating pressure, which causes non-permanent deformation, excessive external leakage, or other resulting malfunction.

Pressure, Rated:  The qualified operating pressure which is recommended for a component or system by the manufacturer.

PSIA:  Pounds per Square Inch Absolute is pressure at a gauge plus the addition of atmospheric pressure at sea level.

PSID:  Pounds per Square Inch Drop-is the amount of pressure loss, in PSI incurred as air flows through a device usually caused by friction and other factors.

PSIG:  Pounds per Square Inch Gauge is an expression of force exerted by pressure, measured by a gauge without taking into consideration atmospheric pressure (14.7 at sea level).


Quick Disconnect Coupling:  A component which can quickly join or separate a fluid line without the use of tools or special devices.

Relief Valve:  A device to reduce the likelihood of over pressurization in a pneumatic system by relieving air at a given pressure set point.


Saturated Air:  Air at 100% relative humidity with a dew point equal to temperature.

SCFM:  Standard Cubic Feet per Minute, converts CFM to standard conditions of 14.7 PSI, 68 degrees F and 35% relative humidity (see CFM).

Seal, Cup:  A sealing device with a radial base integral with an axial cylindrical projection at its outer diameter.

Seal, Dynamic:  A sealing device used between parts that have relative motion.

Seal, Elastomeric:  A material having rubber-like properties; i.e., having the capacity for large deformation and rapid, substantially complete, recovery on release from the deforming force.

Secondary Pressure:  Regulated pressure downstream of a specific device.

Selector Valve:  A device which selects from separate power sources at two input ports and directs the selected source to a single output port.

Sequence Circuit:  An arrangement of devices which establish the order in which two or more phases of a circuit occur.

Shuttle Valve:  A devise that has no supply source and selects either an A or B input (or both) as an output. Sometimes call an “OR”. (A or B or both)

Silencer:  A device for reducing gas flow noise. Noise is decreased by tuned resonant control of gas expansion.

Squeeze:  Cross section diametric compression of O-ring between surface of the groove bottom and surface of the other mating metal part in the gland assembly.

Standard Air:  Air at a temperature of 68.8° F, a pressure of 14.7 pounds per square inch absolute, and a relative humidity of 36% (0.0750 pounds per cubic foot).



Unbalanced Valve:  A term used to describe a condition where primary pressure is allowed to act as a closing force across the valve seat area.


Vacuum:  Pressure less than ambient atmospheric pressure measured in inches of mercury (inHg).

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

Valve, Air:  Usually a valve for controlling air direction or flow.

Valve, Directional Control:  A valve whose primary function is to direct or prevent flow through selected passages.

Valve, Directional Control, 3-way:  A directional control valve whose primary function is to pressurize and exhaust a port.

Valve, Directional Control, 4-way:  Directional valves whose primary function is to pressurize and exhaust two ports.

Valve, Flow Control, (flow metering):  A valve whose primary function is to control flow rate in a single direction and full flow in the opposite direction.

Valve, Needle:  A flow control valve in which the adjustable control element is a tapered needle. Its usual purpose is the bidirectional control of flow.

Valve, Pilot:  A valve applied to operate another valve.