Automated Design and Analysis of a Variable Displacement Linkage Motor

Hydrostatic drives consisting of a variable displacement pump and a low speed high torque (LSHT) motor are frequently used in off-highway vehicles. A variable displacement traction motor is desirable because of the ability to downsize the pump and thereby run the hydrostat at higher efficiency, as well as the possibility of hybridization of the drivetrain. Currently on the market are fixed and discrete speed LSHT radial piston motors and high speed low torque variable displacement axial piston motors. The radial piston motors are displacement dense but are not continuously variable, whereas the axial piston motors are continuously variable but require gearboxes, introducing packaging and robustness concerns. The Variable Displacement Linkage Motor (VDLM) is a LSHT motor that is continuously variable. It offers several benefits over current LSHT motors in that it is highly efficient over its operating range, it has low torque ripple, and it is displacement dense due to its multi-lobed cam and radial packaging. As with the design of any motor, the process is iterative and must be performed whenever performance objectives change. This paper describes an automated method for rapid exploration of the solution space for a variable displacement motor with specific application to the VDLM. This method leads to a motor design that theoretically can achieve 97% efficiency with a torque ripple below 5% at full displacement.