Abstract:
A test body clamping device in a rheometer comprises two clamping jaws between which a test body can be clamped, and an operating device for moving the clamping jaws towards and away from each other. The invention provides that each clamping jaw is pivotably disposed about an axis, wherein the axes extend parallel to each other and parallel to a clamping surface of the respective clamping jaw. The clamping jaws moreover have an associated common drive part for exerting a drive force onto both clamping jaws, which produces a synchronized pivoting motion of both clamping jaws, wherein the clamping jaws are subjected, in their clamped position, to the action of a common clamping force element, in particular a clamping spring.

Description:
[0001]     This application claims Paris Convention priority of DE 10 2005 021 121.6 filed May 6, 2005 the complete disclosure of which is hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     The invention concerns a test body clamping device in a rheometer, comprising two clamping jaws, between which a test body can be clamped, and an operating device for moving the clamping jaws towards each other.  
         [0003]     A rheometer, in particular, a rotational rheometer is conventionally used to determine the rheological values or characteristic values of a viscous substance. The rheometer comprises a lower measuring part and an upper measuring part which can be adjusted relative to each other. The upper measuring part of a rotational rheometer can be rotated and oscillated. A sample space is formed between the measuring parts for receiving a sample of the substance to be investigated. The forces and torques generated through relative adjustment between the upper and lower measuring parts can be determined, from which the desired rheological characteristic values can be calculated.  
         [0004]     A conventional rotational rheometer can be modified to also permit measurement of a test body which consists of a dimensionally stable, rigid material. Towards this end, one test body clamping device replaces each of the upper and lower measuring parts to clamp the test body, being investigated at both its upper and lower ends. The test body is usually cube or rod-shaped. The test body clamping devices are rotated relative to each other, thereby twisting the test body. The forces and torques between the upper and lower test body clamping devices generated through twisting are also determined, from which the desired characteristic material values can be calculated.  
         [0005]     A conventional test body clamping device comprises a fixed contact block and a clamping block which can be adjusted by a threaded rod and whose separation from the contact block can be changed, such that the test body to be investigated can be clamped between the contact block and the clamping block. Since the adjustment path of the clamping block is relatively short, only test bodies having dimensions within a narrow range can be inserted into the clamping device. In order to also enable investigation of test bodies of other dimensions, the contact block must first be removed and exchanged for a different contact block. This procedure is time-consuming and difficult and moreover requires storage of a plurality of different contact blocks.  
         [0006]     A rotational rheometer comprises a shaft that can rotate about a longitudinal axis and can rotate the two test body clamping devices relative to each other. When conventional test body clamping devices are used, the axis of rotation or longitudinal axis of the shaft rarely coincides with the longitudinal axis of the test body. This eccentricity generates moments which can falsify the measuring results.  
         [0007]     The ambient conditions and, in particular, the ambient temperature must be observed during determination of the characteristic material values. It may e.g. be desirable to test the material to be investigated at predetermined ambient temperatures, e.g. at a very low or very high temperatures. For this reason, the surroundings of the test body in the rheometer can be adjusted to a desired temperature within a range of between approximately −170° C. and +700° C. As a result, the test body shrinks or expands after insertion into the test body clamping device in response to the temperature. At low temperatures, the clamping effect is reduced or even eliminated, such that the test body clamping device must be re-tightened. At very high temperatures, very large tensile forces may form in the test body clamping device as the test body expands. These very high localized forces falsify the measuring result, thereby also necessitating subsequent adjustment of the test body clamping device.  
         [0008]     It is the underlying purpose of the invention to provide a test body clamping device in a rheometer of the above-mentioned type which facilitates clamping of test bodies having different dimensions and which holds the test body in a reliable manner even in case of temperature-related deformation.  
       SUMMARY OF THE INVENTION  
       [0009]     This object is achieved in accordance with the invention with a test body clamping device having the characterizing features of the independent claim. Each clamping jaw is thereby disposed to be pivotable about its own axis, wherein the axes extend parallel to each other and parallel to a clamping surface of the respective clamping jaw. Moreover, the clamping jaws have a common drive part which applies a drive force on the clamping jaws that causes synchronized pivoting motion of both clamping jaws. In their clamped position, the clamping jaws are subjected to the action of a common clamping force element. The clamping force element is normally a clamping spring, which is taken as an example in the further description.  
         [0010]     When two pivotably disposed clamping jaws are used, the separation between the clamping surfaces of the clamping jaws can be changed within a relatively large range and test bodies of the most differing dimensions can therefore be received without having to modify the test body clamping device. The common drive part of the two clamping jaws ensures that the pivoting motion of the two clamping jaws is synchronized, wherein the clamping surfaces of the clamping jaws are always oriented parallel to each other. In this fashion, the full surfaces of the clamping jaws uniformly abut on opposite sides of the test body and clamp it between them. The clamping jaws are thereby preferably disposed to be symmetrical with respect to an axis of rotation of the rheometer at any pivot position, such that, during clamping, the test body is centered perpendicularly with respect to the clamping surfaces of the clamping jaws relative to the axis of rotation of the rheometer shaft.  
         [0011]     In accordance with the invention, the test body is not retained in a positive manner at its clamped position, rather the clamping jaws are held in their clamped position in a non-positive manner using a common clamping spring.  
         [0012]     The test body is thereby clamped with a defined clamping force. Dimensional deviations associated with temperature-related expansion or shrinking of the test body and the clamping jaws can be compensated for by the spring force of the clamping spring, so that the test body is also safely retained in this case. It has turned out that even when the geometry changes to a relatively great extent, e.g. in a range of approximately 10%, safe retention of the test body is ensured without having to re-tighten the test body clamping device.  
         [0013]     Temperature-resistant materials and materials having good heat conducting properties should be used as materials for the test body clamping device and, in particular, for the clamping jaws. A mixture of high-temperature resistant special steel and a copper alloy are preferably used, which may also be plated with hard gold.  
         [0014]     The clamping jaws may also be disposed to be exchangeable in order to install appropriate clamping jaws for certain materials of the test body which require e.g. a clamping surface profile or the like.  
         [0015]     In a preferred embodiment of the invention, the clamping spring acts on the clamping jaws via the drive part which is preferably a rotatably disposed drive ring. This ensures that the spring force of the clamping spring acts uniformly and with the same magnitudes on the clamping jaws and therefore on the test body.  
         [0016]     In a preferred embodiment of the invention, the operating device comprises an adjustment screw for clamping the clamping spring. Via the adjustment screw, a user can adjust the pre-tension with which the clamping jaws abut the test body in the clamped position. The clamping spring may be exchangeable to realize different pre-tensioning forces using different, i.e. harder or softer, clamping springs.  
         [0017]     The closing motion of the clamping jaws towards each other is achieved by introducing a rotary motion on the drive part in a positive manner using the operating device while the clamping spring is relaxed. This rotary motion may be generated e.g. when a user adjusts the adjustment screw in such a manner that it abuts a projection of the drive part, rotating the drive part during further adjustment. The rotation of the drive part produces a synchronous pivoting motion of the clamping jaws, thereby clamping the test body with a defined pre-tension. Towards this end, the adjustment screw may comprise a peripheral groove which houses the projection of the drive part.  
         [0018]     In a preferred further development of the invention, the adjustment screw comprises an outer helical toothing which engages, in a preferably self-locking manner, with a toothed profile of the rheometer that is rigid with respect to the frame. The toothed profile of the rheometer that is rigid with respect to the frame, may be formed, in particular, on the shaft of the rheometer or a component rigidly disposed thereon.  
         [0019]     The adjustment screw preferably comprises an inner axial recess or bore by means of which it is seated on a bearing pin of the drive part, such that it can be freely displaced. The clamping spring may thereby be disposed inside the recess and be supported on the bearing pin and also on the bottom of the recess. This has the further advantage that the clamping spring is largely protected from external influences and, in particular, from soiling.  
         [0020]     In a further embodiment, the clamping screw may be seated in a recess of the drive part in such a manner that it can be freely displaced, wherein the clamping spring is pushed onto the clamping screw in an axial direction and is supported on a projection or head of the clamping screw and on the drive part.  
         [0021]     In a further development of the invention, the drive part engages the clamping jaw at its end section facing away from the respective axis in order to obtain adequate clamping using relatively small forces, and to be able to adjust the clamping path in a relatively fine manner.  
         [0022]     Further details and features of the invention can be extracted from the following description of an embodiment with reference to the drawing. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0023]      FIG. 1  shows a perspective view of an inventive test body clamping device;  
         [0024]      FIG. 2  shows a top view of the test body clamping device, partially in section, in accordance with a first embodiment with inserted test body at the start of the clamping process;  
         [0025]      FIG. 3  shows the test body clamping device in accordance with  FIG. 2  during the clamping process;  
         [0026]      FIG. 4  shows the test body clamping device in accordance with  FIG. 3  when the clamped position has been achieved;  
         [0027]      FIG. 5  shows the test body clamping device in accordance with  FIG. 4  after exerting the spring clamping force onto the test body;  
         [0028]      FIG. 6  shows a top view of a test body clamping device, partially in section, in accordance with a second embodiment with inserted test body and at the start of the clamping process;  
         [0029]      FIG. 7  shows the test body clamping device in accordance with  FIG. 6  during the clamping process;  
         [0030]      FIG. 8  shows the test body clamping device in accordance with  FIG. 7  when the clamped position has been reached; and  
         [0031]      FIG. 9  shows the test body clamping device according to  FIG. 8  after exerting the spring clamping force onto the test body. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0032]      FIG. 1  shows a test body clamping device  10  which is disposed on a shaft  11  of a rotational rheometer. The shaft  11  can be rotated together with the test body clamping device  10  about a longitudinal axis D.  
         [0033]     The test body clamping device  10  comprises two clamping jaws  12 ,  13  between which a receiving space  14  is formed for receiving a test body P ( FIG. 2 ).  FIG. 1  shows an adjustment screw  21  having an operating section  15 , the rotation of which moves the two clamping jaws  12 ,  13  towards each other or away from each other, thereby clamping or releasing the test body P.  
         [0034]     In  FIG. 2 , a bottom part  19  is rigidly mounted to the shaft  11  and bears one bearing pin  16  and  17  at each of two locations disposed diametrally opposite relative to the longitudinal axis D. One clamping jaw  12 ,  13  is rotatably disposed on each bearing pin  16  and  17 , such that the clamping jaw  12  on the right in  FIG. 2  can be pivoted about an axis of rotation A 1  and the clamping jaw  13  on the left in  FIG. 2  can be pivoted about an axis of rotation A 2 . The axes of rotation A 1  and A 2  extend parallel to each other and parallel to the longitudinal direction D of the shaft  11  of the rheometer.  
         [0035]     The clamping jaws  12 ,  13  extend substantially parallel to each other, wherein, in particular, their mutually facing clamping surfaces  12   a  and  13   a  extend parallel to each other and define a receiving space  14 , in which the test body P is loosely inserted, such that its longitudinal axis coincides with the longitudinal axis D of the shaft  11 . The test body is thereby supported with its lower side on the bottom part  19 .  
         [0036]     The bottom part  19  moreover comprises a toothed block  28  which bears a toothed profile  29  on its outer side.  
         [0037]     The bottom part  19  and, in particular, the two clamping jaws  12 ,  13  are surrounded by a drive part in the form of a rotatable drive ring  18 , comprising two contact surfaces  18   a  and  18   b  which can be brought into abutment with the clamping jaws  12  and  13  on the rear side of the clamping jaws  12  and  13  facing away from the respective clamping surface  12   a  and  13   a,  wherein further rotation of the drive ring  18  forces the clamping jaw  12  or  13  to pivot about the respective axis of rotation A 1  or A 2 . Since the contact surfaces  18   a  and  18   b  seat on a common drive part, i.e. the drive ring  18 , the pivot motions of the clamping jaws  12  and  13  are synchronized with respect to each other such that clamping surfaces  12   a  or  13   a  remain parallel to each other and symmetrical with respect to the longitudinal axis or axis of rotation D of the shaft  11  of the rheometer in any pivot position of the clamping jaws  12 ,  13 .  
         [0038]     The drive ring  18  which is also disposed to be rotatable about the axis of rotation D, supports a cylindrical bearing pin  22 , which extends in a substantially tangential direction relative to the axis of rotation D, the bearing pin  22  bearing an adjustment screw  21  which is part of an operating device  20  of the test body clamping device  10 . The adjustment screw  21  has a cylindrical basic body having an axial recess or bore  27  via which the adjustment screw  21  is seated on the bearing pin  22  with tight fit but freely displaceable. A helical pressure or clamping spring  25  is disposed within the bore or recess  27 , the axial end of which is supported on the bottom of the bore or recess  27  and the opposite end of which can be brought into abutment with the end face of the bearing pin  22 .  FIG. 2  shows the clamping spring  25  in a relaxed, unloaded state.  
         [0039]     The adjustment screw  21  has an outer helical toothing  23 , which engages in the toothed profile  29  of the toothed block  28  and permits slight relative axial displacement.  
         [0040]     A peripheral depression or annular groove  24  is formed on the end of the adjustment screw  21  facing away from the bearing pin  22 , in which a pin-shaped projection  26  of the drive ring  18  engages with play in the longitudinal direction of the adjustment screw  21 , wherein the projection  26  abuts the bottom of the annular groove  24 .  
         [0041]     The function of the test body clamping device  10  will be described in detail below with reference to  FIGS. 2 through 5 .  FIG. 2  shows the initial state, in which the test body P is inserted with play between the parallel oriented clamping jaws  12  and  13 . The drive ring  18  does not abut the clamping jaws  12 ,  13  and the clamping spring  25  is completely relaxed. The clamping jaws  12  and  13  may be pre-tensioned into their open position by springs (not shown).  
         [0042]     When the adjustment screw  21  is turned, it is axially displaced on the bearing pin  22  until the flank of the annular groove  24  abuts the projection  26  of the drive ring  18 . This state is shown in  FIG. 2 . Further rotation of the adjustment screw  21  produces a rotary motion of the drive ring  18  via abutment of the annular groove  24  and projection  26 , the contact surfaces  18   a  and  18   b  of the drive ring  18  abutting one free end of each clamping jaw  12 ,  13 , facing away from the axes of rotation A 1  and A 2 , and forcing them to pivot, thereby reducing the separation between the clamping surfaces  12   a  and  13   a  of the clamping jaws  12  and  13  ( FIG. 3 ). Since the drive ring  18  and the adjustment screw  21  rotate as a unit, the clamping spring  25  initially remains completely relaxed.  
         [0043]     Rotation of the adjustment screw  21  is continued to further rotate the drive ring  18 , thereby bringing the clamping surfaces  12   a  and  13   a  of the clamping jaws  12  and  13  into abutment with the test body P from opposite sides, clamping it between them. This state is shown in  FIG. 4 .  
         [0044]     The clamping jaws  12  and  13  now firmly abut the test body P, and upon further rotation, the adjustment screw  21  travels away from abutment of its annular groove  24  on the projection  26  of the drive ring  18  towards the bearing pin  22  due to the reaction moment at the location where its outer toothing  23  engages the toothed profile  29 .  
         [0045]     The clamping spring  25  disposed inside the adjustment screw  21  is thereby pressed against the bearing pin  22  and clamped. This state is shown in  FIG. 5 .  
         [0046]     When the clamping force of the clamping jaws  12  and  13  changes during the measurement due to thermal influences on the test body clamping device or the test body to be measured, the clamping spring  25  compensates for the clamping forces on the test body clamped between the clamping jaws, by changing its axial length, i.e. through tension or relaxation, reacting to force changes with approximately the same reaction force. This displacement results from the rotary motion of the drive ring  18  which is effected by opening or closing the clamping jaws  12 ,  13 . This automatic readjustment of the clamping force is possible, since the outer toothing  23  of the adjustment screw  21  is self-locking.  
         [0047]      FIGS. 6 through 9  show an alternative design of the test body clamping device  10  which substantially corresponds to constructive designs of the test body clamping device  10  described in connection with  FIGS. 1 through 5 , wherein identical components have identical reference numerals.  
         [0048]     The essential difference of the test body clamping device  10  of  FIGS. 6 through 9  concerns the operating device  20 . The adjustment screw  21  has a head  21   a  where the user can apply a tool. A bearing block  21   b  is formed on the adjustment screw  21  at the end remote from the head  21   a  and comprises an outer toothing  23  which engages the toothed profile  29  of the toothed block  28  and permits a slight relative axial displacement. The bearing block  21   b  is seated in a recess  18   c  of the drive part  18  in such a manner that the adjustment screw  21  can be rotated and slightly axially displaced within the recess  18   c.    
         [0049]     A helical clamping spring  25  is disposed on the adjustment screw  21  between the head  21   a  and a contact surface of the drive part  18 , which is relaxed and unloaded in the initial state shown in  FIG. 6 .  
         [0050]     The function of the test body clamping device  10  will be explained below with reference to  FIGS. 6 through 9 .  FIG. 6  shows the initial state, wherein the test body P is inserted with play between the clamping jaws  12  and:  13 . The drive ring  18  does not yet exert any force on the clamping jaws  12  and  13  and the clamping spring  25  is completely relaxed.  
         [0051]     When the clamping screw  21  is turned by a user, the outer thread  23  of the bearing block  21   b  engages along the toothed profile  29  of the toothed block  28  to rotate the drive ring  18  in an anticlockwise direction ( FIG. 6 ). Rotation of the drive ring  18  causes the contact surfaces  18   a  and  18   b  of the drive ring  18  to displace the free ends of the clamping jaws  12  and  13  facing away from the axes of rotation A 1  and A 2 , and forces the clamping jaws  12  and  13  to pivot, thereby clamping the test body P between the clamping jaws  12  and  13 . Since the drive ring  18  and the adjustment screw  21  rotate as a unit, the clamping spring  25  is still completely relaxed.  
         [0052]     When the test body P is clamped between the clamping jaws  12  and  13 , further rotation of the drive ring  18  is not possible. During further rotation of the clamping screw  21 , the clamping screw performs an axial motion or displacement relative to the drive ring  18 , thereby compressing and tensioning the clamping spring  25 . This state is shown in  FIG. 9 . The drive ring  18  is pressed against the clamping jaws  12  and  13  by the spring force of the clamping spring  25 , such that the test body P is clamped between them under elastic pre-tension. As was already mentioned above, this causes the test body to be safely retained between the clamping jaws  12  and  13  during the measurement even when its dimensions change due to temperature-related expansion or shrinkage.