Hydraulics Glossary: D

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A glossary of the key terms you may encounter when working with hydraulic systems. Introducing the letter 'D'…

DEAD TIME:
A real, definite delay between two related actions, measured in units of  time.
DEAD-WEIGHT ACCUMULATOR:
A hydraulic component used to store hydraulic fluid. The dead-weight accumulator increases and relieves pressure through the use of a weighted vertical piston. See also under ACCUMULATOR.
The removal of ragged edges from the inside diameter of a fitting or hose end. An essential task to ensure a good seal.
The change in pressure in a hydraulic system from elevated pressure to a lower pressure. Such a change in pressure is normally made in a controlled amount of time to cause an even release of energy in the system.
The volume of fluid discharged by a pump in a given time, usually expressed in gallons per minute (gpm).
Ability of an oil to separate from water. The resistance of a hydraulic fluid to emulsification, or how well a hydraulic fluid resists mixing with water.
A spring device which maintains the spool of a hydraulic directional control valve in position.
A mechanical, hydraulic or electrical device for releasing the detent.
To close the vent connection of a pressure control valve, allowing the valve to operate at its adjusted pressure setting.
Removal of paraffin wax from lubricating oils to improve low temperature properties.

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A differential hydraulic cylinder acts like a normal cylinder when pulling. However, if the hydraulic cylinder has to push, the hydraulic oil from the piston rod side of the cylinder is not returned to the reservoir, but instead goes to the bottom side of the cylinder. In such a way, the cylinder goes much faster, but the maximum force the cylinder can give is like a plunger cylinder. A differential cylinder can be manufactured like a normal cylinder, and only a special control is added.
The value of pressure measured as the absolute difference between the inlet pressure and outlet pressure.
A steady level of electrical current that only flows in one direction in a circuit.
A hydraulic valve whose main function is to prevent or direct flow through particular channels.
The volume or weight of particles that a strainer or filter will hold at the limit of its operation.
An indication of the internal clearance of a hydraulic pump. Hydraulic pumps with high dirt tolerance have the most clearance, are the least efficient, and allow the most backflow. Hydraulic pumps with low dirt tolerance have the least amount of clearance, are the most efficient, and allow little to no back-flow.
The amount of liquid transferred from the inlet of the hydraulic pump to the outlet in a single revolution. Displacement can be fixed or variable.
Two hose assemblies joined by a common elbow.
A hydraulic cylinder which can apply force and motion to the moveable element in either direction.
Other nomenclature “two-way sleeve”. Allows one hand push-to-connect or pull-to-disconnect convenience when the quick-action coupling female half is clamp or bulkhead mounted and connected to a hose.

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The passage beyond a device, normally at the outlet of direction of flow.
A passage in, or line from, a hydraulic component which returns leaked fluid to the reservoir or a vented manifold.    
The amount of movement of a device after a pre-set condition has been applied. Normally drift is measured with varying temperature, although drift may also be plotted against any variable, such as humidity, etc.
The gear in a hydraulic gear pump that is attached to the driving mechanism. The drive gear turns the driven gear.
The gear in a hydraulic gear pump that is rotated by the drive gear.
DRIVING MECHANISM:
A component of a hydraulic gear pump that rotates the driving gear.
A measure of elastomer hardness by use of a durometer.
A removable device that protects the male tip half when disconnected from the female coupling half. Excludes contamination.
A removable device that protects the female body half when disconnected from the male tip half. Excludes contamination.

If you’d like to learn more about hydraulic cylinders such as how they work, or why there are so many different types, our free “ultimate Guide to Hydraulic Cylinders” is a great place to start!

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