Abstract:
In a clamping device ( 1 ) for machine tools ( 2 ) that is 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 ) as well as a force accumulator ( 51 ) for maintaining the clamping force, which comprises several spring packs ( 52 ) supported on an adjustment element ( 32 ) of the motion converter ( 31 ), the spring packs ( 52 ) are each only arranged on one side of the adjustment element ( 32 ). In addition, a pressure piece ( 53 ) interacting with the adjustment element ( 32 ) is firmly connected to several spacer elements ( 57 ), each of which carries a stop disc ( 61 ) and passes through the spring packs ( 52 ) as well as a spacer ( 66 ). The spacer elements ( 57 ) and the spacers ( 66 ) can be adjusted relative to one another and the spacers ( 66 ) interact with spacer pins ( 71 ) which are guided through the pressure piece ( 53 ) and are supported on the wall ( 24 ) of the housing ( 21 ) opposite to the spring packs ( 52 ). 
     Due to this embodiment, the spring packs ( 52 ) are always preloaded and do not have any axial play, and the adjustment movements of the pressure piece can thus be registered and evaluated directly. Rather, the spring packs ( 52 ) have a defined preload at all times, with the effect that the operating method is improved compared to embodiments of prior art.

Description:
REFERENCE TO PENDING PRIOR PATENT APPLICATION 
       [0001]    This patent application claims benefit of European Patent Application No. 15 197 275.9, filed Dec. 1, 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, arranged in a housing, 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 several pre-stressed spring packs arranged around the circumference supported on an adjustment 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 15 150 064. In order to undertake both internal and external clamping by means of the power-operated chuck, in this embodiment several spring packs forming the force accumulator are arranged evenly around the circumference on both sides of pressure pieces acting on the adjustment element of the motion converter. In this case, although the springs are guided on pins screwed into the housing and are alternately clamped by the axial movement of the pressure piece, a defined preload of the springs that are not involved in the particular clamping procedure is not provided. Instead, these springs have the force removed from them and are held with axial play in holes worked in the pressure piece, which as a result of the holes having a diameter larger than the springs, are not suitable for guidance purposes. 
         [0004]    Apart from the fact that this means two force accumulators with a large number of spring packs are required for both clamping directions, the axial play on the force accumulator which is not loaded means that when the clamping direction is reversed there is the requirement for this clearance to be crossed before any clamping force can be established. The adjustment range to be specified for the motion converter must consequently be made relatively large, and also it is necessary to cover a dead travel before clamping force is established. Despite the significant constructional complexity, this means that a satisfactory operating method is not provided. Furthermore, it is a disadvantage that a defined middle position of the pressure piece cannot be achieved because during a clamping procedure, a signal is only available and able to be evaluated once an adjustment movement of the pressure piece to which a signal ring is attached takes place. 
       SUMMARY OF THE INVENTION 
       [0005]    The task of the present invention is therefore to create a force accumulator which does not have any axial play in the spring packs and thus permits the middle position of the pressure piece to be ascertained precisely. The adjustment movement of the pressure piece and thus the clamping force should be able to be established without play as a result. At the same time, it should be guaranteed that a defined preload on the spring packs is provided at all times, thereby allowing short spring travel distances in order for the clamping force to be established. Furthermore, the springs of the force accumulator should always be guided to an adequate extent and their spring travel should be able to be limited according to the working procedures. The operating method of the clamping device should thus be significantly improved compared to the embodiment of prior art. 
         [0006]    In accordance with the present invention, this is achieved in a clamping device of the aforementioned type in that for internal and external clamping of a workpiece, each of the spring packs of the force accumulator is inserted on one side of the adjustment element of the motion converter between the housing and a pressure piece, that several spacer elements are firmly connected to the pressure piece, each of which carries a stop disc and passes through the spring packs as well as a spacer, that the spacer elements and the spacer can be adjusted relative to one another and that the spacer interacts with spacer pins in an axial direction, in which case these spacer pins pass through the pressure piece and are supported against the wall of the housing opposite to the spring packs. 
         [0007]    In a clamping device in which spring packs are arranged on both sides of the adjustment element of the motion converter, the aforementioned task is accomplished in that each of the spring packs is inserted between a pressure piece and stop discs in a mirror-image arrangement, that the stop discs are held by spacer elements attached to the pressure piece in such a way as to allow for limited movement and that stops are arranged on the stop discs on the housing and the pressure piece. 
         [0008]    It is advantageous in both embodiments for the spacer elements to be configured as offset pins which have one end screwed into the pressure piece and are in contact with this by means of a collar, and for the free ends of the spacer elements to be configured as a screw head with a contact surface provided on the inside for the stop discs. The spacer elements can, however, also each be configured as an intermediate piece provided with a collar in contact with the pressure piece, and with one end screwed into the pressure piece, in which case a screw as the stop for the stop discs is inserted on the free end of the intermediate piece and the collar and/or the connecting part of the spacer elements is/are configured as a guide for the spring packs and/or the stop discs. 
         [0009]    It is highly advantageous if the pressure piece, the spacer elements with stop discs and springs as well as the spacer and the spacer pins inserted in the pressure piece or the pressure piece and the spacer elements with stop discs and springs form a structural unit that can be preassembled and clamped into the housing without play. 
         [0010]    The spacer should be configured with an angled or T-piece cross section, with its outer leg guided on the inner wall of the housing in such a way as to be movable, and with the springs of the force accumulator supported against the leg which holds the spacer elements. 
         [0011]    The stop discs in both embodiments can each be configured in the form of a hollow shaft with a T-shaped cross section, the tube-shaped inner part of which accommodates a spacer element and is provided on the outside with a stop for the spacer element and on the inside with a contact surface which interacts with the spacer or the pressure piece. 
         [0012]    Moreover, it is appropriate for the distance between the stop of the tube-shaped part of the stop discs and the spacer or the pressure piece to be adjustable in the neutral position of the clamping device so that the maximum clamping force of the spring packs of the force accumulator can be adjusted and so the spacer can be guided in a movable arrangement with its radially projecting leg along the inner wall of the housing. 
         [0013]    It is also advantageous for the spacer pins to be guided in the pressure piece in an adjustable manner, in which case the pressure piece is preferably configured in two parts, and for the spacer pins to be supported on the opposite side of the housing to the spring packs or for them to be attached to this and for the spacer elements carrying the stop discs to be screwed onto the pressure piece and equipped with a collar configured as a screw head on the outer end area, in which case the collar interacts with a contact surface provided or let into the stop disc. 
         [0014]    The spring packs of the force accumulator can be formed by cup springs or coiled compression springs arranged on the stop discs, and the pressure piece which is configured in a preferred embodiment in two pieces should be supported on the shoulder of the adjustment element of the motion converter by means of anti-friction bearings. 
         [0015]    Moreover, it is advantageous for a sensor to be attached to the pressure piece and/or draw rod, for example in the form of a signal ring, in order to determine the clamping force or the adjustment movements of the draw rod. 
         [0016]    If a clamping device is configured in accordance with the present invention, it is possible in both embodiments for the preload of the spring packs to be set precisely and also for it to be maintained when the load is removed. This means the springs do not have any play when they are slackened, and are instead always kept under a selectable preload. This means that clamping force is established immediately following initiation of a clamping procedure in power-operated chucks, and that this clamping force can also be measured immediately by means of the pressure piece. 
         [0017]    Furthermore, it is possible to use recesses provided on the spacer elements and on the stop discs as well as the housing and the pressure piece to restrict the spring travel of the springs and thus the clamping force, thereby reliably avoiding any overload. Moreover, the maximum clamping force of a spring accumulator can easily be selected and, under certain circumstances, also adjusted to different working procedures. As a result, the method of function of the clamping device is also significantly improved compared to the embodiment of prior art, with a straightforward structural design. 
         [0018]    The components assigned to the force accumulator can be grouped together with it to make one structural unit which can be inserted pre-assembled and as a cartridge into the housing without play, so it is guaranteed that the spring packs of the force accumulator will not have any axial play in the non-loaded condition either, but will always be subject to preload. The force of the preloaded springs is not absorbed by the housing in this case, and because the pressure piece is returned to a middle position automatically after each clamping procedure, its adjustment movements can be registered without play and immediately evaluated in a corresponding manner by means of the machine controller. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    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, 
           [0020]      FIGS. 1 to 3  show the clamping device in an axial section, with a force accumulator arranged on one side of the motion converter, in different operating positions, 
           [0021]      FIG. 4  shows a section from  FIG. 1  in a magnified view, 
           [0022]      FIG. 5  shows the clamping device in accordance with  FIG. 1 , with spring packs as force accumulators arranged on both sides of the motion converter and 
           [0023]      FIG. 6  shows a subsection from  FIG. 5 , in a magnified view. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0024]    The clamping devices illustrated in  FIGS. 1 to 3 and 5  and identified by  1  or  1 ′ are 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  or  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 . An electric motor  4  acts on the machine spindle  3  of the machine tool  2 , by means of which the machine tool  2  can be driven. The clamping device  1  is attached to the machine spindle  3  by means of a flange  9 . 
         [0025]    In order to operate the power-operated check  5  by means of the clamping device  1 , the clamping device  1  has the electric drive motor  11  arranged with its axis parallel to the clamping device  1  so that its rotor shaft  12  is in a driving connection with a belt pulley  14  by means of a belt pulley  13  attached to it and a toothed belt  15 , in such a way that the belt pulley  14  can be connected to a gear unit  86  by means of a gear  16  attached to it using bolts  17  and a sliding sleeve  18 , and the gear unit  86  is connected ahead of the clamping device  1 . 
         [0026]    The clamping device  1  largely consists of the motion converter  31  arranged in a housing  21  for converting the rotational adjustment movement of the rotor shaft  12  of the drive motor  11  into the axial adjustment movements of the draw rod  7  required for actuating the clamping jaws  6  of the power-operated chuck  5 , and a force accumulator  51  for maintaining the clamping force when the drive motor  11  is stopped. The motion converter  31  which is supported on the housing  21  by anti-friction bearings  29  is formed by a recirculating ball spindle  35  in this case which consists of an adjustment element  32  configured with threaded grooves  37  in order to accommodate balls  36  as anti-friction bodies, and has the form of a hollow shaft  33  and counter-rotating grooves  38  worked into the draw rod  7 . The adjustment element  32  is in a driving connection with a gear unit  86  by means of gearing  40 , and the gear unit  86  is connected to an intermediate element  19  that can be linked to the bolt pulley  14 . 
         [0027]    The force accumulator  51  in the sample embodiment shown in  FIGS. 1 to 4 , and particularly in the magnified view in  FIG. 4 , is formed by several spring packs  52  arranged evenly around the circumference comprising coiled compression springs  52 ′, each of which is inserted between stop discs  61  and a spacer  66 . The stop discs  61  in this case can be moved to a limited extent on spacer elements  57  which are screwed into a pressure piece  53 . Anti-friction bearings  39  support the pressure piece  53  on a shoulder  34  formed on the adjustment element  32 , with the effect that adjustment movements of the adjustment element  32  are transmitted directly to the pressure piece  53 . In addition, contact surfaces  59 ′ are provided on the heads  59  of the spacer elements  57 , and the stop discs  61  are supported on these contact surfaces  59 ′. 
         [0028]    The stop discs  61  have a T-shaped cross section with the effect that their radially projecting leg  62  is guided so as to move on the inner wall  22  of the housing  21 , and their tube-shaped parts  63  have the springs  52 ′ arranged on them. On both ends, the stop discs  61  are equipped with contact surfaces  64  and  65  which interact firstly with the spacer  66  and secondly with a contact surface  25  provided on the end wall  23  of the housing  21 . In addition, the legs  62  of the cylindrical holding stop discs  61  are provided with recesses  62 ′ into which the spacer elements  57  engage with their heads  59  and which interact with the contact surface  65 . In addition, contact surfaces  59 ′ are provided on the heads  59  of the spacer elements  57  and have the stop discs  61  supported on them. 
         [0029]    The spacer  66  has an angled cross section into the radially aligned web  67  of which holes  67 ′ are worked for accommodating the spacer elements  57 . The axially projecting leg  68  is, in contrast, in contact with the inner wall  22  of the housing  21  and is guided by it. 
         [0030]    The spacer elements  57  are formed by pins  58  which are provided on one end with a head  59  for supporting on the stop discs  61  and at the other end with a screw thread  70  which are screwed into threaded holes  69  provided in the pressure piece  53 . By means of a collar  60  formed by a shoulder, the spacer elements  57  are located in immovable contact with the two-part pressure piece  54  formed from a ring  54  and a disc  55  which are firmly connected to one another by means of screws  56 . 
         [0031]    A selectable distance a—indicated in  FIG. 4 —between the contact surface  64  of the stop discs  61  and the end face  67 ″ of the web  67  of the spacer  66  means that the maximum clamping force of the coiled compression springs  52 ′ of the spring packs  52  can be determined in advance. This is because as soon as the contact surfaces  64  make contact with the spacer  66 , the coiled compression springs  52 ′ and thus the spring accumulator  51  are blocked. In addition, it is possible to adapt the length of the distance a to the particular conditions by, for example, changing the tube-shaped parts  63  of the stop discs  61 . 
         [0032]      FIG. 1  shows the clamping device  1  in a neutral position, i.e. the drive motor  11  is connected to the motion converter  31  by means of the sliding sleeve  18  and the power-operated chuck  5  is opened so that the workpiece  10  can be placed in the chuck  5 . The springs  52 ′ of the spring packs  52  are preloaded with a predefined preload force between the stop discs  61  and the spacer  66 . 
         [0033]    When the workpiece  10  is inserted in the power-operated chuck  5  as shown in  FIG. 2  and if energy continues to be supplied to the adjustment element  32  of the motion converter  31  by the drive motor  11  when the clamping jaws  6  are in contact with the workpiece  10 , the balls  36  of the recirculating ball spindle  35  rotate in the grooves  38  of the locationally fixed draw rod  7  so that the adjustment element  32  of the motion converter is moved to the left. This causes the springs  52 ′ of the spring packs  52  to be compressed until the tube-shaped parts  63  of the stop discs  61  make contact with the spacer  66 . The heads of the spacer elements  57  engage in this operating status as shown in  FIG. 3  in recesses  28  worked into the sidewall  23 . The adjustment travel is limited in this case by the selected, specified distance a between the contact surfaces  64  and the spacer  66 . When this operating status has been reached, the driving connection between the drive motor  11  and the motion converter  31  is interrupted. 
         [0034]    This is achieved by means of a hydraulically or pneumatically operated servo device  82  acting on the sliding sleeve  18  as well as return springs  83 . The servo device  82  is controlled for this purpose by a control unit  81  to which a sensor  91  is connected in order to determine the particular position of the pressure piece  53 , in such a way that the sliding sleeve  18  is guided back to its starting position and is automatically moved to the left by means of the pressure springs  83  in such a way that the sliding sleeve  18  is released from the gear  18  and the other gearing  84  attached to the sliding sleeve  18  engages in the gearing  85  provided on the housing  21 . Because the sliding sleeve  18  is mounted in a movable arrangement on gearing  20  attached to the intermediate piece  19 , the clamping device  1  is blocked by means of the gear unit  86  such that the clamping force of the force accumulator  51  is maintained. 
         [0035]    If, however, as shown in  FIG. 3 , a workpiece  10 ′ should be clamped internally, the directions of movement must be reversed by means of the drive motor  11  with a switching function. However, in order for the force accumulator  51  to be able to be used effectively in such clamping procedures as well, and for tension to be built up in it, several spacer pins  71  are inserted in holes  72  in the pressure piece  53 , in which case the spacer pins  71  interact with the right end wall  24  which is connected to the housing  21  by means of bolts  27 . As soon as the spacer pins  71  are moved to the right by means of a corresponding adjustment movement initiated via the adjustment element  32  of the motion converter  31 —which is implemented by means of the motion converter  31  when the draw rod  7  is stationary—and make contact with the end wall  24  which serves as a stop  26  for the spacer pins  71 , the stop disc  61  is supported against the spacer pins  71  with the effect that the set, predefined clamping force of the force accumulator  51  is established. The stop discs  61  in turn are supported on the spacer  66 , meaning that the force accumulator  51  arranged on only one side of the pressure piece  53  can be used both for external clamping and for internal clamping of a workpiece  10  or  10 ′. 
         [0036]    However, as shown in  FIG. 3 , the spacer pins  71  can also be firmly connected to the housing  21  by means of a screw thread  74  which engages in threaded holes  73  worked into the end wall  24 . 
         [0037]    The servo device  91  by means of which the particular status of the pressure piece  53  is to be established is influenced by a signal ring  93  which is attached to the pressure piece  53  by means of a holder  94 . For this purpose, slot-shaped recesses  30  are worked into the housing  21  and have the holders  94  carrying the signal ring  93  inserted in them. 
         [0038]    Furthermore, as shown in  FIG. 5 , a signal ring  95  is attached to the draw rod  7  which interacts with a sensor  92  in order to determine the particular position of the draw rod  7  and thus of the clamping jaws  6  of the power-operated chuck  5 . 
         [0039]    The pressure piece  53 , the spacer elements  57  with stop discs  61  and the springs  52  as well as the spacer  66  and the spacer pins  71  inserted in the pressure piece  53  can be preassembled as structural unit B and precisely adapted to the specified installation dimension. Structural unit B can thus be inserted into the housing  21  with an exact fit and without play, and consequently the springs  52  do not have any axial play even when they are in the slackened status. 
         [0040]    In the sample embodiments of the clamping device  1 ′ shown in  FIGS. 5 and 6 , the f force accumulator  101  consists of spring packs  102  and  103  or  102  and  103 ′ arranged on both sides of a pressure piece  104 , in which case these spring packs are also continuously pretensioned. The spring packs  102  or  102 ′ are composed of cup springs  110  or coiled compression springs  110 ′ in this case. 
         [0041]    The stop discs  111  are each held on pin-like spacer elements  121  or  122  which are screwed into holes  105  worked into the pressure piece  104 . The stop discs  111  have a T-shaped cross section and are guided in a moving arrangement with the webs  112  running in a radial direction along the inner wall  22  of the housing, whereas the tube-shaped pins  113  hold the spacer elements  121  or  122  and interact with stops  114  or  115  provided on the end wall  23  of the housing and the pressure piece  104 . 
         [0042]    The spacer elements  121  shown in the upper half of  FIG. 5  and the magnified view shown in  FIG. 6  are configured as offset pins  123  the head  124  of which engage in recesses  116  provided in the stop discs  111  and have their free ends configured as screw threads  125  screwed into the threaded holes  105  in the pressure piece  104 . The spacer elements  121  are supported on the pressure piece  104  by means of a collar  126 , and their heads  124  are in contact with a contact surface provided on the stop discs  111  in the slackened status of the spring packs  102 ,  103  as shown in the right half of  FIG. 6 , involving contact between a contact surface provided on the stop discs  111  and counter-surfaces  117  provided on the heads  124 . 
         [0043]    Each of the spacer elements  122  consists of an intermediate piece  127  provided with a collar  128  for supporting against the pressure piece  104 , and screws  131  inserted in the intermediate pieces  127  by means of which the stop discs  111  are also transported during adjustment movements. Screw threads  130  are used for screwing the intermediate pieces  127  into holes  105  provided in the pressure piece  104 . 
         [0044]    The two spring packs  102  and  103  and  102 ′ and  103 ′ are supported on the stop discs  111  as well as the pressure piece  104  by means of spacer elements  121  or  122 , in which case a selectable preload is always provided even in the non-loaded force accumulator—in  FIG. 6  in the force accumulator  103 —and the individual springs  102 ′,  103 ′ are thus securely held between the stop discs  111  and the pressure piece  104 . 
         [0045]    These embodiments mean that the springs  52 ′;  102 ′,  103 ′ of the force accumulators  51 ,  101  always have a selectable preload and, when in the slackened condition, are arranged without axial play between the stop discs  61  or  111  and the spacer  66  or pressure piece  104 .