Typical connecting rods are shown in Figs 9.1 and 9.2. The connecting rod is generally abbreviated to con-rod. The crankshaft con-rod mechanism transforms reciprocative motion to rotational motion. The con-rod connects the piston to the 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 con-rod must withstand very high forces as the piston moves within the cylinder bore. The shaft portion of the con-rod is subjected to bending as well as tension and compression. The bearing portions receive load from the weight of the piston and the con-rod. To avoid failure of the bearings, the con-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. Figure 9.3 shows the cross-section.
Although con-rods for both four-stroke and two-stroke engines have an Ibeam
shape, the thickness distribution is slightly different in the two engines. The four-stroke con-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 con-rod and square of the rotational velocity. It is bigger than the compressive load above a certain rotational speed.
The science and
technology of
materials in
automotive
engines
Hiroshi Yamagata
Woodhead Publishing and Maney Publishing
on behalf of
The Institute of Materials, Minerals & Mining
CRC Press
Boca Raton Boston New York Washington, DC
WOODHEAD PUBLISHING LIMITED
Cambridge England
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