Editing Connecting rod (section)

== Internal combustion engines ==
[[File:Piston and connecting rod.jpg|thumb|upright=0.75|Connecting rod and piston from a car engine]]

A connecting rod for an internal combustion engine consists of the 'big end', 'rod' and 'small end'. The small end attaches to the [[gudgeon pin]] (also called 'piston pin' or 'wrist pin' in the U.S.), which allows for rotation between the connecting rod and the piston. Typically, the big end connects to the [[crankpin]] using a [[plain bearing]] to reduce friction; however, some smaller engines may instead use a [[rolling-element bearing]], in order to avoid the need for a pumped lubrication system. Connecting rods with rolling element bearings are typically a one piece design where the crankshaft must be pressed together through them, rather than a two piece design that can be bolted around the journal of a one piece crankshaft.{{cn|date=August 2022}}

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]]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]].

=== Materials ===
The materials used for connecting rods widely vary, including carbon steel, iron base sintered metal, micro-alloyed steel, spheroidized graphite cast iron.<ref name="Yamagata 2005 p. 7">{{harvnb|Yamagata|2005|p=7}}</ref> In mass-produced automotive engines, the connecting rods are most usually made of [[steel]]. In high performance applications, "billet" connecting rods can be used, which are machined out of a solid [[Billet (manufacturing)|billet]] 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.
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=== Failure during operation ===
[[File:Biella rotta per fatica.jpg |thumb|right|upright=0.75|Connecting rod that initially failed through fatigue, then was further damaged from impact with the crankshaft]]

During each rotation of the crankshaft, a connecting rod is often subject to large and repetitive forces: [[shear force]]s 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.<ref>{{cite web |title=Causes of Failure With a Connecting Rod |url=https://itstillruns.com/causes-failure-connecting-rod-7260672.html |website=www.itstillruns.com |access-date=21 September 2019 |language=en}}</ref> These forces are proportional to the engine speed (RPM) squared.

Failure of a connecting rod, often called "throwing a rod", often forces the broken rod through the side of the crankcase and thereby renders the engine irreparable.<ref>{{cite web|title = What does it mean to "throw a rod"?|url = http://www.cartalk.com/content/what-does-it-mean-throw-rod|website = Car Talk|access-date = 2016-02-05|date = April 1990}}</ref> 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.<ref>{{cite web |title=Preventing Connecting Rod Failures |url=https://www.enginebuildermag.com/2017/03/preventing-connecting-rod-failures/ |website=www.enginebuildermag.com |access-date=21 September 2019 |date=15 March 2017}}</ref><ref>{{cite web |title=How to eliminate connecting rod failures |url=https://www.hotrod.com/articles/ctrp-0311-connecting-rods/ |website=www.hotrod.com |date=November 2003 |access-date=21 September 2019}}</ref><ref>{{cite web |title=Probable Cause of Most Rod Failures |url=https://arcracing.blogspot.com/1999/07/probable-cause-of-most-rod-failures.html |website=www.arcracing.blogspot.com |access-date=21 September 2019 |date=1 June 1999}}</ref><ref>{{cite web|title = Emerson Bearing Extreme Applications |url = http://www.emersonbearing.com/news-articles/extreme-applications|website = www.emersonbearing.com |access-date = 2016-02-05|language = en-US}}</ref>
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=== Cylinder wear ===
The sideways force exerted on the piston through the connecting rod by the [[crankshaft]] can cause the [[cylinder (engine)|cylinders]] to wear into an oval shape. This significantly reduces engine performance, since the circular [[piston ring]]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.
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=== Master-and-slave rods ===
{{multiple image
| align             = right
| direction         = vertical
| width             = 220
| image1            = Radial_engine_timing-small.gif
| caption1          = Operating principle of a radial engine
| image2            = Renault 190HP conrods fig5.jpg
| caption2          = Master–slave rods in the 1916–1918 [[Renault 8G]] V8 aircraft engine
}}
[[Radial engine]]s 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 engine]]s, 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 stroke lengths of all slave pistons not located 180° from the master piston will always 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.<ref>{{Cite web |url=http://www.flugzeug-lorenz.de/index.php?eID=tx_cms_showpic&file=uploads%2Fpics%2FYY_169-1_Jumo222_Stirnschnitt.jpg&width=10000m&height=10000m&bodyTag=%3Cbody%20style%3D%22margin%3A0%3B%20background%3A%23fff%3B%22%3E&wrap=%3Ca%20href%3D%22javascript%3Aclose%28%29%3B%22%3E%20%7C%20%3C%2Fa%3E&md5=3a4da1957d3bd583a511bb5044efc2d8 |title=Image |access-date=2014-07-11 |archive-date=2014-04-13 |archive-url=https://web.archive.org/web/20140413224411/http://www.flugzeug-lorenz.de/index.php?eID=tx_cms_showpic&file=uploads%2Fpics%2FYY_169-1_Jumo222_Stirnschnitt.jpg&width=10000m&height=10000m&bodyTag=%3Cbody%20style%3D%22margin%3A0%3B%20background%3A%23fff%3B%22%3E&wrap=%3Ca%20href%3D%22javascript%3Aclose%28%29%3B%22%3E%20%7C%20%3C%2Fa%3E&md5=3a4da1957d3bd583a511bb5044efc2d8 |url-status=dead }}</ref> Approximately 300 test engines were built, but the engine did not reach production.
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=== Fork-and-blade rods ===
[[File:Forked connecting rods (Autocar Handbook, 13th ed, 1935).jpg|thumb|upright|Fork and blade rods]]
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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.<ref>{{cite web |title=Drysdale Godzilla V-Twin |url=https://thekneeslider.com/drysdale-godzilla-v-twin/ |website=thekneeslider.com |access-date=26 September 2019}}</ref> 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, [[Electro-Motive Diesel|EMD]] two-stroke Diesel engines, and various [[Harley-Davidson engine timeline|Harley Davidson]] V-twin motorcycle engines.