Abstract: A hydraulic pump apparatus including an outer housing having internal walls which are spherically configured, a hydraulically actuable variable position control ring which cooperates with the spherical housing walls to define the geometry of a hydraulic working cavity, and a rotor assembly having vanes which divide the working cavity into a plurality of working chambers and which follow the position of the control ring to vary the volume of the working chambers so as to move fluid into and out of inlet and outlet ports, respectively. The vanes are supported by a plurality of concentric rings which are attached in gimbal fashion to a rotor hub and pin assembly.