- Balance shaft
piston engineengineering, a balance shaft is an eccentric weighted shaft which offsets vibrations in engine designs that are not inherently balanced (for example, most four-cylinder engines). They were first invented by British engineer Frederick Lanchesterin 1904. [http://www.autozine.org/technical_school/engine/smooth2.htm "Engine Smoothness"] , Mark Wan, "AutoZine Technical School, 1998–2000]
Balance shafts are most common in inline four cylinder engines which, due to the asymmetry of their design, have an inherent second order
vibration(vibrating at twice the engine RPM) which cannot be eliminated no matter how well the internal components are balanced. Flat engines have their pistons horizontally opposed, so they are naturally balanced and do not incur the extra complexity, cost or power loss associated with balance shafts. This vibration is generated because the movement of the connecting rods in an inline engine is not symmetrical throughout the crankshaftrotation; 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. [ [http://www.dinamoto.it/DINAMOTO/on-line%20papers/twin%20motors/twin.html "Shaking forces of twin engines"] , Vittore Cossalter, Dinamoto.it]
The problem increases with larger
engine displacement, since the only ways to achieve larger displacement are with a longer piston stroke, increasing the difference in acceleration, or by a larger bore, increasing the massof the pistons; either way, 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 NVH characteristics.
The basic concept behind balance shafts has existed since 1904, when it was invented and patented by British engineer
Frederick Lanchester. Two balance shafts rotate 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 horizontalplane, but adds in the vertical plane, giving a net forceequal 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 lubricatinga part rotating at twice engine speed at the higher RPMs 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
frictionlosses. It is possible that this is a miscalculation derived from the common use of an inertial dynamometer, which calculates power from angular accelerationrather 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.fact|date=October 2007
Four cylinder applications
Mitsubishi Motorspioneered 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, thereby introducing a torquewhich counteracts the sideways vibrations at double engine RPM, resulting in the exceptionally smooth B234 engine.
ix cylinder applications
Another balance shaft design is found in many
V6engines. While an optimally designed V6 engine would have a 60 degree angle between the two banks of cylinders, many current V6 engines are derived from older V8engines, which have a 90 degree angle between the two banks of cylinders. While this provides for an evenly spaced firing orderin an 8 cylinder engine, in a six cylinder engine this results in a loping rhythm, where during each rotation of the crankshaft three cylinders fire at 90 degree intervals, followed by a gap of 90 degrees with no power pulse. This can be eliminated by using a more complex, and expensive, crankshaft which alters the relationship between the cylinders in the two banks to give an effective 60 degree difference, but recently many manufacturers have found it more economical to adapt the balance shaft concept, using a single shaft with counterweights spaced so as to provide a vibration which cancels out the shake inherent in the 90 degree V6.
Other manufacturers producing engines with one or two balance shafts include(d):
Alfa Romeo2.0L four-cylinder, as fitted to the Alfa Romeo 156
Chrysler K engine
* Chrysler 2.4 L and 2.5 L "Neon" engine
Ford Modular engineV10
Ford Taunus V4 engine
* Buick 3800 V6
General Motors CorporationQuad 4 and Ecotec
GM Atlas enginefour- and five-cylinder engines (two balance shafts)
GM Quad-4 engine, as used in the 1995 Pontiac Sunfire.
GM Vortec engineV-6 (single balance shaft)
Honda2.2 L (F22) four cylinder engine
Mazda's 2.3L MZR engine (two balance shafts)
* Mitsubishi 'Astron' engine
Nissan2.5L (QR25DE) four-cylinder engine
Porsche2.5L, 2.7L and 3.0L inline four-cylinder engines
Subaru EF engine
* Saab H engine
VolvoB234F, B204GT and B204FT (four cylinder, two balance shafts, 16V-head, used in 700 and 900 series) as well as numerous motorcycle engines, particularly vertical twins, and even some small single cylinder engines.
* [http://www.babcox.com/editorial/ar/eb10330.htm "Weighing the Benefits of Engine Balancing"] , Larry Carley, Technical Editor, Babcox.com
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Look at other dictionaries:
balance shaft — An engine will normally vibrate because of the up and down motion of the pistons which turn a crankshaft in one direction. A balance shaft rotates (often in the opposite direction) so that its vibration cancels some of the vibration of the engine … Dictionary of automotive terms
balance shaft — a special shaft with eccentrically mounted weights used in an internal combustion engine to reduce vibrations. * * * … Universalium
balance shaft — a special shaft with eccentrically mounted weights used in an internal combustion engine to reduce vibrations … Useful english dictionary
shaft — A long smooth surfaced bar of metal with a circular (round) cross section. See armature shaft arm shaft auxilliar drive shaft axle shaft balance shaft bevel drive shaft cardan shaft clutch shaft composite propeller shaft … Dictionary of automotive terms
balance —  The state in which weight is evenly distributed.  The action of applying weights or drilling holes in something to establish even weight distribution so that vibration is reduced. Also see balance shaft counter balance crankshaft counter… … Dictionary of automotive terms
shaft — [[t]ʃæft, ʃɑft[/t]] n. 1) a long pole forming the body of various weapons, as lances or arrows 2) something directed at someone or something in sharp attack: shafts of sarcasm[/ex] 3) a ray or beam 4) a long handle serving to balance or… … From formal English to slang
balance staff — noun : the pivoted arbor of a balance wheel * * * Horol. a pivoted axle or shaft on which the balance is mounted. [1880 85] … Useful english dictionary
balance staff — Horol. a pivoted axle or shaft on which the balance is mounted. [1880 85] * * * … Universalium
balance disc — A disc shaped device in a centrifugal pump which is attached to the pump shaft. The disc lifts when a force is applied to the underside of the disc allowing pressure to leak past until the axial forces are balanced … Dictionary of automotive terms
Engine balance — is the design, construction and tuning of an engine to run smoothly. Engine balance reduces vibration and other stresses, and may improve the performance, efficiency, cost of ownership and reliability of the engine, as well as reducing the stress … Wikipedia