Abstract: A power transmission device has a hub portion of a ring gear is fixed by a weld portion to a flange portion on an outer periphery of a transmission member, an annular groove that is recessed in the inner side of the axial direction is formed in a side face of the hub portion for making axial positions of the annular groove and the cavity part partially coincide with each other, the hub portion is narrowed partway along a part sandwiched between the cavity part and weld portion and the annular groove when viewed in a cross section transecting the annular groove, and a narrowed portion is set to have a thickness that alleviates residual stress produced around the weld portion of the flange portion by the force with which the flange portion and the hub portion pull each other in response to thermal shrinkage of the weld portion.

Abstract: The differential device measuring tool measures an inflow amount of lubricating oil flowing into a housing space through a communication hole during the rotation of a differential case having a case main body in which the housing space and the communication hole are formed and a bearing boss having a through-hole protruding from the case main body and communicating with the housing space. The measuring tool has a collecting portion and a deriving portion. The collecting portion does not interfere with the rotating differential case in the housing space in which the differential gear mechanism is not housed, and has a recess opening and collects the lubricating oil flowing into the housing space through the communication hole. The deriving portion is inserted through the through-hole of the bearing boss and have a deriving flow channel. The deriving flow channel communicates with the recess, and extends to the outside.