Page 19 - Tribology in Machine Design

6 Tribology in machine design

fluid films. These layers show various interesting aspects from the
standpoint of tribodesign, or even from that of machine design in general. In
fact, the full fluid film is the most perfect realization of the expedient of the
protective layer. In any full fluid film, pressures must be hydrodynamically
generated, to the extent where their resultant balances the load to be
transmitted through the film from one of the boundary rubbing surfaces to
the other.
These two surfaces are thus kept apart, so that contact prevention is
indeed complete. Accordingly, any kind of mechanical wear that may be
caused by direct contact is eliminated altogether. But, as has already been
observed, only with conformal surfaces will the full fluid film, as an
interposed force transmitting element, be able to reduce substantially the
constriction of the flow of force that would be created in the absence of such
a film. In this respect the diffusion of the flow offeree, in order to protect
both surfaces from the severe surface stressing induced by the constriction
of the flow, is best achieved by a fluid film which is far more effective than
any solid protective layer. Even with counterformal surfaces where
elastohydrodynamic films are exceedingly thin, contact prevention is still
perfectly realizable.
It is quite obvious from the discussion presented above that certain
general principles, typical for machine design, are also applicable in
tribodesign. However, there are certain principles that are specific to
tribodesign, but still hardly known amongst machine designers. It is hoped
that this book will encourage designers to take advantage of the results,
concepts and knowledge offered by tribology.

1.2. Tribological The view that tribology, in general, and tribodesign, in particular, are
problems in machine intrinsic parts of machine design can be further reinforced by a brief review
design of tribological problems encountered in the most common machine
elements.

1.2.1. Plain sliding bearings
When a journal bearing operates in the hydrodynamic regime of lubri-
cation, a hydrodynamic film develops. Under these conditions conformal
surfaces are fully separated and a copious flow of lubricant is provided to
prevent overheating. In these circumstances of complete separation,
mechanical wear does not take place. However, this ideal situation is not
always achieved.
Sometimes misalignment, either inherent in the way the machine is
assembled or of a transient nature arising from thermal or elastic distortion,
may cause metal-metal contact. Moreover, contact may occur at the
instant of starting (before the hydrodynamic film has had the opportunity
to develop fully), the bearing may be overloaded from time to time and
foreign particles may enter the film space. In some applications, internal
combustion engines for example, acids and other corrosive substances may
be formed during combustion and transmitted by the lubricant thus