Connecting rod

The connecting rod , usually a steel or alloy forging ( though sometimes aluminum in small engines ) ,
is fastened to the piston by means of steel piston pin through the rod upper end.

The crankshaft connecting rod mechanism
transforms reciprocative motion to rotational
motion.connecting rod connects The piston
to crankshaft to transfer combustion pressure
to the crankpin.

There are bearing portions at both ends,the
piston side is called the small end,and The
crankshaft side,the big end .

The bearing portions receive load from the
weight of the piston and the conneting rod .

The conneting rod must withstand very high
forces as the piston moves within the cylinder
bore. The shaft portion of the conneting rod is
subjected to Bending as well as tension and
compression.

To avoid failure of the bearings ,the conneting rod should be made as light as possible .to avoid Buckling ,the rod portion usually has an I-beam shape because of the high rigidity-to-weight ratio of this shape.

Although conneting rods for both Four stroke and Two-stroke engines have I-beam shape,the thickness distribution is slightly different in the two engines.

The four stroke conneting rod receives a large Tensile load during the exhaust stroke as well as a Compressive load during the combustion stroke.The inertial force of the reciprocating mass generates
a tensile load which is proportional to the product of the piston assembly weight,reciprocating mass
of the conneting rod and square of the rotational velocity. It is bigger than the compressive load
above a certain rotational speed.

Structure and material :

Multi-cylinder engines for cars and motorcycles use assembled conneting rods like that shown below.
The big end consists of two parts.The bottom part is called the bearing cap ,and this is bolted to the conneting rod body.

Honing finishes the assembled big end boss to an accurate circular shape.The mating planes of the
cap and rod body should be finished accurately in advance because this influences the accuracy of
the boss. The plain bearing is sandwiched between the crankpin and big end.

Hot forging shapes the assembled conneting rod . Cr - Mo steel JIS-SCM435 or carbon steel
JIS-S55C are generally used. Free-cutting steels are frequently used when high machinability is required.

Toughening is a typical heat treatment for carbon steel.The recent tendency to pursue high strength
at reduced weight has led to the use of carburized SCM420 as well , which is very effective if the conneting rod is designed to receive high bending loads .