Glossary and index of fluid power principles and components

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D

DATE CODE
Any combination of numbers, letters, symbols, or other methods used by a manufacturer to identify the time of manufacture of a product.

DEADBAND
The region or band of no response where an error signal will not cause a corresponding actuation of the controlled variable.

DEADTIME
Any definite delay between two related actions. Measured in units of time.

DEBURR
To remove the ragged edges from the inside diameter of a hose end or fitting; an important fabrication step for assembling Teflon® ( PTFE) in order to ensure a good seal.

DECIBEL (dB)
A non-dimensional number used to express sound pressure and sound power. It is logarithmic expression of the ratio of a measure quantity to a reference quantity.

dB (A) & (C)
A sound level reading in decibels made on the A- & C- weighted network, respectively of a sound level meter.

DECOMPRESSION
The name used to describe the change in pressure in a hydraulic system from elevated pressure to a lower pressure. Normally the change in pressure is made in a controlled amount of time to cause an even release of energy in the system. See COMPRESSION.

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

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

DEVICE
A combination of individual components that are arranged to form a unit with a specific set of operating parameters.

DIAGRAM
A formal drawing showing the arrangement of components or devices.

DIE
A tool used to swage or crimp a fitting onto a hose. Swage dies usually consist of two halves machined to a predetermined diameter, designed for a specific hose type or size. A crimp die set is typically six to eight “fingers” designed for infinite diameter settings within a range or preset to a specific diameter for a given hose type and size.

DIFFERENTIAL CURRENT
The algebraic summation of the current in the torque motor; measures in MA (milliamperes).

DIFFERENTIAL CYLINDER
Any cylinder I which the two opposed pistons are not equal.

DIFFERENTIAL PRESSURE
The value or magnitude of pressure measured as the absolute difference of the inlet pressure and outlet pressure.

DIGITAL
The production of a discrete signal based on a change in state. See ANALOG.

DIGITAL DEVICE
A device or component that responds to or produces a discrete function based on a change in state. See ANALOG.

DIGITAL SIGNAL
A voltage or current that varies between two distinct and fixed levels.

DIODE RECTIFIER
A device used to convert AC voltage into DC pulses.

DIRECT CURRENT (DC)
A steady level of electrical current, produced by a power source (e.g. battery, thermocouple, etc.) that flows in only one direction in a circuit.

Direct operated relief valve : Pressure relief valve where full flow passes through the valve chamber. Read more.

DIRECTIONAL VALVE
A valve whose primary function is to direct or prevent flow through selected channels.

DIRT CAPACITY
The measure of volume (or weight) of particles that a filter or strainer will hold at the limit of operation.

DISPLACEMENT
The volume for one revolution or stroke or for one radiant when so stated.

DITHER
A cyclic application of voltage across a solenoid or coil. Most often used to assure that the device driven by the coil or solenoid remains in a state of constant motion, thus reducing breakaway friction.

DOG-LEG ASSEMBLY
Two hose assemblies joined by a common elbow.

DOT
Department of Transportation

DOWNSTREAM
The passage beyond a device, normally at the outlet of direction of flow.

DRAIN
A passage in, or line from a hydraulic component which returns leakage fluid independently to reservoir or to a vented manifold.

DRIFT
The measure of movement of a device after a preset condition is applied. Normally drift is measured with varying temperature, although drift may be plotted against any variable, such as humidity, etc.

DUROMETER
An instrument for measuring the hardness of rubber and plastic compounds.

DUROMETER HARDNESS
A numerical value which indicates the resistance to indentation of the blunt indentor of the durometer.

DYNAMIC BEHAVIOR
Describes how a control system or an individual unit reacts with time when subjected to an input signal.

DYNAMIC ERROR
The error that results during the transient state, that is, the state when the system is moving from one steady state condition to another.