PURPOSE: To make appropriate the torque on a steering wheel at its neutral position and stabilize steering, by a method wherein tooth profile of a central rack tooth is expanded in the direction of tooth width in proximity to the parts located on the axis of symmetry of basic rack for whole depth of the tooth, and is shrinked at the top and the root of the tooth.
CONSTITUTION: When a ball nut 22 is located at its neutral position, the central rack tooth 24 of a rack part 23 engaged with a sector gear 31 so formed that the tooth surface Wa is expanded, beyond the tooth surface W of a reference tooth profile indicated by an imaginary line slanted at a predetermined pressure angle θ, in the direction of the tooth width at parts located on the axis of symmetry of basic rack for whole depth of the tooth 24 and is shrinked at the tip and the root of the tooth 24. When the ball nut 22 is located at its neutral position, the central rack tooth 24 does not interface with the roots of the sector teeth 32, 32 at its tip, and the rack teeth 24W26 are engaged with the tooth surfaces V of the sector teeth 32, 32 at its expanded tooth surfaces Wa over the entire length thereof, so that the preload becomes maximum.
SUDOU SATOSHI
TAKEUCHI KATSUYUKI
