lubrication of bearings is very important to engine service life because it forces oil to
high friction points within the engine. Without lubrication between parts, bearings
overheat and score from friction.
The journal or
shaft must be smaller in diameter than the bearing (fig. 3-71),
so there is clearance (called oil clearance) between the two parts; oil circulates through
the clearance. The oil enters through the oil hole (fig. 3-66) and fills the oil groove in the
bearing. From there, the rotating journal carries the oil around to all moving parts of
the bearing. The oil works its way to the outer edges of the bearing. From there, it is
thrown off and drops back into the oil pan. The oil thrown off helps to lubricate other
engine parts, such as the cylinder walls, the pistons, and the piston rings.
As the oil
moves across the faces of the bearings, it not only lubricates them but also helps keep
them cool. The oil is relatively cool, as it leaves the oil pan. It picks up heat in its
passage through the bearing. This heat is 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 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.
Figure 3-66.Timing gear trains.
clearance between bearing and shaft.