Page 16 - Tribology in Machine Design

Introduction to the concept of tribodesign 3

when compared with the small size of the dispersed contact areas on
conformal surfaces, the thickness of boundary lubricating layers is
negligibly small from the viewpoint of diffusion.
On the one hand, if only by conformal rubbing surfaces, the constric-
tional overstressing can be reduced very effectively by a full fluid film. Such
a film keeps the two surfaces fully separated and offers excellent opportu-
nities for diffusion of the flow offeree, since all of the conjunction area is
covered by the film and is thus entirely utilized for the diffusion concerned.
The result is that again, with the conformal rubbing surfaces with which we
are concerned here, the risk of overstressing the surface material will be
much diminished whenever full fluid film can be established. This means
that a full fluid film will eliminate all those kinds of mechanical wear that
might otherwise be caused by contact between rubbing surfaces. The only
possible kind of mechanical wear under these conditions is erosion,
exemplified by the cavitation erosion that may occur in severely dynami-
cally loaded journal bearings.
On the other hand, the opportunities to create similar conditions in cases
of counterformal surfaces are far less probable. It is now known from the
theory of elastohydrodynamic lubrication of such surfaces that, owing to
the elastic deformation caused by the film pressures in the conjunction area
between the two surfaces, the distribution of these pressures can only be
very similar to the Hertzian distribution for elastic and dry contact. This
means that with counterformal surfaces very little can be gained by
interposing a fluid film. The situation may even be worsened by the
occurrence of the narrow pressure spike which may occur near the outlet to
the fluid film, and which may be much higher than Hertz's maximum
pressure, and may thus result in severe local stress concentration which, in
turn, may aggravate surface fatigue or pitting. Having once conceived the
idea of constriction of the flow offeree, it is not difficult to recognize that, in
conjunction, a similar constriction must occur with the flow of thermal
energy generated as frictional heat at the area of real contact. In fact, this
area acts simultaneously as a heat source and might now, in a double sense,
be called a constrictional area. Accordingly, contact areas on either
conformal or counterformal rubbing surfaces are stress raisers and
temperature raisers.
The above distinction, regarding the differences between conformal and
counterformal rubbing surfaces, provides a significant and fairly sharp line
of demarcation and runs as a characteristic feature through tribology and
tribodesign. It has proved to be a valuable concept, not only in education,
but also in research, development and in promoting sound design. It relates
to the nature of contact, including short-duration temperatures called flash
temperatures, and being indicative of the conditions to which both the
rubbing materials and lubricant are exposed, is also important to the
materials engineer and the lubricant technologist. Further, this distinction
is helpful in recognizing why full fluid film lubrication between counter-
formal rubbing surfaces is normally of the elastohydrodynamic type. It also
results in a rational classification of boundary lubrication.
From the very start of the design process the designer should keep his eye