| 摘要 |
PROBLEM TO BE SOLVED: To make a decision easily whether the yoke attraction face has reached the limit of wear or not. SOLUTION: The pole part attraction face at the forward end of an upper yoke 10 has a two-layer structure of different kinds of plating. The first layer plating 13 has a thickness corresponding to the lifetime of hammer bank generated as the pole part attraction face wears, and the second layer plating 14 has a color different from that of the first layer.
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