Connecting rod

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Typical design of automobile engine connecting rod Undefined w1400.jpg
Typical design of automobile engine connecting rod
Typical aluminium rod (left), oil drip rod (centre), steel rod (right) Bielle.jpg
Typical aluminium rod (left), oil drip rod (centre), steel rod (right)

A connecting rod, also called a 'con rod', [1] [2] [3] 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 crankshaft. [4] 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, 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. [5]

The most common usage of connecting rods is in internal combustion engines or on steam engines.


Hierapolis sawmill schematic Romische Sagemuhle.svg
Hierapolis sawmill schematic

A connecting rod crank has been found in the Celtic Oppida at Paule in Brittany, dated to 69BC [6]

The predecessor to the connecting length is the mechanical linkage used by Roman-era watermills. An early example of this linkage has been found at the late 3rd century Hierapolis sawmill in Roman Asia (modern Turkey) and the 6th century saw mills at Ephesus in Asia Minor (modern Turkey) and at Gerasa in Roman Syria. The crank and connecting rod mechanism of these machines converted the rotary motion of the waterwheel into the linear movement of the saw blades. [7]

An early documentation of the design occurred sometime between 1174 and 1206 AD in the Artuqid State (modern Turkey), when inventor Al-Jazari described a machine which incorporated the connecting rod with a crankshaft to pump water as part of a water-raising machine, [8] [9] though the device was more complex than typical crank and connecting rod designs. [10] :170 There is also documentation of cranks with connecting rods in the sketch books of Taccola from Renaissance Italy and 15th century painter Pisanello. [10] :113

Steam engines

Twin Beam Engine.jpg
Beam engine with twin connecting rods (almost vertical) between the horizontal beam and the flywheel
Walschearts valve gear.jpg
Steam locomotive connecting rod (between the piston and the rear wheel; the largest rod visible)

The 1712 Newcomen atmospheric engine (the first steam engine) used chain drive instead of a connecting rod, since the piston only produced force in one direction. [11] 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 with the hinge between the piston and connecting rod placed outside the cylinder, requiring a seal around the piston rod. [12]

In a steam locomotive, the cranks are usually mounted directly on the driving wheels. The connecting rod is used between the crank pin on the wheel and the crosshead (where it connects to the piston rod). [13] 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, [14] however marine-type rods of circular cross-section have occasionally been used.

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

Internal combustion engines

Connecting rod and piston from a car engine Piston and connecting rod.jpg
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'), 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.[ citation needed ]

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 rings.

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.


The materials used for connecting rods widely vary, including carbon steel, iron base sintered metal, micro-alloyed steel, spheroidized graphite cast iron. [15] 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 of metal, rather than being 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

Broken rod- initial failure due to fatigue, then further damaged by impacting the crankshaft Biella rotta per fatica.jpg
Broken rod- initial failure due to fatigue, then further damaged by impacting the crankshaft

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 forces as the piston moves downwards, and tensile forces as the piston moves upwards. [16] 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 rendering the engine irreparable. [17] 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. [18] [19] [20] [21]

Cylinder wear

The sideways force exerted on the piston through the connecting rod by the crankshaft can cause the cylinders to wear into an oval shape. This significantly reduces engine performance, since the circular piston rings 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 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 engine timing-small.gif
Operating principle of a radial engine
Renault 190HP conrods fig5.jpg
Master-slave rods in the 1916-1918 Renault 8G V8 aircraft engine

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 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. [22] Approximately 300 test engines were built, however the engine did not reach production.

Fork-and-blade rods

Fork and blade rods Forked connecting rods (Autocar Handbook, 13th ed, 1935).jpg
Fork and blade rods

Fork-and-blade rods, also known as "split big-end rods", have been used on V-twin motorcycle engines and V12 aircraft engines. [23] 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 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, EMD two-stroke Diesel engines, and various Harley Davidson V-twin motorcycle engines.

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 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">Piston</span> Machine component used to compress or contain expanding fluids in a cylinder

A piston is a component of reciprocating engines, reciprocating pumps, gas compressors, hydraulic cylinders and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from expanding gas in the cylinder to the crankshaft via a piston rod and/or connecting rod. In a pump, the function is reversed and force is transferred from the crankshaft to the piston for the purpose of compressing or ejecting the fluid in the cylinder. In some engines, the piston also acts as a valve by covering and uncovering ports in the cylinder.

<span class="mw-page-title-main">Reciprocating engine</span> Engine utilising one or more reciprocating pistons

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 features of all types. The main types are: the internal combustion engine, used extensively in motor vehicles; the steam engine, the mainstay of the Industrial Revolution; and the Stirling engine for niche applications. Internal combustion engines are further classified in two ways: either a spark-ignition (SI) engine, where the spark plug initiates the combustion; or a compression-ignition (CI) engine, where the air within the cylinder is compressed, thus heating it, so that the heated air ignites fuel that is injected then or earlier.

<span class="mw-page-title-main">Crosshead</span> Sliding pin joint in a slider-crank linkage, commonly used in engine pistons

In mechanical engineering, a crosshead is a mechanical joint used as part of the slider-crank linkages of long reciprocating engines and reciprocating compressors to eliminate sideways force on the piston. Also, the crosshead enables the connecting rod to freely move outside the cylinder. Because of the very small bore-to-stroke ratio on such engines, the connecting rod would hit the cylinder walls and block the engine from rotating if the piston was attached directly to the connecting rod like on trunk engines. Therefore, the longitudinal dimension of the crosshead must be matched to the stroke of the engine.

<span class="mw-page-title-main">Crank (mechanism)</span> Simple machine transferring motion to or from a rotating shaft at a distance from the centreline

A crank is an arm attached at a right angle to a rotating shaft by which circular motion is imparted to or received from the shaft. When combined with a connecting rod, it can be used to convert circular motion into reciprocating motion, or vice versa. The arm may be a bent portion of the shaft, or a separate arm or disk attached to it. Attached to the end of the crank by a pivot is a rod, usually called a connecting rod (conrod).

<span class="mw-page-title-main">Piston rod</span> Link connecting the piston to the crank in a reciprocating piston mechanism

In a piston engine, a piston rod joins a piston to the crosshead and thus to the connecting rod that drives the crankshaft or the driving wheels.

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">Reciprocating motion</span> Repetitive back-and-forth linear motion

Reciprocating motion, also called reciprocation, is a repetitive up-and-down or back-and-forth linear motion. It is found in a wide range of mechanisms, including reciprocating engines and pumps. The two opposite motions that comprise a single reciprocation cycle are called strokes.

<span class="mw-page-title-main">Crankpin</span> Crankshaft section where connecting rods are attached

A crankpin or crank pin, also known as a rod bearing journal, is a mechanical device in an 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 the connecting rod.

<span class="mw-page-title-main">Dead centre (engineering)</span> The positions of an engines piston at the top or bottom of its stroke

In a reciprocating engine, the dead centre is the position of a piston in which it is either farthest from, or nearest to, the crankshaft. The former is known as Top Dead Centre (TDC) while the latter is known as Bottom Dead Centre (BDC).

<span class="mw-page-title-main">Musgrave non-dead-centre engine</span>

Musgrave's non-dead-centre engine was a stationary steam engine of unusual design, intended to solve the problem of stopping on dead centre. It was designed in 1887 to serve as a marine engine. It used a pair of linked cylinders to prevent the engine from stopping in a position where no turning force can be applied. At least one engine is known to survive.

<span class="mw-page-title-main">Lorraine Pétrel</span> 1930s French piston aircraft engine

The Lorraine 12H Pétrel was a French V-12 supercharged, geared piston aeroengine initially rated at 370 kW (500 hp), but later developed to give 640 kW (860 hp). It powered a variety of mostly French aircraft in the mid-1930s, several on an experimental basis.

<span class="mw-page-title-main">Mechanical joint</span> Section of a machine which is used to connect one mechanical part to another

A mechanical joint is a section of a machine which is used to connect one or more mechanical part to another. Mechanical joints may be temporary or permanent, most types are designed to be disassembled. Most mechanical joints are designed to allow relative movement of these mechanical parts of the machine in one degree of freedom, and restrict movement in one or more others.

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

Grasshopper beam engines are beam engines that are pivoted at one end, rather than in the centre.

<span class="mw-page-title-main">Single- and double-acting cylinders</span> Classification of reciprocating engine cylinders

In mechanical engineering, the cylinders of reciprocating engines are often classified by whether they are single- or double-acting, depending on how the working fluid acts on the piston.

<span class="mw-page-title-main">Return connecting rod engine</span>

A return connecting rod, return piston rod or double piston rod engine or back-acting engine is a particular layout for a steam engine.

<span class="mw-page-title-main">High-speed steam engine</span>

High-speed steam engines were one of the final developments of the stationary steam engine. They ran at a high speed, of several hundred rpm, which was needed by tasks such as electricity generation.

<span class="mw-page-title-main">Internal combustion engine</span> Engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber

An internal combustion engine is a heat engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal combustion engine, the expansion of the high-temperature and high-pressure gases produced by combustion applies direct force to some component of the engine. The force is typically applied to pistons, turbine blades, a rotor, or a nozzle. This force moves the component over a distance, transforming chemical energy into kinetic energy which is used to propel, move or power whatever the engine is attached to. This replaced the external combustion engine for applications where the weight or size of an engine were more important.

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

The Willans engine or central valve engine was a high-speed stationary steam engine used mainly for electricity generation around the start of the 20th century.

<span class="mw-page-title-main">Cyclone Waste Heat Engine</span>

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