Balance shaft

Balance shafts are used in piston engines to reduce vibration by cancelling out unbalanced dynamic forces.

The counter balance shafts have eccentric weights and rotate in the opposite direction to each other, which generates a net vertical force.

This vibration is generated because the movement of the connecting rods in an even-firing inline-four engine is not symmetrical throughout the crankshaft rotation; thus during a given period of crankshaft rotation, the descending and ascending pistons are not always completely opposed in their acceleration, giving rise to a net vertical force twice in each revolution (which increases quadratically with RPM).

The Lanchester design of balance shaft systems was refined with the Mitsubishi Astron 80, an inline-four car engine introduced in 1975.

Any inline engine with an odd number of cylinders has a primary imbalance, which causes an end-to-end rocking motion.