Internal gear

A sintered internal gear comprising:

• cylindrical gear body having teeth in an inner circumferential surface thereof adapted to be in mesh with a pinion;
• a flange portion extending radially inwardly from the cylindrical gear body;
• and
• a boss portion provided at an inner diameter portion of the flange portion and mounted on an outer circumferential surface of a shaft;
• wherein the cylindrical gear body, the flange portion and the boss portion are formed integrally and sintered, wherein the boss portion protrudes beyond both sides of the flange portion at the inner diameter of the flange portion, and wherein all or a part, in a radial direction, of the flange portion has a thickness thereof which progressively increases from an outer diameter side toward an inner diameter side and has a density in the radial direction almost inversely proportional to the thickness.

An internal gear comprising:

• a disk-shaped portion having an engagement hole to be fitted over a rotating shaft and expanding radially of the rotating shaft;
• and
• a cylindrical portion having a tooth portion in an inner circumferential surface thereof and connecting to an outer circumferential part of the disk-shaped portion;
• wherein the cylindrical portion has a density distribution such that the density of an almost central part along a tooth trace of the tooth portions is lower than the density of at least one of end parts in the tooth trace direction of each tooth portion.