Design and testing of a step compensated bi-conic hydrostatic spindle

In high precision machining and grinding applications, high-stiffness, high-load capacity workhead spindles and rotary tables with nanometerlevel error motions are becoming more necessary. The combination of reasonable load capacities and 10,000 RPM maximum speed make the Professional Instruments Company model 4R BLOCK-HEAD air bearing spindle a first choice for many high-accuracy applications. However, there are applications requiring more load capacity in the same package size, making high pressure oil hydrostatic a logical candidate. The present work describes the design and testing of a step compensated oil hydrostatic spindle consisting of two conical bearing surfaces (biconic) with high load capacity—ideal for the rigorous demands of precision machining and grinding. The most compelling argument for this oil hydrostatic spindle is the combination of nanometer-level error motions and near indestructibility. INTRODUCTION Hydrostatic spindles featuring conical surfaces have been shown to be very effective in providing precision rotation and high load capacity in a compact, low profile package [1]. The present biconic oil hydrostatic spindle is unique in that it uses step compensation rather than restrictor compensation with pockets. This results in high load capacity and stiffness since the average pressure of the bearing film is a higher percentage of supply pressure. Because of the excellent performance of the 4R BLOCK-HEAD air bearing spindle, we use its specification as a standard of comparison for the characteristics of this new design. The 4R biconic spindle features a mounting interface identical to the 4R BLOCK-HEAD. Furthermore, with modifications to the step and the bearing clearance, the 4R bi-conic bearing can be designed to operate with air. It is impractical to design an air bearing to use air pressure above 150 PSI, while pressurized oil hydrostatic systems supplying greater than 600 PSI are common. When supplied with high pressure oil, the 4R bi-conic spindle offers significant improvements in load capacity and provides similar performance in error motion and stiffness as compared to the BLOCK-HEAD. In addition, the beneficial squeeze film effects help to prevent damage to the bearing surfaces during a crash. DESIGN The motivation for step compensated bi-conic spindle development at Professional Instruments Company is derived from several potential advantages of a bi-conic design including: • Increased bearing efficiency (load capacity for a given volume) • Increased rotor stiffness (enabling the use of higher bearing pressures) • Lower thermal sensitivity