This article relies largely or entirely on a single source. (February 2019)
In piston engine engineering, a balance shaft is an eccentric weighted shaft that offsets vibrations in engine designs that are not inherently balanced. The balance shaft was invented and patented by British engineer Frederick W. Lanchester in 1904.
The operating principal of a balance shaft system is two shafts with an identical eccentric weight on each shaft. These shafts rotate in opposite directions to each other at twice the engine speed. The centrifugal forces result in vertical forces which are designed to cancel out the vertical second-order forces (at twice the engine RPM) produced by the engine. The horizontal force produced by each balance shaft is equal and opposite to the other shaft, therefore the horizontal forces cancel each other out.
Numerous motorcycle engines— particularly straight-twin engines— have employed balance shaft systems, for example the Yamaha TRX850 and Yamaha TDM850 engines have a 270° crankshaft with a balance shaft. An alternative approach, as used by the BMW GS parallel-twin, is to use a 'dummy' connecting rod which moves a hinged counterweight.
Balance shafts are most commonly utilized in inline four-cylinder engines, which due to their design asymmetry, have an inherent second order vibration (vibrating at twice the engine RPM) that cannot be eliminated no matter how well the internal components are balanced. This vibration is generated because the movement of the connecting rods in an even-firing four-cylinder inline 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 inertial force twice in each revolution whose intensity increases quadratically with RPM, no matter how closely the components are matched for weight.
The problem increases with larger engine displacements, since larger displacement is achieved with a longer piston stroke, which increases the difference in acceleration—or by a larger bore, which increases the mass of the pistons. In all cases, the magnitude of the inertial vibration increases. For many years, two litres was viewed as the 'unofficial' displacement limit for a production inline four-cylinder engine with acceptable noise, vibration, and harshness (NVH) characteristics.
The Mitsubishi Astron 80, an inline-four car engine introduced in 1975 was the first modern engine to use twin balance shafts. A pair of counter-rotating balance shafts at twice engine speed balance second order vertical vibration while cancelling horizontally. This is similar to the original Lanchester design, except the Astron balance shafts were placed at uneven heights to also counteract the second order rolling couple (i.e. about the crankshaft axis) due to the torque exerted by the inertia of the four pistons moving and stopping together).
Balance shafts are more commonly used in inline-four engines with displacements of 2.2 L (134 cu in) or more. In a flat-four engine, the forces are cancelled out by the pistons moving in opposite directions. Therefore balance shafts are not needed in flat-four engines.
V6 engines are inherently unbalanced, regardless of the V-angle. Any inline engine with an odd number of cylinders will exhibit primary imbalance, which causes an end-to-end rocking motion. As each cylinder bank in a V6 has an odd number of cylinders, each cylinder bank experiences this motion. Balance shaft(s) are used on various V6 engines to reduce this rocking motion.
- "Weighing the Benefits of Engine Balancing", Larry Carley, Technical Editor, Babcox.com.