Page 234 - Tribology in Machine Design

Sliding-element bearings 219

bearing, fretting will almost be certain. If flexure is sufficiently great, the
lining material on the bearing surface can also suffer fatigue damage.

5.9.2. Grooving
Both the theoretical and the actual oil film thicknesses are influenced
significantly by the extent of oil grooving in the bearing surface. The
simplest form of oil groove in a bearing is a central, fully circumferential,
type. Some bearings have such grooves misplaced from the centre, but this
should usually be avoided, because oil will flow from this groove
preferentially across the narrower of the two bearing lands. Also, a
multiplicity of circumferential grooves should be avoided, because any
bearing land with pressure fed oil grooves on both ends will not generate
any oil flow across it and will overheat.
A central circumferential groove has the advantage that it allows the
simplest method of supplying oil around the full circumference of the
bearing, and also the simplest method of transferring oil from one location
to another, for example in a diesel engine from the main bearing to the big-
end bearing, then to the small-end bush and so to the piston cooling
passages.
Hydrodynamically, oil grooves are detrimental to load-carrying ca-
pacity, and if the minimum oil film thickness is likely to be so low as to
present a potential wiping problem, ungrooved or partially grooved
bearings may be employed.
Regardless of bearing design, if a fine level of oil filtration is not
maintained throughout the engine life, ferrous debris can contaminate the
bearing surface, but often not be totally embedded. Such particles penetrate
the thin oil film and rub against the crankshaft thereby causing them to
work-harden to a significantly higher level of hardness than the crankshaft.
This inevitably results in wear of the crankshaft surface, and with fully-
grooved bearings a circumferential ridge is eventually produced; the area of
crankshaft corresponding to the oil groove remaining unworn. If the wear is
not excessive no real problem is created, other than eventual excessive
clearance. However, with partially-grooved bearings, a similar ridge is still
produced on the journal surface, caused by wear particles entrapped in the
grooved region of the bearing. This differential wear of the journal surface
then results in wiping, wear and even fatigue of the ungrooved region of the
bearing surface, directly in line with the partial groove.
The final major disadvantage of partially-grooved bearings is cavitation
erosion of the bearing surface. Partially-grooved bearings have become
much more common as engine ratings have increased, and bearing loadings
more severe; a combination which in recent years has given rise to a much
greater incidence of bearing damage caused by cavitation erosion.

5.9.3. Clearance
High values of diametral clearance could lead to excessive damage due to
cavitation erosion. Even where bearings have been designed with a reduced