Theory of Balance Shafts

Theory of Balance Shafts

Equalization of upward and downward forces







In a four-cylinder engine, vibrations are caused by the action of the pistons as they move upwards and downwards in pairs. The two outermost pistons (1 and 4) are at top dead center (TDC) when the inside pair (2 and 3) are at bottom dead center (BDC), and vice versa. However, since the forces produced by each pair of pistons differ, they do not cancel each other completely. (The force due to a piston and crankshaft is greater at TDC than at BDC. At BDC, the mass of that portion of the crankshaft above the center- line is partly counterbalanced by that below.)

The purpose of the balance shafts is to increase the force produced by the pair of pistons reversing at BDC, thereby equalizing that due to the pair at TDC. Since two piston reversals occur during each revolution of the crankshaft, the balance shafts rotate at twice the speed of the engine. The balance weights mounted on the shafts reach their lowest position each time one of the piston pairs reverses at BDC.

As a result, the force developed by this particular pair is increased, equalizing that of the pair at TDC and producing smoother running.


Equalization of lateral forces







The lateral forces produced by the action of the pistons at TDC and BDC cause the engine to pivot about an axis located approximately at the midpoint of the block. The balance shafts are located at different heights on the block to counteract the resultant moment. The forces due to the balance weights combine to exert a moment opposite to that caused by the pistons.

Cancelling the moments produces smoother running in this respect also.