Balance shafts are used in
piston engine
A reciprocating engine, also often known as a piston engine, is typically a heat engine that uses one or more reciprocating pistons to convert high temperature and high pressure into a rotating motion. This article describes the common fea ...
s to reduce
vibration
Vibration is a mechanical phenomenon whereby oscillations occur about an equilibrium point. The word comes from Latin ''vibrationem'' ("shaking, brandishing"). The oscillations may be periodic, such as the motion of a pendulum—or random, suc ...
by cancelling out unbalanced dynamic forces. The counter balance shafts have
eccentric
Eccentricity or eccentric may refer to:
* Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal"
Mathematics, science and technology Mathematics
* Off- center, in geometry
* Eccentricity (graph theory) of a ...
weights and rotate in opposite direction to each other, which generates a net vertical force.
The balance shaft was invented and patented by British engineer
Frederick W. Lanchester
Frederick William Lanchester Doctorate of Law, LLD, Royal Aeronautical Society, Hon FRAeS, Fellow of the Royal Society, FRS (23 October 1868 – 8 March 1946), was an England, English polymath and engineer who made important contributions to au ...
in 1907.
It is most commonly used in
inline-four
A straight-four engine (also called an inline-four) is a four-cylinder piston engine where cylinders are arranged in a line along a common crankshaft.
The vast majority of automotive four-cylinder engines use a straight-four layout (with the e ...
and
V6 engines
A V6 engine is a six-cylinder piston engine where the cylinders share a common crankshaft and are arranged in a V configuration.
The first V6 engines were designed and produced independently by Marmon Motor Car Company, Deutz Gasmotoren Fab ...
used in automobiles and motorcycles.
Overview

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 the
centrifugal forces
In Newtonian mechanics, the centrifugal force is an inertial force (also called a "fictitious" or "pseudo" force) that appears to act on all objects when viewed in a rotating frame of reference. It is directed away from an axis which is parallel ...
produced by the weights cancel the vertical second-order forces (at twice the engine
RPM
Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or with the notation min−1) is a unit of rotational speed or rotational frequency for rotating machines.
Standards
ISO 80000-3:2019 defines a unit of rotation as the dimension ...
) produced by the engine. 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 the
crankshaft.
Applications
Two-cylinder engines
Numerous motorcycle engines— particularly
straight-twin engine
A straight-twin engine, also known as an inline-twin, vertical-twin, or parallel-twin, is a two-cylinder piston engine whose cylinders are arranged in a line along a common crankshaft.
Straight-twin engines are primarily used in motorcycles; ot ...
s— have employed balance shaft systems, for example the
Yamaha TRX850
The Yamaha TRX850 is a sports motorcycle with a 10-valve DOHC 849 cc 270° parallel-twin engine. First released in Japan in 1995, a version for the European market was available from 1996 to 2000.
Design and development
The TRX has a half ...
and
Yamaha TDM850
The Yamaha TDM is an 849 cc DOHC parallel-twin motorcycle that heralded the modern "Sport touring" category. The TDM was first manufactured by the Yamaha Motor Company of Japan in 1991. The TDM has a pressed-steel frame and a dry sump engine ...
engines have a 270° crankshaft with a balance shaft. An alternative approach, as used by the
BMW GS parallel-twin
The BMW F series is a family of parallel-twin engine dual-sport motorcycles manufactured in Berlin, Germany by BMW Motorrad. Launched in 2008, the range comprises the F650GS, F700GS'', ''F800GS, and F800GSA. In 2012, the F700GS replaced the ...
, is to use a 'dummy' connecting rod which moves a hinged counterweight.
Four-cylinder engines
Balance shafts are often used in
inline-four engine
A straight-four engine (also called an inline-four) is a four-cylinder Reciprocating engine, piston engine where cylinders are arranged in a line along a common crankshaft.
The vast majority of automotive four-cylinder engines use a straight-f ...
s, to reduce the
second-order vibration (a vertical force oscillating at twice the engine
RPM
Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or with the notation min−1) is a unit of rotational speed or rotational frequency for rotating machines.
Standards
ISO 80000-3:2019 defines a unit of rotation as the dimension ...
) that is inherent in the design of a typical inline-four engine. This vibration is generated because the movement of the
connecting rod
A connecting rod, also called a 'con rod', is the part of a piston engine which connects the piston to the crankshaft. Together with the crank, the connecting rod converts the reciprocating motion of the piston into the rotation of the cranksha ...
s 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).
["Shaking forces of twin engines"]
Vittore Cossalter, Dinamoto.it
The amount of vibration also increases with engine displacement, resulting in balance shafts often being used in inline-four engines with displacements of or more. Both an increased
stroke or
bore
Bore or Bores often refer to:
*Boredom
* Drill
Relating to holes
* Boring (manufacturing), a machining process that enlarges a hole
** Bore (engine), the diameter of a cylinder in a piston engine or a steam locomotive
** Bore (wind instruments), ...
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.
In a
flat-four engine
A flat-four engine, also known as a horizontally opposed-four engine, is a four-cylinder piston engine with two banks of cylinders lying on opposite sides of a common crankshaft. The most common type of flat-four engine is the boxer-four engine, ...
, the forces are cancelled out by the pistons moving in opposite directions. Therefore balance shafts are not needed in flat-four engines.
Five-cylinder engines
Balance shafts are also used in
straight-five engine
The straight-five engine (also referred to as an inline-five engine; abbreviated I5 or L5) is a piston engine with five cylinders mounted in a straight line along the crankshaft.
Although less common than straight-four engines and straight-six e ...
s such as GM
Vortec 3700.
Six-cylinder engines
In a
straight-six engine
The straight-six engine (also referred to as an inline-six engine; abbreviated I6 or L6) is a piston engine with six cylinders arranged in a straight line along the crankshaft. A straight-six engine has perfect primary and secondary engine bal ...
and
flat-six engine
A flat-six engine, also known as a horizontally opposed-six, is a six-cylinder piston engine with three cylinders on each side of a central crankshaft. The most common type of flat-six engine is the boxer-six engine, where each pair of opposed c ...
, 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 a
primary 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.
Balance shaft(s) are used on various V6 engines to reduce this rocking motion. Unlike 90° V6 engines, 60° V6 engines do not require a balance shaft.
See also
*
Balancing machine
A balancing machine is a measuring tool used for balancing rotating machine parts such as rotors for electric motors, fans, turbine
A turbine ( or ) (from the Greek , ''tyrbē'', or Latin ''turbo'', meaning vortex) is a rotary mechanical devic ...
*
Engine balance
Engine balance refers to how the forces (resulting from combustion or rotating/reciprocating components) are balanced within an internal combustion engine or steam engine. The most commonly used terms are ''primary balance'' and ''secondary bala ...
References
{{Automotive engine
Engine technology