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.
The crankshaft 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 bearings.
TORSIONAL 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
The power impulses of an engine tend to set up
torsional vibration in the crankshaft. If this torsional
vibration were not controlled, the crankshaft might
actually break at certain speeds; a vibration damper
mounted on the front of the crankshaft controls this
There are a few variations of the vibration damper
(fig. 3-44), but they all accomplish their task basically in
Figure 3-44.Vibration damper.