Abstract:
The invention is a torque-limited coupling element including an input and output rotatably mounted about a common rotational axis, at least one Connection which is transferred from a first position into a second position upon exceeding a settable limit torque acting between the input and the output wherein in the first position the at least one connection connects the input and the output in a rotationally fixed manner, and in the second position, the input and the output are rotatably mounted relative to one another. A displacement body is movably mounted within a chamber filled with a magnetorheological fluid with the position of the displacement body resulting from a holding force provided by the magnetorheological fluid being under influence of a magnetic field.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a torque-limited coupling element with an input part and an output part each rotatably mounted about a common rotational axis, and at least one connecting means which from a first position, in which the connecting means interconnects the input part and output part in a rotationally fixed manner subject to the formation of a positive engagement, can be transferred into a second position upon exceeding of a predeterminable limit torque acting between the input part and output part, in which the input part and output part are rotatably mounted relative to each one another. 
     2. Description of the Prior Art 
     Torque-limited coupling elements are employed for example in torque wrenches, preferentially in so-called self-triggering torque wrenches, which allow tightening screw connections manually or actuatorily with a defined predeterminable torque. Such a torque wrench is disclosed in DE 31 40 288 A1, which provides a sleeve-like tool housing which in the interior thereof a coil spring is rotated to provide a variably adjustable spring characteristic on the output side the spring bears against a central ball centrally mounted within the housing, which in axial and radial directions presses against three coupling balls arranged in an equal angular distribution. The balls are radially guided in a cage structure and become operationally connected against a tool bushing radially surrounding the cage structure provided on the output side subject to the formation of a positive engagement. When a predeterminable torque between drive-end tool sleeve and output-end tool sleeve provided through the variably adjustable spring characteristic is exceeded, the coupling balls are radially pushed inwardly via a guide slotted link. As a result, the positive engagement between coupling balls and the output-end tool bushing is released, so that no further force transmission via the torque wrench is possible. 
     In a secondary development stage, devices for the torque transmission provide for the use of magnetorheological fluids, which constitute suspensions of magnetically polarizing particles in a carrier fluid. The viscosity and also other rheological properties of the fluids can be variably adjusted quickly and reversibly in the presence of a magnetic field. Such a magnetorheological torque transmission device is disclosed in DE 10 2007 019 584 A, which in a particular manner is suitable for forming a coupling, break, locking or blocking device or as man-machine interface element. The device comprises at least two device parts relatively mounted rotatably about a common rotational axis, which are separated by at least one torque transmission gap that is filled with a magnetorheological fluid and penetrated by a variably adjustable magnetic field. With controlled increasing magnetic field strength the viscosity of the magnetorheological fluid increases and at the same time also the friction between both device parts rotatably mounted about the rotational axis, between which a maximum torque ultimately dependent on the magnetic field can be transmitted. By contrast, the purely mechanical solution described above making use of the coupling balls described there, is now replaced by the magnetorheological fluid as force-transmitting element. On the one hand, this has the advantage of a simpler design embodiment avoiding moveably mounted, force-transmitting elements while on the other hand the magnetorheological fluid is subjected to high mechanical shear loads, as a result of which the magnetorheological fluid is subjected to degradations capable of sustainably influencing the functional properties of the fluid. 
     A multiplicity of further coupling embodiments, which for the force transmission utilize the variable viscosity adjustment of magnetorheological fluids dependent on the magnetic field, are based on the same coupling principle. In this connection, reference is made to: WO 2006/086807 A1, DE 10 2007 015 053 A1 and DE 698 05 483 T2. 
     DE 11 2005 001 458 T2 describes a rotary fluid coupling which attempts to minimize the disadvantageous degradating effects of the shear loads that occur with the above coupling types on the magnetorheological fluid between the input parts and output parts of the coupling rotating relative to one another. A coupling device releasable in series with the drive element of the coupling is provided which prevents a torque transmission between input part and output part if required. 
     SUMMARY OF THE INVENTION 
     Starting out from the known realization forms of coupling devices for the torque transmission between an input part and output part, the invention ensures that a torque transmission between the input part and output part can be adjusted in a manner that is as quantitatively exact as possible and particularly in those cases in which the predetermination of a maximum limit torque is required. On the other hand, the invention avoids the disadvantages connected with the previously known coupling systems, wherein the force transmission is based on magnetorheological fluids, particularly with respect to degradation of the magnetorheological fluids. 
     The torque-limited coupling element according to the invention is based on utilizing the proven, purely mechanical coupling concept for the general transmission of the torque between an input part and output part and combine this concept with the variably adjustable transmission characteristics of a magnetorheological fluid. As a result, the purely quantitative scaling of a maximally predeterminable limit torque to be transmitted between the input part and output part is possible. Since the magnetorheological fluid with the coupling concept according to the invention is not employed as force-emitting medium between two coupling parts rotation-moveably mounted relative to one another, the fluid is not subjected to any degradating shear loads whatsoever. In addition, the parasitic slipping between the input part and output part through the respective contact with the magnetorheological fluid that is known to occur in conventional fluid couplings can be avoided. As a result, unnecessary energy losses through the coupling element are avoided. The coupling principle according to the invention thus combines the advantages of two types of coupling technology, namely that of the purely mechanical coupling with those of a fluid coupling and at the same time avoids their disadvantages. 
     According to the invention, a torque-limited coupling element with an input part and output part each rotatably mounted about a common rotational axis and at least one connecting means, which from a first position, in which the connecting means interconnects the input part and output part subject to the formation of a positive engagement with one another, can be transferred into a second position upon exceeding of a predeterminable limit torque acting between the input part and output part in which the input part and output part are rotatably mounted relative to one another. The at least one connecting element in the first position is indirectly or directly in non-positive operational connection with a displacement body moveably mounted within a chamber filled with magnetorheological fluid whose position within the chamber is supported by a displacement force brought about by the magnetorheological fluid. To influence the viscosity of the magnetorheological fluid, a unit generating a magnetic field is provided on or in the region of the chamber through which the displacement force of the magnetorheological fluid can be influenced. If the limit torque between the input part and output part is exceeded, the displacement body can be deflected within the chamber against the displacement force brought about by the magnetorheological fluid. As a result, the non-positive connection between the connecting means and the input part and output part is released and the torque transmission is abruptly interrupted. 
     In principle, the torque-limited coupling element according to the invention makes use of a purely mechanical force transmission mechanism for the torque transmission between the input part and output part that is comparable with that, which corresponds to the torque wrench described in DE 31 40 288 A1 discussed above, wherein a non-positive and positive connection between the input part and output part is established by means of coupling balls that are variable in their radial position. The spring element which quantitatively predetermines the limit torque in the case of DE 31 40 288 A1 is replaced in the case of the coupling element according to the invention with a chamber filled with magnetorheological fluid, in which a displacement body is moveable, preferentially mounted to be movable linearly, which via a rod-like extension protruding through a fluid-tight chamber opening enters into operational connection with the connecting elements, which ensures a non-positive and positive connection between the input part and output part. In order to be able to adjust the magnitude of the torque acting between the input part and output part at which the coupling is to open, and the connecting elements are to release their force-loaded non-positive connection, an adjustable limit force directed at the connecting elements has to be generated which originates from an adjustable movability of the displacement body through the magnetorheological fluid within the chamber. The displacement body through its body shape is matched to the geometrical shape of the chamber which divides the chamber volume into two chamber parts. As a result, both chamber parts are fluidically interconnected to one another via a connecting channel. In a preferred embodiment, the chamber encloses a cylindrical volume within which the likewise cylindrically shaped displacement body divides the chamber into a front and a rear chamber part, both of which are interconnected by an annular gap, which is limited by the displacement body and the chamber inner wall. 
     If in a preferred manner, a magnetic field is applied in the region of the connecting channel or annular gap. The viscosity of the magnetorheological fluid is greatly influenced in this region. As a result of the influence of the magnetic field, it is somewhat difficult to force the fluid from one chamber part through the annular gap into the other chamber part. Thus, the displacement body can be somewhat easily displaced in axial direction along the chamber so that by adjusting the magnetic field strength, a scalable holding or supporting force for maintaining the non-positive connection of the connecting elements with the input part and output part of the coupling can be generated. If due to operation the limit torque is exceeded, the displacement body is displaced from its position against the displacement forces acting thereon within the chamber, as a result of which the non-positive connection is released. If this case occurs, it is advantageous to switch off the prevailing magnetic field in order to largely eliminate material-wearing frictional forces within the coupling to which in particular the connecting elements are subjected. 
     With a switched-off magnetic field, the magnetorheological fluid assumes its lowest viscosity so that the displacement body can be returned into its axial starting position within the chamber without major force expenditure. This can be effected using a resetting spring interacting with the displacement body. The coupling is then in a state in which only a very low torque can be transmitted, which is mainly generated by the resetting force of the spring. 
     The coupling concept according to the invention is based on the method for the torque-limited coupling between an input part rotatably mounted about a common rotational axis and an output part. Both parts enter into a releasably fixed non-positive connection with at least one connecting means for purposes of torque transmission so that the at least one connecting means in a first position, interconnects the input part and the output part in a rotationally fixed manner, subject to the formation of a non-positive connection. The at least one connecting means is transferred into a second position upon exceeding of a predeterminable limit torque acting between the input part and output part, in which the input part and output part rotate relative to one another. The method according to the invention is characterized in that a limit torque is influenced by magnetorheological fluid which enters into interaction with a magnetic field. 
     In a particularly advantageous manner, the torque-limited coupling element according to the invention is suitable for application to a torque wrench providing a maximum tightening torque for a screw which can be accurately predetermined. Unintentional excessive tightening of the screw can be prevented in this manner. In addition, the torque-limited coupling element can be supplemented with a suitable sensor device in order to sense the actually applied torque between input part and output part. DMS measuring strips, which are to be provided in a suitable position of the coupling housing, are for example appropriate for this purpose. 
     The coupling elements according to the invention can also be integrated into the drive line of machine tools in a particularly advantageous manner for example to decouple a motor drive from the tool provided on the output side when a certain limit torque is reached. With the help of a magnetic field which is generated to be variable in time and dependent on the application, through which the viscosity of the magnetorheological fluid can be individually adjusted without delay, individual switching characteristics can also be realized which, for example, can be utilized for careful coupling and decoupling between input parts and output parts. 
     In a similar manner, the coupling element can also be employed in a drive train of motor vehicles, in order to realize coupling operations. Particularly in the case of hybrid vehicles for the rapid coupling or decoupling of the combustion engine and electric motor in each case to/from the driveshaft, the coupling principle according to the invention can be advantageously employed. Auxiliary units in motor vehicles, for example generator etc. can be rapidly coupled or decoupled via the coupling principle according to the invention. 
     The coupling principle according to the invention is explained in more detail in the following making reference to two concrete exemplary embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following, the invention is described in terms of exemplary embodiments making reference to the drawings without restriction of the invention: 
         FIG. 1  is a schematic cross-sectional representation through a first preferred exemplary embodiment; and 
         FIG. 2  is a cross-sectional representation through a second preferred exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The coupling element shown in cross section in  FIG. 1 , comprises an input part  1  and an output part  2  which are arranged to be rotatably mounted about a common rotational axis D. For transmitting a torque M on the input  1  to the output  2 , both parts  1  and  2  are in a mutual force-loaded non-positive connection, which is realized through a mechanical locking mechanism. A ball-shaped connecting means V, which is provided both on the drive end and output end, protrudes into corresponding recesses matched to the ball contour and in each case form a positive connection. As an alternative to balls, a cylindrical rolling body with cylinder axes orientated radially relates to the rotational axis D are also suitable. Preferentially, two, three or more connecting means V, which are arranged evenly distributed about the rotational axis D, are provided for the non-positive torque transmission from the input part  1  to the output part  2 . 
     For a secure cohesion between the input part  1  and the output part  2 , an axially directed holding force F, axially holding both parts together is generated, through which ultimately also the maximum limit torque M max  that can be transmitted between the input parts and output parts is determined. If, due to the operation, the maximum torque M max  between input part and output part is exceeded, transverse forces orientated in the rotational direction act on the spherically shaped connecting means V through which the spherical connecting means V are ultimately deflected in rotational direction from their counter-contoured recesses on the part of the input part and output part, as a result of which they simultaneously generate axially orientated displacement forces between the input part and output part which ultimately cancels out the positive connection for transmitting the torque between both parts. 
     To realize the axial holding force F H , which in the state of a non-positive and positive engagement of the connecting means V with the input and output, acts between the input part and output part. A chamber  3  (in contrast with a mechanical tensioning element, for example in the form of a spring generating tensile force) is provided, in which axially to the rotational axis D a longitudinally displacement body  4  divides the chamber  3  into two chamber parts  31  and  32 , wherein both chamber regions  31  and  32  are interconnected via a connecting channel  33 . In the exemplary embodiment shown in  FIG. 1 , it is assumed that the input part  1  and output part  2  are designed to be symmetrically rotatable to the rotational axis D. Thus, also the displacement body  4 , which encloses an annular gap provides the connecting channel with the inner wall of the chamber  3 . The chamber  3  is completely filled with a magnetorheological fluid MRF, so that the magnetorheological fluid MRF circulates around the displacement body  4  on all sides. 
     In the exemplary embodiment shown in  FIG. 1 , the displacement body  4  additionally comprises an electromagnet unit  5  having magnetic coil windings  51  arranged directly facing the ring gap  33 . Obviously it is conceivable to integrate the magnet unit also in the chamber housing of the drive unit  1  (which is not shown in more detail or design) in the form of a permanent magnet unit, which for the variation of the magnetic field has to be suitably deflected. 
     The displacement body  4  in the chamber  3  includes a rod-like extension  6 , which extends through the axial face-end chamber wall  7  via a fluid-tight opening  8 . The face-end of the extension  6  terminates at a force transmission unit  9  of plate-shaped design, which bears against the inner wall of a housing wall  10  provided on the drive side in a torsionally decoupled manner, that is substantially loose, so that the drive-end housing  10  with corresponding axial force action is pulled or pushed in the direction of the output unit through the plate-shaped force transmission unit  9 . To generate the holding force F H  acting in axial direction relative to the rotational axis D, the displacement body  4  is preferably held positionally stable within the chamber  3  through the magnetorheological fluid, which has a variable viscosity which is predeterminable in response to the prevailing magnetic field. The fluid can flow through the ring gap only very slowly or with difficulty. The holding force F H , compensated by the displacement body  4 , causes an evacuation of the magnetorheological fluid MRF from the right chamber part  31  into the left chamber part  32 . If the viscosity is so high that no fluid flow into the right chamber part  32  takes place, the coupling is closed. It is evident that the magnetorheological fluid within the chamber  3  exclusively serves to generate the axially acting holding force F H  and is not exposed to any rotation-induced shear forces whatsoever. 
     In the event that a torque exceeding the holding force F H  occurs between input and output, the displacement body  4  is moved against the displacement forces acting on it from its position axially in the direction of the input part  1 . In this case the coupling is no longer able to transmit any torque between input part and output part. 
     In an advantageous manner, in this state, the strength of the magnetic field in the annular space  33  is reduced or completely switched off in order to reduce or avoid any remaining axially acting holding forces, which would lead to an increased wear on the spherical connecting means  5  and also on the recesses matched to the ball contour. 
     For the purpose of returning the released or opened coupling to the state explained at the outset, the effective magnetic field has to be switched off within the annular gap  33 , as a result of which the viscosity is reduced and the displacement body is returned into its axial starting position preferentially arranged in the middle relative to the chamber subject to the expenditure of minor axial adjusting forces. For generating the resetting forces, a return spring  11  with minor spring forces is sufficient, which in the exemplary embodiment shown in  FIG. 1  is arranged outside the chamber  3  and via a further extension  12 , attached at the back of the displacement body  4 , which protrudes. A rear chamber wall  13 , is operationally connected in a fluid tight manner. Alternatively to such an embodiment version, it is likewise possible to avoid unnecessary fluid-tight chamber openings, to provide the resetting spring  11  within the chamber for example in the region of the chamber part  31  between the chamber wall  7  and the displacement body  4  coaxially to the extension  6 . 
       FIG. 1  shows a situation of the coupling element in which the connecting means V, which typically has a spherical shape, protrude into corresponding recesses matched to the outer contour of each connecting means which are inside the face-end chamber wall  7  and the drive-end chamber wall  10 . As long as both walls  7  and  8  enclose the connection means V, the moment of a torque can be transmitted from the input part  1  to the output part  2 . If the torque exceeds beyond a torque limit value, then the drive and chamber wall  10  moves slightly axially against the holding force F H  so that the connecting means V can slit between both walls  7  and  10 . In such case no torque can be transmitted between input and output. 
     The holding force is generated by a fixed position of the displacement body  4  within the chamber  3 . Assuming that in the situation as shown in  FIG. 1 , the displacement body  4  is positioned in the center of the chamber  3 , which is completely filled with a magnetorheological fluid, which has in the case of the torque transmission, a very high viscosity so that little or no fluid can flow through the very small annulus  33  from the second chamber  32  to the first chamber  31 . Even in a case that the torque M exceeds a torque limit, some magnetorheological fluid is forced to flow through the very small annulus so that the drive and chamber wall  10  can move axially against the holding force F H , so that the connecting means V can leave the fixing position. In such case, the coupling element is open. To close the coupling element again, the viscosity of the magnetorheological fluid has to be reduced so that the displacement body  4  can be moved to its mid position. A resetting spring  11  generates the required resetting force acting onto the displacement body  4 . 
       FIG. 2  shows a further exemplary embodiment for a torque-limited coupling element capable of transmitting a torque M on the input part  1  of sleeve-like design to an output part  2  located coaxially radially on the inside. The output part  2  in comparable arrangement to the exemplary embodiment according to  FIG. 1  comprises a chamber  3  filled with a magnetorheological fluid MRF, in which a displacement body  4  that is bidirectionally deflectable axially to the rotational axis D is introduced. The displacement body  4  divides the chamber  3  likewise into a left and right chamber part  31  and  32 , wherein both chamber parts  31  and  32  are fluidically interconnected through a thin annular channel  33 . In the same way as in the exemplary embodiment according to  FIG. 1 , an electromagnet arrangement with magnet coils  51  facing the annular gap  33  is provided in the displacement body  4 . In an advantageous manner, the displacement body  4  has a magnet core, about which the magnet coils  51  are wound. The extension  6  is connected to the displacement body  4  in this case, which protrudes through the face-end chamber wall  7  through a fluid-tight opening  8 . The continuation ends in a conical contact surface  14  having conical slotted link surfaces against which spherically shaped connecting means V bear. The connection means V in a starting position are radially pushed to the outside by the conical contact surface  14 . The spherical connecting means V terminate in spherical recesses  15  alongside the input part  1  of a sleeve-like design and enter into a force-loaded positive connection with the input part  1 . As a function of the torque M prevailing between the input part and output part and of the geometry of the spherical recesses  15  within the sleeve-like drive  1 , a force in a rotational direction about the rotational axis D is generated, which results in a displacement of the spherical connecting elements V in radial direction. If the displacement is greater than the ball diameter or the emersion depth with which the individual balls terminate in the spherical recesses  15  of the sleeve-like drive  1 , the input part  1  starts to slip relative to the output part  2 . In this case, no torque can be transmitted so that the function of a torque-limited safety coupling is satisfied. 
     For returning the displacement body  4  within the chamber  3  into the original starting position, the effective magnetic field is switched off, as a result of which the magnetorheological fluid becomes less viscous, so that ultimately the displacement body  4  through the force of the resetting spring  11  that becomes effective, which is arranged in this case between the face-end inside of the chamber wall  7  and the displacement body  4 , can be returned. 
     The embodiment shown in  FIG. 2  operates very similarly to the above-explained embodiment in  FIG. 1 . In  FIG. 2 , the holding Force F H  acts perpendicular to the axial direction of the coupling element due to the conical contact surface  14  which contacts the spherical connecting means V. 
     The torque-limited coupling element on which the invention is based can be employed in addition to the mentioned operating purpose for realizing a torque wrench and other technical devices such as, for example, a safety coupling in machine tools, hand-held machines, such as for example drilling machines, in the form of a coupling in drive lines of vehicles, agricultural equipment as well as in construction machinery of any type. 
     LIST OF REFERENCE CHARACTERS 
     
         
           1  Input 
           1 ′ Sleeve element 
           2  Output 
           3  Chamber 
           31  First chamber 
           32  Second chamber 
           33  Annulus 
           4  Displacement body 
           5  Electromagnet arrangement 
           51  Magnet coils 
           6  Rod-like extension 
           7  Face-end chamber wall 
           8  Fluid-tight opening 
           9  plate-shaped force transmission unit 
           10  Drive-end chamber wall 
           11  Resetting spring 
           12  Rear rod-like extension 
           13  Rear chamber wall 
           14  Conical contact surface 
           15  Spherical recess 
         V Connecting means 
         D Rotational axis