Glossary and index of fluid power principles and components A

External damage to the hose assembly caused by rubbing on a foreign object, wearing away by friction.

Absolute : A measure having as its zero point or base the complete absence ot the entity being measured.

Absolute pressure : The pressure above zero absoulte, i.e., the sum of atmospheric and gauge pressure. In vacuum realted work it is usually expressed in millimeters of mercury (mm Hg).

Accumulator : A vessel, normally cylindrical, which is used to store fluid and gas for future release of the energy in the compressed fluid and gas. Normally contains a diaphragm or piston between the fluid (liquid) and gas chambers. Fluid is normally introduced at one end and the gas at the opposite end.

The ability of the servo system to achieve the desired output.

A device for converting hydraulic energy into mechanical energy, i.e., a motor or cylinder.

A mechanical device used to align the shaft of an electric motor (or other rotary device) with the shaft of a hydraulic pump to maintain radial and parallel shaft alignment. Fittings of various sizes and material used to change an end fitting from one type to another type or one size to another    ( i.e., male JIC to male pipe adapter is often attached to a female JIC to create a male end union fitting)

Aeration : Air in the hydraulic medium causes the fluid to appear milky and components to behave erratically due to the compressive nature of air trapped in the fluid.

A valve that is fit into a hydraulic pipe line to facilitate automatic release of air trapped in the pipeline. See also AIR BLEEDER MANUAL.

A valve that is fit into a hydraulic pipe line to facilitate manually initiated release of air trapped in the pipeline. See also AIR BLEEDER AUTOMATIC.

A mechanical device which contains a fine mesh filter element. Normally attached to the top of a reservoir or tank to allow air to pass in and out of the reservoir or tank.

Alternating current (AC) : An electric current which reverses its direction and magnitude at regularly recurring intervals.

The current condition of temperature, humidity and atmospheric pressure.

The noise level in the area surrounding the machine or component to be tested with machine being tested not operating.

American wire gauge : A device used to determine the standard sizes of wire conductors for the ASTM standard B 258, predominantly used in North America.

Amplifier : A device for amplifying either voltage or current.

Amplitude of sound : The loudness of a sound.

An electronic device that requires or produces an infinitely variable signal, usually voltage or current, in response to a state change within the device.

Analog signal : An AC or DC voltage or current that represents continuously variable physical quantities of for example voltage, current, pressure, temperature or velocity.


A restraint applied to eliminate motion and restrain forces.

AND logic gate : A digital circuit in which the ouput state equals a binary 1 if and only if all of its inputs are a binary 1 at the same time.

An electrolytic process used to deposit protective or cosmetic coatings in a variety of colors on metal, primarily used on aluminum.

American National Standards Institute. A mechanical device that is used to connect two pieces of pipe together to form a pressure tight joint. ANSI flanges are round, use through bolts and/or nuts to attach two matched flanges together or to a valve or other mechanical device. See ANSI standards for pressure and temperature ratings.

Annulus area : A ring shaped area that represents the effective area of the cylinder minus the area of the cylinder rod.

American Society for Testing and Materials

Atmosphere (one) : A pressure measurement equal to 14.7 psi (1.01 bar) (101.34 kPa).

Atmospheric pressure : Pressure exerted by the atmosphere at any specific location. Atmospheric pressure at sea level is approximately 14.7 pounds per square inch or 1.01 bar.

Opposite of gain (see gain).

Attenuator (electronic) : a variable resistive device used to reduce the value of current of volatge without distorting its waveform.

Attenuator (hydraulic) : A pressure vessel used for reducing pressure spikes and vibration in hydraulic systems.

Compression or elongation along the longitudinal axis.