those for the 4-cylinder inline type or may have each of
VIBRATION DAMPER.--The power impulses
the four throws fixed at 90 degrees from each other (fig.
of an engine tend to set up torsional vibration in the
3-34) for better balance and smoother operation.
crankshaft. If this torsional vibration were not
controlled, the crankshaft might actually break at
V-type engines usually have two connecting rods
certain speeds; a vibration damper mounted on the front
fastened side by side on one crankshaft throw. With this
of the crankshaft is used to control this vibration.
arrangement one bank of the engine cylinders is set
slightly ahead of the other to allow the two rods to clear
Most types of vibration dampers resemble a
miniature clutch. (See fig. 3-35.) A friction facing is
mounted between the hub face and a small damper
Any piece of rotating machinery has a critical
flywheel. The damper flywheel is mounted on the hub
speed at which it will vibrate. Thus, the thrust of power
face with bolts that go through rubber cones in the
described in the preceding paragraphs creates a
flywheel. These cones permit limited circumferential
torsional or twisting vibration of the crankshaft.
movement between the crankshaft and damper
Torsional vibration is noticeable in long crankshafts,
flywheel. This minimizes the effects of the torsional
and unless it is controlled, the crankshaft could break. If
vibration in the crankshaft. Several other types of
you can imagine a rubber tube being twisted by a
vibration dampers are used. However, they all operate
forceful turn of a wheel to which it is connected, you
in essentially the same way.
can visualize a similar characteristic in the crankshaft
(only to a much lesser degree) as it begins to turn
ENGINE FLYWHEEL.--The flywheel is
against the inertia of the flywheel.
mounted at the rear of the crankshaft near the rear main
bearing. This is usually the longest and heaviest main
The crankshaft rotates in main bearings located at
bearing in the engine, as it must support the weight of
both of its ends and at certain intermediate points. Most
crankshaft bearings are precision bearings (prefit) that
consist of a hard shell of steel or bronze, with a thin
lining of antifrictional metal or bearing alloy. These
bearings often are channeled for oil distribution and
may be lubricated with crankcase oil by pressure
through drilled passages or by splash.
Some main bearings have integral thrust faces that
eliminate crankshaft end play. To prevent loss of oil,
seals are placed at both ends of the crankshaft where it
extends through the crankcase. Similar seals are placed
in the channels provided in the upper half of the
bearings. When replacing main bearings, tighten the
bearing-cap bolts to the proper tension with a torque
wrench and lock them with a cotter pin or safety wire
after they are in place.
Engine crankshafts are large and expensive;
therefore, it is often desirable to repair them rather than
replace them. An engine crankshaft is subjected to
terrific stresses and it may develop minute cracks.
Before extensive repair work is started, check the shaft
carefully for cracks, particularly near the ends of the
connecting rod throws, near the ends of the main
bearing journals, and near the oil feed holes. If the shaft
is sound, the bearing journals may be reground and
oversize bearings fitted. If the crankshaft is ground
down by 10 thousandths of an inch, the new
replacement bearing will be a 10 thousandths of an inch
Figure 3-35.--Sectional view of a typical vibration damper.