Shaft diameter enlargement condition setting method, shaft diameter enlargement method and shaft diameter enlargement apparatus

A method for setting conditions for a shaft diameter enlargement in which an axially intermediate portion a shaft workpiece is enlarged in a radial direction by rotating the shaft workpiece about an axis of the shaft workpiece with axial compressive force being applied to the intermediate portion and with a bending angle being applied to the intermediate portion, the method comprising:

• setting a maximum allowable enlargement ratio based on test data, wherein the test data is obtained by performing the shaft diameter enlargement on test shafts, each of the test shafts having a same material and a same shape as the shaft workpiece, wherein the test data is indicative of a relationship between, for each of the test shafts, an enlargement ratio and a crack occurrence probability at an outer periphery of an axially intermediate portion of the test shaft, the enlargement ratio being a ratio of an outer diameter of the intermediate portion of the test shaft after the shaft diameter enlargement to an outer diameter of the test shaft before the shaft diameter enlargement, and wherein the maximum allowable enlargement ratio is set such that the crack occurrence probability at the outer periphery is equal to or lower than a threshold value;
• and
• setting an enlargement ratio to be obtained by the shaft diameter enlargement to enlarge the intermediate portion of the shaft workpiece to have a predetermined outer diameter, the set enlargement ratio being equal to or smaller than the maximum allowable enlargement ratio.

A shaft diameter enlargement method for enlarging an axially intermediate portion a shaft workpiece in a radial direction, the shaft diameter enlargement method comprising:

• rotating the shaft workpiece about an axis of the shaft workpiece with axial compressive force being applied to the intermediate portion and with a bending angle being applied to the intermediate portion;
• determining whether the shaft workpiece is acceptable with regard to cracking probability based on a number of rotations of the shaft workpiece required for enlarging the intermediate portion of the shaft workpiece to have a predetermined outer diameter;
• and
• setting an allowable number of rotations based on test data, wherein the test data is obtained by performing a shaft diameter enlargement on test shafts, each of the test shafts being made of a same material and having a same shape as the shaft workpiece, wherein the test data is indicative of a relationship between, for each of the test shafts, a number of rotations of the test shaft required for enlarging an axially intermediate portion of the test shaft to a predetermined outer diameter and a boundary crack occurrence probability at a boundary between the intermediate portion of the test shaft and a shaft portion of the test shaft other than the intermediate portion, and wherein the allowable number of rotations is set such that the boundary crack occurrence probability at the boundary is equal to or lower than a threshold value,
• wherein the shaft workpiece is determined as being acceptable when the number of rotations of the shaft workpiece is equal to or less than the allowable number of rotations, and the shaft workpiece is determined as being unacceptable when the number of rotations of the shaft workpiece exceeds the allowable number of rotations.

A shaft diameter enlargement method for enlarging an axially intermediate portion a shaft workpiece in a radial direction, the shaft diameter enlargement method comprising:

• rotating the shaft workpiece about an axis of the shaft workpiece with an axial compressive force being applied to the intermediate portion and with a bending angle being applied to the intermediate portion;
• determining whether the shaft workpiece is acceptable with regard to cracking probability based on a shaft workpiece enlargement ratio, the shaft workpiece enlargement ratio being a ratio of an outer diameter of the intermediate portion of the shaft workpiece after being enlarged to an outer diameter of the intermediate portion before being enlarged;
• and
• setting a maximum allowable enlargement ratio based on test data, wherein the test data is obtained by performing a shaft diameter enlargement on test shafts, each of the test shafts having a same material and a same shape as the shaft workpiece, wherein the test data is indicative of a relationship between, for each of the test shafts, a test shaft enlargement ratio and an outer periphery crack occurrence probability at an outer periphery of an axially intermediate portion of the test shaft, the test shaft enlargement ratio being a ratio of an outer diameter of the intermediate portion of the test shaft after the shaft diameter enlargement to an outer diameter of the test shaft before the shaft diameter enlargement, and wherein the maximum allowable enlargement ratio is set such that the outer periphery crack occurrence probability at the outer periphery is equal to or lower than a threshold value,
• wherein the shaft workpiece is determined as being acceptable when the shaft workpiece enlargement ratio of the shaft workpiece is equal to or smaller than the maximum allowable enlargement ratio, and the shaft workpiece is determined as being unacceptable when the shaft workpiece enlargement ratio of the shaft workpiece exceeds the maximum allowable enlargement ratio.