Induction heater and induction heating method

An induction heater for a work having a gear portion and a stepped shaft, comprising:

• an annular coil surrounding the gear portion;
• and
• a linear coil arranged to face the stepped shaft in an axial direction, and
• wherein alternating currents of different frequencies are supplied to the annular coil and to the linear coil by either separate power sources, or a single power source with the first and second heating coils wired in parallel.

An induction heater for heating a work having a gear portion, a shaft, and a step between the gear portion and the shaft, comprising:

• a first heating coil of an annular shape and arranged so as to surround the gear portion;
• and
• a second heating coil being arranged so as to face the shaft and having a length in an axial direction of the shaft,
• wherein the first heating coil is an annular coil,
• wherein the second heating coil has a plurality of linear parts and a connecting part connecting the linear parts in series by their ends, the linear parts being either straight or lightning-shaped, the connecting part being of an arc shape and being arranged so as to come opposite the step, and
• being adapted to supply a high-frequency current simultaneously to the first and second heating coils,
• wherein a frequency of the high-frequency current supplied to the first heating coil and a frequency of the high-frequency current supplied to the second heating coil are different enough to suppress vibration caused by interference.