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

1. A worm wheel comprising:

an annular worm wheel main body comprising a resin material; and

teeth on an outer periphery of the annular worm wheel main body in a circumferential direction of the annular worm wheel main body, the teeth comprising tooth flanks configured to be brought into meshing engagement with a worm shaft, a tooth crest on an outer peripheral surface of each of the teeth, and a tooth bottom between each adjacent pair of the tooth flanks that face each other in the circumferential direction of the annular worm wheel main body;

a first region on each of the tooth flanks, the first region being offset from a mid-position of a whole depth of each of the teeth in a radially outward direction of the annular worm wheel main body, the first region in at least one end portion of each of the tooth flanks in an axial direction of the annular worm wheel main body, the first region configured to form a predetermined clearance between the first region in the at least one end portion of each of the tooth flanks and the worm shaft when the worm wheel and the worm shaft are in meshing engagement with each other; and

a second region on each of the tooth flanks, the second region being a cut-out portion,

wherein the first region is in a portion of each of the tooth flanks that is on a radial outside of a pitch circle.

2. The worm wheel as claimed in claim 1 , wherein the first region is in at least one end portion of the tooth bottom in the axial direction of the annular worm wheel main body.

3. The worm wheel as claimed in claim 2 , wherein the first region is in both end portions of the tooth bottom in the axial direction of the annular worm wheel main body.

4. The worm wheel as claimed in claim 2 , wherein a shape of each end portion of the tooth bottom, in which the first region is, is formed to be parallel with a rotation axis of the worm wheel.

5. The worm wheel as claimed in claim 1 , wherein the first region is on a circumference of the pitch circle.

6. The worm wheel as claimed in claim 1 , wherein the first region extends over a whole depth of each of the teeth.

7. The worm wheel as claimed in claim 1 , wherein the first region is only in one of opposite end portions of each of the tooth flanks in the axial direction of the annular worm wheel main body which is on a side of a leading end of each of the tooth flanks in a direction of meshing the worm wheel with the worm shaft.

8. An electric power steering apparatus comprising:

a steering mechanism coupled to steerable road wheels;

a reduction mechanism coupled to the steering mechanism, the reduction mechanism including a worm wheel and a worm shaft meshing with the worm wheel;

an electric motor coupled to the reduction mechanism; and

a control circuit which drivingly controls the electric motor,

wherein the worm wheel according to claim 1 is used as the worm wheel of the reduction mechanism, and

the worm shaft is disposed such that a rotation axis of the worm shaft is inclined relative to a plane perpendicular to a rotation axis of the worm wheel to make an acute angle between the rotation axis of the worm shaft and the plane perpendicular to the rotation axis of the worm wheel.

9. An electric power steering apparatus comprising:

a steering mechanism coupled to steerable road wheels;

a reduction mechanism coupled to the steering mechanism, the reduction mechanism including a worm wheel and a worm shaft meshing with the worm wheel;

an electric motor coupled to the reduction mechanism; and

a control circuit which drivingly controls the electric motor,

wherein the worm wheel according to claim 1 is used as the worm wheel of the reduction mechanism, and the worm shaft is disposed such that a rotation axis of the worm shaft is parallel to a plane perpendicular to a rotation axis of the worm wheel.

10. The worm wheel as claimed in claim 1 , wherein the first region is at an end portion on a side of the tooth crest in the radially outward direction of the annular worm wheel main body.