EP3694659A1

EP3694659A1 - Rack bar manufacturing method and rack bar

Info

Publication number: EP3694659A1
Application number: EP18796138.8A
Authority: EP
Inventors: Kenichi Aoki
Original assignee: Neturen Co Ltd
Current assignee: Neturen Co Ltd
Priority date: 2017-10-10
Filing date: 2018-10-10
Publication date: 2020-08-19
Anticipated expiration: 2038-10-10
Also published as: KR20200066285A; KR102462187B1; WO2019074022A1; JP7037906B2; CN111201096A; JP2019069463A; EP3694659B1; CN111201096B

Abstract

A rack bar (14) is manufactured by sequentially inserting a plurality of mandrels into a hollow shaft member with a teeth forming die being pressed against an external surface of a teeth base portion of the shaft member, such that the size of the mandrel in the height direction is increased in a stepwise manner and such that the size of the pressing portion of the mandrel in the width direction is reduced in a stepwise manner. The manufactured rack bar (14) has a first worked surface (15) and a second worked surface (16), both formed on an internal surface of the teeth base portion in a recessed manner and extending in the axial direction. The second worked surface is formed at a central portion of the first worked surface with respect to a tooth-width direction.

Description

RACK BAR MANUFACTURING METHOD AND RACK BAR

The present invention relates to a rack bar manufacturing method and a rack bar.

Examples of a rack bar used for a rack-and-pinion type steering apparatus include a rack bar having a solid shaft member on which teeth are formed by cutting and a hollow rack bar having a hollow shaft member to reduce weight.

Hollow rack bars may be manufactured in the following manner. First, a flat teeth base portion is formed on the hollow shaft member by partially crushing the shaft member. Then, a teeth forming die is pressed onto an external surface of the teeth base portion. Further, mandrels are inserted into the shaft member in a state where a teeth forming die is pressed onto the teeth base portion. When the mandrels are inserted into the shaft member, the material of the teeth base portion of the shaft member is plastically worked by the mandrels and enters the grooves of the teeth forming die. Further, the insertions of the mandrels are repeated by increasing the size of the mandrels in a stepwise manner, so that the teeth profile of the teeth forming die is transferred on the teeth base portion of the shaft member to form a plurality of teeth.

The tooth width of the teeth formed according to the hollow rack bar manufacturing method is associated with the number of insertions of the mandrels. Generally, the tooth width becomes wider as the number of insertions increases. However, as the number of insertions increases, a lifetime of the teeth forming die is shortened, and the manufacturing cost increases. According to a related art method of manufacturing a hollow rack bar, ultrasonic vibration is applied to the teeth forming die so as to achieve the reduction of the number of insertions of the mandrels and the enlargement of the tooth width (see, e.g., JP2013-006189A).

This related art hollow rack bar manufacturing method however requires means for applying the ultrasonic vibration.

Illustrative aspects of the present invention provide a rack bar and a manufacturing method by which a tooth width is easily enlarged with reduced number of insertions of mandrels.

According to an illustrative aspect of the present invention, method for manufacturing a rack bar is provided. The method includes pressing a teeth forming die against an external surface of a teeth base portion of a hollow shaft member, the teeth base portion being flat and extending in an axial direction of the shaft member, and forming teeth on the teeth base portion by causing a material of the teeth base portion to plastically flow toward the teeth forming die. The forming of the teeth includes sequentially inserting a plurality of mandrels into the shaft member with the teeth forming die being pressed against the external surface of the teeth base portion. Each of the mandrels has a pressing portion configured to contact an internal surface of the teeth base portion when inserted into the shaft member. The plurality of mandrels includes a first mandrel and a second mandrel to be inserted into the shaft member after inserting the first mandrel into the shaft member. With respect to a height direction perpendicular to the teeth base portion, a size of the second mandrel is greater than a size of the first mandrel. With respect to a width direction perpendicular to the axial direction and the height direction, the size of the pressing portion of the second mandrel is smaller than the size of the pressing portion of the first mandrel.

According to another illustrative aspect of the present invention, a rack bar has a hollow shaft member. The hollow shaft member includes a teeth base portion extending in an axial direction of the shaft member, teeth formed on an external surface of the teeth base portion, and a first worked surface and a second worked surface, both formed on an internal surface of the teeth base portion in a recessed manner and extending in the axial direction. The second worked surface is formed at a central portion of the first worked surface with respect to a tooth-width direction.

Fig. 1 is a plan view of an example of a rack bar according to an embodiment of the present invention. Fig. 2 is a longitudinal sectional view of the rack bar of Fig. 1. Fig. 3A is a diagram illustrating one step of a method for manufacturing the rack bar of Fig. 1. Fig. 3B is a diagram illustrating another step of the method. Fig. 3C is a diagram illustrating another step of the method. Fig. 4 is a side view of a mandrel used for the method. Fig. 5 is a front view of the mandrel of Fig. 4. Fig. 6 is a sectional view taken along the line VI-VI of Fig. 1. Fig. 7 is a sectional view of a rack bar of a reference example. Fig. 8 is a sectional view of a modification of the rack bar of Fig. 1. Fig. 9 is a diagram illustrating an evaluation of tooth widths of a test example. Fig. 10 is a graph showing a result of the evaluation of the tooth widths of the test example.

Figs. 1 and 2 illustrate an example of a rack bar according to an embodiment of the present invention.

A rack bar 10 is made of a metal material such as steel. The rack bar 10 includes a cylindrical hollow shaft member 11 having axial ends 11a, 11b, each having an opening, and a flat teeth base portion 12 extending in the axial direction, and teeth 13 are formed on an external surface of the teeth base portion 12. For example, female screws for joining a joint in which a tie rod of a steering apparatus is connected are formed in the inner circumferential portions of the ends 11a, 11b on both sides.

In this example, a tooth width direction of the teeth 13 is orthogonal to the axial direction of the shaft member 11, and the pitch of the teeth 13 is constant. However, the tooth width direction of the teeth 13 may be tilted with respect to the axial direction of the shaft member 11. For example, the pitch of the teeth 13 may be changed to be relatively narrow in the central portion of the teeth base portion 12 and to be relatively widened in both the end portions of the teeth base portion 12.

Figs. 3A to 3C illustrate a manufacturing method of the rack bar 10.

First, as illustrated in Fig. 3A, the cylindrical hollow shaft member 11 is partially crushed inward by a press processing, and the flat teeth base portion 12 which extends in the axial direction of the shaft member 11 is formed in advance.

Next, as illustrated in Fig. 3B, the shaft member 11 is held by an upper die 21 and a lower die 22 of a die 20, and a teeth forming die 23 attached at the upper die 21 is pressed against the external surface of the teeth base portion 12 of the shaft member 11. Further, one of a plurality of mandrels 25 housed in a mandrel holder 24 is pushed by a first push rod 26 to be inserted into the shaft member 11. Subsequently, as illustrated in Fig. 3C, the mandrel 25 inserted into the shaft member 11 is pushed back by a second push rod 27 to be discharged from the shaft member 11 and be housed back in the mandrel holder 24.

During the course of pushing in and pushing back of the mandrel 25 along the entire length of the teeth base portion 12, the material of the teeth base portion 12 is plastically deformed by the mandrel 25 and plastically flows toward the teeth forming die 23 which is pressed against the teeth base portion 12. The insertion of the mandrel 25 with respect to the shaft member 11 is repeated while the size in the height direction perpendicular to the teeth base portion 12 of the mandrel which is selected from the plurality of mandrels 25 housed in the mandrel holder 24 to be inserted into the shaft member 11 increases gradually. Thus, the material of the teeth base portion 12 gradually enters the molding surface of the teeth forming die 23, and the teeth profile of the molding surface of the teeth forming die 23 is transferred so that the teeth 13 are formed on the teeth base portion 12.

Figs. 4 and 5 illustrate one example of the mandrel 25.

The mandrel 25 is a rod-shaped member made of a metal material, and is inserted into the shaft member 11 in a state where the central axis of the mandrel 25 is arranged substantially in parallel with the central axis of the shaft member 11. The mandrel 25 includes a pressing portion 30 and a support portion 31. When the mandrel 25 is inserted into the shaft member 11, the pressing portion 30 contacts the internal surface of the teeth base portion 12. The support portion 31 is formed in an arc shape in the sectional surface perpendicular to the central axis of the mandrel 25, and a curvature radius r of the support portion 31 is almost the same as the internal radius (a half an inner diameter D) of the shaft member 11. When the mandrel 25 is inserted into the shaft member 11, the support portion 31 contacts an arc-shaped back surface 14 (see Fig. 2) on the opposite side of the teeth base portion 12 with the central axis of the shaft member 11 interposed therebetween.

The pressing portion 30 is generally flat, but has a plurality of protrusions 32a, 32b, 32c provided on the surface of the pressing portion 30. The protrusions 32a, 32b, 32c may be formed to have the same height h from the surface. Alternatively, the protrusion 32c arranged on the rear side in an inserting direction when the mandrel 25 is inserted into the shaft member 11 in the teeth forming step may be formed to be higher than the protrusion 32a arranged on the tip side in the inserting direction, or the central protrusion 32b may be formed to be higher than the protrusions 32a, 32c on both sides. The size H of the mandrel 25 in the height direction is defined as a distance from a center point of the arc of the support portion 31 in the sectional surface to the top of the highest one of the protrusions 32a, 32b, 32c.

With respect to the width direction perpendicular to the axial direction and the height direction of the mandrel 25, the size W of the protrusions 32a, 32b, 32c is substantially the same. Hereinafter, the size W of the protrusion 32a, 32b, 32c will be described as the width of the pressing portion 30. The plurality of mandrels 25 to be inserted into the shaft member 11 in the teeth forming step has two or more different widths W of the pressing portion 30. The mandrels 25 are sequentially inserted into the shaft member 11, such that the height H of the mandrel 25 is increased in a stepwise manner, and such that the width W of the pressing portion 30 reduced in a stepwise manner.

All the mandrels 25 are configured such that, regardless of the height H of the mandrel 25 and the width W of the pressing portion 30, corners 30a, 30b at both sides of the pressing portion 30 in the width direction of the pressing portion 30 are located within a circle C1 having the curvature center O of the support portion 31 as the circle C1, and the curvature radius r of the support portion 31 is a radius of the circle C1.

Fig. 6 illustrates one example of the sectional surface of the rack bar 10.

In Fig. 6, the one or more mandrel 25A in which the width W of the pressing portion 30 is W1 and one or more mandrel 25B in which the width W of the pressing portion 30 is W2 (W1 > W2) are used. Fig. 6 is an example in which in the teeth forming step, with respect to the shaft member 11, the mandrel 25A is inserted first, and the mandrel 25B is inserted later. The internal surface of the teeth base portion 12 is formed with a first worked surface 15 and a second worked surface 16 which extend in the axial direction of the shaft member 11.

The first worked surface 15 is a recessed worked surface formed by a relatively wide pressing portion 30 of the first inserted mandrel 25A. The second worked surface 16 is a recessed worked surface formed by a relatively narrow pressing portion 30 of the later-inserted mandrel 25B. The second worked surface 16 is formed at the central portion of the first worked surface 15 with respect to the tooth-width direction. Small protrusions 17 protruded along the side surfaces of the both sides of the later-inserted mandrel 25Band toward the inside of the shaft member 11 is formed at the boundary between the first worked surface 15 remaining on respective sides of the second worked surface 16 in the tooth-width direction and the second worked surface 16.

In a case where the mandrels 25 to be inserted into the shaft member 11 are all configured as the mandrel 25A in which the width W of the pressing portion 30 is W1, as illustrated in Fig. 7, the corners of the pressing portion 30 at both sides in the width direction of the pressing portion 30 may interfere with the inner circumferential portion of the end 11a of the shaft member 11. In the other words, the corners 15a, 15b at both sides of the first worked surface 15 in the tooth-width direction may extend to the outside of an inner diameter circle C2 of the shaft member 11. In this case, the inner circumferential portion of the end 11a of the shaft member 11 may be damaged and may hinder the formation of the female screw for joining the joint, for example.

With respect thereto, by making the width W of the pressing portion 30 of the mandrel 25 smaller in a stepwise manner in order of being inserted into the shaft member 11, it is possible to avoid the interference between the corners of the pressing portion 30 at both sides of the pressing portion 30 with respect to the width direction of the pressing portion 30 and the inner circumferential portion of the end 11a of the shaft member 11. In the other words, the first worked surface 15 and the second worked surface 16 are located within the inner diameter circle C2 of the shaft member 11. Further, the material of the teeth base portion 12 is plastically worked over the wide range by the relatively wide pressing portion 30 of the initially inserted mandrel 25A, so as to easily enlarge the tooth width of the teeth 13 and to reduce the number of insertions of the mandrel 25 which is required to obtain a desired tooth width. Further, it is possible to extend the lifetime of the teeth forming die 23.

The plurality of mandrels 25 in which the width W of the pressing portion 30 is three or more different types may be used. For example, in addition to the mandrel 25A and the mandrel 25B, one or more mandrel 25C in which the width W of the pressing portion 30 is W3 (W1 > W2 > W3) may be further used. In a case where the mandrel 25C is inserted into the shaft member 11 after the mandrel 25B is inserted, as illustrated in Fig. 8, a third worked surface 18 is formed to be recessed at the central portion of the second worked surface 16 with respect to the tooth-width direction.

Hereinafter, test examples with be described.

In a first test example, the rack bar 10 is manufactured such that nineteen mandrels 25 are inserted into the shaft member 11 having an outer diameter of 38 mm and an inner diameter of 26.5 mm in the teeth forming step. The mandrel in which the width W of the pressing portion 30 is 20.35 mm is used as all the nineteen mandrels 25. In addition, in order of being inserted into the shaft member 11, the height H of the mandrel 25 of No. 1 is 19.04 mm, the height H of the mandrel 25 of No. 19 is 22.70 mm, and the height increases by a gap of 0.4 mm to 0.05 mm.

Similarly to the first test example, in a second test example, the rack bar 10 is manufactured such that the seventeen mandrels 25 are inserted into the shaft member 11 in the teeth forming step. In order of being inserted into the shaft member 11, the mandrel in which the width W of the pressing portion 30 is 22.3 mm is used as the mandrels 25 of Nos. 1 to 13, and the mandrel in which the width W of the pressing portion 30 is 20.35 mm is used as the mandrels 25 of Nos. 14 to 17. The height H of the first mandrel 25 of No. 1 is 19.04 mm, the height H of the mandrel 25 of No. 17 is 22.60 mm, and the height increases by a gap of 0.4 mm to 0.05 mm.

The tooth widths of the teeth 13 of the rack bar 10 of the first test example and the rack bar 10 of the second test example are respectively evaluated by a pin contact length. The teeth 13 which are evaluating targets are six teeth (tooth Nos. 1 to 6) on one end side of the teeth base portion 12 in the axial direction, two teeth (tooth Nos. 11 and 12) of the axial central portion of the teeth base portion 12, and six teeth (tooth Nos. 17 to 22) on one the other end side of the teeth base portion 12 in the axial direction. In addition, as illustrated in Fig. 9, the pin contact length indicates a length L of the contact portion between the tooth 13 and the pin gauge 40 at the time of inserting a pin gauge 40 having a dimeter of 4.5 mm between the two adjacent teeth 13. In the evaluation by the pin contact length, the tooth width of the portion which is closer to a portion engaged with the pinion than the tooth tip is evaluated in the tooth 13. The evaluation can be considered as an evaluation adapted to the actual circumstances.

Fig. 10 illustrates the evaluation result of the tooth width of the first test example and the second test example.

As illustrated in Fig. 10, the width W of the pressing portion 30 of the mandrel 25 inserted in the initial stage of the teeth formation step is relatively wider than the width W of the pressing portion 30 of the mandrel 25 inserted in the later stage of the teeth formation step. With regard to all teeth 13 that were evaluated, the tooth width of the rack bar 10 of the second test example in which the seventeen mandrels 25 are inserted is wider than the tooth width of the rack bar 10 of the first test example in which the nineteen mandrels 25 having the same width W of the pressing portion 30 are inserted.

From the above results, by sequentially inserting the plurality of mandrels 25 having two or more different widths W of the pressing portion 30 such the width W of the pressing portion 30 of the mandrel 25 reduced in a stepwise manner in the order of being inserted into the shaft member 11, in the other words, by making the width W of the pressing portion 30 of the mandrel inserted in the initial stage of the teeth formation step wider than the width W of the pressing portion 30 of the mandrel inserted in the later stage of the teeth formation step, the width of the teeth 13 can be easily enlarged and with reduced number of insertions of the mandrel 25 required to obtain a desired tooth width.

This application is based on Japanese Patent Application No. 2017-196921 filed on October 10, 2017, the entire content of which is incorporated herein by reference.

Claims

A method for manufacturing a rack bar, the method comprising:
pressing a teeth forming die against an external surface of a teeth base portion of a hollow shaft member, the teeth base portion being flat and extending in an axial direction of the shaft member; and
forming teeth on the teeth base portion by causing a material of the teeth base portion to plastically flow toward the teeth forming die, wherein the forming of the teeth comprises sequentially inserting a plurality of mandrels into the shaft member with the teeth forming die being pressed against the external surface of the teeth base portion,
wherein each of the mandrels has a pressing portion configured to contact an internal surface of the teeth base portion when inserted into the shaft member,
wherein the plurality of mandrels includes a first mandrel and a second mandrel to be inserted into the shaft member after inserting the first mandrel into the shaft member,
wherein, with respect to a height direction perpendicular to the teeth base portion, a size of the second mandrel is greater than a size of the first mandrel, and
wherein, with respect to a width direction perpendicular to the axial direction and the height direction, the size of the pressing portion of the second mandrel is smaller than the size of the pressing portion of the first mandrel.
The method according to claim 1, wherein each of the mandrels has a support portion having a same curvature radius as an inner radius of the shaft member, and
wherein corners of the pressing portion at both sides of the pressing portion in the width direction are located within a circle having a curvature center of the support portion as a center of the circle and a curvature radius of the support portion as a radius of the circle.
A rack bar comprising a hollow shaft member, the hollow shaft member comprising:
a teeth base portion extending in an axial direction of the shaft member;
teeth formed on an external surface of the teeth base portion; and
a first worked surface and a second worked surface, both formed on an internal surface of the teeth base portion in a recessed manner and extending in the axial direction,
wherein the second worked surface is formed at a central portion of the first worked surface with respect to a tooth-width direction.
The rack bar according to claim 3, wherein the first worked surface and the second worked surface are located within an inner diameter circle of the shaft member.