The minimum size of hydrostatic transmissions for locomotion

• Manufacturers’ literature does not look for the minimum size as procedure does.
• The proposed procedure defines: pump and motor displacement, pump gear ratio.
• If transmission layout is known, the procedure is a benchmark to check performance.
• If transmission layout is unknown, the procedure is a tool to review alternatives.
• The minimum size paradigm can drive the progress of HT and their extensions.

The hydrostatic transmissions are often used to drive the locomotion devices of off-road machines (wheeled or tracked). A key factor of their feasibility, in addition to their cost, is the size of the volumetric units (two or more) in the fluid loop. The procedure presented in the paper aims to find the minimum displacement of one pump and one or more motors in parallel, from a set of inputs, among which are the rated engine power and the power ratio (a synthetic figure of merit of the transmission performance). The procedure is applied to one pump and one motor, a simple and popular layout, by using data representative of the commercial units in the heavy duty range and emphasizing the differences between the fixed and variable displacement motor. Moreover, the sensitivity of the results to some critical parameters – in particular, the number of motors and their speed limits – is surveyed. The minimum size also stands as a powerful paradigm to evaluate on a rigorous ground the alternative, and sometimes rather complex, layouts conceived to extend the field of application of the hydrostatic transmissions.