Abstract:
A method of producing a rack is proposed, which comprises aligning a first bar piece, an intermediate piece and a second bar piece in order along a common axis; carrying out a first process in which the first and second bar pieces are stationarily held while showing a predetermined intersection angle between the first rack teeth on the first bar piece and the second rack teeth on the second bar piece; and carrying out a second process in which each of the first and second bar pieces and the intermediate piece are brought into contact with each other at mutually facing ends and subjected to a relative rotation therebetween for a friction welding thereby to join the first bar piece, the intermediate piece and the second bar piece together by the friction welding.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates in general to a method of producing racks used in electric power steering devices, and more particularly to the method of producing the racks of a type that is meshed with both a first pinion driven by a steering wheel system and a second pinion driven by a power assisting system thereby to take part in transmitting both a steering force from a driver and an assisting force from the assisting system. 
         [0003]    2. Description of the Related Art 
         [0004]    One of the methods of producing racks of the electric power steering device is disclosed in Japanese Laid-open Patent Application (tokkai) 2002-154442. 
         [0005]    The disclosed method is as follows. First, an elongate bar is prepared and the elongate bar is subjected to a hot working to form, on axially opposed portions of the bar, first rack teeth for the first pinion and second rack teeth for the second pinion respectively. Then, the first rack teeth are subjected to a cutting work. With this cutting work, undesired axial misregistration of the first rack teeth relative to the second rack teeth, which would be caused by a thermal strain produced in the first rack teeth at the time of formation of the first and second rack teeth by the hot working, is eliminated and corrected. 
       SUMMARY OF THE INVENTION 
       [0006]    However, in the above-mentioned known method, the cutting work is used as an additional work, which causes a complicated and time-consuming production process for the rack. 
         [0007]    The present invention is provided by taking the above-mentioned drawback of the conventional rack production method into consideration and aims to provide a method of producing a rack, which is free of the above-mentioned drawback. 
         [0008]    That is, in accordance with the present invention, there is provided a method of producing a rack, which can easily produce a precision rack without usage of a complicated and time-consuming production process. 
         [0009]    In accordance with the present invention, there are further provided a rack that is produced through the method of the invention and an electric power steering device that employs therein the rack produced through the method of the invention. 
         [0010]    In accordance with a first aspect of the present invention, there is provided a method of producing a rack for use in an electric power steering device of a vehicle, which comprises preparing a first bar piece ( 31 ) that has first rack teeth (R 1 ) adapted to be meshed with a first pinion (P 1 ) directly driven by a steering wheel, a second bar piece ( 32 ) that has second rack teeth (R 2 ) adapted to be meshed with a second pinion (P 2 ) driven by a steering assist electric motor (M) and an intermediate piece ( 33 ,  33 ′) that has one end to be welded to the first bar piece ( 31 ) and the other end to be welded to the second bar piece ( 32 ); aligning the first bar piece ( 31 ), the intermediate piece ( 33 ) and the second bar piece ( 32 ) in order along a common axis (Z); carrying out a first process in which the first and second bar pieces ( 31 ,  32 ) are stationarily held while showing a predetermined intersection angle (θ) between the first rack teeth (R 1 ) on the first bar piece ( 31 ) and the second rack teeth (R 2 ) on the second bar piece ( 32 ); and carrying out a second process in which each of the first and second bar pieces ( 31 ,  32 ) and the intermediate piece ( 33 ,  33 ′) are brought into contact with each other at mutually facing ends and subjected to a relative rotation therebetween for a friction welding thereby to join the first bar piece ( 31 ), the intermediate piece ( 33 ,  33 ′) and the second bar piece ( 32 ) together by the friction welding. 
         [0011]    In accordance with a second aspect of the present invention, there is provided a rack for transmitting a steering operation of a steering wheel to steered road wheels of a vehicle, which comprises a first bar piece ( 31 ) that has first rack teeth (R 1 ) adapted to be meshed with a first pinion (P 1 ) directly driven by the steering wheel; a second bar piece ( 32 ) that has second rack teeth (R 2 ) adapted to be meshed with a second pinion (P 2 ) driven by a steering assist electric motor (M) and; an intermediate piece ( 33 ,  33 ′) that has one end to be secured to the first bar piece ( 31 ) and the other end to be secured to the second bar piece ( 32 ), in which the rack is produced through the following steps: aligning the first bar piece ( 31 ), the intermediate piece ( 33 ) and the second bar piece ( 32 ) in order along a common axis (Z); carrying out a first process in which is the first and second bar pieces ( 31 ,  32 ) are stationarily held while showing a predetermined intersection angle “θ” between the first rack teeth (R 1 ) on the first bar piece ( 31 ) and the second rack teeth (R 2 ) on the second bar piece ( 32 ); and carrying out a second process in which each of the first and second bar pieces ( 31 ,  32 ) and the intermediate piece ( 33 ,  33 ′) are brought into contact with each other at mutually facing ends and subjected to a relative rotation for a friction welding thereby to join the first bar piece ( 31 ), the intermediate piece ( 33 ,  33 ′) and the second bar piece ( 32 ) together by the friction welding. 
         [0012]    In accordance with a third aspect of the present invention, there is provided a rack for transmitting a steering operation to steered road wheels of a vehicle, which comprises a first bar piece ( 31 ) that has first rack teeth (R 1 ) adapted to be meshed with a first pinion (P 1 ) directly driven by a steering wheel; a second bar piece ( 32 ) that has second rack teeth (R 2 ) adapted to be meshed with a second pinion (P 2 ) driven by a steering assist electric motor (M); and an intermediate piece ( 33 ,  33 ′) that has one end welded to the first bar piece ( 31 ) and the other end welded to the second bar piece ( 32 ). 
         [0013]    In accordance with a fourth aspect of the present invention, there is provided an electric power steering device which comprises a first pinion (P 1 ) directly driven by a steering wheel; an electric motor (M) for producing a steering assist power; a second pinion (P 2 ) driven by the steering assist power; a rack ( 1 ,  1 ′) including a first bar piece ( 31 ) having first rack teeth (R 1 ) meshed with the first pinion (P 1 ), a second bar piece ( 32 ) having second rack teeth (R 2 ) meshed with the second pinion (P 2 ) and an intermediate piece ( 33 ) having one end secured to the first bar piece ( 31 ) and the other end secured to the second bar piece ( 32 ), so that the rack is axially moved to transmit the power from the steering wheel and the electric motor (M) to steered road wheels; in which the rack is produced through the following steps: aligning the first bar piece ( 31 ), the intermediate piece ( 33 ) and the second bar piece ( 32 ) in order along a common axis (Z); carrying out a first process in which the first and second bar pieces ( 31 ,  32 ) are stationarily held while showing a predetermined intersection angle “θ” between the first rack teeth (R 1 ) on the first bar piece ( 31 ) and the second rack teeth (R 2 ) on the second bar piece ( 32 ); and carrying out a second process in which each of the first and second bar pieces ( 31 ,  32 ) and the intermediate piece ( 33 ,  33 ′) are brought into contact with each other at mutually facing ends and subjected to a relative rotation for a friction welding thereby to join the first bar piece ( 31 ), the intermediate piece ( 33 ,  33 ′) and the second bar piece ( 32 ) together by the friction welding. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    Other objects and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings, in which: 
           [0015]      FIG. 1  is a front view of an electric power steering device that practically employs therein a rack produced through the method of the present invention; 
           [0016]      FIG. 2  is a vertically sectional view of one part of the electric power steering device of  FIG. 1 , where a steering system constituting section is provided; 
           [0017]      FIG. 3  is a vertically sectional view of the other part of the electric power steering device of  FIG. 1 , where an assist system constituting section is provided; 
           [0018]      FIG. 4  is a perspective view of the rack produced through the method of the present invention; 
           [0019]      FIG. 5  is a plan view of the rack, which is taken from a direction of the arrow “A” of  FIG. 4 ; 
           [0020]      FIG. 6  is an enlarged sectional view taken along the line “a-a” of  FIG. 5 ; 
           [0021]      FIG. 7  is an enlarged sectional view taken along the line “b-b” of  FIG. 5 ; 
           [0022]      FIG. 8  is a rough sketch showing a relative angular displacement (or phase difference) between a first bar piece and a second bar piece; 
           [0023]      FIG. 9  is a schematic view of a production equipment by which the rack production method of the present invention is practically carried out; 
           [0024]      FIG. 10  is a rough sketch showing a condition in which the first and second bar pieces are held by first and second chucks respectively at both sides of an intermediate bar piece; 
           [0025]      FIG. 11  is an enlarged sectional view taken along the line “c-c” of  FIG. 10 ; 
           [0026]      FIG. 12  is an enlarged sectional view taken along the line “d-d” of  FIG. 10 ; 
           [0027]      FIG. 13  is a rough sketch showing a condition in which the second bar piece is held and positioned by the second chuck with the aid of positioning pins; 
           [0028]      FIG. 14  is a partially sectional plan view of the rack showing a condition in which the first and second bar pieces and the intermediate bar piece are just joined by a friction welding; 
           [0029]      FIG. 15  is a view similar to  FIG. 14 , but showing a condition in which ugly burrs have been removed from the rack of  FIG. 14 ; 
           [0030]      FIG. 16  is a view corresponding to  FIG. 14 , but showing a rack produced through a rack production method of a second embodiment of the present invention; and 
           [0031]      FIG. 17  is a view similar to  FIG. 16 , but showing a condition in which ugly burrs have been removed from the rack of  FIG. 16 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0032]    In the following, a method of producing a rack according to the present invention will be described in detail with reference to the accompanying drawings. 
         [0033]    Referring to  FIGS. 1 to 15 , there is shown a method of producing a rack, which is a first embodiment of the present invention. 
         [0034]    For ease of description, before explaining the method of producing the rack, the description will be commenced with respect to an electric power steering device of dual pinion type that practically uses therein the rack. 
         [0035]    Furthermore, for ease of description, various directional terms, such as, right, left, upper, lower, upwardly and the like are used in the following description. However, such terms are to be understood with respect to only drawing or drawings on which a corresponding element or portion is shown. 
         [0036]    In  FIG. 1 , there is shown the electric power steering device that employs therein the rack produced through the method of the present invention. 
         [0037]    The electric power steering device shown generally comprises a steering system constituting section  10  that is connected to a steering wheel (not shown) to transmit a steering force of a driver to steered road wheels (not shown) of an associated motor vehicle and an assist system constituting section  20  that is connected to an electric motor M to transmit a steering assist force produced by electric motor M to the steered road wheels. 
         [0038]    As will be understood from  FIG. 1 , the electric power steering device is tightly mounted to a vehicle body (not shown) through first and second brackets BR 1  and BR 2  mounted on the device. 
         [0039]    As is seen from  FIGS. 1 to 4 , the electric power steering device further comprises a first rack-and-pinion mechanism  11  that is connected to the steering wheel and a second rack-and-pinion mechanism  21  that is connected to the electric motor M. 
         [0040]    As is seen from  FIGS. 2 and 4 , the first rack-and-pinion mechanism  11  comprises a first pinion P 1  driven by the steering wheel (not shown) and first rack teeth R 1  formed on the left half of the rack  1  (see  FIG. 4 ) and meshed with first pinion P 1 , and the second rack-and-pinion mechanism  12  comprises a second pinion P 2  driven by the electric motor M and second rack teeth R 2  formed on the right half of the rack  1  (see  FIG. 4 ) and meshed with second pinion P 2 . That is, the steering force produced by the driver and the steering assist force produced by electric motor M are both transmitted to the steered road wheels through the rack  1 . 
         [0041]    More specifically, as is seen from  FIG. 2 , steering system constituting section  10  comprises mainly an input shaft  12  that has one end connected to the steering wheel to effect an integral rotation together with the steering wheel, a first output shaft  14  (or first pinion P 1 ) that has one end pivotally connected through a torsion bar  13  to the other end of input shaft  12  and the other end connected through the first rack-and-pinion mechanism  11  to the steered road wheels, and a torque sensor  15  that is arranged around input shaft  12  to detect a steering input torque. Thus, in response to rotation of first output shaft  14 , rack  1  is urged to move axially in one or the other direction. Both ends of rack are linked to the steered road wheels, so that the axial movement of rack  1  induces a steering movement of the steered road wheels of the vehicle. 
         [0042]    As is seen from  FIG. 3 , assist system constituting section  20  comprises mainly the electric motor M that generates a steering assist force in accordance with the steering force of the driver and a second output shaft  22  (or second pinion P 2 ) that has one end connected through a speed reduction device to an output shaft of electric motor M and the other end connected through the second rack-and-pinion mechanism  21  to the steered road wheels. The speed reduction device comprises a warm shaft  23  connected to the output shaft of electric motor M and a warm wheel  24  that is tightly mounted on second output shaft  22  and operatively engaged with warm shaft  23 . 
         [0043]    As is seen from  FIGS. 3 and 4 , the second rack-and-pinion mechanism  21  comprises second pinion P 2  that is formed on a leading portion of second output shaft  22  and second rack teeth R 2  (see  FIG. 7 ) that are formed on the right half of rack  1 . Thus, like in the above-mentioned first rack-and-pinion mechanism  11 , in response to rotation of second output shaft  22 , rack  1  is urged to move axially in one or other direction. Under this movement of rack  1 , the assist force produced by electric motor M is controlled by various control factors which are, for example, the steering input torque detected by torque sensor  15 , a vehicle speed detected by a vehicle speed sensor mounted to the road wheel, etc.,. With this control, a suitable assist force is produced by electric motor M and transmitted to the steered road wheels. 
         [0044]    As is seen from  FIGS. 4 and 5 , the rack  1  consists of three bar pieces  31 ,  33  and  32  that are aligned and joined together. More specifically, rack  1  consists of a first bar piece  31  that is formed with first rack teeth R 1 , a second bar piece  32  that is formed with second rack teeth R 2  and an intermediate bar piece  33  that is coaxially disposed between first and second bar pieces  31  and  32 . These three bar pieces  31 ,  32  and  33  are jointed together through a friction welding as will be described in detail hereinafter. 
         [0045]    As is seen from  FIGS. 4 ,  5  and  8 , first and second bar pieces  31  and  32  are angled by a given angle “θ”. In the illustrated embodiment, the given angle “θ” is 90 degrees, as is seen from  FIG. 8 . 
         [0046]    As is seen from  FIGS. 4 and 6 , first bar piece  31  is a m cylindrical rod of untempered steel, such as carbon steel or the like and has within a given axial range thereof first rack teeth R 1  that are forged. That is, first bar piece  31  has a circular cross section at axially opposed end portions where first rack teeth R 1  are not formed. 
         [0047]    More specifically, as is seen from  FIGS. 4 ,  5  and  6 , a first rack teeth forming portion T 1  is constructed by first rack teeth R 1  and a semicylindrical portion B 1  (see  FIG. 6 ) provided at the back of first rack teeth R 1 . 
         [0048]    As will be described in detail hereinafter, first rack teeth forming portion T 1  receives a steering force (or torque) from a driver. The steering force from the driver is small as compared with a steering assist force produced by electric motor M. Thus, as is seen from  FIG. 4 , first rack teeth R 1  are small in tooth size as compared with second rack teeth R 2  of an after-mentioned second rack teeth forming portion T 2  that receives the steering assist force of electric motor M. Accordingly, first rack teeth forming portion T 1  can be made simpler than the after-mentioned second rack teeth forming portion T 2 . 
         [0049]    As is seen from  FIGS. 4 and 5 , a cylindrical right end of first bar piece  31  is welded to a cylindrical left end of intermediate bar piece  33 . The welded portion is designated by reference  1   a . As shown, cylindrical right end of first bar piece  31  and cylindrical left end of intermediate bar piece  33  are the same in diameter and thus, the two bar pieces  31  and  33  are suppressed from having an ugly gap therebetween at the welded portion  1   a.    
         [0050]    As is shown in  FIG. 10 , a circular right end face of first bar piece  31  is denoted by reference  31   a , and as is shown in  FIG. 14 , a right end portion of first bar piece  31  has a blind bore  31   b.    
         [0051]    As is seen from  FIGS. 4 and 7 , the second bar piece  32  is a rod of untempered steel, such as carbon steel or the like and has within a given axial range thereof second rack teeth R 2  that are forged. As shown, second rack teeth R 2  are larger in size than to the above-mentioned first rack teeth R 1 . Each tooth of second rack teeth R 2  is also larger than that of first rack teeth R 1 . 
         [0052]    More specifically, as is seen from  FIG. 8 , a lateral size of the given axial range of second bar piece  32  is larger than the diameter of first and intermediate bar pieces  31  and  33 . 
         [0053]    That is, as is seen form  FIGS. 4 ,  5  and  7 , a second rack teeth forming portion T 2  is constructed by second rack teeth R 2  and an elongate roof portion B 2  (see  FIG. 7 ) provided at the back of second rack teeth R 2 . Elongate roof portion B 2  is formed with an axially extending ridge B 2 - 1 , and thus the given axial range of second bar piece  32  has a generally Y-shaped cross section as is seen from  FIG. 7 . As will be understood from  FIG. 4 , both ends of second bar piece  32  are cylindrical in shape. 
         [0054]    As has been mentioned hereinabove, second rack teeth forming portion T 2  receives the steering assist force produced by electric motor M. Thus, second rack teeth R 2  are constructed to have a size larger than that of the above-mentioned first rack teeth R 1 . More specifically, second rack teeth R 2  are so sized as to sufficiently withstand the larger steering assist force applied thereto from electric motor M. Accordingly, second rack teeth forming portion T 2  is suppressed from having damages caused by strength poverty. 
         [0055]    As is seen from  FIGS. 4 and 5 , a cylindrical left end of second bar piece  32  is welded to a cylindrical right end of intermediate bar piece  33 . The welded portion is designated by reference  1   b . As shown, cylindrical left end of second bar piece  32  and cylindrical right end of intermediate bar piece  33  are the same in diameter and thus, the two bar pieces  32  and  33  are suppressed from having an ugly gap therebetween at the welded portion  1   b.    
         [0056]    As is shown in  FIG. 10 , a circular left end face of second bar piece  32  is denoted by reference  32   a , and as is shown in  FIG. 14 , a left end portion of second bar piece  32  has a blind bore  32   b.    
         [0057]    As will become apparent as the description proceeds, blind bore  31   b  of first bar piece  31  and the other blind bore  32   b  of second bar piece  32  are coaxially connected to a through bore  33   c  provided by intermediate bar piece  33  when these three bar pieces  31 ,  33  and  32  are properly joined via friction welding. 
         [0058]    As is shown in  FIG. 14 , blind bores  31   b  and  32   b  and through bore  33   c  are the same in diameter. 
         [0059]    As is described hereinabove, first and second bar pieces  31  and  32  are preferably made of untempered steel, such as carbon steel or the like. Due to usage of such untempered steel, the bar pieces  31  and  32  can stand a high heat produced under the friction welding, and thus, reduction in hardness of the pieces  31  and  32  is suppressed. 
         [0060]    As is seen from  FIGS. 4 and 15 , the intermediate bar piece  33  is a cylindrical rod of untempered steel, such as carbon steel or the like, like in the above-mentioned first and second bar pieces  31  and  32 . If desired, intermediate bar piece  33  may be made of an aluminum alloy. 
         [0061]    As is seen from  FIG. 10 , intermediate bar piece  33  has a circular left end face  33   a  that is to be welded to circular right end face  31   a  of first bar piece  31  and a circular right end face  33   b  that is to be welded to circular left end face  32   a  of second bar piece  32 . 
         [0062]    As is seen from  FIG. 14 , intermediate bar piece  33  is formed with the through bore  33   c  that extends axially. The through bore  33   c  and the above-mentioned blind bores  31   b  and  32   b  of first and second bar pieces  31  and  32  are same in diameter. Thus, as will be understood from  FIG. 10 , the four circular end faces  31   a ,  33   a ,  33   b  and  32   a  of the three bar pieces  31 ,  33  and  32  have the same contact area. With this, the high heat produced under the friction welding is equally and uniformly applied to the welded part  1   a  where the two end faces  31   a  and  33   a  contact and the other welded part  1   b  where the two end faces  33   b  and  32   a  contact. That is, poor welding between first and intermediate bar pieces  31  and  33  and between intermediate and second bar pieces  33  and  32  is suppressed. 
         [0063]    As will be understood from  FIG. 4 , an imaginary elongate plane of first bar piece  31  on which first rack teeth R 1  are formed is denoted by GF 1  and an imaginary elongate plane of second bar piece  32  on which second rack teeth R 2  are formed is denoted by GF 2 . These two imaginary elongate planes GF 1  and GF 2  extend along an axis “Z” of rack  1 . Thus, in the illustrated embodiment, the two imaginary elongate planes GF 1  and GF 2  intersect each other at the angle “θ” (or 90 degrees). 
         [0064]    As will be described in detail herein next, for producing the rack  1 , first, intermediate and second bar pieces  31 ,  33  and  32  are aligned in order and first and second chucks C 1  and C 2  are used to hold first and second bar pieces  31  and  32  in a manner to contact these first and second bar pieces  31  and  32  to opposed ends  33   a  and  33   b  of intermediate bar piece  33  while causing these two bar pieces  31  and  32  to show an intersection angle “θ” (viz., 90 degrees) therebetween. Then, only intermediate bar piece  33  is rotated at a given high speed against the first and second bar pieces  31  and  32  to effect the friction welding of these three bar pieces  31 ,  33  and  32  at the contact portions. Each chuck C 1  or C 2  is of a split type. That is, first chuck C 1  includes a pair jaw rods OD 1  and UD 1  and second chuck C 2  includes a pair jaw rods OD 2  and UD 2 . 
         [0065]    In the following, the method of producing the rack  1  will be described in detail with reference to the drawings, particularly  FIGS. 9 to 15 . 
         [0066]    In  FIG. 9 , there is schematically shown a production equipment that is used for carrying out the method of the present invention. 
         [0067]    The production equipment shown comprises a rotating device  40  that rotates intermediate bar piece  33  at a given high speed, a first pressing device  41  that is located at a left side of rotating device  40  to press first bar piece  31  against a left end of intermediate bar piece  33 , and a second pressing device  42  that is located at a right side of rotating device  40  to press second bar piece  32  against a right end of intermediate bar piece  33 . These three devices  40 ,  41  and  42  are thus aligned with respect to the axis “Z” of rack  1  properly set in the production equipment. 
         [0068]    Rotating device  40  comprises an electric motor  40   a  that produces a power for rotating intermediate bar piece  33 , a rotor unit  40   b  that actually rotates intermediate bar piece  33  about its axis by using the power from electric motor  40   a  and a speed reducer  40   c  that is arranged between electric motor  40   a  and rotor unit  40   b  to transmit the rotation from electric motor  40   a  to rotor unit  40   b  while reducing the speed of the rotation. Rotor unit  40   b  may be, for example, a device that rotates intermediate bar piece  33  with the aid of rollers or shoes that hold the outer surface of intermediate bar piece  33 . 
         [0069]    As is seen from  FIG. 9 , first pressing device  41  comprises a first holding unit  41   a  that is equipped with the above-mentioned chuck C 1  (or first chuck) for holding first bar piece  31 , a first hydraulic power unit  41   b  that moves first chuck C 1  rightward in the drawing toward rotating device  40  when operated, and a first position sensor  41   c  that is connected to first holding unit  41   a  to detect an axial position of first bar piece  31  held by first chuck C 1 . 
         [0070]    Like first pressing device  41 , second pressing device  42  comprises a second holding unit  42   a  that is equipped with the above-mentioned chuck C 2  (or second chuck) for holding second bar piece  32 , a second hydraulic power unit  42   b  that moves second chuck C 2  leftward in the drawing toward rotating device  40  when operated, and a second position sensor  42   c  that is connected to second holding unit  42   a  to detect an axial position to of second bar piece  32  held by second chuck C 2 . 
         [0071]    As has been mentioned hereinabove, each chuck C 1  or C 2  is of a split type including a pair of jaw rods OD 1  and UD 1  (or OD 2  and UD 2 ). 
         [0072]    Rotating device  40  and first and second pressing devices  41  and  42  are controlled by a known controller (or servomechanism) for carrying out the friction welding of the three bar pieces  31 ,  33  and  32 . More specifically, with the work of the controller, the rotation speed and braking level of rotating device  40  are controlled, and with the work of the controller and first and second position sensors  41   c  and  42   c , the axial movement of first and second chucks C 1  and C 2  and the pressing force of first and second bar pieces  31  and  32  against intermediate bar piece  33  are also controlled. 
         [0073]    In the following, steps for carrying out the friction welding will be described in detail with the aid of the drawings, particularly  FIG. 9 . 
         [0074]    First, first bar piece  31  is held by first chuck C 1  of first holding unit  41   a  that has been set at a predetermined position. That is, as is seen from  FIG. 11 , first bar piece  31  is tightly put between the paired jaw rods OD 1  and UD 1  in such a manner that first rack teeth R 1  of the bar piece  31  are meshed with positioning pins  50  (not shown in  FIG. 11 ) possessed by jaw rod OD 1  and semicylindrical portion B 1  of the bar piece  31  is put in a V-shaped groove  51  formed in the other jaw rod UD 1 . With this, an angular position of first bar piece  31  relative to first chuck C 1  is tightly set. More specifically, an angular position of first rack teeth forming portion T 1  of first bar piece  31  is tightly set. Arrangement of positioning pins  50  of jaw rod OD 1  relative to first rack teeth R 1  of the bar piece  31  will be imaged from  FIG. 13  that shows the angular positioning of second bar piece  32  relative to chuck C 2 . 
         [0075]    Then, second bar piece  32  is held by second chuck C 2  of second holding unit  42   a  that has been set at a predetermined position. That is, as is seen from  FIG. 12 , second bar piece  32  is tightly put between the paired jaw rods OD 2  and UD 2  in such a manner that second rack teeth R 2  of the bar piece  32  are meshed with positioning pins  50  (not shown in  FIG. 12 ) possessed by jaw rod OD 2  and elongate roof portion B 2  of the bar piece  32  is snugly put in a V-shaped groove  52  formed in the other jaw rod UD 2 . With this, an angular position of second bar piece  32  relative to second chuck C 2  is tightly set. More specifically, an angular position of second rack teeth forming portion T 2  of second bar piece  32  is tightly set. Arrangement of the positioning pins  50  of jaw rod OD 2  relative second rack teeth R 2  of the bar piece  32  will be well understood from  FIG. 13 . 
         [0076]    Then, as is seen from  FIGS. 9 and 10 , intermediate bar piece  33  is set to rotor unit  40   b  of rotating device  40  and then rotated about its axis at a given high speed. 
         [0077]    Then, first and second hydraulic power units  41   b  and  42   b  are energized for axially moving, through first and second chucks C 1  and C 2 , first and second bar pieces  31  and  32  toward the rotating intermediate bar piece  33 , more specifically, toward circular left and right end faces  33   a  and  33   b  of the rotating intermediate bar piece  33  respectively. 
         [0078]    When respective circular right and left ends  31   a  and  32   a  of first and second bar pieces  31  and  32  are brought into contact with circular left and right end faces  33   a  and  33   b  of the rotating intermediate bar piece  33 , the contacting portions start to generate a high frictional heat. When the contacting portions show a predetermined high temperature, the rotation of intermediate bar piece  33  is instantly stopped. Upon this, welding between the right end  31   a  of first bar piece  31  and the left end  33   a  of intermediate bar piece  33  as well as welding between the left end  32   a  of second bar piece  32  and the right end  33   b  of intermediate bar piece  33  are effected by the generated high heat. Until the time when the welding is sufficiently made, first and second bar pieces  31  and  32  are kept pressed against intermediate bar piece  33  with a given force by the work of first and second hydraulic power units  41   b  and  42   b.    
         [0079]    With the above-mentioned steps, a rough rack  1  as shown in  FIG. 14  is produced. As shown, the rough rack  1  just produced has ugly burrs  1   x  around the welded portions. 
         [0080]    The ugly burrs  1   x  are then removed through a known finishing method. With this, a finished rack  1  is produced as is seen from  FIG. 15 . Due to removal of the burrs  1   x , the two welded portions is and  1   b  of rack  1  are smoothed, which reduces a stress that would be applied to such portions  1   a  and  1   b  under practical use of rack  1 . 
         [0081]    In the following, advantages provided by the first embodiment of the present invention will be itemized. 
         [0082]    Due to employment of intermediate bar piece  33 , the joining of the three bar pieces  31 ,  32  and  33  through the friction welding is achieved by only turning intermediate bar piece  33 . That is, it is not necessary to turn first and second bar pieces  31  and  32  that have thereon first and second rack teeth R 1  and R 2  respectively. Thus, the intersection angle “θ” between first and second rack teeth R 1  and R 2  can be stably and assuredly kept even under rotation of intermediate bar piece  33 , which eliminates an additional machining that would be applied to the rack teeth R 1  and R 2  if the intersection angle “A” is changed due to the rotation of intermediate bar piece  33 . 
         [0083]    Before making the friction welding, first and second bar pieces  31  and  32  are tightly held by respective first and second chucks C 1  and C 2  in such a manner as to allow the two bar pieces  31  and  32  to show the angular displacement of “θ” therebetween, and then, the friction welding is carried out by turning only the intermediate bar piece  33 . This method is quite effective for eliminating or at least minimizing a deviation of the predetermined relative angular displacement of “θ”. 
         [0084]    For holding first and second bar pieces  31  and  32  by respective chucks C 1  and C 2 , the imaginary elongate planes GF 1  and GF 2  provided by the bar pieces  31  and  32  are used as a reference for positioning. Since such planes GF 1  and GF 2  have a high positioning accuracy, the relative angular displacement of “θ” between the two bar pieces  31  and  32  can have a high accuracy. 
         [0085]    Furthermore, since the imaginary elongate planes GF 1  and GF 2  provided by the bar pieces  31  and  32  are used as a reference for setting the relative angular displacement of “θ” between the two bar pieces  31  and  32 , the engagement between first pinion P 1  and first rack teeth R 1  as well as the engagement between second pinion P 2  and second rack teeth R 2  are improved in meshing accuracy. 
         [0086]    Furthermore, in the friction welding, the rotation speed of rotating device  40  and the pressing force of first and second hydraulic power units  41   b  and  42   b  are controlled by the controller (or servomechanism) and at the same time, the movement of first and second chucks C 1  and C 2  is controlled by the controller and first and second position sensors  41   c  and  42   c . Thus, the friction welding between each of first and second bar pieces  31  and  32  and intermediate bar piece  33  can be accurately made. That is, dimensional errors of the produced rack  1  in the axial direction are suppressed or at least minimized. More specifically, the frictional welding can be sufficiently and equally made in the two welded portions  1   a  and  1   b  of the rack  1 . 
         [0087]    Furthermore, due to usage of two pressing units  41  and  42  for pressing first and second bar pieces  31  and  32  against intermediate bar piece  33 , the pressing force with which first bar piece  31  is pressed against intermediate bar piece  33  as well as the other pressing force with which second bar piece  32  is pressed against intermediate bar piece  33  are much effectively controlled. Thus, the friction welding at the two welded portions  1   a  and  1   b  of the rack  1  is effectively made. 
         [0088]    In the following, another method of producing the rack  1 ′, which is a second embodiment of the present invention, will be described with reference to  FIGS. 16 and 17 . 
         [0089]    Since the method of the second embodiment is similar to the above-mentioned first embodiment, only part or portion that is different from that of the first embodiment will be explained in the following. 
         [0090]    In this second embodiment, a thin circular piece  33 ′ is used in place of the above-mentioned intermediate bar piece  33  that is used in the first embodiment. 
         [0091]    As shown in  FIG. 16 , the thin circular piece  33 ′ has an outer diameter larger than that of first and second bar pieces  31  and  32  and is formed with a circular opening  33 ′ c  of which diameter is the same as that of blind bores  31   b  and  32   b  of first and second bar pieces  31  and  32 . The thickness of thin circular piece  33 ′ is determined so as to allow the rotating device  40  (see  FIG. 9 ) to stably rotate the thin circular piece  33 ′. 
         [0092]    For joining these three pieces  31 ,  32  and  33 ′ by the friction welding, substantially same steps as those of the above-mentioned first embodiment are carried out. 
         [0093]    That is, first bar piece  31  (see  FIG. 9 ) is held by first chuck C 1 , second bar piece  32  is held by second chuck C 2 , and thin circular piece  33 ′ is set to rotating device  40  and rotated about its center at a given high speed. Then first and second bar pieces  31  and  32  are moved toward the rotating thin circular piece  33 ′ and brought into contact with left and right faces of the rotating piece  33 ′ respectively. Upon this, a high frictional heat is generated at the contacting portions, so that welding between first bar piece  31  and the left face  33 ′ a  of thin circular piece  33 ′ and welding between second bar piece  32  and the right face  33 ′ b  of thin circular piece  33 ′ are effected by the generated high heat. Until the time when the welding is sufficiently made, first and second bar pieces  31  and  32  are kept pressed against thin circular piece  33 ′ with a given force by the work of first and second hydraulic power units  41   b  and  42   b.    
         [0094]    With the above-mentioned steps, a rough rack  1 ′ as shown is in  FIG. 16  is produced. As shown, the rough rack  1 ′ just produced has ugly burrs  1   x  around the welded portions. 
         [0095]    The ugly burrs  1   x  are then removed together with a radially projected part of thin circular piece  33 ′ through a known finishing method. With this, a finished rack  1 ′ is produced as is seen from  FIG. 17 . Due to removal of the burrs  1   x , the two welded portions (no numerals) of rack  1 ′ are smoothed, which reduces a stress that would be applied to such welded portions under practical use of rack  1 ′. 
         [0096]    In the following, advantages provided by the method of the second embodiment of the invention will be itemized. 
         [0097]    Also in this second embodiment, the advantages provided by the above-mentioned first embodiment are almost obtained. 
         [0098]    Furthermore, in this second embodiment, usage of the thin circular piece  33 ′ brings about a light weight construction of rack  1 ′. In other words, the thin circular piece  33 ′ (and the above-mentioned intermediate bar piece  33  of the first embodiment) is a so-called stopgap member that is used for suppressing a mutual rotation between first and second bar pieces  31  and  32 . More specifically, the piece  33 ′ or  33  is not a member that can play an important part of rack  1 ′ or  1 . 
         [0099]    For carrying out the friction welding, the thin circular piece  33 ′ of which diameter is larger than that of first and second bar pieces  31  and  32  is used. Accordingly, the friction welding between each of first and second bar pieces  31  and  32  and thin circular piece  33 ′ is easily and assuredly made. Due to the larger size of the piece  33 ′, a displacement between first and second bar pieces  31  and  32  in a radial direction can be suitably compensated by the piece  33 ′ when the displacement is not so large. 
         [0100]    The present invention is not limited to the above-mentioned first and second embodiments. That is, the shape of first and second bar pieces  31  and  32 , the relative angle “θ” between these bar pieces  31  and  32  and the size and shape of first and second rack teeth R 1  and R 2  may be changed or adjusted in accordance with specifications of an electric power steering device to which the rack of the invention is practically applied. 
         [0101]    Furthermore, if desired, the pressing force for pressing first and second bar peaces  31  and  32  against intermediate bar piece  33  (or thin circular piece  33 ′) may be produced by only one of first and second pressing devices  41  and  42 . That is, for example, first hydraulic power unit  41   b  (see  FIG. 9 ) may be removed from first pressing device  41 . In this case, a modification is so made that the pressing force produced by second hydraulic power unit  42   b  causes a pressing of the piece  33  (or  33 ′) against first bar piece  31  as well as a pressing of second bar piece  32  against the piece  33  (or  33 ′). Of course, in this modification, the production equipment for producing the rack  1  or  1 ′ can be simplified and downsized. 
         [0102]    Furthermore, if desired, for achieving the friction welding of the three pieces  31 ,  32  and  33  (or  33 ′), first and second bar pieces  31  and  32  may be rotated keeping intermediate bar piece  33  (or thin circular piece  33 ′) stationary. In this case, once the friction heat produced at the contacting portions  1   a  and  1   b  becomes sufficient, the rotation of first and second bar pieces  31  and  32  is instantly stopped and just after this stopping, the relative angle between these two bar pieces  31  and  32  is adjusted to “θ”. 
         [0103]    Furthermore, for accurately aligning the three pieces  31 ,  32  and  33  before the friction welding in case of the first embodiment, first and second positioning pins (not shown) may be used. That is, as will be imaged or understood from  FIG. 14 , the first positioning pin extends in and between blind bore  31   b  of first bar piece  31  and through bore  33   c  of intermediate bar piece  33  and the second positioning pin extends in and between blind bore  32   b  of second bar piece  32  and through bore  33   c  of intermediate bar is piece  33 . Of course, in this case, such first and second positioning pins are left in a finished rack  1 . 
         [0104]    The entire contents of Japanese Patent Application 2012-191074 filed Aug. 31, 2012 are incorporated herein by reference. 
         [0105]    Although the invention has been described above with reference to the embodiments of the invention, the invention is not limited to such embodiments as described above. Various modifications and variations of such embodiments may be carried out by those skilled in the art, in light of the above description.