Patent Publication Number: US-2023146914-A1

Title: Clamping device for tool holder

Description:
FIELD OF THE INVENTION AND PRIOR ART 
     The present invention relates to a clamping device according to the preamble of claim  1 , which is intended to be used for connecting a tool holder to a machine tool. 
     Within the field of machine tools for metal cutting, the cutting tools, for instance in the form of drills, milling tools or lathe tools, used for machining work pieces of metallic material are often fixed to a tool holder, which in its turn may be detachably fixed to a clamping device that is mounted to a machine tool. It is previously know to clamp a shank of such a tool holder to the housing of a clamping device by means of a clamping mechanism arranged in the housing. Tool holders for cutting tools in the form of drills and milling tools are normally clamped to a rotatable housing in the form of a spindle, whereas tool holders for lathe tools may be clamped to a non-rotatably arranged housing. When the cutting tool needs to be exchanged, the tool holder is released from the housing of the clamping device and a new tool holder with another cutting tool is clamped to the housing. A clamping device of this type may for instance be detachably fixed at the periphery of a tool turret included in a machine tool. 
     Many previously known clamping devices of the above-mentioned type require manual operation in order to effect clamping or releasing of a tool holder. There also exist clamping devices adapted for automatic tool changing operations, where for instance a hydraulic piston is used to control the state of the clamping mechanism. However, such automatically operated clamping mechanisms are normally rather bulky and require much space, particularly in a longitudinal direction, which make them unsuitable for use with a tool turret of a machine tool where the available axial space for the clamping device is limited. 
     A clamping device comprising a housing in the form of a spindle with a clamping mechanism adapted for an automatic tool changing operation is previously known from EP 1 468 767 B1. In the clamping device according to EP 1 468 767 B1, an actuating member in the form of a first drawbar is slidably mounted inside the spindle and configured to effect an axial displacement of a second drawbar via a force amplifying mechanism comprising a number of co-operating wedges arranged between the drawbars. A gas spring inside the spindle is configured to urge the two drawbars into a retracted locking position, in which a tool holder is clamped to the spindle, and a hydraulic piston may be configured to act on a piston at the rear end of the gas spring in order to achieve a displacement of the two drawbars into an advanced releasing position, in which the tool holder may be released from the spindle. However, this previously known clamping device has a relatively long axial extent and it is therefore not suitable to use this type of clamping device when tool holders are to be detachably fixed at the periphery of a tool turret where the available axial space for the clamping device is limited. 
     OBJECT OF THE INVENTION 
     The object of the present invention is to provide a clamping device of the above-mentioned type that has a new and favourable design and that is suitable for use with a tool turret of a machine tool. 
     SUMMARY OF THE INVENTION 
     According to the invention, said object is achieved by means of a clamping device having the features defined in claim  1 . 
     The clamping device according to the invention comprises: 
     a housing having a front end, a rear end and a bore which intersects the front end and extends rearwardly therefrom, wherein a mounting portion for receiving the tool holder shank is provided at a front end of the bore; 
     a drawbar slidably mounted inside the bore so as to be reciprocally moveable in the bore along a longitudinal axis thereof between an advanced releasing position and a retracted locking position; 
     engagement members arranged around the drawbar at a front end thereof, wherein the engagement members, under the effect of a movement of the drawbar from the advanced releasing position to the retracted locking position, are moveable from a first position, in which the engagement members allow the tool holder shank to move into and out of said mounting portion of the bore, to a second position, in which the engagement members are in locking engagement with the tool holder shank and keep it secured to the housing; 
     an actuating member rotatably arranged in the bore so as to be rotatable about the longitudinal axis, wherein the actuating member is axially fixed in relation to the housing so as to be prevented from moving along the longitudinal axis; and 
     a motion transferring mechanism arranged inside the housing, wherein the motion transferring mechanism is configured to transfer a rotary movement of the actuating member in a first rotary direction in relation to the housing into an axial movement of the drawbar from the advanced releasing position to the retracted locking position. 
     The above-mentioned motion transferring mechanism comprises: 
     a first motion transferring element, which is fixed to or fixedly connected to the drawbar so as to be moveable along the longitudinal axis together with the drawbar and which is provided with a first pressure receiving contact surface facing towards the front end of the housing; and 
     a second motion transferring element, which is fixed to the actuating member so as to be rotatable in relation to the housing and the drawbar together with the actuating member and which is provided with a first pressure applying contact surface facing towards the rear end of the housing for contacting the first pressure receiving contact surface, wherein the first pressure applying contact surface is configured to press the drawbar from the advanced releasing position to the retracted locking position by sliding and pressing against the first pressure receiving contact surface when the actuating member is rotated in the first rotary direction. 
     By using the above-mentioned actuating member and motion transferring mechanism for controlling the axial movement of the drawbar, no gas spring or the similar that takes up a lot of space in the axial direction is required for controlling the axial movement of the drawbar, which implies that the clamping device can be made compact in axial direction. The clamping device is therefore suitable for use in tool turrets. 
     According to the invention, at least one of the above-mentioned contact surfaces, i.e. at least one of the first pressure receiving contact surface and the first pressure applying contact surface, comprises a first contact surface section and a second contact surface section arranged at different positions along the associated motion transferring element, such that, upon a movement of the actuating member in the first rotary direction, the first contact surface section is configured to slide against the other contact surface during a first phase of the movement and the second contact surface section is configured to slide against the other contact surface during a final phase of the movement, wherein the first contact surface section has a larger pitch than the second contact surface section such that the same angular displacement of the actuating member in connection with a rotation thereof in the first rotary direction in relation to the housing results in a longer axial movement of the drawbar in said first phase than in said final phase. Hereby, the drawbar may be quickly moved in axial direction during the initial phase of the clamping under the effect of the larger pitch. This initial clamping phase does not require much force. However, the final phase of the clamping will require larger force. When the actual clamping occurs, i.e. when the engagement members assume the above-mentioned second position, the drawbar is moved in axial direction under the effect of the smaller pitch such that the axial movement of the drawbar will be small as compared to the angular displacement of the actuating member, which results in a force-amplifying effect, also referred to as “power boost”. By using a larger pitch for the initial axial movement of the drawbar, it will be possible to achieve the entire axial movement of the drawbar from the advanced releasing position to the retracted locking position with a rather small angular displacement of the actuating member. 
     The above-mentioned housing of the clamping device may have the form of a rotatable spindle, which may be connected or connectable to a drive mechanism in a tool turret of a machine tool. However, the clamping device of the present invention is not limited to use in a tool turret and such a housing in the form of a rotatable spindle could as an alternative constitute the main spindle of a machine tool or be connected to such a main spindle without any intermediate tool turret. When the housing has the form of a rotatable spindle, the clamping device may be used for clamping tool holders provided with drilling or milling tools or other types of rotating tools. The housing included in the clamping device of the present invention may as a further alternative be a non-rotatable housing. In the latter case, the clamping device may be used for clamping tool holders provided with lathe tools or other types of non-rotating tools. 
     According to an embodiment of the invention, the pitch is constant along said second contact surface section. The pitch is preferably also constant along said first contact surface section, but of course larger than the pitch of the second contact surface section. The use of a constant first pitch along the first contact surface section and/or a constant second pitch along the second contact surface section will facilitate the manufacturing of the motion transferring elements. However, as an alternative, the first contact surface section and/or the second contact surface section may have a pitch that varies along the contact surface section in question. 
     Said second contact surface section is preferably positioned on the associated motion transferring element adjacent to said first contact surface section, such that the second contact surface section follows directly after the first contact surface section. However, as an alternative, one or more intermediate contact surface sections with a constant or varying pitch that deviates from the pitches of the first and second contact surface sections may be provided between the first and second contact surface sections on the motion transferring element in question. 
     According to another embodiment of the invention, said second contact surface section has such a pitch that the first and second motion transferring elements will keep the actuating member in a self-locking rotary position in relation to the drawbar when the drawbar has been forced into the retracted locking position under the effect of the actuating member and the first and second motion transferring elements. Hereby, the drawbar can be held in the retracted locking position without requiring any external force from an actuator, which implies that the actuating member only needs to be subjected to external force from an actuator, for instance in the form of a hydraulic actuator, in connection with a tool changing operation. 
     According to another embodiment of the invention, the motion transferring mechanism is configured to transfer a rotary movement of the actuating member in a second rotary direction, opposite to the first rotary direction, into an axial movement of the drawbar from the retracted locking position to the advanced releasing position. Hereby, the actuating member may also be used for effecting a movement of the drawbar from the retracted locking position to the advanced releasing position in connection with a tool changing operation. 
     According to another embodiment of the invention, the motion transferring mechanism comprises two or more sets of such motion transferring elements, each set comprising a first motion transferring element and an associated second motion transferring element, wherein these sets are spaced apart from each other in the circumferential direction of the actuating member. Said sets are preferably evenly distributed in the circumferential direction of the actuating member. Hereby, a well-balanced clamping device with good force distribution is obtained. 
     Further advantageous features of the clamping device according to the present invention will appear from the description following below and the dependent claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       With reference to the appended drawings, a specific description of embodiments of the invention cited as examples follows below. In the drawings: 
         FIG.  1    is a longitudinal section through a clamping device according to an embodiment of the present invention and a tool holder, with the drawbar of the clamping device in a retracted locking position and with the tool holder clamped to the housing of the clamping device, 
         FIG.  2    is a lateral view of the clamping device and tool holder shown in  FIG.  1   , 
         FIG.  3    is a cross-section according to the line III-III in  FIG.  2   , 
         FIG.  4    is a longitudinal section corresponding to  FIG.  1   , with the drawbar shown in an advanced releasing position, 
         FIG.  5    is a lateral view of the clamping device and tool holder shown in  FIG.  4   , 
         FIG.  6    is a cross-section according to the line VI-VI in  FIG.  5   , 
         FIG.  7    is an exploded view of the clamping device and tool holder of  FIG.  1   , 
         FIG.  8    is an exploded view from another direction of the clamping device and tool holder of  FIG.  1   , 
         FIG.  9    is a partly cut perspective view of the clamping device of  FIG.  1   , 
         FIG.  10    is a lateral view of a component included in the clamping device of  FIG.  1   , 
         FIG.  11    is a lateral view of a part of the actuating member included in the clamping device of  FIG.  1   , 
         FIG.  12    is a perspective view of a clamping device according to another embodiment of the invention and a tool holder, 
         FIG.  13    is a cross-section through the clamping device of  FIG.  12   , with an actuator vane of the clamping device shown in a first position, 
         FIG.  14    is a longitudinal section through a part of the clamping device according to the line XIV-XIV in  FIG.  13   , 
         FIG.  15    is a cross-section corresponding to  FIG.  13   , but with the actuator vane shown in a second position, 
         FIG.  16    is a longitudinal section through a part of the clamping device according to the line XVI-XVI in  FIG.  15   , 
         FIG.  17    is an exploded view of the clamping device and tool holder of  FIG.  12   , 
         FIG.  18    is an exploded view of a part shown in an assembled state in  FIG.  17   , 
         FIG.  19    is an exploded view from another direction of the clamping device and tool holder of  FIG.  12   , 
         FIG.  20    is an exploded view corresponding to  FIG.  18    but seen from a another direction, 
         FIG.  21    is a partly cut perspective view of the clamping device of  FIG.  12   , 
         FIG.  22    is a lateral view of a component included in the clamping device of  FIG.  12   , and 
         FIG.  23    is a lateral view of a part of the actuating member included in the clamping device of  FIG.  12   . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     A clamping device  1  according to different embodiments of the present invention is illustrated in  FIGS.  1 - 23   . The clamping device  1  is configured to releasably clamp a tool holder  90  (very schematically illustrated in the drawings) to a housing  2  of the clamping device and enable the machining of a work piece by means of a cutting tool (not shown) fixed to the tool holder  90 . 
     The housing  2  has a front end  2   a,  a rear end  2   b  and a bore  3  which intersects the front end  2   a  and extends rearwardly therefrom. Thus, the bore  3  has an entrance opening  3   a  at the front end  2   a  of the housing. 
     In the embodiment illustrated in  FIGS.  1 - 11   , the housing  2  has the form of a machine spindle, and this spindle-shaped housing  2  may be rotatably mounted in a surrounding casing by means of rolling bearings. In this case, the housing  2  may be connectable to a drive mechanism of a machine tool, for instance a drive mechanism in a tool turret of a machine tool, in order to allow the housing  2  to be driven in rotation by the drive mechanism. 
     In the embodiment illustrated in  FIG.  12 - 23   , the housing  2  is designed as a non-rotatable housing. In this case, the housing  2  is connectable to a tool turret of a machine tool via a connection member  4 , which extends from the housing  2  at the rear end thereof. 
     A drawbar  5  is slidably mounted inside the bore  3  so as to be reciprocally moveable in the bore  3  along a longitudinal axis L thereof between an advanced releasing position (see  FIGS.  4  and  16   ) and a retracted locking position (see  FIGS.  1  and  14   ). The drawbar  5  has a front end facing the entrance opening  3   a  of the bore  3  and an opposite rear end. A head portion  6  and a neck portion  7  are provided at the front end of the drawbar  5 . The head portion  6  is located in front of the neck portion  7  as seen in the longitudinal direction of the drawbar, wherein the head portion  6  is connected to the neck portion  7  via a rearwardly facing bevelled surface  8  on the head portion  6 . A sealing ring  9  is arranged between the drawbar  5  and an inner surface of the bore  3 . In the illustrated examples, this sealing ring  9  is received in a groove on the outer side of the drawbar  5 . 
     A mounting portion  10  for receiving a mounting shank  91  on the tool holder  90  is provided at a front end of the bore  3 . This mounting shank  91  is here referred to as tool holder shank. 
     In the embodiment illustrated in  FIGS.  12 - 23   , the housing  2  comprises a base part  12  and an end piece  13  mounted to the base part  12  at the front end  2   a  of the housing. The end piece  13  has the form of a sleeve with an axial through hole that forms part of the above-mentioned bore  3 . In this case, the above-mentioned mounting portion  10  is located within the end piece  13 . 
     In the embodiment illustrated in  FIGS.  1 - 11   , the part of the housing  2  accommodating the bore  3  is formed in one piece without any end piece  13  of the above-mentioned type. 
     The tool holder shank  91  is insertable into the mounting portion of the bore  3  via the entrance opening  3   a  at the front end  2   a  of the housing  2 . The head portion  6  of the drawbar is received in an engagement bore  92  in the tool holder shank  91  and a tubular wall  93  of the tool holder shank is received in a space between the head portion  6  and an inner surface of the bore  3 . In the illustrated embodiments, the mounting portion  10  of the bore  3  is conically shaped and has a somewhat “triangular” or polygonal, non-circular cross-sectional shape adapted to receive a similarly shaped tool holder shank  91 . The conical shape ensures a connection free from play in the radial as well as the axial direction between the tool holder shank  91  and the housing  2 , whereas the non-circular cross-section ensures a non-rotatable fixation of the tool holder shank  91  to the housing  2 . However, the mounting portion  10  of the bore  3  could also have any other suitable shape for receiving other types of tool holder shanks. 
     Engagement members  20  in the form of segments are arranged around the drawbar  5  at a front end thereof. Under the effect of a movement of the drawbar  5  from the advanced releasing position to the retracted locking position, the engagement members  20  are moveable from a first position (see  FIGS.  4  and  16   ), in which the engagement members  20  allow the tool holder shank  91  to move into and out of the mounting portion of the bore  3 , to a second position (see  FIGS.  1  and  14   ), in which the engagement members  20  are in locking engagement with an engagement groove  94  in the engagement bore  92  in the tool holder shank  91  and thereby keep the tool holder shank  91  secured to the housing  2 . 
     In the illustrated embodiments, the engagement members  20  are arranged around the neck portion  7  of the drawbar  5  and held in place around the neck portion by means of a retainer ring  21  and an elastic O-ring  22 , which are arranged in the bore  3  and surround the neck portion  7 . Each engagement member  20  has an outwardly facing flange portion  23  engaged in an inner groove in the retainer ring  21 . The O-ring  22  is received in an outwardly facing groove at the rear end of each engagement member  20 . A compression spring  24  is also arranged in the bore  3  and configured to surround the drawbar  5 . The compression spring  24  is mounted between a shoulder on the drawbar  5  and a thrust ring  25  in the embodiment illustrated in  FIGS.  1 - 11    between a shoulder on the drawbar  5  and the retainer ring  21  in the embodiment illustrated in  FIGS.  12 - 23   . The compression spring  24  is configured to urge the retainer ring  21  and the engagement members  20  forwards. In the embodiment illustrated in  FIGS.  1 - 11   , the forward movement of the retainer ring  21  towards the entrance opening of the bore  3  is limited by a stop ring  26 , which is mounted in a groove in an inner surface of the bore  3 . In the embodiment illustrated in  FIGS.  12 - 23   , the forward movement of the retainer ring  21  towards the entrance opening of the bore  3  is limited by a shoulder on an inner end surface of the end piece  13 . 
     At its front end, each engagement member  20  is provided with an outwardly directed engagement flange  27 , which is configured to be in engagement with the engagement groove  94  in the tool holder shank  91  when the engagement member  20  is in the above-mentioned second position. When the drawbar  5  is in the advanced releasing position, the front ends of the engagement members  20  are located behind the head portion  6  of the drawbar  5  and the engagement flanges  27  are out of engagement with the engagement groove  94  in the tool holder shank  91 , as illustrated in  FIGS.  4  and  16   . When the drawbar  5  is moved axially rearwards in the bore  3  along the longitudinal axis L thereof, the bevelled surface  8  on the head portion  6  of the drawbar will come into contact with the front ends of the engagement members  20 , wherein the front ends of the engagement members  20  will slide on this bevelled surface  8  and be pressed outwards so that the engagement flanges  27  on the engagement members come into engagement with the engagement groove  94  in the tool holder shank  91 , whereupon the tool holder shank  91  will be pulled by the drawbar  5  into firm contact with inner surfaces of the housing  2  at the above-mentioned mounting portion  10 . 
     The clamping device  1  further comprises an actuating member  14 , which is rotatably arranged in the bore  3  so as to be rotatable in relation to the drawbar  5  and the housing  2  about the longitudinal axis L and which is axially fixed in relation to the housing  2  so as to be prevented from moving in relation to the housing  2  along the longitudinal axis L. The actuating member  14  is concentric with the drawbar  5 . 
     A motion transferring mechanism is arranged inside the housing  2  and configured to transfer a rotary movement of the actuating member  14  in a first rotary direction D 1  in relation to the housing  2  into an axial movement of the drawbar  5  from the advanced releasing position to the retracted locking position. This motion transferring mechanism comprises: 
     a first motion transferring element  40 , which is fixed to or fixedly connected to the drawbar  5  so as to be moveable along the longitudinal axis L together with the drawbar and which is provided with a first pressure receiving contact surface  41  facing towards the front end  2   a  of the housing; and 
     a second motion transferring element  50 , which is fixed to the actuating member  14  so as to be rotatable in relation to the housing  2  and the drawbar  5  together with the actuating member and which is provided with a first pressure applying contact surface  51  facing towards the rear end  2   b  of the housing for contacting the first pressure receiving contact surface  41 . 
     The first pressure applying contact surface  51  is configured to press the drawbar  5  from the advanced releasing position to the retracted locking position by sliding and pressing against the first pressure receiving contact surface  41  when the actuating member  14  is rotated in the first rotary direction D 1 . 
     The first pressure receiving contact surface  41  comprises a first contact surface section  41   a  and a second contact surface section  41  b arranged at different positions along the first motion transferring element  40 . The first pressure applying contact surface  51  comprises a first contact surface section  51   a  and a second contact surface section  51   b  arranged at different positions along the second motion transferring element  50 . Upon a movement of the actuating member  14  in the first rotary direction D 1 , the first section  51   a  of the first pressure applying contact surface  51  is configured to slide and press against the first section  41   a  of the first pressure receiving contact surface  41  during a first phase of the movement and the second section  51   b  of the first pressure applying contact surface  51  is configured to slide and press against the second section  41   b  of the first pressure receiving contact surface  41  during a final phase of the movement. The first section  41   a  of the first pressure receiving contact surface  41  has a larger pitch than the second section  41   b  thereof. Likewise, the first section  51   a  of the first pressure applying contact surface  51  has a larger pitch than the second section  51   b  thereof. Hereby, the same angular displacement of the actuating member  14  in connection with a rotation thereof in the first rotary direction D 1  in relation to the housing  2  will result in a longer axial movement of the drawbar  5  along the longitudinal axis L in said first phase than in said final phase. The pitch of the first section  41   a  of the first pressure receiving contact surface  41  is preferably in the order of 5-8 times as large as the pitch of the second section  41   b  thereof, and the pitch of the first section  51   a  of the first pressure applying contact surface  51  is preferably in the order of 5-8 times as large as the pitch of the second section  51   b  thereof. 
     The first section  51   a  of the first pressure applying contact surface  51  preferably has the same pitch as the first section  41   a  of the first pressure receiving contact surface  41 . Likewise, the second section  51   b  of the first pressure applying contact surface  51  preferably has the same pitch as the second section  41   b  of the first pressure receiving contact surface  41 . 
     In the illustrated embodiments, the pitch is constant along the first section  41   a  of the first pressure receiving contact surface  41  and also constant, but smaller, along the second section  41   b  thereof. In the illustrated embodiments, the pitch is also constant along the first section  51   a  of the first pressure applying contact surface  51  and also constant, but smaller, along the second section  51   b  thereof. However, the pitch may as an alternative vary along one or both of the above-mentioned sections  41   a,    41   b  of the first pressure receiving contact surface  41  and along one or both of the above-mentioned sections  51   a,    51   b  of the first pressure applying contact surface  51 . 
     In the illustrated embodiments, the first and second sections  41   a,    41   b  of the first pressure receiving contact surface  41  are positioned adjacent to each other on the first motion transferring element  40 , such that the second section  41   b  follows directly after the first section  41   a.  Likewise, the first and second sections  51   a,    51   b  of the first pressure applying contact surface  51  are positioned adjacent to each other on the second motion transferring element  50 , such that the second section  51   b  follows directly after the first section  51   a.  However, as an alternative, the first pressure receiving contact surface  41  could comprise an intermediate section positioned on the first motion transferring element  40  between the first and second sections  41   a,    41   b  of the first pressure receiving contact surface  41 . Likewise, the first pressure applying contact surface  51  could comprise an intermediate section positioned on the second motion transferring element  50  between the first and second sections  51   a,    51   b  of the first pressure applying contact surface  51 . 
     In the illustrated embodiments, the motion transferring mechanism is also configured to transfer a rotary movement of the actuating member  14  in a second rotary direction D 2  in relation to the housing  2 , opposite to the first rotary direction D 1 , into an axial movement of the drawbar  5  from the retracted locking position to the advanced releasing position. For this purpose, the first motion transferring element  40  is provided with a second pressure receiving contact surface  42 , which faces towards the rear end  2   b  of the housing and which is configured to co-operate with a second pressure applying contact surface  62  provided on a third motion transferring element  60 . This element  60  is fixed to the actuating member  14  so as to be rotatable in relation to the housing  2  and the drawbar  5  together with the actuating member. The second pressure applying contact surface  62  faces towards the front end  2   a  of the housing for contacting the second pressure receiving contact surface  42 , wherein the second pressure applying contact surface  62  is configured to press the drawbar  5  from the retracted locking position to the advanced releasing position by sliding and pressing against the second pressure receiving contact surface  42  when the actuating member  14  is rotated in relation to the housing  2  in the second rotary direction D 2 . 
     The second pressure receiving contact surface  42  may comprises a first contact surface section  42   a  and a second contact surface section  42   b  arranged at different positions along the first motion transferring element  40 , and the second pressure applying contact surface  62  may likewise comprise a first contact surface section  62   a  and a second contact surface section  62   b  arranged at different positions along the third motion transferring element  60 . Upon a movement of the actuating member  14  in the second rotary direction D 2 , the first section  62   a  of the second pressure applying contact surface  62  is configured to slide and press against the first section  42   a  of the second pressure receiving contact surface  42  during a first phase of this movement and the second section  62   b  of the second pressure applying contact surface  62  is configured to slide and press against the second section  42   b  of the second pressure receiving contact surface  42  during a final phase of this movement. The first section  42   a  of the second pressure receiving contact surface  42  has a smaller pitch than the second section  42   b  thereof. Likewise, the first section  62   a  of the first pressure applying contact surface  62  has a smaller pitch than the second section  62   b  thereof. Hereby, the same angular displacement of the actuating member  14  in connection with a rotation thereof in the second rotary direction D 2  in relation to the housing  2  will result in a longer axial movement of the drawbar  5  along the longitudinal axis L in the final phase of this rotary movement than in the first phase of this rotary movement. The pitch of the second section  42   b  of the second pressure receiving contact surface  42  is preferably in the order of 5-8 times as large as the pitch of the first section  42   a  thereof, and the pitch of the second section  62   b  of the second pressure applying contact surface  62  is preferably in the order of 5-8 times as large as the pitch of the first section  62   a  thereof. 
     The first section  62   a  of the second pressure applying contact surface  62  preferably has the same pitch as the first section  42   a  of the second pressure receiving contact surface  42 . Likewise, the second section  62   b  of the second pressure applying contact surface  62  preferably has the same pitch as the second section  42   b  of the second pressure receiving contact surface  42 . 
     In the illustrated embodiments, the pitch is constant along the first section  42   a  of the second pressure receiving contact surface  42  and also constant, but larger, along the second section  42   b  thereof. In the illustrated embodiments, the pitch is also constant along the first section  62   a  of the second pressure applying contact surface  62  and also constant, but larger, along the second section  62   b  thereof. However, the pitch may as an alternative vary along one or both of the above-mentioned sections  42   a,    42   b  of the second pressure receiving contact surface  42  and along one or both of the above-mentioned sections  62   a,    62   b  of the second pressure applying contact surface  62 . 
     In the illustrated embodiments, the first and second sections  42   a,    42   b  of the second pressure receiving contact surface  42  are positioned adjacent to each other on the first motion transferring element  40 , such that the second section  42   b  follows directly after the first section  42   a.  Likewise, the first and second sections  62   a,    62   b  of the second pressure applying contact surface  62  are positioned adjacent to each other on the third motion transferring element  60 , such that the second section  62   b  follows directly after the first section  62   a.  However, as an alternative, the second pressure receiving contact surface  42  could comprise an intermediate section positioned on the first motion transferring element  40  between the first and second sections  42   a,    42   b  of the second pressure receiving contact surface  42 . Likewise, the second pressure applying contact surface  62  could comprise an intermediate section positioned on the third motion transferring element  60  between the first and second sections  62   a,    62   b  of the second pressure applying contact surface  62 . 
     The first motion transferring element  40  is received in a gap between the second and third motion transferring elements  50 ,  60 . Thus, the second and third motion transferring elements  50 ,  60  are arranged on either side of the first motion transferring element  40 , as seen in the axial direction of the actuating member  14 . 
     Motion transferring elements  40 ,  50 ,  60  of the above-mentioned types may be arranged in two or more sets, where each set comprises a first motion transferring element  40  and associated second and third motion transferring elements  50 ,  60 . These sets are spaced apart from each other in the circumferential direction of the actuating member  14 . In the illustrated embodiments, the motion transferring mechanism of the clamping device  1  comprises two such sets of motion transferring elements. 
     In the illustrated embodiments, the motion transferring elements  40 ,  50 ,  60  form part of two threads, which are arranged in threaded engagement with each other. In this case, the first and second pressure receiving contact surfaces  41 ,  42  and the first and second pressure applying contact surfaces  51 ,  62  have the form of thread surfaces. As an alternative, each one of the first and second pressure applying contact surfaces  51 ,  62  could have the form of a rather short and convex surface arranged on a motion transferring element of a pin-like shape, with the first and second pressure receiving contact surfaces  41 ,  42  acting as cam surfaces for the first and second pressure applying contact surfaces. In the latter case, only the first and second pressure receiving contact surfaces  41 ,  42  are divided into first and second contact surface sections  41   a,    41   b,    42   a,    42   b  in the manner described above. 
     As a further alternative, each one of the first and second pressure receiving contact surfaces  41 ,  42  could have the form of a rather short and convex surface arranged on a first motion transferring element of a pin-like shape, with the first and second pressure applying contact surfaces  51 ,  62  acting as cam surfaces for the first and second pressure receiving contact surfaces. In the latter case, only the first and second pressure applying contact surfaces  51 ,  62  are divided into first and second contact surface sections  51   a,    51   b,    62   a,    62   b  in the manner described above. 
     The second contact surface section  41   b  of the first pressure receiving contact surface  41  and/or the second contact surface section  51   b  of the first pressure applying contact surface  51  preferably has such a pitch that the first and second motion transferring elements  40 ,  50  will keep the actuating member  14  in a self-locking rotary position in relation to the drawbar  5  when the drawbar has been forced into the retracted locking position under the effect of the actuating member  14  and the first and second motion transferring elements  40 ,  50 . In the self-locking rotary position, frictional forces between the first pressure applying contact surface  51  and the first pressure receiving contact surface  41  prevent the actuating member  14  from being rotated in the above-mentioned second rotary direction D 2 . 
     In the illustrated embodiments, the first and second pressure receiving contact surfaces  41 ,  42  are provided on the outer side of a shaft  15 ,  15 ′, which is fixed to the drawbar  5  and configured to move along the longitudinal axis L together with the drawbar. The drawbar  5  and the shaft  15  may be formed in one piece, as illustrated in  FIGS.  1  and  4   . However, the shaft  15 ′ may as an alternative be formed as a separate component which is fixed to the drawbar  5 , for instance through a threaded joint formed by an internal thread  16   a  in the shaft  15 ′ and a corresponding external thread  16   b  on the drawbar  5 , as illustrated in  FIGS.  14  and  16   . However, the shaft  15 ′ may of course also be fixed to the drawbar  5  in any other suitable manner. In the illustrated examples, each one of the first motion transferring elements  40  has the form of an external projection on the envelop surface of the shaft  15 ,  15 ′. 
     In the illustrated embodiments, the actuating member  14  is sleeve-shaped and surrounds said shaft  15 ,  15 ′, wherein the first and second pressure applying contact surfaces  51 ,  62  are provided on the inner side of the actuating member. In order to facilitate the assembling of the clamping device  1 , the sleeve-shaped actuating member  14  is suitably formed by two separate sleeve sectors  14   a,    14   b,  which are joined in order to form a sleeve and which are clamped to each other by means of clamping rings  17 . In the illustrated examples, each one of the second and third motion transferring elements  50 ,  60  has the form of an internal projection on the inner side of the actuating member  14 . 
     In the illustrated embodiments, the actuating member  14  and the second and third motion transferring elements  50 ,  60  together form a nut, which is configured to co-operate with a screw formed by the shaft  15 ,  15 ′ and the first motion transferring elements  40 . 
     As an alternative, the actuating member  14  may have the form of a shaft, wherein the first and second pressure applying contact surfaces  51 ,  62  are provided on the outer side of the actuating member. In this case, the first and second pressure receiving contact surfaces  41 ,  42  are provided on the inner side of a sleeve, which surrounds the shaft-shaped actuating member and which is fixed to the drawbar  5  and configured to move along the longitudinal axis L together with the drawbar. The drawbar  5  and this sleeve may be formed in one piece. However, the sleeve may as an alternative be formed as a separate component which is fixed to the drawbar  5 , for instance through a threaded joint. 
     The drawbar  5  is prevented from rotating in relation to the housing  2 . In the embodiment illustrated in  FIGS.  1 - 11   , the drawbar  5  is provided with a number of stop lugs  30 , which project in radial direction from a peripheral wall of the drawbar and which are distributed about the centre axis of the drawbar. These stop lugs  30  are slidably received in a respective longitudinal groove  31  on a wall surface in the bore  3  in order to prevent mutual rotation between the drawbar  5  and the housing  2 . In the embodiment illustrated in  FIGS.  12 - 23   , an elongated stop member  32  extends through an aperture  33  in a plate  34  which is non-rotatably mounted inside the housing  2 , wherein this stop member  32  is slidably received in an axially extending recess  35  in the shaft  15 ′ in order to prevent mutual rotation between the drawbar  5  and the housing  2 . Mutual rotation between the drawbar  5  and the housing  2  may of course also be prevented in any other suitable manner. 
     The clamping device  1  may also comprise additional components, such as one or more hydraulic, pneumatic or electric actuators, for effecting the rotation of the actuating member  14  in the first and second rotary directions D 1 , D 2 . The control of the clamping device  1  may be automatized by means of such an actuator. Thus, the clamping device  1  according to the present invention is suitable for use in automatic tool changing operations. However, the clamping device  1  according to the present invention may also be adapted for manual operation. 
     In the illustrated embodiments, the clamping device  1  comprises a hydraulic actuator  70 ,  70 ′ arranged in or mounted on the housing  2  and configured to rotate the actuating member  14  in relation to the housing  2 . In the illustrated embodiments, the hydraulic actuator  70 ,  70 ′ is integrated in the housing  2  of the clamping device. However, the hydraulic actuator  70 ,  70 ′ may as an alternative comprise a separate actuator casing that is fixed to the housing of clamping device. 
     In the embodiment illustrated in  FIGS.  1 - 11   , the hydraulic actuator  70  comprises a rotatable actuator shaft  71 , which is arranged with its axis of rotation in alignment with the axis of rotation of the actuating member  14 . The actuating member  14  is configured to be rotatable in relation to the housing  2  together with the actuator shaft  71  in the above-mentioned first and second rotary directions D 1 , D 2 . The hydraulic actuator  70  also comprises two actuator vanes  72 , which are fixed to the actuator shaft  71  and configured to be rotatable together with the actuator shaft  71  about the axis of rotation of the actuator shaft. The actuator vanes  72  project radially from the actuator shaft  71  in opposite directions. The actuator vanes  72  and the actuator shaft  71  are preferably formed in one piece. Each actuator vane  72  is moveably received in an internal space  73  of the hydraulic actuator  70  and configured to divide this internal space  73  into a first hydraulic chamber  74   a  on a first side of the actuator vane and a second hydraulic chamber  74   b  on an opposite second side of the actuator vane. Hydraulic fluid, for instance in the form of hydraulic oil, can be fed into and discharged from the first hydraulic chambers  74   a  through a respective first inlet and outlet port  75   a  and fed into and discharged from the second hydraulic chambers  74   b  through a respective second inlet and outlet port  75   b.  The actuating member  14  is rotatable together with the actuator shaft  71  and the actuator vanes  72  in the first rotary direction D 1  by feeding of hydraulic fluid into the first hydraulic chambers  74   a  and in the second rotary direction D 2  by feeding of hydraulic fluid into the second hydraulic chambers  74   b.    
     In embodiment illustrated in  FIGS.  1 - 11   , the hydraulic actuator  70  comprises a cylindrical cavity  76 , which is limited in radial direction outwards by a cylindrical wall  77  of the housing  2 , wherein the above-mentioned internal spaces  73  form part of this cylindrical cavity  76 . The actuator shaft  71  is rotatably arranged in the cylindrical cavity  76  with its longitudinal axis coinciding with the centre axis of the cylindrical cavity. A longitudinal outer edge  72   a  of each actuator vane  72  is in contact with the cylindrical wall  77 . If so desired, an elongated sealing member may be arranged along the longitudinal outer edge  72   a  of each actuator vane  72  in order to form a fluid-tight seal at the interface between the actuator vane  72  and the cylindrical wall  77  and thereby prevent hydraulic fluid from leaking from one hydraulic chamber to the other through this interface. However, a smaller leakage of hydraulic fluid between the hydraulic chambers  74   a,    74   b  may be accepted without impairing the functionality of the hydraulic actuator  70 , and such sealing members are therefore not necessary. 
     Each one of the above-mentioned internal spaces  73  extends in the circumferential direction of the cylindrical cavity  76  between first and second chamber walls  78   a,    78   b,  which are arranged inside the cylindrical cavity  76  and extend in the longitudinal direction thereof along the hydraulic chambers  74   a,    74   b.  The first chamber wall  78   a  faces the first hydraulic chamber  74   a  and is configured to limit this hydraulic chamber  74   a  in a circumferential direction of the cylindrical cavity  76 . The second chamber wall  78   b  faces the second hydraulic chamber  74   b  and is configured to limit this hydraulic chamber  74   b  in a circumferential direction of the cylindrical cavity  76 . The actuator vanes  72  are configured to come into contact with the first chamber walls  78   a  when the drawbar  5  has reached the advanced releasing position, as illustrated in  FIG.  6   , to thereby prevent further movement of the drawbar  5  towards the front end  2   a  of the housing  2 . 
     The precise axial position of the drawbar  5  in the bore  3  depends on the tolerances of the tool holder shank  91  and the mounting portion  10  and it is therefore not possible to predict the precise end position of the actuator vanes  72  in the first rotary direction D 1 . Thus, the actuator vanes  72  can not be configured to reach the second chamber walls  78   b  when the drawbar  5  assumes its retracted locking position. Consequently, each actuator vane  72  is configured to assume a position somewhere between the associated first and second chamber walls  78   a,    78   b  when the drawbar  5  has reached the retracted locking position, as illustrated in  FIG.  3   . 
     The first and second chamber walls  78   a,    78   b  are with advantage configured to form part of elongated partition elements  79 , preferably in the form of solid bodies, which are mounted inside the cylindrical cavity  76  and extend along the cylindrical cavity  76  in the longitudinal direction thereof. These partition elements  79  are fixed to the cylindrical wall  77 , for instance by means of screws  80 . 
     In the embodiment illustrated in  FIGS.  1 - 11   , the above-mentioned internal spaces  73  are limited in a first axial direction by a rear end wall  14   c  on the actuating member  14  and in the opposite axial direction by an end piece  81 , which is fixed to the housing  2  at the rear end  2   b  thereof. 
     In the embodiment illustrated in  FIGS.  1 - 11   , an end part  72   b  at the front end of each actuator vane  72  is received in a radially extending recess in the rear end wall  14   c  of the actuating member  14  in order to allow transmission of torque from the actuator vanes  72  to the actuating member  14  and prevent the actuating member from rotating in relation to the actuator vanes  72  and the actuator shaft  71 . However, the actuating member  14  may of course also be non-rotatably connected to the actuator vanes  72  and/or the actuator shaft  71  in any other suitable manner. 
     In embodiment illustrated in  FIGS.  1 - 11   , the hydraulic actuator  70  comprises two actuator vanes  72 . However, the hydraulic actuator may as an alternative comprise more than two actuator vanes  72  evenly distributed about the actuator shaft  71  and received in a respective internal space of the hydraulic actuator, or one single actuator vane  72 . 
     In the embodiment illustrated in  FIGS.  12 - 23   , the hydraulic actuator  70 ′ comprises an actuator vane  72 ′, which is fixed to the actuating member  14  and configured to be rotatable in relation to the housing  2  together with the actuating member  14 . The actuator vane  72 ′ projects radially from the actuating member  14 . The actuator vane  72 ′ and one of the sleeve sectors  14   a  of the actuating member  14  are preferably formed in one piece. The actuator vane  72 ′ is moveably received in an internal space  73 ′ of the hydraulic actuator and configured to divide this internal space  73 ′ into a first hydraulic chamber  74   a′  on a first side of the actuator vane and a second hydraulic chamber  74   b′  on an opposite second side of the actuator vane. Hydraulic fluid, for instance in the form of hydraulic oil, can be fed into and discharged from the first hydraulic chamber  74   a′  through a first inlet and outlet port  75   a′  and fed into and discharged from the second hydraulic chamber  74   b′  through a second inlet and outlet port  75   b′.  The actuating member  14  is rotatable together with the actuator vane  72 ′ in the first rotary direction D 1  by feeding of hydraulic fluid into the first hydraulic chamber  74   a′  and in the second rotary direction D 2  by feeding of hydraulic fluid into the second hydraulic chamber  74   b′.    
     A first chamber wall  78   a′  is arranged at a first end of the internal space  73 ′ and a second chamber wall  78   b′  is arranged at an opposite second end of the internal space  73 ′, as seen in the rotary directions of the actuator vane  72 ′, wherein the first chamber wall  78   a′  faces the first hydraulic chamber  74   a′  and the second chamber wall  78   b′  faces the second hydraulic chamber  74   b′.  The actuator vane  72 ′ is configured to come into contact with the first chamber walls  78   a′  when the drawbar  5  has reached the advanced releasing position, as illustrated in  FIG.  16   , to thereby prevent further movement of the drawbar  5  towards the front end  2   a  of the housing  2 . The actuator vane  72 ′ is configured to assume a position somewhere between the first and second chamber walls  78   a′,    78   b′  when the drawbar  5  has reached the retracted locking position, as illustrated in  FIG.  14   . 
     In the embodiment illustrated in  FIGS.  12 - 23   , the housing  2  comprises an insert piece  82 , which is mounted to the above-mentioned base part  12  of the housing through an aperture  83  (see  FIG.  19   ) in an external wall of the base part. The insert piece  82  comprises two legs  84 , and an essentially U-shaped seat  85  for the actuating member  14  is formed between these legs, wherein the actuating member  14  is rotatably received in this seat  85 . The actuator vane  72 ′ extends through an opening provided between the free ends of the legs  84 . The above-mentioned first chamber wall  78   a′  is formed by a surface at the free end of one of the legs  84  and the second chamber wall  78   b′  is formed by a surface at the free end of the other leg  84 . 
     When a tool holder  90  is to be clamped to the housing  2 , the tool holder shank  91  is inserted into the mounting portion  10  of the bore  3  with the drawbar  5  positioned in the advanced releasing position, as illustrated in  FIGS.  4  and  16   . Hereby, the head portion  6  of the drawbar is received in the engagement bore  92  in the tool holder shank  91  and the engagement groove  94  in the tool holder shank  91  is positioned on the outside of the engagement flanges  27  of the engagement members  20 . Thereupon, hydraulic fluid is fed into the first hydraulic chamber  74   a′  or chambers  74   a  in order to rotate the actuator shaft  14  in the first rotary direction D 1 . During a first phase of this rotary movement of the actuating member  14 , the first section  51   a  of the first pressure applying contact surface  51  will slide and press against the first section  41   a  of the first pressure receiving contact surface  41 . Hereby, the drawbar  5  will be axially displaced rearwards in the bore  3  towards the retracted locking position. The relatively large pitch of said sections  41   a,    51   a  will result in a relatively quick displacement of the drawbar  5 . The relatively large pitch is advantageous since the initial displacement of the drawbar  5  does not require much force. The first and second sections  51   a,    51   b  of the first pressure applying contact surface  51  and the first and second sections  41   a,    41   b  of the first pressure receiving contact surface  41  are so arranged that when the actuating member  14  has been subjected to such an angular displacement that the first section  51   a  of the first pressure applying contact surface  51  has passed the first section  41   a  of the first pressure receiving contact surface  41  and the second section  51   b  of the first pressure applying contact surface  51  reaches the second section  41   b  of the first pressure receiving contact surface  41 , i.e. at the transition between these respective sections, the drawbar  5  has almost reached its final retracted position in the bore  3 . Hence, for the final clamping phase, in which a large force is beneficial, the second sections  51   b,    41   b  of the first pressure applying and receiving contact surfaces  41 ,  51  are active. In this phase, a relatively large angular displacement of the actuating member  14  will result in a very small axial displacement of the drawbar  5 , which consequently will provide a force amplifying effect that will make it possible for the drawbar  5  to pull the tool holder shank  91  with a large force into firm engagement with the housing  2 . Furthermore, the small pitch of the second sections  51   b,    41   b  of the first pressure applying and receiving contact surfaces  51 ,  41  will provide a self-locking effect and make sure that the clamping device  1  will remain in the clamped state without requiring any additional locking means. Hereby, the force on the actuating member  14  may be released when the drawbar  5  has reached the retracted locking position. 
     When a tool changing operation is to be performed and the tool holder  90  is to be released from the housing  2 , hydraulic fluid is fed into the second hydraulic chamber  74   b′  or chambers  74   b  in order to rotate the actuator member  14  in the second rotary direction D 2 . When the actuating member  14  is subjected to a sufficient force in the second rotary direction D 2 , the self-locking frictional engagement between the second sections  51   b,    41   b  of the first pressure applying and receiving contact surfaces  51 ,  41  will be released, whereupon the actuating sleeve  14  is rotatable in relation to the housing  2  in the second rotary direction D 2  in order to push the drawbar  5  axially forwards in the bore  3  towards the advanced releasing position. During a first phase of this rotary movement of the actuating member  14 , the first section  62   a  of the second pressure applying contact surface  62  will slide and press against the first section  42   a  of the second pressure receiving contact surface  42 , whereupon the second section  62   b  of the second pressure applying contact surface  62  will slide and press against the second section  42   b  of the second pressure receiving contact surface  42  during a subsequent final phase of this rotary movement of the actuating member  14 . When the drawbar  5  is moved towards the advanced releasing position, the outer end of the head portion  6  of the drawbar  5  will hit against a surface  95  in the engagement bore  92  in the tool holder shank  91  and thereby release the tool holder shank  91  from the housing  2 . 
     The invention is of course not in any way restricted to the embodiments described above. On the contrary, many possibilities to modifications thereof will be apparent to a person with ordinary skill in the art without departing from the basic idea of the invention such as defined in the appended claims.