Abstract:
A clamping device ( 1 ) for machine tools ( 2 ), equipped with a power-operated chuck ( 5 ) and an electric drive motor ( 11 ) with a changeover function for triggering clamping movements, a motion converter ( 31 ) for converting the adjustment movements into the axial movements of a draw rod ( 7 ) as well as a force accumulator ( 41 ), is configured as a play and slip-free functional unit, and to determine the axial adjustment movements of the draw rod ( 7 ) when clamping and unclamping a workpiece ( 10 ), an electronic rotary encoder ( 101 ) in a stationary arrangement is allocated to one of the components ( 53; 54 ) that is involved in the force transmission. Due to this embodiment, it is possible to configure the clamping device ( 1 ) with inherent rigidity, having neither play nor slip, as a result of which both the axial adjustment movements of the draw rod ( 7 ) and consequently the particular operating positions of the clamping jaws ( 6 ) of the power-operated chuck ( 5 ) can be used without restrictions. The machine tool ( 2 ) can be controlled without problems in this way with great accuracy without having to accept inaccuracies.

Description:
REFERENCE TO PENDING PRIOR PATENT APPLICATION 
       [0001]    This patent application claims benefit of European Patent Application No. 15 150 064.2, filed Jan. 5, 2015, which patent application is hereby incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a clamping device, especially for machine tools, that are for example equipped with a power-operated chuck for holding a workpiece and the clamping jaws of which can be adjusted using the clamping device by means of an axially moveable draw rod as the actuating element, in which the clamping device possesses an electric drive motor with a changeover function for triggering clamping movements, a motion converter for converting the adjustment movements of the rotor shaft of the drive motor into the axial adjustment movements of the draw rod required for actuating the clamping jaws as well as a force accumulator for maintaining the clamping force, which comprises pre-stressed spring packs supported on an adjusting element of the motion converter that is configured as a hollow shaft and is provided with a projection projecting radially outward. 
       BACKGROUND OF THE INVENTION 
       [0003]    A clamping device of this kind is disclosed in EP 2 548 681 A1. In this embodiment, the movement converter has several planetary roller spindles as transmission elements which have a certain amount of slip due to their angled position in the thread turns provided in the draw rod and a hollow shaft in each case, depending on the pitch angle of the thread. This is detrimental because the draw rod thus occupies a different position on each rotation of the hollow shaft. The exact location of the draw rod and thus of the clamping jaws consequently cannot be determined by means of a travel sensor. 
         [0004]    In addition, there is an idle movement in each change of direction between clamping and unclamping of a workpiece, which also means it is not possible to determine the axial location of the draw rod. 
         [0005]    Furthermore, in this clamping device, it is a disadvantage that the cup springs used in the spring packs of the force accumulator only have a short spring travel, which is not suitable for providing satisfactory control of the machine tool. Also, a change in the clamping force of the power-operated chuck using the cup springs is only possible to an inadequate extent. 
         [0006]    The clamping device of prior art consequently cannot be used in machine tools that should be controlled with the help of operating parameters resulting from the working sequences. 
         [0007]    The task of the present invention is therefore to create a clamping device of the aforementioned type such that no slip occurs in the motion converter and that play-free adjustment movements are always provided which can thus be used for controlling the machine tool. Above all, the play-free rotary movements of one of the components of the clamping device should be able to be used as a measuring parameter for the corresponding adjustment movements of the draw rod in a simple manner. In addition, the force accumulator should have a large adjustment range that can also be used for controlling the machine tool, and its clamping force should be easily adaptable to changeable operating conditions without difficulties. 
       SUMMARY OF THE INVENTION 
       [0008]    In accordance with the present invention, this is achieved in a clamping device of the aforementioned type in that the clamping device is configured as a play and slip-free functional unit and that to determine the axial adjustment movements of the draw rod when clamping and unclamping a workpiece, an electronic rotary encoder in a stationary arrangement is allocated to a component of the clamping device that is involved in the force transmission. 
         [0009]    For play-free configuration of the clamping device, it is necessary to connect the adjusting element of the motion converter to the actuation element in a direct, driveable connection by means of a pre-stressed ball screw drive; moreover, it is necessary to configure the spring packs of the force accumulator that act on one or both sides on the adjusting element equipped with a return channel for the balls of the ball screw drive by means of a plurality of coil compression springs arranged evenly around the circumference, and to use the motion converter and the force accumulator in a first housing in a fixed location, which is provided with a projection element or a carrier configured as a hollow shaft facing away from the power-operated chuck, on which transmission elements allocated to the drive motor are mounted, and are connected in a driveable connection to the adjusting element of the motion converter. 
         [0010]    The transmission elements arranged on the projection element should in this case be able to be locked on the projection element or be in a positive-locking connection by means of a second housing that accommodates them, or a carrier that supports them. 
         [0011]    The second housing or the carrier, for example, can be provided as a component of the clamping device interacting with the rotary encoder, in which case they are provided on an outer jacket surface with one or more barcodes or toothed profiles, in the rotational plane of which the sensor of the rotary encoder is arranged. 
         [0012]    In accordance with a different embodiment, it is also possible for the rotary encoder to be allocated to the drive motor of the clamping device in that a cylindrical disk is arranged on its rotor shaft in a rotationally fixed arrangement, one or more barcodes or toothed profiles are attached to its outer jacket surface that interact with the sensor of the rotary encoder. 
         [0013]    By means of the electronic rotary encoder, it is consequently possible in a simple way to register the speed of rotation, direction of rotation and angle of rotation of a component of the clamping device. Play and slip-free transmission is provided, as a result of which the particular operating status of the draw rod can be determined precisely, in particular during clamping or unclamping of a workpiece. By means of this measuring parameter, it is thus ensured that the machine tool can be controlled and monitored highly accurately. 
         [0014]    It is also advantageous in this case for the coil compression springs of the force accumulator to be inserted in a one or two-part pressure piece in which the projection of the adjusting element of the motion converter engages and is supported in this in an axially rotating arrangement, in which case the coil compression springs of the force accumulator that can be inserted in holes preferably provided in the pressure piece on one or both sides of the projection of the adjusting element have a rectangular, preferably square, elliptical or circular cross sectional surface. 
         [0015]    This embodiment of the force accumulator makes it possible to configure its clamping force for the maximum clamping force of the power-operated chuck when the pressure piece is completely equipped with coil compression springs, and by removing individual coil compression springs or using springs with a low spring force, the corresponding clamping force of the force accumulator can be adapted to specified operating conditions. 
         [0016]    Furthermore, it is advantageous for the ball screw drive to be provided with lubricant from the interior of the first housing in a forced feed by means of adjusting movements of the adjusting element of the motion converter, for example through holes provided in it. 
         [0017]    It is also appropriate for the pressure piece to be provided with a signal transmitter that passes through the first housing, which interacts with a travel sensor in order to determine the particular clamping force of the force accumulator. In addition, the draw rod can be equipped with a signal transmitter configured as a stroke ring in the area between the machine tool and the first housing of the clamping device, in which case the signal transmitter interacts with a further travel sensor to determine the particular position of the draw rod during working procedures. By means of these signal transmitters, it is thus possible to pick up measurement parameters that allow reliable control of the machine tool. 
         [0018]    Moreover, it is advantageous for the transmission elements allocated to the drive motor of the clamping device to be inserted in the second housing which is connected in a fluid-tight connection to the first housing, or for them to be supported on the carrier. 
         [0019]    The transmission elements should be configured as a play-free double-planetary gear unit with different numbers of teeth on the planetary gears in order to create step-down or step-up ratios, in which case the planetary gears must be mounted in a rotating arrangement on a pin supported in the second housing and must engage in sun gears, of which one sun gear is firmly connected to the projection element and the other sun gear interacts with an intermediate gear that engages in the intermediate elements. 
         [0020]    The transmission elements can also be configured by a gear rim provided on the carrier. The transmission elements in this case should be able to be connected to the adjusting element of the motion converter by means of one or more intermediate elements passed through the adjacent end wall of the first housing. 
         [0021]    Each of the intermediate elements can be configured as eccentrically mounted double gears which are mounted in a rotating arrangement on a pin supported on the first and/or second housing, and are in a driving connection with the sun gear of one of the gear sets of the planetary gear unit or with the gear rim provided on the carrier or the adjusting element of the motion converter. 
         [0022]    In order to lock the second housing or the carrier with the projection, it is possible to provide a sliding sleeve that is axially adjusted on it and mounted in a non-rotating arrangement, which can be activated by means of a servo device and/or the force of springs. 
         [0023]    Moreover, the draw rod should be mounted in the projection element of the first housing with its end facing away from the power-operated chuck in an axially adjustable arrangement, and the first and the second housings should be filled completely or partially with oil or a lubricant. 
         [0024]    If a clamping device is configured in accordance with the present invention and has a plurality of design features that are also partially known, it is possible to configure the clamping device with inherent rigidity, having neither play nor slip, as a result of which both the axial adjustment movements of the draw rod and consequently the particular operating positions of the clamping jaws of the power-operated chuck can be used without restrictions. The machine tool can consequently be controlled without problems using the measuring parameters ascertained in this way, and it can be controlled with great accuracy without having to accept inaccuracies. Above all, it is an advantage in this case that no slip or play exists between the rotating components of the clamping device, as a result of which the position of the draw rod can be measured without difficulties by means of the rotary encoder outside the clamping device, thereby guaranteeing monitoring and control during operation. 
         [0025]    The rigid connection between the components involved in the force transmission which is achieved in spite of the simple configuration and low complexity of design thereby guarantees a correct correlation of the particular operating data of the clamping device, and consequently accurate positioning of the machine tool. There is no need to undertake differential measurements in this case, because no slip occurs and there is no play either. Furthermore, the particular measurement values can be ascertained without difficulties outside the clamping device, as a result of which trouble-free operation is guaranteed over a long operating period. 
         [0026]    Furthermore, it is an advantage that the preload of the force accumulator can be adapted without difficulties to the particular requirements by the plurality of coil compression springs which can have a high clamping force. And, because the coil compression springs have a relatively long spring travel that can be used as a measuring parameters for the clamping force, it is possible to determine the particular clamping force applied in a reliable manner. 
         [0027]    In the clamping device configured in accordance with the proposal, it is thus possible to ascertain the precise clamping force and the location of the draw rod at any time, as a result of which this operating data enables the machine tool to be controlled without any incorrect measurements. In addition, the interior of the functional unit can be sealed and filled with oil or a lubricant, consequently guaranteeing trouble-free operation over a long period with a high degree of operational reliability. Also, operating the clamping device requires little energy, because during operation the components involved in the force transmission are clamped against one another so the drive motor does not have to absorb any energy. The clamping device can thus be used in a wide range of applications in an economic manner. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    The drawing shows two sample embodiments of the clamping device configured in accordance with the present invention, the details of which are explained below. In the drawing, 
           [0029]      FIG. 1  shows the clamping device in an axial section in the clamping position, thus during a working procedure, 
           [0030]      FIG. 2  shows a modification of the clamping device in accordance with  FIG. 1 , also in the locked operating condition, 
           [0031]      FIG. 3  shows a section through the clamping device in accordance with  FIG. 1 , in a magnified view with a decoupled drive motor and a different kind of locking mechanism, as well as 
           [0032]      FIG. 4  shows the force accumulator of the clamping device in a magnified perspective view. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0033]    The clamping device illustrated in  FIGS. 1 to 3  and identified by  1  is used for actuating a power-operated chuck  5  arranged on a machine tool  2 , by means of the radially adjustable clamping jaws  6  of which a workpiece  10  to be machined can be clamped in the power-operated chuck  5 . The clamping jaws  6  of the power-operated chuck  5  in this case can be actuated via relay levers  8  by an axially adjustable, two-part draw rod  7 ,  7 ′ that is in a driven connection with an electric drive motor  11  that has a changeover function by means of a motion converter  31 . The rotational adjustment movements of the drive motor  11  are converted into axial adjustment movements of the draw rod  7 ,  7 ′ by means of the motion converter  31 . 
         [0034]    The drive motor  11  consists of a stator  12  arranged parallel to the longitudinal axis A of the clamping device  1  in a stationary location and a rotor  13  which has a gear  15  attached to its rotor shafts  14  in a rotationally fixed arrangement, by means of which a toothed belt  16  is guided which interacts with a geared drive wheel  17  allocated to the clamping device  11 . However, the drive motor  11  can also be arranged at right angles to the longitudinal axis A of the clamping device and is in a driving connection with the drive wheel  17  via bevel gears. 
         [0035]    The clamping device  1  has a housing  21  in which the motion converter  31  and a force accumulator  41  are accommodated. On the side facing the machine tool  2 , the housing  21  is provided with a projecting web  27  to which a flange  28  is attached by means of screws  29 . Additional screws  30  attach the flange  28  to another flange  9  that is formed on the machine spindle  3 . An electric motor  4  acts on the machine spindle  3 , by means of which the machine tool  2  can be driven. 
         [0036]    In the embodiment shown in  FIG. 1 , on the side facing away from the power-operated chuck  5 , a projection element  24  is attached to the housing  21  and carries a second housing  53 . The second housing  53  accommodates a double-planetary gear unit  55  as the transmission element  51 , which comprises both sun gears  56  and  57  as well as several planetary gears  58  and  59  that engage in the sun gears  56  and  57 . In this case, as shown in detail in  FIG. 3 , each of the planetary gears  58  and  59  is mounted on an anti-friction bearing with  70  on a pin  60  that is supported in the housing  53 . The planetary gears  58  engage with the sun gear  56  which is firmly connected to the projection element  24 . The planetary gears  59 , on the other hand, engage in an intermediate gear  97  which is in a driving connection with intermediate elements  62 . The double planetary gear unit  55  thus forms the transmission element  51  by means of which the drive energy taken from the drive motor  11  can be delivered to the motion converter  31  via the intermediate elements  62 , thereby causing a step-down ratio to be applied to the input rotation because the numbers of teeth on the planetary gears  58  and  59  are different. 
         [0037]    In this case, the intermediate elements  62  consist of double-toothed gears  63  that are mounted on anti-friction bearings  69  so as to rotate on pins  64 . The intermediate elements  62  pass through the end wall  23  of the first housing  21  as well as a flange  25  of the projection element  24 , with the effect that the double-toothed gears  63  can engage in gearing  69  provided on the adjusting element  32  of the motion converter  31 , as a result of which the gearing  68  can be placed in a driving connection with the drive motor  11  for actuating the power-operated chuck  5 . 
         [0038]    The motion converter  31  has the adjusting element  32  configured as a hollow shaft  33 , which is provided with a projection  34  projecting radially outwards, and has a pre-stressed ball screw drive  35  worked into it. A channel  37  is provided in order to return the balls  36  that interact with a thread  38  worked into the draw rod  7 ′. Anti-friction bearings  50  support the adjusting element  32  on the projection element  24  in a rotating arrangement. 
         [0039]    The force accumulator  41  has a plurality of spring packs  42  and  43  that are inserted in a pressure piece  45 . Furthermore, as can be seen in particular in  FIG. 4 , a plurality of holes  46  are worked into the pressure piece  45 , and are distributed evenly around the circumference; these holes  46  have the coil compression springs  44  or  44 ′ inserted in them, which have a square or circular cross-section and are guided on pins  47 . Anti-friction bearings  48  and  49  support the pressure piece  45  in the axial and radial direction on the adjusting element  32  of the motion converter  31 . 
         [0040]    This configuration makes it possible to arrange a large number of coil compression springs  44  or  44 ′ in the pressure piece  45 , and due to their cross-sectional shape these springs have a high spring force. Also, the coil compression springs  44  or  44 ′ can easily be exchanged or partially removed within a short period of time, thereby allowing the force of the force accumulator  41  to be adapted to the particular required clamping force of the power-operated chuck  5  without difficulty. Also, if individual coil compression springs  44  or  44 ′ are damaged for any reason and needed to be replaced quickly this means the clamping force of the force accumulator  41  is not significantly impaired. 
         [0041]    Furthermore, because the coil compression springs  44  or  44 ′ also have a large spring travel, it is possible to use them for precise control of the machine tool  2 . To make this possible, a signal transmitter  92  is attached to the pressure piece  45  and passes through an opening  93  provided in the first housing  21 , interacting with a travel sensor  91  in a fixed location. In this way, the force prevailing in the force accumulator  41  can easily be determined. 
         [0042]    Also, the particular location of the clamping jaws  6  of the power-operated chuck  5  can be determined in a similar manner during a working procedure. To do this, a stroke ring  95  is attached as a signal transmitter on the draw rod  7  in the area between the first housing  21  and the machine tool  2 , and also interacts with a travel sensor  94 . In this case, the stroke ring  95  passes through the web  27  that is provided with an opening  96 , by means of which the first housing  21  is fixed to the flange  28  that is attached to the flange  9  formed on the machine spindle  3 . 
         [0043]    In the operating position shown in  FIGS. 1 and 2 , corresponding to a working position in which the workpiece  10  clamped in the power-operated chuck  5  is machined, the projection element  24  is locked with the first housing  21 . This is done using a sliding sleeve  71  that is provided with gearing at  72  on its end face that faces towards the projection element  24 , with the gearing  72  engaging in gearing  19  attached to a flange  18  that is itself connected to the second housing  53  in a rotationally fixed arrangement in this operating condition. The second housing  53  is thus linked to the projection element  24  on which the sliding sleeve  71  is supported in a rotationally fixed arrangement but is axially movable, meaning that a rigid connection is established between all components involved in the force transmission. 
         [0044]    The sliding sleeve  71  is able to be operated automatically by the force of springs  74  that are supported against the sliding sleeve  71  as well as against a flange  76  attached to the projection element  24 . However, a servo device  73  is provided for disengaging the sliding sleeve  71  and operates against the force of the springs  74  on the sliding sleeve  71 . The sliding sleeve  71  can be controlled by means of a control unit  81  to which the travel sensors  91  and  94  are also connected. In order for the workpiece  10  to be clamped or unclamped, it is consequently possible to release the locking of the gearings  19  and  72 , while the drive motor  11  can supply energy to the motion converter  31  by means of the second housing  53 , the planetary gear unit  55  and the intermediate elements  62 , in order to act on the draw rod  7 ,  7 ′ and thus on the clamping jaws  6  of the power-operated chuck  5  by means of the adjusting element  32  of the movement converter  31  in a corresponding manner. 
         [0045]    In order to allow the ball screw drive  35  of the motion converter  31  to be supplied adequately with lubricant at all times, the interior  22  of the housing  21  filled with a lubricant is sealed towards the outside by means of seals  65  and  66 . 
         [0046]    In addition, the bearings  69  that carry the second housing  53  which, in addition, is sealed against the first housing  21  by means of a seal  67 , are configured with a fluid-tight seal. And, because several radial holes  39  are worked into the adjusting element  32  of the motion converter  31 , each adjustment procedure towards the right of the adjusting element  31  and of the pressure piece  45  in a driving connection with the adjusting element  32  causes lubricant to be supplied to the transmission element  51  which is sealed on one side by a seal  40 , with the effect that the ball screw drive  35  is always adequately lubricated. 
         [0047]    In the embodiment shown in  FIG. 3 , the drive motor  11  is decoupled from the clamping device  1 . In this case, the gearing  72  attached to the sliding sleeve  71  is not engaged with the gearing  19  provided on the drive gear  17  that is supported on the projection element  24  by means of bearings  98 . The sliding sleeve  71  which can in turn be actuated by means of the servo device  73  is supported on the end wall  23  of the first housing  21  by means of pins  77 , however, with gearing  79  and  80  provided in this housing  21  engaging with one another. Screws  78  attach the pins  77  to the sliding sleeve  71 , and they are thus guided in the second housing  53 . 
         [0048]    In the embodiment shown in  FIG. 2 , the drive gear  17  and also the transmission elements  52  configured in the form of a gear rim  61  are attached to a T-shaped carrier  54 . The drive energy for carrying out adjusting movements of the clamping jaws  6  of the power-operated chuck  5  is thus supplied to the motion converter  31  by means of the carrier  54  and, in turn, by means of intermediate elements  62 , and via the ball screw drive  35  of the motion converter  31 , to the draw rod  7 ,  7 ′ and from this to the clamping jaws  6  of the power-operated chuck  5 . In this case, a gear rim  61  attached to the carrier  54  is provided as the transmission element  52 . 
         [0049]    The play and slip-free configuration of the motion converter  31 , of the force accumulator  41  and of the other components of the clamping device  1  involved in the force transmission makes it possible to use the adjusting movements of rotating parts for exact control and monitoring of the machine tool  2 . To make this possible, the embodiment shown in  FIGS. 1 and 2  are each provided with an electronic rotary encoder  101  which consists of a sensor  103  and a one or multiple-part barcode  104  attached to an outer jacket surface of the second housing  53  or of the carrier  54 . Instead of the barcode  104 , it is also possible to provide a correspondingly configured gearing. 
         [0050]    By means of the rotary encoder  101 , it is thus not only possible to determine the rotation speeds and the particular rotation direction of the second housing  53  or of the carrier  54 , but also slight changes in the rotation angle of these components. These measuring parameters, which can also be sent to the control unit  81 , correspond precisely to the changes in the draw rod  7 ,  7 ′ or the power-operated chuck  5 , because the design embodiment of the clamping device  1  means that no slip or play has to be accepted, as a result of which the machine tool  2  can be controlled with very great accuracy. 
         [0051]    For the same purpose, the drive motor  11  can also be equipped with an electronic rotary encoder  102 . For this purpose, only a disc  105  needs to be attached to its rotor shaft  14 , and the disc must be provided with a corresponding barcode  107  on its outer jacket surface. The measurement results are to be sent to the control unit in turn by means of a sensor  106 .