Abstract: The pump has a fixed gear axis eccentricity and varies displacement by moving controlling elements linearly along the drive shaft. A pressure compensator may be employed to displace the controlling elements. The pump includes a housing penetrated by a drive shaft rotating internal elements. The internal components slide along the longitudinal axis established by the drive shaft to vary the fluid displacement of the pump. The axially-moving elements include the drive shaft, inner gerotor element, port plug, thrust bearing and retainer sleeve. The drive shaft includes an internal flanged forming a piston. The outward face of the piston is in contact with fluid at system pressure. The internal gerotor element and port plug are retained against the piston by a thrust bearing that slides over the drive shaft and is held in place by a retainer sleeve. The port plug has a rear face that also functions as a piston and this face is the same size as the flanged piston.