The design and development of high speed externally pressurized air bearings for an internal grinder

Briefly, the basic theory of externally pressurized air bearings which provides shaft support, with very low friction in high speed devices, has been discussed in this paper. Some considerations have been given to the various factors such as axial flow, circumferential flow, dispersion effect, compressibility,. effect and hydrodynamic effect. The main objective was to produce an air bearing system capable of running between 45,000 to 80,000 rev/min., driven by a belt, with the radial and axial stiffnesses of 250,000 lb/in. and 150,000 lb/in. respectively. Moreover, the air consumption rate had to be as low as possible, at a maximum supply pressure of 80 lb/in2 • (gauge), while at the same time the manufacturing method had to be as simple as possible. With the aim of satisfying these requirements, an experimental internal grinding head was designed and manufactured. Throughout the manufacturing stages, the most conventional production techniques were used; even the radial clearance of 0.0005:ins. between the shaft and the journal bearing was achieved by grinding and lapping, and also the smallest journal orifice hole diameter of 0.004 ins. was successfully produced by using a small drill. ·The tests consisted mainly of the verification of the predicted values of stiffness, volumetric air flow and free running and the effect of the driving method. The complete tests were carried out in two test rigs one for direct drive and another for belt drive. The test results revealed a satisfactory correlation between predicted an measured values. At the supply pressure of 80 lb/in. (gauge), the stiffnesses of 235,000 lb/in. and 244,000 lb/in. at the centre position of the rear and_ the front journal bearings, and 190,500 lb/in. and 200 7 000 lb/in. for the front and the rear thrust bearings respectively, were obtained. Without any rotation, the total air consumption was 0.95 ft3/min., which was 1{-% higher than the predicted value but, at 60 7 000 rev/min. due to hydrodynamic effects, the reduction of air consumption of 7½f; of the no rotational condition was observed. An actual speed of 75,000 rev/min. was successfully achieved by the experimental head. Further increase of speed was limited by the pull and the vibration induced by the belt.