Air Bearings for High-Speed Mirrors Rotating in a Vacuum

Abstract

Current laser technology employs several types of scanners. A scanner consists of a high-speed rotating mirror attached to, or part of, a shaft. Supported on bearings and driven by a turbine or an electric motor, the scanner is used to deflect a light beam onto a recording medium. Rotor speeds of 120,000 r/min are common. If a mirror is rotated in air at such speeds turbulence will degrade the recording spot profile. The spot degradation is not apparent, however, if the mirror is rotated in a vacuum of 15 torr or less. This work has experimentally demonstrated the feasibility of rotating a 2-in. OD pyramidal mirror in a vacuum at 120,000 r/min. The mirror was fastened to an air turbine-driven rotor which in turn was supported on porous wall, carbon-graphite air bearings. The air bearings were supported by rubber O-rings to increase the operational stability range of the rotor. The mirror was located in a cavity whose pressure was maintained at 3.0 torr. A noncontacting dynamic face seal was used to isolate the air bearing area from the mirror cavity. Good correlation was obtained between the predicted and measured optimum journal load carrying capability. Static and dynamic methods were employed in determining the inversion point of the rotor. In the static approach the rotor was struck with a sharp radial blow, and the damped natural frequency of the system was recorded. This natural frequency coincides with the inversion point. In the dynamic analysis the inversion point manifested itself at a speed at which the orbiting amplitude of the rotor was maximum. The static approach predicted inversion at a speed of 48,000 rpm and was confirmed by dynamic testing which indicated a maximum rotor orbit at 50,000 rpm. Attitude tests conducted at speeds ranging from 0–120,000 r/min showed that the bearings and rotor constantly shifted in opposite directions as the speed was increased; however, the shift locations did repeat as the rotor was cycled between zero and maximum speed. It was noticed that throughout the entire speed range the eccentricity of the rotor was always in phase with that of the rubber O-ring supported bearings. At 120,000 r/min, the rotor orbit diameter was 15 microin., which is much less than that which would be encountered in the vicinity of the half-frequency whirl speed if the bearings were rigidly mounted.