Abstract:
A machine tool deceleration device, in particular a hand-held machine tool deceleration device, for a portable machine tool, includes at least one magnetic deceleration unit. The magnetic deceleration unit includes at least one movably mounted claw segment element that is configured to change at least one parameter of a magnetic field of the magnetic deceleration unit.

Description:
This application is a 35 U.S.C. §371 National Stage Application of PCT/EP2013/057799, filed on Apr. 15, 2013, which claims the benefit of priority to Serial No. DE 10 2012 210 133.0, filed on Jun. 15, 2012 in Germany, the disclosures of which are incorporated herein by reference in their entirety. 
     BACKGROUND 
     DE 199 32 578 B4 already discloses a machine tool braking apparatus of a portable machine tool, which machine tool braking apparatus has a magnetic field braking unit. 
     SUMMARY 
     The disclosure proceeds from a machine tool braking apparatus, in particular from a hand-held machine tool braking apparatus, of a portable machine tool, having at least one magnetic field braking unit. 
     It is proposed that the magnetic field braking unit has at least one movably mounted claw segment element for changing at least one characteristic variable of a magnetic field of the magnetic field braking unit. In this document, a “magnetic field braking unit” is intended to be understood to mean, in particular, a braking unit which uses a magnetic field to reduce and/or limit a speed, in particular a rotation speed, of a moving component, in particular of a rotating component, in comparison to a working speed at least substantially without friction, in particular in addition to purely friction-related reduction and/or limiting of the speed as a result of mounting of the component. In this document, “reduce and/or limit substantially without friction” is intended to be understood to mean, in particular, braking of a component which takes place without frictional forces, with the exception of bearing-related frictional forces and/or flow-related resistance forces. In particular, the component is braked by means of the magnetic field braking unit in a manner decoupled from a contact between the component and a friction lining of a braking element. In principle however, it is also feasible for a frictional braking unit which is coupled to or uncoupled from the magnetic field braking unit to be provided in addition to the at least substantially friction-free magnetic field braking unit. Furthermore, the magnetic field braking unit is, in particular, in the form of a magnetic field braking unit which is separate from a drive. In this document, a “magnetic field braking unit which is separate from a drive” is intended to be understood to mean a magnetic field braking unit which brakes a component by means of a magnetic field in a manner decoupled from a magnetic field of a drive unit, such as an electric motor for example. A stator and/or a rotor of the drive unit are preferably decoupled from the magnetic field of the magnetic field braking unit. The magnetic field braking unit is preferably provided for braking the component in a braking state of the magnetic field braking unit, in particular in a period of greater than 0.1 s, preferably greater than 0.5 s and particularly preferably less than 3 s starting from a working speed, in particular braking said component to a speed which is less than 50% of the working speed, is preferably less than 20% of the working speed, and particularly preferably to a speed of 0 m/s. 
     The magnetic field braking unit may be in the form of an assembly module. In this document, the expression “assembly module” is intended to define, in particular, a design of a unit in which a plurality of components are pre-mounted and the unit as a whole is mounted in an overall system, in particular in a portable machine tool. The assembly module preferably has at least one fastening element which is intended to connect the assembly module to the overall system in a releasable manner. The assembly module can advantageously be removed from the overall system in particular with less than 10 fastening elements, preferably with less than 8 fastening elements and particularly preferably with less than 5 fastening elements. The fastening elements are particularly preferably in the form of screws. However, it is also feasible for the fastening elements to be in the form of other elements which appear to be expedient to a person skilled in the art, such as quick-action clamping elements, fastening elements which can be operated without tools etc. for example. At least one function of the assembly module, in particular a change in the pole position of the permanent magnets for activating the magnetic field braking unit, can preferably be ensured in a state in which said assembly module is removed from the overall system. The assembly module can be removed particularly preferably by an end user. Therefore, the assembly module is in the form of a replaceable unit which can be replaced with a further assembly module, such as in the event of a defect in the assembly module or an extension of the functions and/or a change in the functions of the overall system for example. 
     In this document, the expression “movably mounted” is intended to define, in particular, mounting of a unit and/or of an element relative to at least one further unit and/or relative to a further element, wherein the unit and/or the element is able to move along at least one axis along a distance of greater than 1 mm, preferably greater than 10 mm and particularly preferably greater than 20 mm, and/or is able to move about at least one axis by an angle of greater than 10°, preferably greater than 45°, and particularly preferably greater than 60°, in particular in a manner decoupled from an elastic deformation of the unit and/or of the element and in a manner decoupled from abilities to move which are created by bearing play. In this case, the claw segment element is preferably mounted such that it can rotate about a movement axis of the claw segment element. The movement axis of the claw segment element preferably runs at least substantially parallel or coaxially to a rotation axis of an output drive element, in particular of a spindle, of an output drive unit of the portable machine tool. In this document, “substantially parallel” is intended to be understood to mean, in particular, orientation of a direction relative to a reference direction, in particular in one plane, wherein the direction exhibits a deviation in particular of less than 8°, advantageously less than 5° and particularly advantageously less than 2°, in relation to the reference direction. However, it is also feasible for the movement axis of the output drive element to have another orientation which appears to be expedient to a person skilled in the art 
     In this document, the term “claw segment element” is intended to define, in particular, an element which has at least one magnetically permeable projection, which runs at least substantially parallel to the movement axis of the element, for changing a magnetic field, which projection has, as viewed along a direction which runs about the movement axis, an extent which is smaller than an overall extent of the element along the direction which runs about the movement axis. The claw segment element particularly preferably has a large number of projections which, as viewed along the direction which runs about the movement axis, are arranged on the claw segment element in a manner uniformly distributed or spaced apart relative to the following projection. In this case, in each case two projections which directly follow one another along the direction which runs about the movement axis can be separated from one another by an air gap or connected to one another by means of a magnetically impermeable connecting element, such as a plastic connecting element etc., for example. The claw segment element preferably comprises at least four projections which, for the most part, are separated from one another by air gaps along the direction which runs around the movement axis. However, it is also feasible for the claw segment element to have a number of projections which differs from four. The claw segment element is preferably different from a magnet element of the magnetic field braking unit, in particular from a permanent magnet of the magnetic field braking unit. Therefore, the magnetic field braking unit preferably comprises at least one claw segment element, which is different from a magnet element of the magnetic field braking unit, for changing at least one characteristic variable of a magnetic field of the magnetic field braking unit, which claw segment element is movably mounted. In this document, a “characteristic variable of a magnetic field” is intended to be understood to mean, in particular, a parameter which defines a magnetic field, such as a magnetic flux, a magnetic induction, a magnetic resistance, a magnet voltage, a magnetic return path etc. for example. In this document, the term “change” is intended to define, in particular, “set” and/or “influence”. The claw segment element is preferably formed at least partially from a ferromagnetic material, such as iron, iron-cobalt and/or iron-nickel alloys for example. However, it is also feasible for the claw segment element to be formed entirely from a ferromagnetic material. The claw segment element is particularly preferably intended to change or to influence a profile of a magnetic flux or of a magnetic return path in at least one operating state or in at least one operating position. In this document, “intended” is intended to be understood to mean, in particular, specially designed and/or specially equipped. A change between a braking state and a freewheeling state of the magnetic field braking unit can be implemented in a structurally simple manner by means of the refinement according to the disclosure. As a result of a relative movement of the claw segment element, a magnetic return path of the magnetic field braking unit can advantageously be changed, and as a result an intensity of a braking magnetic field can be influenced. In addition, a compact machine tool braking apparatus can advantageously be obtained. 
     It is further proposed that the magnetic field braking unit comprises at least one further claw segment element for changing at least one characteristic variable of a magnetic field of the magnetic field braking unit. Therefore, the magnetic field braking unit preferably comprises at least one further claw segment element, which is different from a magnet element of the magnetic field braking unit, for changing at least one characteristic variable of a magnetic field of the magnetic field braking unit. The further claw segment element preferably has magnetically permeable projections. In this case, the further claw segment element is preferably at least partially formed from a ferromagnetic material, such as iron, iron-cobalt and/or iron-cobalt-nickel alloys for example. The further claw segment element is particularly preferably intended to change or to influence a profile of a magnetic flux or of a magnetic return path in at least one operating state or in at least one operating position by means of interaction with the claw segment element. The claw segment element and the further claw segment element can preferably be moved relative to one another in at least one operating state. The further claw segment element is preferably movably mounted. In this case, the further claw segment element can preferably be moved together with the claw segment element in at least one operating state. A large magnetic return path area which allows advantageous braking of a moving component by means of the magnetic field unit can advantageously be obtained in at least one operating state, in particular in a braking state, of the magnetic field braking unit by means of the refinement according to the disclosure. 
     It is further proposed that the claw segment element and the further claw segment element can be moved relative to one another in at least one operating state. The claw segment element and the further claw segment element can preferably be rotated relative to one another in a transition state of the magnetic field braking unit starting from a freewheeling state of the magnetic field braking unit into a braking state of the magnetic field braking unit. Furthermore, the claw segment element and the further claw segment element can preferably be rotated relative to one another in a transition state of the magnetic field braking unit starting from a braking state of the magnetic field braking unit into a freewheeling state of the magnetic field braking unit. The machine tool braking apparatus comprises at least one mechanical activation unit for moving the claw segment element and the further claw segment element relative to one another. In this document, a “mechanical activation unit” is intended to mean, in particular, a unit which, as a result of a relative movement, initiates a tripping process and/or an activation process, in particular of the magnetic field braking unit, wherein the relative movement differs from a pure switching movement of a switching element for generating an electrical signal and is formed, in particular, by a movement of a magnet element and/or by an inertia-related movement, in particular by an inertia-related movement of a rotating drive element, output drive element and/or of an operating tool. In this connection, a “tripping process” is intended to be understood to mean, in particular, mechanical, electrical, magnetic and/or electronic signaling of a state which is intended to initiate an activation process. In this document, an “activation process” is intended to be understood to mean, in particular, mechanical, electrical, magnetic and/or electronic activation of the magnetic field braking unit for generating forces and/or torques for braking a component. 
     In a preferred embodiment of the machine tool braking apparatus according to the disclosure, the activation unit is intended to initiate the tripping process and the activation process as a result of the relative movement, in particular at least substantially without a time delay. In this case, the activation unit can be intended to signal, for example, a tripping process and to initiate an activation process of the magnetic field braking unit at least substantially at the same time as a result of the relative movement. A refinement of the mechanical activation unit in which a switch is operated by the relative movement as the tripping process and an activation process which follows the tripping process is initiated by means of an actuator and/or a spring force and/or by means of other operating elements which appear to be expedient to a person skilled in the art is likewise feasible. Furthermore, it is likewise feasible for the activation unit to comprise a sensor unit which senses the relative movement and, as a result of this, initiates a tripping process, wherein the activation process is initiated, for example, by means of an actuator. 
     A further inventive refinement of the machine tool braking apparatus may involve the activation unit being mechanically, electrically, magnetically and/or electronically connected to a solenoid of the magnetic field braking unit, wherein the solenoid is intended to influence a magnetic field of the magnetic field braking unit in at least one operating mode. The solenoid can generate an additional magnetic field to an existing magnetic field of the magnetic field braking unit. In this case, it is feasible for the additional magnetic field to at least partially compensate for at least magnetic forces of the existing magnetic field of the magnetic field braking unit in a working mode and/or to at least partially attenuate said magnetic forces at least in comparison to an intensity of the magnetic force of the magnetic field in a braking mode. The solenoid of the magnetic field braking unit can advantageously likewise be intended to allow an additional torque for achieving a working rotation speed of the drive unit in a short period of time, such as preferably for achieving boost operation, during run-up of a drive unit of the portable machine tool in an operating mode. Reliable tripping and/or activation of the magnetic field braking unit can advantageously be obtained by means of the mechanical activation unit. Furthermore, in a preferred embodiment of the machine tool braking apparatus according to the disclosure, electrical components for tripping and/or activating the magnetic field braking unit can advantageously be dispensed with. As a result, the susceptibility of the magnetic field braking unit to faults can advantageously be kept low. A switching process for switching between a braking state of the magnetic field braking unit and a freewheeling state of the magnetic field braking unit can be implemented in a structurally simple manner by means of the two claw segment elements which move relative to one another in at least one operating state. 
     It is further proposed that the magnetic field braking unit comprises at least one eddy current element which, as viewed along a direction which runs at least substantially perpendicular to a movement axis of the claw segment element, is arranged between the claw segment element and the further claw segment element in at least one operating state. In this document, the expression “arranged between” is intended to be understood to mean, in particular, a physical arrangement in which components are arranged one behind the other at least along a straight line and, as viewed along the straight line, at least partially overlap or the straight line intersects the components. As viewed along the direction which runs at least substantially perpendicular to a movement axis of the claw segment element, the eddy current element is preferably arranged between the claw segment element and the further claw segment element at least in a braking state of the magnetic field braking unit. Therefore, a high braking force for braking at least one moving component can advantageously be obtained in at least one operating state, in particular a braking state, of the magnetic field braking unit. 
     In addition, it is proposed that the magnetic field braking unit comprises at least one eddy current element which is arranged on a return path element of the magnetic field braking unit. In this document, an “eddy current element” is intended to be understood to mean, in particular, an element which is intended to generate a magnetic field for braking at least one moving component as a result of eddy currents. Friction-free braking of a moving component can advantageously be implemented in this way. 
     Furthermore, it is proposed that the magnetic field braking unit has at least one braking element which is stationary relative to a gear mechanism housing and is in the form of a permanent magnet. A compact magnetic field braking unit can advantageously be obtained in this way. An output drive element can advantageously be designed to be decoupled from a weight of the braking element in the event of rotation. 
     It is further proposed that the magnetic field braking unit has at least one braking element which is in the form of a permanent magnet. However, it is also feasible for the magnetic field braking unit to have, in an alternative refinement, a braking element, which is in the form of a coil, for generating a magnetic field. The braking element, which is in the form of a permanent magnet, is preferably in the form of a rare-earth magnet, such as a rare-earth magnet comprising hard ferrite, neodymium-iron-boron (NdFeB), samarium-cobalt (SmCo) etc. for example. However, it is also feasible for the permanent magnet to be formed from another material which appears to be expedient to a person skilled in the art. The permanent magnet is preferably in the form of a ring. Furthermore, the permanent magnet preferably has an inner magnetic field which runs along a direction which runs at least substantially parallel to the movement axis of the claw segment element. The braking element, which is in the form of a permanent magnet, is preferably in the form of an axially magnetized magnet. A magnetic field braking unit can be obtained in a structurally simple manner. Furthermore, a magnetic field braking unit which is independent of a voltage supply can advantageously be obtained. Therefore, a high degree of functional reliability can advantageously be achieved since the magnetic field braking unit is advantageously functional without separately supplied electrical energy. 
     In an advantageous refinement of the machine tool braking apparatus according to the disclosure, the braking element is connected in a rotationally fixed manner to a further claw segment element of the magnetic field braking unit. “In a rotationally fixed manner” is intended to be understood to mean, in particular, a connection which transfers a torque and/or a rotary movement in an at least substantially unchanged manner. In this document, “transfer in an at least substantially unchanged manner” is intended to be understood to mean, in particular, complete transmission of forces and/or torques from one component to a further component, apart from a loss as a result of friction and/or of tolerances. The braking element is particularly preferably firmly fixed to the further claw segment element. In this case, the braking element can be fixed to the further claw segment element by means of an interlocking, force-fitting and/or cohesive connection. The braking element is preferably fixed to the further claw segment element by means of a cohesive connection, in particular by means of adhesive bonding. An installation space-saving arrangement of the braking element can advantageously be obtained by means of the refinement according to the disclosure. 
     In an alternative refinement of the machine tool braking apparatus according to the disclosure, the braking element is connected to an eddy current element of the magnetic field braking unit by means of a return path element of the magnetic field braking unit. In this case, the braking element can be fixed to the return path element by means of an interlocking, force-fitting and/or cohesive connection. The braking element is preferably fixed to the return path element by means of a cohesive connection, in particular by means of adhesive bonding. 
     In addition, the eddy current element is preferably fixed to the return path element by means of a cohesive connection, in particular by means of adhesive bonding. However, it is also feasible for the eddy current element to be fixed to the return path element by means of another connection which appears to be expedient to a person skilled in the art, such as by means of an interlocking connection and/or by means of a cohesive connection for example. An advantageous magnetic return path for braking a moving component can be implemented in at least one operating state by means of the refinement according to the disclosure. 
     The disclosure further proceeds from a portable machine tool, in particular from a portable hand-held machine tool, having a machine tool braking apparatus according to the disclosure, in particular having a hand-held machine tool braking apparatus. In this document, a “portable machine tool” is intended to be understood to mean, in particular, a machine tool, in particular a hand-held machine tool, which can be transported by an operator without a transportation machine. The portable machine tool has, in particular, a mass which is less than 50 kg, preferably less than 20 kg and particularly preferably less than 10 kg. In this case, the portable machine tool may be in the form of an angle grinder, a drill, a hand-held circular saw, a chipping hammer and/or a percussion drill etc. The portable machine tool is particularly preferably in the form of an angle grinder. A safety function for an operator of the portable machine tool can advantageously be obtained, it advantageously being possible for a risk of injury to an operator to be kept low by said safety function. 
     In addition, it is proposed that the portable machine tool has at least one output drive unit which comprises at least one output drive element on which the claw segment element is arranged in a rotationally fixed manner. In this document, an “output drive unit” is intended to be understood to mean, in particular, a unit which can be driven by means of a drive unit and transmits forces and/or torques which are generated by the drive unit to a processing tool. The output drive unit is preferably in the form of an angular gear mechanism. However, it is also feasible for the output drive unit to have another design which appears to be expedient to a person skilled in the art, such as a design in the form of a worm gear mechanism, in the form of a toothed belt mechanism, in the form of a planetary gear mechanism etc. for example. In addition, it is likewise feasible for the magnetic field braking unit to be arranged on a drive unit of the portable machine tool. The refinement according to the disclosure advantageously allows a compact arrangement of the machine tool braking apparatus on the portable machine tool. 
     The machine tool braking apparatus according to the disclosure and the portable machine tool according to the disclosure are not intended to be limited to the above-described application and embodiment in this case. In particular, the machine tool braking apparatus according to the disclosure and the portable machine tool according to the disclosure for fulfilling a manner of operation described in this document can have a number of individual elements, components and units which differs from the number cited in this document. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantages can be gathered from the following description of the drawings. The drawings illustrate exemplary embodiments of the disclosure. The drawings, the description and the claims contain numerous features in combination. A person skilled in the art will expediently consider the features on their own and combine said features to form expedient further combinations. 
       In the drawings: 
         FIG. 1  shows a schematic illustration of a machine tool according to the disclosure having a machine tool braking apparatus according to the disclosure, 
         FIG. 2  shows a schematic illustration of a view of a detail of the machine tool braking apparatus according to the disclosure from  FIG. 1 , 
         FIG. 3  shows a schematic illustration of a view of a detail of a magnetic flux profile of a magnetic field braking unit of the machine tool braking apparatus according to the disclosure, 
         FIG. 4  shows a schematic illustration of a view of a detail of an output drive element of an output drive unit of the portable machine tool according to the disclosure, 
         FIG. 5  shows a schematic illustration of a view of a detail of a driver element of an activation unit of the machine tool braking apparatus according to the disclosure, 
         FIG. 6  shows a schematic illustration of a sectional view of the magnetic field braking unit in a freewheeling state along line A-A from  FIG. 2 , 
         FIG. 7  shows a schematic illustration of a sectional view of the magnetic field braking unit in a braking state along line A-A from  FIG. 2 , and 
         FIG. 8  shows a schematic illustration of a view of a detail of an alternative machine tool braking apparatus according to the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a portable machine tool  12   a,  which is in the form of an angle grinder, having a machine tool braking apparatus  10   a.  The angle grinder comprises a protective hood unit  32   a,  a machine tool housing  34   a  and a main handle  36   a  which extends in the direction of a main direction  42   a  of extent of the portable machine tool  12   a  on a side  40   a  of the machine tool housing  34   a  which is averted from a processing tool  38   a.  In this case, the processing tool  38   a  is in the form of a grinding disk. However, it is also feasible for the processing tool  38   a  to be in the form of a cutting disk or polishing disk. The machine tool housing  34   a  comprises a motor housing  44   a  for accommodating a drive unit  46   a  of the portable machine tool  12   a  and a gear mechanism housing  48   a  for accommodating an output drive unit  28   a  of the portable machine tool  12   a.  The drive unit  46   a  is intended to drive the processing tool  38   a  in a rotatable manner by means of the output drive unit  28   a.  The output drive unit  28   a  is connected to the drive unit  46   a  by means of a drive element  52   a  of the drive unit  46   a,  which drive element is driven in a rotating manner about a rotation axis. The drive element  52   a  is in the form of an armature shaft ( FIG. 2 ). Furthermore, the output drive unit  28   a  comprises a spindle  66   a  which can rotate about a rotation axis  50   a , a bearing flange  68   a  and a bearing element  70   a,  which is arranged in the bearing flange  68   a,  for bearing the spindle  66   a.  The bearing flange  68   a  is connected in a releasable manner to the gear mechanism housing  48   a  by means of fastening elements (not illustrated in any detail here) of the output drive unit  28   a.  Furthermore, the processing tool  38   a  can be connected in a rotationally fixed manner to the spindle  66   a  by means of a fastening element (not illustrated in any detail here) for processing a workpiece. The processing tool  38   a  can therefore be driven in a rotatable manner during operation of the portable machine tool  12   a . Furthermore, an auxiliary handle  54   a  is arranged on the gear mechanism housing  48   a.  The auxiliary handle  54   a  extends transverse to the main direction  42   a  of extent of the portable machine tool  12   a.    
     The machine tool braking apparatus  10   a  is arranged in the gear mechanism housing  48   a  of the portable machine tool  12   a.  In this case, the machine tool braking apparatus  10   a  has at least one magnetic field braking unit  14   a  ( FIG. 2 ). The magnetic field braking unit  14   a  comprises at least one movably mounted claw segment element  16   a  for changing at least one characteristic variable of a magnetic field of the magnetic field braking unit  14   a.  The claw segment element  16   a  is arranged in a rotationally fixed manner on an output drive element  30   a  of the output drive unit  28   a.  In this case, the claw segment element  16   a  is integrally formed with the output drive element  30   a  ( FIGS. 2 and 4 ). The output drive element  30   a  is therefore formed from a magnetically permeable material, such as a ferromagnetic material for example, in order to change at least one characteristic variable of a magnetic field of the magnetic field braking unit  14   a.  The output drive element  30   a  is in the form of a crown gear. In this case, the output drive element  30   a  is arranged on the spindle  66   a  of the output drive unit  28   a  by means of a clearance fit. The output drive unit  28   a  has at least one driver element  72   a  for transmitting torque between the spindle  66   a  and the output drive element  30   a.  The driver element  72   a  is connected in a rotationally fixed manner to the spindle  66   a.  In this case, the driver element  72   a  can be connected in a rotationally fixed manner to the spindle  66   a  by means of an interlocking, force-fitting and/or cohesive connection in a manner which is already known to a person skilled in the art. 
     The output drive element  30   a  has three rotary driver projections  78   a,    80   a,    82   a  on a side of the output drive element  30   a  which is averted from a tooth system  76   a  of the output drive element  30   a  in order to couple the output drive element  30   a  and the driver element  72   a  in a rotationally fixed manner ( FIG. 4 ). However, it is also feasible for the output drive element  30   a  to have a number of rotary driver projections  78   a,    80   a,    82   a  which differs from three. A person skilled in the art will provide a suitable number of rotary driver projections  78   a,    80   a,    82   a  on the output drive element  30   a  depending on the field of application. The rotary driver projections  78   a,    80   a,    82   a  are arranged on that side of the output drive element  30   a  which is averted from the tooth system  76   a  in a manner distributed uniformly along a circumferential direction  84   a.  In this case, the circumferential direction  84   a  runs in a plane which extends at least substantially perpendicular to the rotation axis  50   a  of the spindle  66   a  or of the output drive element  30   a.  Furthermore, the rotary driver projections  78   a,    80   a,    82   a  extend perpendicular to that side of the output drive element  30   a  which is averted from the tooth system  76   a.  The rotary driver projections  78   a,    80   a,    82   a  extend in the direction of the driver element  72   a  in a mounted state of the output drive unit  28   a.    
       FIG. 5  shows a view of a detail of the driver element  72   a.  The driver element  72   a  has rotary driver recesses  86   a,    88   a,    90   a  for receiving the rotary driver projections  78   a,    80   a,    82   a  ( FIG. 5 ). Therefore, in a mounted state, the rotary driver projections  78   a,    80   a ,  82   a  extend along the rotation axis  50   a  of the spindle  66   a  into the rotary driver recesses  86   a,    88   a,    90   a.  The rotary driver recesses  86   a,    88   a,    90   a  are arranged on the driver element  72   a  in a manner distributed uniformly along the circumferential direction  84   a.    
     Furthermore, the rotary driver recesses  86   a,    88   a,    90   a  have an extent which is greater than that of the rotary driver projections  78   a,    80   a,    82   a  along the circumferential direction  84   a.  This results in rotary play being obtained between the output drive element  30   a  and the driver element  72   a  along the circumferential direction  84   a.  The rotary play is formed by an angular range around which the output drive element  30   a  can be rotated relative to the driver element  72   a  about the rotation axis  50   a  of the spindle  66   a.  The angular range is in this case formed by a distance between projections  58   a,    60   a,    62   a,    64   a  of the claw segment element  16   a  which directly follow one another along the circumferential direction  84   a,  as viewed along the circumferential direction  84   a  ( FIGS. 6 and 7 ). The rotary driver projections  78   a,    80   a,    82   a  can therefore be moved along the circumferential direction  84   a  in the rotary driver recesses  86   a,    88   a ,  90   a  relative to edge regions of the rotary driver recesses  86   a,    88   a,    90   a.  The driver element  72   a  couples the output drive element  30   a  to the spindle  66   a  in a rotationally fixed manner when the rotary driver projections  78   a,    80   a,    82   a  bear against edge regions of the rotary driver recesses  86   a,    88   a,    90   a.  However, it is also feasible for the rotary driver projections  78   a ,  80   a,    82   a  to be arranged on the driver element  72   a  and for the rotary driver recesses  86   a,    88   a,    90   a  to be arranged on the output drive element  30   a.  The rotary driver projections  78   a,    80   a,    82   a  of the output drive element  30   a  and the rotary driver recesses  86   a,    88   a ,  90   a  of the driver element  72   a  form a mechanical activation unit  56   a  of the machine tool braking apparatus  10   a.  The activation unit  56   a  is intended to switch the magnetic field braking unit  14   a  from a braking state to a freewheeling state, and vice versa. 
     Furthermore, the magnetic field braking unit  14   a  has at least one further claw segment element  18   a  for changing at least one characteristic variable of a magnetic field of the magnetic field braking unit  14   a.  To this end, the further claw segment element  18   a  is formed from a ferromagnetic material. The further claw segment element  18   a  is fixed in a rotationally fixed manner to the driver element  72   a.  In this case, the driver element  72   a  is fixed in a rotationally fixed manner on the spindle  66   a  by means of a clearance fit. However, it is also feasible for the driver element  72   a  to be fixed in a rotationally fixed manner on the spindle  66   a,  for example by means of a screw connection, by means of a rivet connection, by means of an adhesive bonding connection, by means of a welded connection, by means of a feather key connection etc. The driver element  72   a  is formed from a magnetically impermeable material, such as stainless steel, plastic etc. for example, for the purpose of magnetic insulation. However, it is also feasible to arrange an insulation element between the output drive element  30   a  and the driver element  72   a  and/or the spindle  66   a.  The insulation element can be in the form of, for example, a Teflon sliding bushing etc., which mounts the output drive element  30   a  on the spindle  66   a.  The claw segment element  16   a  and the further claw segment element  18   a  are intended to change at least one characteristic variable of a magnetic field of the magnetic field braking unit  14   a  by means of interaction. In this case, the activation unit  56   a  is intended to move the claw segment element  16   a  and the further claw segment element  18   a  relative to one another in at least one operating state. Therefore, the claw segment element  16   a  and the further claw segment element  18   a  can be moved relative to one another in at least one operating state. The further claw segment element  18   a  is fixed to the driver element  72   a  in a rotationally fixed manner. 
     The claw segment element  16   a  has the four projections  58   a,    60   a,    62   a,    64   a  for changing at least one characteristic variable of a magnetic field of the magnetic field braking unit  14   a.  However, it is also feasible for the claw segment element  16   a  to have a number of projections  58   a,    60   a,    62   a,    64   a  which differs from four. The projections  58   a,    60   a,    62   a,    64   a  of the claw segment element  16   a  are arranged on the claw segment element  16   a  in a manner spaced apart relative to one another along the circumferential direction  84   a . In addition, the projections  58   a,    60   a,    62   a,    64   a  of the claw segment element  16   a  are arranged on the claw segment element  16   a  in a manner uniformly distributed along the circumferential direction  84   a.  The further claw segment element  18   a  likewise comprises four projections  92   a,    94   a,    96   a,    98   a  ( FIGS. 6 and 7 ). The projections  92   a,    94   a,    96   a,    98   a  of the further claw segment element  18   a  are arranged on the further claw segment element  18   a  in a manner uniformly distributed along the circumferential direction  84   a.  In addition, the projections  92   a,    94   a,    96   a,    98   a  of the further claw segment element  18   a  are arranged on the further claw segment element  18   a  in a manner spaced apart relative to one another along the circumferential direction  84   a . In a mounted state of the claw segment element  16   a  and of the further claw segment element  18   a,  the projections  92   a,    94   a,    96   a,    98   a  of the further claw segment element  18   a  extend at least substantially parallel to the rotation axis  50   a  of the spindle  66   a  in the direction of the claw segment element  16   a.  The projections  58   a,    60   a,    62   a,    64   a  of the claw segment element  16   a  extend at least substantially parallel to the rotation axis  50   a  of the spindle  66   a  in the direction of the further claw segment element  18   a  in a mounted state. 
     Furthermore, the magnetic field braking unit  14   a  comprises at least one eddy current element  20   a  which is arranged between the claw segment element  16   a  and the further claw segment element  18   a  of the magnetic field braking unit  14   a,  as viewed along a direction which runs at least substantially perpendicular to a movement axis  24   a  of the claw segment element  16   a,  in at least one operating state. The eddy current element  20   a  is formed from an electrically conductive material, such as aluminum and/or copper for example. The movement axis  24   a  of the claw segment element  16   a  which is integrally formed with the output drive element  30   a  runs coaxially to the rotation axis  50   a  of the spindle  66   a  in this case. The eddy current element  20   a  is in the form of a ring. In addition, the eddy current element  20   a  is fixed on the bearing flange  68   a  of the output drive unit  28   a.  In this case, the eddy current element  20   a  is fixed in a force-fitting and/or interlocking manner on the bearing flange  68   a  in a ring-like recess  74   a  of the bearing flange  68   a.  The ring-like recess  74   a  is in the form of an annular groove which runs along the circumferential direction  84   a.  Therefore, the claw segment element  16   a  and the further claw segment element  18   a  are moved relative to the eddy current element  20   a  by means of the spindle  66   a  during operation of the portable machine tool  12   a.    
     Furthermore, the magnetic field braking unit  14   a  has at least one braking element  26   a  which is in the form of a permanent magnet. The braking element  26   a  is connected in a rotationally fixed manner to the further claw segment element  18   a  of the magnetic field braking unit  14   a.  Therefore, the braking element  26   a  is connected in a rotationally fixed manner to the driver element  72   a . The braking element  26   a  is arranged between the projections  92   a,    94   a,    96   a,    98   a  of the further claw segment element  18   a  and the driver element  72   a,  as viewed along the direction which runs at least substantially perpendicular to the movement axis  24   a  of the claw segment element  16   a.  In addition, the braking element  26   a  exhibits axial magnetization which is oriented, as viewed along an at least substantially parallel to the rotation axis  50   a  of the spindle  66   a . One side of the braking element  26   a  forms a magnetic north pole of the braking element  26   a,  and one side of the braking element  26   a  forms a magnetic south pole of the braking element  26   a.  The braking element  26   a  is therefore in the form of an axially magnetized permanent magnet, with respect to the rotation axis  50   a  of the spindle  66   a.  In addition, it is also feasible for the magnetic field braking unit  14   a  to have a large number of braking elements  26   a  which are in the form of permanent magnets. 
     The machine tool braking apparatus  10   a  is in a braking mode in an idle state of the portable machine tool  12   a  in which no current is supplied to the electric motor unit of the drive unit  46   a.  The magnetic field braking unit  14   a  is therefore in a braking state. In the braking state of the magnetic field braking unit  14   a , the projections  92   a,    94   a,    96   a,    98   a  of the further claw segment element  18   a  and the projections  58   a,    60   a,    62   a ,  64   a  of the claw segment element  16   a  are situated opposite one another, as viewed along a direction which runs at least substantially perpendicular to the rotation axis  50   a  of the spindle  66   a.  A straight line which runs at least substantially perpendicular to the rotation axis  50   a  of the spindle  66   a  intersects at least one of the projections  92   a,    94   a,    96   a,    98   a  of the further claw segment element  18   a,  the eddy current element  20   a  and at least one of the projections  58   a ,  60   a,    62   a,    64   a  of the claw segment element  16   a  in a braking state of the magnetic field braking unit  14   a . Therefore, the projections  58   a,    60   a,    62   a,    64   a  of the claw segment element  16   a,  the eddy current element  20   a  and the projections  92   a,    94   a,    96   a,    98   a  of the further claw segment element  18   a  overlap as viewed along the direction which runs at least substantially perpendicular to the rotation axis  50   a  of the spindle  66   a  ( FIG. 7 ). In this case, it is feasible for the magnetic field braking unit  14   a  to have at least one spring element, for a spring force to be applied to the claw segment element  16   a  and/or the further claw segment element  18   a  in the direction of a braking position in which the projections  58   a,    60   a,    62   a,    64   a  of the claw segment element  16   a  and the projections  92   a ,  94   a,    96   a,    98   a  of the further claw segment element  18   a  overlap. This could result in automatic or supporting movement of the claw segment element  16   a  and/or of the further claw segment element  18   a  to a braking position when a torque of the electric motor unit of the drive unit  46   a  drops. 
     In this case, a magnetic flux of the magnetic field braking unit  14   a  or of the braking element  26   a  which is in the form of a permanent magnet runs, starting from the braking element  26   a,  along a direction which runs at least substantially parallel to the rotation axis  50   a  of the spindle  66   a,  across an air gap, into the output drive element  30   a.  From the output drive element  30   a,  the magnetic flux in the output drive element  30   a  runs to the projections  58   a,    60   a,    62   a,    64   a  of the claw segment element  16   a.  The magnetic flux runs further across an air gap into the eddy current element  20   a.  In this case, the magnetic flux enters the eddy current element  20   a  along a direction which runs at least substantially perpendicular to the rotation axis  50   a  of the spindle  66   a.  Starting from the eddy current element  20   a,  the magnetic flux runs across an air gap into the projections  92   a,    94   a,    96   a,    98   a  of the further claw segment element  18   a.  The flux exits the eddy current element  20   a  along the direction which runs at least substantially perpendicular to the rotation axis  50   a  of the spindle  66   a,  and enters the projections  92   a,    94   a ,  96   a,    98   a  of the further claw segment element  18   a.  From the projections  92   a,    94   a,    96   a,    98   a  of the further claw segment element  18   a,  the magnetic flux runs, across the further claw segment element  18   a,  back to the braking element  26   a  ( FIG. 3 ). 
     When the portable machine tool  12   a  is started up by current being supplied to the electric motor unit of the drive unit  46   a,  the output drive element  30   a  is driven by the drive element  52   a.  In this case, the output drive element  30   a  is rotated about the rotation axis  50   a  of the spindle  66   a  relative to the driver element  72   a  until the rotary driver projections  78   a ,  80   a,    82   a  bear against edge regions of the rotary driver recesses  86   a,    88   a,    90   a.  As a result, the output drive element  30   a  is coupled to the spindle  66   a  in a rotationally fixed manner. The spindle  66   a  is consequently driven in rotation. The processing tool  38   a  which is fastened to the spindle  66   a  is therefore likewise driven in rotation. As a result of the relative movement between the output drive element  30   a  and the driver element  72   a,  the claw segment element  16   a  is rotated relative to the further claw segment element  18   a.  As a result, the magnetic field braking unit  14   a  is switched to a freewheeling state in which low magnetic forces of the brake element  26   a,  which is in the form of a permanent magnet, act on the eddy current element  20   a.  As a result of the relative movement between the claw segment element  16   a  and the further claw segment element  18   a,  the projections  58   a ,  60   a,    62   a,    64   a  of the claw segment element  16   a  are rotated about the movement axis  24   a  of the claw segment element  16   a  relative to the projections  92   a,    94   a,    96   a ,  98   a  of the further claw segment element  18   a.  As a result, overlapping of the projections  58   a,    60   a,    62   a ,  64   a  of the claw segment element  16   a  and the projections  92   a,    94   a,    96   a,    98   a  of the further claw segment element  18   a  along the direction which runs at least substantially perpendicular to the rotation axis  50   a  of the spindle  66   a  is removed ( FIG. 6 ). A straight line which runs along the at least substantially perpendicular to the rotation axis  50   a  of the spindle  66   a  in this case intersects either one of the projections  58   a,    60   a,    62   a,    64   a  of the claw segment element  16   a  and the eddy current element  20   a  or one of the projections  92   a,    94   a,    96   a,    98   a  of the further claw segment element  18   a  and the eddy current element  20   a.    
     When the portable machine tool  12   a  is switched off, the drive element  52   a  is braked by the electric motor unit of the drive unit  46   a.  The processing tool  38   a  which is fastened on the spindle  66   a  continues to rotate on account of a mass inertia. The spindle  66   a  is therefore likewise further rotated about the rotation axis  50   a.    
     The processing tool  38   a  has larger moments of mass inertia than the drive element  52   a  and/or the losses in the drive element  52   a  during operation are higher than in the spindle  66   a,  for example on account of bearing losses, power consumption by a fan of the drive unit  46   a.  The drive element  52   a  therefore brakes the output drive element  30   a.  The output drive element  30   a  is rotated about the rotation axis  50   a  of the spindle  66   a  relative to the driver element  72   a  until the rotary driver projections  78   a,    80   a,    82   a  bear against edge regions of the rotary driver recesses  86   a,    88   a,    90   a . The magnetic field braking unit  14   a  is therefore switched, starting from a freewheeling state, to a braking state. As a result, the claw segment element  16   a  is rotated relative to the further claw segment element  18   a  on account of a relative movement between the output drive element  30   a  and the driver element  72   a.  In this case, the projections  58   a,    60   a,    62   a,    64   a  of the claw segment element  16   a  are rotated relative to the projections  92   a,    94   a,    96   a,    98   a  of the further claw segment element  18   a  until the projections  58   a,    60   a ,  62   a,    64   a  of the claw segment element  16   a  are opposite the projections  92   a,    94   a,    96   a,    98   a  of the further claw segment element  18   a.  Eddy currents are produced in the stationary eddy current element  20   a  on account of a relative movement between the projections  58   a,    60   a ,  62   a,    64   a  of the claw segment element  16   a  and the projections  92   a,    94   a,    96   a,    98   a  of the further claw segment element  18   a.  The eddy currents generate a magnetic flux in a perpendicular and eddying manner in relation to a magnetic flux of the magnetic field braking unit  14   a.  Therefore, a magnetic field which opposes a magnetic field of the braking element  26   a  which is in the form of a permanent magnet is generated in the eddy current element  20   a.  This generates a braking torque which brakes the claw segment element  16   a  which rotates with the spindle  66   a  relative to the eddy current element  20   a  and the further claw segment element  18   a  which rotates with the spindle  66   a  relative to the eddy current element  20   a.  The spindle  66   a  and the processing tool  38   a  are likewise braked. The claw segment element  16   a  and the further claw segment element  18   a  are therefore intended to change or to influence at least one profile of a magnetic flux of a magnetic field of the magnetic field braking unit  14   a  by means of interaction. 
     Furthermore, the magnetic field braking unit  14   a , together with the output drive unit  28   a,  is in the form of an assembly module  100   a  ( FIG. 2 ). The assembly module  100   a  comprises four fastening elements (not illustrated here) which are in the form of screws. The screws are intended to connect the assembly module  100   a  to the gear mechanism housing  48   a  in a releasable manner. An operator can remove the assembly module  100   a  from the gear mechanism housing  48   a  as required and replace it with a further assembly module, not illustrated in any detail here, which is decoupled from a magnetic field braking unit and comprises only an output drive unit. The further assembly module can therefore be mounted on the gear mechanism housing  48   a  by the operator as an alternative to the assembly module  100   a.  An operator therefore has the option of equipping the portable machine tool  12   a  with the assembly module  100   a  having the magnetic field braking unit  14   a  and the output drive unit  28   a,  or with the further assembly module having a drive unit. For an application in which the portable machine tool  12   a  is intended to be operated in a manner uncoupled from the machine tool braking apparatus  10   a,  the assembly module  100   a  can be replaced by the further assembly module by an operator. To this end, the operator removes only the assembly module  100   a  from the gear mechanism housing  48   a  and mounts the further assembly module on the gear mechanism housing  48   a.    
       FIG. 8  illustrates an alternative exemplary embodiment. Components, features and functions which remain substantially the same are denoted by the same reference symbols in principle. In order to distinguish between the exemplary embodiments, the letters a and b are added to the reference symbols of the exemplary embodiments. The following description is limited substantially to the differences from the first exemplary embodiment in  FIGS. 1 to 6 , wherein reference can be made to the description of the first exemplary embodiment in  FIGS. 1 to 6  in respect of components, features and functions which remain the same. 
       FIG. 8  shows a machine tool braking apparatus  10   b  which is arranged, as an alternative, in a gear mechanism housing  48   b  of a portable machine tool  12   b.  The machine tool braking apparatus  10   b  comprises a magnetic field braking unit  14   b  which comprises at least one movably mounted claw segment element  16   b  for changing at least one characteristic variable of a magnetic field of the magnetic field braking unit  14   b.  The claw segment element  16   b  is fixed in a rotationally fixed manner to an output drive element  30   b  of an output drive unit  28   b  of the portable machine tool  12   b.  In this case, the claw segment element  16   b  is separated from the output drive element  30   b  and fixed in a rotationally fixed manner to the output drive element  30   b  by means of a type of connection which appears to be expedient to a person skilled in the art, such as by means of an adhesive bonding connection, by means of a screw connection, by means of a rivet connection etc. for example. Apart from fastening to the output drive element  30   b,  the claw segment element  16   b  is designed in at least substantially the same way as the claw segment element  16   a  which is described in  FIGS. 1 to 7 . The magnetic field braking unit  14   b  further comprises at least one further claw segment element  18   b  for changing at least one characteristic variable of a magnetic field of the magnetic field braking unit  14   b . The further claw segment element  18   b  is fixed in a rotationally fixed manner to a driver element  72   b  of the output drive unit  28   b.  The further claw segment element  18   b  is designed in at least substantially the same way as the further claw segment element  18   a  which is described in  FIGS. 1 to 7 . 
     Furthermore, the magnetic field braking unit  14   b  comprises at least one eddy current element  20   b  which is arranged on a return path element  22   b  of the magnetic field braking unit  14   b.  The return path element  22   b  is intended to compress a magnetic field of the magnetic field braking unit  14   b  in the region of the magnetic field braking unit  14   b  and to keep stray flux low. In this case, the return path element  22   b  is fixed to a bearing flange  68   b  of the output drive unit  28   b.  The return path element  22   b  is in the form of a ring. The magnetic field braking unit  14   b  further comprises at least one eddy current element  20   b  which is arranged on the return path element  22   b  of the magnetic field braking unit  14   b.  In addition, the magnetic field braking unit  14   b  has at least one braking element  26   b  which is in the form of a permanent magnet. The braking element  26   b  is connected to the eddy current element  20   b  of the magnetic field braking unit  14   b  by means of the return path element  22   b  of the magnetic field braking unit  14   b.  Therefore, the claw segment element  16   b  and the further claw segment element  18   b  are moved relative to the braking element  26   b  during operation of the portable machine tool  12   b.    
     The magnetic field braking unit  14   b  therefore has at least one braking element  26   b  which is stationary in relation to the gear mechanism housing  48   b  and is in the form of a permanent magnet. 
     A magnetic flux of the magnetic field braking unit  14   b  or of the braking element  26   b  which is in the form of a permanent magnet runs, starting from the braking element  26   b,  along a direction which runs at least substantially parallel to a rotation axis  50   b  of a spindle  66   b  of the output drive unit  28   b,  across an air gap, into the claw segment element  16   b.  From the claw segment element  16   b,  the magnetic flux in the claw segment element  16   b  runs to projections  58   b,    62   b  of the claw segment element  16   b  (only two projections are illustrated in  FIG. 8 ). The magnetic flux runs further across an air gap into the eddy current element  20   b.  In this case, the magnetic flux enters the eddy current element  20   b  along a direction which runs at least substantially perpendicular to the rotation axis  50   b  of the spindle  66   b.  Starting from the eddy current element  20   b,  the magnetic flux runs across an air gap into projections  92   b,    96   b  of the further claw segment element  18   b  (only two projections are illustrated in  FIG. 8 ). The flux exits the eddy current element  20   b  along the direction which runs at least substantially perpendicular to the rotation axis  50   b  of the spindle  66   b,  and enters the projections  92   b,    96   b  of the further claw segment element  18   b.  From the projections  92   b,    96   b  of the further claw segment element  18   b,  the magnetic flux runs across an air gap into the return path element  22   b  and back to the braking element  26   b . Reference may be made to the machine tool braking apparatus  10   a  described in  FIGS. 1 to 7  in respect of further features and functions of the machine tool braking apparatus  10   b.