Patent ID: 7748492

Claim:
An electric power steering device which transmits output rotation of an electric motor for generating a steering assist force to a steering mechanism via a driving gear and a driven gear meshed with the driving gear, the electric power steering device comprising: a housing which accommodates the driving gear and the driven gear; a biasing member for resiliently biasing the driving gear toward the driven gear; and a pair of bearings which rotatably support opposite end portions of a shaft co-rotatable with the driving gear, respectively, wherein the housing has a support hole which supports one of the end portions of the shaft via a corresponding one of the bearings in such a manner as to permit the one end portion to be offset toward the driven gear radially of the driving gear, and a recess formed in an inner peripheral surface of the support hole, the recess having a bottom and a pair of interior walls opposed to each other circumferentially of the support hole; the biasing member includes a main portion of a circular arc cross section fitted around an outer peripheral surface of an outer ring of the corresponding bearing in opposed relation to the inner peripheral surface of the support hole, a pair of circular arc ends provided on the main portion, and a pair of movement restriction portions provided at the respective circular arc ends of the main portion, only one of the pair of movement restriction portions having a resilient tongue which biases the driving gear toward the driven gear via the corresponding bearing, the one of the movement restriction portions having a smaller stress load than the other of the movement restriction portions; the pair of movement restriction portions are engaged with the respective interior walls of the recess to restrict movement of the main portion in corresponding circumferential directions; the resilient tongue extends from the one of the movement restriction portions and is received by the bottom of the recess; the shaft comprises a worm shaft; the driving gear comprises a worm provided around the worm shaft; the driven gear comprises a worm wheel; the worm shaft has a first end portion located relatively far from the electric motor and a second end portion located relatively close to the electric motor; the one end portion of the shaft comprises the first end portion of the worm shaft; the worm shaft is rotated in first and second rotation directions which are opposite from each other; a driving counter force to be received by the worm shaft from the worm wheel when the worm shaft is rotated in the first rotation direction includes an axial component which acts axially of the worm shaft to move the worm shaft away from the electric motor; the pair of circular arc ends of the main portion comprise front and rear ends with respect to the first rotation direction; and the one movement restriction portion from which the resilient tongue extends is provided at the front end as seen in the first rotation direction.