Steering rack and method for manufacturing the same

Abstract

A steering rack meshes with a pinion rotatably driven by the input shaft of a steering gear constituting an automotive steering device. The steering rack is provided with a an axially extending rod part of round cross section, and a plurality of rack teeth formed on a radial one side surface of an axial portion of the rod part, the rack teeth meshing with the pinion. At least one dummy tooth is formed in portions that are axial parts of the rod part and are adjacent to both axial sides of the plurality of rack teeth. The dummy tooth has a tooth height less than the rack teeth and does not mesh with the pinion.

Claims

1. A steering rack configured to mesh with a pinion configured to be rotationally driven by an input shaft of a steering gear configuring an automotive steering device, the steering rack comprising: a rod part extending in an axial direction and having a round cross section; and a plurality of rack teeth formed on a radial one side surface of a portion of the rod part in the axial direction and configured to mesh with the pinion; and dummy teeth each of the dummy teeth having a tooth height less than the plurality of rack teeth and configured not to mesh with the pinion, wherein the dummy teeth are formed at an end of the portion of the rod part in the axial direction and are adjacent to the plurality of rack teeth in the axial direction, wherein each of the dummy teeth comprises: an axially inner surface and an axially outer surface, which are configured to incline toward each other in a radial direction, a radially outer surface configured to connect the axially inner surface and the axially outer surface via a pair of connection portions, and the pair of connection portions comprising a first connection portion which is between the axially inner surface and the radially outer surface, and a second connection portion which is between the axially outer surface and the radially outer surface, wherein each of the pair of connection portions has a curved shape, wherein an n number of dummy teeth are formed, the n number being a natural number, wherein each of the plurality of rack teeth comprises: an axially inner surface and an axially outer surface, which are configured to incline toward each other in the radial direction, and a radially outer surface configured to connect the axially inner surface and the axially outer surface, wherein a relationship of .sub.1<.sub.2.sub.3<.sub.4.sub.5< . . . <.sub.2n.sub.2n+1 is satisfied, in which: .sub.1 denotes a first inclination angle of the axially outer surface of a first rack tooth, from among the plurality of rack teeth, where the first rack tooth is adjacent to the dummy tooth and is positioned at one of axial ends of the plurality of rack teeth, .sub.2, .sub.4, . . . , .sub.2n respectively denote inclination angles of axially inner surfaces of the dummy teeth such that the dummy teeth are arranged in an order of a first dummy tooth having the .sub.2 inclination angle is closest to the first rack tooth, from among the dummy teeth, and an nth dummy tooth having the .sub.2n inclination angle is furthest from the first rack tooth, from among the dummy teeth, and .sub.3, .sub.5, . . . , .sub.2n+1 respectively denoting inclination angles of axially outer surfaces of the dummy teeth.

2. The steering rack according to claim 1, wherein a second relationship of H.sub.1>H.sub.2>H.sub.3> . . . >H.sub.n+1 is satisfied, in which H.sub.1 denotes a first height of the first rack tooth and the height of a second rack tooth at another one of the axial ends of the plurality of rack teeth and H.sub.2, H.sub.3, . . . , H.sub.n+1 denote heights of the plurality of dummy teeth in an order from the first dummy tooth having the height H.sub.2 to the n.sup.th dummy teeth having the height H.sub.n+1.

3. The steering rack according to claim 2, wherein a third relationship of L.sub.2=H.sub.1L.sub.3, L.sub.4=H.sub.3L.sub.5, . . . , L.sub.2n=H.sub.2n1L.sub.2n+1 is satisfied, in which: L.sub.1 is a first half value of the first tooth height of the first rack tooth and the second rack tooth, and L.sub.2, L.sub.3, . . . , L.sub.n+1 are half values of the tooth heights H.sub.2, H.sub.3, . . . , H.sub.n+1 of the first dummy tooth to the n.sup.th dummy tooth, and wherein a fourth relationship of L.sub.1f.sub.1=L.sub.2f.sub.2, L.sub.3f.sub.3=L.sub.4f.sub.4, . . . , and L.sub.nf.sub.n=L.sub.n+1f.sub.n+1 is satisfied, in which: f.sub.1 denotes a first force, which is to be generated toward the axially outer surface of the first rack tooth and the second rack tooth, at the L.sub.1 height, f.sub.2 denotes a second force, which is to be generated toward the axially inner surface of the first dummy tooth the L.sub.2 height, f.sub.n is a third force, which is to be generated toward the axially outer surface of an (n1).sup.th dummy tooth at a L.sub.n1 height, and f.sub.n+1 is a fourth force, which is to be toward the axially inner surface of the n.sup.th dummy tooth at the L.sub.n+1 height.

4. The steering rack according to claim 1, wherein the radially outer surface of the at least one dummy tooth is formed with at least one groove portion.

5. The steering rack according to claim 4, wherein the groove portion is formed in total of three or more on the plurality of dummy teeth having the same tooth height.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 depicts a rack and a tooth-forming punch according to a first embodiment of the present invention.

(2) FIG. 2 depicts a stress distribution that is to occur in a tooth-forming punch of the related art.

(3) FIG. 3 depicts a stress distribution that is to occur in a tooth-forming punch of the present invention.

(4) FIG. 4 is an SN diagram of a lifetime of the tooth-forming punch of the present invention.

(5) FIG. 5 depicts a rack and a tooth-forming punch according to a second embodiment of the present invention.

(6) FIG. 6 depicts a rack and a tooth-forming punch according to a modified embodiment.

(7) FIG. 7 depicts a rack and a tooth-forming punch according to a modified embodiment.

(8) FIG. 8 is a perspective view depicting a rack according to a modified embodiment.

(9) FIG. 9 is a perspective view depicting a rack according to a modified embodiment.

(10) FIG. 10 is a partially sectional view depicting a first example of the conventional structure of an automotive steering device having a steering gear in which a steering rack is incorporated.

(11) FIG. 11 is a partially sectional view depicting a second example of the conventional structure of an automotive steering device having a steering gear in which a steering rack is incorporated.

(12) FIG. 12 is a perspective view depicting the steering rack.

(13) FIG. 13 is a view, as seen from a XIII arrow direction of FIG. 12.

(14) FIG. 14 is a view, as seen from a XIV arrow direction of FIG. 12.

(15) FIG. 15 is a sectional view taken along a line XV-XV of FIG. 14.

(16) FIGS. 16A to 16F are sectional views as seen from the same direction as FIG. 15, depicting a method of manufacturing the steering rack relating to the conventional structure in a process sequence.

(17) FIGS. 17A and 17B are partial perspective views depicting shapes of rack teeth before and after sizing.

(18) FIG. 18 is a sectional view for illustrating problems of a method and an apparatus for manufacturing the rack of the related art.

(19) FIG. 19 is an enlarged sectional view corresponding to an area A of FIG. 18.

(20) FIG. 20 is an enlarged sectional view corresponding to an area B of FIG. 18.

DETAILED DESCRIPTION OF EMBODIMENTS

(21) Hereinafter, a steering rack according to each embodiment of the present invention will be described in detail with reference to the drawings.

(22) A steering rack and a method for manufacturing the same according to the present invention have features that when forming the rack teeth 16 on the radial one side surface of a part in the axial direction of the rod part 15 configuring the steering rack 11a (processes corresponding to FIGS. 16C and 16D described above), the stress is prevented from being concentrated on the teeth 39 of the tooth-forming punch 32 and the long lifetime of the tooth-forming punch 32 is thus implemented. Since the other configurations and operational effects are the same as the conventionally known steering rack manufacturing method and manufacturing apparatus, including the conventional manufacturing method and manufacturing apparatus shown in FIGS. 16 to 18, the illustration and description on the equivalent parts are omitted or simplified.

First Embodiment

(23) As shown in FIG. 1, a tooth-forming punch 32 of a first embodiment has not only the above-described rack tooth processing concave and convex 40 but also dummy tooth processing concave and convex 41 at parts adjacent to both axial sides of the rack tooth processing concave and convex 40 (in FIG. 1, only the dummy tooth processing concave and convex 41 on one side in the axial direction is shown). A tooth height 41L of the dummy tooth processing concave and convex 41 is formed less than a tooth height 40L of the rack tooth processing concave and convex 40.

(24) Therefore, when strongly pushing the intermediate material 23 into the holding hole 28 by the tooth-forming punch 32 (processes corresponding to FIGS. 16C and 16D), the flat surface part 25 of the intermediate material 23 is plastically deformed in conformity to the rack tooth processing concave and convex 40 and the dummy tooth processing concave and convex 41 and is thus processed to a base rack 33 having rack teeth 16 and dummy teeth 42 as shown in FIG. 1.

(25) More specifically, the base rack 33 has an axially extending rod part 15 having a round cross section and a plurality of rack teeth 16 formed on a radial one side surface of a part in the axial direction of the rod part 15 and configured to mesh with the pinion. A dummy tooth 42 is formed one by one at parts that are parts of the rod part 15 in the axial direction and are adjacent to both sides of the plurality of rack teeth 16 in the axial direction. Since a tooth height H.sub.3 of the dummy tooth 42 is formed less than a tooth height H.sub.1 of the rack tooth 16 (H.sub.3=41 L<H.sub.1=40 L), the dummy tooth 42 does not mesh with the pinion.

(26) Also, the dummy tooth 42 has an axially inner surface 42a and an axially outer surface 42b configured to be inclined in a direction of axially coming close to each other as they go toward a radially outer side, and a radially outer surface 42c configured to connect the axially inner surface 42a and the axially outer surface 42b. The radially outer surface 42c is a curved surface having an R-shaped cross section of which an axially intermediate part is convex, and is configured to smoothly connect the axially inner surface 42a and the axially outer surface 42b. Therefore, a pair of connection parts 42d consisting of the axially inner surface 42a and axially outer surface 42b and the radially outer surface 42c also has an R shape. In the meantime, as described above, since the dummy tooth 42 is configured not to mesh with the pinion, there is no problem even when the radially outer surface 42c, which is a tooth tip, and the pair of connection parts 42d are made to have the R shape.

(27) The rack tooth 16 has an axially inner surface 16a and an axially outer surface 16b configured to be inclined in the direction of axially coming close to each other as they go toward the radially outer side, and a radially outer surface 16c configured to connect the axially inner surface 16a and the axially outer surface 16b. Herein, since the rack tooth 16 is configured to mesh with the pinion, the radially outer surface 16c is formed to be a planar shape, and a pair of connection parts 16d between the axially inner surface 16a and the radially outer surface 16c and between the axially outer surface 16b and the radially outer surface 16c has an angled shape.

(28) When an inclined angle of the axially outer surface 16b, which is adjacent to the dummy tooth 42, of the rack tooth 16 (the rack tooth 16 positioned at the rightmost side in FIG. 1), which is positioned at each of both axial ends, of the plurality of rack teeth 16 is denoted as .sub.1, an inclined angle of the axially inner surface 42a of the dummy tooth 42 is denoted as .sub.2, and an inclined angle of the axially outer surface 42b of the dummy tooth is denoted as .sub.3, the angles are set so that a relation of .sub.1<.sub.2=.sub.3 is satisfied.

(29) Also, a half value of the tooth height H.sub.1 of the rack tooth positioned at each of both axial ends is denoted as L.sub.1 (L.sub.1=0.5H.sub.1), a half value of the tooth height H.sub.3 of the dummy tooth 42 is denoted as L.sub.3 (L.sub.3=0.5H.sub.3), and L.sub.2=H.sub.1L.sub.3 is denoted. In this case, a force f.sub.1 is generated on the axially outer surface 16b of the rack tooth 16 positioned at each of both axial ends toward an axially outer side at a position of L.sub.1 distant from a radially outer end part of the corresponding rack tooth 16 toward a radially inner side. Also, a force f.sub.2 is generated on the axially inner surface 42a of the dummy tooth 42 toward an axially inner side at a position of L.sub.2 distant from the radially outer end part of the rack tooth 16 positioned at each of both axial ends toward the radially inner side. Also, a force f.sub.3 is generated on the axially outer surface 42b of the dummy tooth 42 toward the axially outer side at a position of L.sub.3 distant from the radially outer end part of the dummy tooth 42 toward the radially inner side. Herein, since the angles are set so that the relation of .sub.1<.sub.2=.sub.3 is satisfied, as described above, a relation of f.sub.1>f.sub.2=f.sub.3 is satisfied by a wedge effect. Also, variation of 0.5H.sub.1<H.sub.10.5H.sub.3 is made due to H.sub.1>H.sub.3, and a relation of L.sub.1<L.sub.2 is satisfied due to L.sub.1=0.5H.sub.1, L.sub.3=0.5H.sub.3 and L.sub.2=H.sub.1L.sub.3.

(30) Therefore, the moment L.sub.1f.sub.1 is generated on the axially outer surface 16b of the rack tooth 16, the moment L.sub.2f.sub.2 is generated on the axially inner surface 42a of the dummy tooth 42, and the moment L.sub.3f.sub.3 is generated on the axially outer surface 42b of the dummy tooth 42. Herein, since .sub.2, L.sub.2 (H.sub.3) and the like can be freely designed, the moments are designed so that a relation of L.sub.1f.sub.1=L.sub.2f.sub.2 is satisfied.

(31) According to the above configuration, the tooth 39 of the tooth-forming punch 32 configured to form the rack tooth 16 of each of both axial ends and positioned at the axially outer side of the corresponding rack tooth 16 is applied with the moment L.sub.1f.sub.1 from the rack tooth 16 of the axially inner side and the moment L.sub.2f.sub.2 from the dummy tooth 42 of the axially outer side. Therefore, the moments applied to the tooth 39 of the tooth-forming punch 32 are balanced (L.sub.1f.sub.1=L.sub.2f.sub.2).

(32) Also, the tooth 39 of the tooth-forming punch 32 positioned at each of both ends is applied with the moment L.sub.3f.sub.3 from the dummy tooth 42 of the axially inner side. The moment L.sub.3f.sub.3 is less than the moment L.sub.1f.sub.1, which is to be applied to the tooth 39 of the tooth-forming punch 32 when manufacturing the steering rack of the related art (refer to FIG. 20) (L.sub.3f.sub.3<L.sub.1f.sub.1 because L.sub.3<L.sub.1, f.sub.3<f.sub.1). Since the balance of the moments is improved in this way, it is possible to prevent the stress from being concentrated on the teeth 39, thereby implementing the long lifetime of the tooth-forming punch 32.

(33) Further, since the radially outer surface 42c and the pair of connection parts 42d of the dummy tooth 42 have the R shape, the stress concentration on the roots of the teeth 39 of the tooth-forming punch 32 in contact with the connection parts 42d is relieved, so that it is possible to implement the additional long lifetime of the tooth-forming punch 32.

(34) The reduction in the tensile stress, which is to be generated on the roots of the teeth 39 of the tooth-forming punch 32, by the dummy tooth 42 is also clear from evaluation results of an elastic-plastic analysis shown in FIGS. 2 and 3. FIG. 2 depicts the tooth-forming punch 32 (corresponding to the tooth-forming punch 32 of FIG. 18) where the dummy tooth processing concave and convex 41 is not provided like the related art, and FIG. 3 depicts the tooth-forming punch 32 of the first embodiment. In FIGS. 2 and 3, the dark part indicates that the tensile stress is high. It can be seen that the considerably high stress is generated at the roots of the teeth 39 of both axial ends of the tooth-forming punch 32 of the related art but the stress to be generated at the roots of the teeth 39 is remarkably reduced in the tooth-forming punch 32 of the first embodiment.

(35) Also, FIG. 4 depicts an SN diagram of the lifetime of the tooth-forming punch 32. Since the stress to be generated at the roots of the teeth 39 is remarkably reduced in the tooth-forming punch 32 of the first embodiment, as compared to the tooth-forming punch 32 of the related art, the number of cycles to fracture considerably increases and the long lifetime can be thus implemented.

Second Embodiment

(36) In the first embodiment, the dummy tooth 42 is formed one by one at the part that are the part of the rod part 15 in the axial direction and are adjacent to both axial sides of the plurality of rack teeth 16 (refer to FIG. 1). However, as shown in FIG. 5, a plurality of dummy teeth 42 may be formed at a part that is the part of the rod part 15 in the axial direction and is adjacent to each of both axial sides of the plurality of rack teeth 16.

(37) In FIG. 5, the n (n: natural number of 2 or greater) dummy teeth 42 are formed at one side in the axial direction. When inclined angles of the axially inner surfaces 42a of the plurality of dummy teeth 42 are denoted as .sub.2, .sub.4, . . . , .sub.2n2, .sub.2n in order from first to n.sup.th dummy teeth 42 close to the rack tooth 16, and inclined angles of the axially outer surfaces 42b of the plurality of dummy teeth 42 are denoted as .sub.3, .sub.5, . . . , .sub.2n1, .sub.2n+1 in order from the first to n.sup.th dummy teeth 42 close to the rack tooth 16, the inclined angles are set so that a relation of .sub.1<.sub.2=.sub.3<.sub.4=.sub.5< . . . <.sub.2n2=.sub.2n1<.sub.2n=.sub.2n+1 is satisfied. Also, when tooth heights of the plurality of dummy teeth 42 are denoted as H.sub.3, H.sub.5, . . . , H.sub.2n1, H.sub.2n+1 in order from the first to n.sup.th dummy teeth 42, the tooth heights are set so that a relation of H.sub.1>H.sub.3>H.sub.5> . . . >H.sub.2n1>H.sub.2n+1 is satisfied. Also, half values of the tooth heights H.sub.3, H.sub.5, . . . , H.sub.2n1, H.sub.2n+1 of the dummy teeth 42 are denoted as L.sub.3, L.sub.5, . . . , L.sub.2n1, L.sub.2n+1, and L.sub.2=H.sub.1L.sub.3, L.sub.4=H.sub.3L.sub.5, . . . , L.sub.2n=H.sub.2n1L.sub.2n+1 are denoted.

(38) In this case, a force f.sub.1 is generated on the axially outer surface 16b of the rack tooth 16 positioned at each of both axial ends toward an axially outer side at a position of L.sub.1 distant from a radially outer end portion of the corresponding rack tooth 16 toward a radially inner side. Also, a force f.sub.2 is generated on the axially inner surface 42a of the dummy tooth 42 toward an axially inner side at a position of L.sub.2 distant from the radially outer end portion of the rack tooth 16 positioned at each of both axial ends toward the radially inner side. Also, a force f.sub.3 is generated on the axially outer surface 42b of the first dummy tooth 42 toward the axially outer side at a position of L.sub.3 distant from the radially outer end portion of the first dummy tooth 42 toward the radially inner side. Also, a force f.sub.4 is generated on the axially inner surface 42a of the second dummy tooth 42 toward the axially inner side at a position of L.sub.4 distant from the radially outer end portion of the first dummy tooth 42 toward the radially inner side. Also, a force f.sub.5 is generated on the axially outer surface 42b of the second dummy tooth 42 toward the axially outer side at a position of L.sub.5 distant from the radially outer end portion of the second dummy tooth 42 toward the radially outer side. Also, a force f.sub.2n2 is generated on the axially inner surface 42a of the (n1).sup.th dummy tooth 42 toward the axially inner side at a position of L.sub.2n2 distant from the radially outer end portion of the (n2).sup.th dummy tooth 42 (not shown) toward the radially inner side. Also, a force f.sub.2n1 is generated on the axially outer surface 42b of the (n1).sup.th dummy tooth 42 toward the axially outer side at a position of L.sub.2n1 distant from the radially outer end portion of the (n1).sup.th dummy tooth 42 toward the radially inner side. Also, a force f.sub.2n is generated on the axially inner surface 42a of the n.sup.th dummy tooth 42 toward the axially inner side at a position of L.sub.2n distant from the radially outer end portion of the (n1).sup.th dummy tooth 42 toward the radially outer side. Also, a force f.sub.2n+1 is generated on the axially outer surface 42b of the n.sup.th dummy tooth 42 toward the axially outer side at a position of L.sub.2n+1 distant from the radially outer end portion of the n.sup.th dummy tooth 42 toward the radially inner side.

(39) Herein, since the inclined angles are set so that the relation of .sub.1<.sub.2=.sub.3<.sub.4=.sub.5< . . . <.sub.2n2=.sub.2n-1<.sub.2n=.sub.2n+1 is satisfied, as described above, a relation of f.sub.1>f.sub.2=f.sub.3>f.sub.4=f.sub.5> . . . >f.sub.2n2=f.sub.2n1>f.sub.2n=f.sub.2n+1 is satisfied by a wedge effect. Also, a relation of L.sub.1<L.sub.2, L.sub.3<L.sub.4, . . . , L.sub.2n1<L.sub.2n is satisfied due to H.sub.1>H.sub.3>H.sub.5> . . . >H.sub.2n1>H.sub.2n+1.

(40) Therefore, the moment L.sub.1f.sub.1 is generated on the axially outer surface 16b of the rack tooth 16, the moment L.sub.2f.sub.2 is generated on the axially inner surface 42a of the first dummy tooth 42, the moment L.sub.3f.sub.3 is generated on the axially outer surface 42b of the first dummy tooth 42, the moment L.sub.4f.sub.4 is generated on the axially inner surface 42a of the second dummy tooth 42, the moment L.sub.2n1f.sub.2n1 is generated on the axially outer surface 42b of the (n1).sup.th dummy tooth 42, and the moment L.sub.2nf.sub.2n is generated on the axially inner surface 42a of the n.sup.th dummy tooth 42. Herein, the moments are designed so that L.sub.1f.sub.1=L.sub.2f.sub.2, L.sub.3f.sub.3=L.sub.4f.sub.4 and L.sub.2n1f.sub.2n1=L.sub.2nf.sub.2n and the moments from the left and right are balanced.

(41) According to the above configuration, the tooth 39 of the tooth-forming punch 32 configured to form the rack tooth 16 at each of both ends in the axial direction and positioned at the axially outer side of the corresponding rack tooth 16 is applied with the moment L.sub.1f.sub.1 from the rack tooth 16 of the axially inner side and the moment L.sub.2f.sub.2 from the dummy tooth 42 of the axially outer side. Also, the tooth 39 (the second tooth 39) adjacent to the axially outer side of the tooth 39 is applied with the moment L.sub.3f.sub.3 from the first dummy tooth 42 of the axially inner side and the moment L.sub.4f.sub.4 from the second dummy tooth 42 of the axially outer side. Also, the n.sup.th tooth 39 is applied with the moment L.sub.2n1f.sub.2n1 from the (n1).sup.th dummy tooth 42 and the moment L.sub.2nf.sub.2n from the n.sup.th dummy tooth 42 of the axially outer side. Therefore, the moments that are to be applied to the plurality of teeth 39 of the tooth-forming punch 32 are balanced (L.sub.1f.sub.1=L.sub.2f.sub.2, L.sub.3f.sub.3=L.sub.4f.sub.4, L.sub.2n1f.sub.2n1=L.sub.2nf.sub.2n).

(42) Also, the tooth 39 of the tooth-forming punch 32 positioned at each of both ends is applied with the moment L.sub.2n+1f.sub.2n+1 from the dummy tooth 42 of the axially inner side. However, the moment L.sub.2n+1f.sub.2n+1 is less than the moment L.sub.3f.sub.3, which is to be applied to the tooth 39 of the tooth-forming punch 32 positioned at each of both ends in the first embodiment (L.sub.2n+1f.sub.2n+1<L.sub.3f.sub.3 because L.sub.2n+1<L.sub.3, f.sub.2n+1<f.sub.3). Since the balance of the moments is improved in this way, as compared to the first embodiment, it is possible to prevent the stress from being concentrated on the teeth 39, thereby implementing the long lifetime of the tooth-forming punch 32.

(43) In the meantime, the present invention is not limited to the respective embodiments and can be appropriately changed and modified.

(44) For example, in the first embodiment (refer to FIG. 1), the tooth 39 of the tooth-forming punch 32 positioned at each of both end parts is applied with the moment L.sub.3f.sub.3 from the dummy tooth 42 of the axially inner side and is not applied with the moment from the rod part 15 of the axially outer side. However, the present invention is not limited to the corresponding configuration. For example, as shown in FIG. 6, the moment may be applied from the rod part 15 of the axially outer side.

(45) In this case, the tooth 39 of the tooth-forming punch 32 positioned at each of both end parts and the rod pan 15 are axially contacted to each other. Also, a force f.sub.0 is generated on the axially inner surface 15a of the rod part 15 toward the axially inner side at a position of L.sub.0 distant from the radially outer end portion of the dummy tooth 42 positioned at each of both ends toward the radially inner side. Also, the moment L.sub.0f.sub.0 is generated on the axially inner surface 15a of the rod part 15. Meanwhile, in the shown example, L.sub.0 is an arbitrary value satisfying a relation of 0<L.sub.0<H.sub.3.

(46) Therefore, the tooth 39 of the tooth-forming punch 32 positioned at each of both end parts is applied with the moment L.sub.3f.sub.3 from the dummy tooth 42 of the axially inner side and the moment L.sub.0f.sub.0 from the rod part 15 of the axially outer side. Therefore, since the balance of the moments is improved, as compared to the moment L.sub.3f.sub.3 that is to be applied to the teeth 39 of both ends of the tooth-forming punch 32 in the first embodiment, it is possible to prevent the stress from being concentrated on the teeth 39, thereby implementing the long lifetime of the tooth-forming punch 32. This configuration is particularly efficient when it is difficult to increase the number of the dummy teeth 42.

(47) In the meantime, also in the second embodiment, the tooth 39 of the tooth-forming punch 32 positioned at each of both end parts may be applied with the moment L.sub.2n+1f.sub.2n+1 from the dummy tooth 42 of the axially inner side and the moment L.sub.0f.sub.0 from the rod part 15 of the axially outer side.

(48) Also, in the above embodiments, the inclined angles .sub.2, .sub.4, . . . , .sub.2n, of the axially inner surfaces 42a of the plurality of dummy teeth 42 and the inclined angles .sub.3, .sub.5, . . . , .sub.2n+1 of the axially outer surfaces 42b are the same (.sub.2=.sub.3<.sub.4=.sub.5< . . . <.sub.2n2=.sub.2n1<.sub.2n=.sub.2n+1). However, the inclined angles are not necessarily required to be the same. In this case, the inclined angles are set so that at least a relation of .sub.1<.sub.2.sub.3<.sub.4.sub.5< . . . <.sub.2n2.sub.2n1<.sub.2n<.sub.2n+1 is satisfied.

(49) Also, the radially outer surface 42c of the dummy tooth 42 is not necessarily required to have the curved surface of R-shaped cross section of which the axially intermediate part is convex. For example, as shown in FIG. 7, the radially outer surface 42c of the dummy tooth 42 may have a planar cross section. Also in this configuration, when the pair of connection parts 42d consisting of the axially inner surface 42a and axially outer surface 42b and the radially outer surface 42c is formed to have an R shape, it is possible to relieve the stress concentration on the roots of the teeth 39 of the tooth-forming punch 32 in contact with the connection parts 42d.

(50) Also, as shown in FIG. 8, the radially outer surface 42c of the at least one dummy tooth 42 is preferably formed with at least one groove portion 43. FIG. 8 depicts an example where a pair of groove portions 43 is formed adjacent to both ends in a width direction of the radially outer surface 42c of the dummy tooth 42, in the first embodiment where the dummy tooth 42 is formed one by one at the parts that are adjacent to both axial sides of the rack teeth 16. The pair of groove portions 43 has a substantially linear shape axially extending and enabling the axially inner surface 42a and the axially outer surface 42b to communicate with each other. Also, the groove portion 43 has a width size of about 0.5 to 1.0 mm and a depth size of about 0.5 to 1.0 mm. In the meantime, since the dummy tooth 42 is configured not to mesh with the pinion, as described above, there is no problem even when the groove portion 43 is provided on the radially outer surface 42c which is a tooth end.

(51) In this way, when the radially outer surface 42c of the at least one dummy tooth 42 is formed with the at least one groove portion 43, it is possible to use the groove portion 43 as a reference when positioning the steering rack 11a in a post process after the forging. For example, the positioning is performed by operating a probe having the same shape as the groove portion 43 toward the groove portion 43 and engaging the same with the groove portion 43. Particularly, in this example, each of the pair of dummy teeth 42 provided at the parts adjacent to both axial sides of the rack teeth 16 (in FIG. 8, only one axial dummy tooth 42 is shown) is provided with the pair of groove portions 43, so that a total of four groove portions 43 are provided. Therefore, the four probes are engaged with the four groove portions for positioning, so that the positioning can be made with higher precision.

(52) Also, in FIG. 8, the pair of dummy teeth 42 provided at the portions adjacent to both axial sides of the rack teeth 16 has the same tooth height. Therefore, it is possible to level the steering rack 11a by engaging the four probes with the four groove portions 43 to detect positions of the respective groove portions 43 and confirming that the horizontal positions of the at least three groove portions 43 are the same. Therefore, for the leveling, it is necessary to form a total of three or more groove portions 43 on the plurality of dummy teeth 42 having the same tooth height.

(53) In order to form the groove portion 43, the dummy tooth processing concave and convex 41 of the tooth-forming punch 32 (refer to FIG. 1 and the like) is formed with a protrusion (not shown) having a shape corresponding to the groove portion 43. According to this configuration, since the tooth-forming punch 32 has the rack tooth processing concave and convex 40, the dummy tooth processing concave and convex 41 and the protrusion, it is possible to form the rack teeth 16, the dummy tooth 42 and the groove portion 43 on the steering rack 11a at the same time. Therefore, since it is not necessary to process the groove portion 43 in a separate process, it is possible to prevent increases in the working hours and costs.

(54) Meanwhile, in FIG. 8, the groove portions 43 are provided on the dummy teeth 42 of the first embodiment. However, also in the second embodiment (refer to FIG. 5) where the plurality of dummy teeth 42 is provided at the parts adjacent to both axial sides of the rack teeth 16, the groove portions 43 may be provided on the dummy teeth 42. In this case, when the radially outer surface 42c of the at least one dummy tooth 42 is formed with the at least one groove portion 43, the positioning of the steering rack 11a can be performed. Also, when a total of three or more groove portions 43 are formed on the plurality of dummy teeth 42 having the same tooth height, the leveling is possible.

(55) The shape of the groove portion 43 is not particularly limited inasmuch as it can be used as a reference of the positioning or leveling of the steering rack 11a. For example, a substantially semispherical shape as shown in FIG. 9 is also possible. In this case, the groove portion 43 has a diameter size of about 0.5 to 1.0 mm and a depth size of about 0.5 to 1.0 mm.

(56) The subject application is based on a Japanese Patent Application No. 2014-009670 filed on Jan. 22, 2014, which is herein incorporated for reference.

DESCRIPTION OF REFERENCE NUMERALS

(57) 1: steering wheel, 2: steering shaft, 3: universal joint, 4: intermediate shaft, 5: steering gear, 6: input shaft, 7: tie-rod, 8: steering column, 9: gear housing, 10, 10a: electric motor, 11, 11a: steering rack, 12: second input shaft, 13: housing, 14: decelerator, 15: rod part, 15a: axially inner surface, 16: rack tooth, 16a: axially inner surface, 16b: axially outer surface, 16c: radially outer surface, 16d: connection part, 17: backside part, 18: cylindrical part, 19: material, 20: receiving die, 21: concave groove portion, 22: pressing punch, 23: intermediate material, 24: partially cylindrical surface part, 25: flat surface part, 26: curved surface part, 27: die, 28: holding hole, 29: bottom, 30: inner surface, 31: inclined guide surface portion, 32: tooth-forming punch, 33: base rack, 34: relief flat surface part, 35: sizing die, 36: sizing concave and convex surface part, 37: press die, 38: pressing concave groove, 39: tooth, 40: rack tooth processing concave and convex, 40L: tooth height, 41: dummy tooth processing concave and convex, 41L: tooth height, 42: dummy tooth, 42L: tooth height, 42a: axially inner surface, 42b: axially outer surface, 42c: radially outer surface, 42d: connection part, 43: groove portion, A, B: area, C: crack, R.sub.17, r.sub.18: radius of curvature