Worm wheel, power steering apparatus including the worm wheel, and method of forming the worm wheel

A worm wheel comprising:

• an annular worm wheel main body formed from a resin material by injection molding;
• and
• a plurality of teeth disposed on an outer periphery of the worm wheel main body in a circumferential direction of the worm wheel main body, the plurality of teeth comprising tooth flanks which are brought into meshing engagement with a worm shaft, a tooth crest disposed on an outer peripheral surface of the respective teeth, and a tooth bottom disposed between respective adjacent pairs of the tooth flanks which face to each other in the circumferential direction of the worm wheel main body,
• wherein a non-cutting region is provided in at least one end portion of the respective tooth flanks in an axial direction of the worm wheel main body which is located at an end portion on a side of the tooth crest, the non-cutting region being not formed by cutting by a hob after completion of the injection molding, and
• a cutting region is provided on the respective tooth flanks, the cutting region being formed by cutting by the hob after completion of the injection molding.

A method of forming a worm wheel, comprising:

• a first step of forming an annular worm wheel main body and a primary teeth portion on an outer periphery of the worm wheel main body from a resin material by injection molding, the primary teeth portion having a plurality of primary teeth disposed in a circumferential direction of the worm wheel main body, the plurality of primary teeth including primary tooth flanks, a primary tooth crest disposed on an outer peripheral surface of the respective primary teeth, and a primary tooth bottom disposed between respective adjacent pairs of the primary tooth flanks which face to each other in the circumferential direction of the worm wheel main body, and
• a second step of cutting a predetermined portion of the respective primary tooth flanks formed in the first step by a hob to thereby provide a plurality of secondary teeth comprising secondary tooth flanks which are brought into meshing engagement with a worm shaft, a secondary tooth crest disposed on an outer peripheral surface of the respective secondary teeth, and a secondary tooth bottom disposed between respective adjacent pairs of the secondary tooth flanks which face to each other in the circumferential direction of the worm wheel main body, the respective secondary tooth flanks comprising a cutting region formed by the cutting by the hob and a non-cutting region provided in a remaining portion of the respective secondary tooth flanks exclusive of the cutting region which is not formed by the cutting by the hob, the non-cutting region being located offset from a mid-position of a whole depth of the respective secondary teeth in a radially outward direction of the worm wheel main body.