Abstract:
A spindle drive ( 1 ), drive spindle ( 8 ), and a production method for such a spindle, particularly for a seat adjustment or a servo steering mechanism, having a spindle thread ( 19 ) for receiving a spindle nut ( 9 ), wherein a drive gear ( 5 ) made from synthetic material is injection-moulded in a non-rotatable manner on the drive spindle ( 8 ), said cog having external toothing ( 21 ), by means of which the drive spindle ( 8 ) can be caused to rotate, wherein within an axial extension ( 27 ) of the drive cog ( 5 ) the threaded spindle ( 8 ) has a recessed region ( 31 ) with a smaller external diameter ( 30 ) between two holding regions ( 28, 38 ) with a larger outer diameter ( 29, 39 ).

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a gear spindle, a spindle gear, and a method for producing a gear spindle, with an improved transmission of axial forces that can arise in particular in the event of an accident. 
         [0002]    EP 0 759 374 A2 has disclosed a device for the adjustment of a seat in a motor vehicle, which device can absorb forces considerably higher than those in normal operation, such as are generated for example in the event of a traffic accident. Here, it is important that the vehicle seat remains fixedly connected to the vehicle body in order to ensure the function of the protective measures provided for the vehicle occupants (safety belts, air bag). Here, a spindle nut that receives a threaded spindle is connected fixedly to the vehicle body. The threaded spindle is driven, via a worm wheel, by an electric motor, which in turn is fixedly connected to the seat. The gear housing of the worm wheel is manufactured from plastic and is connected via a further housing part to the drive motor. When the drive motor is actuated, the threaded spindle rotates and displaces the gear housing, including drive motor and seat, relative to the spindle nut. To prevent the gear housing from breaking away from the threaded spindle for example in the event of a collision, an additional metallic, U-shaped support part is provided which connects the gear housing, via a pivot pin, to the drive motor and thus to the seat. If the gear housing composed of plastic cannot withstand the high force flux, it is held, by means of an additional spindle nut, by the metallic support part. 
         [0003]    A disadvantage of said embodiments is that, in addition to the complete gear housing, a cumbersome support structure is required, which increases the number of components and takes up additional structural space. 
         [0004]    EP1223073 B1 presents a geared drive unit in which a worm wheel composed of plastic is arranged rotationally conjointly on a threaded spindle. Here, within the worm wheel, an annular support element is fastened to the threaded spindle, said support element being enclosed by the worm wheel. In the case of this embodiment, however, the external diameter of the worm wheel is enlarged, whereby the gear housing as a whole must be of correspondingly larger dimensions. 
       SUMMARY OF THE INVENTION 
       [0005]    The gear spindle according to the invention, and the method for producing a gear spindle of said type, have the advantage that the crash resistance of the spindle gear is increased considerably through the formation of a recess region on the threaded spindle, into which recess region the material of the gearwheel engages. As a result of the formation of the recess region, there is formed, in addition to a first axial collar of the first retention region, a second axial collar of the second retention region, which second axial collar prevents the gearwheel manufactured from plastic from sliding off the threaded spindle. Owing to the plastics material that is used, the shear behavior between the gearwheel and the threaded spindle is different to that in the case of a purely metal-metal combination, because the plastic exhibits a certain flow behavior, or greater elastic deformability than metal. By contrast to the situation in the case of a purely metal-metal connection, the second collar of the second retention element created by the recess region yields an increase in strength of for example 1 to 3 kN (kilonewtons). As a result of the formation of said second mechanical barrier, a further shear shoulder is created, which absorbs additional tensile forces. Said additional shear shoulder is generated by the difference in external diameter of the second retention region in relation to the recess region. The spacing between the face surface of the radial bearing journal and the gear housing is advantageously selected such that, in the event of a crash, said face surface is supported on the gear housing, and thus the entire axial extent of the gearwheel serves as an effective shear length with the threaded spindle. 
         [0006]    For example, the spindle gear can be a particularly compact construction if the gearwheel on the threaded spindle is in the form of a worm wheel whose worm toothing, arranged on the outer circumference, meshes with a corresponding drive worm, for example of an electric motor. By means of this worm toothing, it is also possible, if appropriate, to realize a desired self-locking action of the gear. 
         [0007]    The two shear shoulders of the first and second retention regions can be produced in a particularly expedient manner in terms of manufacturing by virtue of the spindle thread, which extends over the entire length of the threaded spindle, being subjected to material removal—for example by turning—in each case in front of the first and second retention regions with respect to the spindle nut. Here, the diameter of said material-removed regions of the recess region and of the mounting region approximately corresponds to the base circle diameter of the spindle thread, whereby the mechanical stability of the threaded spindle is not impaired. The thread turns formed in the retention regions yield additional positive locking with the molded-on gearwheel, and this further increases the tensile strength. Through the removal of material of the spindle thread, it is not necessary for an additional retention element of relatively large external diameter to be molded on, whereby overall, the external diameter of the gearwheel is not enlarged. 
         [0008]    In order for the spindle gear to be of compact construction, the gearwheel is arranged on an axial end of the gear spindle, which gear spindle is supported axially on a thrust element. To minimize friction, the threaded spindle bears against the thrust element by way of a domed surface. For this purpose, it is for example possible for a metal ball to be received on the axial end of the gearwheel—this being realized in particular by insert molding. In this way, the threaded spindle composed of metal is supported directly on the thrust element composed of metal, without plastics elements being situated in the force flux. 
         [0009]    For the mounting of the gearwheel, there are integrally formed on the latter two bearing journals which are mounted directly in the housing main body of the gear housing. In this way, no additional radial bearing elements are required for the gearwheel. Owing to the larger external diameter of the external toothing, an additional collar is formed on the gearwheel between the bearing journal and the external toothing, which collar is for example supported on a support disk when axial tensile forces act on the threaded spindle. 
         [0010]    In the region in which the gear spindle emerges from the housing wall of the gear housing, the gear housing has a further support element for retaining the gear spindle in the gear housing in the event of a crash. For this purpose, at least in the region around the threaded spindle, the housing wall is formed from metal, in particular in the form of a metallic housing part which is enclosed by a gear housing main body composed of plastic. For the mounting of the gear spindle, the latter has, in the region of the housing wall, a mounting region of reduced diameter, such that the internal diameter of the passage opening of the housing wall is smaller than the external diameter of the gearwheel, in particular than the bearing journals thereof. The internal diameter of the passage opening of the housing wall may preferably be smaller than the external diameter of the first retention region which is arranged within the gearwheel, such that there is a radial overlap between the retention region and the passage opening. 
         [0011]    For the embodiment according to the invention for seat adjustment drives, the recess region extends in the axial direction over approximately 2-12 mm, wherein, corresponding to the illustration in  FIG. 4 , an axial extent of approximately 6-8 mm is preferred. Here, the axial edges of the recess region may be beveled, wherein the dimensions for the axial extent of the recess region relate to the region of fully reduced diameter. 
         [0012]    The external diameter of the recess region preferably lies in a similar range to the core diameter of the spindle thread. In a variation of the invention, however, said diameter of the recess region may also be larger or smaller than the core diameter of the spindle thread, as long as it is smaller than the external diameter of the retention regions. 
         [0013]    If material is removed from the thread in order to form the recess region, a maximum radial recess depth is realized without the mechanical stability of the threaded spindle being reduced as a result. The retention regions may be formed by the external diameter of the spindle gear or by a radial material protrusion formed in some other way. The tensile strength of the spindle gear is in this case increased considerably as a result of the formation of the encircling shear shoulders with a radial depth of 1-2 mm. 
         [0014]    It is particularly expedient if, at the external diameter at least of the first retention region, radial positive locking with the gearwheel in the circumferential direction is generated, said positive locking serving for twist prevention. Said positive locking may be realized by way of a knurling or a radial depression, such that the plastics material of the gearwheel engages radially into the retention region. Here, the knurling is preferably pressed onto the external diameter of the spindle thread on the first and/or second retention region. 
         [0015]    Owing to the design of the spindle gear according to the invention, it is possible, in the event of an accident, for higher crash forces to be absorbed without the gear spindle being torn out of the gear housing. Owing to the increase, according to the invention, of the strength with which the gearwheel is fastened to the threaded spindle, it is thereby possible, in combination with a support element on the housing wall of the gear housing—in particular in the region of the passage opening for the gear spindle—for the position of the seat, and of the passenger, to be held in a defined position. In this way, the optimum effect of the restraint systems (airbags, belt tensioners, pre-crash measures) is ensured. 
         [0016]    For the absorption of high crash forces, it is particularly expedient for the gear housing composed of plastic to be enclosed by a more stable frame, such as can be realized for example by way of a metal bracket or metal cage. The spindle gear is driven in a particularly efficient manner by an electric motor. Here, it is for example possible for a worm to be arranged on an armature shaft, which worm drives the gearwheel which is in the form of a worm wheel. Alternatively, the gearwheel may also be driven via a flexible shaft that is connected to the electric motor. 
         [0017]    In the method according to the invention for producing the gear spindle, it is advantageously the case that no additional components and also no additional material is required because, taking a continuous threaded rod as a starting point, certain regions of the thread are cut out. A mounting and recess region formed in this way, which is entirely or partially encapsulated by the gearwheel, considerably increases the crash resistance of the spindle gear. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    Preferred exemplary embodiments of the invention will be described in detail below with reference to the appended drawing. In the drawing: 
           [0019]      FIG. 1  is a perspective exploded illustration of a spindle gear according to an exemplary embodiment of the invention, 
           [0020]      FIG. 2  shows a sectional view of a further embodiment of a spindle gear according to the invention, 
           [0021]      FIG. 3  shows a schematic sectional view of a threaded spindle with different embodiments of recess regions, and 
           [0022]      FIG. 4  is a graphic illustration of the spindle strength as a function of the recess length. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    As can be seen in particular from  FIG. 1 , a geared drive unit  1  in the form of a spindle gear comprises a drive motor  2  with a drive output shaft  3  on which a worm  4  is provided. The geared drive unit  1  furthermore comprises a threaded spindle  8  which is arranged perpendicular to the drive output shaft  3 . On the threaded spindle  8  there is arranged a gearwheel  5  in the form of a worm wheel which engages with the worm  4  of the drive output shaft  3 . Also provided on the threaded spindle  8  is a spindle nut  9 . The spindle nut  9  forms a first interface, for example to a vehicle seat. 
         [0024]    The geared drive unit  1  also comprises a first housing part  6  composed of plastic and a second housing part  7  composed of metal, preferably aluminum or steel. The first housing part  6  in turn is of two-part form with a housing main body  61  and a housing cover  62 . A clip-type connection may for example be provided between the housing main body  61  and the housing cover  62 . The housing cover  62  furthermore comprises a tab  68  which is arranged at approximately a 90° angle with respect to the cover surface. The first and second housing parts  6 ,  7  form a gear housing  20  in which the worm  4  and the worm wheel  5  are accommodated. 
         [0025]    All of the functional surfaces for the mounting of the threaded spindle  8  are provided on the first housing part  6 . More precisely, a first radial bearing  63 , a second radial bearing  64 , a first axial bearing  65  and a second axial bearing  66  are provided on the first housing part  6 . On the second housing part  7 , which is produced from metal, there are provided further connection interfaces, for example to a vehicle body, in the form of a first flange  76  and a second flange  77 . For this purpose, the two flanges  76 ,  77  each have a passage opening into which a spacer sleeve  13  is inserted. The second, metallic housing part  7  is of encircling, closed form and comprises a first side part  71 , a second side part  72 , a third side part  73  and a fourth side part  74 . A passage opening  75  for the leadthrough of the threaded spindle  8  is provided in the fourth side part  74 . Furthermore, an overlap  70  is provided on the fourth side part  74 . Here, the second housing part  7  can be opened in the region of the overlap  70  and thus engaged over the threaded spindle  8 . The overlap  70  is subsequently restored, and the two sub-regions of the second side part  74  are connected to one another, for example by means of two screws  12 . 
         [0026]    In the assembled state, at least a sub-region of the first housing part  6 , which is produced from plastic, is surrounded by the second housing part  7 . The second housing part  7  is in this case used in particular for the fixing of the geared drive unit  1  to, for example, a body or a seat part of a vehicle. 
         [0027]    Furthermore, the geared drive unit  1  comprises a plate element  11  which is arranged, as viewed in the axial direction  16  of the threaded spindle  8 , at a first end  81  of the threaded spindle  8 . The plate element  11  is in this case situated parallel to the tab  68  of the housing cover  62 , and abuts against said tab  68 . The plate element  11  is in this case arranged in a groove  67  in the housing main body  61 . Furthermore, a thrust disk  14  is provided which is arranged directly adjacent to the worm wheel  5  and against which the spring element  10  abuts. 
         [0028]    It is thus possible according to the invention for interfaces of the geared drive unit  1  on the spindle nut  9  and the flanges  76 ,  77  to be separated from functional surfaces that are arranged exclusively on the first housing part  6 . It is thus possible for the first housing part  6  to be molded from plastic, wherein the respective functional surfaces can also be directly molded conjointly without reworking of the functional surfaces being necessary. The force-absorbing interfaces on the second housing part  7  may in this case be formed from metal, such that, for example, forces in the event of an accident do not lead directly to destruction of the geared drive unit  1 . The forces can in particular be transmitted onward via the flanges  76 ,  77  into a customer structure. If axial tensile or compressive forces  54  act on the threaded spindle  8 , the threaded spindle  8  or the worm wheel  5  is always supported in the axial direction  16  on one of the axial bearings on the housing main body  61  or on the housing cover  62 . Radial support is realized at the radial bearings  63 ,  64  of the housing main body  61 . Since it is furthermore the case that interfaces of the geared drive unit  1  to external components are provided only on the spindle nut  9  and on the second housing part  7 , it is the case for example in the automotive field that only variants of said two components need be stocked for different customers. 
         [0029]      FIG. 2  shows a section through a further geared drive unit  1 , wherein the core of the threaded spindle  8  is not shown in section. The gearwheel  5  in the form of a worm wheel has an external toothing  21  in the form of a worm wheel toothing  22 , and meshes with the toothing of the drive output shaft  3 . In the exemplary embodiment, the drive output shaft  3  has a four-flight worm  4 . Axially adjacent to the external toothing  21 , the gearwheel  5  has a first bearing journal  23  facing the receiving region  17 , said first bearing journal being mounted in the first radial bearing  63  of the housing main body  61 . On the opposite side, there extends a second bearing journal  24  which is mounted correspondingly in the second radial bearing  64  of the housing main body  61 . The gearwheel  5  is molded rotationally conjointly onto the threaded spindle  8 . For this purpose, the threaded spindle  8  has, within an axial extent  27  of the gearwheel  5 , a first retention region  28  whose external diameter  29  is larger than the diameter  30  of the adjoining recess region  31 . Axially adjacent thereto, and facing away from the receiving region  17 , there is arranged a second retention region  38  with an external diameter  39  which is likewise larger than the diameter  30  of the recess region  31 . Thus, the recess region  31  of relatively small diameter  30  is arranged between the two retention regions  28 ,  38 , with positive locking being generated between the recess region and the gearwheel  5  with respect to axial force action  54 . Arranged between the first retention region  28  and the receiving region  17  there is a mounting region  34  with a diameter  34  smaller than the external diameter  29  of the first retention region  28 . On the mounting region  34  there is arranged a housing wall  36  of the gear housing  20 , said housing wall being formed, in particular as part of the second housing part  7 , from metal. Said housing wall  36  encloses the threaded spindle  8  and forms an abutment  37  against which the gearwheel  5  is supported axially under the action of tensile load  54 . The gearwheel  5  extends axially over the entire axial width  42  of the first retention region  28  and over the entire axial width  43  of the recess region  31 , and in particular over the entire axial width  44  of the second retention region  38 . In the exemplary embodiment, on that end  46  of the gear spindle  8  which is situated opposite the receiving region  17 , there is arranged a spherical element  48  which bears by way of a domed surface  47  against the plate element  11  as abutment. The spherical element  48  is in this case enclosed radially by the gearwheel  5 —preferably by the second bearing journal  24 . For additional axial support, an annular thrust disk  14  is arranged between the external toothing  21  and the housing main body  61 , against which thrust disk an axial collar  15  of the external toothing  15  abuts axially when the spindle is subjected to axial tensile load  54 . On the outer circumference, the first retention region  28  and/or the second retention region  38  has a knurling  50  or radial formations  51  which prevent a rotation of the molded-on gearwheel  5  relative to the threaded spindle  8 . In the exemplary embodiment, the retention regions  28 ,  38  are formed as part of the spindle thread  19 , with the thread material having been retroactively removed in the recess region  31  and in the mounting region  34 . The material removal may be performed for example by way of cutting processes, for example by turning. Here, a bevel  52  may be formed in each case on both axial sides of the first retention region  28  and/or of the second retention region  38 . 
         [0030]      FIG. 3  illustrates, once again schematically, an embodiment of a threaded spindle  8 . Here, different embodiments are shown, in which the axial width  43  of the recess region  31  is varied. For example, the axial width  43  of the recess region  31  is X2=4 mm in a first embodiment, X3=7 mm in a second embodiment, X4=10 mm in a third embodiment and X5=13 mm in a fourth embodiment. The axial width  43  of the recess region  31  is preferably always larger than the pitch of the spindle thread  19 . The recess region  31  is in each case entirely encapsulated by the gearwheel  5 , such that the plastics material of the gearwheel  5  within the recess region  31  varies correspondingly. 
         [0031]    With regard to these different embodiments, tensile tests were carried out in each case, in which the threaded spindle  8  was pulled out of the gear housing  20  with a high axial force  54  until the gearwheel  5  broke off. In the process, the maximum tensile strength  60  of the gearwheel  5  was measured and plotted, in the diagram of  FIG. 4 , versus the axial width  43  of the recess region  31 . In the upper curve  55  with the square measurement points  56 , the first retention region  28  has a knurling  50  on the outer circumference in each case, and by contrast, in the case of the second, lower curve  57  with the rhomboidal measurement points  58 , the retention regions  28 ,  38  have no knurling  50  and no radial formations  51 . It can be seen that the two curves  55 ,  56  have a maximum in the case of an axial width  43  of the recess region  31  of approximately 7 mm. Therefore, the recess region  31  of the threaded spindle  8  according to the invention is preferably formed with an axial width  43  of approximately 7 mm, or in a range from 6-8 mm. Depending on the dimensions of the threaded spindle  8  and of the gearwheel  5 , it is also possible for the recess region  31  to select axial widths  43  that lie in the range from 5-9 mm or even in a range from 2-12 mm. In a typical embodiment, the recess region  31  has in this case for example a diameter  30  of 8-10 mm, wherein the external diameters  29  and  39  of the first and second retention regions  28 ,  38  are approximately 11-13 mm. That is to say, the tooth height  18  of the thread flights of the spindle thread  19  is approximately 1-2 mm. The external diameters  29 ,  39  of the two bearing journals  23 ,  24  typically lie in the range of 14-18 mm, and the external toothing  21  has for example a base circle diameter  26  in the range from 18-22 mm. For reasons relating to structural space, the gearwheel  5  has a maximum axial extent  27  of for example 20-30 mm, in particular approximately 25 mm. The gearwheel  5  is preferably manufactured from POM plastic, which is for example molded onto a threaded spindle  8  composed of metal. 
         [0032]    From the diagram of  FIG. 4 , it can be seen that forming a knurling  50  or a radial formation  51  on the outer circumference of the first or second retention region  28 ,  38  not only effects rotational locking between the gearwheel  5  and the threaded spindle  8  but also, at least in the range of the optimum axial width  43  of the recess region  31 , increases the maximum tensile strength of the threaded spindle  8  in the gear housing  20 . 
         [0033]    The exemplary embodiments explain the invention on the basis of a seat adjustment device, but the invention may also be used for other adjustment movements, for example as a steering assistance drive, in which the drive output shaft  3  should be prevented from becoming detached from the gear housing  19  under the action of high forces. A combination of the individual features of the different exemplary embodiments also realizes a threaded spindle  9  and geared drive unit  1  according to the invention.