Reduction gear unit

A reduction gear unit comprising:

• a pinion that is fixed to a pinion shaft;
• a bearing that is located on each of both sides of the pinion;
• a large gear that is fixed to an axle, formed with a diameter larger than a diameter of a pinion, and meshes with the pinion;
• a gear case that stores therein an amount of lubricant oil, and accommodates therein the pinion and the large gear;
• and
• pinion bearing caps each of which is detachably fixed by a fastener screwed into a side surface of the gear case, and includes an annular portion interposed between the gear case and an outer peripheral surface of an outer race of the bearing and surrounding the outer peripheral surface, wherein
• a plate is provided on an end surface of the annular portion, and the plate includes a base portion that is fixed by a fastener screwed from inside of the gear case into the end surface, and an inclined portion that extends from the base portion towards the inside of the gear case to an end surface thereof which is provided on a same side as the pinion with respect to an extended line of a gear-side end surface of an inner race of the bearing,
• the bearing includes a tapered roller that includes a large-diameter-side end surface provided on a side of the gear, and
• the plate is manufactured from shape-memory alloy, and is formed so that a diameter of an inner peripheral edge of the inclined portion becomes larger at a room temperature, and becomes smaller at a temperature higher than a room temperature, than a diameter of an area where an outer peripheral edge of the large-diameter-side end surface of the tapered roller comes into contact with a raceway surface of the outer race.