Abstract:
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.
