connecting rod
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A connecting rod is the part of a
piston engine , internal combustion, gasoline Gasoline () or petrol () (see the #Etymology, etymology for naming differences and the use of the term ''gas'') is a transparent, petroleum-derived flammable liquid that is used primarily as a fuel in most spar ...
which connects the
piston A piston is a component of reciprocating engine , internal combustion piston engine. A reciprocating engine, also often known as a piston engine, is typically a heat engine (although there are also pneumatic Pneumatics (from Greek ‘wind, ...

piston
to the
crankshaft A crankshaft is a shaft Shaft may refer to: Rotating machine elements * Shaft (mechanical engineering), a rotating machine element used to transmit power * Line shaft, a power transmission system * Drive shaft, a shaft for transferring torque ...

crankshaft
. Together with the crank, the connecting rod converts the
reciprocating motion Double-acting stationary steam engine demonstrating conversion of reciprocating motion to rotary motion. The piston is on the left, the Crank (mechanism), crank is mounted on the flywheel axle on the right Reciprocating motion, also called reci ...
of the piston into the rotation of the crankshaft. The connecting rod is required to transmit the compressive and tensile forces from the piston. In its most common form, in an
internal combustion engine An internal combustion engine (ICE) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal combustion engine, t ...

internal combustion engine
, it allows pivoting on the piston end and rotation on the shaft end. The predecessor to the connecting rod is a mechanic linkage used by water mills to convert rotating motion of the water wheel into reciprocating motion. The most common usage of connecting rods is in
internal combustion engine An internal combustion engine (ICE) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal combustion engine, t ...

internal combustion engine
s or on
steam engine from Stott Park Bobbin Mill, Cumbria, England A steam engine is a heat engine In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energ ...

steam engine
s. __TOC__


Origins

The earliest evidence for a connecting rod appears in the late 3rd century AD
Roman Roman or Romans usually refers to: *Rome, the capital city of Italy *Ancient Rome, Roman civilization from 8th century BC to 5th century AD *Roman people, the people of ancient Rome *''Epistle to the Romans'', shortened to ''Romans'', a letter in ...

Roman
Hierapolis sawmill The Hierapolis sawmill was a List of Roman watermills, Roman water-powered stone sawmill at Hierapolis, Asia Minor (modern-day Turkey). Dating to the second half of the 3rd century AD, the sawmill is considered the earliest known machine to combine ...
. It also appears in two 6th century
Eastern Roman The Byzantine Empire, also referred to as the Eastern Roman Empire, or Byzantium, was the continuation of the Roman Empire in its eastern provinces during Late Antiquity and the Middle Ages In the history of Europe, the Middle Ages ...

Eastern Roman
saw mill A sawmill (saw mill, saw-mill) or lumber mill is a facility where logs are cut into lumber Lumber, also known as timber, is wood Wood is a porous and fibrous structural tissue found in the Plant stem, stems and roots of trees and ...
s excavated at
Ephesus Ephesus (; gr, Ἔφεσος, Éphesos; tr, Efes; may ultimately derive from hit, 𒀀𒉺𒊭, Apaša) was a city in ancient Greece Ancient Greece ( el, Ἑλλάς, Hellás) was a civilization belonging to a period of History of Greece ...

Ephesus
respectively
Gerasa Jerash ( ar, جرش ''Ǧaraš''; grc, Γέρασα ''Gérasa'') is a city in northern Jordan Jordan ( ar, الأردن; tr. ' ), officially the Hashemite Kingdom of Jordan,; tr. ') is a country in Western Asia Western Asia, also West A ...
. The crank and connecting rod mechanism of these Roman watermills converted the rotary motion of the waterwheel into the linear movement of the saw blades.: In
Renaissance Italy The Italian Renaissance ( it, Rinascimento ) was a period in Italian history The history of Italy covers the Ancient Period, the Middle Ages and the modern era. Since classical times, ancient Phoenicians, Magna Graecia, Greeks, Etruscan civil ...
, the earliest evidence of a − albeit mechanically misunderstood − compound crank and connecting-rod is found in the sketch books of
Taccola Mariano di Jacopo (1382 – c. 1453), called Taccola (" the jackdaw"), was an Italian polymath A polymath ( el, πολυμαθής, ', "having learned much"; Latin Latin (, or , ) is a classical language belonging to the Italic languages ...
. A sound understanding of the motion involved displays the painter
Pisanello Pisanello (c. 1380/1395c. 1450/1455), known professionally as Antonio di Puccio Pisano or Antonio di Puccio da Cereto, also erroneously called Vittore Pisano by Giorgio Vasari, was one of the most distinguished painters of the early Italian Rena ...
(d. 1455) who showed a piston-pump driven by a water-wheel and operated by two simple cranks and two connecting-rods. By the 16th century, evidence of cranks and connecting rods in the technological treatises and artwork of
Renaissance The Renaissance ( , ) , from , with the same meanings. is a period Period may refer to: Common uses * Era, a length or span of time * Full stop (or period), a punctuation mark Arts, entertainment, and media * Period (music), a concept in m ...

Renaissance
Europe becomes abundant;
Agostino Ramelli Agostino Ramelli (1531–ca. 1610) was an Italian Italian may refer to: * Anything of, from, or related to the country and nation of Italy ** Italians, an ethnic group or simply a citizen of the Italian Republic ** Italian language, a Romanc ...
's ''The Diverse and Artifactitious Machines'' of 1588 alone depicts eighteen examples, a number which rises in the ''Theatrum Machinarum Novum'' by to 45 different machines. An early documentation of the design occurred sometime between 1174 and 1206 AD in the Artuqid State (modern Turkey), when inventor
Al-Jazari Badīʿ az-Zaman Abu l-ʿIzz ibn Ismāʿīl ibn ar-Razāz al-Jazarī (1136–1206, ar, بديع الزمان أَبُ اَلْعِزِ إبْنُ إسْماعِيلِ إبْنُ الرِّزاز الجزري,), ) was a Muslim polymath ...
described a machine which incorporated the connecting rod with a crankshaft to pump water as part of a water-raising machine.


Steam engines

The 1712
Newcomen atmospheric engine The atmospheric engine was invented by Thomas Newcomen in 1712, and is often referred to simply as a Newcomen engine. The engine was operated by condensing steam drawn into the cylinder, thereby creating a partial vacuum which allowed the atmosp ...
(the first steam engine) used chain drive instead of a connecting rod, since the piston only produced force in one direction. However, most steam engines after this are double-acting, therefore the force is produced in both directions, leading to the use of a connecting rod. The typical arrangement uses a large sliding bearing block called a
crosshead File:Hudswell Clarke Nunlow at Lafarge Hope Cement Works 4.jpg, Typical steam locomotive; crosshead and two slide bars A crosshead is a mechanism used as part of the slider-crank linkages of long reciprocating engines and reciprocating compressors ...
with the hinge between the piston and connecting rod placed outside the cylinder, requiring a seal around the
piston rod In a piston engine, a piston rod joins a piston to the crosshead and thus to the connecting rod that drives the crankshaft or (for steam locomotives) the driving wheels. Internal combustion engines, and in particular all current automobile engines ...
. In a
steam locomotive A steam locomotive is a rail vehicle A railroad car, railcar (American English, American and Canadian English), railway wagon, railway carriage, railway truck, railwagon, railcarriage or railtruck (British English and International Union o ...

steam locomotive
, the cranks are usually mounted directly on the
driving wheel locomotive. On a steam locomotive A steam locomotive is a rail vehicle A railroad car, railcar (American English, American and Canadian English), railway wagon, railway carriage, railway truck, railwagon, railcarriage or railtruck (Brit ...
s. The connecting rod is used between the crank pin on the wheel and the crosshead (where it connects to the
piston rod In a piston engine, a piston rod joins a piston to the crosshead and thus to the connecting rod that drives the crankshaft or (for steam locomotives) the driving wheels. Internal combustion engines, and in particular all current automobile engines ...
). The equivalent connecting rods on diesel locomotives are called 'side rods' or 'coupling rods'. On smaller steam locomotives, the connecting rods are usually of rectangular cross-section, however marine-type rods of circular cross-section have occasionally been used. On
paddle steamer A paddle steamer is a steamship A steamship, often referred to as a steamer, is a type of steam-powered vessel, typically ocean-faring and seaworthy, that is propelled by one or more steam engines that typically move (turn) propellers or Pad ...

paddle steamer
s, the connecting rods are called 'pitmans' (not to be mistaken for pitman arms).


Internal combustion engines

A connecting rod for an internal combustion engine consists of the 'big end', 'rod' and 'small end' (or 'little end'). The small end attaches to the
gudgeon pin#REDIRECT Gudgeon pin In internal combustion engine An internal combustion engine (ICE) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working flu ...
(also called 'piston pin' or 'wrist pin'), which can swivel in the piston. Typically, the big end connects to the
crankpin upright=1.35, A crankpin is a mechanical device in an reciprocating engine, engine which connects the crankshaft to the connecting rod for each cylinder. It has a cylindrical surface, to allow the crankpin to rotate relative to the "big end" of t ...
using a
plain bearing A plain bearing, or more commonly sliding bearing and slide bearing (in railroading sometimes called a solid bearing, journal bearing, or friction bearing), is the simplest type of bearing, comprising just a bearing surface and no rolling eleme ...
to reduce friction; however some smaller engines may instead use a
rolling-element bearing A rolling-element bearing, also known as a rolling bearing, is a bearing Bearing may refer to: * Bearing (angle), a term for direction * Bearing (mechanical), a component that separates moving parts and takes a load * Bridge bearing, a component s ...
, in order to avoid the need for a pumped lubrication system. Typically there is a pinhole bored through the bearing on the big end of the connecting rod so that lubricating oil squirts out onto the thrust side of the cylinder wall to lubricate the travel of the pistons and
piston ring A piston ring is a metallic split ring that is attached to the outer diameter of a piston in an internal combustion engine or steam engine. The main functions of piston rings in engines are: # Sealing the combustion chamber so that there is minim ...
s. A connecting rod can rotate at both ends, so that the angle between the connecting rod and the piston can change as the rod moves up and down and rotates around the
crankshaft A crankshaft is a shaft Shaft may refer to: Rotating machine elements * Shaft (mechanical engineering), a rotating machine element used to transmit power * Line shaft, a power transmission system * Drive shaft, a shaft for transferring torque ...

crankshaft
.


Materials

The materials used for connecting rods widely vary, including carbon steel, iron base sintered metal, micro-alloyed steel, spheroidized graphite cast iron. In mass-produced automotive engines, the connecting rods are most usually made of
steel Steel is an alloy An alloy is an admixture of metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appea ...

steel
. In high performance applications, "billet" connecting rods can be used, which are machined out of a solid
billet A billet is a living quarters to which a soldier A soldier is one who fights as part of a military A military, also known collectively as armed forces, is a heavily armed, highly organized force primarily intended for warfare. It ...
of metal, rather than being casting, cast or forged. Other materials include T6-2024 aluminium alloy or T651-7075 aluminium alloy, which are used for lightness and the ability to absorb high impact at the expense of durability. Titanium is a more expensive option which reduces the weight. Cast iron can be used for cheaper, lower performance applications such as motor scooters.


Failure during operation

During each rotation of the crankshaft, a connecting rod is often subject to large and repetitive forces: shear forces due to the angle between the piston and the crankpin, Compression (physics), compression forces as the piston moves downwards, and Tension (physics), tensile forces as the piston moves upwards. These forces are proportional to the engine speed (RPM) squared. Failure of a connecting rod, often called "throwing a rod", is one of the most common causes of catastrophic engine failure in cars, frequently driving the broken rod through the side of the crankcase and thereby rendering the engine irreparable. Common causes of connecting rod failure are tensile failure from high engine speeds, the impact force when the piston hits a valve (due to a valvetrain problem), rod bearing failure (usually due to a lubrication problem), or incorrect installation of the connecting rod.


Cylinder wear

The sideways force exerted on the piston through the connecting rod by the
crankshaft A crankshaft is a shaft Shaft may refer to: Rotating machine elements * Shaft (mechanical engineering), a rotating machine element used to transmit power * Line shaft, a power transmission system * Drive shaft, a shaft for transferring torque ...

crankshaft
can cause the cylinder (engine), cylinders to wear into an oval shape. This significantly reduces engine performance, since the circular
piston ring A piston ring is a metallic split ring that is attached to the outer diameter of a piston in an internal combustion engine or steam engine. The main functions of piston rings in engines are: # Sealing the combustion chamber so that there is minim ...
s are unable to properly seal against the oval-shaped cylinder walls. The amount of sideways force is proportional to the angle of the connecting rod, therefore longer connecting rods will reduce the amount of sideways force and engine wear. However, the maximum length of a connecting rod is constrained by the engine block size; the Stroke (engine), stroke length plus the connecting rod length must not result in the piston travelling past the top of the engine block.


Master-and-slave rods

Radial engines typically use master-and-slave connecting rods, whereby one piston (the uppermost piston in the animation), has a master rod with a direct attachment to the crankshaft. The remaining pistons pin their connecting rods' attachments to rings around the edge of the master rod. Multi-bank engines with many cylinders, such as V12 engines, have little space available for many connecting rod journals on a limited length of crankshaft. The simplest solution, as used in most road car engines, is for each pair of cylinders to share a crank journal, but this reduces the size of the rod bearings and means that matching (i.e. opposite) cylinders in the different banks are slightly offset along the crankshaft axis (which creates a Couple (mechanics), rocking couple). Another solution is to use master-and-slave connecting rods, where the master rod also includes one or more ring pins which are connected to the big ends of slave rods on other cylinders. A drawback of master-slave rods is that the slave pistons' strokes will be slightly longer than that of the master piston, which increases vibration in V engines. One of the most complicated examples of master-and-slave connecting rods is the 24-cylinder Junkers Jumo 222 experimental airplane engine developed for World War II. This engine consisted of six banks of cylinders, each with four cylinders per bank. Each "layer" of six cylinders used one master connecting rod, with the other five cylinders using slave rods. Approximately 300 test engines were built, however the engine did not reach production.


Fork-and-blade rods

Fork-and-blade rods, also known as "split big-end rods", have been used on V-twin engine, V-twin motorcycle engines and V12 engine, V12 aircraft engines. For each pair of cylinders, a "fork" rod is split in two at the big end and the "blade" rod from the opposing cylinder is thinned to fit into this gap in the fork. This arrangement removes the couple (mechanics), rocking couple that is caused when cylinder pairs are offset along the crankshaft. A common arrangement for the big-end bearing is for the fork rod to have a single wide bearing sleeve that spans the whole width of the rod, including the central gap. The blade rod then runs, not directly on the crankpin, but on the outside of this sleeve. This causes the two rods to oscillate back and forth (instead of rotating relative to each other), which reduces the forces on the bearing and the surface speed. However the bearing movement also becomes reciprocating rather than continuously rotating, which is a more difficult problem for lubrication. Notable engines to use fork-and-blade rods include the Rolls-Royce Merlin V12 aircraft engine and various Harley-Davidson engine timeline, Harley Davidson V-twin motorcycle engines.


See also

*Steam locomotive components *Hydrolock


References

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