Balance shafts are used inpiston engines to reducevibration by cancelling out unbalanced dynamic forces. The counter balance shafts haveeccentric weights and rotate in the opposite direction to each other, which generates a net vertical force.
The balance shaft was invented and patented by British engineerFrederick W. Lanchester in 1907.[1][2] It is most commonly used ininline-four andV6 engines used in automobiles and motorcycles.
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The operating principle of a balance shaft system is that two shafts carrying identical eccentric weights rotate in opposite directions at twice the engine speed. The phasing of the shafts is such that thecentrifugal forces produced by the weights cancel the vertical second-order forces (at twice the engineRPM) produced by the engine.[3] The horizontal forces produced by the balance shafts are equal and opposite, and so cancel each other.
The balance shafts do not reduce the vibrations experienced by thecrankshaft.[4]
Numerous motorcycle engines— particularlyparallel-twin engines— have employed balance shaft systems, for example theYamaha TRX850 andYamaha TDM850 engines have a 270° crankshaft with a balance shaft. An alternative approach, as used by theBMW GS parallel-twin, is to use a 'dummy' connecting rod which moves a hinged counterweight.
Balance shafts are often used ininline-four engines, to reduce thesecond-order vibration (a vertical force oscillating at twice the engineRPM) that is inherent in the design of a typical inline-four engine. This vibration is generated because the movement of theconnecting 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 increasesquadratically with RPM).[5]
The amount of vibration also increases with engine displacement, resulting in balance shafts often being used in inline-four engines with displacements of 2.2 L (134 cu in) or more. Both an increasedstroke orbore cause an increased secondary vibration; a larger stroke increases the difference in acceleration and a larger bore increases the mass of the pistons.
The Lanchester design of balance shaft systems was refined with theMitsubishi Astron 80, an inline-four car engine introduced in 1975. This engine was the first to locate one balance shaft higher than the other, to counteract the second order rolling couple (i.e. about the crankshaft axis) due to the torque exerted by the inertia caused by increases and decreases in engine speed.[6][7]
In aflat-four engine, the forces are cancelled out by the pistons moving in opposite directions. Therefore balance shafts are not needed in flat-four engines.
Balance shafts are also used instraight-five engines such as GMVortec 3700.
In astraight-six engine andflat-six engine, the rocking forces are naturally balanced out, therefore balance shafts are not required.
V6 engines are inherently unbalanced, regardless of the V-angle. Any inline engine with an odd number of cylinders has aprimary imbalance, which causes an end-to-end rocking motion. As each cylinder bank in a V6 has three cylinders, each cylinder bank experiences this motion.[8] Balance shaft(s) are used on various V6 engines to reduce this rocking motion.
Examples are theMercedes-Benz OM629 andVolvo B8444S engine.[9]