Balance shaft

Balance shaft in Ford Taunus V4 engine.

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

Lanchester's vertical force balancer. Masses C & D rotate with an opposed and balanced component sideways, and an overall vertical component

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. 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. 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 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.

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

Valve timing gears on a Ford Taunus V4 engine. The balance shaft runs off the small gear on the left (the large gear is for the camshaft, causing it to rotate at half the speed of the crankshaft).

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.

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

• Balancing machine

External links

• "Weighing the Benefits of Engine Balancing", Larry Carley, Technical Editor, Babcox.com.