CRANKSHAFT, PISTON AND CONNECTING ROD
73
Balance Shaft (483 & 490)
There are two primary motions that generate most
of the vibrations in single-cylinder engines; the rotation
of the crankshaft, and the reciprocating motion of the
piston.
See Figure 10.17.
The connecting rod translates the linear motion of
the piston to the rotating motion of the crankshaft. Two-
thirds of its mass can be attributed to rotating motion,
and one-third of its mass can be attributed to recipro
-
cating motion. See Figure 10.18.
Figure 10.17
Piston travel
Crankshaft
travel
Connecting rod
Figure 10.18
Reciprocating
mass
Rotating mass
counter-weight
If you balance the rotational mass perfectly by
adding weight to the crankshaft
counter-weights, there
will still be a large force generated by the
reciprocating
masses (the piston, piston pin and one-third of the con
-
necting rod). The crankshafts on MTD engines are not
balanced. Extra weight
is added to the crankshaft
counter-weight to reduce the total reciprocating and
rotational forces. This causes
the engine to shake side
to side
(perpendicular to the crankshaft), but reduces
overall vibrations.
As an extra feature, the 483 and 490 series
engines come with a balance shaft
. The balance shaft
further reduces side to side shaking forces
by having
an eccentric weight attached to it. The shaft is geared
to the crankshaft and rotates at the same speed as the
crankshaft, but in the opposite direction.
The eccentric
weight helps counter act the shaking forces
as well as
dynamically balances the crankshaft
.
See Figure 10.19.
NOTE: The timing of the balance shaft to the
crankshaft will be covered in the Reassembly
section of this chapter.
Figure 10.19
Balance shaft
Piston’s axis
of movement
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