Page 226 - Tribology in Machine Design

Sliding-element bearings 211

(ii) Metrology. Air bearings are used for precise linear and rotational
indexing without vibration and oil contamination,
(iii) Dental drills. High-speed air-bearing dental drills are now a standard
equipment in the profession.
(iv) Airborne air-cycle turbomachines. Foil-type bearings have been
successfully introduced for air-cycle turbomachines on passenger
aircraft. Increased reliability, leading to reduced maintenance costs, is
the benefit derived from air bearings.
(v) Computer peripheral devices. Air lubrication makes possible high-
packing-density magnetic memory devices, including tapes, discs and
drums. Read-write heads now operate at submicrometer separation
from the magnetic film with practically no risk of damage due to wear.
In the development of each of these successful applications, effective
utilization of analytical design tools was crucial. This section gives only an
introduction to the problems associated with gas bearing design. There is a
quite sophisticated theory of gas lubrication, which forms the foundation of
all analytical design tools. However, detailed presentation and discussion of
this theory is beyond the scope of this text and reader is referred to the
specialized books listed at the end of this chapter. It is, however,
appropriate to review briefly, lessons that were learned in the past so that
future designers will not be misled by too optimistic views of supporters of
gas lubrication.
Most important problems identified in the past can be summarized as
follows:
I. Inadvertent contact between the bearing surfaces is unavoidable. Even
if the surfaces are coated with a boundary lubricant, the coefficient of
friction is expected to be at least 0.3. This is more than three times as
large as that between oil-lubricated metal surfaces. Thus, a gas bearing
is substantially more vulnerable to wear damage than an oil-lubricated
bearing. For this reason, the gas bearing surface is usually a hard
material.
II. Even when a nominal separation between the bearing surfaces is
maintained under normal operation, particulate debris may occasion-
ally enter the bearing clearance and cause solid-debris-solid contact
with high normal and tangential local stresses. In a conventional oil
lubricated bearing, one of the surfaces is usually a soft material such as
bronze or babbitt; the intruding debris become embedded in the soft
surface with no damage done to the bearing. Since the wear-life
requirement precludes use of a soft gas bearing surface, one has to
resort to the other extreme; the bearing surface, together with its
substrate, must be hard enough to pulverize the debris.
III. Gas bearings generally operate at very high sliding velocities; 50 m s~ l
is quite common, and this is at least ten times higher than the sliding
speed of a typical oil-lubricated bearing. Intense local heating results
when dry contact occurs or debris is encountered. Together with the
three times higher coefficient of friction, the thermal-mechanical
distress in a gas bearing is potentially thirty times more severe than
that in an oil-lubricated bearing under the same normal load. An even