The connecting rod connects the piston and the crankshaft, and transmits the force acting on the piston to the crankshaft, converting the reciprocating motion of the piston into the rotational motion of the crankshaft.
Brief introduction
The connecting rod assembly consists of a connecting rod body, a connecting rod big end cap, a connecting rod small end bushing, a connecting rod big end bearing shell, and connecting rod bolts (or screws). The connecting rod group bears the gas force transmitted by the piston pin, as well as the swinging and reciprocating inertial forces of the piston group. The magnitude and direction of these forces vary periodically. 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 often leads to the fracture of connecting rod bodies or connecting rod bolts, resulting in major accidents of complete machine damage. If the stiffness is insufficient, it will cause bending deformation of the rod body and out of roundness deformation of the connecting rod big end, resulting in eccentric wear of the piston, cylinder, bearing, and crank pin.
Structural composition
The connecting rod body consists of three parts, and the part connected to the piston pin is called the connecting rod small head; The part connected to the crankshaft is called the connecting rod big end, and the rod part connecting the small end and the big end is called the connecting rod body.
The small head of the connecting rod is mostly a thin-walled circular ring structure. To reduce wear with the piston pin, a thin-walled bronze liner is pressed into the small head hole. Drill or mill grooves on the small head and liner to allow splashing oil droplets to enter the mating surface between the lubricating liner and the piston pin.
The connecting rod body is a long member that experiences significant stress during operation. To prevent bending and deformation, the rod body must have sufficient rigidity. For this reason, most of the connecting rod bodies of automotive engines adopt an I-shaped cross-section, which can minimize mass while maintaining sufficient rigidity and strength. High reinforcement engines also use an H-shaped cross-section. Some engines use connecting rod small head oil injection to cool the piston, which requires drilling a through hole longitudinally in the rod body. To avoid stress concentration, the connection between the connecting rod body and the small and large ends adopts a large circular arc smooth transition.
To reduce engine vibration, it is necessary to limit the mass difference of each cylinder connecting rod to the minimum range. When assembling the engine in the factory, it is generally measured in grams and grouped according to the mass of the large and small ends of the connecting rod. The same set of connecting rods is selected for the same engine.
On a V-shaped engine, the corresponding cylinders in the left and right columns share a crank pin, and there are three types of connecting rods: parallel connecting rods, fork shaped connecting rods, and main and auxiliary connecting rods
Main forms of damage
The main forms of damage to connecting rods are fatigue fracture and excessive deformation. The usual locations of fatigue fracture are three high stress areas on the connecting rod. The working conditions of the connecting rod require it to have high strength and fatigue resistance; It also requires sufficient rigidity and toughness. In traditional connecting rod processing techniques, materials such as 45 steel, 40Cr, or 40MnB quenched and tempered steel are generally used, which have higher hardness. Therefore, new connecting rod materials produced by German automotive companies such as C70S6 high carbon microalloyed non quenched and tempered steel, SPLITASCO series forged steel, FRACTIM forged steel, and S53CV-FS forged steel are used (all of which are German DIN standards). Although alloy steel has high strength, it is sensitive to stress concentration. Therefore, strict requirements should be placed on the shape and excessive fillet of the connecting rod, and attention should also be paid to the surface processing quality to improve fatigue strength. Otherwise, the application of high-strength alloy steel may not achieve the expected results.