Firstly, the approximate formulas for the motion errors of hydrostatic guideways under quasi-static condition were derived. Equation (13) and Equation (15) illustrate the direct relationship among structural parameters, geometric errors, and motion errors of hydrostatic guideways, reducing the difficulty of calculation. Secondly, the discussion of the formulas was made. The overall motion errors can be obtained from the linear superposition of the parts caused by the harmonic wave errors of the upside and downside guide rails. Besides, when the two phase angles of the harmonic waves on the upside and downside rails are reversed the motion errors can be totally or partially offset. When the number of the harmonic waves on the guide rails in the space of two adjacent pads is 0.5, 1.5, 2.5, 3.5..., the straightness errors are zero, and when the number of the harmonic waves on the guide rails in the space of two adjacent pads is 1, 2, 3, 4..., the yaw errors are zero.
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