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 opposite direction to each other, which generates a net vertical force.

A balance shaft is manufactured through either forging or casting. Balance shafts can be forged from a steel bar, usually through roll forging. 

Balance shafts are often used in inline-four engines, to reduce the second-order vibration (a vertical force oscillating at twice the engine RPM) that is inherent in the design of a typical inline-four engine. 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