As the pistons
collectively might be regarded as the heart of the engine, so the crankshaft (fig. 3-40) may be considered its backbone. The
crankshaft is the part of the engine that transforms the reciprocating motion of the
piston to rotary motion. It transmits power through the flywheel, the clutch, the
transmission, and the differential to drive your vehicle.
are made from forged or cast steel. Forged steel is the stronger of the two and is used in
commercial and military engines. The cast unit is primarily used in light- and
regular-duty gasoline engines. After the rough forging or casting is produced, it becomes
a finished product by going through the following steps:
hole is located and drilled.
surface is rough machined
crankshaft, with the exception of the bearing journals, is plated with alight coating of
bearing journals are case-hardened.
bearing journals are ground to size.
are cut into necessary bolt holes.
Crank throw arrangements for four-, six-, and eight-cylinder
engines are shown in figure 3-41. The arrangements
of throws determine the firing order of the engine. The position of the throws for each
cylinder arrangement is paramount to the overall smoothness of operation. For the various
engine configurations, typical throws are arranged as follows:
is supported in the crankcase and rotates in the main bearings (fig. 3-42). The connecting rods are supported on
the crankshaft by the rod bearings. Crankshaft bearings are made as precision inserts that
consist of a hard shell of steel or bronze with a thin lining of antifrictional metal or
be able to support the crankshaft rotation and deliver power stroke thrust under the most
rotates in the main bearings located at both ends of the crankshaft and at
certain intermediate points. The upper halves of the bearing fit right into the crankcase
and the lower halves fit into the caps that hold the crankshaft in place (fig. 3-43).
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
an integral thrust face that eliminates crankshaft end play. To prevent the loss of oil,
place the seals at both ends of the crankshaft where it extends through the crankcase.
When main bearings are replaced, tighten the bearing cap to the proper tension with a
torque wrench and lock them in place with a cotter pin or safety wire after they are in
DUE TO IMBALANCE is an inherent problem with a crankshaft that is made with offset
throws. The weight of the throws tend to make the crankshaft rotate elliptically. This is
aggravated further by the weight of the piston and the connecting rod. To eliminate the
problem, position the weights along the crankshaft. One weight is placed 180 degrees away
from each throw. They are called counterweights and are usually part of the crankshaft but
may be a separate bolt on items on small engines.
has a tendency to bend slightly when subjected to tremendous thrust from the piston. This
deflection of the rotating member causes vibration. This vibration due to deflection is
minimized by heavy crankshaft construction and sufficient support along its length by
VIBRATION occurs when the crankshaft twists because of the power stroke thrusts. It is
caused by the cylinders furthest away from the crankshaft output. As these cylinders apply
thrust to the crankshaft, it twists and the thrust decreases. The twisting and unwinding
of the crankshaft produces a vibration. The use of a vibration damper at the end of the
crankshaft opposite the output acts to absorb torsional vibration.