Worm wheel and electric power steering apparatus

A worm wheel comprising:

• a disk-shaped core made of metal;
• and
• an annular rim portion made of synthetic resin and covering an outer circumferential portion of the core;
• wherein a gear is formed on an outer circumferential portion of the rim portion,
• wherein the rim portion is integrally formed with the core by synthetic resin injection molding,
• wherein the core comprises a center hole into which a shaft is fitted, an axial end surface formed with a first annular recess, and another axial end surface formed with a second annular recess,
• wherein the first annular recess is formed at a radially inner side with respect to the outer circumferential portion of the core and is recessed toward an axially inner side with respect to the axial end surface of the outer circumferential portion of the core, and the second annular recess is formed at a radially inner side with respect to the outer circumferential portion of the core and is recessed toward an axially inner side with respect to the other axial end surface of the outer circumferential portion of the core,
• wherein an outer circumferential wall surface of a radially outer region of the second annular recess comprises a tapered surface extending from a bottom surface of the second annular recess to form an obtuse angle relative to the bottom surface, and a third annular recess formed radially outside the tapered surface, a cross-sectional shape of the third annular recess being a right-angled triangle having, as two sides of the triangle, a surface parallel to the bottom surface of the second annular recess and a surface perpendicular to the bottom surface of the second annular recess,
• wherein the rim portion comprises a first inner circumference extending from an axial end surface of the rim portion to a radially outer region of the first annular recess and fixed to the radially outer region of the first annular recess, and a second inner circumference extending from another axial end surface of the rim portion to the radially outer region of the second annular recess and fixed to the radially outer region of the second annular recess,
• wherein the rim portion is injection-molded in a state in which the core is arranged in a mold such that the second annular recess faces a gate and such that an axial direction of a sprue and an axial direction of the core are aligned with each other, and
• wherein the outer circumferential wall surface of the second annular recess further comprises an outer tapered surface formed radially outside the third annular recess, wherein the tapered surface and the outer tapered surface are arranged on a same line.