Abstract:
The chuck includes a centering clamping member having a variable diameter, a radially extending clamping surface, which is adjacent to the centering clamping member, a movable clamping element, a first spring element, via which the centering clamping member can be moved into a clamping position, a second spring element, via which the clamping element can be loaded with a first force, a third spring element, via which the clamping element can be loaded with a second, higher force, and an axially movable actuating device by which the centering clamping member and the clamping element can be moved into a release position against the force of the first and third spring elements.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of and Applicant claims priority under 35 U.S.C. §§120 and 121 of parent U.S. patent application Ser. No. 12/218,003 filed Jul. 10, 2008, which application is based upon and claims the benefit of priority under 35 U.S.C. §119 of German Patent Application No. 10 2007 032 608.6, filed on Jul. 11, 2011, the disclosures of each of which are incorporated herein in their entirety by reference. 
    
    
     BACKGROUND AND SUMMARY OF THE INVENTION 
     The invention relates to a method for centering and clamping a workpiece, in particular an articulated shaft, in a balancing machine and a chuck for carrying out the method. 
     For balancing, workpieces must be rotatably mounted in a balancing machine in such a manner that the rotation axis which the workpieces each assume in their subsequent operating position corresponds as precisely as possible to the rotation axis of the balancing machine. When the workpieces are received in a chuck, in order to determine the rotation axis, the workpieces are generally provided with a mostly cylindrical centering surface and with a rotationally symmetrical mostly planar contact surface which extends radially and by means of which the rotation axis of the workpiece is determined. In the chuck there are provided receiving members which co-operate with these surfaces, for example, a radially supporting centering clamping member, axially supporting clamping surfaces or the like, to which the workpiece is securely clamped by means of clamping elements. When securely clamping, it may be necessary for the clamping at the centering surface of the workplace and the clamping at the radial contact surface not to be able to take place at the same time, so that particular attention must be paid in order to prevent the workpiece from being clamped in an inclined manner. 
     For balancing, articulated shafts are generally secured in a horizontal position, by the ends thereof, to two spindles of a balancing machine, each spindle having a chuck in which the flange which is arranged at the end of the articulated shaft is centered and clamped. In this instance, chucks are used which are configured in such a manner that the flange of the articulated shafts becomes centered in the conical clamping device thereof, which engages in a hole of the flange, and is radially clamped with significant actuating force. Subsequently, the flange is pressed with a radial contact surface, by means of connecting rods and clamping elements which surround the flange from the outer side, against a clamping surface of the chuck. In this instance, it may be the case that, owing to the radial clamping with significant actuating force which has been previously carried out, the axial pressing force of the clamping elements is no longer sufficient to press the contact surface of the flange in a uniform manner against the clamping surface of the chuck. The flange may therefore be in an inclined position which results in an inadmissibly significant and irreproducible error during the balancing operation. 
     An object of the invention is to provide a method of the type mentioned in the introduction which prevents clamping errors and which ensures very precise clamping of the workpieces. The method is further intended to be able to be carried out automatically. Furthermore, an object of the invention is to provide a chuck for a balancing machine which ensures automatic centering and clamping of workpieces, in particular articulated shafts, with a high level of precision. 
     According to the invention, the method for achieving the object set out involves the steps of positioning a workpiece with a centering surface on a centering clamping member of a chuck, pressing a radial contact surface of the workpiece against a clamping surface of the chuck with a first force which still allows radial movement of the workpiece, centering the workpiece by clamping the centering clamping member against the centering surface of the workpiece, and pressing the radial contact surface of the workpiece against the clamping surface of the chuck with a greater second force which securely clamps the workpiece. Preferably, the second force is at least three times greater than the first force. 
     In accordance with the method according to the invention, the planar radial contact surface of the workpiece is first pressed with a relatively small first force against the clamping surface of the chuck. A parallel orientation of the rotation axes of the chuck and workpiece is thereby achieved, but with the workpiece still being able to be radially displaced for the subsequent centering operation. Owing to the subsequent clamping of the centering clamping member, the workpiece can therefore be precisely centered, the contact between the planar contact surface and the clamping surface and consequently the correct orientation of the rotation axis of the workpiece being maintained owing to the axial effect of the first force. After the radial clamping of the centering clamping member, which can also be carried out with a high degree of force, the rotation axis of the workpiece is orientated in an optimal manner relative to the rotation axis of the chuck so that owing to the subsequent axial clamping of the workpiece with the significantly greater second force, the workpiece can be definitively fixed in the chuck. Inclined clamping of the workpiece is prevented in this instance 
     In accordance with the invention, one advantageous device for carrying out the method according to the invention comprises a centering clamping member with a centering portion having a variable diameter, a radially extending clamping surface, which is adjacent to the centering clamping member, a movable clamping element for axially pressing the workpiece against the clamping surface, a first spring element via which the centering clamping member can be moved into a clamping position, a second spring element via which the clamping element can be loaded with a first force, a third spring element, via which the clamping element can be loaded with a second force, and an axially movable actuating device by which the centering clamping member and the clamping element can be moved into a release position against the force of the first and third spring elements. 
     The device according to the invention allows automatic clamping and centering of a workpiece with defined forces and reliably prevents the occurrence of clamping errors. The device further has the advantage that it is kept play-free by means of the spring elements and therefore no oscillations occur which impair the accuracy of the unbalance measurement. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in greater detail below with reference to embodiments which are illustrated in the drawings, in which: 
         FIG. 1  is an axial section of a chuck which can be connected to the spindle of a balancing machine, 
         FIG. 2  is a section of the chuck according to  FIG. 1 , 
         FIG. 3  is an axial section of a second configuration of a chuck which can be connected to the spindle of a balancing machine and 
         FIG. 4  is a section of a third configuration of a chuck which can be connected to the spindle of a balancing machine. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The chuck  1  which is illustrated in  FIGS. 1 and 2  has a chuck housing  2  which is composed of a flange portion  3  and a housing portion  4 . On the flange portion  3  there is formed a centering ring  5  which serves to centre the flange portion  3  on a spindle flange of a balancing machine. The flange portion  3  can be secured to the spindle flange by means of screws  6 . 
     The chuck housing  2  has a central hole  7  in which an actuating element  8  is mounted so as to be longitudinally movable. There are secured to the actuating element  8  at least two arms  9  which extend radially outwards in the chuck housing  2 . The radially outer ends of the arms  9  are each connected to a connecting rod  10  which is mounted so as to be rotatable and longitudinally movable in a hole  11  in the chuck housing  2  that is axially parallel to the hole  7 . The connection between an arm  9  and a connecting rod  10  is constructed in such a manner that the connecting rod  10  can be rotated relative to the arm  9 . In an axial direction, the connection is positive-locking and substantially play-free. At the side of the arms  9  facing away from the flange portion  3 , the actuating element  8  carries a spring plate  12 . A plurality of biased compression springs  13  are arranged at a uniform distance from one another around the actuating element  8  and are clamped between the spring plate  12  and a wall  14  of the housing portion  4 . 
     Each connecting rod  10  has, in the hole  11 , a drive portion  15  with a groove  16  which has a helical portion  16   a  and a straight portion  16   b . A ball  17  which is supported in a spherical formation in the hole wall engages in the groove  16  and, by means of the helical portion  16   a , causes the connecting rod  10  to rotate when it is moved in the longitudinal direction of the hole  11 . If the ball  17  engages in the straight portion  16   b , the connecting rod  10  is secured against rotation. The front end of the connecting rod  10  remote from the flange portion  3  protrudes from the chuck housing  2 . At this end of the connecting rod  10 , a clamping element  18  is secured by means of a screw. The clamping element  18  has a hole which is parallel to the axis of the connecting rod  10  and in which a shaft  20  of a dome-headed clamping member  19  and a compression spring  21  are arranged. The compression spring  21  surrounds the shaft  20  and is supported at one end on the dome-headed clamping member  19  and at the other end on the clamping element  18 . At the free end of the shaft  20  protruding from the hole of the clamping element  18 , two mutually tightened stop nuts  22  are arranged and limit a movement of the dome-headed clamping member  19  in the direction towards the chuck housing  2  brought about by the compression spring  21 . In the opposite direction, the dome-headed clamping member  19  can be directly supported on the clamping element  18 . 
     At the side facing the flange portion  3 , the housing portion  4  of the chuck housing  2  has a cup-shaped recess  24  in which a mandrel housing  25  is arranged. The mandrel housing  25  is precisely centered in the recess  24  by means of centering surfaces and has an annular collar  26  which is securely connected to the housing portion  4  by means of screws which are not illustrated. The collar  26  is centered with a conical lateral surface  28  in a conical portion  29  of the housing portion  4  and supported in an axial direction. The end face of the mandrel housing  25  facing away from the housing portion  4  forms a planar, annular clamping surface  27  against which a workpiece  50  can be clamped using the clamping element  18  and the dome-headed clamping member  19 . 
     An actuating bushing  30  with an actuating edge  31  formed in the bushing hole is axially movably arranged in a cylindrical hole of the mandrel housing  25 . The actuating bushing  30  actuates a cylindrical clamping member  32  which protrudes into the actuating bushing  30  and has a conical actuating surface  33  which co-operates with the actuating edge  31 . The cylindrical clamping member  32  has the shape of a bushing provided with slots and is mounted on a base element  34  which is arranged in the mandrel housing  25  and is rigidly connected thereto. One, front end of the cylindrical clamping member  32 , at the side facing away from the housing portion  4 , protrudes from the mandrel housing  25  and is provided with a centering portion of a smaller diameter which forms a clamping edge  35  with the outer side thereof. On the inner side, the centering portion has a supporting edge  36  which adjoins the base element  34  and a stop edge  37  which adjoins a shoulder of the base element  34  in an axial direction. Owing to a projection  38  which is formed on the base element  34  and which engages in an annular groove  39  in the hole of the cylindrical clamping member  32 , the cylindrical clamping member  32  is also secured against displacements in an axial direction. 
     In a stepped hole  40  of the base element  34 , an actuating member  41  is longitudinally movably arranged. The end of the actuating member  41  adjacent to the housing portion  4  is provided with a flange  42 . Between the flange  42  and a shoulder of the base element  34 , a biased compression spring  43  is arranged and is guided on the actuating member  41 . The compression spring  43  serves to actuate the actuating bushing  30  which is securely connected to the flange  42  of the actuating member  41  by means of at least one, preferably several connecting bolt(s)  44 . The connecting bolt  44  extends through a slot  45  in the base element  34 . The length of the slot  45  is adapted to the actuating path of the actuating bushing  30 . 
     The operating method of the chuck  1  is as follows: 
     In the release position provided for inserting or removing a workpiece  50 , the actuating element  8  is pushed so far to the right-hand side in the drawing by means of the actuating device of the balancing spindle that the spring plate  12  abuts the wall  14 , as indicated by the dot-dash line  12   a . The actuating member  41  adjacent to the actuating element  8  is displaced to the same extent so that the actuating bushing  30  is in the position  30   a . The movement of the actuating element  8  and the actuating member  41  into the position mentioned is carried out counter to the force of the compression springs  13  and  43  which are compressed to a greater extent during this movement and reach their maximum bias. The actuating edge  31  has, relative to the actuating surface  33  of the cylindrical clamping member  32 , a large axial spacing which is required in order to achieve a sufficiently large pivot angle of the clamping elements  18 . The arms  9  which are connected to the actuating element  8  and the connecting rods  10  which are connected thereto are also displaced to the right to a corresponding extent and, owing to the co-operation of the groove  16  and ball  17 , are rotated into a position in which the dome-headed clamping member  19  of the respective clamping element  18  is pivoted radially outwards away from the clamping surface  27 . 
     The workpiece  50  is inserted into the chuck  1 , which is in the release position, in such a manner that it surrounds the clamping edge  35  of the cylindrical clamping member  32  with the hollow cylindrical centering surface  51  of said workpiece and abuts the clamping surface  27  with the planar contact surface  52  thereof. If this is the case, the actuating device of the balancing spindle is returned to the retracted initial position thereof in order to clamp and centre the workpiece  50 . In this case, the connecting rods  10 , which are connected to the actuating element  8 , and the actuating bushing  30  are moved to the left by the force of the compression springs  13 ,  43 . This first results in the connecting rods  10  being rotated into the clamping position, the clamping elements  18  with the dome-headed clamping member  19  thereof pivoting over the workpiece  50  and the dome-headed clamping members  19  pressing the workpiece  50  against the clamping surface  27  owing to the smaller force of the compression springs  21 . The force of the compression springs  21  in this instance is of such a size that the workpiece  50  is held in abutment against the clamping surface  27  but the friction present can be overcome and the workpiece can still be radially displaced relative to the clamping surface  27 . After this clamping state has been achieved, owing to the continuing movement of the actuating element  8 , the actuating edge  31  of the actuating bushing  30  comes into contact with the actuating surface  33  of the cylindrical clamping member  32  and acts on the cylindrical clamping member  32 . The outer diameter thereby increases in the region of the clamping edge  35  and the clamping edge  35  is pressed against the centering surface  51  of the workpiece  50 , the cylindrical clamping member  32  being supported radially inwards on the base element  34  by the supporting edge  36  and thereby ensuring precise centering of the workpiece  50 . 
     When the centering operation which is brought about exclusively by the compression spring  43  is complete, the actuating element  8  is raised from the actuating member  41  and moves the connecting rods  10  further to the left under the action of the compression springs  13 . The compression spring  21  is thereby compressed to an even greater extent and the rear side of the dome-headed clamping member  19  comes into contact with the clamping element  18 . The significantly greater actuating force of the compression spring  13  is thereby transferred to the dome-headed clamping member  19  and the workpiece  50  is pressed against the clamping surface  27  with a greater force. Owing to the greater pressing force, the workpiece  50  is securely and reliably retained in the chuck  1  for the balancing operation which is subsequently to be carried out. The drawing illustrates the chuck  1  in the achieved clamping position, in which the workpiece  50  is retained in the chuck  1  in a precisely centered state. 
     The operating method described for the chuck has the advantage that the workpiece can be centered and clamped with a high level of precision and clamping errors, such as an inclined position of the workpiece, are prevented. Furthermore, it is advantageous that the entire clamping operation is carried out in a completely automated manner after the workpiece has been inserted and no specific adjustment measures are required. The chuck is further characterized by a compact structure which can also be used in place of chucks which are already present. 
       FIG. 3  illustrates a modified chuck  1  a which differs from the chuck  1  described above owing to a different arrangement of the compression spring  21  but which otherwise substantially corresponds to the chuck  1 . The same reference numerals are therefore used below for corresponding components. 
     With the chuck  1   a , the arms  9  which form an integral component are arranged axially movably on the actuating element  8 . At the side of the arms  9  facing the flange portion  3 , the actuating element  8  is provided with a stop  46  on which the arms  9  can be supported in an axial direction. Between the stop  46  and the arms  9 , in the clamping position illustrated in  FIG. 3 , there is an axial play S. At the opposite side, the arms  9  have an annular collar which is in abutment against the spring plate  12  under the action of the compression springs  13 . At the side thereof facing the spring plate  12 , the arms  9  each have a blind hole in which a biased compression spring  47  is arranged and is supported on the spring plate  12 . The compression spring  47  replaces the compression spring  21  which is arranged in the clamping element  18  in the case of the chuck  1 . In the chuck  1   a , the dome-headed clamping member  19  is therefore securely connected to the clamping element  18 . 
     In the clamping position illustrated in  FIG. 3 , the compression springs  47  are biased to the maximum extent. The sum of their bias forces is considerably smaller than the sum of the bias forces of the compression springs  13 . 
     If the actuating element  8  is actuated in order to loosen the chuck  1   a , the spring plate  12  is first moved towards the right-hand side in the drawing by a step  48  of the actuating element  8 , the compression springs  13  being clamped to a greater extent. Owing to the bias of the compression springs  47 , which is slightly decreased, the arms  9  are retained in their clamping position. After overcoming the play S, the arms  9  are also carried by the stop  46  by the movement of the actuating element  8  and the chuck is moved into the release position, in which the spring plate  12  is at the location  12   a  and in which the clamping elements  18  are pivoted away from the workpiece  50 . 
     The clamping of the workpiece  50  is carried out using the reverse movement sequence, the clamping elements  18  with the dome-headed clamping members  19 , driven by the compression springs  13  and the compression spring  43  which is arranged inside the mandrel housing  25 , first being positioned against the workpiece  50  and pressed with the force of the compression springs  47 , then the cylindrical clamping member  32  being clamped and finally, when the spring plate  12  abuts the arms  9 , the workpiece  50  being securely clamped by the dome-headed clamping members  19  with the significantly greater force of the compression springs  13 . 
       FIG. 4  illustrates a further configuration of a chuck  55  according to the invention which is configured to clamp and centre the workpiece on a cylindrical outer face. The chuck  55  has a cup-shaped clamping member housing  56  with a hole  57  in which a cylindrical clamping member  58  and an actuating bushing  59  are arranged. The cylindrical clamping member  58  has the shape of a substantially cylindrical bushing which is provided with axially parallel slots and which has reinforced end portions. The front end portion forms a radially outwardly protruding shoulder which engages in an annular centering recess  60  at the open end of the hole  57  and which is supported with a supporting edge  61  radially outwardly on the wall of the centering recess  60  and with a stop edge  62  in an axial direction on a shoulder  63  of the clamping member housing that delimits the centering recess  60 . The front end face of the cylindrical clamping member  58  is flush with the front end face of the clamping member housing  56  and forms, together with the hole face of the front end portion, a clamping edge  64  for centering and clamping a workpiece. The rear end portion of the cylindrical clamping member  58  forms a radially inwardly protruding collar with an inner conical actuating surface  65  which faces the clamping edge  64  and which co-operates with a shoulder of the actuating bushing  59 . The actuating bushing  59  is axially movably mounted on a cylindrical tube member  66  which is arranged in the hole of the clamping member housing  56  and which is securely connected to the clamping member housing  56  by means of screws  67 . At the front end of the tube member  66 , a plate  68  is secured by means of screws  69 . The plate  68  extends radially outwards as far as the proximity of the clamping edge  64  and has, at that location, a radial clamping surface  70  which is formed by an annular collar. The mutually opposing regions of the plate  68  and the actuating bushing  59  are provided with coaxial blind holes in which biased compression springs  71  are arranged with uniform peripheral spacing. The compression springs  71  press the actuating bushing  59  against the actuating surface  65 , whereby the cylindrical clamping member  58  is clamped. 
     In the hole of the tube member  66 , a cup-like actuating member  72  is arranged and is loaded by a compression spring  73  which is supported on the plate  68 . Opposite the actuating member  72  is an actuating member  74  by means of which the actuating member  72  can be actuated counter to the force of the compression spring  73 . The actuating member  72  is operationally connected to the actuating bushing  59  by means of balls  75 . The balls  75  are located with uniform mutual spacing in radial through-holes of the tube member  66  and their centre points are located in a common radial plane. The diameter of the balls  75  is approximately two to three times as large as the wall thickness of the tube member  66  so that the balls  75  can protrude inwardly and outwardly from the wall of the tube member  66 . The balls  75  engage in an annular groove  76  in the hole face of the actuating bushing  59  and in an annular groove  77  in the lateral surface of the actuating member  72 . One side wall  78  of the annular groove  76 , which is adjacent to the end of the actuating bushing  59  acted on by the compression spring  71 , is constructed in a conical manner with an inclination of approximately 45 [deg.] relative to the longitudinal axis. The annular groove  77  has a conical side wall  79  with a corresponding inclination, which abuts the contact locations between the balls  75  and the locations of the balls  75  diametrically opposite the side wall  78 . The balls  75  form with the annular grooves  76 ,  77  a transmission means which transmits the movement of the actuating member  72  to the actuating bushing  59  and after reaching a predetermined actuating distance, disconnects the transmission connection between the actuating member  72  and the actuating bushing  59  so that the actuating member  72  can be moved further independently of the actuating bushing  59 . 
     As with the chuck  1  illustrated in  FIG. 1 , with the chuck  55  the actuating element  74  is also connected by means of arms  9  and connecting rods  10 , which are not illustrated, to clamping elements  18 , which clamp the workpiece  50  against the clamping surface  70  by means of dome-headed clamping members  19 , which are supported on springs  21 . 
       FIG. 4  illustrates the chuck  55  in the clamping position in which a workpiece  50  is clamped in a precisely centered manner for the subsequent balancing operation. The compression springs  71  act, via the actuating bushing  59 , on the cylindrical clamping member  58 , whereby the clamping edge  64  of said member is pressed against the cylindrical centering surface  51  of the workpiece  50 . In an axial direction, the workpiece  50  is securely clamped between the clamping surface  70  and the dome-headed clamping member  19  with a significant actuating force which is produced by means of a resilient element which is not illustrated and which acts on the actuating element  74 . Between the actuating element  74  and the actuating member  72 , there is provided a gap and the actuating member  72  is supported, with the edge  80  of the annular groove  77  that faces the side wall  79  and is provided with a conical face, on the balls  75  under the action of the compression spring  73 . The balls  75  are thereby pressed radially outwards against the side wall  78  and are thereby kept play-free. The force of the compression spring  73  is significantly smaller than the force of the compression spring  71  so that the play-free retention of the balls  75  has no significant influence on the actuation of the cylindrical clamping member  58 . 
     The workpiece  50  is unclamped from the chuck  55  by moving the actuating element  74  in the direction towards the plate  68 . Owing to this movement, which is also transmitted to the connecting rods  10 , the axial clamping force is first reduced since the dome-headed clamping members  19  are raised from the clamping elements  18  and then still pressed against the workpiece  50  only by the weaker compression springs  21 . The actuating element  74  then moves into engagement with the actuating member  72  and presses it with the side wall  79  against the balls  75 . The balls  75  are thus pressed radially outwards into the annular groove  76 , whereby the actuating bushing  59  moves past the plate  68  and the compression springs  71  are compressed. The actuating bushing  59  is thus released from the actuating surface  65  of the centering clamping member  58  so that, owing to the inherent resilience thereof, it assumes its release position in which the inner diameter of the clamping edge  64  is greater than the outer diameter of the centering surface  51  of the workpiece  50 . When this position of the actuating element  74  is reached, the dome-headed clamping members  19  still hold the workpiece  50  securely under the action of the compression springs  21 . In order to completely release the workpiece  50 , the actuating element  74  is moved further in the direction towards the plate  68 . In this case, the balls  75  are forced completely from the annular groove  77  so that the actuating member  72  can be pushed past the balls  75  further in the direction towards the plate  68 , without this movement being transmitted to the actuating bushing  59 . The actuating path of the actuating bushing  59  and the compression springs  71  which are supported thereon therefore remains relatively small, which has structural advantages. Furthermore, the force of the compression springs  71  no longer has to be overcome by the drive device which moves the actuating element  74 , but instead only by the friction resistance brought about by the force of these springs. Owing to this last portion of the actuating path of the actuating element  74 , the dome-headed clamping members  19  are released from the workpiece and the clamping elements  18  are pivoted out of the supply region for the workpiece by the rotation of the connecting rods  10 . 
     The clamping of a new workpiece is carried out by reversing the movement sequence described, the dome-headed clamping members  19  first being activated under the action of the compression springs  21 , then the cylindrical clamping member  58  under the action of the compression springs  71  and finally the greater force of a resilient element which acts on the actuating element  74  by means of the dome-headed clamping members  19 , again in succession.