carried down to the oil pan and released to the air
passing around the oil pan. The oil also flushes and
cleans the bearings. It tends to flush out particles of grit
and dirt that may have worked into the bearing. The
particles are carried back to the oil pan by the circulating
oil. The particles then drop to the bottom of the oil pan
or are removed from the oil by the oil screen or filter.
The greater the oil clearance, the faster the oil flows
through the bearing; however, excessive oil clearance
causes some bearings to fail from oil starvation. Heres
the reason: If oil clearances are excessive, most of the
oil passes through the nearest bearings. There is not
enough oil for the most distant bearings; these bearings
eventually fail from lack of oil. An engine with
excessive bearing oil clearance usually has low oil
pressure; the oil pump cannot build up normal pressure
because of the excessive oil clearance in the bearings.
On the other hand, when the bearings have
insufficient oil clearances, there is metal-to-metal
contact between the bearings and the journal.
Extremely rapid wear and quick failure is the end result.
Also, there is not enough throw off for adequate
lubrication of cylinder walls, pistons, and rings.
Engine bearings must operate under tremendous
loads, severe temperature variations, abrasive action,
and corrosive surroundings.
characteristics include the following.
BEARING LOAD STRENGTH is the ability of a
bearing to withstand pounding and crushing during
engine operations. The piston and rod can produce
several TONS of downward force. The bearing must
not fatigue, flatten, or split under these loads. If the
bearing load resistance is too low, the beating can
smash, fail, and spin in its bore. This ruins the bore or
BEARING CONFORMABILITY is the ability
of a bearing to move, shift, conform to variations in
shaft alignment, and adjust to imperfections in the
surface of the journal. Usually, a soft metal is placed
over hard steel. This lets the bearing conform to the
defects in the journal.
BEARING EMBEDABILITY refers to the ability
of a bearing to permit foreign particles to become
embedded in it (fig. 3-72). Dirt and metal are sometimes
carried into the bearings. The bearing should allow the
particles to sink beneath the surface into the bearing
material. This prevents the particles from scratching,
wearing, and damaging the surface of the crankshaft or
BEARING CORROSION RESISTANCE is the
ability of a bearing to resist corrosion from acid, water,
and other impurities in the engine oil. Combustion
blow-by gases cause engine oil contamination that can
also corrode engine bearings. Aluminum-lead and
other alloys are commonly being used because of their
excellent corrosion resistance.
As discussed earlier, there are three basic types of
engine bearingsconnecting rod bearings, crankshaft
main bearings, and camshaft bearings. The backing
material (body of the bearing that contacts stationary
parts) for engine bearings is normally steel. Softer
alloys are bonded over the backing to form the bearing
surface. Any one of three basic types of metal alloys can
be plated over the top of the steel backingBabbitt
(lead-tin alloy), copper, or aluminum (fig. 3-73). These
three metals may be used in different combinations to
design bearings for either light-, medium-, or heavy-
duty applications. The engine designer selects the
combination of ingredients that will best suit the engine.
Figure 3-72.Effect of a metallic particle embedded in
bearing material (Babbitt lining).
Figure 3-73.Bearing materials