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Glossary of Terms - All Terms

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Glossary Subsets

DC Power (Direct Current)

Electrical flow in only one direction.


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


A separator that removes air from the system.


Removing air from a liquid, usually by ultrasonic or vacuum methods.


A separator that removes water from the system fluid.


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

Delta Connection

A common three phase connection shaped schematically like the Greek Delta. The end of one phase is connected to the beginning of the next phase, or vice versa.

Delta P

The change in pressure between two points in a system, such as in a passageway or between the inlet and outlet of a componet. Also expressed as DP.


Separate water from oil.

Design Horsepower (DHP)

A rating achieved by multiplying horsepower by the service factor.


An additive that neutralizes contaminants in oil before they become insoluble.

Dielectric Strength

The voltage an insulating material can withstand before breaking down.

Differential Pressure Indicator

An indicator which signals the difference in pressure between any two points of a system or a component.

Direct Current (DC)

Electrical flow in only one direction.

Directional Valve

A valve which selectively directs (or prevents) fluid to specific ports.


Often used as a rectifier, a diode is a two electrode electron tube containing an anode and a cathode.


A term usually used interchangeably with detergent. An additive which keeps fine particles of insoluble materials in an oil. Particles are not permitted to settle or accumulate.


The quantity of fluid which can pass through a pump, hydraulic motor, or cylinder in a single revolution or stroke.

Double Acting Cylinder

A cylinder in which fluid force can be applied in either direction.

Double Row Bearing

A bearing with two rows of rolling elements.


A port or a line from a hydraulic component which returns fluid independently to a reservoir or vented manifold.

Drawn Cup Needle Roller Bearing

A needle roller radial bearing with thin pressed steel outer ring. This design, also referred to as a drawn cup needle bearing, usually has both ends open, but may have one closed end.

Drip Cover

A cover on the top of a motor that protects it from solids and/or liquids that might drop onto it.

Dropping Point

The temperature at which a grease changes from a semi-solid to a liquid state under test conditions. It may be considered an indication of the high temperature limitation for application purposes.

Duplex Bearing

A bearing with controlled axial location of faces of inner and outer rings. This makes it suitable for mounting in various combinations with one or more bearings controlled in the same manner.

Duplex Filter

An assembly of two filters with valving that allows use of either or both filters.

Duty Cycle

The ratio of the lengths of the positive to negative halves of a waveform expressed in percentages.

Dynamic Braking

• A quick way to stop a motor by disconnecting the power source. The rotating motor then becomes a generator.
• When connected to a resistor, the energy of rotation is then dissipated as heat in the resistor.

Dynamic Load

A load exerted on a bearing that is in motion.