Abstract:
A rail guide that has at least one rail pair consisting of a floor rail ( 22 ) and a seat rail ( 24 ). The seat rail ( 24 ) has connectors ( 30 ) for fastening a seat frame to be arranged above the rail pair. The rails can be moved relative to each other in a longitudinal direction along a movement path ( 20 ). The floor rail ( 22 ) and the seat rail ( 24 ) are curved by the same circular radius of curvature ( 42 ) that ranges between 1,500 and 3,000 mm, in particular between 1,800 and 2,500 mm. The center point of curvature ( 44 ) is on the side of the rail pair facing away from the seat frame. The rail guide is a longitudinal adjustment device having a stationary spindle ( 52 ) and an associated spindle nut ( 54 ) that is longitudinally movable thereon, the spindle ( 52 ) being curved at the same circular radius of curvature ( 42 ). Also described is a suitable method for producing said curved rails by at least one stamping mold ( 68 ) or by bending strips ( 96 ).

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a United States National Phase Application of International Application PCT/EP2012/065064 filed Aug. 1, 2012 and claims the benefit of priority under 35 U.S.C. §119 of German Patent Application 10 2011 080 221.5 filed Aug. 1, 2011, the entire contents of which are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The invention relates to a rail guide for a longitudinal adjustment mechanism of a motor vehicle seat with at least one pair of rails, which comprises a floor rail, a seat rail and guiding means disposed between these two rails, the rails of the pair of rails being displaceable relative to each other along a displacement path in a longitudinal direction, which is substantially parallel to an x-axis, and the seat rail comprising means for fastening a seat frame to be disposed above the pair of rails. 
     BACKGROUND OF THE INVENTION 
     Such a rail guide is known from DE 42 38 486 A1, for example FIG. 2 of DE 42 38 486 A1. Other rail guides are known from U.S. Pat. No. 5,941,495 A and DE 10 2006 048 786 A1. Additionally, reference is made to DE 10 2006 047 525 A1. 
     The two rails of a pair of rails in the rail guides according to the prior art extend in a straight line. This has great advantages; the rails can be manufactured and, if necessary, aligned with precision, attachments are simple and a consistently good adjustability over the entire length of the adjustment distance can be obtained. 
     In the case of straight rails, there is, however, the disadvantage that the seat is only moved in a linear manner in space when adjusted, but that no other movements are possible. However, it was now found out that a tall passenger, who sets the seat far towards the rear and low in a downward direction, generally also inclines the backrest more towards the rear relative to the vertical than a smaller passenger, who will set his seat more towards the front and higher. According to the prior art, the inclination of the backrest can only be adjusted by means of a separate backrest joint. However, if such a joint is provided, this means a certain expenditure. The inclination of the seat surface is also chosen differently by tall and small persons. In order to be able to adjust the inclination of seat surface, there are seat inclination adjustment mechanisms. They offer good comfort but also require added expenditure. A height adjustment mechanism with which the height of the seat surface can be adjusted is also frequently provided. In principle, these and any other additional adjustment device mean added weight and expenditure as regards material and assembly. 
     SUMMARY OF THE INVENTION 
     The invention is based on the above background. The invention has the object to provide a rail guide that provides by itself as good a setting as possible for the entire range of passengers so that additional setting devices beyond the longitudinal adjustment mechanism can be omitted as far as possible. In this case, the rail guide is supposed to still have a simple structure; the rail guide is supposed to satisfy the requirements made to a rail guide according to the prior art. The rail guide is supposed to be suitable for manual and motor-operated adjustment devices. 
     Based on the rail guide of the type mentioned in the introduction, this object is achieved by the floor rail and the seat rail each being curved with the same circular radius of curvature, that the radius of curvature is between 1,500 and 3,000 mm, in particular between 1,800 and 2,500 mm, and that the center of curvature is located on the side of the pair of rails facing away from the seat frame. 
     In contrast to the prior art, the adjustment direction along the displacement path no longer extends on a straight line, but on an arc of a circle. The particular choice of radii of curvature within the specified limits results in a seat that is located far back in the rail guide being positioned slightly more obliquely towards the rear and, additionally, lower than a seat that is set in the front region of the rail guide. This makes it possible to dispense with additional setting devices, such as a seat height adjustment mechanism and/or backrest inclination adjustment mechanism. This means that a considerable amount of weight can be saved. In this way, a motor vehicle seat can be formed to be significantly lighter than in accordance with the prior art. 
     The curvature following an arc of a circle has a center of curvature which, in the installed state of the motor vehicle seat, is located underneath this seat. The center of curvature is located on the side of the rail guide that faces away from the seat region and the backrest. 
     Rails with such a radius of curvature can be manufactured with precision. Despite the displacement no longer being linear, good synchronism, precise guidance and a high degree of manufacturing precision are achieved. One of the rails still preferably is the clasping rail and the other the clasped rail. Preferably, the two rails rest against each other with an elastic bias. The two rails rest against each other by means of rolling or sliding members. They form the guiding means. Production inaccuracies are compensated and the guiding means held in their position due to this elasticity. The rails both have a length of at least 25 cm; one of the two rails has a length of at least 30 cm. Preferably, the rails are made from a steel sheet having a yield point of at least 600 N/mm2. Thus, metal sheets with a thickness of, for example, 1.5 mm can be used. The range of thickness is 1.2 to 1.8 mm, preferably 1.5+10% and −30%. The thinner the metal sheet, the lower the weight of the rail. 
     Preferably, the two rails are each produced individually as a stamped-bent part. This is started with a steel sheet blank, in particular a wide band. This blank is elongated; it is either banana-shaped or saber-shaped if the rail is to be mounted in such a way that a base flange lies in the x-z-plane, or substantially rectangular if the rail is to be mounted in such a way that a base flange lies in the x-y-plane. This blank is preferably bent in several individual bending steps about two parallel bending lines, respectively. These are realized by means of bending bars. Bending processes are carried out along and in the vicinity of the longitudinal edges. Preferably, the profile of the finished rail is axially symmetrical or substantially axially symmetrical relative to an axis that is a midperpendicular to the base flange. 
     In order to produce a rail which is to be mounted in such a way that the base flange lies in the x-y-plane, the following procedure is preferably used: In a first processing step, the blank is bent, with the bending line having a radius matching the radius of curvature or deviating by up to 3%. The center point is located in the vicinity of or on the center point line; the latter extends parallel to the short edges of the blank. The blank then follows a cylinder jacket. In a second bending step, one lateral first leg, respectively, is bent on the left and the right, parallel to the respective longitudinal edge of the blank. This is done about a bending line lying on an arc of a circle. The bending line has a radius that can deviate from the radius of curvature by up to 3%. Generally, it does not match the radius of curvature. Its center point is located in the vicinity of or on the center point line. At least one further leg is bent in at least one further bending step. Here, the procedure is the same as in the second bending step. When producing a rail that is to be mounted in such a way that the base flange lies in the x-z-plane, the first processing step is omitted and the process is started directly with the second bending step. 
     A distortion or warping occurs due to the use of high-strength steel material and the bending steps. The legs and, if applicable, also the base flange of the subsequent rail show irregularities; they do not run exactly on a cylinder jacket surface or a smooth, regularly curved surface. The more high-strength the steel, the greater the yield point, the greater the inner stresses, and thus the warpage, in the material during the bending steps. Therefore, the warpage and stresses have to be compensated after at least one bending step, preferably after two or three bending steps. This is done in a calibrating step. In the process, the rail is straightened. The distortion is eliminated. This is done by bending over towards the opposite side, i.e. in the direction opposite to the previous bending direction. A plastic deformation is carried out in the process. The lateral flank is straightened in this manner. Optionally, another region is also straightened. 
     The bending processes take place by applying bending bars to the blank and bending around the bending bars. The bending bars have a curvature about the center of curvature with a deviation with respect to the radius of less than +/−3% of the radius of curvature. Bending steps are carried out as they are also carried out in the case of rails extending in a straight line. However, bending bars are being used now that do not extend in a straight line but are curved with the bending radius. 
     Preferably, the rail guide is associated with an actuating drive for longitudinal adjustment. This actuating drive preferably comprises a rotary drive having an output shaft and a gear unit comprising a spindle nut rotationally connected to the output shaft and having a spindle; in this case, the spindle comprises a spindle thread with which the spindle nut is in engagement; the spindle is curved with the same circular radius of curvature as the rails; the spindle nut is a straight nut. 
     Preferably, the motor vehicle seat is provided with a motor-operated adjustment device which provides for the adjustment along the displacement path. The rotary drive unit can also be designed for manual adjustment. In that case, the rotary drive unit is to be actuated by hand It comprises, for example, a crank which is within the reach of a passenger and via which the passenger introduces the rotary movement into the gear unit. In contrast to a rail guide according to the prior art, which lacks a rotary drive unit and is equipped with a locking device to be operated manually, no spring is required that biases the seat forward within its rail guide. This spring must have a considerable spring force; it contributes to the overall weight. Such a spring is saved. 
     It is also an object of the invention to provide a method for producing such a rail guide. According to this method, the floor rail and the seat rail are each manufactured from a suitable blank; this blank extends along the radii of curvature that the finished rail has. The blanks are stamped in at least one respective stamping die; stamping takes place in at least one stamping step. In this way, the finished rail is prepared. 
     This method is advantageous in that the desired accuracies are achieved. Rails can be produced in this manner that are easily adjustable despite the curved profile of the displacement path, that are guided within each other precisely, and that have good sliding properties. 
     Usually, a longitudinal adjustment mechanism has two rail guides which are largely constructionally identical. In the case of a motor vehicle seat with two rail guides, both rail guides are bent with the same radius of curvature. 
     The floor rail preferably has fastening regions; with these, it can be connected to a floor assembly of the motor vehicle. 
     The invention also relates to a motor vehicle seat with a rail guide as described above. In this motor vehicle seat, the seat frame comprises connecting parts; the connecting parts are preferably connected to the means for fastening the two seat rails of each side. 
     Preferably, such a motor vehicle seat has no further setting device, in particular no seat height adjustment mechanism and/or no backrest inclination adjustment mechanism. 
     Finally, the invention also relates to a motor vehicle equipped with a rail guide or a motor vehicle seat as described above. In this motor vehicle, the rail guide is disposed in such a way that, in the rearmost position of the longitudinal adjustment mechanism, the motor vehicle seat, particularly its backrest, is inclined rearwards at an angle of 10° to 15° measured from the vertical, and in the foremost position is inclined rearwards at an angle of 5° to 10°, again measured from the vertical, with the change of angle between these two positions being at least 4°, preferably 6°. In this way, the desired adjustment positions are obtained directly by means of the rail guide without any additional setting devices being required. This results in a significant advantage with regard to cost and weight. 
     In the motor vehicle seat, the seat region is preferably rigidly connected to the rails. Preferably, no setting devices are provided between the seat rail and the seat region. This saves the components of a seat height adjustment mechanism and/or seat inclination adjustment mechanism which are usually provided. These components include, for example, a seat-lifting spring that has to have a certain spring force, which means a considerable amount of weight; joint arms and setting devices, for example wobble joint fittings, are saved. 
     Preferably, all centers of curvature of partial areas of the rails and, if applicable, an associated curved spindle, lie on a center point line which is a parallel to the y-axis. The individual radii extend perpendicular to this center point line. Their center points lie on the center point line. In the case of a longitudinal guide with two rail guides, the centers of curvature of both rail guides lie on the same center point line, but at different points thereon. In each individual rail of a rail guide, the centers of the radii of curvature of the edges lie on the center point line. There, they have the same spacing from each other as they are spaced in the rail itself in the y-direction. The radii of curvature of different partial regions of the rails are different. They extend concentrically with respect to each other. This particularly applies to those partial regions of a rail that are in contact with the other rail. 
     Other advantages and features of the invention become apparent from the other claims as well as from the following description of an exemplary embodiment of the invention, which shall be understood not to be limiting and which will be explained below with reference to the drawing. The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  is a schematic side view of a motor vehicle seat with a curved rail guide; 
         FIG. 2  is a perspective view in the form of an assembly view of a rail guide with an additional spindle drive; 
         FIG. 3  is an axial cross sectional view through a spindle of a spindle drive with a spindle nut, housing and accessories, only the region around the spindle nut is shown; 
         FIG. 4  is a top view of a blank for a rail; 
         FIG. 5  is a schematic view of a stamping die with a top view of the die cavity; 
         FIG. 6  is a perspective view of a blank for an outer rail; 
         FIG. 7  is the blank according to  FIG. 6  after a first bending stage; 
         FIG. 8  is the blank according to  FIG. 6  after a second bending stage; 
         FIG. 9  is a perspective view of a blank for an inner rail; 
         FIG. 10  is the blank according to  FIG. 9  after a first bending stage; 
         FIG. 11  is the blank according to  FIG. 9  after a second bending stage; 
         FIG. 12  is the blank according to  FIG. 9  after a third bending stage; 
         FIG. 13  is the blank according to  FIG. 9  after a fourth bending stage; and 
         FIG. 14  is a schematic perspective view of a bending die as it can be used for the production of the rails according to  FIGS. 1 and 3  as an alternative to the stamping die according to  FIG. 5 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings in particular, the motor vehicle seat shown in  FIG. 1  is shown in two different extreme positions, namely in continuous lines in a rearmost position and in dash-dotted lines in a foremost position. Both positions are reached along a displacement path  20  extending on an arc of a circle. 
     In the known manner, the motor vehicle seat has at least one pair of rails comprising a floor rail  22 , a seat rail  24  and guiding means  26 . In the exemplary embodiment shown, two rail guides disposed parallel to each other are provided. In the exemplary embodiment, they are constructionally identical. The guiding means  26  are configured as balls; other configurations are possible. The floor rail  22  is substantially C-shaped. It is formed as a grasping rail. It comprises means for fastening to a floor assembly  28  of a motor vehicle that is otherwise not shown in more detail. The seat rail  24  is a rail which is grasped; it is also substantially C-shaped. It comprises means  30  which are in this case configured as openings; they serve for connection to a front connecting part  32  and a rear connecting part  34  for each seat side. These connecting parts  32 ,  34  support a seat region  36 . The latter is rigidly connected to the seat rail  24 . A backrest  38  is provided. As  FIG. 1  shows, no means are provided for seat inclination adjustment, seat height adjustment and inclination adjustment of the backrest  38 . 
     A right-handed x-y-z coordinate system is used for the description. The x-axis and the y-axis lie in the horizontal. The x-axis points in the direction of normal straight travel of the motor vehicle. The z-axis extends parallel to a vertical. 
     The two rails  22 ,  24  of each rail guide delimit a channel-shaped cavity  40 . It is open only in the x-direction. 
     The floor rail  22  and the seat rail  24  are curved each with the same circular radius of curvature. The displacement path  20  has at least a length of 150 mm and at most a length of 350 mm. Preferably, the displacement path  20  is in the range of 250 mm to 300 mm. The radius of curvature R  42  is between 1,500 and 3,000 m, in particular between 1,800 and 2,500 mm. A center  44  of curvature lies on a center point line  46 , which is a parallel to the y-axis. The radii  42  of curvature for the individual edges of a rail  22  or  24  have different lengths and also different centers of curvature. All centers of curvature lie on a center point line  46 . The radii each extend perpendicular to the center point line  46 ; in  FIG. 2 , this is represented by a corresponding symbol. The centers  44  of curvature of two different longitudinal edges of a rail  22  or  24  have, in the rail itself, the same spacing in the y-direction as on the center point line  46 . Each rail  22  or  24  has a constant cross section, seen over its length. The cross section is determined on a cross-sectional plane; the center point line  46  lies in the respective cross-sectional plane. The individual cross-sectional planes are at an angle relative to each other. 
     Both rails  22 ,  24  each have a base flange  48 . This base flange extends in a plane parallel to the x-z-plane. The base flange  48  has the shape of a circular disc sector. 
     The two pairs of rails of the motor vehicle seat are offset only in the y-direction; in the x-direction and z-direction, they have matching coordinates. The offset in the y-direction corresponds to at least half the width of the seat region and is at most 120% of the width in the y-direction of the seat region. 
       FIG. 1  shows the state of the motor vehicle seat installed in the motor vehicle (not shown). In this state of installation, the rear end of the rails  20 ,  22  is the lowermost point. The designation front corresponds to the positive direction of the x-direction, and top corresponds to the positive direction of the z-axis. 
     The rail guide is disposed in the motor vehicle so as to rise towards the front. A tangent drawn to the foremost region of the floor rail  22  substantially extends parallel to the x-y-plane. The tangent deviates from this plane by preferably only ±° 5, particularly only ±° 3. 
     In a foremost position, the seat region  36  is inclined rearwards by an angle relative to the horizontal x-y-plane that is approximately 3°. The angle range can be 0 to 6°. Within the displacement path  20 , a change  50  of angle by about 10° is achieved. This change of angle can be between 4 and 12°. 
     In the exemplary embodiment, the length of the front connecting parts  32  is slightly less, for example, 5 to 30% less, than the length of the rear connecting parts  34 . The front connecting parts  32  of each seat side and also the rear connecting parts  34  of each seat side are of equal length in the longitudinal direction, which substantially matches the z-direction. 
     The center point line  46  is the location of the centers  44  of curvature. The latter are located underneath the rail guide and thus on the other side of the rail guide from the seat region  36  and the backrest  38 . 
     A motor-operated adjustment device is provided which enables an adjustment of the motor vehicle seat within the displacement path  20 . In a manner known per se, it comprises a spindle  52  which is non-rotatably connected at its ends to the floor rail  22  via brackets. In contrast to the prior art, the spindle  52  is curved along a circular line. This circular line has a radius of curvature corresponding to the radius of curvature of the rails  22 ,  24 . The center of curvature lies on the center point line  46 . Spindles with a diameter of 6 to 10 mm are being used, for example a spindle with an M8 thread. In a manner known per se, the spindle  52  is grasped by a spindle nut  54 . This spindle nut  54  is straight. It has a length of 10 to 20 mm. Preferably, it is selected to be short. A length of 10 to 15 mm is preferred. The spindle nut  54  is a nut in accordance with the prior art. Preferably, however, the spindle nut  54  has a starting region  56  on each axial side that is configured in a conical manner or a manner that expands outwards in another manner. Due to the starting regions  56 , a jamming of the spindle nut  54  is counteracted. 
     In a manner known per se, an external worm wheel is formed on the spindle nut, which is in engagement with a worm  58 . The latter is connected to an electric motor  60  via an elastic output shaft  70 . The electric motor  60  is responsible for both seat sides; it is connected to the rail guide of the other seat side via another elastic shaft. This is the prior art. 
     In a preferred alternative, a manual drive is used instead of an electric motor; in this case, a crank can be used, for example. The latter can be disposed underneath the front edge of the seat and have an axis of rotation substantially extending in the x-direction. It is connected to the two seat sides via elastic shafts. 
     The spindle nut  54  and the worm  58  are accommodated in a housing  62 . The housing  62  is grasped by a clamp  64  that is substantially U-shaped. It has fastening areas; they are connected to the seat rail  24 ; for this purpose, the seat rail  24  comprises fastening holes. A passage of the seat rail  24  for the elastic output shaft  70  is located in the middle between the two fastening holes. 
     A first method for producing the seat rail  22 ,  24  as it is suitable for the exemplary embodiment according to  FIGS. 1 and 3  is described below, i.e. for fastening to a component substantially extending in the x-y-plane. 
       FIG. 4  shows a banana-shaped or saber-shaped blank  66 . It is cut from a metal sheet material. Steel sheet grades and thicknesses as in the prior art are used. In contrast to the prior art, the blank  66  is delimited by arcs of a circle having a radius of curvature corresponding to that of the finished rail. The blanks are laterally delimited by longitudinal edges  67  in the form of radial lines. As  FIG. 4  shows, the centers of curvature  44  substantially lie on a line extending in the paper plane which corresponds to the subsequent center point line  46 . The centers of curvature  44  are distributed over a length corresponding to the thickness of the blank  66 . The radius deviates from the radius of curvature by less than 3%. The blank has short transverse edges  69 . 
       FIG. 5  shows a part of a stamping die  68 ; only a schematic view is being shown. The stamping die  68  is associated with a corresponding counter die; the latter is not shown. Stamping the blank  66  can take place in a single stamping die  68  with a female and male die; however, several stamping steps in different stamping dies  68  can also be carried out. The blank  66  is inserted into the stamping die  68 , then the counter die is added and the stamping process is carried out. 
     A production of the rail  22 ,  24  by gradually forming it along the curvature profile is possible; such a process would be, for example, a rolling or bending process. 
     Another method will now be described with reference to the  FIGS. 6-8  and  9 - 13 .  FIG. 6  shows a perspective view of a blank  66 , from which an outer rail is to be manufactured. It is intended for fastening to a component that substantially extends in the x-y-plane. The blank  66  is already bent so as to follow a cylinder jacket whose axis is determined by the center point line  46  (with a maximum deviation of 3% of the length of the radius). In other words, the longitudinal edges  67  now run along an arc of a circle with a radius that deviates by maximally 3% from the radius  42  of curvature. The transverse edge  69  extends parallel to the center point line  46 . 
     A first bending stage is carried out in  FIG. 7 . In the process, first legs  73  have been bent parallel to the two longitudinal edges  67  and in their vicinity. A first bending line  79  extends along an arc of a circle with a radius that deviates by maximally 3% from the radius  42  of curvature. 
       FIG. 8  shows the condition after a second bending step along second bending lines  81 ; the latter are located within the first bending lines  79  of the first bending step; second legs  74  are being bent. The high-strength material used has warped due to the bending steps; it has warped and twisted slightly. The irregularities are eliminated by a calibrating step. The calibrating step is also referred to as a compensating step or correction bending step. In the process, the first leg  73  and/or the second leg  74  are each bent over and plastically deformed in the direction opposite to the previous respective bending step in such a way that the warpage is reduced and the outer surface of the respective leg  73 ,  74  has a regular, smooth profile. 
     The inner rail is prepared analogously to the outer rail; this is described below; however, more than two bending steps are carried out. Generally,  92  refers to a bending line. 
       FIG. 9  shows the rectangular blank  76  of the inner rail; like the blank  66  for the outer rail, it is also already curved. 
       FIG. 10  shows the blank  76  after carrying out the first bending step; in the process, first inner legs  78  have been formed. The bending process was carried out along a third bending line  83 ; it extends along an arc of a circle with a radius that deviates by maximally 3% from the radius  42  of curvature. After a second bending step shown by  FIG. 11 , second outer legs  80  are formed which lie inward of the first outer legs  78 . After a third bending step, third outer legs  82  are bent out from the plane of the blank  76 ; they lie inward of the second outer legs  80 . 
     In a fourth and last bending step, fourth outer legs  84  are formed; they lie inward of the third outer legs  82 . Each bending step described for the outer rail and the inner rail respectively takes place about an arc of a circle with a radius that deviates from the radius  42  of curvature by maximally 3%. 
     The blank  71  for the outer rail has a plurality of latching openings  86  which are periodically disposed one behind the other in the longitudinal direction. Each blank  71 ,  76  has fastening means  30  which are in this case configured as holes. With them, the rail can be screwed directly to a component of the floor assembly that extends substantially in the x-y-plane. The screwed used in the process extend in the z-direction. The blank  76  for the inner rail comprises pin holes  90 ; here, individual locking pins of a multi-pin lock as it is known, for example, from EP 1 316 465 B1, are passed through.  FIG. 14  schematically shows a bending device as it can be used as an alternative to the stamping die  68  according to  FIG. 5  in order to gradually deform into a rail a blank  66  according to  FIG. 4 , which is curved in the shape of a banana or a saber. The bending device has a base  94  from which two parallel bending bars  96  protrude upwards. With their top smooth edges, they form a bending line  92 . The latter extends on an arc of a circle with the radius  98 ; this radius  98  differs by a maximum of 3% from the radius  42  of curvature. Its center lies above the base  94 ; the radius  98  extends parallel to the plane of the base  94 . The bending bars  96  form bending lines  92 . A blank  66  is already placed on the upper edges of the bending bars  96 ; a first bending step has been carried out along the bending lines  92 ; a first leg  73  has already been prepared on both sides. The bending line  92  extends in the vicinity of the longitudinal edge  67 . Like the finished rail, the intermediate condition reached is axially symmetrical to an axis  100  which is a midperpendicular to the base flange  48  of the subsequent rail. 
     A bending die for the production of the rails according to  FIGS. 6-13  differs from the bending die according to  FIG. 14  as follows: The bending bars now extend in a straight line. They remain parallel to each other. The distance of the bending line from the base  94  is no longer constant, as in  FIG. 14 , but changes; it still lies on an arc of a circle. The radius  98  now extends perpendicular to the base  94  in the plane of the bending bar. 
     While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.