Hydraulic gear machines (HGM) are a type of machines applied in the hydraulic drive and control systems where gears are the main unit both in terms of the design and operation principle. HGMs are utilized for converting mechanical energy into hydraulic energy or vice versa. In the former case, the machines are pumps, and in the latter, hydraulic motors.\nThe author has been dealing with HGMs for over 40 years, first individually, and for the last several years, as a member of the Fluid Power Research Group constituted at the Mechanical Engineering Department of Wroclaw University of Science and Technology, Poland.