Bush bearing and rack-and-pinion type steering apparatus for automobile using the same

A bush bearing for allowing a rack shaft having rack teeth meshing with teeth of a pinion to be supported movably in an axial direction of the rack shaft, comprising:

• a synthetic resin-made bush having at least one circumferential groove in an outer peripheral surface thereof;
• and an endless annular elastic member which is fitted in the circumferential groove of said bush,
• wherein said bush has an inner peripheral surface defining a through hole for the insertion and fitting therein of the rack shaft;
• at least one slit which partially severs the inner peripheral surface in a circumferential direction;
• a first contact portion which is formed on the inner peripheral surface for slidably coming into contact with an outer peripheral surface of said rack shaft on a line extending in a direction perpendicular to the axial direction of said rack shaft and an axial direction of the pinion;
• a second contact portion which is formed on the inner peripheral surface for slidably coming into contact with the outer peripheral surface of said rack shaft;
• and a third contact portion which is formed on the inner peripheral surface for slidably coming into contact with the outer peripheral surface of said rack shaft,
• wherein the second and third contact portions are arranged with an interval with respect to the first contact portion in the circumferential direction with the first contact portion interposed therebetween in the circumferential direction,
• the second and third contact portions are arranged bilaterally symmetrically about the line,
• the second and third contact portions are each arranged at an equiangular interval with respect to the first contact portion in the circumferential direction,
• the first contact portion has a first inside inner peripheral surface for slidably coming into contact with the outer peripheral surface of said rack shaft, the second contact portion has a second inside inner peripheral surface for slidably coming into contact with the outer peripheral surface of said rack shaft, and the third contact portion has a third inside inner peripheral surface for slidably coming into contact with the outer peripheral surface of said rack shaft,
• the inner peripheral surface of said bush has a first outside inner peripheral surface which is disposed outwardly of the first to third contact portions in a radial direction and for being arranged with a first gap with respect to the outer peripheral surface of said rack shaft between the first and second contact portions, a second outside inner peripheral surface which is disposed outwardly of the first to third contact portions in a radial direction and for being arranged with a second gap with respect to the outer peripheral surface of said rack shaft between the first and third contact portions, and a third outside inner peripheral surface which is disposed outwardly of the first to third contact portions in a radial direction and for being arranged with a third gap with respect to the outer peripheral surface of said rack shaft between the second and third contact portions,
• wherein the first inside inner peripheral surface at a portion thereof is adapted to slidably come into linear contact with a first outer peripheral surface of said rack shaft on an opposite side to the rack teeth side, excluding a second outer peripheral surface of said rack shaft on the rack teeth side, in the outer peripheral surface of the rack shaft,
• the second inside inner peripheral surface at a portion thereof is adapted to slidably come into linear contact with a third outer peripheral surface on one side of the line, excluding the first and second outer peripheral surfaces of the rack shaft, in the outer peripheral surface of the rack shaft,
• the third inside inner peripheral surface at a portion thereof is adapted to slidably come into linear contact with a fourth outer peripheral surface on the other side of the line, excluding the first, second and third outer peripheral surfaces of the rack shaft, in the outer peripheral surface of the rack shaft,
• each of the first, second and third inside inner peripheral surfaces has a flat surface shape, and
• an angle at which a first perpendicular line in an isosceles triangle formed by two sides respectively extending in the radial direction toward a center of where said rack shaft would be positioned, in use from both edges in the circumferential direction of the first inside inner peripheral surface and by a side connecting the both edges and a second perpendicular line in an isosceles triangle formed by two sides respectively extending in the radial direction toward the center of where said rack shaft would be positioned, in use, from both edges in the circumferential direction of the second inside inner peripheral surface and by a side connecting the both edges intersect each other, is identical to an angle at which the first perpendicular line and a third perpendicular line in an isosceles triangle formed by two sides respectively extending in the radial direction toward the center of where said rack shaft would be positioned, in use, from both edges in the circumferential direction of the third inside inner peripheral surface and by a side connecting the both edges intersect each other.