Page 15 - Tribology in Machine Design

2 Tribology in machine design

the engineering task of mechanical engineers in general, and of machine
designers in particular. The task of a mechanical engineer consists of the
control, by any suitable means, of flows of force, energy and matter,
including any combination and interaction of these different kinds of flow.
Conversion from one form of energy to another may also result in kinetic
energy, which in turn involves motion. Motion also comes into play when
one aims not so much at kinetic energy as at a controlled time-variation of
the position of some element. Motion is also essential in converting
mechanical energy into thermal energy in the form of frictional heat.
Certain similar operations are .also important in tribology, and par-
ticularly in tribodesign. For instance, from the present point of view, wear
may be regarded as an undesirable flow of matter that is to be kept within
bounds by controlling the flows of force and energy (primarily frictional
heat), particularly where the force and energy have to pass through the
contact area affected by the wear.
In order to provide further examples illustrating the operations in
mechanical engineering, let us consider the transmission of load from one
rubbing surface to its mating surface under conditions of dry contact or
boundary lubrication. In general, the transmission of load is associated
with concentration of the contact pressure, irrespective of whether the
surfaces are conformal, like a lathe support or a journal in a sleeve bearing,
or whether they are counterformal, like two mating convex gear teeth, cams
and tappets or rolling elements on their raceways. With conformal surfaces,
contact will, owing to the surface roughness, confine itself primarily to, or
near to, the summits of the highest asperities and thus be of a dispersed
nature. With counterformal surfaces, even if they are perfectly smooth, the
contact will still tend to concentrate itself. This area of contact is called
Hertzian because, in an elastic regime, it may be calculated from the Hertz
theory of elastic contact. Because of surface roughness, contact will not in
general be obtained throughout this area, particularly at or near its
boundaries. Therefore, the areas of real contact tend to be dispersed over
the Hertzian area. This Hertzian area may be called a conjunction area as it
is the area of closest approach between the two rubbing surfaces.
It is clearly seen that, with both conformal and counterformal contacting
surfaces, the cross-sectional area presented to the flow offeree (where it is to
be transmitted through the rubbing surfaces themselves) is much smaller
than in the bulk of the two contacting bodies. In fact, the areas of real
contact present passages or inlets to the flow of force that are invariably
throttled to a severe extent. In other words, in the transmission of a flow of
force by means of dry contact a rather severe constriction of this flow
cannot, as a rule, be avoided. This is, in a way, synonymous with a
concentration of stress. Thus, unless the load to be transmitted is unusually
small, with any degree of conformity contact pressures are bound to be high
under such dry conditions. Nothing much can be done by boundary
lubricating layers when it comes to protecting (by means of smoothing of
the flow offeree in such layers), the surface material of the rubbing bodies
from constrictional overstressing, that is, from wear caused by mechanical
factors. Such protection must be sought by other expedients. In fact, even