Abstract: An internally meshing gear-pump, or motor comprises inlet and outlet ports 27,28 for the working fluid through a sealing plate 5 mounted on the shaft 1, an externally-toothed gear 2 is longitudinally displaceable relative to internally-toothed gears 3,4 to vary the pump or motor displacement, a sealing member 6 is secured to the end-face of the gear 2 which separates the displacement chambers 29,30 from the varying volume spaces which may be filled by controlled working fluid formed in both longitudinal and diametrically opposite regions 11,12. When the gear 2 is moved longitudinally from the maximum displacement position to the extreme left to a reduced capacity position and thus varies the output of the pump or motor mounting these machines back-to-back or in other combinations, a continuously variable and or differential transmission of working fluid may be obtained. Additionally electrical windings 85,86 may be mounted in the casings 7,8.