Crankshaft

[0028]Hereinafter, one embodiment in which a crankshaft is embodied will be described with reference to FIG. 1 to FIG. 4. As shown in FIG. 1, a crankshaft 1 of the present embodiment is a flat plane crankshaft for an in-line four cylinder engine. The crankshaft is made of a manganese steel that is one of alloy steels for machine structure use, and is produced by forging.

[0029]A pulley or the like used for driving a timing belt or a fan belt is attached to a shaft end 4 of the crankshaft 1 on the left side of the drawing, for example. A flywheel averaging a shaft torque of an engine so as to attain smooth rotation is attached to a shaft end 5 of the crankshaft 1 on the right side of the drawing.

[0030]The crankshaft 1 is provided with five crank journals (hereinafter, referred to as journals) serving as a rotational center of the crankshaft 1. Hereinafter, a journal at a first position counted from the shaft end 4 is referred to as a first journal J1, a journal at a second position count