Balance shaft

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Balance shaft in Ford Taunus V4 engine. Balanceshaft.jpg
Balance shaft in Ford Taunus V4 engine.

Balance shafts are used in piston engines to reduce vibration by cancelling out unbalanced dynamic forces. The counter balance shafts have eccentric weights and rotate in opposite direction to each other, which generates a net vertical force.

Contents

The balance shaft was invented and patented by British engineer Frederick W. Lanchester in 1907. [1] [2] It is most commonly used in inline-four and V6 engines used in automobiles and motorcycles.

Overview

Lanchester's vertical force balancer. The eccentric masses are labelled "C" and "D". Lanchester patent harmonic balancer (Modern Motors, I) 031.jpg
Lanchester's vertical force balancer. The eccentric masses are labelled "C" and "D".

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 produced by the weights cancel the vertical second-order forces (at twice the engine RPM) produced by the engine. [3] 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. [4]

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). FordtaunusV4front.jpg
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).

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). [5]

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. [6] [7]

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.

Five-cylinder engines

Balance shafts are also used in straight-five engines such as GM Vortec 3700.

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.[ citation needed ] 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. [8] 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.[ citation needed ]

See also

Related Research Articles

<span class="mw-page-title-main">Crankshaft</span> Mechanism for converting reciprocating motion to rotation

A crankshaft is a mechanical component used by in a piston engine to convert the reciprocating motion into rotational motion. The crankshaft is a rotating shaft containing one or more crankpins, that are driven by the pistons via the connecting rods.

<span class="mw-page-title-main">V-twin engine</span> Piston engine with two cylinders in "V" configuration

A V-twin engine, also called a V2 engine, is a two-cylinder piston engine where the cylinders share a common crankshaft and are arranged in a V configuration.

<span class="mw-page-title-main">V6 engine</span> Piston engine with six cylinders in a "V" configuration

A V6 engine is a six-cylinder piston engine where the cylinders share a common crankshaft and are arranged in a V configuration.

The engine configuration describes the fundamental operating principles by which internal combustion engines are categorized.

<span class="mw-page-title-main">Flat engine</span> Combustion engine using pistons facing to the sides on a common crankshaft

A flat engine, also known as a horizontally opposed engine, is a piston engine where the cylinders are located on either side of a central crankshaft. A flat engine should not be confused with the opposed-piston engine, in which each cylinder has two pistons sharing a central combustion chamber.

<span class="mw-page-title-main">Flat-twin engine</span> Piston engine with two cylinders in opposing directions

A flat-twin engine is a two-cylinder internal combustion engine with the cylinders on opposite sides of the crankshaft. The most common type of flat-twin engine is the boxer-twin engine, where both pistons move inwards and outwards at the same time.

<span class="mw-page-title-main">Flat-four engine</span> Horizontally opposed four-cylinder piston 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, each pair of opposed pistons moves inwards and outwards at the same time.

<span class="mw-page-title-main">Straight-twin engine</span> Inline piston engine with two cylinders

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.

<span class="mw-page-title-main">Straight-four engine</span> Inline piston engine with four cylinders

A straight-four engine is a four-cylinder piston engine where cylinders are arranged in a line along a common crankshaft.

<span class="mw-page-title-main">Straight-three engine</span> Type of engine

A straight-three engine is a three-cylinder piston engine where cylinders are arranged in a line along a common crankshaft.

<span class="mw-page-title-main">Frederick W. Lanchester</span>

Frederick William Lanchester LLD, Hon FRAeS, FRS, was an English polymath and engineer who made important contributions to automotive engineering and to aerodynamics, and co-invented the topic of operations research.

<span class="mw-page-title-main">U engine</span>

A U engine is a piston engine made up of two separate straight engines placed side-by-side and coupled to a shared output shaft. When viewed from the front, the engine block resembles the letter "U".

<span class="mw-page-title-main">Motorcycle engine</span> Engine that powers a motorcycle

A motorcycle engine is an engine that powers a motorcycle. Motorcycle engines are typically two-stroke or four-stroke internal combustion engines, but other engine types, such as Wankels and electric motors, have been used.

Engine balance refers to how the forces are balanced within an internal combustion engine or steam engine. The most commonly used terms are primary balance and secondary balance. First-order balance and second-order balance are also used. Unbalanced forces within the engine can lead to vibrations.

<span class="mw-page-title-main">Crossplane</span> Crankshaft with throws extending in two planes

The crossplane or cross-plane is a crankshaft design for piston engines with a 90° angle between the crank throws. The crossplane crankshaft is the most popular configuration used in V8 road cars.

A flat-eight engine, also called a horizontally-opposed eight, is an eight-cylinder piston engine with two banks of four inline cylinders, one on each side of a central crankshaft, 180° apart.

<span class="mw-page-title-main">Yamaha TX750</span> Two-cylinder standard motorcycle built by Yamaha

The TX750 is a two-cylinder standard motorcycle built by Yamaha. The bike was released in 1972. Significant reliability problems affected the engines in early bikes. Yamaha made several changes to solve the problems but the bike was withdrawn from most markets after 1974 and production stopped in the home market after 1975.

<span class="mw-page-title-main">Harmonic damper</span>

A harmonic damper is a device fitted to the free end of the crankshaft of an internal combustion engine to counter torsional and resonance vibrations from the crankshaft. This device must be interference fit to the crankshaft in order to operate in an effective manner. An interference fit ensures the device moves in perfect step with the crankshaft. It is essential on engines with long crankshafts and V8 engines with cross plane cranks, or V6 and straight-three engines with uneven firing order. Harmonics and torsional vibrations can greatly reduce crankshaft life, or cause instantaneous failure if the crankshaft runs at or through an amplified resonance. Dampers are designed with a specific weight (mass) and diameter, which are dependent on the damping material/method used, to reduce mechanical Q factor, or damp, crankshaft resonances.

A big bang engine is an unconventional engine designed so that some of the power strokes occur simultaneously or in close succession. This is achieved by changing the ignition timing, changing or re-timing the camshaft, and sometimes in combination with a change in crankpin angle. The goal is to change the power delivery characteristics of the engine. A regular firing multi-cylinder engine fires at approximately even intervals, giving a smooth-running engine. Because a big-bang engine has uneven power delivery, they tend to run rougher and generate more vibration than an even-firing engine.

The Michel engine was an unusual form of opposed-piston engine. It was unique in that its cylinders, instead of being open-ended cylinders containing two pistons, were instead joined in a Y-shape and had three pistons working within them.

References

  1. "Frederick William Lanchester, 1868-1946". Obituary Notices of Fellows of the Royal Society. The Royal Society. 5 (16): 761. 1948. doi: 10.1098/rsbm.1948.0010 . ISSN   1479-571X.
  2. Frederick William Lanchester (1912-10-31). "US1163832A - Balancing means for reciprocating engines". Google Patents. Retrieved 2021-04-03.
  3. "Engine Balance and the Balance Shafts". www.zzperformance.com. Retrieved 20 August 2019.
  4. "Weighing the Benefits of Engine Balancing". www.babcox.com. Archived from the original on 24 February 2009. Retrieved 12 December 2004.
  5. "Shaking forces of twin engines", Vittore Cossalter, Dinamoto.it
  6. Carney, Dan (2014-06-10). "Before they were carmakers". UK: BBC. Retrieved 2018-11-01.
  7. Nadel, Brian (June 1989). "Balancing Act". Popular Science. p. 52.
  8. "The Physics of: Engine Cylinder-Bank Angles". www.caranddriver.com. 14 January 2011. Retrieved 22 August 2019.