Abstract:
A clamping unit ( 1 ) for machine tools ( 2 ) with a housing ( 11 ) connected in a rotationally fixed arrangement with a machine spindle, ( 5 ) and a screw drive ( 13 ) that interacts with a draw rod ( 6 ) such that rotational movements are converted into translational movements, the housing ( 11 ) being provided with an output element by which rotational movements are transmitted to an actuator ( 21 ). The housing ( 11 ) has an input element in a rotating mounting, with the actuator ( 21 ) acting on the input element, and the input element ( 29 ) is in driving connection with a shaft ( 14 ) by means of intermediate elements ( 31 ), whereby to direct rotational movements of the machine spindle ( 5 ) via the clamping unit ( 1 ), and for adjustment movements of differently configured clamping devices, to be converted into axial adjustment movements and transmitted directly onto the draw rod ( 6 ). No complicated electrical control devices and programs are required for adjusting and adapting a power chuck ( 3 ) linked to the draw rod ( 3 ) to different operating conditions.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a clamping unit for machine tools with a housing that can be connected to a machine spindle in a rotationally fixed arrangement, and a screw drive arranged in the housing, wherein the screw drive interacts with a draw rod linked to a power chuck, by means of which rotational movements transmitted to the clamping unit can be converted into translational adjustment movements of the draw rod. 
     2. Description of the Prior Art 
     An electrical chuck with a clamping unit of this kind is disclosed in DE 10 2007 044 309 A1. In this case, the circumferential housing only serves to support the ring gear of a harmonic drive; in this configuration, there is no transmission of the rotational movement of the machine spindle onto an actuator. Instead, the actuator takes the form of an electric motor acting on a shaft generator of the harmonic drive, by means of which the rolling screw drive of the draw rod can be driven by means of a steel bush that is provided with external gearing and a dog clutch. The rotational adjustment movements of the electric motor connected to the drive gear of the spline shaft gear unit by means of a belt drive, can be converted into translational adjustment movements of the draw rod, although the electric motor needs to be controlled according to the rotation speed and direction of rotation of the power chuck in order to trigger adjustment movements of the clamping jaws, or in order to retain or change the clamping force. In order to achieve this, so that the clamping unit can be used together with the servomotor as an electric chuck, it is necessary to have an extraordinarily complicated control device, as well as numerous programs in which a large number of parameters must be incorporated. 
     Quite apart from the very great control complexity, this not only leads to a high susceptibility to malfunctions, but also the investment costs, in particular for the control device and the various programs, are considerable. However, the principal disadvantage is that the control unit has to be adjusted to take account of numerous different existing conditions, thereby placing very exacting requirements on the operating personnel. As a result, the electric chuck of the prior art only has limited practical applications. 
     SUMMARY OF THE INVENTION 
     The task of the present invention is, therefore, to provide a clamping unit for machine tools for transmitting and converting movements which can be used in a very advantageous manner with electrical clamping devices in order to create an axially vectored clamping force that is directed towards the power chuck, thereby permitting safe and trouble free operation at all times over a long period. The design must be straightforward such as to allow for economical manufacture. Above all, however, the clamping unit must be capable of being used in electrical clamping devices with different configurations, and it should be possible to adjust the clamping force, even whilst the machine spindle is rotating. Furthermore, there should be no need for complicated control units and programs in order to operate the clamping unit. 
     In accordance with the present invention, this is achieved in a clamping unit for machine tools of the aforementioned type in that the housing is provided with an output element that is firmly connected to the housing, with rotational movements of the machine spindle being transmitted via the output element onto an actuator located after the clamping unit, in that the housing has an input element assigned to it that is mounted, in a preferred embodiment, in a rotating arrangement on the housing, with the downstream actuator acting on the input element, and in that the input element is located in a driving connection with a rotating, but axially fixed, hollow shaft by means of one or more intermediate elements, in which case the hollow shaft is supported on the housing and interacts with a screw drive. 
     It is advantageous for the housing to comprise a hollow body that is stepped and configured as one piece, which is closed by a cover on a side facing toward the machine spindle, and for the drive element and the input element to be secured, or in a rotating arrangement, on the side opposite the cover, preferably in the end area, in which case the cover allows the hollow shaft to be axially locked in the housing, in a preferred embodiment by means of anti-friction bearings, and for the cover to be configured with a T-shaped cross-section and for parts of the screw drive to be supported on a shoulder that protrudes into the housing. 
     The intermediate elements provided between the input element of the clamping unit and the screw drive can easily be arranged out-of-centre and configured as splined shafts, or as a one or multi-stage gear unit, in which case the carrier shafts of the gear unit should be mounted in an intermediate wall of the housing in a rotating arrangement, and the gears of the gear unit should be in a driving connection with a hollow shaft carrying the input element and supported in a rotating arrangement on the housing and directly with the hollow shaft assigned to the screw drive. 
     The screw drive of the clamping unit preferably is composed of one or more drive rollers in a rotating mounting and provided with projections arranged in parallel to one another, and of a threaded spindle arranged on the draw rod, and the drive rollers should engage with their projections in guide grooves worked into the inner jacket surface of the hollow shaft and in thread grooves of the threaded spindle, such that the threaded spindle can be axially adjusted using the drive rollers during rotation of the hollow shaft. 
     In a screw drive provided with several drive rollers, it is appropriate for these to be arranged evenly over the circumference and for them to be supported in end areas on both sides with one guide cage each. 
     The thread grooves of the threaded spindle preferably are configured as single or multi-thread grooves, and with a double thread. 
     Furthermore, it is advantageous for the clamping unit and/or the actuator assigned to the clamping unit to have one securing device each assigned to them, by means of which the intermediate elements of the clamping unit and/or the actuator can be blocked in the event of disruptions in operation. 
     Each of the securing devices can consist of a magnetic coil inserted in a separate housing, or of an adjustment piston on which a pressurised agent acts, preferably compressed air, and of a securing pin attached to an armature which can be moved axially by the adjustment piston against the force of a spring, in which case the securing pin can be inserted directly, or via intermediate elements, into openings provided on the assigned components. 
     In a securing device assigned to the intermediate elements, it is appropriate to have a securing pin act on a disc mounted on the housing, which pin can be moved against the force of a spring, in which case the disc is provided with a detent pin attached to it which can be inserted into openings disposed in the end face of a gear, in order to maintain the clamping force in the power chuck in the event of an electrical power failure, and/or a disruption in operation of the transmission elements of the clamping device, in which case the transmission elements of the actuator should have capacitive sensors, optical sensors, or light barriers, assigned to them, by means of which the operational status of the transmission elements can be monitored, and which are connected to the securing device assigned to the intermediate elements. 
     In a securing device that is assigned to an actuator provided with a circumferential housing for accommodating a step-down gear unit, the securing pin therefor should be able to be inserted directly and automatically into one of the openings disposed in the end faces of the housing, in order to maintain the clamping force in the power chuck in the event of an electrical power failure. 
     In a securing device that is assigned to an actuator provided with circumferential belt sheaves, however, the securing pin should interact with a belt sheave provided with openings, either directly or via a disc provided with a detent pin that can be moved axially against the force of a spring. 
     Furthermore, it is advantageous for the housing to be equipped with a flat or stepped contact surface on its side facing away from the power chuck for a stop shaft to be inserted into the central hole in the housing, or for a guide tube. 
     If a clamping unit for machine tools is embodied in accordance with the present invention, it is possible for the rotational movements of the machine spindle to be directed through the clamping unit and for the adjustment movements of differently configured electrical clamping devices to be converted into axial adjustment movements, with the help of the clamping unit, and for them to be transmitted directly to a draw rod. The design complexity required to achieve this is very low, although the principal advantage is that no complicated electrical control devices and programs are required for control purposes in order to adjust and adapt the power chuck to different operating conditions. The configuration in accordance with the proposal therefore permits safe and trouble-free operation, with straightforward handling over a long period, as well as provide for a variety of applications. 
     In addition, an assurance is given that if there is an electrical power failure, and/or any other kind of operational disruptions in the clamping devices, then the draw rod will be blocked immediately with the help of securing devices. Also, given coasting of the machine spindle, this ensures that the clamping force of the power chuck will not be lessened, and that the clamping jaws will not come open. In this way, accidents during operation are reliably avoidable. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings show two configuration variants of the clamping unit for machine tools configured in accordance with the present invention, the details of which are explained below. In the drawings, 
         FIG. 1  shows the clamping unit mounted on a machine spindle of a machine tool, with an electrical clamping device as an actuator, in an axial section, 
         FIG. 2  shows the clamping unit in accordance with  FIG. 1 , with a different kind of electrical clamping device and a different intermediate element, 
         FIG. 3  shows the clamping unit with clamping device in accordance with  FIG. 2 , in a side view, and 
         FIG. 4  shows the screw drive used in the clamping unit in accordance with  FIGS. 1 and 2 , in a magnified view and in an axial section. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The clamping unit  1  illustrated in  FIGS. 1 and 2 , is used for transmitting a rotational movement of a machine spindle  5  of a machine tool  2  onto an actuator  21  or  21 ′ configured as an electrical clamping device, as well as for converting the rotational adjustment movements supplied by the actuators  21 ,  21 ′ to the clamping unit  1 , into translational movements which are to be supplied to a power chuck  3  arranged on the machine tool  2 , in which case a workpiece  10  is clamped in the power chuck  3  for the purpose of machining. The power chuck  3  is provided with radially adjustable clamping jaws  4  for this purpose, which can be adjusted via relay levers  7  by means of a draw rod  6  that can be actuated in an axial direction. 
     The clamping unit  1  comprises a housing  11  which is attached by bolts  9 ′ to a flange  9  provided on the machine spindle  5 , and of a screw drive  13  that is in a driving connection with the actuator  21  or  21 ′, as well as the draw rod  6  by means of an intermediate element  31  or  31 ′. The housing  11 , which is configured as a stepped hollow shaft  11 ′ and is closed by a T-shaped cover  12 , is firmly coupled to the machine spindle  5 , adapted to be driven by an electric motor  8 , and the housing  11  rotates in conjunction with the spindle  5 . In addition, an output element in the form of a belt sheave  26  is arranged on the housing  11  in the end area opposite to the flange  9 , by means of which the rotational movements of the machine spindle  5  are transmitted to the actuator  21  or  21 ′. A wedge  74  guided in a longitudinal groove  75  worked into the hollow shaft  11 ′, also connects the housing  11  in a rotationally fixed arrangement to the axially movable draw rod  6 . 
     As shown in detail in  FIG. 4 , the screw drive  13  is provided with a hollow shaft  14  which is supported in a rotating, but axially immobile, arrangement in the housing  11  by means of bearings  15 , as well as several drive rollers  17 , which are held in guide cages  17 ′ supported on the hollow shaft  14 , and which engage in a threaded spindle  19  arranged on the draw rod  6 . Projections  18  arranged in parallel to one another, which interact with guide grooves  20  worked into the inner jacket surface of the hollow shaft  14 , provide a driving connection between the drive rollers  17  and the hollow shaft  14 . If the hollow shaft  14  is driven, then the drive rollers  17  are set rotating and, by means of their projections  18 , which engage tangentially in thread grooves  19 ′ worked into the threaded spindle  19 , they are pushed to the right, or the left, to a greater or lesser extent, depending on the direction of rotation of the hollow shaft  14  and the selected pitch of thread grooves  19 ′. A shoulder  12 ′ of the cover  12  projecting into the housing  11  also supports the screw drive  13 , and in particular the drive rollers  17 . 
     The actuator  21  is provided with a servomotor adapted to act on a gear unit  25 , installed in a housing  24 , by means of a belt drive  22 ′. An output shaft  24 ′ of the gear unit  25  in this case is mounted in a stationary carrier  23 , and two belt sheaves  27  and  28  are supported on the output shaft  24 ′ which interact via drive belts  30  and  30 ′ with belt sheaves  26  and  29  that are attached to the housing  11 , or to a hollow shaft  38 , arranged in a rotating mounting by means of bearings  39 . As a result, the rotational movements of the machine spindle  5  are transmitted from the housing  11  via the belt sheave  26  as the input element, and the drive belt  30  onto the belt sheave  27 . The signals sent by a servomotor  22  and the gear unit  25 , depending on the signals sent by a control unit  81  to the servomotor  22 , effect adjustment movements triggered for increasing or reducing the clamping force in the power chuck  3  and for changing the position of the clamping jaws  4 , and are transmitted via the belt sheaves  28  and  29  as input element of the clamping unit  1 , as well as via the drive belt  30 ′ onto the hollow shaft  38 . 
     In order to transmit the adjustment movements further, the hollow shaft  38  is provided with gearing  40  which, as is shown in  FIG. 1 , interacts with a gear  36  of the intermediate elements  31 , a plurality of which are provided. Each of the gears  36  disposed in a shaft  34  are mounted in a rotating arrangement off-centre in the hollow shaft  11 ″ of the housing  11  by means of bearings  35  and carry an additional gear  37  which engages with gearing  16  worked into the hollow shaft  14  of the screw drive  13 . The rotational adjustment movements of the servomotor  22  are therefore converted into axial adjustment movements of the draw rod  6  by means of the screw drive  13 . 
     In a different embodiment of the actuator  21 , as shown in  FIG. 2 , the servomotor on the motor shaft  24 ″ on which the belt sheaves  27  and  28  are arranged, is not shown. Also, the intermediate element  31 ′ is configured as a two-stage step-down gear unit  33 , the gears  36 ′″ and  37 ′ of which are in turn arranged on a shaft  34 ′. The gear  36 ′ which interacts with the hollow shaft  38  is in a driving connection with the gear  36 ′″ by means of another gear  36 ″ mounted in the intermediate wall  11 ″ of the housing  11  in a rotating arrangement. In this way, it is possible to achieve a significant step-down ratio of the input rotational movements. 
     In order to maintain the clamping force in the power chuck  3  in the event of an electrical power failure, and/or any kind of operational disruptions in the area of the actuator  21  or  21 ′, for example, in case of a breakage of the drive belts  30  and/or  30 ′, with the effect that during coasting of the machine spindle  5  no change in the clamping status of the clamping jaws  4  can be occasioned, the clamping unit  1 , as well as the actuators  21  or  21 ′, are each provided with a securing device  51  or  51 ′ or  51 ″. The securing devices  51 ,  51 ′ and  51 ″ in this case principally comprise a separate housing  52  located in a stationary position, in which a magnetic coil  53  and an armature  54 , that can be actuated by the coil  53 , are located. The armature  54  is adapted to be adjusted against the force of a compression spring  55  and is provided with a securing pin  56 , which can be inserted into openings  60 ,  61  or  62  in the corresponding assigned component, either directly or by means of intermediate elements. 
     In the securing device  51 , shown in  FIG. 1 , the securing pin  56  acts on a disc  57 , which is mounted on the housing  11 , and can be moved against the force of a compression spring  58 , and has another pin  59  attached to it, which can be inserted into one of the openings  60  disposed in the gear  36 . If the magnetic coil  53  is energised, this holds the armature  54  in the operating position indicated, against the force of the spring  55 . However, if there is an electrical power failure and no more magnetic force is exerted on the armature  54 , the force of the spring  55  immediately pushes the armature  54  and presses it against the disc  57  which is also moved, together with the pin  59  that the disc  57  carries. This causes the pin  59  to engage in one of the openings  60  provided in the gear  36  or  36 ′, such that the intermediate element  31  or  31 ′, and the components connected thereto, namely the screw drive  13  and the draw rod  6 , are blocked, with the effect that it is not possible for the clamping jaws  4  of the power chuck  3  to change their positions, and therefore the clamping status cannot be changed, even when the machine spindle  5  is coasting. 
     The securing device  51 ′ is assigned to the gear unit  25  of the clamping device  1  and has the same structure. In this case, the securing pin  56  engages directly in one of the openings  61  worked directly into an end face of the housing  11 , with the effect that the actuator  21  is blocked in this way and there cannot be any adjustment of the clamping jaws  4  of the power chuck. 
     The securing device  51 ″, shown in  FIG. 2 , interacts with the belt sheave  28  of the actuator  21 ′ to maintain the tension in the power chuck  3  in the event of an electrical power failure and/or other operational disruption. A disc  57 ′ moveable axially against the force of a spring  58 ′, against which a securing pin  56 ′ of the securing device  51 ″ is pressed in case of an electrical power failure, in turn moves a pin  59 ′ against the force of the spring  58 ′, and the pin  59 ′ is adapted to engage in one of the openings  62  provided in the belt sheave  29 . 
     In order to monitor the operational status of the drive belts  30  and/or  30 ′, capacitive sensors  63  or  64  are assigned to them as shown in  FIG. 3 , the signals of which are sent to the securing device  51  via signal cables  65 . In the event of a break in drive belts  30  and/or  30 ′, the clamping unit  1  is blocked, providing a reliable means of preventing accidents caused by the workpiece  10  in the power chuck  3  coming loose. 
     The housing  11  is configured as a hollow body  11 ′, and therefore has a central hole, which means it is possible to arrange a stop shaft  72  or a guide tube in the housing  11  and the draw rod  6  whilst machining rod stock. As a result, workpieces  10  clamped in the power chuck  3 , or rods passing through the clamping unit  1 , are supported by the stop shaft  72  or a guide tube. A stepped contact surface  71  is disposed in the hollow body  11 ′ for this purpose, and the stop shaft  72  makes contact with it. Bolts  73  secure the stop shaft  72  on the hollow body  11 ′. 
     It is possible for the clamping unit  1  to be followed by actuators with an alternative configuration, e.g. in the form of an electric motor, in order to convert the actuator movements in the clamping unit  1  into axial adjustment movements for the draw rod  6 .