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Hydraulic Terms Glossary

Hydraulic Info

Find the most common hydraulic terms and what they mean.

Hydraulic Terms: A-C

ACCUMULATOR: A container which stores fluid under pressure. Used as an energy source or to absorb hydraulic shock. Common types are piston, bladder and diaphragm.

BLEEDER (BLEED VALVE): A device for removal of pressurized fluid. Used to bleed air from system. 
CAVITATION: A gaseous condition within a liquid stream caused when pressure is reduced to the vapor pressure. To be avoided due to destructive effects on pumps and motors.

CIRCUIT; PILOT: Used to control a main circuit or component.

CIRCUIT; REGENERATIVE: Used to increase cylinder speed by directing rod end discharge to the piston side of the cylinder. Can be incorporated into directional control valve as fourth position.

CLOSED CENTER TYPE: Has the inlet port blocked from the outlet and work ports in neutral.

CYLINDER: A device which converts hydraulic energy into linear mechanical motion and force.

CYLINDER; DOUBLE ACTING: A cylinder which can apply force and motion in either direction.

CYLINDER; SINGLE ACTING: A cylinder which can apply force in one direction only.

CYLINDER; DEPTH CONTROL: A mechanical or hydraulic device, adjustable, for limiting cylinder stroke.

CYLINDER; REPHASING: A cylinder design which permits the use of two or more cylinders in series, automatically synchronizing cylinder position at the end of each stroke.

Hydraulic Terms: D-F

DETENT: A spring device which maintains the spool of a directional control valve in position.

DETENT RELEASE: A mechanical, hydraulic or electrical device for releasing the detent.

FILTER: A device incorporated into a hydraulic system to remove contaminants from the oil.

FITTING: A device for connecting hose or pipe to hydraulic components.

FLOAT SPOOL (POSIT10N): A spool valve design which connects all ports to the tank (return) port, usually in a detented fourth position, allowing a cylinder or motor to “float”.

FLOW RATE: The volume of fluid passing through the system or component in gal. per min. (or 11m).

FLUID POWER SYSTEM: The transmission and control of power through the use of fluid pressure.

FOUR-WAY: A 4 port valve (in, out and 2 work) used with double acting cylinders, bi-directional motors.

Hydraulic Terms:L-P

LOAD CHECK (LIFT CHECK): A device which prevents a load from dropping when a valve is shifted, until ample pressure and flow is available to hold or move the load.

MOTOR: A device which converts hydraulic energy into rotary motion, either fixed or variable.

OPEN CENTER (TANDEM) TYPE: Has the inlet port connected to the outlet (tank) port in neutral.

PARALLEL TYPE: A multiple spool valve in which the inlet oil is connected to all spools simultaneously. If more than one spool is actuated, the function requiring the lowest pressure will operate first.

PORT: The internal or external terminus of a passage. The point where the fitting is attached.

POWER BEYOND (HIGH PRESSURE CARRYOVER):A sleeve attachment which permits the oil flow from one valve (when in neutral) to be used by another valve downstream. Hence, a 3-spool valve cou1d be connected to a 2-spool valve to create a 5-spool valve, The first valve takes priority and thrust have a separate outlet port to return oil from an activator back to the reservoir.

PRESSURE: The force per unit area, expressed in pounds per square inch (psi), bars, or atmospheres.

PRESSURE; BACK: The pressure encountered on the downstream or return side of a component.

PRESSURE; CRACKING: The pressure at which a pressure operated valve begins to pass fluid.

PRESSURE; MAXIMUM RATED: The maximum pressure at which a component should be operated on a continuous basis, usually the relief valve setting at maximum flow rate.

PUMP: A device which converts mechanical energy into hydraulic. energy, either fixed or variable.

Hydraulic Terms: R-V

RESERVOIR: A container which stores the liquid in a fluid power system.

SEAL: A device which prevents or controls the escape or passage of hydraulic fluid.

SERIES TYPE: A multiple spool valve in which the return oil from the first spool is directed to the inlet of the second spool (and from the second to the third, etc.). This type valve permits simultaneous operation of two or more functions with the same oil flow. However, the total pressure requirements of all functions are accumulative.

SERIES-PARALLEL TYPE: A multiple spool valve which has all spools connected to the open center passage in neutral. However, when actuated, the upstream valve takes full priority. The return oil is directed to downstream spools as in a series type valve.

THREE-WAY: A 3 port valve (in, out ~r,d work) normally used with a single acting cylinder or uni-directional motor.

TWO, THREE, FOUR POSITION: The number of positions in which a valve can be positioned.

TWO-WAY: A 2 port valve with inlet and outlet ports.

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

VALVE; DIRECTIONAL CONTROL: A device for directing or preventing the oil flow in a system.