Direction of the load that there is a certain

,

CHAPTER SIXTEEN Plain Surface Bearings 565
cause rapid heating of the bearing/journal interface, and sei-
zure would likely occur very rapidly.
16–7 DESIGN OF FULL-FILM
HYDRO-DYNAMICALLY
LUBRICATED BEARINGS
The following discussion presents several guidelines
for  bearing design for typical industrial applications. The
design  procedure is based primarily on information in
References 1–5.
Surface Roughness
A ground journal with a surface roughness average of 16–32
microinches ( μ in), or 0.40 to 0.80 μ m, is recommended
for bearings of good quality. The bearing should be equally
smooth or made from one of the softer materials so that
“wearing in” can smooth the high spots, creating a good
fit between the journal and the bearing. In high-precision
equipment, polishing or lapping can be used to produce a
surface finish of the order of 8 to 16 μ in (0.20 to 0.40 μ m).
bearing. The clearance between the bearing and the journal
is highly exaggerated. Part (a) of the figure shows the rise of
pressure as the rotating shaft draws oil into the converging
wedge, approaching the point of minimum film thickness.
The maximum pressure occurs there and then falls rapidly
to nearly zero as the space between the journal and the bear-
ing diverges again. The integrated effect of the pressure dis-
tribution is a force sufficient to support the shaft on a film of
oil without metal-to-metal contact.
Figure 16–6(b) shows the pressure distribution axially
along the shaft through the line of minimum film thickness
or maximum pressure. The highest pressure value occurs
in the middle of the bearing length, and it falls rapidly as
the ends are approached because the pressure outside the
bearing is ambient pressure, typically atmospheric pressure.
There is continual leakage flow from both ends of the bear-
ing. This illustrates the importance of providing a means
of continually supplying oil to the bearing to maintain the
full-film operation. Without a steady, adequate supply of oil,
the system would not be able to create the pressurized film
to carry the shaft, and boundary lubrication would result.
The significantly higher frictional forces thus created would
( a ) Stationary ( b ) Start-up ( c ) Full-film operation
Bearing
Journal
Radial load, F
F
F
Oil fills
the space
between
bearing
and
journal
e
h
o
FIGURE 16–5
Position of the journal relative to the bearing as a function of
mode of operation
Bearing
Oil
Journal
e
h
o
P
max
Oil film pressure
distribution
End leakage
( b ) Diametral section through line
of minimum film thickness
Bearing
Oil
Journal
( a ) Section through middle
of bearing
FIGURE 16–6
Pressure distribution in the oil film for hydrodynamic lubrication

566 PART THREE