Figure 12-20.-Crankshaft and throw arrangements commonly used.
bearing surfaces for the connecting rods and the main
bearings. It is case-hardened (coated in a furnace with
copper alloyed and carbon). These bearing surfaces are
called journals. The crankshaft counterweights impede
the centrifugal force of the connecting rod and assembly
attached to the throws or points of bearing support.
These throws must be placed so that they counter-
balance each other.
Crankshaft and throw arrangements for four-, six-,
and eight-cylinder engines are shown in figure 12-20.
Four-cylinder engine crankshafts have either three or
five main support bearings and four throws in one plane.
As shown in the figure, the four throws for the number
1 and 4 cylinders (four-cylinder engine) are 180° from
those for the number 2 and 3 cylinders. On six-cylinder
engine crankshafts, each of the three pairs of throws is
arranged 120° from the other two. Such crankshafts may
be supported by as many as seven main bearingsone
at each end of the shaft and one between each pair of
crankshaft throws. The crankshafts of eight-cylinder
V-type engines are similar to those of the four-cylinder
in-line type. They may have each of the four throws
fixed at 90° from each other (as in fig. 12-20) for better
balance and smoother operation.
V-type engines usually have two connecting rods
fastened side by side on one crankshaft throw. With this
arrangement, one bank of the engine cylinders is set
slightly ahead of the other to allow the two rods to clear
The power impulses of an engine result in torsional
vibration in the crankshaft. A vibration damper mounted
on the front of the crankshaft controls this vibration (fig.
12-21). If this torsional vibration were not controlled,
the crankshaft might actually break at certain speeds.
Most types of vibration dampers resemble a
miniature clutch. A friction facing is mounted between
the hub face and a small damper flywheel. The damper
flywheel is mounted on the hub face with bolts that go
through rubber cones in the flywheel. These cones
permit limited circumferential movement between the
crankshaft and damper flywheel. That reduces the
effects of the torsional vibration in the crankshaft.
Several other types of vibration dampers are used;
however, they all operate in essentially the same way.