Abstract:
A device for transforming a rotational movement into a reciprocating to-and-fro movement has a cam element on a driven carrier shaft. The cam element has an eccentric control area, or lobe. A cam follower can be displaced or pivoted by the cam element. The cam element is rotationally mounted in a flexible encompassing element which is movably connected to the cam follower perpendicular to the axis of rotation of the cam element. The flexible encompassing element surrounds the eccentric control surface of the cam element and a non-driven bearing surface for the cam follower.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application is a continuation, under 35 U.S.C. § 120, of copending International Application No. PCT/AT02/00096, filed Mar. 28, 2002, which designated the United States. The application also claims the benefit, under 35 U.S.C. § 119, of Austrian patent application A  1224 / 2001 , filed Aug. 6, 2001; the entire disclosure of the priority application is herewith incorporated by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    Field of the Invention  
           [0003]    The invention relates to a device for converting a rotational movement into a reciprocating movement, in particular cam control, valve timing gear for internal combustion engines of motor vehicles or the like. The device has at least one cam element which is disposed on a driven support shaft and has an eccentric control surface and having a cam follower element, in particular a valve tappet or the like, which can be displaced or pivoted by the cam element. The cam element is rotatably disposed in a flexible enclosing element which is connected to one end of the cam follower element in a manner enabling it to move in a plane which is perpendicular with respect to the axis of rotation of the cam element.  
           [0004]    Since customary valves of internal combustion engines require, for them to be closed, restoring springs which have to apply considerable forces, constrained guides have also already been proposed, these requiring weaker restoring springs or rendering them superfluous. One particular embodiment of a constrained guide of this type can be found, for example, in German published patent application DE 37 00 715 A1. There, the cam element is surrounded in a loosely adjacent manner by a flexible enclosing element that is connected to the valve actuating element. The cam element therefore revolves in the enclosing element.  
           [0005]    I have previously described various developments of the foregoing type of constrained guide. See, for example, my international publications WO 01/12958 A (US 2002/0073947 A1) and WO 01/12959 A (US 2002/0185092 A1). When these enclosing elements are used, friction occurs between the circumferential surface of the cam element and the inner surface of the enclosing element, and it has therefore also been proposed to insert a friction-reducing medium between the circumferential surface of the cam element and the enclosing element via radial ducts in the cam element.  
           [0006]    Since the enclosing element is subjected to relatively high tensile forces by the reciprocating cam follower element particularly when the push-off acceleration is braked, that part of the enclosing element which lies opposite the connecting region is pressed fixedly against the circumference of the cam element. Conversely, that part of the enclosing element which encloses the connecting region is exposed, shortly before it returns into the starting position, to correspondingly high compressive forces, since the restoring acceleration is braked, and is pressed onto the circumference of the cam element. In both cases, outlet openings situated in these regions are tightly closed by the enclosing element, and a very high pressure would be required to feed in the lubricating medium. For example, there is a pressure of 2 to 5 bar in conventional cylinder heads, and at least 10 times the pressure would have to be able to be applied in order to push the enclosing element away from the circumference and to allow the medium to emerge. (The values of this example relate to lubrications using oil). Only partial lubricant films are produced, and a mixed friction occurs, the coefficient of friction of which is not smaller than 0.1.  
         SUMMARY OF THE INVENTION  
         [0007]    It is accordingly an object of the invention to provide a novel device for converting a rotational movement to a reciprocating movement, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which, specifically, substantially improves the frictional ratios in a device of the above-mentioned type.  
           [0008]    With the foregoing and other objects in view there is provided, in accordance with the invention, a device for converting a rotational movement into a reciprocating movement, such as a cam control device, or a drive for a valve tappet of valve in an internal combustion engine of a motor vehicle. The device comprises:  
           [0009]    at least one cam element mounted on a driven support shaft for rotation about an axis of rotation, the cam element having an eccentric control surface driven by the support shaft;  
           [0010]    a cam follower element mounted for displacement or pivoting by the cam element and for bearing on a non-driven bearing surface;  
           [0011]    a flexible enclosing element connected to the cam follower element, the flexible enclosing element enclosing the cam element while allowing the cam element to rotate therein, the flexible enclosing element moving in a plane perpendicular to the axis of rotation of the cam element and surrounding the eccentric control surface of the cam element and the non-driven bearing surface for the cam follower element.  
           [0012]    In other words, the objects are achieved by the fact that the flexible enclosing element surrounds the eccentric control surface of the cam element and a nondriven bearing surface for the cam follower element. A nondriven bearing surface is understood above all to mean a cylindrical bearing surface fixed on the device, for example a bearing surface on a bearing element of the support shaft. This enables, depending on the shape of the cam, the contact surface, which produces a substantial part of the friction, between the cam element and the enclosing element to be reduced in length by at least one third, even by up to two thirds in the case of conventional cam shapes. Since the cam element is additionally also narrower than the enclosing element—at least on one side, preferably on both sides, the cam element is adjoined by an, in particular a cylindrical end region of a bearing element—the contact surface producing friction is also narrower than in conventional designs.  
           [0013]    However, the nondriven bearing surface may also be formed on a ring or the like mounted rotatably on the bearing element, for example, so that minimal revolving of the bearing surface is possible, this arising owing to the slightly alternating and changing geometrical ratios between the connecting point of the enclosing element with the cam element and the migrating control surface.  
           [0014]    As previously described in my earlier international PCT publication WO 98/26 161 A, it is possible to divide the control cam region into two components, namely into the cam element and a bearing element. This takes place there, however, owing to reasons concerned with easier manufacturing and setting of the closing position stop, since machining of the base circle of a cam element is not required.  
           [0015]    Further friction-reducing measures may comprise the placement of roller bearings between each bearing element and the support shaft and/or the cam element, and/or the mounting of a rotatably mounted roller in the eccentric control surface of the cam element and/or the formation of feed ducts for feeding a friction-reducing medium, in particular lubricating oil, to the contact surfaces producing friction.  
           [0016]    In the above-mentioned cases, in which high tensile or compressive forces occur, the forces are transmitted by the design according to the invention directly to the bearing elements, so that the sliding or rolling bearings between the bearing elements and the support shaft are relieved of load. In order to relieve the mounting of the cam follower element of load, provision is made, in a further preferred embodiment, for that end of the cam follower element which is connected to the enclosing element to be guided in a guide which is fixed on the device.  
           [0017]    The reduction in size of the friction-producing contact surfaces furthermore reduces the quantity of heat which arises, the dissipation of which is facilitated if the upright base circle region is part of the camshaft bearing and can be connected directly to the housing, in particular the cylinder head, and reduces the requirement for lubricant. The preferably cylindrical bearing surface may furthermore also have a central flat point from which the enclosing element is slightly spaced apart, so that a heat-induced compensation of play for the cam follower element is also provided in a simple manner. The cam element is restricted to the eccentric region, i.e. the customary base circle region is only formed in part, if at all.  
           [0018]    The constrained guidance of the cam follower element renders the customary, solid restoring springs, which have to have, for example, conventional valve timing gears, superfluous. Nevertheless, a small restoring spring may be advantageous. In one preferred embodiment, in which the cam follower element is articulated on the enclosing element by means of a bearing pin, the restoring force can act on the bearing pin by the bearing pin being pressed against the bearing surface, which is fixed on the device, by an elastic element. In order to produce the restoring force, use may be made, for example, of a leg spring or the like which is supported, on the one hand, on the bearing pin, and on the other hand, on the bearing element or the like. One preferred embodiment makes provision for the bearing pin to have at least one exposed end region, and for an elastically flexible strip of steel, rubber or the like to be guided around the exposed end region and the bearing element.  
           [0019]    The device according to the invention therefore contains at least two constrained strips or loops, namely the extension-resistant enclosing element for the constrained guidance of the cam follower element and the elastic strip for resetting the cam follower element.  
           [0020]    In a further preferred embodiment, the enclosing element is also formed by an elastic strip which is preferably provided with an elongation limit and interacts with a radially retractable and extendable cam element on the support shaft in order to change the size of the cam stroke. In the case of valve timing gears, devices of this type are also referred to as variable valve operating mechanisms, it being possible for the radial displaceability of the cam element to be obtained by rising control surfaces which are provided between the cam element and the driven support shaft, if the support shaft is, for example, axially displaceable, or are provided between the cam element and a control shaft which are arranged rotatably in the hollow-cylindrical support shaft. In a further embodiment, the cam element may also be guided in a constrained manner, for example by a crank mechanism or the like.  
           [0021]    Other features which are considered as characteristic for the invention are set forth in the appended claims.  
           [0022]    Although the invention is illustrated and described herein as embodied in a device for converting a rotational movement into a reciprocating movement, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.  
           [0023]    The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    [0024]FIG. 1 shows an exploded illustration of the individual components of a first embodiment of a device according to the invention,  
         [0025]    [0025]FIG. 2 shows a side view,  
         [0026]    [0026]FIG. 3 shows a section according to the line III-III of FIG. 2, and  
         [0027]    [0027]FIG. 4 shows a longitudinal section through the first embodiment.  
         [0028]    [0028]FIG. 5 shows a longitudinal section through a second embodiment of a device according to the invention,  
         [0029]    [0029]FIG. 6 shows a section according to the line VI-VI of FIG. 5,  
         [0030]    [0030]FIG. 7 shows, on an enlarged scale, the control cam region of FIG. 5, and  
         [0031]    [0031]FIG. 8 shows an oblique view of the second embodiment,  
         [0032]    [0032]FIG. 9 shows a side view of a third embodiment of the device according to the invention,  
         [0033]    [0033]FIG. 10 shows a section according to the line X-X of FIG. 9, and  
         [0034]    [0034]FIG. 11 shows a longitudinal section through the third embodiment according to FIG. 9.  
         [0035]    [0035]FIG. 12 shows an exploded illustration of the individual components of a fourth embodiment of the device according to the invention,  
         [0036]    [0036]FIG. 13 shows a longitudinal section through the fourth embodiment,  
         [0037]    [0037]FIG. 14 and FIG. 16 each show, on an enlarged scale, the connecting region between the enclosing element and the cam following element, and  
         [0038]    [0038]FIG. 15 shows a section according to the line XV-XV of FIG. 13.  
         [0039]    [0039]FIG. 17 shows an exploded illustration of the individual components of a fifth embodiment of the device according to the invention,  
         [0040]    [0040]FIG. 18 shows the longitudinal section through the fifth embodiment, the cam follower element bearing against the bearing surface,  
         [0041]    [0041]FIG. 19 shows, on an enlarged scale, the connecting region between the enclosing element and the follower element,  
         [0042]    [0042]FIG. 20 shows a section according to the line XX-XX of FIG. 18,  
         [0043]    [0043]FIG. 21 shows a longitudinal section similar to FIG. 18, in which the cam element is rotated through 180°,  
         [0044]    [0044]FIG. 22 shows, on an enlarged scale, the bearing region of the cam element from FIG. 21,  
         [0045]    [0045]FIG. 23 shows a section according to the line XXIII-XXIII of FIG. 22, and  
         [0046]    [0046]FIG. 24 shows a schematic oblique view of the device.  
         [0047]    [0047]FIG. 25 shows an exploded illustration of the individual components of a sixth embodiment of the device according to the invention,  
         [0048]    [0048]FIG. 26 shows a longitudinal section through the sixth embodiment, the cam follower element bearing against the bearing surface,  
         [0049]    [0049]FIG. 27 shows, on an enlarged scale, the connecting region between the enclosing element and the cam follower element,  
         [0050]    [0050]FIG. 28 shows a section according to the line XXVIII-XXVIII of FIG. 26,  
         [0051]    [0051]FIG. 29 shows a longitudinal section similar to FIG. 26, in which the cam follower element is rotated through 180°,  
         [0052]    [0052]FIG. 30 shows a section according to the line XXX-XXX of FIG. 29, and  
         [0053]    [0053]FIG. 31 shows a side view of the sixth embodiment.  
         [0054]    [0054]FIGS. 32 and 33 show longitudinal sections through a seventh embodiment of the device according to the invention, the cam follower element bearing in each case against the bearing surface,  
         [0055]    [0055]FIG. 34 shows oblique views of the support shaft and of the cam element in three different positions, and  
         [0056]    [0056]FIGS. 35 and 36 show sections according to the line XXXV-XXXV of FIG. 32 and the line XXXVI-XXXVI of FIG. 33.  
         [0057]    [0057]FIGS. 37 and 38 show sections through an eighth embodiment of the device according to the invention, the cam follower element bearing in each case against the bearing surface.  
         [0058]    [0058]FIGS. 39 and 40 show schematic end views of a cam element guided in a constrained manner by means of a crank mechanism, in two different positions. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0059]    Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is shown a device according to the invention for converting a rotational movement into a reciprocating, rectilinear, to-and-fro, or pivoting movement. The exemplary device comprises a driven support shaft  1  on which a cam element  5  having an eccentric control surface  4  is fixed. The eccentric control surface  4 , also referred to as cam lobe and cam valley surface, enables a cam follower element  9 , which is held in a bearing manner against it, to be moved in a reciprocating manner in accordance with its guide or mounting. In all of the exemplary embodiments, the preferred use of the device is shown, namely as a valve operating mechanism of internal combustion engines. However, devices of this type may also be used, for example, in cam controls of machine tools, in particular gears or the like, in which case the cam follower element  9 , which forms a valve tappet in the exemplary embodiments shown, is designed in accordance with the use.  
         [0060]    A central, hub-like region  16  of the cam element  5  is rotatably mounted on one or both sides on or in a bearing element  10 , on which an annular or sleeve-shaped end region  11  having an in particular cylindrical outer surface is formed. A flexible surround element, or enclosing element  6 , for example a fabric strip or the like, surrounds the eccentric control surface  4  of the cam element  5  and the outer surface of the end region  11  of each bearing element  10 , and has a holder  12  on which the cam follower element  9  is arranged in an articulated manner. The axis of articulation runs parallel to the axis of rotation  8  of the support shaft  1 . The rotation of the cam element  5  results in an oscillating movement of the enclosing element but the latter, owing to its connection to the cam follower element  9 , is not able to rotate but rather is lifted up continuously all around from the outer surface of the end region  11 . In the process, the cam follower element  9  is transferred from a bearing surface  3 , in which the cam follower element  9  is at the shortest distance from the axis of rotation  8 , and which forms part of the outer surface of the end region  11 , into a position at maximum distance from the axis of rotation  8 , if the maximum amount of the eccentric control surface  4  of the cam element  5  is effective, and, on further rotation, is pulled back into the basic position again. In the case of the valve timing gear, the closed position is therefore the basic position and the position at maximum distance is the open position of the valve disk  13 .  
         [0061]    FIGS.  1  to  4  show a first embodiment in which the bearing elements  10  are only shown schematically in the form of a length of casing pipe with rings on the end sides which are fixed, for example, in securing means  6  on the housing or—as FIG. 8 shows—are provided with corresponding fastening parts. The cam element  5  has a cam region which bears the eccentric control surface  4  and the axial extent of which around the two annular end regions  11  of the bearing elements  10  is shorter than its central region  16 , which is fixed on the support shaft. The enclosing element  6  is approximately of a width which corresponds to the axial extent of the central cam region  16 , so that the enclosing element  6  surrounds part of the cylindrical circumferential surface of the two end regions  11  and the eccentric control surface  4  of the cam element  5 . Since only the eccentric control surface  4  has to slide along the inner surface of the enclosing element  6 , the friction-producing contact surface is smaller than half of the inner surface of the enclosing element  6 . As already mentioned, the latter is connected in an articulated manner via its holder  12  to the cam follower element  9 , so that friction does not occur between the enclosing element  6  and the cylindrical outer surface, which serves as a bearing surface  3 , of the two end regions  11  which are fixed on the housing as parts of the bearing elements  10 . The division into contact surfaces with friction and those without friction can readily be seen in particular in FIG. 3, in which the cam element  5  having the eccentric control surface  4  can be seen cutaway, and, in contrast, the axially offset end region can be seen in plan view.  
         [0062]    In the embodiment according to FIGS.  5  to  8 , the support shaft is formed by a bundle of supporting rods  2 , thereby providing a simple, form-fitting connection between the support shaft  1  and the cam element  5 . The support shaft  1  is driven via a drive wheel (not shown) which, like the cam element  5 , has a corresponding pattern of holes in the center. The cam element  5  has a lateral annular groove in which the end region  11  of a bearing element  10  engages. A rolling bearing  15 , for example a needle bearing or the like, is inserted between the core region  16 , as FIG. 7 shows on an enlarged scale. Owing to the intermeshing of the end region  11  and of the cam element  10 , the enclosing element  6  bears over its entire width against the bearing surface  3  of the end region  11  and surrounds the eccentric control region  4  of the cam element  5 . As can be seen from the enlarged illustration of FIG. 7, the bearing surface  3  can have a central flattened section  17 , so that compensation of play, for example in the case of heat-induced changes in length of the cam follower element  9 , is possible. The oblique view of FIG. 8 shows the embodiment from the side which faces away from the bearing element  10 .  
         [0063]    In the embodiment according to FIGS.  9  to  11 , which largely corresponds to the embodiment according to FIGS.  1  to  4 , two recesses are formed in the cam element  5  and the remaining central web accommodates a pin  14  on which one roller  7  per recess is mounted rotatably by means of a rolling bearing, the arrangement and the cam shape being selected in such a manner that the circumferential surface of the two rollers  7  drop into the central region of the eccentric control surface  4 . As is apparent in particular from FIG. 10, on both sides of the rollers  7  there remains only a short transition section  18  in which the circumferential surface of the cam element  5  comes into contact with the enclosing element  6 . Since, during rotation of the cam element  5 , the rollers  7  roll in the enclosing element  5 , the friction-producing contact surfaces are once again substantially reduced. Of course, the installation of a rolling bearing  15  between the support shaft  1  and the core region  16  of the cam element  5  is also possible in this embodiment.  
         [0064]    FIGS.  12  to  16  show an embodiment in which two cam elements  5  having a common central region  16  are formed, each cam element  5  having a radial recess  20  and forming a complete ring  22  in this region. The central region  16  of the cam element  5  is connected in a rotationally fixed manner to the support shaft  1 , and the two rings  22 , which each accommodate a rolling bearing  15 , are mounted rotatably on the two tubular bearing elements  10 . As is apparent from FIG. 14 or  16 , an annular gap  23  remains between the support shaft  1  and the bearing elements  10 , so that production inaccuracies in the support shaft  1  do not require any further processing. The recess  20  leaves a clearance for a guide sleeve  81  which is raised between the enclosing element  6  and the cam follower element  9  as far as the holder  12 , is restricted at two mutually diametrically opposite webs  83  of the cylinder head  80  and the width of which corresponds to the recess  20 . The two parts of the rotating cam element  5  rotate past on both sides of the raised guide sleeve  81  for the cam follower element  9 . In this embodiment, the enclosing element  6  is provided with a central cutout which corresponds with the recess  20  or, as FIG. 12 shows, is formed from two loops which are held together by the holder  12  or by the bearing pin  62  of the tappet head  61 . The cam element  5  may have an edge shoulder in order to avoid the enclosing element  6  slipping. The bearing elements  10  are fixed on protruding webs of the cylinder head  80  by means of holding-down devices  84 .  
         [0065]    FIGS.  17  to  24  illustrate an embodiment in which a common cam element  5  is assigned to two cam follower elements  9 . The cam element  5  which is shown in oblique view in FIG. 17 therefore has a ring  22  at each end and a central region  16  with a five-sided opening. The cam element  5  is arranged in a rotationally fixed manner on a five-sided support shaft  1  which is mounted via the bearing elements  10  and via rings which are arranged fixedly or loosely on the end regions  11  of the bearing elements and on which the two rings  22  of the cam element  5  are mounted rotatably, in each case by means of a rolling bearing  15 .  
         [0066]    The enclosing element  6  does not have any cutouts and has, in the holder  12 , a plug-in opening which is formed in a sleeve  19  and into which a bearing pin  62  is inserted, the bearing pin protruding on both sides and being connected at each end to a tappet head  61 . In this embodiment too, the guide  81  for each cam follower element  9  is raised to reach the bearing sleeve  10 .  
         [0067]    The clearance between the two guides  81  is of such a size that the cam element  5  can spin around, FIGS. 21 and 23 showing that position in which the valve disk  13  is open the greatest distance away from the valve seat.  
         [0068]    This embodiment also shows a possibility for feeding a friction-reducing medium, for example lubricating oil, to the individual bearing surfaces. For this purpose, the support shaft  1  has a central feed duct and radial outlet openings  25  which merge into holes  26  of the cam element  5 . The holes  26  open into the contact surface with the enclosing element  6  on the circumference of the cam element  5  and in the region of the rolling bearings  15  (FIG. 22). A continuing hole  27  extends through the holder  12  to a hole  28  in the sleeve  19 , in which the bearing pin  32  having a circumferential groove  29  is arranged. The bearing pin  62  is provided with an axial duct  30  which is connected to the circumferential groove  29  by a hole (not designated). The medium emerging from the duct  30  is distributed over the sliding surfaces of the guide sleeves  81  for the tappet head  62 .  
         [0069]    FIGS.  25  to  31  show a sixth exemplary embodiment in which two cam elements  5  are again provided on the central region  16 , said cam elements being surrounded by a common enclosing element  6 . The central region  16  is provided with a noncircular hole  21  and is arranged in a rotationally fixed manner on the support shaft  1 , the cross-section shape of which is composed from three more sharply curved arcs and three less sharply curved arcs which alternate with one another. Two cylindrical extensions which have outer bearing surfaces are formed on the central region  16  and are mounted inside two sleeve-shaped bearing elements  10 . The bearing sleeves  10  are each fixed in two closed bearing rings  85  of the cylinder block  80 , on which, in turn, raised guide sleeves  81  are provided.  
         [0070]    An extended bearing pin  62  is inserted into the holder  12  of the enclosing element  6  and a tappet head  61  of a cam follower element  9  is mounted rotatably on both sides of it, in a manner similar to the embodiment according to FIG. 17. The ends  63  of the bearing pin  62  protrude in each case through a slot  82  in the bearing rings  85  and are pressed in the protruding part against the bearing sleeves  10  by a rubber band, a spring steel clip or another elastic element  31 . The lateral slipping of the element  31  is prevented by a collar  64  (FIG. 27). As the comparison of FIGS. 26 and 29 and also  28  and  30  shows, the elastic elements  31  are expanded by the cam element  5  during the downward movement of the cam follower elements  9 , i.e. during the opening of the valves, and produce a force which assists the return and which may be advantageous in many applications. Substantially stronger restoring springs which engage directly on the cam follower elements  9  are rendered superfluous by the constrained guidance of the enclosing element  6 . Instead of the strip which is shown, other spring devices, for example leg springs or the like, may also be provided.  
         [0071]    It is apparent in particular from FIGS. 25 and 27 that the tappet head  61  has an undercut insertion groove for the tappet of the cam follower element  9 . Said element can be inserted from the side and is thereby mounted rotatably in the tappet head  61 .  
         [0072]    FIGS.  32  to  40  show embodiments which permit the cam stroke to be adjusted, and can therefore be used especially as a variable valve operating mechanism.  
         [0073]    In the embodiment according to FIGS.  32  to  36 , the support shaft  1  is arranged in a longitudinally displaceable manner in the bearing elements  10  and has, in each region in which a cam follower element  9  is to be actuated, a cutout  41  which is provided with an oblique surface  42 , which rises in the longitudinal direction, and with lateral, parallel flattened sections. A cam element  5  which has an approximately U-shaped cutout on the side lying opposite the eccentric control surface  4  is guided on the parallel flattened sections in a manner enabling it to be pushed out and pushed in vertically. FIGS. 34 and 36 clearly show that the cam element  5 , which does not protrude beyond the circumference of the bearing element  10  in a lowest position, is raised, when the support shaft  1  is displaced to the left, by the oblique surface  42 , which rises in a wedge-shaped manner, and are transferred into the position which is shown at the bottom in FIG. 34 and in FIG. 35 and in which it is extended to the maximum.  
         [0074]    The rest of the structural design corresponds essentially to that of FIG. 25, and so these details do not have to be repeated here. Only the enclosing element  6  is of elastically expandable design, since it has to be lengthened and shortened, as is apparent in particular from the comparison of FIGS. 35 and 36.  
         [0075]    An enclosing element  6  which can be lengthened reversibly has already been described by me in my above-mentioned earlier PCT publication WO 01/12959 A, and its substantially corresponding U.S. Patent application publication US 2002/0185092 A1, which are herewith incorporated by reference. The enclosing element  6  is, for example, a seamless loop which is produced from threads or fibers in a textile circular working technique. The enclosing element preferably has threads made from an extension-resistant material which extend in the circumferential direction and form an elongation limit. A fabric-loop may be provided with a friction-reducing coating at least in each case in the region of the inwardly protruding bumps which are formed by the intersecting threads.  
         [0076]    The elastic enclosing element  6  can render the elastic elements  31  shown in FIGS. 32 and 33 superfluous, since it likewise exerts a restoring force on the bearing pin  62 . Owing to the elasticity of the enclosing element  6 , it may be advantageous if it contains stiffenings in the transverse direction, i.e. in the axial direction of the support shaft, for example in the form of reinforcing ribs  43  which have pins inserted or bonded into them. The transverse stiffenings prevent unsupported parts of the enclosing element  6  from being pulled in in the region of the cam element  5 .  
         [0077]    In the embodiment according to FIGS. 37 and 38, a rotatable control shaft  44  is arranged in the support shaft  1  for the radial movement of the cam element  5  and has an eccentric, spirally rising control surface  49  formed on it. From the comparison of the two FIGS. 37 and 38, the adjustment sequence of the cam element  5  can be seen. The cam element  5  is held in the hook-like core region of the control shaft  34  in the pushed-out position according to FIG. 37. If the control shaft  34  is rotated anticlockwise in the support shaft  31 , then the cam element  5 , which bears against the spiral-shaped control surface  49 , migrates inward until the position without any lift according to FIG. 38 is reached. In this position, the cam element  5  is situated within the cylindrical outer surface of the bearing element  10  or the annular region  11  of the bearing element  10 , so that the gathered enclosing element  6  bears all around the annular region  11  and all friction is avoided since the cam element  5  revolves without any contact.  
         [0078]    [0078]FIGS. 39 and 40 show an embodiment in which the cam element  5  is extended and retracted while being guided in a constrained manner. A control shaft  44  in the interior of the support shaft  1  has a slot  45  in which a link  48  is mounted rotatably on a bearing pin  46 . The second end of the link  48  is arranged on a bearing pin  47  which is mounted in the interior of the cam element  5 , the cam element  5  being of approximately U-shaped design and being arranged in a guide of the support shaft  1 , or in a guide sleeve arranged on the support shaft  1 , in a manner such that it can be pushed out and in. The constrained guide therefore constitutes a crank mechanism which can be rotated over an angle of approximately 120°. FIG. 39 shows a partial stroke and FIG. 40 the full stroke of the cam element  5 .  
         [0079]    In the embodiments according to FIGS.  32  to  40 , the enclosing element  6  forms on both sides a rectilinear bridging of the transition region between the nonrotatable bearing surface  3  and the eccentric control surface  4  which changes as the stroke changes. Furthermore, the embodiments according to FIGS.  32  to  40  can also be used for adjusting the stroke of the cam element  5  if the bearing surface  3  is provided on the driven support shaft  1  or on a part rotating at the same time as the support shaft.