Connecting rod

From Academic Kids

Missing image
piston + connecting rod

In a reciprocal piston engine, the connecting rod or con rod connects the piston to the crankshaft. They are most usually made of steel for production engines, but can be made of aluminium or titanium for high performance engines. They are not rigidly fixed at either end, so that the angle between the con rod and the piston can change as the rod moves up and down and rotates around the crankshaft.

The small end attaches to the piston pin or wrist pin, which is most typically press fit into the con rod but can swivel in the piston, a "floating wrist pin" design. The big end connects to the bearing journal on the crank throw, running on replacable bearing shells accessible via the con rod bolts which hold the "cap" onto the big end; typically there is a pinhole bored through the bearing and the big end of the con rod so that pressurized lubricating motor oil squirts onto the cylinder wall to lubricate the travel of the pistons and piston rings.

The con rod is under tremendous stress from the reciprocating load represented by the piston, and the load increases rapidly with increasing engine speed. Failure of a connecting rod is one of the most common causes of catastrophic engine failure in cars, frequently putting the broken rod through the side of the crankcase and thereby rendering the engine irreparable; it can result from overheating, a physical defect, or failure of the rod bolts from a defect or improper tightening. Luckily, despite their frequent occurrence on televised competitive automobile events, such failures are quite rare on production cars during daily driving. When building a high performance engine, great attention is paid to the con rods, eliminating stress risers by such techniques as grinding the edges of the rod to a smooth radius, shotpeening to relieve internal stress, and Magnafluxing to reveal otherwise invisible small cracks which would cause the rod to fail under stress. In addition, great care is taken to torque the con rod bolts to the exact value specified; often these bolts are replaced rather than reused. The big end of the rod is fabricated as a unit and cut or cracked in two to establish precision fit around the big end bearing shell. Therefore, the big end "caps" are not interchangable between con rods, and when rebuilding an engine, care must be taken to ensure that the caps of the different con rods are not mixed up.

A source of engine wear is the sideways force exerted on the piston through the con rod by the crankshaft, which typically wears the cylinder into an oval cross-section rather than circular, making it impossible for piston rings to correctly seal against the cylinder walls. Geometrically, it can be seen that longer con rods will reduce the amount of this sideways force, and therefore lead to longer engine life. However, for a given engine block, the sum of the length of the con rod plus the piston stroke is a fixed number, determined by the fixed distance between the crankshaft axis and the top of the cylinder block where the head fastens; thus, longer stroke, giving greater engine displacement and power, requires a shorter connecting rod, resulting in accelerated cylinder wear.cs:Ojnice de:Pleuel it:Biella (meccanica)


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