A sprocket wheel, which is a machine component configured to slide relative to a bushing while being in contact with the bushing in an outer peripheral surface, includes a base made of a first metal, and an overlay that covers the base so as to constitute the outer peripheral surface. The surface of the overlay constituting the outer peripheral surface has been smoothed. Such a smoothed surface of the overlay makes the sprocket wheel less damaging to the bushing.

A sprocket wheel, which is a machine component configured to slide relative to a bushing while being in contact with the bushing in an outer peripheral surface, includes a base made of a first metal, and an overlay that covers the base so as to constitute the outer peripheral surface. The surface of the overlay constituting the outer peripheral surface has been smoothed. Such a smoothed surface of the overlay makes the sprocket wheel less damaging to the bushing.

A nitrided part having excellent fatigue strength which has predetermined constituents, has structures comprised of ferrite and pearlite, has ferrite grains with an aspect ratio of a ratio of a long axis direction and short axis direction of 4.5 or more present in the entire region at a depth from the surface of the part where stress is expected to concentrate of (0.09+0.05) mm or less, and has an average concentration of N of 5000 ppm or more at a surface layer part from the surface down to 200 m in the depth direction.

The disclosure is directed to a method of manufacturing a power transmission device that can include forging to form a planetary carrier plate integrated with dog teeth, sintering to form a planetary carrier, and joining the planetary carrier plate to the planetary carrier by brazing the planetary carrier plate to the planetary carrier at support parts arranged in a circumferential direction at a peripheral edge part of the planetary carrier so that the planetary carrier plate is fixed to face the planetary carrier on a side of the planetary carrier where a pinion gear is positioned. The method can further include sintering the planetary carrier plate and the planetary carrier, forming a pinion shaft hole for mounting a pinion shaft that constitutes the pinion gear in the planetary carrier plate and the planetary carrier, and performing high-frequency quenching on the dog teeth of the planetary carrier plate.

The disclosure is directed to a method of manufacturing a power transmission device that can include forging to form a planetary carrier plate integrated with dog teeth, sintering to form a planetary carrier, and joining the planetary carrier plate to the planetary carrier by brazing the planetary carrier plate to the planetary carrier at support parts arranged in a circumferential direction at a peripheral edge part of the planetary carrier so that the planetary carrier plate is fixed to face the planetary carrier on a side of the planetary carrier where a pinion gear is positioned. The method can further include sintering the planetary carrier plate and the planetary carrier, forming a pinion shaft hole for mounting a pinion shaft that constitutes the pinion gear in the planetary carrier plate and the planetary carrier, and performing high-frequency quenching on the dog teeth of the planetary carrier plate.

A method of manufacturing a hypoid gear includes face hobbing a gear blank and forming a green hypoid gear with gear teeth, heat treating the green hypoid gear to form a heat treated hypoid gear with heat treated gear teeth, and hard hobbing the heat treated gear teeth to form a hard finished hypoid gear. Critical non-tooth features on the heat treated hypoid gear are hard finished. Also, the critical non-tooth features on the heat treated hypoid gear can be hard finished prior to hard hobbing the heat treated gear teeth. The heat treating includes at least one of carburizing and induction hardening the green hypoid gear, a surface of the heat treated gear teeth has a hardness greater than or equal to 58 HRC, and the hard hobbing removes heat distortion from the heat treated gear teeth.

A method of manufacturing a hypoid gear includes face hobbing a gear blank and forming a green hypoid gear with gear teeth, heat treating the green hypoid gear to form a heat treated hypoid gear with heat treated gear teeth, and hard hobbing the heat treated gear teeth to form a hard finished hypoid gear. Critical non-tooth features on the heat treated hypoid gear are hard finished. Also, the critical non-tooth features on the heat treated hypoid gear can be hard finished prior to hard hobbing the heat treated gear teeth. The heat treating includes at least one of carburizing and induction hardening the green hypoid gear, a surface of the heat treated gear teeth has a hardness greater than or equal to 58 HRC, and the hard hobbing removes heat distortion from the heat treated gear teeth.

A steel for induction hardening according to the present invention includes a chemical composition consisting of, in mass percent: C: 0.58 to 0.68%, Si: 0.70 to 1.40%, Mn: 0.20 to 1.40%, P: less than 0.020%, S: less than 0.020%, Al: 0.005 to 0.060%, N: 0.0020 to 0.0080%, O: 0.0015% or less, V: 0.01 to 0.25%, B: 0.0003 to 0.0040%, Ti: 0.010 to 0.050%, and Ca: 0.0005 to 0.005%, with the balance being Fe and impurities, and satisfies Formulae (1) to (3). The steel microstructure is made up of ferrite and pearlite. A ratio of a number of composite inclusions is 20% or more.
C+Si/7+Mn/5+Cr/9+Mo/2.51.05(1)
C+Si/10+Mn/20+Cr/250.70(2)
Cr/Si0.20(3)

A rack bar includes: a rack tooth row including a plurality of rack teeth meshing with pinion teeth; a hardened layer provided continuously over an entire circumference of the rack tooth row; and a center portion provided inside the hardened layer and having lower hardness than the hardened layer. When the rack bar is viewed in an axial direction of the rack bar, a depth of the hardened layer from the following positions i), ii), and iii) increases in this order: i) a bottom land of the rack teeth; ii) a side of the rack bar relative to the bottom land; and iii) a back of the rack bar relative to the bottom land.

A steel for induction hardening according to an aspect includes, as a chemical composition, predetermined amounts of alloy elements and a remainder including Fe and impurities, in which AlN is 0.000330 to 0.000825, Mn/S is 4.6 to 14.0, a machinability index M is 15.5 to 25.65, an area fraction of AlN having an equivalent circle diameter of more than 200 nm at a position of a diameter of the steel is 20% or less of an area fraction of all AlN having an equivalent circle diameter of 40 nm or more, and a number density of Mn sulfide-based inclusions having a maximum diameter of 0.3 m or more and 10 m or less at the position of the diameter is 100 pieces/mm.sup.2 or more and 2500 pieces/mm.sup.2 or less.