CRANK SHAFT

PROBLEM TO BE SOLVED: To provide a crank shaft capable of suppressing degradation of the torsional rigidity below the permissible level attributable to the depth of a recessed part provided in an arm while enhancing the rotational balance by reducing the weight of the arm. SOLUTION: In an arm 4 to connect a crank journal 2 to a crank pin 3, a recessed part 8 is formed in a part corresponding to an overlap of the crank journal 2 with the crank pin 3. The depth of the recessed part 8 is set so that the position of a bottom surface 8a is located outside the tangential line of the outside diameter of the crank journal 2 with the outside diameter of the crank pin 3, and the width in the rotational direction of the arm 4 is larger than the outside diameter of the crank journal 2. When the depth of the recessed part 8 is set, the width is larger than the value when the degradation ratio of the torsional rigidity of the crank shaft 1 is abruptly increased as the width is reduced. When the degradation ratio of the torsional rigidity is abruptly increased as the width is reduced, it is determined that the torsional rigidity of the crank shaft 1 is below the permissible level. COPYRIGHT: (C)2005,JPO&NCIPI