Ball nut type steering gear

A ball nut type steering gear for a motor vehicle comprising;

• a threaded shaft formed with helical groove sections;
• a ball nut formed with helical groove sections and mounted over said threaded shaft for movement axially of the threaded shaft through two trains of balls;
• a rack formed on the outer periphery of said ball nut;
• and a sector gear in meshing engagement with said rack;
• said two trains of balls being confined in passageways formed by respective ones of the helical groove sections in said threaded shaft and said ball nut so as to move the ball nut axially of the threaded shaft as the latter is rotated to thereby rotate said sector gear to change the direction of travel of the motor vehicle;
• wherein the improvement comprises:
• two major diameter portions formed in said threaded shaft and spaced apart from each other by a minor diameter portion, so that said two trains of balls are brought into engagement in the helical groove sections in the major diameter portions of the threaded shaft and the ball nut only when the motor vehicle travels along a straight course and there is no play between the balls and the helical groove sections in which the balls are brought into rolling engagement.