CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to German Patent Application No. 10 2009 053 555.1 filed on Nov. 18, 2009, the entire disclosure of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a folding-top drive for actuating a motor vehicle folding top. 
     2. Description of the Related Art 
     DE 102 17 784 B4 discloses a drive device for actuating an adjustable vehicle part, such as a collapsible folding top, a hinged folding-top compartment or a luggage compartment flap. The drive device disclosed in DE 102 17 784 B4 has a gearing and a drive motor that interacts with the gearing so that the gearing transmits an actuating force of the drive motor, or an actuating moment of the drive motor, to the adjustable vehicle part to be actuated. The gearing is preferably a worm gear, and accordingly a self-locking gearing. Further drive devices for actuating a motor vehicle folding top are known from DE 38 26 411 A1 and DE 101 08 769 A1. 
     The drive devices or folding-top drives of the prior art have enabled a folding top of a motor vehicle to open and close. However, it has been difficult to protect a folding-top drive from overloading. Such an overloading may occur for, example, if a folding top to be opened and closed abuts against an obstruction during the opening process or closing process. Such an obstruction constitutes a resistance for the folding-top drive during the opening or closing process, and the folding-top drive then attempts to move the folding top counter to the resistance. As a result, the folding-top drive and the entire folding top can be damaged. Such damage to the folding-top drive or to the entire folding top must, however, be prevented. Furthermore, it hitherto has proven difficult to manually open or close a folding top in the event of failure of the or each drive motor of a folding-top drive. Folding-top drives that permit manual actuation of a folding top have a complex design. 
     It is an object of the invention to provide a novel folding-top drive that addresses the problems described above. 
     SUMMARY OF THE INVENTION 
     The invention relates to a folding-top drive that comprises at least one self-locking gearing. The self-locking gearing couples the folding-top drive to a folding-top linkage of the motor vehicle folding top. The self locking gearing has first and second gearing parts. The first gearing part is actuable by a drive motor and the actuation of the first gearing part is transmissible via the second gearing part to the folding-top linkage. The first gearing part is mounted pivotably on a housing of the gearing. A guide element is provided for the first gearing part and is acted on by a spring. The guide element for the first gearing part also is mounted pivotably on the housing of the gearing. The first gearing part is guided by the guide element so that when a force or a moment on the first gearing part exceeds a defined limit value, the guide element can be moved automatically together with the first gearing part counter to the spring force of the spring from a first position, in which the first and second gearing parts are engaged, into a second position in which the first and second gearing parts are out of engagement. Additionally, when the force or the moment acting on the first gearing part falls below the defined limit value, the guide element can be moved automatically together with the first gearing part by the spring force of the spring element from the second position into the first position. 
     The folding-top drive of the invention is of simple construction. Additionally, the folding-top drive of the invention makes it possible with simple means to protect the folding-top drive against overloading. The or each self-locking gearing of the folding-top drive of the invention comprises at least two gearing parts. The first gearing part preferably is mounted pivotably at one end on the housing of the gearing and is guided at an opposite end on the guide element, which also is mounted pivotably on the housing of the gearing. 
     The folding-top drive of the invention enables both an automatic movement of the self-locking gearing from the first position, in which the first and second gearing parts are in engagement to a second position in which the first and second gearing parts are out of engagement, and also conversely an automatic movement from the second position to the first position. Hence, if the gearing has been moved from the first position into the second position for overload protection in the event of exceeding the limit value for the moment or force acting on the first gearing part, no manual access to the folding-top drive is necessary after the limit value is undershot. The restoring movement from the second position to the first position also takes place automatically when the limit value is undershot. 
     A stop preferably is assigned to the guide element. The stop restricts the relative movement between the first gearing part and the guide element when the guide element is moved automatically together with the first gearing part counter to the spring force of the spring from the first position to the second position. The stop assigned to the guide element simply and reliably ensures that a relative movement between the first gearing part and the guide element is restricted when the gearing is moved from the first position to the second position. There is no risk of the first gearing part passing out of engagement with the guide element when the gearing is moved from the first position to the second position. 
     The guide element preferably is assigned an actuating element that enables the guide element to be moved manually together with the first gearing part of the respective self-locking gearing counter to the spring force of the spring from the first position into the second position. The actuating element provides a simple emergency unlocking function. Therefore, if one or each drive motor of the folding-top drive of the invention fails, then the actuating element enables the respective self-locking gearing to be moved manually from the first position to the second position for manually opening or closing the motor vehicle folding top by eliminating the self-locking action of the gearing. 
     Preferred refinements of the invention will emerge from the subclaims and from the following description. An exemplary embodiment of the invention will, without being restricted thereto, be explained in more detail on the basis of the drawing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a detail of a folding-top drive according to the invention in a first state of the folding-top drive. 
         FIG. 2  shows the detail of  FIG. 1  in a second state of the folding-top drive. 
         FIG. 3  shows the detail of  FIG. 1  in a third state of the folding-top drive. 
         FIG. 4  shows a further detail of the folding-top drive according to the invention. 
         FIG. 5  shows the detail V of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIGS. 1 to 4  show different details of a folding-top drive  10  according to the invention. The folding-top drive  10  has, at each side of the motor vehicle folding top to be actuated, one gearing  11  via which the folding-top drive  10  can be coupled or attached to a folding-top linkage  12  of the motor vehicle folding top. In the preferred illustrated embodiment, in each case one drive motor  13  interacts with each gearing  11 . However, it is also possible for both gearings  11  to be assigned a common drive motor, which then acts on both gearings  11  via a driveshaft. 
     Each gearing  11  is designed as a self-locking gearing, specifically in the exemplary embodiment shown as a worm gearing. A further, non-self locking gearing such as a planetary gear set may be connected between the respective self-locking worm gearing  11  and the folding-top linkage  12 . 
     Each self-locking gearing  11  has first and second gearing parts  14  and  15 . In the case of a worm gearing, the first gearing part  14  of the respective gearing  11  has a worm housing  16  and a worm  17  held in the worm housing  16 . The second gearing part  15  has a worm gear  18  that interacts with the worm  17  of the first gearing part  14 . The worm  17  and the worm gear  18  are shown in more detail in  FIGS. 4 and 5 . 
     The first gearing part  14  of each self-locking gearing  11  is mounted pivotably on a housing  19  of the respective gearing  11 .  FIG. 1  shows that the worm housing  16  of the first gearing part  14  is mounted on the housing  19  of the gearing  11  so as to be pivotable about a first joint  20 . Thus, the worm  17  held in the worm housing  16  and the drive motor  13  that acts on the worm  17  also are pivotable with the worm housing  16  about the first joint  20 . The joint  20  is in proximity to a first end  21  of the first gearing part  14  and therefore, in the illustrated embodiment, to a first end  21  of the worm housing  16 . 
     As an alternate to the illustrated embodiment, the drive motor  13  may be stationary and a flexible shaft may be arranged between the drive motor  13  and first gearing part  14 . Thus, the first gearing part  14  of the alternate embodiment (e.g. the worm housing  16  and the worm  17 ) can be pivoted relative to the stationary drive motor  13  about the joint  20 . 
     A guide element  23  also is mounted pivotably on the housing  19  of the respective gearing  11 . The guide element  23  is acted on by a spring  22  for the first gearing part  14  for the worm housing  16 . A first end  24  of the guide element  23  for the first gearing part  14  is mounted on the housing  19  so as to be pivotable about a second joint  25 . 
     The first gearing part  14 , which is mounted on the housing  19  of the respective gearing  11  to be pivot about the joint  20 , interacts with the guide element  23 , which is mounted on the housing  19  of the respective gearing  11  to pivot about the joint  25 . 
     The first gearing part  14  and the guide element  23  interact in such a way that the first gearing part  14  is guided on the guide element  23 . More particularly, when a force or moment on the first gearing part  14  exceeds a limit value, the guide element  23  can be moved automatically together with the first gearing part  14  counter to the spring force provided by the spring from the position shown in  FIG. 1 , in which the first and second gearing parts  14  and  15  are engaged, into the position shown in  FIG. 2 , in which the first and second gearing parts  14  and  15  of the respective gearing  11  are not engaged. 
     Similarly, when the force or moment acting on the first gearing part  14  of the respective gearing  11  falls below the defined limit value, the spring force of the spring element  22  automatically moves the guide element  23  together with the first gearing part  14  of the respective gearing  11  from the position shown in  FIG. 2  into the position shown in  FIG. 1 . 
     As noted above, the first gearing part  14  of the respective self-locking gearing  11  is mounted on the housing  19  of the gearing  11  so as to be pivotable about the first joint  20 , and first joint  20  is in proximity to the first end  21  of the first gearing part  14 . The guide element  23  is mounted on the housing  19  of the respective self-locking gearing  11  so as to be pivotable about the joint  25 , and the joint  25  is in proximity to the first end  24  of the guide element  23 . 
     The first gearing part  14  has a second end  27  opposite the first end  21  thereof, and the guide element  23  has a second end  28  opposite the first end  24  thereof. The second end  27  of the first gearing part  14  is guided on the second end  28  of the guide element  23 . More particularly, a guide pin  29  at the second end  27  of the first gearing part  14  is guided in a slotted guide  30  in proximity the second end  28  of the guide element  29 . As shown in  FIGS. 1 to 3 , the slotted guide  30  has two angularly aligned sections  32 ,  33 . 
     The guide pin  29  is guided in the upper, relatively flat section  32  of the slotted guide  30  when the gearing  11  or the gearing parts  14  and  15  thereof assume the first position ( FIG. 1 ). In contrast, the guide pin  29  is in the lower relatively steep section  33  of the slotted guide  30  when the two gearing parts  14  and  15  of the gearing  11  are moved relative to one another into the second position ( FIGS. 2 ,  3 ). 
     The slotted guide of the guide element  29  alternatively may be designed to run straight with a single obliquely running section that has no bend. 
     A stop  31  is associated with the second end  28  of the guide element  23  on which the first gearing part  14  of the respective self-locking gearing  11  is guided. In the illustrated embodiment, the lower relatively steep section  33  of the slotted guide  30  is closed so that the portion of the guide element  23  at the closed lower section  33  of the slotted guide  30  forms the stop  31 . 
     The stop  31  ensures that relative movement between the first gearing part  14  and the guide element  23  is restricted when the first guide element  23  is moved together with the first gearing part  14  of the respective self-locking gearing  11  counter to the force of the spring  22  from the first position to the second position. Thus, the first gearing part  14  is always in engagement with the guide element  23 . 
     In the illustrated embodiment, the stop  31  holds the guide pin  29  in the slotted guide  30  at all times, and accordingly the guide pin  29  of the first gearing part  14  can never be moved out of the slotted guide  13  of the guide element  23 . 
     The joint  20  could have a rotational angle delimiting means on the housing  19  instead of having the stop  31 . such a rotational angle delimiting means would restrict relative movement between the first gearing part  14  and the guide element  23 , and would ensure that the first gearing part  14  would always remain engaged with the guide element  23 . 
     The limit value for the force or moment acting on the first gearing part  14  is selected to exceed a maximum drive force of the respective drive motor  13  required for a regular opening or closing of the motor vehicle folding top. This takes place by coordinating the design of the spring  22  and the slotted guide  30 . Influential factors on the limit value are: the spring constant of the spring  22 ; and the angle of the slotted guide  30  with respect to the tangent of a pivoting radius about the joint  20 , regardless of whether the slotted guide  30  has two angled sections  32 ,  33  or a single straight-running slot. 
     The limit value for the force or moment acting on the first gearing part  14  preferably is selected so as not to be exceeded when a folding top that is to be opened or closed by the folding-top drive  10  abuts against a stop and no additional force is exerted on the folding top counter to the opening or closing movement. In this case, the two gearing parts  14 ,  15  of the respective self-locking gearing  11  then remain engaged and the drive motor  13  simply comes to a standstill but continues to be supplied with current, but eventually is deactivated to prevent overheating. 
     The limit value is exceeded only when an additional force is exerted on the folding top counter to the opening or closing movement. In this case, the two gearing parts  14 ,  15  of the respective self-locking gearing  11  then pass out of engagement. This occurs, for example, if the folding top to be opened or closed is actuated while the vehicle is driving and the folding top impacts against an obstruction, or if pressure inadvertently is applied to the moving folding top when the vehicle is at a standstill. 
     The spring  22  may be an extension spring. One end of the extension spring  22  engages on the housing  19  and the other end engages on the guide element  23 . Thus, the spring  22  pulls the guide element  23  into the position shown in  FIG. 1  with the two gearing parts  14  and  15  of the self-locking gearing  11  engaged. 
     The spring  22  may alternatively be a leg spring or some other spring. A leg spring may be arranged around the joint  25 . 
     The above-described design of the folding-top drive  10  simply and reliably provides overload protection for the folding-top drive  10  both in the event of exceeding the limit value for overload protection and also in the event of undershooting the limit value and automatically places the two gearing parts  14  and  15  of the respective self-locking gearing  11  into engagement or out of engagement. Thus, manual access to the folding-top drive  10  is not required to implement or cancel overload protection. 
     The guide element  23  may have an actuating element  26  that is used if the drive motor  13  fails. The actuating element  26  enables the guide element  23  to be moved manually together with the first gearing part  14  of the respective self-locking gearing  11  counter to the spring force of the spring element  22  from the position shown in  FIG. 1  into the second position shown in  FIG. 3 . The two gearing parts  14  and  15  of the self-locking gearing  11  are not engaged with one another in the second position, so as to eliminate the self-locking action of the gearing  11 . 
     The stop  31  restricts displacement of the guide element  23  relative to the first gearing part  14  during the displacement of the two gearing parts  14  and  15  of the respective self-locking gearing  11  relative to one another, including displacement that is initiated manually via the actuating element  26 . Thus, even if the self-locking of the gearing  11  is overcome manually, there is no risk of the first gearing part  14  passing out of engagement with the guide element  23 . 
     In the illustrated embodiment, the actuating element  26  is an actuating screw with an external thread that is guided in a recess of the guide element  23  that has an internal thread. 
     The actuating element or screw  26  is moved manually further in the direction of the housing  19  to manually overcome the self-locking of the gearing  11 . Thus, the actuating screw  26  is supported rotatably in the guide element  23  with an end of the actuating screw  26  engaged against the housing  19 . Thus, progressive rotation of the actuating screw  26  pivots the guide element  23  about the joint  25 . Accordingly, the guide pin  29  of the first gearing part  14  in the slotted guide  30  of the guide element  23  passes into the lower, relatively steeply running section of the slotted guide  30 , and the two gearing parts  14  and  15  of the respective self-locking gearing  11  are placed out of engagement to eliminate the self-locking action of said gearing  11 .
A folding-top drive has a motor ( 13) that transmits forces to a first gear ( 14) and then to a folding-top linkage via a second gear ( 15). The first gear ( 14) and a guide ( 23) are pivoted to a housing ( 19) and a spring ( 22) acts on the guide ( 23). The first gear is guided on the guide ( 23) so that the guide ( 23) can be moved automatically with the first gear ( 14) counter to the force of the spring ( 22) from a first position where the gears ( 14, 15) engage to a second position where the gears ( 14, 15) are disengaged if a force on the first gear exceeds a defined limit. Additionally, the spring ( 22) can move the guide ( 12) automatically with the first gear ( 14) from the second position to the first position if the force on the first gear falls below the defined limit.