Balance shaft
 | 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.
Overview
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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.[1] The horizontal force produced by each balance shaft is equal and opposite to the other shaft, therefore the horizontal forces cancel each other out.
Applications
Two-cylinder engines
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.
Four-cylinder engines
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 (which increases quadratically with RPM).[2]
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 increased stroke or bore 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 the Mitsubishi 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.[3][4]
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.
Six-cylinder engines
In a straight-six engine and flat-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 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.
See also
References
- ^ "Engine Balance and the Balance Shafts". www.zzperformance.com. Retrieved 20 August 2019.
- ^ "Shaking forces of twin engines", Vittore Cossalter, Dinamoto.it
- ^ Carney, Dan (2014-06-10). "Before they were carmakers". UK: BBC. Retrieved 2018-11-01.
- ^ Nadel, Brian (June 1989). "Balancing Act". Popular Science. p. 52.
External links
- "Weighing the Benefits of Engine Balancing", Larry Carley, Technical Editor, Babcox.com.