Uneven Preload for Improving Motion Straightness in Closed Hydrostatic Guideways

Hydrostatic guideways have varied applications in precision or ultra-precision machine tools due to their high motion accuracy and low friction coefficient. Slider motion straightness is an important accuracy index for evaluating precision of hydrostatic guideways. It can be defined as the linear deviation of the slider center within the entire or any assigned stroke. This paper mainly focuses on the relationship between the keeper rail (upper guiderail) preload and slider motion straightness in the closed hydrostatic guideways, where the slider is enveloped by the guiderails. A finite element method was used to calculate keeper rail deformation under different screw preloads, slider motion straightness was measured by laser interferometer. Mapping relationship between screw preload and slider motion straightness was obtained. Experimental results shown that the end of the keeper rail had larger deformation for same preload conditions on all screws. This had an obvious effect on slider motion straightness. When oil supply pressure was 0.85 Mpa, slider motion straightness was 2.08 μm and 1.85 μm (within the entire stroke 330 mm) with torque on all screws as 15Nm and 20Nm, respectively. After uneven preload was employed, slider motion straightness was 0.87 μm and 0.84 μm for the above mentioned two cases. Accuracy increased by more than 50 percent. Screw preload approach proposed in this study can be used to guide the assembly processes of closed hydrostatic guideways with other forms for improving the motion straightness.