A hydraulic drive carries out a method for discretely changing a positional output on the hydraulic drive. At least one displacement cylinder incrementally supplies or discharges the cylinder volume to or from the drive by displacing the cylinder piston element from a starting to an end position depending on at least one input signal. The piston element is repeatedly displaced from starting to end position and back to discretely change the positional output correspondingly to the supplied or discharged displacement volume. The cylinder is hydraulically connected to a pressure medium source supply or return line while displacing the piston element from starting to end position. The changed positional output is restored in a successive step. The cylinder chambers of the displacement cylinder are hydraulically short-circuited by a short-circuit line hydraulically separated from the supply or return line when returning the piston element from the end to the starting position.

A technique provides enhanced control over a variety of hydraulically actuated devices, e.g. flow control valves. A hydraulic control module is placed in hydraulic communication with an actuator of a hydraulically actuated device. The hydraulic control module comprises features which prevent hydraulic locking of the system. Additionally, the control module may comprise metering features to enable metered flow of actuating fluid. The features may include valves, mini-indexers, flowline configurations, or other features which maintain the ability to shift the hydraulically actuated device and/or provide metering of the actuating fluid.

Apparatus and methods for metering fluid to a fluid actuator. An example apparatus may include a hydraulic actuator, a fluid chamber, and a hydraulic directional control valve. The fluid chamber may include a piston slidably movable between first and second ends of the fluid chamber and dividing the chamber into first and second chamber portions. The hydraulic directional control valve may direct a fluid from a fluid source into the first chamber portion to cause a volume of fluid to be discharged out of the second chamber portion into the hydraulic actuator to actuate the hydraulic actuator by a distance corresponding to the volume of fluid received by the hydraulic actuator.

Apparatus and methods for metering fluid to a fluid actuator. An example apparatus may include a hydraulic actuator and a fluid chamber. The fluid chamber may include a piston slidably movable between first and second ends of the fluid chamber and dividing the chamber into first and second chamber portions, a first port extending into the first chamber portion, and a second port extending into the second chamber portion. The apparatus may further include a hydraulic directional control valve operable to direct a fluid from a fluid source into one of the first and second ports to cause a volume of fluid to be discharged out of the other of the first and second ports into the hydraulic actuator to actuate the hydraulic actuator by a distance corresponding to the volume of fluid received by the hydraulic actuator.

An inner oil passage (45) is formed inside a slidable spool (22). The slidable spool (22) defines, in its circumferential face, a first opening (46), a second opening (47) and a third opening (48). The first opening (46), the second opening (47) and the third opening (48) are communicated to the inner oil passage (45). When the slidable spool (22) is switched to a floating position (F), the first the first opening (46) is aligned with a first port end portion (25a), the second opening (47) is aligned with a second port end portion (26a), and the third opening (48) is aligned with a fourth port end portion (24a), respectively, and a first cylinder port and a second cylinder port are communicated to a tank port (24) via the inner oil passage (45).

An inner oil passage (45) is formed inside a slidable spool (22). The slidable spool (22) defines, in its circumferential face, a first opening (46), a second opening (47) and a third opening (48). The first opening (46), the second opening (47) and the third opening (48) are communicated to the inner oil passage (45). When the slidable spool (22) is switched to a floating position (F), the first the first opening (46) is aligned with a first port end portion (25a), the second opening (47) is aligned with a second port end portion (26a), and the third opening (48) is aligned with a fourth port end portion (24a), respectively, and a first cylinder port and a second cylinder port are communicated to a tank port (24) via the inner oil passage (45).