Influences of moisture expansion on motion errors of granite hydrostatic guideways

Granite is a kind of high damping material with low thermal expansion, so it is widely used in aerostatic guideways. However, moisture expansion may occur when the granite works in a moist environment, so the applicability of granite to oil hydrostatic guideways is investigated in this paper. By error modeling for a typical closed hydrostatic guideway with four pads, it is shown that the motion errors are mainly determined by geometric errors of the rails when film stiffness of each pads are equal. Experiments on rails of Jinan Dark Green granite are carried out, and the results show the rail straightness variation by the moisture expansion may be less than 1μm/300mm. According to the previous analysis on the error averaging mechanism of hydrostatic guideways, the motion straightness variation may be less than 0.1μm/300mm. In addition, the thickness variation may be less than one ten-thousandth of the rail thickness and the viscosity of liquid has little influence on the moisture expansion.