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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 crankshaft. The connecting rod is required to transmit the compressive and tensile forces from the piston, and rotate at both ends.

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.[1]

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

Fork-and-blade rodsMulti-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 (ie. 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 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.[17] Approximately 300 test engines were built, however the engine did not reach production.

Fork-and-blade rods, also known as "split big-end rods", have been used on V-twin motorcycle engines and V12 aircraft engines.[18] 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 and various Harley Davidson V-twin motorcycle engines.

See also