Abstract:
A tool clamping fixture includes at least one clamping unit, at least one operating unit, and at least one return damping unit. The at least one clamping unit is configured to fixedly clamp a machining tool in an axial direction. The at least one operating unit is configured to actuate the at least one clamping unit. The at least one return damping unit is at least configured to damp a return motion of the at least one operating unit. The at least one return damping unit is also configured to generate friction torques of different magnitudes in opposite directions of motion of the at least one operating unit.

Description:
This application is a 35 U.S.C. §371 National Stage Application of PCT/EP2011/074108, filed on Dec. 27, 2011, which claims the benefit of priority to Serial No. DE 10 2011 003 098.0, filed on Jan. 25, 2011 in Germany, the disclosures of which are incorporated herein by reference in their entirety. 
     BACKGROUND 
     Tool clamping fixtures, in particular oscillation tool clamping fixtures, which have a clamping unit for clamping a machining tool in an axial direction, a control unit for actuating the clamping unit, and a return damping unit, which is provided to damp a return motion of the control unit, are already known. 
     SUMMARY 
     The disclosure is based on a tool clamping fixture, in particular an oscillation tool clamping fixture, having at least one clamping unit for clamping a machining tool in an axial direction, having at least one control unit for actuating the clamping unit, and having at least one return damping unit, which is provided to at least damp a return motion of the control unit. 
     It is proposed that the return damping unit is provided to generate friction torques of different magnitude in oppositely directed motional directions of the control unit. In this context, the term “provided” is intended to define specially equipped and/or specially configured. By a “clamping unit” should here be understood, in particular, a unit which secures a machining tool by means of a form closure and/or by means of a force closure along the axial direction, in particular to a tool holder of a portable machine tool. Preferably, in a clamping mode of the clamping unit, a clamping force acts along the axial direction on the machining tool. The term “axial direction” is here intended to define, in particular, a direction which runs preferably at least substantially parallel to a pivot axis and/or rotation axis of a drive shaft and/or spindle of a portable machine tool, which drive shaft and/or spindle is/are provided to drive the machining tool. By “substantially parallel” should here be understood, in particular, an alignment of a direction relative to a reference direction, in particular in one plane, wherein the direction has in relation to the reference direction a deviation of, in particular, less than 8°, advantageously less than 5°, and particularly advantageously less than 2°. The term “control unit” is here intended to define, in particular, a unit which has at least one control element, that can be actuated directly by an operator, and which is provided to influence and/or alter, by an actuating action and/or by the inputting of parameters, a process and/or a state of a unit coupled to the control unit. 
     By a “return damping unit” should here be understood, in particular, a unit which is specifically provided to convert one energy form, in particular a kinetic energy of a control lever of the control unit, in at least one operating state into another energy form, in particular a thermal energy, and in particular is provided to generate a friction torque which is greater than a friction torque which is generated upon a motion decoupled from the return damping unit. In particular, the friction torque generated by means of the return damping unit is at least more than twice as large as a friction torque generated in the motion decoupled from the return damping unit, preferably at least more than four times greater, and particularly preferably more than six times greater. Preferably, a magnitude of the friction torque generated by means of the return damping unit is direction-dependent. Preferably, the return damping unit generates in at least one operating state a friction torque which, in addition to a bearing-conditioned friction torque, acts on the control lever. Preferably, the return damping unit is provided to generate in a clamping direction, upon a motion of the control unit, in particular upon a rotary motion about at least one axis, a friction torque which, upon a motion of the control unit, in particular upon a rotary motion about at least one axis, is generated in a release direction. In particular, the friction torque which is generated in the clamping direction upon a motion of the control unit, in particular upon a rotary motion about at least one axis, is at least more than twice as large as a friction torque generated in the release direction upon the motion of the control unit, in particular upon a rotary motion about at least one axis, preferably at least more than four times greater and particularly preferably more than six times greater. By a “clamping direction” should here be understood, in particular, a direction in which the control unit is movable for actuation of the clamping unit, wherein, as a consequence of the movement of the control unit, a clamping force for clamping of the machining tool is generated by means of the clamping unit. By a “release direction” should here be understood, in particular, a direction in which the control unit can be moved for actuation of the clamping unit, wherein, as a consequence of the movement of the control unit, a clamping force generated by the clamping unit is released. By means of the disclosed configuration of the tool clamping fixture, a situation in which the control unit snaps back into a starting position after a clamping operation can advantageously be avoided. Good ease of operation can thus advantageously be achieved. 
     Advantageously, the return damping unit has at least one return damping element, which is disposed on a transmission element of the control unit. Preferably, the return damping element is configured as a spring element. In an alternative configuration, the return damping element is configured as a wedge element. In a further alternative configuration of the disclosed tool clamping fixture, the return damping element is preferably configured as a bearing element. It is also conceivable, however, for the return damping element to have a different configuration which appears sensible to a person skilled in the art. The term “transmission element” is here intended to define, in particular, an element which is provided to transmit forces and/or torques from a control lever of the control unit to the return damping element and/or to transmit forces and/or torques from a control lever of the control unit to the clamping unit for actuation of this same. Preferably, the transmission unit is connected in a rotationally fixed manner to the control lever of the control unit. By “connected in a rotationally fixed manner” should here be understood, in particular, a connection which invariably transmits a torque and/or a rotary motion. A compact return damping unit can advantageously be achieved. 
     It is further proposed that the return damping element is disposed along a peripheral direction on the transmission element of the control unit. Preferably, the return damping element has an extent which runs along the peripheral direction. The transmission element is thus preferably enclosed along the peripheral direction by the return damping element. Particularly preferably, the return damping element entwines the transmission element along an angular range greater than 60°, preferably greater than 180°, and particularly preferably greater than 350° along the peripheral direction. By a “peripheral direction” should here be understood a direction which runs at least substantially in a plane extending at least substantially perpendicular to the axial direction. It is also conceivable, however, for the return damping element to be disposed at a different location which appears sensible to a person skilled in the art. Preferably, the transmission element has a fine surface structure. By a “fine surface structure” should here be understood, in particular, a surface structure which has an average roughness R a  less than 1 μm. A high efficiency of the return damping element for damping the return motion of the control unit can advantageously be achieved. 
     Preferably, the return damping element is configured as a leg spring. By a “leg spring” should here be understood, in particular, an elastic component which preferably has at least one leg, extending tangentially to at least one coil and configured, in particular, as a bending beam, and which preferably in at least one operating state is subjected to torsional stress about at least one axis, in particular an axis running at least substantially parallel to the axial direction. The leg spring preferably has a plurality of coils, which entwine the transmission element along the peripheral direction. Particularly preferably, the leg spring has at least two legs, which are preferably supported in receiving elements of a housing of a portable machine tool. In particular, at least one leg of the leg spring, at least in an operating state, transmits forces and/or torques to the housing. Preferably, at least one leg of the leg spring is provided for pretensioning of the leg spring. A return damping element can be achieved in a constructively simple manner, in particular a return damping element which can advantageously generate friction torques of different magnitude in oppositely directed motional directions of the control unit. 
     It is additionally proposed that the return damping element is pivotably mounted. Preferably, the return damping element is mounted pivotably relative to the transmission element. A pivot axis of the return damping element here runs preferably, in a mounted state, at least substantially parallel to the axial direction. It is also conceivable, however, for the pivot axis of the return damping element to run along a different direction which appears sensible to a person skilled in the art. Preferably, the return damping element is pivoted about the pivot axis as a consequence of a motion of the transmission element. The return damping element preferably acts directly and/or indirectly on the transmission element in order to damp, in particular as a consequence of a friction torque, a return motion of the control unit. A direction-dependent action of the return damping element can be achieved in a constructively simple manner. 
     Advantageously, the return damping unit has at least one spring element, which is provided to subject the return damping element to a spring force in at least one direction. Preferably, the spring element is configured as a tension spring. It is also conceivable, however, for the spring element to have a different configuration which appears sensible to a person skilled in the art, such as, for example, as a compression spring, as a cup spring, as a volute spring, etc. The return damping element is preferably subjected to a spring force of the spring element along a direction running at least substantially perpendicular to the axial direction. Particularly preferably, the return damping element is subjected to a spring force of the spring element along the clamping direction. Advantageously, a spring force which is provided to move the return damping element into a return damping position can be generated. 
     It is further proposed that the return damping unit has at least one stop element, which is provided to limit a motion of the return damping element in at least one direction. By a “stop element” should here be understood, in particular, an element which is provided to limit a motional path of a further element along at least one motional direction. Preferably, the stop element is provided to limit a motional path of the return damping element as a consequence of a motion of the return damping element induced by a spring force of the spring element and/or as a consequence of a motion of the return damping element induced by a motion of the control unit in the clamping direction. It can thus advantageously be ensured that the return damping element, upon a return motion of the control unit, is secured in a position necessary for damping the return motion of the control unit. 
     Preferably, the return damping element is configured as a freewheeling roller bearing, which is provided to prevent a rotary motion at least in one rotational direction. By a “freewheeling roller bearing” should here be understood, in particular, a roller bearing which has at least one clamping body provided to prevent and/or block a rotary motion of the roller bearing in at least one rotational direction. Particularly preferably, the freewheeling slide bearing is provided to prevent and/or block a rotary motion in the clamping direction. Preferably, the transmission element bears in a mounted state, along the axial direction, against an inner race of the roller bearing. A large friction torque for damping a return motion of the control unit can advantageously be achieved. 
     The disclosure is further based on a portable machine tool, in particular a portable machine tool having an oscillatingly drivable spindle, having at least one disclosed tool clamping fixture. By a “portable machine tool” should here be understood, in particular, a machine tool, in particular a hand-operated machine tool, which can be transported without a transport machine by an operator. The portable machine tool has, in particular, a weight which is less than 40 kg, preferably less than 10 kg, and particularly preferably less than 5 kg. Good ease of operation for an operator of the machine tool can advantageously be achieved. 
     The disclosed tool clamping fixture is here not intended to be limited to the above-described application and embodiment. In particular, the disclosed tool clamping fixture, in order to effect a working method described herein, can have a number of individual elements, components and units which differs from a number stated herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantages emerge from the following drawing description. In the drawing, illustrative embodiments of the disclosure are represented. The drawing and the description contain numerous features in combination. The person skilled in the art will expediently also view the features individually and combine them into sensible further combinations, wherein: 
         FIG. 1  shows a machine tool having a tool clamping fixture in a schematic representation, 
         FIG. 2  shows a detailed view of the tool clamping fixture in a schematic representation, 
         FIG. 3  shows a further detailed view of the tool clamping fixture in a schematic representation, 
         FIG. 4  shows a detailed view of an alternative tool clamping fixture in a schematic representation, 
         FIG. 5  shows a further detailed view of the alternative tool clamping fixture from  FIG. 4  in a damping mode of a return damping unit of the alternative tool clamping fixture in a schematic representation, 
         FIG. 6  shows a detailed view of a further alternative tool clamping fixture in a schematic representation, and 
         FIG. 7  shows a further detailed view of the further alternative tool clamping fixture from  FIG. 6  in a damping mode of a return damping unit of the further alternative tool clamping fixture in a schematic representation. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an electrically operated portable machine tool  42   a  having a tool clamping fixture  10   a . The portable machine tool  42   a  comprises a machine tool housing  44   a , which encloses an electric motor unit  46   a  and a gearbox unit  48   a  of the portable machine tool  42   a . The machine tool housing  44   a  here comprises two housing half shells  50   a ,  52   a , which are detachably connected to each other along a plane running through an axial direction  16   a . It is also conceivable, however, for the machine tool housing  44   a  to have two or more cup-shaped housing parts, which can be detachably connected to each other. The axial direction  16   a  runs along and/or parallel to a rotation axis  54   a  of a hollow shaft (not represented in detail here) of the portable machine tool  42   a , which hollow shaft is configured as a spindle. The hollow shaft is provided to oscillatingly drive, in a mounted state, a machining tool  14   a . An oscillating drive of the machining tool  14   a  is here effected in a manner which is already known to a person skilled in the art, such as, for example, by means of a journal (not represented in detail here) of the gearbox unit  48   a , which journal is disposed eccentrically on a drive shaft of the electric motor unit  46   a  and, by means of a link and a vibrating sleeve (not represented in detail here) of the gearbox unit  48   a , drives the hollow shaft during operation of the portable machine tool  42   a . The hollow shaft configured as a spindle can thus be driven oscillatingly. For the metal cutting of workpieces, the machining tool  14   a  can be fastened to a tool holder  56   a  of the portable machine tool  42   a . The tool holder  56   a  is connected in a rotationally fixed manner to the hollow shaft by means of a positive and/or non-positive connection. It is also conceivable, however, for the tool holder  56   a  to be configured in one piece with the hollow shaft. A pivot motion of the hollow shaft can thus be transmitted to the tool holder  56   a.    
       FIG. 2  shows a detailed view of the tool clamping fixture  10   a . The tool clamping fixture  10   a  comprises a clamping unit  12   a  for clamping the machining tool  14   a  in the axial direction  16   a  and a control unit  18   a  for actuating the clamping unit  12   a . The control unit  18   a  has a control lever  58   a . The control lever  58   a  is disposed on a side  60   a  of the machine tool housing  44   a  facing away from the tool holder  56   a . In addition, the control lever  58   a  is mounted rotatably about the rotation axis  54   a  in the machine tool housing  44   a . For actuation of the clamping unit  12   a  in order to generate and/or release a clamping force generated by means of the clamping unit  12   a , the control lever  58   a  can be actuated by an operator. 
     The tool clamping fixture  10   a  further comprises a return damping unit  20   a , which is provided to damp a return motion of the control unit  18   a . The return damping unit  20   a  is here provided to generate friction torques of different magnitude in oppositely directed motional directions  22   a ,  24   a  of the control unit  18   a . Viewed along the axial direction  16   a , the return damping unit  20   a  is arranged between the control lever  58   a  of the control unit  18   a  and the tool holder  56   a . In addition, the return damping unit  20   a  has a return damping element  26   a , which is disposed on a transmission unit  28   a  of the control unit  18   a  ( FIG. 3 ). Viewed in a plane running perpendicular to the axial direction  16   a , the transmission element  28   a  is of circular configuration. In addition, the transmission element  28   a  is configured as a cover cap  62   a . The cover cap  62   a  is of cup-shaped configuration. It is also conceivable, however, for the transmission element  28   a  to have a different configuration which appears sensible to a person skilled in the art. The cover cap  62   a  is here configured rotationally fixed to the control lever  58   a  by means of a screw connection. It is also conceivable, however, for the cover cap  62   a  to be configured rotationally fixed to the control lever  58   a  by means of a different type of connection which appears sensible to a person skilled in the art, such as, for example, by means of an integral and/or positive connection. 
     The return damping element  26   a  is disposed along a peripheral direction  30   a  on the transmission element  28   a , configured as a cover cap  62   a , of the control unit  18   a . The peripheral direction  30   a  runs in a plane extending perpendicular to the axial direction  16   a . The return damping element  26   a  surrounds the transmission element  28   a  along the peripheral direction  30   a  along an angular range of 360°. The return damping element  26   a  is here configured as a leg spring  32   a . The leg spring  32   a  has a plurality of coils  64   a , which entwine the transmission element  28   a , configured as a cover cap  62   a , in an operating state along the peripheral direction  30   a . The individual coils  64   a  of the leg spring  32   a  entwine the transmission element  28   a  respectively through an angle greater than 300°. The leg spring  32   a  further has a first leg  66   a  and a second leg  68   a . The first leg  66   a  and the second leg  68   a  are respectively disposed with one end in receiving elements  70   a ,  72   a  of the machine tool housing  44   a . The first leg  66   a  and the second leg  68   a  are thus connected to the machine tool housing  44   a  ( FIG. 3 ). The first leg  66   a  extends tangentially away from the coils  64   a . One end of the first leg  66   a  is bent over and ends in one of the receiving elements  70   a ,  72   a . The first leg  66   a  is provided to transmit forces and/or torques to the machine tool housing  44   a . The second leg  68   a  is bent in a tangential region  74   a  and runs, starting from the bend, rectilinearly in the direction of one of the receiving elements  70   a ,  72   a . The second leg  68   a  is provided to pretension the leg spring  32   a.    
     When the control lever  58   a  of the control unit  18   a  is actuated by an operator in the motional direction  22   a  corresponding to a release direction, the control lever  58   a  is rotated about the rotation axis  54   a . The transmission element  28   a  configured as a cover cap  62   a , as a consequence of the rotationally fixed connection to the control lever  58   a , is jointly rotated. As a consequence of the connection of the first leg  66   a  and second leg  68   a  to the machine tool housing  44   a , the leg spring  32   a  disposed along the peripheral direction  30   a  on the cover cap  62   a  is secured against twisting about the rotation axis  54   a . Upon a motion of the control lever  58   a  along the motional direction  22   a  corresponding to the release direction, as a consequence of a friction between a side of the leg spring  32   a  which is facing toward the transmission element  28   a  configured as a cover cap  62   a  and a side of the transmission element  28   a  which is facing toward the leg spring  32   a , a force acts along a course of the coils  64   a  of the leg spring  32   a  or a torque acts about the rotation axis  54   a . By means of the force resulting from the friction or by means of the torque acting about the rotation axis  54   a , the leg spring  32   a  is hereupon expanded, as a consequence of the twistproof connection of the leg spring  32   a  to the machine tool housing  44   a , along a direction running perpendicular to the axial direction  16   a . Between the leg spring  32   a  and the transmission element  28   a  configured as a cover cap  62   a , a small friction torque thus acts in that motional direction  22   a  of the control lever  58   a  which corresponds to the release direction. 
     The control lever  58   a  is acted on by means of a spring force of a spring element (not represented in detail here) of the clamping unit  12   a , which spring force, after a force effect of the operator has been neutralized following a movement of the control lever  58   a  in the motional direction  22   a  corresponding to the release direction, moves the control lever  58   a  via a mechanism (not represented in detail here) of the control unit  18   a  in a motional direction  24   a  corresponding to a clamping direction. The mechanism of the control unit  18   a  is provided to convert a rotary motion of the control lever  58   a  into a pivot motion and/or a translatory motion of a clamping element (not represented in detail here) of the clamping unit  12   a  for clamping the machining tool  14   a . The clamping element is here captively disposed in the hollow shaft of the portable machine tool  42   a . As a consequence of the spring force, the control lever  58   a  is rotated about the rotation axis  54   a . By virtue of the rotationally fixed connection to the control lever  58   a , the transmission element  28   a , configured as a cover cap  62   a , is jointly rotated about the rotation axis  54   a . Upon a motion of the control lever  58   a  along the motional direction  24   a  corresponding to the clamping direction, as a consequence of a friction between that side of the leg spring  32   a  which is facing toward the transmission element  28   a  configured as a cover cap  62   a  and that side of the transmission element  28   a  which is facing toward the leg spring  32   a , a force acts along a course of the coils  64   a  of the leg spring  32   a  or a torque acts about the rotation axis  54   a . By means of the force resulting from the friction or by means of the torque acting about the rotation axis  54   a , the leg spring  32   a  is hereupon contracted, as a consequence of the twistproof connection of the leg spring  32   a  to the machine tool housing  44   a , along a direction running perpendicular to the axial direction  16   a . Between the leg spring  32   a  and the transmission element  28   a  configured as a cover cap  62   a , in that motional direction  24   a  of the control lever  58   a  which corresponds to the clamping direction, is generated a friction torque which is many times greater than a friction torque which is generated upon a motion of the control lever  58   a  along the motional direction  22   a  corresponding to the release direction. The return motion of the control lever  58   a  of the control unit  18   a  along the motional direction  24   a  corresponding to the clamping direction is hereby damped. The acting friction torque between the leg spring  32   a  and the transmission element  28   a  is dependent on a wrap angle of the leg spring  32   a  and/or on a number of coils of the leg spring  32   a  and/or a friction coefficient between the leg spring  32   a  and the transmission element  28   a.    
     In  FIGS. 4 to 7 , two alternative illustrative embodiments are represented. Substantially constant components, features and functions are basically denoted by the same reference symbols. In order to differentiate between the illustrative embodiments, the letters a to c are added to the reference symbols of the illustrative embodiments. The following description is substantially confined to the differences from the first illustrative embodiment in  FIGS. 1 to 3 , while, with respect to constant components, features and functions, reference can be made to the description of the first illustrative embodiment in  FIGS. 1 to 3 . 
       FIG. 4  shows a detailed view of an alternative tool clamping fixture  10   b . The tool clamping fixture  10   b  is disposed in a portable machine tool (not represented in detail here), which has a structure analogous to the portable machine tool  42   a  from  FIG. 1 . In addition, the tool clamping fixture  10   b  comprises a clamping unit  12   b  for clamping a machining tool  14   b  in an axial direction  16   b  and a control unit  18   b  for actuating the clamping unit  12   b . Furthermore, the tool clamping fixture  10   b  has a return damping unit  20   b , which is provided to damp a return motion of the control unit  18   b . The return damping unit  20   b  is further provided to generate friction torques of different magnitude in oppositely directed motional directions  22   b ,  24   b  of the control unit  18   b . The return damping unit  20   b  here comprises a return damping element  26   b , which is disposed on a transmission element  28   b  of the control unit  18   b . The return damping element  26   b  is pivotably mounted. A pivot axis  76   b  of the return damping element  26   b  runs at least substantially parallel to the axial direction  16   b . The return damping element  26   b  is configured as a wedge element  78   b . The wedge element  78   b  extends along a direction running at least substantially perpendicular to the axial direction  16   b.    
     Furthermore, the return damping element  20   b  has a spring element  34   b , which is provided to subject the return damping element  26   b  to a spring force in a direction  36   b . The spring element  34   b  is configured as a tension spring  80   b . One end of the tension spring  80   b  is connected to the return damping element  26   b . A further end of the tension spring  80   b  is connected to a machine tool housing  44   b  of the portable machine tool. The return damping element  20   b  further comprises a stop element  38   b , which is provided to limit a motion of the return damping element  26   b  in the direction  36   b . The stop element  38   b  is configured in one piece with the machine tool housing  44   b . It is also conceivable, however, for the stop element  38   b  to be configured separate from the machine tool housing  44   b.    
     When a control lever (not represented in detail here) of the control unit  18   a , which control lever is connected in a rotationally fixed manner to the transmission element  28   b , is actuated by an operator in a motional direction  22   b  corresponding to a release direction, the transmission element  28   b  is rotated about a rotation axis  54   b . The return damping element  26   b  configured as a wedge element  78   b  is hereupon moved away from the stop element  38   b , as a consequence of a friction between the transmission element  28   b  and the wedge element  78   b , counter to the spring force of the tension spring  80   b  of the return damping unit  20   b . A small friction torque thus acts between the wedge element  78   b  and the transmission element  28   b  in that motional direction  22   b  of the control lever of the control unit  18   b  which corresponds to the release direction. The tension spring  80   b  is provided to prevent a loss of contact between the transmission element  28   b  and the wedge element  78   b.    
     The control lever is acted on by means of a spring force of a spring element (not represented in detail here) of the clamping unit  12   b , which spring force, after a force effect of the operator has been neutralized following a movement of the control lever in the motional direction  22   b  corresponding to the release direction, moves the control lever via a mechanism (not represented in detail here) of the control unit  18   b  in a motional direction  24   a  corresponding to a clamping direction. The mechanism of the control unit  18   b  is provided to convert a rotary motion of the control lever into a pivot motion and/or a translatory motion of a clamping element (not represented in detail here) of the clamping unit  12   b  for clamping the machining tool  14   b . As a consequence of the spring force of the spring element of the clamping unit  12   b , the control lever is rotated about the rotation axis  54   b . By virtue of the rotationally fixed connection to the control lever, the transmission element  28   b  is jointly rotated about the rotation axis  54   b  ( FIG. 5 ). Upon a motion of the transmission element  28   b  along the motional direction  24   b  corresponding to the clamping direction, the wedge element  78   b , as a consequence of a friction between the transmission element  28   b  and the wedge element  78   b , is pivoted about the pivot axis  76   b  in the direction of the stop element  38   b  until the wedge element  78   b  bears against the stop element  38   b . The pivot motion of the wedge element  78   b  in the direction of the stop element  38   b  is supported by means of the spring force of the tension spring  80   b . Upon a bearing contact of the wedge element  78   b  against the stop element  38   b  and a further motion of the transmission element  28   b  in the motional direction  24   b  corresponding to the clamping direction, a wedge effect between the wedge element  78   b  and the transmission element  28   b  is produced. Starting from the wedge element  78   b , a large normal force here acts on the transmission element  28   b . As a consequence of the motion of the transmission element  28   b  in the motional direction  24   b  corresponding to the clamping direction, the large normal force generates a friction torque which is many times greater than a friction torque which is generated upon a motion of the control lever along the motional direction  22   b  corresponding to the release direction. As a result, the return motion of the control lever of the control unit  18   b  along the motional direction  24   b  corresponding to the clamping direction is damped. The acting friction torque between the wedge element  78   b  and the transmission element  28   b  is dependent on a friction coefficient between the wedge element  78   b  and the transmission element  28   b.    
       FIG. 6  shows a detailed view of a further alternative tool clamping fixture  10   c . The tool clamping fixture  10   c  is disposed in a portable machine tool (not represented in detail here), which has a structure analogous to the portable machine tool  42   a  from  FIG. 1 . In addition, the tool clamping fixture  10   c  comprises a clamping unit  12   c  for clamping a machining tool  14   c  in an axial direction  16   c  and a control unit  18   c  for actuating the clamping unit  12   c . Furthermore, the tool clamping fixture  10   c  has a return damping unit  20   c , which is provided to damp a return motion of the control unit  18   c . The return damping unit  20   c  is further provided to generate friction torques of different magnitude in oppositely directed motional directions  22   c ,  24   c  of the control unit  18   c . The return damping unit  20   c  here comprises a return damping element  26   c , which is disposed on a transmission element  28   c  of the control unit  18   c . The return damping element  26   c  is configured as a freewheeling roller bearing  40   c , which is provided to prevent a rotary motion at least in one rotational direction  22   c ,  24   c . The transmission element  28   c  is here arranged at a distance from the roller bearing  40   c  along a direction running substantially perpendicular to the axial direction  16   c . In a mounted state, the transmission element  28   c  bears along the axial direction  16   c  against an inner race  82   c  of the roller bearing  40   c . The roller bearing  40   c  has a blocking unit (not represented in detail here) already known to a person skilled in the art, which is provided to block a rotary motion of the roller bearing  40   c  and/or of the inner race  82   c  in the motional direction  24   c . A rotary motion of the roller bearing  40   c  and/or of the inner race  82   c  about the rotation axis  54   c  is prevented by means of the blocking unit. 
     When a control lever  58   c  of the control unit  18   c , which control lever is connected to the transmission element  28   c , is actuated by an operator in a motional direction  22   c  corresponding to the release direction, the transmission element  28   c  is rotated about a rotation axis  54   c . Between the control lever  58   c  and the roller bearing  40   c , viewed along the axial direction  16   c , is arranged a spacer sleeve  88   c . As a result of the bearing of the transmission element  28   c  along the axial direction  16   c  against the inner race  82   c  of the roller bearing  40   c , the inner race  82   c  is rotated jointly with the transmission element  28   c  about the rotation axis  54   c . A small friction torque thus acts between the inner race  82   c  of the roller bearing  40   c  and the transmission element  28   c  in that motional direction  22   c  of the control lever  58   c  of the control unit  18   c  which corresponds to the release direction. 
     The control lever  58   c  is acted on by means of a spring force of a spring element  84   c , configured as a compression spring  86   c , of the clamping unit  12   c , which spring force, after a force effect of the operator has been neutralized following a movement of the control lever  58   c  in the motional direction  22   c  corresponding to the release direction, moves the control lever  58   c  via a mechanism (not represented in detail here) of the control unit  18   c  in a motional direction  24   c  corresponding to a clamping direction. The mechanism of the control unit  18   c  is provided to convert a rotary motion of the control lever  58   c  into a pivot motion and/or a translatory motion of a clamping element (not represented in detail here) of the clamping unit  12   c  for clamping the machining tool  14   c . As a consequence of the spring force of the spring element  84   c  of the clamping unit  12   c , the control lever  58   c  is rotated about the rotation axis  54   c . By virtue of the rotationally fixed connection to the control lever  58   c , the transmission element  28   c  is jointly rotated about the rotation axis  54   c  ( FIG. 7 ). The roller bearing  40   c  and/or the inner race  82   c  is barred by means of the blocking unit of the roller bearing  40   c  from jointly rotating along the motional direction  24   c  about the rotation axis  54   c . The transmission element  28   c  is subjected by the spring element  84   c  to a spring force in the direction of the roller bearing  40   c . The transmission element  28   c  is hereby pressed against the inner race  82   c . As a consequence of the pressing of the transmission element  28   c  against the inner race  82   c  and a rotary motion, prevented by means of the blocking unit, of the inner race  82   c  along the motional direction  24   c , upon a rotary motion of the transmission element  28   c  about the rotation axis  54   c  a friction torque is generated between the transmission element  28   c  and the inner race  82   c , which friction torque is many times greater than a friction torque which is generated upon a motion of the control lever  58   c  along the motional direction  22   c  corresponding to the release direction. The return motion of the control lever  58   c  of the control unit  18   c  along the motional direction  24   c  corresponding to the clamping direction is hereby damped. The acting friction torque between the inner race  82   c  and the transmission element  28   c  is dependent on a friction coefficient between the inner race  82   c  and the transmission element  28   c . The acting friction torque can additionally be increased by means of a pretensioned cup spring arranged, as an alternative to the spacer sleeve  88   c , between the control lever  58   c  and the roller bearing  40   c.