PURPOSE: To make possible the superior product to be simply and cheaply manufactured, by surrounding approximate half circumference of the journal of the crankshaft with the circular arc type high frequency coil, and by rotating this crankshaft in one direction in order to give the specified heating at the prearranged angular position.
CONSTITUTION: The crankshaft 1 is made freely rotatable, and the upper portion of the journal 2 is surrounded by the circular are type high frequency coil 5 so as to cover approximate half of the outer circumference; the high frequency coil 5 is constituted so as to be freely switched to the forward position for heating and to the backward position for unheating. The above stated prearranged angular position of the crankshaft 1 is detected by the detector 6, and when the crankpin 4 of the crankshaft 1 rotating in one direction moves from the position higher than the coil 5 to the lower position that the coil 5, the coil 5 starts heating for the semicircumference (a) of the journal 2 on the counterweight 3 side. Next, the semicircumference (b) is heated, and the heatings for (a) and (b) are alternately repeated so as to heat (a) by one time more than the (b), and the heating work of the coil 5 is stopped. Thus the whole assembly is uniformly heat treated, followed by the quenching with cold water, finishing the hardening work.
OOBA YASUHIRO
KUBOTA ZENKAN
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