Electric power steering apparatus equipped with worm gear mechanism

1. An electric power steering apparatus for a vehicle, comprising:

an electric motor for generating a torque in accordance with a steering input; and

a worm gear mechanism for transmitting the torque generated by the electric motor to a steered road wheel of the vehicle,

wherein the worm gear mechanism has a worm gear made of metal and a worm wheel formed of plastic material and meshing with the worm gear, the worm gear mechanism further having a line of action along which contact between a plurality of adiacent teeth of the worm gear and a corresponding number of adiacent teeth of the worm wheel gear occurs when the worm gear and the worm wheel are meshing together,

wherein the teeth of the worm wheel have an involute profile,

wherein the worm gear has a tooth profile including a dedendum part formed in an involute shape and an addendum part formed in a circular arc shape, the circular arc shape of the addendum part having a center located on the straight line of action,

wherein the worm gear has a dedendum width decreasing progressively at a first decreasing rate in a direction from a bottom of each tooth of the worm gear toward a pitch circle of the worm gear, and

wherein the circular arc shape of the addendum part of the tooth profile is such that the worm gear has an addendum width decreasing progressively in a direction from the pitch circle of the worm gear toward a top of each worm gear tooth at a second decreasing rate greater than the first decreasing rate of the dedendum width of the worm gear.

2. The electric power steering apparatus as recited in claim 1 , wherein the worm gear mechanism further includes an auxiliary worm wheel rotatable relative to the worm wheel about the same rotation axis as the worm wheel and urged into meshing engagement with the worm gear for eliminating undesired backlash between the worm gear and the worm wheel.