Balancer Shaft
A balance shaft is an eccentric weighted shaft which offsets vibrations in engine designs that are not inherently balanced.
It is an auto part designed so that, as it rotates, it vibrates in a way that reduces or cancels some of the vibration produced by an engine. Not essential to an engine's operation, balance shafts are most commonly used as means for engine refinement. Balance-shafted four-cylinder engines use two shafts turning in opposite directions on either side of the engine's crankshaft. A single balance shaft is used when fitted to three-cylinder and V-6 engines