Abstract:
In a method for producing flat-blade windshield wipers with curved flat blades in particular, and for obtaining dimensionally stable bending radii with exact tolerances, a spring band formed by flat blades individually arranged one next to the other, is bent between three support sites spaced from each other in the longitudinal direction of the spring band and resting successively in an alternating manner on one of the two sides of the spring band, and the spring band is thereafter re-bent on a subsequent support site in the opposite direction by a re-bending degree lower than the bending degree. The flat blade so treated is subsequently separated from the spring band, combined with a rubber-elastic wiping strip, and provided with a connecting device for a wiper arm.

Description:
CROSS-REFERENCE TO A RELATED APPLICATION  
       [0001]    This application is a continuation-in-part of application Ser. No. 09/445,880. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The invention relates to a method of producing flat-blade windshield wipers for motor vehicles with curved flat blades.  
           [0003]    Such flat-blade windshield wipers are known from U.S. Pat. No. 3,192,551, wherein one single back of the spring band, being engaged by the wiper arm of the windshield wiper in the center, produces a nearly uniform contact pressure of the rubber wiping strip, secured on its back, on the surface of the windshield over the entire wiping range, such windshield being curved, as a rule. For this purpose, the curved back of the spring band has a material thickness changing over its length, such thickness being the greatest in the center of the back and decreasing toward the two ends of the back, and, furthermore, has a changing back width, which decreases from the center of the back toward the ends thereof.  
           [0004]    A method and a device for producing metal strips with varying material thickness are known from WO 94/17932. Such metal strips are used as components of windshield wipers. In this process, a strip material having a constant thickness and width is drawn through a pair of counteropposed rolls, whereby the spacing of the rolls is varied in order to shape in this way the strip material with a thickness varying by sections over the length of said material. The material is then finally subjected to a heat treatment.  
         SUMMARY OF THE INVENTION  
         [0005]    The method as defined by the invention for producing flat-blade windshield wipers with curved flat blades offers the advantage that flat blades required directly as components of flat-blade windshield wipers, are produced in one single step from a heat-treated spring band blank with varying material thickness.  
           [0006]    Flexing and re-flexing takes place in this process as the spring band is continuously advanced, such feed being interrupted briefly periodically for separating the flat blades. The method as defined by the invention supplies final flat-blade products with preset bending radii and largely within exact tolerances, which are absolutely stable dimensionally, and will no longer deform even as they are separated from the band material. Owing to the fact that the center one of the three support sites for the bending of the spring band, and the fourth support site for the re-bending of the spring band each are transversely displaceable relative to the spring band in the direction of the thickness of the band, and that their transverse displacements relative to the spring band are controlled according to preset programs, taking into account the changing material thickness within the flat blades, the spring band undergoes uniform plastic deformation in all regions. It is possible at the same time to realize different bending radii within one flat blade. In the programs for controlling the displacement motion of the two support sites, other parameters influencing the manufacturing result are taken into account as well, in addition to the variable material thickness and, if need be, different bending radii of the flat blades based on zones, such other parameters being, for example the band hardness, the band width, prior alignment of the spring band, sense of winding of the spring band on a supply coil, etc.  
           [0007]    For maintaining most exacting manufacturing tolerances, provision is made according to an advantageous further development of the method as defined by the invention that at least some of the separated flat blades are optically measured and compared with nominal value specifications. The mean deviations from the nominal values are used for correcting the bending and re-bending programs.  
           [0008]    The novel features which are considered as characteristic for the present invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The invention is explained in greater detail in the following description with the help of the exemplified embodiment of a device shown in the drawing, where the figures show the following schematic representations:  
         [0010]    [0010]FIG. 1 is a side view of a device for producing curved flat blades for flat-blade windshield wipers.  
         [0011]    [0011]FIG. 2 shows by a cutout an enlarged side view of a spring band with varying material thickness used for producing the curved flat blades; and  
         [0012]    [0012]FIG. 3 is a side view of a flat-blade windshield wiper.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0013]    The device schematically shown in FIG. 1 in side view, for producing curved flat blades for flat-blade windshield wipers, is supplied with a spring band  10 , the latter being shown schematically and enlarged by the cutout in FIG. 2. Individual flat blades  11  of the same type are consecutively fixed one after the other in spring band  10 , with their heads and ends being marked in spring band  10  by so-called trigger holes  12 . The material thickness or band thickness “d” of the spring band  10  varies within each flat blade  11 , as clearly shown in FIG. 2. In a realistic exemplified embodiment, the band thickness “d” varies between a maximum value of 1.2 mm and a minimum value of 0.4 mm, whereby the band thickness “d” decreases from the center of each flat blade  11  toward the two ends. The material thickness or the band thickness “d” of the spring band  10  can be also constant from one end to the other end, as not shown in the drawing.  
         [0014]    Viewed in the direction in which the spring band travels through or is advanced through the device, the latter has two feed rolls  13 ,  14  engaging the two opposite sides of the spring band  10 . Said feed rolls are motor-driven and numerically controlled (NR), and pull the spring band  10  from a supply coil  15 . Furthermore, the device has a plurality of guide rollers, in the present case four guide rollers  16  to  19  (or also six guide rolls, if need be), which engage in pairs the opposite sides of spring band  10 , and which are not driven. Furthermore, the device has a bending unit  20 , a separation unit  21 , and a re-bending unit  22 . The two guide rollers  16 ,  18  engaging the spring band  10  on the top side of the latter are displaceable in the vertical direction, i.e. at right angles relative to spring band  10 , and can be adjusted manually in order to assure tight guidance of spring band  10  without permitting the latter to “buckle out”.  
         [0015]    The bending unit  20  comprises three support sides  23  to  25 , which are spaced from each other and successively rest alternatingly against the one and the other side of the band, viewed in the direction in which the latter travels through the device. Viewed in the direction of feed of the spring band  10 , the first and second support sites  23 ,  24  each are formed by the circumference of a roll or roller  26  and  27 , respectively, the axles thereof being transversely displaceable relative to spring band  10  in the direction of band thickness “d”. The first roller  26  is adjustable manually, whereas the roller  27 , which is hereinafter referred to as the bending roller  27 , is adjusted by a servo-motor  28 , which is controlled by an NC control unit  29 . The third support site  25  of the bending unit  20  is formed by a cutting edge  30 , which is greater than the width of the spring band  10 , and which, for separating a flat blade  11 , cooperates with a cutter  31 , which is moved vertically, i.e. transversely relative to the spring band  10 , and guided past the cutting edge  30 . The cutting edge  30  and the cutter  31  jointly form the separation unit  21 , which thus is arranged at the end of the bending unit  20  and integrated in the bending unit  20  due to the design of the fourth support side  25  in the form of cutting edge  30 .  
         [0016]    The re-bending unit  22 , which is located downstream of the separation unit  21 , consists of a fourth support site  32  engaging the same band side of the spring band  10  as the center support site  24  of the bending unit  20 , thus as the bending roller  27 . Here, too, said fourth support site  32  is formed by the circumference of a roll or roller, the so-called re-bending roller  33 . The axle of said re-bending roller  33  is also displaceable transversely to the spring band  10  in the direction of the band thickness “d”. The displacement of the re-bending roller  33 , i.e. the motion setting it in its acting position, is again effected by means of a servo-motor  34 , which is controlled by the NC control unit  29 .  
         [0017]    With the device described above, the curved flat blades are produced as follows:  
         [0018]    The spring band  10  continuously taken off from the supply coil  15  is bent by sections between the three support sites  23  to  25  of the bending unit  20 , whereby the bending radius is determined by adjusting the center support site  24  in its operating positions as required. Subsequently, the flat blade  11  so bent is bent back on the following fourth support site  32  in the opposite direction, whereby the degree of re-bending is substantially smaller than the bending degree. The degree of re-bending required in order to assure the dimensional stability of the bent flat blade  11  is determined empirically. A degree of re-bending amounting to about 10 to 20% of the bend previously produced was found to be the mean value depending on the quality of the spring band  10 . Since the bending radius of the finished flat blade  11  is predetermined, bending of the spring band  10  between the three support sites  23  to  25  is carried out with a bending degree increased by the degree of re-bending on the fourth support site  32 .  
         [0019]    The feeding motion for the center support site  24  for bending the band, and for the fourth support site  32  for re-bending the spring band is controlled according to preset programs, which take into account especially the changing material thickness of the flat blade. In addition, it is possible to take into account in said programs also different bending radii desired within the flat blade in certain zones. Furthermore, other parameters influencing the production result such as, for example the hardness and width of the band, prior alignment of the spring band, winding sense of the spring band on the supply coil  15 , etc., are taken into account in the programs. The described control of the feeding movements of the support sites  24  and  32  is effected with the NC control unit  29 , which controls the servomotors  28  and  34  depending on the NC-axis (x-axis) of the feed rolls. The latter, in turn, effect a displacement of the axis of the bending roller  27  forming the center support site  24 , or of the axis of the re-bending roller  33  forming the support site  32 , by a corresponding amount toward the spring band  10  or away from the latter. The NC control unit  29  is triggered by the trigger holes  12  in the spring band  10 . Said trigger holes  12  are detected by an optical sensor  35 , which cooperates with a light source  38 , and which is arranged upstream of the guide rollers  16  to  19 . Upon detection of said trigger holes, said sensor  35  transmits s triggering signal to the control unit  29  and thereby calls up the execution of the program. The trigger holes  12  consequently determine the start and the end of the bending and re-bending programs to which the individual flat blades  11  are subjected, as well as the subsequent separation process in that the control unit  29  also triggers the cutting motion of the cutter  31  after the program has been completed, or executed. For separating the finished curved flat blades  11  from the spring band  10 , the continuous feed of the spring band  10  is briefly interrupted for each cut.  
         [0020]    For the purpose of obtaining tight manufacturing tolerances, at least some of the flat blades  11  are optically measured, which can be done prior to or after the separation step, and compared with preset nominal values. The mean deviations from the rated values are used for correcting the bending and re-bending programs. For this purpose, provision is made for a graphic acquisition system  36 , with which the finished flat blades  11  are optically measured and the deviations from the specified nominal values are determined. The graphic acquisition system  36  is connected to a correction unit  37 , which changes suitable parameters in the programs of the NC control unit  29  in accordance with the deviations from nominal values, so that a control circuit is formed for correcting the bending degree. The programs stored in the NC-unit are adapted to a defined shape of the finished flat blades  11  and have to be newly generated or modified for changed shapes of the flat blades  11 .  
         [0021]    For producing a flat-blade windshield wiper  40  as shown by a side view in FIG. 3, the flat blade  11  produced as described above is provided with a connecting device  41  for a wiper arm  42 ; its surface is hardened, and combined with a wiping strip  43  having the elasticity of rubber. The wiper arm  42  indicated in FIG. 3 by dash-dotted lines is put into an oscillating motion in the known manner by a wiper gearing, which drives the wiping edge  431  of the wiping strip  43  across the windshield indicated by the dash-dotted line and denoted by reference numeral  44 . The wiping strip  43  can be mounted by pressing the flat blade  11  into a plane position and gluing or vulcanizing the wiping strip  43  to the concave side in the unstressed condition. However, it is possible also to clamp the wiping strip  43  between two flat blades produced in the same way, whereby the two flat blades are then kept together by the connecting device, which still needs to be mounted, as well as by additional clips, if need be. It is important in this connection that the two flat blades have exactly the same bending curvature so as to avoid stresses in the wiping blade.  
         [0022]    The novel features of the present are set forth on the appended claims.