Page 17 - Tribology in Machine Design

4 Tribology in machine design

constantly upon the ultimate goal, that is, the satisfactory, or rather the
optimum, fulfilment of all the functions required. Since many machine
designers are not sufficiently aware of all the really essential functions
required in the various stages of tribodesign, on many occasions, they
simply miss the optimum conceivable design. For instance, in the case of
self-acting hydrodynamic journal bearings, the two functions to be fulfilled,
i.e. guidance and support of the journal, were recognized a long time ago.
But the view that the hydrodynamic generation of pressure required for
these two functions is associated with a journal-bearing system serving as
its own pump is far from common. The awareness of this concept of
pumping action should have led machine designers to conceive at least one
layout for a self-acting bearing that is different from the more conventional
one based on the hydrodynamic wedging and/or squeezing effect. For
example, the pumping action could be achieved through suitable grooving
of the bearing surface, or of the opposite rubbing surface of the journal, or
collar, of a journal of thrust bearing.

1.1. Specific principles Two principles, specific to tribodesign, that is, the principle of preventing
of tribodesign contact between rubbing surfaces, and the equally important principle of
regarding lubricant films as machine elements and, accordingly, lubricants
as engineering materials, can be distinguished.
In its most general form the principle of contact prevention is also taken
to embody inhibiting, not so much the contact itself as certain consequences
of the contact such as the risk of constrictional overstressing of the surface
material of a rubbing body, i.e. the risk of mechanical wear. This principle,
which is all-important in tribodesign, may be executed in a number of ways.
When it is combined with yet another principle of the optimal grouping of
functions, it leads to the expediency of the protective layer. Such a layer,
covering the rubbing surface, is frequently used in protecting its substrate
from wear. The protective action may, for example, be aimed at lowering
the contact pressure by using a relatively soft solid for the layer, and thereby
reducing the risk of constrictional overstressing of the mating surface.
The protective layer, in a variety of forms, is indeed the most frequently
used embodiment of the principle of contact prevention. At the same time,
the principle of optimal grouping is usually involved, as the protective layer
and the substrate of the rubbing surface each has its own function. The
protective function is assigned to the layer and the structural strength is
provided by the substrate material. In fact, the substrate serves, quite often,
as support for the weaker material of the layer and thus enables the further
transmission of the external load. Since the protective layer is an element
interposed in the flow of force, it must be designed so as not to fail in
transmitting the load towards the substrate. From this point of view, a
distinction should be made between protective layers made of some solid
material (achieved by surface treatment or coating) and those consisting of
a fluid, which will be either a liquid or a gaseous lubricant.
Solid protective layers should be considered first. With conformal
rubbing surfaces, particularly, it is often profitable to use a protective layer