Influence of the perpendicularity error direction angle on the hydrostatic thrust bearing motion error

In ultraprecision machine tools, the motion accuracy of a hydrostatic thrust bearing directly affects the machining accuracy. The perpendicularity error of the shaft and bearing bush will affect the motion accuracy of the hydrostatic thrust bearing, but the error direction angle is also the main factor. In this study, a relationship model of the perpendicularity error and the error direction angle with respect to the additional oil film force and tilting moment is established, and the influence of the error direction angle on the motion error is thoroughly examined. The results show that when the perpendicularity errors of the two end surfaces of the shaft and bearing bush are similar, the error direction angle greatly influences the axial and tilt errors. Conversely, the error direction angle has little influence on the axial error and a relatively significant influence on the tilt error. Based on this, the adjustment method and calculation model of the error direction angle are proposed. When the error direction angle of the thrust surface changes from −1.268 to −0.897 rad, the additional oil film force amplitude decreases by 0.11%, and the tilting moment amplitudes in both the x and y directions are reduced by 8.28%. The results show that adjusting the perpendicularity error direction angle of the thrust surface reduces the axial and tilt errors at different speeds. The axial error is reduced to a lesser extent than the tilt error. The results from this study have important guiding significance for the design, processing, and assembly of hydrostatic thrust bearings. In the design stage, the design tolerance requirements of perpendicularity can be appropriately reduced. In the assembly stage, the calculation model, which is based on the error direction angle, can be used to improve the motion accuracy by placing precision shims.