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    Hydraulics Glossary – U

    A hydraulic pump that consists of a rotor mounted off-centre in a circular cavity. As the rotor spins, vanes trap and move hydraulic fluid.

    The position of a spool and body in a servo valve or other spool valve wherein the spool is displaced a specified amount (the underlap) to expose two adjacent cavities to each other.

    To release flow (usually directly to the reservoir), to prevent pressure being imposed on the system or portion of the system.

    A valve that is normally closed and opens from a separate fluid source on rising pressure that is balanced against a precision spring. Unloading valves are installed in hydraulic systems using accumulators. The function of this hydraulic valve is to “unload” the pump of pressure until such time as a device on the hydraulic system, such as a hydraulic cylinder, actually begins operation. In this way, the hydraulic pump is allowed to operate without load (pressure) until it is needed. The unloading valve is not intended to replace the pressure relief valve. In fact, the pressure setting of the unloading valve is much lower than the setting on the pressure relief valve.