Electric power steering apparatus

1. An electric power steering apparatus, comprising:

an electric motor for producing a steering assist force;

a steering shaft;

a speed reduction mechanism for decelerating the rotation of an output shaft in the electric motor, the speed reduction mechanism including an input pulley driven by the electric motor, an output pulley connected to the steering shaft, and a belt for connecting the input pulley and the output pulley;

a first housing accommodating the input pulley, the first housing being cylindrical and having a first section;

a second housing accommodating the output pulley and the steering shaft, the second housing being cylindrical and having a second section opposed to the first section; and

a spacer for changing a distance between centers of the input pulley and the output pulley and for adjusting a spacing between the first and second sections, the spacer having a plate shape and being interposed between the first and second sections,

wherein:

the belt includes a helical toothed belt; and

the input pulley and the output pulley respectively include helical toothed pulleys meshed with the helical toothed belt.

2. The electric power steering apparatus according to claim 1 , wherein the helical toothed belt comprises teeth, and an angle formed between a tooth trace of each of the teeth of the helical toothed belt and a width direction of the helical toothed belt is set to not more than 10 degrees.

3. The electric power steering apparatus according to claim 1 , wherein the helical toothed belt comprises teeth, and an angle formed between a tooth trace of each of the teeth of the helical toothed belt and a width direction of the helical toothed belt is set in a range of 5 degrees to 10 degrees.

4. The electric power steering apparatus according to claim 1 , wherein each of the helical toothed pulleys comprises teeth, and an angle of torsion of a tooth trace of each of the teeth of each of the helical toothed pulleys is set to not more than 10 degrees.

5. The electric power steering apparatus according to claim 1 , wherein each of the helical toothed pulleys comprises teeth, and an angle of torsion of a tooth trace of each of the teeth of each of the helical toothed pulleys is set in a range of 5 degrees to 10 degrees.

6. The electric power steering apparatus according to claim 1 , wherein the output pulley has an annular shape surrounding the steering shaft.

7. The electric power steering apparatus according to claim 1 , further comprising:

a first shaft connected to a steering wheel so as to be integrally rotatable,

the steering shaft including a second shaft connected to the first shaft through a torsion bar so as to be relatively rotatable.

8. The electric power steering apparatus according to claim 1 , further comprising:

a first shaft connected to a steering wheel so as to be integrally rotable, a second shaft connected to the first shaft through a torsion bar so as to be relatively rotatable, and a rack extending along a width direction of a vehicle,

the steering shaft including a pinion shaft meshed with the rack shaft.

9. The electric power steering apparatus according to claim 1 , Wherein the spacer is a shim.

10. The electric power steering apparatus according to claim 1 , wherein

the speed reduction mechanism includes a pair of belt pulley mechanisms, and

each of the belt pulley mechanisms is provided with the helical toothed pulley serving as the input pulley, the helical toothed pulley serving as the output pulley, and the helical toothed belt.

11. The electric power steering apparatus according to claim 10 , wherein

respective tooth traces of teeth of the helical toothed belts in the pair of belt pulley mechanisms are inclined in a same direction.

12. The electric power steering apparatus according to claim 10 , wherein

respective tooth traces of teeth of the helical toothed belts in the pair of belt pulley mechanisms are inclined in opposite directions.

13. The electric power steering apparatus according to claim 1 , wherein the steering shaft includes a rack shaft extending along a width direction of a vehicle.

14. The electric power steering apparatus according to claim 13 , further comprising a conversion mechanism for converting a rotation of the output pulley into an axial movement of the rack shaft.

15. The electric power steering apparatus according to claim 14 , wherein:

the conversion mechanism includes a ball screw mechanism;

the ball screw mechanism comprises a ball nut surrounding the rack shaft and rotatable integrally with the output pulley;

a screw groove is formed on a peripheral surface of the rack shaft; and

a ball is interposed between the screw groove and the ball nut.