BALL SCREW MECHANISM AND STEERING SYSTEM

• US 20170334479A1
• Filed: 05/16/2017
• Published: 11/23/2017
• Est. Priority Date: 05/23/2016
• Status: Active Grant

• Current Assignee: Jtekt Corporation (Toyota Motor Corporation)
• Original Assignee: Jtekt Corporation (Toyota Motor Corporation)
• Inventors: ASAKURA, Toshihiro, NAKAYAMA, Takuya, OGAWA, Keisuke
• Granted Patent: US 10,246,122 B2
• Time in Patent Office: Days
• CPC Class Codes: B62D 5/0424 the axes of motor and final driven element of steering gear, e.g. rack, being parallel
• CPC Class Codes: B62D 5/0448 Ball nuts
• CPC Class Codes: F16H 25/2233 with cages or means to hold the balls in position
• CPC Class Codes: F16H 25/24 Elements essential to such mechanisms, e.g. screws, nuts F16H25/22 takes precedence

Abstract

A ball screw mechanism that allows a reduction in contact and friction between a retainer and a nut is provided. In a ball screw mechanism, a nut is threadedly engaged with a screw groove of a rack shaft through balls retained in a retainer groove of a retainer, and an axial force is applied to the rack shaft in accordance with rotation of the nut. The nut includes a uniform diameter portion having a uniform inside diameter and enlarged diameter portions each having an inside diameter larger than that of the uniform diameter portion. The enlarged diameter portions are provided at opposite axial ends of a region on an inner circumferential surface of the nut. The region faces the retainer groove and retainer ends adjoining to opposite axial ends of the retainer groove.

Claims

1. A ball screw mechanism comprising:

a screw shaft having an outer circumferential surface provided with a helical screw groove;

a cylindrical nut having an inner circumferential surface provided with a helical screw groove corresponding to the screw groove of the screw shaft;

a plurality of balls; and

a cylindrical retainer provided between the screw shaft and the nut, wherein

the retainer has a retainer groove that retains the plurality of balls to allow the plurality of balls to avoid contact with each other,

the nut is threadedly engaged with the screw groove of the screw shaft through the plurality of balls,

an axial force is applied to the screw shaft in accordance with rotation of the nut,

the nut includes a uniform diameter portion and an enlarged diameter portion, the uniform diameter portion having a uniform inside diameter, the enlarged diameter portion having an inside diameter larger than the inside diameter of the uniform diameter portion, and

the enlarged diameter portion is provided at at least one of opposite axial ends of a region on the inner circumferential surface of the nut, the region facing the retainer groove and retainer ends adjoining to opposite axial ends of the retainer groove.

2. The ball screw mechanism according to claim 1, wherein

the enlarged diameter portion includes a tapered portion in the region on the inner circumferential surface of the nut, the region facing the retainer groove and the retainer ends adjoining to the opposite axial ends of the retainer groove, the tapered portion having an inside diameter that increases toward an end face of one of opposite axial ends of the nut, the tapered portion located closer to the one of the opposite axial ends of the nut than to the other one of the opposite axial ends of the nut.

3. The ball screw mechanism according to claim 2, wherein

out of a first enlarged diameter portion and a second enlarged diameter portion, the enlarged diameter portion includes at least the second enlarged diameter portion, the first enlarged diameter portion configured to avoid contact with a tool for providing the screw groove on the inner circumferential surface of the nut, the second enlarged diameter portion configured to avoid contact with the retainer and having an inside diameter different from an inside diameter of the first enlarged diameter portion.

4. The ball screw mechanism according to claim 3, wherein

the first enlarged diameter portion adjoins to the end face of the one of the opposite axial ends of the nut,

the second enlarged diameter portion adjoins to the first enlarged diameter portion and includes a screw thread projecting between adjacent turns of the screw groove on the inner circumferential surface of the nut,

the inside diameter of the tapered portion of the second enlarged diameter portion is measured at a crest of the screw thread of the nut and increases toward the end face of the closer one of the opposite axial ends of the nut, and

a tapered angle of the tapered portion of the second enlarged diameter portion is smaller than a tapered angle of the tapered portion of the first enlarged diameter portion.

5. The ball screw mechanism according to claim 1, wherein

the enlarged diameter portion is provided at each of the opposite axial ends of the region on the inner circumferential surface of the nut, the region facing the retainer groove and the retainer ends adjoining to the opposite axial ends of the retainer groove.

6. A steering system comprising:

the ball screw mechanism of claim 1; and

a motor for supplying the nut with a force that rotates the nut, wherein

the screw shaft is a steered shaft that performs reciprocating axial motion and that has opposite ends, each end connected to a corresponding one of steered wheels that are steered in accordance with the reciprocating axial motion, and

the ball screw mechanism steers the steered wheels by converting rotary motion of the motor into the reciprocating axial motion of the steered shaft.