Action of Connecting Rods

The connecting rod group is composed of connecting rod body, connecting rod big head cover, connecting rod small head bushing, connecting rod big head bearing bushing and connecting rod bolt (or screw). The connecting rod group is subjected to the gas force from the piston pin, its own oscillation and the reciprocating inertia force of the piston group, the magnitude and direction of these forces are periodically changed. Therefore, the connecting rod is subjected to alternating loads such as compression and tension. The connecting rod must have sufficient fatigue strength and structural stiffness. Insufficient fatigue strength will often cause the connecting rod body or connecting rod bolt to break, and then produce a major accident of the whole machine damage. If the stiffness is insufficient, it will cause the bending deformation of the rod body and the circular deformation of the big head of the connecting rod, resulting in the partial wear of the piston, cylinder, bearing and crank pin.

The connecting rod body is composed of three parts, and the part connected with the piston pin is called the connecting rod small head; The part connected with the crankshaft is called the big head of the connecting rod, and the part connecting the small head and the big head is called the connecting rod body.

In order to reduce the wear between the small head and the piston pin, the thin-walled bronze bushing is pressed into the small head hole. Drill or mill grooves into the small heads and bushings to allow the splash of oil to enter the mating surface of the lubricating bushing-piston pin.

The connecting rod body is a long rod, and the force in the work is also large, in order to prevent its bending deformation, the rod body must have enough stiffness. For this reason, the connecting rod body of the vehicle engine mostly adopts the shape I section, which can minimize the mass under the condition that the stiffness and strength are sufficient, and the high-strength engine has the H-shaped section. Some engines use connecting rod small head injection oil cooling piston, which must be drilled through the longitudinal hole in the rod body. In order to avoid stress concentration, the connecting rod body, small head and big head are connected by a large circular smooth transition.

In order to reduce the vibration of the engine, the quality difference of the cylinder connecting rod must be limited to the minimum range, in the factory assembly of the engine, generally in grams as a unit of measurement according to the large and small mass of the connecting rod, the same engine to choose the same group of connecting rod.

On the V-type engine, the corresponding cylinders in the left and right columns share a crank pin, and the connecting rod has three types: parallel connecting rod, fork connecting rod and main and auxiliary connecting rod.