Balance Shaft - MTD P90 series Shop Manual

P90 Series Vertical Shaft Engines

Balance Shaft

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 reciprocating motion.
See Figure 10.18.
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 connecting 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 4P90 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 crank-
shaft will be covered in the Reassembly sec-
tion of this chapter.
98
Connecting rod
Crankshaft
travel
Figure 10.17
Rotating mass
counter-weight
Figure 10.18
Figure 10.19
Piston travel
Reciprocating
mass
Piston's axis
of movement
Balance shaft