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Micromachines rotating at high speeds require low drag bearings with adequate load capacity and stability. Such bearings must be compatible with the capabilities of microfabrication technology. A self-acting (hydrodynamic) gas thrust bearing was designed, fabricated and tested on a silicon microturbine. Conventional thrust bearing design techniques were adapted from macroscale literature. Microbearing design charts are presented that relate bearing performance to geometry. Such bearings exhibit a design tradeoff between load bearing capability and maximum operating speed (as limited by instabilities). The specific geometry described herein was intended to replace externally pressurized, hydrostatic thrust bearings in an existing device (a 4-mm-diameter silicon microturbine), thus the hydrodynamic bearing design was constrained to be compatible in geometry and fabrication process. The final design consisted of 2.2-μm deep by 40-μ wide spiral grooves around the 700-μm diameter bearing. The bearings were fabricated in silicon with standard RIE and DRIE techniques. Test devices demonstrated lift-off and operation up to 450,000 rpm with a load capacity of 0.03 N. Measurements of load capacity and stiffness were consistent with the analysis.