Ball screw, method for manufacturing ball screw, power steering device, and method for manufacturing power steering device

Patent metadata

Field	Value
Publication number	US-2022128135-A1
Application number	US-202017434915-A
Country	US
Kind code	A1
Filing date	Mar 3, 2020
Priority date	Mar 15, 2019
Publication date	Apr 28, 2022
Grant date	—

Abstract

An object of the present invention is to provide a ball screw having a structure in which a screw shaft and a nut satisfy required performance and having improved durability, a method for manufacturing the ball screw, a power steering device, and a method for manufacturing the power steering device.
A ball screw according to the present invention includes: a nut made of a steel substrate having an inner peripheral surface on which a female screw is formed; a screw shaft that is combined with the nut and is made of a steel substrate having an outer peripheral surface on which a male screw facing the female screw is formed; and a plurality of balls arranged between the female screw and the male screw. A surface of the nut in contact with the ball and a surface of the screw shaft in contact with the ball each have carburized layers. A carbon concentration in the carburized layer of the nut is higher than a carbon concentration in the carburized layer of the screw shaft.

First claim

1 . A ball screw comprising:
a nut made of a steel substrate having an inner peripheral surface on which a female screw is formed; a screw shaft that is combined with the nut and is made of a steel substrate having an outer peripheral surface on which a male screw facing the female screw is formed; and a plurality of balls arranged between the female screw and the male screw, wherein a surface of the nut in contact with the ball and a surface of the screw shaft in contact with the ball each have carburized layers, and a carbon concentration in the carburized layer of the nut is higher than a carbon concentration in the carburized layer of the screw shaft.

2 . The ball screw according to claim 1 , wherein
the screw shaft includes: a cementite dispersion region in which carbide particles containing cementite are dispersed in a martensite structure on the surface in contact with the ball; a martensite structure formed inside the cementite dispersion region; and a sorbite structure formed inside the martensite structure, the carbide particle containing the cementite includes a first cementite layer formed on a surface of a particle having a martensite structure and a second cementite layer formed on a surface of the first cementite layer, and a carbide concentration of the second cementite layer is lower than a carbide concentration of the first cementite layer.

3 . The ball screw according to claim 2 , wherein a unit cell volume of the second cementite layer is smaller than a unit cell volume of the first cementite layer.

4 . The ball screw according to claim 1 , wherein a Vickers hardness of a surface of the nut is larger than a Vickers hardness of a surface of the screw shaft.

5 . A power steering device comprising:
a ball screw including a nut made of a steel substrate having an inner peripheral surface on which a female screw is formed; a screw shaft that is combined with the nut and is made of a steel substrate having an outer peripheral surface on which a male screw facing the female screw is formed; and a plurality of balls arranged between the female screw and the male screw; a steering gear case that accommodates the ball screw mechanism; an electric motor used to drive the ball screw mechanism; and a power transmission mechanism that transmits a rotational driving force of the electric motor to the nut, wherein the ball screw is the ball screw according to claim 1 .

6 . A method for manufacturing a ball screw, which includes: a nut made of a steel substrate having an inner peripheral surface on which a female screw is formed; a screw shaft that is combined with the nut and is made of a steel substrate having an outer peripheral surface on which a male screw facing the female screw is formed; and a plurality of balls arranged between the female screw and the male screw, the method comprising:
performing a radio frequency carburization treatment of heating a surface of the screw shaft in contact with the ball in a carburizing gas atmosphere while applying a radio frequency to form a carburized layer; and performing an induction hardening treatment of heating the screw shaft subjected to the radio frequency carburization treatment while applying a radio frequency to the screw shaft to quench the screw shaft.

7 . The method for manufacturing a ball screw according to claim 6 , wherein
a precipitate including a particle having an austenite structure, a first cementite layer formed on a surface of the particle having the austenite structure, and a second cementite layer formed on a surface of the first cementite layer is formed by the radio frequency carburization treatment, and the induction hardening treatment is performed to change the austenite structure to a martensite structure.

8 . The method for manufacturing a ball screw according to claim 6 , wherein the radio frequency carburization treatment includes a step of performing heating from 400° C. to 1100° C. for 20 seconds, keeping 1100° C. for 45 seconds while repeating introduction of a carburizing gas and stop of the introduction, and then, performing cooling with an Ar gas.

9 . The method for manufacturing a ball screw according to claim 6 , wherein the induction hardening treatment includes a step of performing heating from 4400° C. to 900° C. for 20 seconds, keeping 900° C. for 50 seconds, and then, performing quenching.

10 . A method for manufacturing a power steering device, which includes:
a ball screw including a nut made of a steel substrate having an inner peripheral surface on which a female screw is formed; a screw shaft that is combined with the nut and is made of a steel substrate having an outer peripheral surface on which a male screw facing the female screw is formed; and a plurality of balls arranged between the female screw and the male screw; a steering gear case that accommodates the ball screw mechanism; an electric motor used to drive the ball screw mechanism; and a power transmission mechanism that transmits a rotational driving force of the electric motor to the nut, the method comprising manufacturing the ball screw by the method for manufacturing a ball screw according to claim 6 .

Assignees

• Hitachi Astemo Ltd

Inventors

• Komuro Matahiro
• Hatou Hisamitsu
• Shiino Kotaro
• Numakura Koji
• Tsujimi Shintaro

Classifications

• F16H2025/249

  Materials or coatings for screws or nuts · CPC title

• F16H2025/2081

  Parallel arrangement of drive motor to screw axis · CPC title

• F16H25/2204

  with balls · CPC title

• B62D5/0448

  Ball nuts · CPC title

• C21D1/10 Primary

  by electric induction · CPC title