Abstract:
A slip clutch ( 19 ) for a hand-held power tool ( 1 ) that serves to uncouple a tool socket ( 2 ) from a driving motor ( 5 ) in the event of an overload is provided. An annular running surface ( 23 ) has cams ( 24 ) projecting radially inward. The slip clutch ( 19 ) has a set of pairs consisting of a preloaded coil spring ( 28 ) and a pawl ( 29 ). The coil spring ( 28 ) presses a head ( 38 ) of the pawl ( 29 ) in the radial direction outward onto the running surface ( 23 ). The pawl ( 29 ) has a sheet-metal strip ( 34 ) whose one section is bent into a hollow prism that is annularly closed, except for a gap, in order to form the head ( 38 ). An insert ( 35 ) is placed into the gap ( 41 ) so as to fill said gap ( 41 ).

Description:
BACKGROUND 
       [0001]    The present invention relates to a slip clutch for a hand-held power tool of the type known from German patent application DE 10 2009 046 475 A. 
       SUMMARY OF THE INVENTION 
       [0002]    The present invention provides a slip clutch for a hand-held power tool that serves to uncouple a tool socket from a driving motor in the event of an overload. An annular running surface has cams projecting radially inward. The slip clutch has a set of pairs, each consisting of a preloaded coil spring and a pawl. The coil spring presses a head of the pawl in the radial direction outward onto the running surface. The pawl has a sheet-metal strip whose one section is bent into a hollow prism that is annularly closed, except for a gap, in order to form the head. An insert is placed into the gap so as to fill said gap. 
         [0003]    The slip clutch has a running surface that, in its normal position, transfers a torque to the pawls by means of a non-positive and frictional fit. In the event of an overload, the pawls are deflected against the force of the coil springs until the pawls are no longer in contact with the running surface. As a result, the slip clutch opens the drive train of the hand-held power tool. The coil springs push the pawls back into the normal position. The loads on the pawls during the opening and the return into the normal position depend, among other things, on the rotational speed of the slip clutch, especially if resonances of the pawl are excited. 
         [0004]    The preferably loosely placed insert causes a damping of the resonances, thereby significantly increasing the service life of the pawls. A classic reinforcement, for example, due to thicker sheet-metal strips, does not entail any major improvement of the service life. 
         [0005]    One embodiment provides that the width of the gap is smaller than the thickness of the sheet-metal strip. The insert is clamped in the gap. 
         [0006]    The head can have a contact surface and a support surface, whereby the contact surface is in contact with the running surface, and a coil spring that is associated in pairs with the pawl is in contact with the support surface. The contact surface and the support surface are formed by the sheet-metal strip. One end of the support surface facing away from the contact surface is in contact with the insert. One embodiment provides that the end is clamped under pre-tensioning against the insert. The open end of the surface impinged upon by the coil spring is supported on the insert. The end can slide on the insert and can damp vibrations. 
         [0007]    One embodiment provides that the sheet-metal strip is bent to form a cylindrical bearing head, a flat lever arm and the prismatic head. The insert can be held with a positive fit in the cylindrical bearing head. This prevents the insert from sliding. 
         [0008]    One embodiment provides that the head is hollow. The hollow space occupies at least 25%, preferably at least 50%, of the volume of the head. 
         [0009]    One embodiment provides that the insert is made of plastic or titanium. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The description below explains the invention with reference to embodiments and figures given by way of example. The figures show the following: 
           [0011]      FIG. 1  a hammer drill; 
           [0012]      FIG. 2  a slip clutch; 
           [0013]      FIG. 3  the base body of a pawl; 
           [0014]      FIG. 4  a pawl; 
           [0015]      FIG. 5  a pawl; 
           [0016]      FIG. 6  a pawl. 
       
    
    
       [0017]    Unless otherwise indicated, identical or functionally equivalent elements are designated by the same reference numerals in the figures. 
       DETAILED DESCRIPTION 
       [0018]      FIG. 1  schematically shows a hammer drill  1  as an example of a chiseling hand-held power tool. The hammer drill  1  has a tool socket  2  into which the shank end  3  of a tool, e.g. a drill chisel  4 , can be inserted. A motor  5 —which drives a striking mechanism  6  and a driven shaft  7 —constitutes the primary drive of the hammer drill  1 . A user can hold the hammer drill  1  by means of a handle  3  and can start up the hammer drill  1  using a system switch  8 . During operation, the hammer drill  1  continuously rotates the drill chisel  4  around a working axis  9 , and in this process, it can hammer the drill chisel  4  into a substrate in the striking direction  10  along the working axis  9 . 
         [0019]    The striking mechanism  6  is, for example, a pneumatic striking mechanism  6 . An exciter  11  and a striker  12  are installed in a guide tube  13  of the striking mechanism  6  so as to be movable along the working axis  9 . The exciter  11  is coupled to the motor  5  via an eccentric  14  or a toggle element, and it is forced to execute a periodic, linear movement. An air cushion formed by a pneumatic chamber  15  between the exciter  11  and the striker  12  couples a movement of the striker  12  to the movement of the exciter  11 . The striker  12  can strike a rear end of the drill chisel  4  directly, or else indirectly via an essentially stationary intermediate striker  16  and can transmit part of its pulse to the drill chisel  4 . The striking mechanism  6  and preferably the other drive components are arranged inside a machine housing  17 . 
         [0020]    The drive shaft  7  is coupled to the motor  5  via a gear  18  and a slip clutch  19 . The slip clutch  19  is actuated when the torque that is present on the drive shaft  7  exceeds a threshold value, for example, when the drill chisel  4  is blocked. The slip clutch  19  shown by way of example has a hollow gear wheel  20  on the drive side and a bevel gear  21  on the driven side ( FIG. 2 ). The slip clutch  19  is inserted into the hollow gear wheel  20  and couples the hollow gear wheel  20  to a shaft  22  of the bevel gear  21 . 
         [0021]    The drive side  20  has an annular running surface  23  that is arranged coaxially to the shaft  22 . The running surface  23  has cams  24  projecting radially inwards. As seen in the direction of rotation  25 , the cams  24  have a front flank  27  as well as a rear flank  26 . The front flank  27  can be steeper than the rear flank  26 . 
         [0022]    The driven side  21  has several coil springs  28  and the same number of pawls  29 , which are arranged on a disc  30 . The disc  30  has several pockets  32  that are arranged at identical angular distances  31  and into each of which a coil spring  28  is placed. The coil springs  28  are oriented radially relative to the shaft  22 . Each of the coil springs  28  defines an axis  33  that runs perpendicularly through the shaft  22 . The forces exerted by the coil springs  28  act perpendicularly on the shaft  22 . 
         [0023]    Each of the coil springs  28  is paired with a pawl  29 . The pawls  29  each comprise a bent sheet-metal strip  34  and an insert  35 . The insert  35  is preferably made of a material that differs from that of the sheet-metal strip  34 . 
         [0024]    The sheet-metal strip  34  is bent to form a bearing head  36 , a lever arm  37  and a hollow head  38  ( FIG. 4 ). A middle section of the sheet-metal strip  34  forms the lever arm  37  and is configured to be essentially flat. An outer section of the sheet-metal strip  34 , directly adjacent to the middle section, is bent to form a hollow cylindrical outer contour that forms the bearing head  36 . The cylindrical outer contour encloses an angle of between 200° and 300°, for example, 270°. The other outer section of the sheet-metal strip  34  that is adjacent to the middle section is shaped to form the hollow head  38 . The head  38  has a prismatic, triangular shape. One side is closed off by the flat lever arm  37 . A second side  39 , which is connected to the lever arm  37 , is slanted by between 90° and 135° relative to the lever arm  37 . The third side  40 —referred to below as the support surface  40 —that adjoins the contact surface  39  is slanted by between 22° and 45° relative to the contact surface  39 . The support surface  40  and the lever arm  37  enclose an acute angle of between 22° and 45°. The support surface  40  is at a distance from the lever arm  37  by a gap  41 . The size  42  of the gap  41  is preferably not greater than the thickness  43  of the sheet-metal strip  34 , for example, between 25% and 100% of the thickness  43 . The edge  44  of the sheet-metal strip  34  facing the lever arm  37  can be bent open or rounded off. The hollow space  45  has a volume amounting to approximately 50% of the entire head  38 . 
         [0025]    The insert  35  is arranged in the gap  41  between the lever arm  37  and the support surface  40  ( FIG. 5 ). The thickness  46  of the insert  35  is preferably greater than the size  42  of the gap  41 , for example, 10% to 50% greater. The insert  35  is clamped to the lever arm  37  by the edge  44 . 
         [0026]    The insert  35  given by way of example is a flat platelet whose entire surface is in contact with the lever arm  37 . One end  47  of the insert  35  is thickened, for example, to form a cylindrical contour. The end  47  is inserted with a positive fit into the hollow space  45  that is surrounded by the bearing head  36 . This prevents the insert  35  from shifting along the lever arm  37 . The length  48  of the insert  35  preferably corresponds to the distance of the bearing head  36  from the gap  41 . The hollow head  38  is only negligibly filled by the insert  35  or not at all. The inertia is only slightly increased by the insert  35 . The width of the insert  35  is approximately the same as the width of the lever arm  37 . 
         [0027]    The pawl  29  is inserted into the disc  30  so that it can pivot around a pivot axis  49 . The bearing pedestal  50  is configured, for instance, as a cylindrical recess in the disc  30 . The bearing head  36  can be inserted with a positive fit into the recess along the pivot axis  49 . The pivot axis  49  is coaxial to the shaft  22 . The distance  51  between the pivot axis  49  and the shaft  22  is preferably greater than 75% of the inner radius of the running surface  23 . The bearing  50  is offset downstream from the shaft relative to the associated coil springs  28  arranged in pairs in the running direction  25 . In the embodiment given by way of example, the bearing  50  is arranged in the center between the associated coil spring  28  and the coil spring  28  downstream in the running direction  25 . 
         [0028]    The support surface  40  of the pawl  29  is on the axis  33  of the paired coil springs  28 . Preferably, the support surface  40  is oriented perpendicular to the axis  33 . The coil spring  28  is inserted so as to be preloaded between the support surface  40  and the pocket  32 . As a result, the pawl  29  is pressed with its contact surface  39  against the running surface  23 . The slant of the contact surface  39  is preferably approximately the same as the slant of the front flank  27  of the cam  24 . The running surface  23  can be rotated torque-free relative to the pawls  29  until the spring-loaded contact surface  39  comes to lie against the flank  26 . 
         [0029]    The support surface  40  has a projection  52  protruding in the direction of the coil spring  28 . The projection  52  is embossed into the sheet-metal strip  34 . With its last winding or with a ring formed by the last winding, the coil spring  28  surrounds the projection  52 . This prevents the coil spring  28  from sliding along the support surface  40  when the pawl is deflected. 
         [0030]    An applied torque leads to a deflection of the pawl  29  against the force of the coil spring  28 . If the torque exceeds a critical value, the pawl  29  is deflected by the entire height of the cam  24  and the running surface  23  can continue to rotate torque-free. The pawl  29  follows the rear flank  26  of the radially widening running surface  23 , until the pawl  29  strikes the next front flank  27 . In this process, the head  38  of the pawl  29  is dynamically loaded by the coil spring  28 . The gap  41  in the head  38  permits a displacement, whereby it is mainly the open edge  44  that shifts parallel to the lever arm  37 . The friction between the edge  44  and the insert  35  damps natural vibrations of the pawl  29 . Natural vibrations can be excited in the case of the uniformly rotating slip clutch  19  and can cause fatigue fractures. 
         [0031]    The insert  35  is preferably made of titanium. The coefficient of friction between titanium and iron (sheet metal) is advantageously high for the damping properties. An alternative is plastic, which has a lower coefficient of friction with iron, but which brings about a damping due to thermal losses in case of elastic deformations. 
         [0032]      FIG. 5  shows another embodiment of the insert  53 . The insert  53  is an elongated platelet that is bent at the lengthwise ends  54 . The length  56  of the insert  53  corresponds to the length of the lever arm  37 , as a result of which the two lengthwise ends  54  in the bearing head  36  and in the head  38  are in contact at the angle formed between the lever arm  37  and the contact surface  39 . The thickness  46  and the width of the insert  53  can be selected as in the preceding embodiment. 
         [0033]    The insert  53  can be put in place under pre-tensioning. The insert  53  is pre-tensioned perpendicularly to the lever arm  37 . This can be achieved, for instance, by an appropriate basic shape of the insert  53 . The insert  53  is curved over its length  45 , whereby the side facing the lever arm  37  is concave. 
         [0034]      FIG. 6  shows another embodiment of the insert  56 . The insert  56  runs from the bearing head  36  to the angle between the contact surface  39  and the support surface  40 . Thus, the insert  56  runs along the entire lever arm  37  and the entire contact surface  39 . The insert  56  is put in place with a positive fit and optionally, the insert  35  can be put in place under pre-tensioning. The insert  56  can be excited to natural vibrations under the dynamic loads of the slip clutch  19 . Due to the fact that the material is different from that of the sheet-metal strip  34  and in view of the slightly different dimensions, different natural vibrations are expected from the insert  56  and from the sheet-metal strip  34 . The positive fit or non-positive fit permits a frictional coupling to each other, thereby suppressing a vibration of the resonances. 
         [0035]    The surface  40  opposite from the support surface  40  is uncovered. The support surface  40 , especially the edge  44 , is freely movable relative to the insert  35  along the lever arm  37 . This embodiment of the insert  56  has the highest degree of filling of the hollow head  38 . The degree of filling is less than 50%. The degree of filling is the volume ratio of the section of the insert  35  that is situated in the head  38  relative to the defined hollow volume of the head  38  that is delineated by the sheet-metal strip  34 . 
         [0036]    The thickness  46  and the width of the inner  56  can be selected as in the preceding embodiment.