Delashed worm gear assembly and electric power assist apparatus

摘要

An electric power assist apparatus for motor vehicle power steering including an electric motor and a speed reducer. The speed reducer includes a worm and a molded plastic worm gear on an output shaft of the speed reducer. The polymer from which the worm gear and the integral worm gear teeth thereon is molded has substantial structural rigidity after becoming cured. A plastic tooth ring is insert molded on the worm gear from a polymer having substantially greater resilience after becoming cured. The plastic tooth ring includes a plurality of resiliently flexible gear teeth having flanks which circumferentially overlap the flanks of the worm gear teeth. When the electric motor is off, the gear teeth on the worm mesh with the resiliently flexible gear teeth on the tooth ring with zero clearance to suppress noise attributable to rattling between the worm and resilient flexure of the flexible gear teeth on the tooth ring until the worm teeth engage the structurally more rigid worm gear teeth.