LO 7-7 Identify and explain the purpose, function, and construction features of connecting rods

Connecting Rods

A connecting rod links a piston to the crankshaft, which enables the conversion of the piston’s reciprocating motion into rotational movement of the crankshaft. Connecting rods are the most stressed component of the engine because they experience either tensional loading or compression loading every time the piston changes direction Figure 45.

Tensional loading occurs at the end of the upstroke at TDC, when the weight of the piston assembly would naturally continue in a straight line through the cylinder head but is stopped by the connecting rod. Without the cushioning effect of pushing exhaust gases out or compressing the gases in the cylinder, the tensional forces would be even greater. Compression loading occurs during the power stroke when the combustion forces push the connecting rod down with total cylinder forces in excess of 30,000 lb (13,608 kg). Because of these forces, connecting rods are built to typically withstand forces more than 16 times greater than those expected during normal operation. These forces could be from abnormal combustion conditions or engine over-speeding. Engines that use heavier articulating pistons have larger connecting rods to handle the increased tensional forces Figure 46.

FIGURE 07-45: Connecting rod features and nomenclature.

FIGURE 07-46: The cross-sectional area of these connecting rods is much larger because they must resist tensional loading from heavier articulating pistons.