Properties and sizing methods of 2-motor transmissions

The hydrostatic transmissions are often used to operate the drive train of off-road machines (wheeled or tracked) due to their advantages, e.g. compact size, little inertia, wide speed range, dynamic braking. Since the typical transmission with one pump and one motor (named Type A for convenience) features a rather limited application range - i.e. compatibility of specifications in terms of engine power, maximum traction and top speed - the paper reviews three improvementes to expand it without introducing too much complexity: the addition of a gearbox in series (Type B), and two alternative schemes with two motors: fixed and variable (Type C), or both variable (Type D). The constraints on size, setting and speed of the main hydraulic units dictated by each of the configurations are disclosed and their consistent design rules are discussed in detail on the basis of discrete series of heavy duty pumps and motors taken from the technical literature. To highlight the realistic implementation of the transmissions with two motors, the conventional traction vs. speed curves of three Type C and D transmissions are compared by interpolating 2-D and 3-D efficiency tables.