Troy-Bilt 78 Benutzerhandbuch

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 10.16.

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 10.17.

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 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 10.18.

NOTE: The timing of the balance shaft to the crank-

shaft will be covered in the Reassembly sec-
tion of this chapter.

Piston travel

Crankshaft
travel

Connecting rod

Reciprocating

mass

Rotating mass

counter-weight

Balance shaft

Piston’s axis

 of movement

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