The crankshaft's function is to convert the gas pressure from the piston and connecting rod assembly into torque for external output, driving the valve train and other auxiliary devices. It is mostly cast from high-quality medium carbon steel, medium alloy carbon steel, or ductile iron. The crankshaft structure includes the front shaft, main journals, connecting rod journals, crank, counterweight, and rear shaft, and contains oil passages running through the main journals, crank, and connecting rod journals to lubricate them. Because it is subjected to various forces during use, it will experience a certain amount of wear, with different parts exhibiting different wear characteristics.

The wear characteristics of the main journals and bearings: The axial wear of the main journal should be uniform, but the radial wear is uneven, showing a slight ellipticity. Since the intermediate journals often bear greater forces, their wear is usually greater than that of the ends. The radial wear of the bearings corresponds to that of the main journals, with the greatest wear occurring on the lower bearing, and the wear of the intermediate bearings being greater than that of the ends.

Wear characteristics of connecting rod journals and bearings: Radial wear of connecting rod journals and bearings is uneven, with greater wear on the inner side. This uneven wear causes the connecting rod journal to form a certain ellipse radially, while axial wear is generally uniform. Connecting rods with an asymmetrical large-end structure will become conical after wear due to uneven load distribution on the connecting rod journal; connecting rods with a symmetrical large-end structure will also experience the same result if bending occurs.