Abstract:
The invention relates to a device for limiting torque to be transferred, having first and second shafts having a longitudinal axis; at least first and second transfer elements arranged coaxial with the longitudinal axis of the first and second shafts, the first transfer element associated with the first shaft and the second transfer element associated with the second shaft; the two transfer elements having substantially complementary engaging faces; at least one elastic element configured to bias the engaging faces of the first and second transfer elements into engagement with each other; wherein an engaging face of the first transfer element has a first engageable contour, and an engaging face of the second transfer element has a second engageable contour, wherein first and second engageable contours each have first and second active angles and a first passive angle.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of International Application No. PCT/CH01/00517, filed on Aug. 23, 2001. 

   FIELD OF THE INVENTION 
   The invention relates to a device for limiting a torque, which is to be transferred. 
   BACKGROUND OF THE INVENTION 
   In the case of osteosynthetic treatments of fractures or the implantation of artificial joints, implants, such as bone plates or implant parts, are frequently fixed in a position at the bone by means of bone screws. Such bone screws frequently are self-drilling and self-cutting, and are screwed with a drill or a similar device into the bone. In so doing, the maximum torque, which is exerted on the bone screw and, accordingly, on the threaded connection between the bone screw and the bone, is to be limited to a particular value. 
   A device for limiting the maximum torque, which can be applied on a screw during a surgical intervention, is described in U.S. Pat. No. 6,132,435 to YOUNG. This known device comprises a driving shaft, which contains a disk-shaped segment with boreholes, disposed on a circle concentric with the axis of rotation, for accommodating balls, a driving shaft segment, which also contains a disk-shaped segment with spherical-shaped depressions, disposed complementary to the boreholes, and a housing, in which the driving shaft and the driving shaft segment are mounted. The balls are pressed by means of springs into the depressions in the disk-shaped segment of the driving shaft segment, the force of the springs being adjustable by means of loosening or tightening a threaded connection between the disk-shaped segment at the driving shaft and the housing. 
   It is a disadvantage of this known device that the same maximum torque is transferred in both directions of rotation. This is disadvantageous in surgical technique if a bone screw has become ingrown and, as a result, the torque for loosening it during removal has increased. 
   The invention is to provide a remedy here. It is an object of the invention to provide a device, which permits the transfer of a maximum torque, once set, only in a desired direction of rotation, while in the other direction of rotation, for example, for loosening screws, the torque, which can be transferred, may be larger or smaller. 
   SUMMARY OF THE INVENTION 
   The inventive device comprises first and second shafts having a longitudinal axis; at least first and second transfer elements arranged coaxial with the longitudinal axis of the first and second shafts, the two transfer elements having substantially complementary engaging faces; and at least one elastic element configured to bias the engaging faces of the first and second transfer elements into engagement with each other. The first transfer element may be associated with the first shaft, and the second transfer element may be associated with the second shaft. 
   In one embodiment, each engaging face of the first and second transfer elements may have at least one engageable contour, each contour having a first and second active angle and a first passive angle. The first and second active angles of at least one engageable contour may be substantially unequal. 
   In another embodiment, each engaging face of the first and second transfer elements may have at least one engageable contours, each contour having a first active angle and a first passive angle. The first active angle and first passive angle of at least one engageable contour may be substantially unequal. 
   In yet another embodiment, each engaging face of the first and second transfer elements may have at least one engageable contours, each contour having a first and second active angle and a first and second passive angle. The first and second active angles of at least one engageable contour may be substantially unequal. The first and second passive angles of at least one engageable contour may be substantially unequal. 
   The slopes of the teeth of the two contours, which may be passive during the transfer of torque in the desired direction of rotation, may enclose an angle β, which may be between 45° and 90°, with the diameter of the transfer elements, which may be perpendicular to the longitudinal axis. The angle β selected affects the maximum torque to be transferred (the loosening torque at β=90° is infinite) at a particular contacting force of the transfer elements. The slopes of the teeth of the two contours, which may be active during the transfer of torque in the desired direction of rotation, may enclose an angle α, which may be between 1° and 45°, with the diameter of the transfer elements, which may be perpendicular to the longitudinal axis. 
   In a different embodiment of the inventive device, the slopes of the teeth may be configured so that the angle α A  at the external diameter of the contours and the angle α I  at the internal diameter of the contours are unequal in size. As a result, there may be a linear contact over the whole width of the contours and there may be no point stress, which could have a negative effect on the device, especially on the wear properties. 
   In a further embodiment, the teeth of the two contours may be rounded off as they terminate in the respective end surface. The radii of these roundings-off may be between 0.1 mm and 1.0 mm. The wear behavior may be optimized by the choice of this radius. 
   Further advantageous embodiments of the invention are characterized in the dependent claims. 
   The advantages, achieved by the invention, are to be seen essentially therein that, due to the inventive device, a transfer of torque with a particular, maximum transferable torque may be enabled in only one direction of rotation, whereas the maximum transferable torque may assume a different value in the other direction of rotation. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Numerous features of the present invention are disclosed in the accompanying drawings, wherein similar reference characters denote similar elements throughout the several views, and wherein: 
       FIG. 1  is a longitudinal cross-sectional view of the inventive device. 
       FIG. 2  is a side view of the transfer elements of the inventive device. 
       FIG. 3  is a detail view of the contours of transfer elements of  FIG. 2 . 
       FIG. 4  is a perspective view of a transfer element in an alternative embodiment of the inventive device. 
       FIG. 5  is a longitudinal cross-sectional view of an alternative embodiment of the inventive device. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In  FIG. 1 , an embodiment of the inventive device is shown, which may permit a transfer of torque between a first shaft  1  and a second shaft  2 . The two shafts  1 ,  2  may be disposed coaxially, sharing a longitudinal axis  5 . The two shafts  1 ,  2  may also be coupled directly or indirectly with one of the transfer elements  3 ,  4 . The transfer of torque may take place over the mutually complementary contours  11 ,  12  mounted at mutually opposite end faces  8 , 9  of the transfer elements  3 , 4 . A first transfer element  3  may be mounted in the housing  10 , so that it may rotate about the longitudinal axis  5 , whereas a second transfer element  4  may be rotatably attached with the housing  10 . The front end  13  of the second shaft  2  may be fastened to the second end  24  of the housing  10 . 
   The housing  10  may have a hollow space  19 , which may be coaxial with the longitudinal axis  5 , a first end  25  and a second end  24 . The second end  24  may adjoin the second shaft  2 . The hollow space  19  may be open at the first end  25  of the housing  10 , so that the transfer elements  3 , 4  may be disposed in the hollow space  19 . An axial bearing  26 , which may be contacted, axially adjoining, by a first transfer element  3 , may be disposed in the hollow space  19  at the second end  24  of the housing  10 . 
   A borehole  27  may be drilled concentrically through each of the transfer elements  3 , 4 . The diameter of the borehole  27  in a second transfer element  4  may be larger than the diameter in a first transfer element  3 , so that the front segment  28  of the first shaft  1  may be passed through the second transfer element  4  and subsequently may be connected axially and detachably with the first transfer element  3  at the second end  24  of the housing  10 . This may be achieved, for example, by means of a pin  29 , which may be inserted in a transverse borehole  33  perpendicular to the longitudinal axis  5  through the front segment  28  of the shaft  1  and the first transfer element  3 . The second transfer element  4  may be mounted axially and may be rotationally movable on the middle segment  30  of the first shaft  1 , so that the end face  8  of the first transfer element  3  may lie opposite to the end face  9  of the second transfer element  4  and the contours  11 , 12 , disposed at the end faces  8 , 9  may engage one another. 
   The middle segment  30  of the first shaft  1  may have a larger diameter than the front segment  28 , so that a shoulder  31  may be formed at the first shaft  1  between the front and middle segment  30 . The shoulder  31  may also contact the first transfer element  3 . Therefore, the first shaft  1  may be held axially by the first transfer element  3 , while the transfer of torque may be brought about by the pin  29 . The transfer of torque between the housing  10 , which may be firmly connected with the second shaft  2 , and the second transfer element  4 , may take place over sliding blocks  18 . The sliding blocks  18  may be mounted peripherally at the second transfer element  4 , and may be axially disposed in the housing  10  in grooves  20  parallel to the longitudinal axis  5 . 
   The second transfer element  4  may be pressed axially against the first transfer element  3  by elastic means  7 , which may be constructed in the embodiment shown here as a compression spring disposed coaxially with the longitudinal axis  5 . The elastic means  7  may be clamped axially between the second transfer element  4  and tensioning means  6 , the tensioning means  6  in this embodiment consisting of a screw. The screw may be screwed into a complementary inner thread  32  at the first end  25  of the housing  10 . The tensioning means  6  may also be disposed concentrically to the longitudinal axis  5 , and a hole may be drilled through them coaxially, so that the middle segment  30  of the first shaft  1  may be passed through the tensioning means  6 . The middle segment  30  of the first shaft  1  may be mounted radially in the tensioning means  6  by means of a radial bearing  34 , which may be constructed, for example, as a ball bearing. The contacting force of the elastic means  7  may be adjusted by its pre-tension, and the pre-tension may be adjusted by axially turning the clamping means  6  in or out in the hollow space  19  of the housing  10 . The tensioning means  6  may be operated at the first end  25  of the housing  6 , with depressions  35  for accommodating a screwdriver. 
   In  FIG. 2 , transfer elements  3 , 4  of an embodiment of the inventive device are shown. The contours  11 , 12  at the transfer elements  3 , 4  may be constructed asymmetrically. The active slopes of the teeth  16  of the two contours  11 , 12  may enclose an angle α with the diameter of the transfer elements  3 ,  4 , which is approximately 20° in the and which may be is perpendicular to the longitudinal axis  5 . The passive slopes  15  of the teeth may enclose an angle β, which is approximately 45° in the embodiment shown here, with the diameter of the transfer elements  3 , 4 . 
     FIG. 3  shows enlargement of the contours  11 , 12 , as shown in  FIG. 2 . The tips of the teeth  17  in this embodiment are rounded off with a radius R. 
   An alternative embodiment of a transfer element is shown in  FIG. 4 . The tips of the teeth of the contours  11  may rest on the circular end surface  8  of a first transfer element  3 . This circular end surface  8  may have an outer diameter D A  and an inner diameter D I . The teeth  17  may be constructed asymmetrically. The slopes  16  of the teeth, which may be active in the desired direction of rotation for transferring torque, may have a varying angle α between the outer diameter D A  and the inner diameter D I  of the end surface  8 . The angle α A  at the outer diameter D A  may be smaller than the angle α I  at the inner diameter D I . The slopes  15  of the teeth, which may be passive in the desired direction of rotation, may enclose an angle β of 90° with the end surface  8 . The contours  12  at a second transfer element  4  (not shown) may be complementary to the contours  11  at the first transfer element  3 . The tips of the teeth  17  may also be rounded off, as shown in  FIG. 3 . 
   An alternative embodiment of the inventive device shown in  FIG. 5 , comprises a driving first shaft  1 , which may be firmly connected with a first transfer element  3 . As compared to the embodiment shown in  FIG. 1 , the first shaft  1  has a different second end  24  of the housing  10 , where the torque applied by the first shaft  1  may be transferred to a second shaft  2  and a tool or instrument (not shown). The second shaft  2  may be fastened detachably to the second end  24  of the housing  10  by means of a locking mechanism  23 . The locking mechanism  23  may be constructed of a slide block. The locking mechanism  23  may be operated by means of an operating handle  36 , which, axially displaceable with respect to the longitudinal axis  5 , may be connected with the second end  24  of the housing  10 . A bolt  22 , which may be movable perpendicularly to the longitudinal axis  5 , may be fastened in the operating handle  36  perpendicularly to the longitudinal axis  5 . Furthermore, the bolt  22 , which also may be movable in a notch  21  extending at an angle to the longitudinal axis  5 , may be connected with the second end  24  of the housing  10 . Thus, through an axial movement of the operating handle  36  in the direction of the first end  25  of the housing  10 , bolt  22  may be disengaged from the notch  21  due to the inclined position of the latter. For connecting the second shaft  2  with the second end  24  of the housing  10 , the second shaft  2  may be equipped at its front end  13  with a peripheral groove  39 , which may be disposed at the front end  13  of the shaft  2 . In the engaged state, the bolt  22  may engage the groove  39 , as a result of which the second shaft  2  may be fixed axially in the housing  10 . For transferring the torque from the housing  10  to the second shaft  2 , a cam  41  may be mounted in the borehole  40 , which may accommodate the second shaft  2  and may be coaxial with the longitudinal axis  5 . The cam  41  may be brought into engagement with a groove or a flattening  42  at the second shaft  2  and may, at least in part, protrude radially into the borehole  40 . Furthermore, when the operating handle  36  is in the inoperative state, it may be held in the engaged position of the locking mechanism  23  by a spring  38 , which may exert a compressive force from the housing  10  in the direction of the second shaft  2 . For this purpose, a slide  37  may be inserted between the end of the spring  38 , acting on the bolt  22 . 
   While the invention has been shown and described herein with reference to particular embodiments, it is to be understood that the various additions, substitutions, or modifications of form, structure, arrangement, proportions, materials, and components and otherwise, used in the practice and which are particularly adapted to specific environments and operative requirements, may be made to the described embodiments without departing from the spirit and scope of the present invention. Accordingly, it should be understood that the embodiments disclosed herein are merely illustrative of the principles of the invention. Various other modifications may be made by those skilled in the art which will embody the principles of the invention and fall within the spirit and the scope thereof.