So you need to replace an unlabeled external gear pump originally sold by Cro-Magnon Hydraulics during the First Crusades, and sadly you have no idea of the displacement? Find the root and tip diameters of one of the gears, assuming the gears are identical, and the length of one gear, one side to the other. Figure the volume of two cylinders based on those two diameters and that gear height, and subtract the smaller volume from the larger. This will be very close to the displacement of the pump (or motor). Just because you have two gears, don’t be tempted to multiply the answer by two. The total displacement is the sum volume of all teeth in the pump (or all voids between the teeth), and that single hollow cylinder you worked out contains the entire volume of teeth (or voids) from both shafts.
by Stephen L. White, CFPHS, Parker Hannifin Corp., (firstname.lastname@example.org)