Analysis of an hydrostatic worm rack drive for large machine tools

The subject of this study is a drive equipped by an hydrostatic worm rack transmission, used to move axes of very large machine tools. The axis is a mechatronic system composed by an electrical motor, a mechanical transmission, the hydrostatic worm rack unit and the surrounding machine structure (Figure 1). The continuous growth of required performance, in terms of speed and acceleration, asked for additional investigations, in order to evaluate the static and dynamic interactions between all involved components and optimized their design. In this work, an integrated numerical model of the hydrostatic drive has been developed and experimentally tested on a milling machine built by INNSE BERARDI Machine Tools in Brescia, Italy. The integrated drive model considers the hydrostatic system non-linearity, the pressure drops between the output of the capillary restrictors and the pad pockets, fluid compressibility, varying number of opposed hydrostatic bearings simultaneously engaged, the effect of geometrical tolerances and assembly errors that may cause unwanted variation of the pads gap.