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    Hydraulics Glossary – R

    A hydraulic rotary actuator that directs energy in a circular motion through the use of a toothed piston that turns a toothed gear.

    A hydraulic mechanism that uses the kinetic energy of a flowing liquid to force a small amount of the liquid to a reservoir contained at a higher level.

    The rate of change of a specific output, e.g. the ramp of a pressure compensator.

    An electronic device that controls the rate of rise of a servo or proportional valve by using capacitors to limit the rate of voltage or current change to the servo or proportional valve.

    The maximum flow assigned to a specific component as the maximum desired flow at which the device will function properly.

    The maximum pressure at which a device is designed to operate.

    A physical law governing hydraulic intensifiers stating that a given pressure in a larger hydraulic cylinder can be intensified by the same pressure exerted in a smaller hydraulic cylinder. In general, the ratio of areas is inversely proportioned to the ratio of pressures, meaning the larger area has a smaller pressure and the smaller area has a larger pressure.

    Movement characterized by repeatable back-and-forth action.

    An adjustable hydraulic control valve that regulates pressure in a particular zone of a hydraulic system. A valve that decreases the downstream pressure (at the valve outlet) in order to control the flow and therefore the outlet pressure to some preset level. Normally accomplished by balancing the outlet pressure against a precision spring.

    A piping arrangement for a differential type cylinder in which discharge fluid from the rod end combines with pump delivery to be directed into the head end.

    A term used to describe a valve or device that limits the pressure in a passage.

    A non-adjustable hydraulic control valve that regulates pressure in a hydraulic system. When a critical pressure is exceeded, the hydraulic relief valve releases or exhausts flow present at its inlet port to another chamber of lower pressure potential through its outlet port.

    The addition of fluid to maintain a full hydraulic system.

    Rephasing hydraulic cylinders are two or more cylinders plumbed in series or parallel, with the bores and rods sized such that all rods extend and/or retract equally when flow is directed to the first, or last, cylinder within the hydraulic system.

    In “parallel” applications, the bore and rod sizes are always the same, and the hydraulic cylinders are always used in pairs.

    In “series” applications, the bore and rod sizes are always different, and two or more hydraulic cylinders may be used. In these applications, the bores and rods are sized such that all rods extend or retract equally when flow is applied to the first or last cylinder within the system. This hydraulic synchronization of rod positions eliminates the need for a flow divider in the hydraulic system, or any type of mechanical connection between the cylinder rods to achieve synchronization.


    A container for the storage of liquid in a hydraulic system. The size of a reservoir will depend on the capacity of the hydraulic system, as well as what is required by the hydraulic system. The reservoir should contain a large volume of oil and should provide ample oil to the hydraulic pump.

    In hydraulics, the condition created by an obstruction or restriction in the flow path.

    A device that reduces the cross-sectional flow area.

    A passage that is used to route fluid to a reservoir or tank after use in another function.

    A four-way directional valve used to reverse a double-action cylinder or reversible motor.

    A unit of measurement that indicates the number of revolutions a hydraulic pump component makes in one minute.

    The standard unit of absolute viscosity in the English system. It is expressed in pounds-seconds per square inch.

    A connector or fitting that is equipped with seals or ‘o’ rings that allow it to rotate while passing one or more fluid paths through sealed internal passages.

    An actuator that directs force in a circular motion.