Hydraulic Terminology

ACCUMULATOR: A container that 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 the removal of pressurized fluid. Used to bleed air from the system.

CAVITATION: A gaseous condition within a liquid stream caused when pressure is reduced to the vapor pressure. This is 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. It can be incorporated into the directional control valve as the fourth position.

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

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

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

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

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

DETENT: A spring device that 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 a hose or pipe to hydraulic components.

FLOAT SPOOL (POSIT10N): A spool valve design that 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.

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

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

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

PRESSURE; BACK: The pressure encountered on a component’s downstream or return side.

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 that converts mechanical energy into hydraulic. Energy, either fixed or variable.

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

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

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

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

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

CLOSED CENTER TYPE: The inlet port is blocked from the outlet, and the work ports are neutral.

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

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

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

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

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 of valve permits simultaneous operation of two or more functions with the same oil flow. However, the total pressure requirements of all functions are accumulative.

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.

SERIES-PARALLEL TYPE: A multiple spool valve that 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.

POWER BEYOND (HIGH-PRESSURE CARRYOVER): A sleeve attachment that 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.

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