The invention relates to a hydraulic controller (1), in particular for the control of the rotating mechanism of an excavator. A hydraulic motor (5) is driven by means of a hydraulic pump (2) in a hydraulic drive circuit, the hydraulic pump (2) and the hydraulic motor (5) being connected by a first and a second working line (3, 4). The hydraulic controller (1) includes an adjustment arrangement (26) for adjusting a setting piston (28a), acting on the displacement volume of the hydraulic pump (2), arranged between two setting pressure chambers (26, 27), in dependence upon the pressure difference between two control lines (20, 21). In accordance with the invention, a respective separate brake valve (24; 25) is provided in each connection between each of the two setting pressure chambers (26; 27) with the pressure fluid tank (10).

A positive-displacement machine includes a hollow housing provided with two diametrically opposed bores each having a first end communicating with an interior of the housing and a second end closed by a cover. A rotor member is adjustably displaceable in the interior of the housing due to displacement of two pistons positioned in the bores. The pistons are connected to a high pressure chamber, to thereby create in the pistons pressure sufficient to move the latter, to thereby move the rotor member. The housing is provided with control devices to change the pressure in the bores and create a pressure differential in the bores sufficient to move the rotor member into a predetermined position.

A hydraulic drive system for driving a platen in forward and return reciprocatory strokes includes a hydraulically driven reciprocable piston and a reversible rotary hydraulic motor having a crank coupled to the platen by a link. A hydraulic circuit employs a variable output pump connected via a reversible direction control valve to drive the piston and crank in unison to cooperatively drive the platen. The piston and crank motor are connected in parallel to the pump and the pump output is advantageously distributed variably between the piston and motor during the stroke cycle. Where the stroke of the platen is vertical, the system may include a hydraulic counterbalance circuit to counterbalance the weight of the platen. The pump output may be varied to control the velocity of the platen at selected points in its stroke.

The hydrostatic actuator in a constant displacement type radial piston pump or engine includes two telescopically arranged component parts separating a high pressure side from a low pressure side for a working medium, and defining a variable pressure space therebetween. The pressure space communicates with the high pressure side via a flow restrictor and with the low pressure side via a pressure limiting valve. The telescopically arranged components parts act as piston-like members and adjust the gaps between the outer end faces of the component parts and their control surfaces of the actuator in response to the pressure in the pressure space.

A control arrangement for preventing overloading of a prime mover which drives a pump comprises a pair of hydraulically operating adjusting units operative for adjusting the volume of fluid discharged by the pump. The arrangement further comprises a negative-feedback stabilizing circuit connected to the adjusting units and to a multi-setting regulating valve. The stabilizing circuit is operative for causing the valve to regulate the operation of the pump by controlling the operation of the adjusting units in dependence upon the output pressure of the pump. In order to protect the prime mover when the loading thereof has reached a predetermined value, an overload signal is generated which actuates an over-ride device. The override device is operative in response to the overload signal to cause the valve to assume a setting such that the adjusting units decrease the quantity of fluid discharged by the pump so as to unload the prime mover.