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.
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 pistons in four-cylinder opposed engines move in opposite directions,thus cancelling reciprocating forces--although the slight offset of the cylinders introduces a rocking couple. Hence the extra complexity, cost and frictional losses associated with balance shafts are avoided.
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. One can use both techniques to maximize engine displacement. 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 basic concept has a pair of balance shafts rotating in opposite directions at twice the engine speed. Equally sized eccentric weights on these shafts are sized and phased so that the inertial reaction to their counter-rotation cancels out in the horizontal plane, but adds in the vertical plane, giving a net force equal to but 180 degrees out-of-phase with the undesired second-order vibration of the basic engine, thereby cancelling it. The actual implementation of the concept, however, is concrete enough to be patented. The basic problem presented by the concept is adequately supporting and lubricating a part rotating at twice engine speed where the second order vibration becomes unacceptable.
There is some debate as to how much power the twin balance shafts cost the engine. The basic figure given is usually around 15 hp (11 kW), but this may be excessive for pure friction losses. It is possible that this is a miscalculation derived from the common use of an inertial dynamometer, which calculates power from angular acceleration rather than actual measurement of steady state torque. The 15 hp (11 kW), then, includes both the actual frictional loss as well as the increase in angular inertia of the rapidly rotating shafts, which would not be a factor at steady speed. Nevertheless, some owners modify their engines by removing the balance shafts, both to reclaim some of this power and to reduce complexity and potential areas of breakage for high-performance and racing use, as it is commonly (but falsely) believed that the smoothness provided by the balance shafts can be attained after their removal by careful balancing of the reciprocating components of the engine.
Numerous motorcycle engines, particularly parallel twins, have employed balance shaft systems. The Yamaha TRX850 and TDM parallel twins both have a 270° crankshaft with a balance shaft. Some twins employ other systems rather than balance shafts, such as a "dummy connecting rod" connected to a hinged counterweight as used in BMW F800 motorcycles, not dissimilar to that used in the Ducati Supermono single cylinder engine. Some larger single-cylinder engines have also used balance shaft systems.
Mitsubishi Motors developed the design in the modern era with its "Silent Shaft" Astron engines in 1975, with balance shafts located low on the side of the engine block and driven by chains from the oil pump, and they subsequently licensed the patent to Fiat, Saab and Porsche.
Saab has further refined the balance shaft principle to overcome second harmonic sideways vibrations (due to the same basic asymmetry in engine design, but much smaller in magnitude) by locating the balance shafts with lateral symmetry, but at different heights above the crankshaft. This introduces a torque that counteracts the sideways vibrations at double engine RPM, resulting in the exceptionally smooth B234 engine.
Toyota also began to use balance shafts in their 3RZ-FE engines in the mid 90s. These engines started as a 2RZ-FE, but creating greater torque and horsepower required a longer stroke. That longer stroke required balance shafts to counterbalance the added vibration. The longer stroke transformed the displacement in the 2RZ-FE from 2.4L to the 2.7L for the 3RZ-FE.
In an inline six-cylinder layout, the two ends of the engine are mirror images of each other and compensate every rocking motion. The only significant vibration in a properly balanced inline six will be odd harmonic torsional vibration that can be suppressed with a harmonic damper.
V6 designs are inherently unbalanced due to the odd number of cylinders in each bank, regardless of the V-angle. Only a 60 degree V6 with a six-throw crankshaft achieves acceptable primary balance. 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, this rocking motion will be present unless steps are taken to mitigate it. A balance shaft, as well as appropriate crankshaft counterweighting, can minimize the rocking.
In an opposed six cylinder layout, the rocking motions of the two rows of cylinders offset each other, while the reciprocating forces are cancelled due to the pistons moving in opposite directions.
Other manufacturers having produced engines with one or two balance shafts include: