Thrust needle bearings (16A, 16B), radial needle bearings (15A, 15B), and a shaft (14) are processed so that each part of them has 0.3 weight percent or more nitrogen concentration and a Vickers hardness (Hv) of 653 to 832. Therefore, wear at a contact point is less, early wear can be kept to a minimum level even in use under severe lubrication conditions, and as a consequence, life is longer.
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