Abstract: Gear pump or motor to deliver fluid medium or use fluid medium as power source, respectively. The pump or motor has at least two gears contained within a housing. At least one gear is in a fixed position while the other gear is in slidable engagement with the fixed gear. In the pump configuration, fluid is introduced into the housing on one side of the engaged gears and is drawn through decreasingly wide passages by the rotating gears. The fluid thereby passes to the other side of the gears and exits the pump at a greater pressure. A control medium is used to control the position of the slidable gear. The greater the overlap between the gears the greater the volume of pumped fluid.