Abstract: A welding apparatus includes a housing having facing surfaces in the axial direction, and a welding head turning gear and first to third ring gears inside of the housing. A guide surface is formed on the facing surfaces. The welding head turning gear is coupled to a guide member, and the first to third ring gears are supported by the welding head turning gear and the guide member. Since a roller provided to the guide member rolls on the guide surface to rotatably support the first to third ring gears with respect to the housing, the welding apparatus can be lighter in weight and more compact compared to a conventional welding apparatus.

Abstract: A differential device including a planetary gear mechanism having a sub-gear, an annular gear and a planetary gear is more compact than a differential gear using a bevel gear mechanism. Since revolution torque of the planetary gear is transmitted to a free gear, and torque from the main gear is reduced to be transmitted, the differential device can use a small-sized drive unit in comparison with a conventional one. Hence, a machining apparatus including such a differential device is also compact. If the machining apparatus has three drive units, a pair of ring gears, first and second power transmission shafts, a radial movement mechanism and an axial movement mechanism, a tool holder, namely, a tool coupled to the radial movement mechanism and the axial movement mechanism, can be freely moved in axial and radial directions with respect to the pipe.