Page 225 - Tribology in Machine Design

210 Tribology in machine design

Again, taking z = 1.69 and assuming an axial length large compared with
the breadth B, so that leakage may be neglected

Hence the virtual coefficient of friction for the journal is

5.7. Gas bearings Fluid film lubrication is an exceptional mechanical process in which
viscous shear stress contributes directly to the useful function of developing
a load capacity. Although viscosity also causes bearing friction, the
equivalent lift-to-drag ratio of a typical hydrodynamic wedge is of the order
of 1000 to 1, which compares favourably to a high-performance wing. In the
case of gas bearings, in contrast to the more common liquid lubricated
bearings, lubricant compressibility is the distinctive feature.
Although basic concepts such as the hydrodynamic wedge are still
applicable to gas bearings despite gas compressibility, many additional
features of gas bearings are unique and require separate attention. The
potential for large-scale industrial application of gas bearings was recog-
nized in the late 1950s. Advocates of gas lubrication have emphasized the
following advantages:
- the gaseous lubricant is chemically stable over a wide temperature range;
atmospheric contamination is avoided by the use of gas bearings;
- the viscosity of a gas increases with temperature so that the heating effect
in overloading a gas bearing tends to increase the restoring force to
overcome the overload;
- a gas bearing is more suitable for high-speed operation;
- there is no fire hazard;
- use of gas bearings can reduce the thermal gradient in the rotor and
enhance its mechanical integrity and strength;
- for high-speed applications, the gas bearing is inherently more noise-free
than the rolling-contact bearing;
- system simplicity is enhanced by the use of self-acting gas bearings, which
do not require cooling facilities.
These optimistic views must be tempered with more subtle engineering
considerations before one can confidently substitute gas bearings for more
conventional oil lubricated bearings in actual applications.
Intense development of gas lubrication technology was triggered by the
demands of sophisticated navigation systems, by the prospects for gas-
cooled nuclear reactors, by the proliferation of magnetic peripheral devices
in the computer industry and by the everlasting quest for machinery and
devices in aerospace applications.
Although not all the early expectations have been realized, the ad-
vantages of gas lubrication are fully established in the following areas:
(i) Machine tools. Use of gas lubrication in grinding spindles allows
attainment of high speeds with minimal heat generation.