What is the connecting rod

The piston and the crankshaft are connected, and the force received by the piston is transmitted to the crankshaft, which converts the reciprocating motion of the piston into the rotational motion of the crankshaft.

Introduction of connecting rod

The connecting rod group consists of connecting rod body, connecting rod cap, connecting rod bushing, connecting rod bush and connecting rod bolts (or screws). The connecting rod group bears the gas force from the piston pin and its own swing and the reciprocating inertial 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 rigidity. Insufficient fatigue strength will often cause the connecting rod body or connecting rod bolts to break, which in turn will cause a major accident in the whole machine. If the stiffness is insufficient, it will cause bending deformation of the rod body and out-of-round deformation of the big end of the connecting rod, resulting in uneven wear of the piston, cylinder, bearing and crank pin.

Structural composition of connecting rod

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

The small end of the connecting rod is mostly a thin-walled circular ring structure. In order to reduce the wear between the piston pin, a thin-walled bronze bush is pressed into the small-end hole. Drill or mill grooves on the small head and bushing to allow splashed oil foam to enter the mating surface of the lubricating bushing and the piston pin.

The connecting rod shaft is a long rod, which is also subject to large forces during work. In order to prevent its bending and deformation, the shaft must have sufficient rigidity. For this reason, most of the connecting rod shafts of automotive engines adopt I-shaped cross-sections. I-shaped cross-sections can minimize the mass when the rigidity and strength are sufficient. High-strength engines have H-shaped cross-sections. Some engines use connecting rod small-head injection engine oil to cool the piston, and a through hole must be drilled in the longitudinal direction of the shaft. In order to avoid stress concentration, large circular arcs are used for smooth transitions between the connecting rod shaft and the small and large heads.

In order to reduce the vibration of the engine, the mass difference of each cylinder connecting rod must be limited to the minimum range. When the engine is assembled in the factory, it is generally grouped according to the large and small head mass of the connecting rod in grams. The same engine uses the same Group connecting rod.

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

The main damage form of connecting rod

The main damage forms of connecting rods are fatigue fracture and excessive deformation. Usually the location of fatigue fracture is the three high stress areas on the connecting rod. The working conditions of the connecting rod require the connecting rod to have high strength and fatigue resistance; it also requires sufficient rigidity and toughness. In the traditional connecting rod processing technology, the material is generally quenched and tempered steel such as 45 steel, 40Cr or 40MnB, and the hardness is higher. Therefore, new connecting rod materials produced by German automobile companies such as C70S6 high-carbon microalloyed non-tempered steel and SPLITASCO series Forged steel, FRACTIM forged steel and S53CV-FS forged steel, etc. (the above are German din standards). Although alloy steel has high strength, it is sensitive to stress concentration. Therefore, strict requirements are required in terms of connecting rod shape, excessive fillet, etc., and attention should be paid to the surface processing quality to improve fatigue strength, otherwise the application of high-strength alloy steel cannot achieve the desired effect.