JP2006103644A - Hollow steering rack bar and manufacturing method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steering rack bar having tooth width larger than that of a current one in response to electric power steering required to improve rigidity as much as possible against a large load to be applied to a rack, in regard to a steering rack bar formed with the rack by performing plastic machining to a steel pipe as a raw material. <P>SOLUTION: Quantity of the steel pipe to be flattened to form a rack is increased so that a part of the periphery of a cross section at a right angle against the axial direction in a length part formed with the rack exists outside of a circle forming the periphery of the steel pipe as a raw material. With this structure, a flattened surface is widened, and tooth width becomes larger than that of a current one. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a steering rack shaft that is a part of a steering apparatus for an automobile and a method for manufacturing the same. In recent years, for the purpose of reducing the weight of automobiles, it has been attempted to make hollow shafts and the like that have been conventionally solid, but the present invention is directed to such hollow steering rack shafts.

  FIG. 10 is an external view showing a conventional hollow steering rack shaft 51, in which a rack 52 is formed in a part of the length of the shaft having a circular cross section, for example, about half. Further, in the steering rack shaft of FIG. 10, the ball groove screw groove 3 is provided in a part of the length other than the part where the rack 52 is formed. This thread groove is for electric power steering which has been widely adopted recently in order to simplify the mechanism of the steering device. The conventional general steering rack shaft is compatible with hydraulic power steering, and in this case, the portion other than the rack is formed in a simple cylinder (round bar if not hollow) in the total length. ing.

  Although a mechanism of the electric power steering is not shown, a nut is coupled to the screw groove 3 via a ball to form a ball screw, and a motor provided with a steering rack shaft passing through the outer periphery of the nut. It is combined with the inner circumference of the rotor. Since the motor itself is fixed at a fixed position, the rotational force of the motor is converted into the thrust force of the rack shaft by the ball screw, and the action of power steering occurs.

  As a manufacturing method of the hollow steering rack shaft as described above, a method of making a hole after forming a rack portion on a round bar by cutting has been performed. It is conceivable to use a hollow material, that is, a steel pipe, and crush the rack after flattening the portion where the rack is to be formed, but in reality the thickness of the portion where the rack is formed is reduced. It is difficult to ensure the required strength. For this reason, a method of forming a rack portion by plastic working using a steel pipe as a raw material to form a steering rack shaft having a hollow overall length was proposed in Japanese Patent Publication No. 3-5892, and a further improved method is also considered. As a result, production is expanding rapidly.

  The method disclosed in Japanese Patent Publication No. 3-5892 is a two-stage process in which a portion of a steel pipe rack to be formed is first crushed into a flat surface, and then a rack is formed in this portion. For this purpose, first, a split mold for primary forming that can be opened to the left and right is provided to accommodate the portion of the steel pipe to be processed, but this die penetrates the portion where the upper rack is to be formed in the combined state. A hole is provided. Then, a press die having a flat tip is inserted into the through hole from above, and the portion of the steel pipe is processed into a flat surface, which is used as a primary molded body.

  Next, the said primary molded object is accommodated in the split mold for secondary shaping | molding which inner surface shape corresponds with the external shape of the finished product of a rack bar, and can open to right and left. That is, the secondary molding split mold is configured such that when the left and right molds are combined, a female mold of rack teeth is formed at the top. Then, the mandrel is inserted into the steel pipe, and the previously flattened portion is ironed from the inside. By repeating this a plurality of times, the material sequentially rises, and the rack is formed in accordance with the shape of the secondary forming split mold.

On the other hand, the thread groove is provided only in the case of electric power steering. At present, there are few examples of the thread groove in the hollow rack bar in which the rack as described above is formed by plastic working. However, there are a method by cutting and a method by plastic working such as rolling in the thread groove. There are both methods for the process of forming the thread groove and the process of forming the rack, which is performed first.
Japanese Examined Patent Publication No. 3-5892

  By the way, in the method using the plastic processing as described above, that is, the method of ironing the inside of the tube with a mandrel and raising the tooth portion, the inclination from the tooth tip to the tooth root at both ends of the tooth width is loosened, and effective teeth The width becomes smaller and you win. In particular, in the case of electric power steering, the load on the rack is often larger than that of hydraulic power steering, and the tooth width may be required to be as large as possible from the viewpoint of rack strength. However, in the plastic working method, it is difficult to increase the tooth width by increasing the inclination from the tooth tip to the tooth base. SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to obtain a hollow steering rack shaft produced by plastic working having a tooth width larger than that of the conventional one.

  The present invention solves the above-mentioned problems, and in a steering rack shaft in which a rack is formed by plastic working using a steel pipe as a raw material, a part of the outer periphery of the longitudinal section of the length portion where the rack is formed is A hollow steering rack shaft characterized by being outside the circle that was the outer periphery of the steel pipe of the material. Here, the hollow steering rack shaft is also used for an electric power steering apparatus.

  The present invention also includes a steel pipe that has been processed into a flat shape by pressing a part of the length of the steel pipe to make a part of the outer circumference thereof flat and accommodated in a split mold having a rack forming die on the inner surface. A hollow for forming a rack according to the rack forming die by bringing the rack forming die into contact with the flattened portion and pressing the mandrel into the steel pipe to sequentially iron the inner surface of the flattened portion. In a manufacturing method for a steering rack shaft, a part of the outer periphery of the cross section perpendicular to the axial direction of the length portion of the steel pipe that has been processed to be planar is outside the circle that was the outer periphery of the steel pipe before being planarized. There is provided a method for manufacturing a hollow steering rack shaft. Here, in the process of making the flat shape, the steel pipe is accommodated in a split mold that accommodates at least a length portion of the steel pipe that is to be flat, and a punch having a flat tip is inserted into a hole in the split mold, It is also characterized by being pressed and deformed.

  Furthermore, in the present invention, a part of the length of the steel pipe is pressed by the split mold having the rack forming mold on the inner surface to make the portion in contact with the rack forming mold flat, and the steel pipe is held in a state pressed by the split mold. A method of forming a rack according to the rack forming mold by sequentially ironing the inner surface of the flattened portion by pushing a mandrel into the steel pipe while the rack is formed. A method for manufacturing a hollow steering rack shaft is characterized in that a part of the outer periphery of the axial section perpendicular to the axial direction is outside the circle that was the outer periphery of the steel pipe before forming the rack. In each of the hollow steering rack shaft manufacturing methods described above, the crushing stroke for flattening is 30 to 60% of the diameter of the steel pipe before flattening.

  When manufacturing a hollow steering rack shaft using a steel pipe as a raw material by a method of raising the rack teeth by inserting a mandrel, it has been difficult to secure a sufficient tooth width by the conventional method. A rack with a tooth width can be formed. Therefore, it is possible to provide a hollow steering rack shaft that is particularly suitable for electric power steering that is generally heavy on the rack.

  1A is a cross-sectional view of the steering rack shaft 1 of the present invention, and FIG. 1B is a cross-sectional view perpendicular to the axial direction of the rack portion in the conventional steering rack shaft 51 shown in FIG. In any case, it is assumed that a rack is formed by plastic working using a steel pipe as a raw material, but in the conventional one, the entire cross section is contained in a circle that is the outer periphery of the steel pipe of the raw material. That is, in FIG. 5B, the entire cross section is contained in a virtual circle 46 obtained by extending a missing circle 45 that constitutes the outer periphery of the rack 52 and the axially symmetric side. On the other hand, in the present invention, a part of the cross section extends to the outside of the circle which was the outer periphery of the steel pipe of the material, and in FIG. A part 4 of the cross section protrudes from the extended virtual circle 46.

  In the past, in the cross section perpendicular to the axial direction in the rack portion, the entire cross section was contained in a circle that was the outer periphery of the steel pipe of the material. Before the method of manufacturing a hollow steering rack shaft by plastic working was developed as described above, a round bar was cut to form a rack, but in this case the outer shape of the completed steering rack shaft was Inevitably enter the outline of the original round bar. Thereafter, in the manufacturing method by plastic working that has been put to practical use, the shape of the steering rack shaft itself has followed the conventional cutting processing.

  However, a rack formed by a method in which a tooth portion is raised by inserting a mandrel has a problem that it is difficult to make the tooth width full of the contour of a steel pipe that is a material. FIG. 11 is a cross-sectional view perpendicular to the steering rack shaft 51 for explaining the raised state of the rack teeth when the mandrel is repeatedly inserted into the flattened steel pipe (this cross-sectional view shows a conventional steering rack shaft). However, the phenomenon itself is the same in the present invention). 41, 42, and 43 are tooth formation states at each stage of ironing by a mandrel, 41 is an initial stage, 42 is an intermediate stage, and 43 is a final stage. As can be seen, the material bulge begins at the center of the rack tooth width, and after reaching a predetermined tooth tip height at the center, the tooth width gradually increases. Then, the tooth forming process is terminated when the tooth root portion slightly protrudes at both ends of the width of the rack forming mold. Therefore, the inclination 44 from the tooth tip to the tooth root at both ends of the tooth width is loose, and the tooth width at the tooth tip portion which is an effective tooth width is considerably smaller than the width of the rack forming mold.

  If the rack portion is provided on the round bar by cutting, the surfaces at both ends in the width direction of the rack teeth can be secured up to the round bar surface of the material. In order to secure sufficient tooth width in the same way as in the manufacturing method by plastic working, it should be filled with metal until it reaches both ends in the tooth width direction of the rack forming mold, but the current manufacturing method by plastic working It is difficult. This is because the rack forming mold used in the manufacturing method currently practiced industrially has an open shape without closing both ends in the tooth width direction.

  That is, in the above manufacturing method, the steel pipe that has been processed into a flat surface is accommodated in a split mold having a rack forming mold on the inner surface, the rack forming mold is brought into contact with the flattened portion, and the mandrel is pushed into the steel pipe in order. Although the ironing is performed, the rack forming mold is detachable from the split mold. This is because cold processing is applied to the rack-forming mold repeatedly, which causes cracks and shortens the service life, so that if it is integrated with the split mold, the cost increases. This detachable rack forming mold processes the tooth portion by electric discharge machining or cutting, but in any case, it is easier to process and lower cost if the width direction ends are open. . Therefore, when such a rack forming mold is mounted in the split mold, the metal that has reached the end in the tooth width direction due to the bulge must be received by the inner surface of the split mold itself, not the rack forming mold. This is difficult because the gap between the rack forming mold and the split mold is widened due to stress during processing.

  As described above, in the manufacturing method by plastic working, the shape of both ends of the width of the teeth of the rack is not defined by the inner surface of the rack forming mold, but is a surface freely formed by the metal bulge accompanying the insertion of the mandrel. Consists of. Therefore, in the present invention, as described above, in the cross-section perpendicular to the axial direction of the length portion where the rack of the steering rack shaft is formed, a part of the outer periphery extends to the outside of the circle which was the outer periphery of the steel pipe of the material. I decided to make it overhang. In this way, even when a processing method is applied in which both ends of the rack teeth have a surface formed freely by the metal bulge accompanying insertion of the mandrel, a sufficient tooth width can be obtained. It can be secured.

  Further, the steering rack shaft of the present invention may be provided with a thread groove 3 for electric power steering as shown in FIG. However, the feature of the present invention resides in the configuration of the rack portion as described above, and the configuration of the thread groove itself is not particularly specified. In the hydraulic steering rack shaft, since the perfect circle portion constitutes a part of the hydraulic cylinder, a packing such as an oil seal is attached. When installing this packing, it may be necessary to insert from the side of the rack. In this case, a part of the outer periphery of the rack section perpendicular to the axial direction extends to the outside of the circle that was the outer periphery of the steel pipe of the material. And insertion becomes difficult. In such a case, the use of the steering rack shaft of the present invention is inappropriate. However, in the case of an electric power steering, there is no such problem because no packing is used. On the other hand, in an electric power steering, the load applied to the teeth of the rack is often large, and there is a great demand for increasing the tooth width. Therefore, the steering rack shaft of the present invention is particularly suitable for electric power steering.

  The above is the form of the hollow steering rack shaft of the present invention, which is premised on being manufactured by plastic working using a steel pipe as a raw material. The steering rack shaft of the present invention can be manufactured by the method described below. 2 is a cross-sectional view parallel to the steering rack shaft showing the flattening process, FIG. 3 is a cross-sectional view at the AA position in FIG. 2, and FIG. 4 is a cross-sectional view at the BB position in FIG. The split molds 12 and 13 surrounding the entire circumference of the steel pipe 11 accommodate at least a length portion of the steel pipe to be planar, that is, a length portion to be planar and a portion adjacent to the length portion. The split mold upper mold 12 has a vertical hole 14 passing through a portion corresponding to a portion where a steel pipe rack is to be formed. In this state, a punch 15 is inserted into the vertical hole 14 to press the steel pipe 11 in this portion. To make it flat. In this way, a part of the outer circumference of the steel pipe is flattened for a part of the length of the steel pipe.

  FIG. 5 (a) is a cross-sectional view perpendicular to the axial direction of the steel pipe after the flattening process of the present invention, and FIG. 5 (b) is a cross-sectional view perpendicular to the axial direction of the steel pipe. The manufacturing method of the present invention is characterized by the above-described flattening process, and as shown in FIG. 5A, a part 17 of the cross section of the steel pipe 11 where flattening is performed is the outer periphery of the steel pipe before the flattening process. So that it is outside the virtual circle 46. That is, in the conventional flattening process, the shape of the outer periphery of the original steel pipe 11 is maintained as it is except for the portion crushed in the plane of FIG. There is no going outside the virtual circle 46. On the other hand, in the present invention, portions other than those processed into the plane of the steel pipe are also deformed and spread from the outer periphery of the steel pipe before being processed into a plane. As a result, the width W of the portion processed into a flat surface is wider than that in the conventional method, and the tooth width of the rack formed on the flat surface can be increased.

  The crushing stroke L in the flattening process in the present invention is suitably 30 to 60%, preferably 35 to 50%, of the diameter of the steel pipe before the flat surface is provided. In the conventional processing method in which a flat surface is provided and a cross section after processing is inside a circle that is the outer periphery of the steel pipe before processing, the crushing stroke L is usually less than 30%. On the other hand, in the present invention, even if the area of the flattened portion is increased by increasing the crushing stroke as described above, the thickness of this portion is secured to form rack teeth with sufficient strength. It is.

  Further, in the process of providing a flat surface on the steel pipe, it is also possible to change the raised state of the material during the ironing process by the mandrel by slightly raising the central portion in the width direction, for example, rather than a complete flat surface. Furthermore, the transition portion between the circumference and the plane portion is not shown as a clear ridge line between the side portions at both ends in the width direction of the plane portion, as shown in a cross-sectional view perpendicular to the steering rack shaft in FIG. 18 may be a gently curved surface. The term planar as used in the claims and part of this specification is particularly conscious of such cases.

  In the flattening process in the conventional method of manufacturing a steering rack shaft by plastic working, the upper and lower mold contact surfaces were at the position of the central axis of the steel pipe. However, in the present invention, as a result of the flattening process spreading the steel pipe from the outer periphery before processing, as shown in FIG. 3, the upper and lower mold contact surfaces 16 are above the central axis O of the steel pipe and processed. Sometimes it is necessary to set the cross section to the maximum width. This is because the steel pipe cannot be removed from the mold after processing unless this is done. However, this is the length position where the flattening process of the steel pipe shown in FIG. 3 is performed. In the portion where the flattening process shown in FIG. Without the surface 16, it cannot be completely held by the upper and lower molds.

  For this reason, as shown in a cross-sectional view parallel to the steering rack shaft in FIG. 2, the vertical position of the die contact surface 16 changes depending on the axial position. For this reason, in the transition part 20 of the flattened part and the perfect circle part which is not flattened, an inclination may be provided so that the height position of the die contact surface gradually changes. Although not shown, it is necessary to gradually shift the width dimension at the transition portion 20 between the flattened portion and the perfect circle portion with an inclination.

  As another method, the die contact surface is positioned at the position where the cross-section becomes the maximum width when processed in the same manner as in FIG. 3 over the entire length in the axial direction, and the shape of the split die is changed at the place where flattening is not performed. May be. That is, FIG. 7 is a cross-sectional view perpendicular to the axial direction of the perfect circle portion in the flattening process similar to FIG. 4, but the lower die 23 is like a U-shaped groove and the center axis O of the steel pipe 11 is the die contact surface 25. Even if it is lower, it can be put in and out of the lower mold. On the other hand, since the cross section of the upper die 22 is an arc having a length shorter than a semicircle, a gap 24 is formed on both sides when held by the upper die 22 and the lower die 23, but this is practically acceptable. In this case, the width dimension at the transition portion 20 (FIG. 2) between the flattened portion and the perfect circle portion also needs to be gradually shifted with an inclination.

  Further, as disclosed in Japanese Patent Publication No. 3-5892 described as the prior art, a split mold that can be opened on a vertical dividing surface at a position including the central axis of the steel pipe can be used. If such a split mold of the right and left split type is used, as a result of the steel pipe expanding from the outer periphery before processing, even if the position where the cross section becomes the maximum width moves above the central axis of the steel pipe, Extraction is possible. In addition, it is possible to use a split mold with three or more divisions instead of the split mold with two divisions as described above. Still further, without using a split mold, only the lower side of the steel pipe is supported by a groove type, and the upper side is open, but it can be flattened by pressing the punch, but it was flattened as explained so far. In order to prevent deformation of the steel pipe other than the place, it is preferable to accommodate the whole in a split mold.

  A rack forming step is then performed on the steel pipe that has been flattened as described above. FIG. 8 is a cross-sectional view parallel to the steering rack shaft showing the rack forming step, and FIG. 9 is a cross-sectional view perpendicular to the steering rack shaft at the rack. As shown in FIGS. 8 and 9, portions where the flat surface of the steel pipe is formed are accommodated in the split dies 26 and 27 different from the flattening step surrounding the entire circumference of the steel pipe 11. A rack forming die 28 is provided inside the split die so as to come into contact with the portion where the flat surface of the steel pipe is formed. In this state, the mandrel 29 is pushed into the steel pipe, and the inner surface of the flattened portion is sequentially ironed to raise the material from the inside, and a rack is formed according to the rack forming mold 28. 8 and 9 show a state where the rack formation is completed.

  The mandrel 29 is slightly different in size and is processed by a plurality of strokes, for example, a dozen strokes. However, if a mandrel having protrusions 30 that are ironing portions at a plurality of positions on the entire length is used. The number of strokes can be reduced. There are a method of inserting a mandrel from one side of a steel pipe and a method of improving efficiency by providing insertion devices on both sides and alternately inserting from both sides. These plastic workings are all performed cold. The portion of the rack forming die 28 may be integrated with the split die upper die 26 in principle, but as described above, since it receives a large force, its life is relatively short. As shown in FIG. 9, it is preferable that only the rack-forming part is independently attached to the split mold for use.

  In the rack forming step, the width of the rack forming die 28 is increased in accordance with the fact that the width of the plane generated by the flattening step is larger than that in the conventional method. In addition, the width of the action part of the mandrel is wider than the conventional one so that the material is raised widely. In the method of the present invention, the end point of the tooth formation process by mandrel insertion is where the tooth portion slightly protrudes at both ends of the width of the rack forming mold, and is the same as the conventional method. For this reason, the state of inclination from the tooth tip to the tooth root at both ends of the tooth width is the same as the conventional one, but the tooth width of the rack can be sufficiently increased corresponding to the increase in the width of the rack forming mold.

  In the split molds 26 and 27 in the rack forming process, as in the split molds 12 and 13 in the flattening process, the upper and lower mold contact surfaces 31 in the rack forming part are above the central axis O of the steel pipe. It is necessary to set the cross section at the maximum width when processed. On the other hand, except for the rack forming portion, it is necessary to set the die matching surface to the position of the central axis of the steel pipe as in FIG. 4, and it is necessary to change the height position of the die matching surface during this period (FIG. The description of the contact surface is omitted). As another method, the shape of the split mold other than the rack forming portion can be made the same as in FIG. 7, and the position of the mold contact surface 31 can be the same as the position in FIG. It is the same.

  Although the steering rack shaft can be manufactured by the above steps, it can also be manufactured by a method different from the above. That is, it is the same as the previous process in that a part of the steel pipe is flattened and the rack is raised on the generated plane, but a rack forming mold is provided without using a dedicated split mold and punch for flattening. Direct flattening is performed by the split mold. That is, a part of the length of the steel pipe is pressed by the split mold having the rack forming mold on the inner surface so that the part in contact with the rack forming mold is flat, and the mandrel is held in the state pressed by the split mold while holding the steel pipe. A rack is formed according to a rack forming mold by sequentially ironing the inner surface of the flattened portion by being pushed into the interior of the rack.

  In this method, even when pressed by a rack-forming mold having unevenness, the unevenness due to the rack-forming mold hardly occurs due to the rigidity of the material, and becomes almost flat. Therefore, the rack teeth are formed in the ironing process by inserting the mandrel in the same manner as the previous method. In this way, a portion of the outer periphery of the cross section perpendicular to the axial direction of the length portion in which the rack is formed is outside the circle that was the outer periphery of the steel pipe before forming the rack. The width can be made sufficiently large.

  In the case of a hollow steering rack shaft for use in electric power steering, a rack is formed on a part of the length as shown in FIG. 10, and a screw groove 3 for a ball screw is formed on a part of another length. It is necessary to provide it. Although there are cutting, rolling, and the like as a method for processing the thread groove, the method for forming the thread groove is not limited in the present invention. Further, the thread groove may be formed after the rack is formed, or conversely, the process of forming the rack may be performed after the thread groove is formed first.

(A) is the steering rack shaft of the present invention, (b) is a cross-sectional view perpendicular to the axial direction of the rack portion in the conventional steering rack shaft. Sectional view parallel to the steering rack shaft showing the flattening process Sectional drawing in the AA position of FIG. Sectional drawing in the BB position of FIG. (A) is the present invention, (b) is a cross-sectional view perpendicular to the axial direction of the steel pipe after the flattening process in the prior art. Cross section perpendicular to the steering rack shaft after flattening Sectional view perpendicular to the axial direction of the perfect circle in the flattening process Sectional view parallel to the steering rack axis showing the rack forming process Sectional view perpendicular to the steering rack axis at the rack location showing the rack forming process External view showing a conventional hollow steering rack shaft Cross-sectional view perpendicular to the steering rack shaft explaining the rack teeth

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Steering rack axis | shaft 2 Rack 3 Thread groove 4 Part of cross section 11 Steel pipe 12, 13 Split type 14 Vertical hole 15 Punch 16 Mold contact surface 17 Part of cross section 18 Transition part of circumference and plane part 20 Flattened part And upper part 23 Lower part 24 Gap 25 Mold contact surface 26, 27 Split mold 28 Rack forming mold 29 Mandrel 30 Projection 31 Mold contact surface 41, 42, 43 Teeth at each stage of ironing 44 Inclination from tooth tip to tooth root 45 Missing circle 46 Virtual circle 51 Steering rack shaft 52 Rack

Claims (6)

In a steering rack shaft in which a rack is formed by plastic processing using a steel pipe as a raw material, a part of the outer periphery of the longitudinal section of the length portion where the rack is formed is outside the circle that was the outer periphery of the steel pipe of the material. A hollow steering rack shaft characterized by being. 2. The hollow steering rack shaft according to claim 1, wherein the hollow steering rack shaft is used for an electric power steering apparatus. A part of the length of the steel pipe is pressed to make a part of the outer circumference flat, and the processed steel pipe is accommodated in a split mold having a rack forming mold on the inner surface to make the rack forming mold flat. A method for manufacturing a hollow steering rack shaft in which a rack is formed in accordance with the rack forming die by sequentially ironing the inner surface of the flattened portion by bringing the mandrel into contact with the formed portion and pushing the mandrel into the steel pipe The part of the outer periphery of the cross section perpendicular to the axial direction of the length portion of the steel pipe that has been processed into a flat shape is outside the circle that was the outer periphery of the steel pipe before the flat processing. A method for manufacturing a hollow steering rack shaft. In the flattening process, the steel pipe is accommodated in a split mold that accommodates at least a length portion of the steel pipe to be planar, and a punch having a flat tip is inserted into a hole of the split mold, and the steel pipe is pressed. 4. The method for manufacturing a hollow steering rack shaft according to claim 3, wherein the hollow steering rack shaft is deformed. A part of the length of the steel pipe is pressed by a split mold having an inner surface with a rack forming mold so that a portion in contact with the rack forming mold is planar, and the mandrel is held by the steel pipe while being held pressed by the split mold. A method of forming a rack in accordance with the rack forming mold by sequentially ironing the inner surface of the flattened portion by pushing into the inside of the rack, wherein the rack is formed at a right angle in the axial direction. A method for manufacturing a hollow steering rack shaft, wherein a part of the outer periphery of the cross section is outside the circle that was the outer periphery of the steel pipe before forming the rack. The hollow steering wheel according to any one of claims 3 to 5, wherein a crushing stroke of the flattening process is 30 to 60% of a diameter of the steel pipe before the flat portion is provided. A manufacturing method of a rack shaft.

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