Page 69 - The Tribology Handbook

A7 1 Crankshaft bearinas

Automotive diesel and petrol engines
In smaller engines, thinner overlay plating can be used and the maximum loading is then no longer limited by fatigue of the
overlay. With the exception of white metals the specific loading limits for smaller automotive diesel and petrol engines may
by considerably higher than the values quoted in the table for medium/large diesel engines. Lead bronzes and tin-
aluminium materials are available suitable for loads up to 7500 psi. in these applications.

EFFECT OF FILM THICKNESS ON PERFORMANCE
Many well-designed bearings of modern engines tend to reach their limit of performance because of thin oil film
conditions rather than by fatigue breakdown.
A vital factor determining the film thickness is the precise shape of the polar load diagram compared with the magnitude
of the load vector. With experience it is possible to assess, from the diagram, whether conditions are likely to be critical
and thus to determine whether computation ofoil film thickness, peak pressures and power losses is advisable. The existence of'
load vectors rotating at half shaft speed is undesirable since they tend to reduce the oil film thickness.

Factors promoting thin oil films
Polar load diagrams, with features, are shown below.

Two firing peaks combined with arc where load vector
is travelling approximately at half shaft speed. (Diagram
is typical of V-engine main bearing).

No half-speed vector but forces oflarge magnitude mostly
directed in a limited quadrant. Journal dwells in one
position in bearing during cycle, giving no chance for
squeeze-film effects to assist.

Big-end diagram with large inertia loop, promoting
heavy loading on one area of crankpin throughout cycle.
The resulting local overheating reduces the oil viscosity and
gives thin oil films.

A11.3