Abstract:
A belt retractor comprises a belt spool rotatably mounted in a frame, a locking mechanism for the belt spool with at least one locking disc connected to the belt spool, and a torque limiting member arranged in a force flow path between the belt spool and the locking disc. The torque limiting member is constructed as a friction clutch comprising two clutch faces lying opposite each other and a friction member arranged between the clutch faces. The friction member is constructed as a flat spring part having shaped sections. The shaped sections lie at least partially against at least one of the clutch faces.

Description:
TECHNICAL FIELD 
     The invention relates to a friction clutch and to a belt retractor incorporating such a friction clutch. 
     BACKGROUND OF THE INVENTION 
     Known friction clutches have as friction member a metal disc which is arranged between two clutch plates and is provided on both sides with a clutch lining. The clutch lining ensures on the one hand a high frictional force to the clutch plates and on the other hand is designed such that it can deal with the stresses of pressure and temperature during operation. The friction member is hence always composed of several parts of different material. In order to ensure the necessary contact pressure and hence a sufficient frictional force, the clutch plates and the friction member are pressed together by means of a separate spring. 
     In known belt retractors, a belt force limitation is achieved in that in the force flow path between the belt spool and the locking disc a torsion rod is arranged, which is twisted on exceeding a predetermined torque and in so doing is plastically deformed. A controlled torque limitation is only possible here once and in one direction. 
     The object of the invention is to provide a friction clutch and a belt retractor with torque limitation, which operate in two directions in a force-limiting manner and at the same time are constructed simply and are able to be manufactured at a favorable cost. 
     BRIEF SUMMARY OF THE INVENTION 
     This is achieved in a friction clutch which comprises two clutch faces lying opposite each other and a friction member arranged between the clutch faces. The friction member is constructed as a flat spring part having shaped sections, the shaped sections lying at least partially against at least one of the clutch faces. Such a friction clutch is able to be produced particularly simply and at a favorable cost, because the necessary contact pressure between friction member and clutch faces is applied by the friction member itself, because this is constructed as a spring part. A separate pressure spring can therefore be dispensed with. The friction member can be produced for example in a simple manner as a sheet metal stamped member at low cost. The torque which is able to be transferred through the friction clutch is codetermined substantially by the elastic force of the spring part. 
     In a further embodiment of the invention, the shaped sections have a cross-section in the form of a circular arc as seen in a direction parallel to the clutch faces. Good elastic characteristics are achieved with a cross-section in the form of a circular arc. In tangential direction to the cross-section of the shaped sections in the form of a circular arc, the transferable friction force is, moreover, independent of direction. 
     Advantageously, the spring part is constructed as a corrugated band. Such a corrugated band is constructed in a simple manner and is suitable for arrangement between concentric clutch faces. Both the shaped sections, lying partially against the first clutch face and also the shaped sections of the spring part lying partially against the second clutch face then have a cross-section in the form of a circular arc. These measures also allow the friction member to consist of one single part. 
     It is likewise advantageous if the spring part is constructed as a cylinder with elastic shaped sections. Here, also, the friction member consists of a single part and is able to be mounted in a particularly simple manner between two concentric clutch faces, because the cylinder has a certain inherent stability and can thus be easily pressed in between the clutch faces. 
     In a further advantageous embodiment, the spring part is constructed as a plate with elastic shaped sections. Such an embodiment permits the arrangement between two clutch plates. The spring part likewise consists of a single part and can be produced in a particularly simple manner as a sheet metal stamped part. 
     According to the invention, there is also provided a belt retractor comprising a belt spool rotatably mounted in a frame, a locking mechanism for the belt spool with at least one locking disc connected to the belt spool, and a torque limiting member arranged in a force flow path between the belt spool and the locking disc, in which the torque limiting member is constructed as a friction clutch. Such a belt retractor acts in a force-limiting manner in two directions, because the friction clutch limits the torque in two directions. Hence, peak values of the belt force can be avoided both with the locking of the belt spool in a vehicle-sensitive or belt webbing-sensitive manner and also during operation of a belt tensioner engaging on the belt spool via the friction clutch. In contrast to conventional belt retractors with torsion rods as torque limiting member, the belt retractor according to the invention can also be re-used after a torque limitation has been occurred. 
     Advantageously, a first clutch plate is provided which is connected non-rotatably with the belt spool and presents a first clutch face and a second clutch plate is provided which is connected non-rotatably with the locking disc and presents a second clutch face, a plate being arranged between the first and second clutch faces which has elastic shaped sections. These measures make possible a space-saving construction of the belt retractor, because the friction clutch is constructed so as to be flat and extends in radial direction. The friction clutch can be arranged for example outside the frame of the belt retractor on the belt spool axle. 
     An advantageous embodiment is also produced in that a first cylinder face is provided which is connected non-rotatably with the belt spool and presents the first clutch face and a second cylinder face is provided which is connected non-rotatably with the locking disc and presents the second clutch face, a cylinder being arranged between the first and second clutch faces which has elastic shaped sections. In such a belt retractor, the spring part is stressed uniformly, because the same relative speed occurs everywhere between the concentric clutch faces. 
     In a further embodiment of the invention, provision is finally made that the first clutch face is formed by a side face of an extension of a belt spool axle non-rotatably connected to the belt spool, and the second clutch face is formed by an inner face of a hollow cylinder non-rotatably connected to the locking disc. To realize the friction clutch, such a belt retractor requires only a small effort as regards construction, because it is merely necessary to alter already existing parts, namely the belt spool axle and the locking disc, and also the spring part. The belt spool axle merely has to be provided with an extension, extending for example outside the frame, and a hollow cylinder, concentric to the belt spool axle, must be attached to the locking disc, The spring part is then pushed onto the extension of the belt spool axle and into the hollow cylinder of the locking disc. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a diagrammatic illustration to explain the functioning principle of the friction clutch according to the invention; 
     FIG. 2 shows a perspective illustration of an embodiment of the spring part for the friction clutch according to the invention; 
     FIG. 3 shows a perspective view, partially in section, of a first embodiment of the belt retractor according to the invention; 
     FIG. 4 shows a perspective detail view, partially in section, of the belt retractor of FIG. 3; and 
     FIG. 5 shows a perspective view, partially in section, of a second embodiment of the belt retractor according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The diagrammatic illustration of FIG. 1 shows two clutch faces  10  and  12  arranged concentrically to each other. On the right-hand side of FIG. 1, a band-shaped spring part  14  is arranged between these clutch faces  10  and  12 . The band-shaped spring part  14  has shaped sections which form first sections  16  lying partially against the first clutch face  10 , and second sections  18  lying partially against the second clutch face  12 . Both the first sections  16  and also the second sections  18  have a cross-section in the form of a circular arc as seen in a direction parallel to the clutch faces  10  and  12 . Through the shaped sections, in the form of a circular arc, of the spring part  14 , the latter assumes the form of a corrugated band. The spring part  14  is pressed in between the clutch faces  10  and  12 , so that through the spring action of the spring part  14  the first sections  16  exert a force onto the clutch face  10  and the second sections  18  exert a force onto the clutch face  12 . The forces exerted onto the clutch faces  10  and  12  are symbolized in FIG. 1 by arrows. The spring action of the spring part  14  between the clutch faces  10  and  12  is symbolized in the left-hand part of FIG. 1 by helical springs  20  which are arranged between the clutch faces  10  and  12  and exert a force onto them. Each corrugation of the spring part  14  behaves like a spring  20 , the force of which in the elastic range is proportional to the spring constant and to the deflection of the spring. The friction force which is able to be transferred between the spring part  14  and the clutch faces  10  or  12  results from the radial force indicated by arrows in FIG. 1, which is perpendicular to the clutch faces  10  or  12 , and the friction coefficient between the spring part  14  and the clutch face  10  or  12 . The torque which is able to be transferred as a whole through the friction clutch then follows from the radial dimensions of the clutch. On exceeding a predetermined torque between the spring part  14  and the clutch faces  10  or  12 , a transition takes place from static friction to sliding friction. Thereby, the transferable torque decreases and the force in a belt webbing connected for example to the clutch face  10  is limited. The friction clutch illustrated in FIG. 1 can limit the transferred torque here in two directions, clockwise and anticlockwise. 
     In FIG. 2, a spring part  22  is illustrated, which is constructed as a cylinder with elastic shaped sections  24 . The shaped sections  24  have an elongated shape, and several shaped sections  24  are arranged around the periphery of the cylinder  22  with their respective long side adjoining each other. The cylinder  22  has a certain inherent stability, so that it can easily be pressed in between two concentric clutch faces. 
     FIG. 3 shows a first embodiment of a belt retractor  30  according to the invention. The belt retractor  30  has a frame  32 , a belt spool  34  mounted in the frame  32  and a locking mechanism for the belt spool  34 . The locking mechanism consists of a locking disc  36  with a locking toothing  38 . To lock the belt spool  34  with respect to the frame  32 , a locking pawl  40  can be caused to engage the locking toothing  38  by a conventional driving mechanism, which is not illustrated, acting in a vehicle-sensitive and belt webbing sensitive manner. To limit the belt force, the belt retractor  30  is provided with a friction clutch  42  which is arranged in the force flow path between the belt spool  34  and the locking disc  36 . The friction clutch  42  consists of the locking disc  36 , a clutch plate  44  and a spring plate  46  arranged between the locking disc  36  and the clutch plate  44 . The spring plate  46  is constructed as a sheet metal stamped part and has elastic shaped sections  48 , which are arranged annularly in two rows around the mid-point of the circular plate  46 . A belt spool axle  50  is firmly connected to the belt spool  34  and extends through the frame  32 , the locking toothing  38 , the locking disc  36 , the spring plate  46  and the clutch plate  44 . Whereas the locking disc  36  is rotatably mounted on the belt spool axle  50 , the clutch plate is fastened non-rotatably, but so as to be displaceable longitudinally, on the belt spool axle  50 . For this, the belt spool axle  50  has several longitudinal grooves  52 , into which noses  54  of the clutch plate  44  engage. 
     As can be seen in FIG. 4, the clutch plate  44  is prestressed by a nut  56 , screwed onto the belt spool axle  50 , against the spring plate  46  and the locking disc  36 . Depending on how great the torque is to be which is transferable by the friction clutch  42 , the nut  56  is tightened to a greater or lesser extent. 
     In FIG. 4 it can also be seen that the shaped sections  48  of the spring plate  46  have a cross-section in the form of a circular arc as seen in a direction parallel to the clutch faces. The shaped sections  48  thereby have a good spring action, and the friction clutch  42  can act in two directions in a torque-limiting manner. The individual shaped sections  48  of the spring plate  46  are connected to each other by straight sections  58 . Whereas the shaped sections  48  form first sections of the spring plate  46  which lie against the first clutch face formed by the face of the clutch plate  44  facing the frame  32 , the straight sections  58  form second sections which lie against the second clutch face, which is formed by the face of the locking disc  36  facing away from the frame  32 . 
     In a vehicle impact, the locking pawl  40  is caused to engage in a vehicle-sensitive or belt webbing-sensitive manner into the locking toothing  38  of the locking disc  36 , so that the belt spool  34  is locked with respect to the frame  32  and no more belt webbing can be unwound from the belt spool  34 . When a vehicle occupant moves forward, the force acting in the belt webbing thereby increases. Owing to the force acting in the belt webbing, the friction clutch  42  must transfer a torque. If this torque which is to be transferred by the friction clutch  42  exceeds a predetermined value, the maximum frictional force transferable between the first sections, formed by the shaped sections  48 , and the first clutch face or the maximum frictional force transferable between the second sections formed by the straight sections  58  and the second clutch face, is also exceeded, so that the clutch disc  44  can rotate relative to the locking disc  36 . Thereby, also, belt webbing can be unwound again from the belt spool  34 , so that the belt force in the belt webbing is also reduced. If the maximum torque transferable by the friction clutch  42  is fallen below again, static friction occurs again between the spring plate  46  and the locking disc  36  and the clutch disc  44 , respectively, so that no more relative rotation of the clutch disc  44  with respect to the locking disc  36  is possible. 
     A second embodiment of a belt retractor  60  according to the invention is illustrated in FIG.  5 . In the same way as the belt retractor illustrated in FIGS. 3 and 4, the belt retractor  60  has a frame  62 , a belt spool  64 , a locking disc  66  with a locking toothing  68  and a locking pawl  70  which is able to be brought into engagement with the locking toothing  68 . The belt spool  64  has an axle which is firmly connected to the belt spool  64  and which has an extension  72  extending beyond the frame  62 . The locking disc  66  is fixedly provided with a hollow cylinder  74 , in which the extension  72  of the belt spool axle extends. A spring part consisting of a cylinder  76  with elastic shaped sections  78  is pressed in between the extension  72  of the belt spool axle and the hollow cylinder  74  of the locking disc  66 . The friction clutch for torque limiting, provided in the belt retractor  60 , is thereby formed by a side face of the extension  72 , the cylinder  76  with elastic shaped sections  78  and the inner face of the hollow cylinder  74 . With the arrangement of the friction clutch shown in FIG. 5, the relative speed between the clutch faces is identical over the entire clutch faces, so that the spring part consisting of the cylinder  76  with the shaped sections  78  is stressed uniformly throughout. The additional structural expenditure which is necessary to realize the friction clutch in the belt retractor  60  is small here, because the belt spool axle only has to be provided with the extension  72  and the locking disc  66  only has to be provided with the hollow cylinder  74 . Apart from the cylinder  76  as friction member, therefore no additional parts are necessary compared with a conventional belt retractor.