Abstract:
The invention relates to a device for adjusting the phase angle of a camshaft of an internal combustion engine with a drive gear for driving a camshaft accommodated in a coaxial arrangement relative to the camshaft and with an electric motor for rotating the camshaft relative to the drive gear, the electric motor having two concentrically arranged rotors, of which the one is connected to the camshaft and the other to the drive gear. Simple and safe adjusting is achieved by having a first coil arrangement non rotatably linked to one of the rotors and electromagnetically interacting with a stationary coil arrangement in order to induce or to transmit the energy needed for the operation of the electric motor.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a device for adjusting the phase angle of a camshaft of an internal combustion engine with a drive gear for driving a camshaft accommodated in a coaxial arrangement relative to the camshaft and with an electric motor for rotating the camshaft relative to the drive gear, wherein the electric motor has two concentrically arranged rotors, of which the one is connected to the camshaft and the other to the drive gear. 
     To obtain optimum values for fuel consumption and exhaust emissions in different areas of the internal combustion engine&#39;s operating characteristics, the valve timing must be varied in dependence of different operating parameters. An elegant manner of varying the valve timing is realized by rotating the camshaft relative to its driving gear. The camshaft of an internal combustion engine is usually driven by a sprocket wheel, which is connected to the crankshaft via a drive chain, or by a drive gear configured as a pulley, which is connected to the crankshaft via a toothed belt. 
     DESCRIPTION OF THE PRIOR ART 
     In GB 2 221 513 A a camshaft adjusting mechanism is described wherein an electric motor operates a set of link arms turning the camshaft relative to its driving gear. To this purpose an actuating element carrying the pivoted arms is shifted in axial direction. This solution however involves considerable expense and play on account of the large number of bearings. 
     In DE 41 10 088 C1 and DE 39 29 619 A1 adjusting mechanisms are described wherein an adjusting element is provided between a member connected to the camshaft and a member connected to the drive gear, which element has two helical threads meshing with corresponding threads of the camshaft or the drive gear. By axially displacing this adjusting element, the camshaft can be rotated relative to its drive gear. Axial displacement of the adjusting element may be obtained by actuating a hydraulic plunger which is operated in dependence of the desired adjustment. The disadvantage of this solution is that the forces required can only be attained with a large hydraulic plunger necessitating considerable constructional expense. Moreover, a comparatively large quantity of oil is required for operating the plunger, which will necessitate a suitably sized pump and thus add to the engine load. As a further drawback of this known type of mechanism, adjustment of the camshaft is possible only between two extreme positions. 
     An electric adjusting device also is presented in DE 41 01 676 A1, wherein an electric motor is provided for displacing the adjusting element by means of a threaded spindle. As the adjusting element rotates essentially at camshaft speed, an axial thrust bearing must be provided between the electric motor and the adjusting element, which takes up the relative movement between the non rotating and the rotating member. In the above solution, the thrust bearing is more or less permanently subject to load throughout the entire operating period, since the torsional moments acting between drive gear and camshaft will produce a force acting on the adjusting element in axial direction. For this reason the thrust bearing is a critical component which will limit the useful life of the engine. A similar solution is disclosed in DE 33 20 835 A1, wherein the same disadvantages are encountered. 
     In DE 36 07 256 A a mechanism is described, wherein a stepping motor is provided for adjusting the camshaft, which stepping motor being connected to both camshaft and drive gear. As the stepping motor must take up the entire driving torque for the camshaft, such a solution cannot be achieved within reasonable limits of expense. 
     This disadvantage is avoided in the adjusting device disclosed in DE 41 10 195 for two structural components connected in their rotating drive, in which an electric motor with a stationary stator is driving a planet carrier supporting a couple of concentric planet gears. Such a stationary electric motor however always has to run during operation, adjustment of the two structural components relative to one another being carried out by faster or slower run. To adjust such a device is complicated. 
     EP 0 596 860 A discloses a device for adjusting the valve opening times in which the camshaft has a hollow configuration and comprises an inner shaft. The cams are bipartite, wherein each single cam section can be turned relative to the other by a determined angle. The rotation of the two cam sections is executed by a revolving electric motor, which is supplied via slip rings. Similar solutions are disclosed in U.S. Pat. Nos. 5,417,186 and 4,770,060. 
     A former suggestion of the applicant, published in EP-A 0 903 471 presents an adjusting mechanism for the phase angle of a camshaft with a planetary gear set, in which the adjustment is executed by an electric motor that is supplied with current by means of slip contacts. 
     Adjusting the phase angle of a camshaft by means of an electric motor proved an advantageous solution in practical operation. To supply the electric motors by means of slip contacts however is the weak point of such devices. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to avoid these drawbacks and to develop a device as described above in such a manner that slip rings, slip contacts and the like can be omitted. 
     It also is an object of the invention to have a first coil arrangement non rotatably linked to one of the rotors, said first coil arrangement electromagnetically interacting with a stationary coil arrangement in order to induce or to transmit the energy needed for the operation of the electric motor. 
     The main point of the invention is that the electric motor that occasions the adjustment of the camshaft is energized without any contact by a coil arrangement. The energy may hereby either be transmitted like in a transformer via an air core or be induced like in a generator. Since the electric motor is a rotating member, the following description will not distinguish stator and rotor of the electric motor, as this is generally the case, but will rather speak of two rotors. One of the rotors is basically connected to the camshaft, whereas the other rotor is connected to the drive gear which may be configured as a sprocket wheel or as a pulley wheel for receiving a toothed belt. Since the driving torque needed to adjust the phase angle of the camshaft is quite big, and since, on the other side, the adjusting angle is quite small, it commonly is necessary to provide a set of gears that converts a relative movement of the two rotors of some revolutions into a rotation of the camshaft relative to the drive gear of approximately 15° to 20°. 
     In a particularly preferred embodiment of the present invention the set of gears is composed of a rigid, circular hollow gear and of a flexible externally toothed gear, which is accommodated on a roller bearing with an elliptical inner ring that meshes with the hollow gear. Such a gear is commonly called a Harmonic-Drive. As already described above, a quite big step-up ratio is needed between the electric motor and the member it is driving, which may be achieved by such a set of gears. If for example the teeth of the hollow gear amount to 100 and the teeth of the elliptical gear amount to 98, the step-up ratio obtained is of about 1:50. Since this is achieved by a single-stage gear without using planet gear or the like, the device according to the invention may thus be of an extremely compact design. 
     To have a rotor directly connected to the elliptical inner ring of the roller bearing constitutes a particularly favorable solution in this connection. It is also of advantage when the internally toothed gear has entered a rigid connection with a rotor. Thanks to these measures, a simple and compact layout may be achieved. 
     The disadvantage of the Harmonic Drives of the art is that they only can be loaded to a limited extent because of their relatively fine teeth. Such gears are particularly sensitive to impulsive loads. In order to avoid this disadvantage, the gear may be made of the following component parts: a first plane of action arranged on the inner periphery of a first engaging part, a second plane of action arranged on the outer periphery of a flexible engaging part and engaging the first plane of action and a driving member arranged coaxially to the first engaging part and to the flexible engaging part, a roller bearing provided with a non circular inner ring being accommodated on said driving member and having a flexible outer ring connected to the flexible engaging part and pushing it by preferably two points against the first engaging part, wherein the first plane of action of the first engaging part frictionally engages the second plane of action of the flexible engaging part. 
     In such a gear, the evident allocation of the different component parts with regard to the phases and the exact transmission ratio of a toothed gearing is no longer given, but resistance to overload may thus be achieved, which is not possible with a toothed gearing. Furthermore, the gear according to the invention is unaffected by dirt and requires little lubrication. 
     A further advantage of the invention is that the eccentricity of the wave generator may be considerably smaller than in a toothed gearing of the art. In those traditional toothed gearings, it is necessary to make the eccentricity so big that the teeth of the first and of the second plane of action do not come into conflict outside the engaging areas. In the solution according to the invention, the eccentricity is only defined by the small path needed to establish a frictional engagement. That is why the deformation of the flexible engaging part during operation is considerably smaller, which reduces losses and increases service life. 
     In principle it is possible to have the first and the second plane of action meshing on one, two, three or more points. It proved particularly advantageous however to provide two opposite engaging points. In such a solution, the inner ring of the roller bearing is essentially elliptical in cross section. 
     Secure transmission of force may particularly be achieved by giving the first plane of action a conical shape with a small aperture angle. Eventual wear can thus be compensated, too. In this connection, it is particularly advantageous to have the aperture angle amounting to between 1° and 10°, preferably between 2° and 6°. In this case, the wave generator preferably is embodied in such a manner that the outer ring of the roller bearing has got a conical shape with a small aperture angle. The aperture angle of the outer ring should thereby essentially match the aperture angle of the first plane of action. 
     It is particularly advantageous to provide a pressure means that presses the first plane of action in axial direction against the second plane of action. An independent adjustment of the pressure force and with it of the transmissible torque may thus be achieved. The pressure means preferably is provided with a spring that prestresses the first engaging part and the flexible engaging part against each other in axial direction. 
     In a particularly preferred variant a rotor of the electric motor is essentially configured as a tube-shaped sleeve that supports on its inner circumference a winding electromagnetically interacting with a winding or with a permanent magnet arranged on the other rotor, the sleeve having on its external circumference a coil arrangement cooperating with a stationary coil arrangement. A particularly simple bearing of the movable component parts is thus made possible. The inner rotor may for example support permanent magnets interacting with windings arranged on the inner circumference of the outer rotor. By feeding the outer rotor with alternate current of an appropriate frequency, the electric motor may be operated like a synchronous machine. In theory, it is also possible to configure the inner rotor as a squirrel-cage rotor and to operate the electric motor as an asynchronous motor. These variants have the advantage that the inner rotor needs not to be fed with external power. Higher efficiency may be achieved however by energizing the inner rotor electromagnetically, too. 
     The coil arrangements comprise at least one winding each that is essentially accommodated in circumferential direction. Thus, the electric current transmitted by the coil arrangement becomes independent of the camshaft&#39;s speed. This is important, since the transmission executed by the coil arrangement not only supplies the power for the operation of the electric motor, but also includes the control data. 
     In this connection it is particularly advantageous to have the sleeve rigidly connected to the drive gear. The rigid connection is also to be used in case the sleeve is configured integral with the drive gear. 
     Thanks to the preferred configuration of the inner rotor as a tube-shaped sleeve, the construction of the device and particularly its erection may be considerably facilitated, since the whole adjusting device may essentially be fastened onto the camshaft by means of one single screw. 
     In another variant of the present invention one rotor is connected to the other rotor via a ribbon cable in order to transmit electric power for the energization of a rotor. In this solution, advantage has been taken of the fact that the two rotors move in opposite direction during only a few revolutions. In this solution, one rotor is fed with power via a coil arrangement. The other rotor is communicating with the first rotor via the ribbon cable. In this connection, it is particularly advantageous to have one rotor fitted with an electronic control unit for evaluating the control signals transmitted through the ribbon cable. In this way, the current needed to supply a rotor may be changed by the control unit, in its frequency for example. It is thus possible to imitate for example the functioning of a stepping motor. 
     In an alternative variant of the invention the two rotors are each provided with an independent coil arrangement, each of which interacting with a corresponding stationary coil arrangement. Thanks to the independent energization, an independent energization of windings in both rotors may be achieved easily, too. 
     It also is advantageous to have the first coil arrangement and the stationary coil arrangement configured as a rotary current generator. Electric current may thus be produced in a particularly easy and reliable manner. The current production in this generator depends on the energizing current so that the motor may be driven in a simple way. If need be, a second field winding may be provided, which brings the rotor to slow down in order to also effect in a fast and reliable way a backward adjustment. As an alternative, the current produced by the generator may be phase shifted in a corresponding circuit element before it is fed to the electric motor, which also makes a return of motion possible. 
     A compact design and a particularly fast response characteristic may also be achieved even at low speed when the stationary coil arrangement is provided with a number of poles which is superior to the one of the rotors of the electric motor. The slip of the squirrel-cage rotor may thus efficiently be compensated. 
     A preferred variant of the invention has the drive gear communicating with the crankshaft via a frictionally engaged drive. A frictionally engaged drive, for example a V-ribbed belt, is of advantage for being considerably cost-saving. However, the clear phase relation of the drive gear to the crankshaft of the internal combustion engine gets lost. This may be compensated by the adjusting device according to the invention, which offers unlimited adjusting possibilities. 
     The present invention will be described more explicitly in the following with the help of the embodiments illustrated in the figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a longitudinal section through a first variant of the invention; 
     FIG. 1A is a slightly modified variant of the embodiment in FIG. 1; 
     FIG. 2 is an exploded view of a common Harmonic Drive; 
     FIGS. 3A, B, C, D are representations intended to explain the way a Harmonic Drive operates; 
     FIG. 4 to  8  are longitudinal sections or partially axonometric representations in vertical section of further variants of the invention. 
     FIG. 9,  10 ,  11  and  11 A are details of a further variant of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The device of FIG. 1 consists of a camshaft  1  and of a sleeve  2  which is rotatable relative to the camshaft  1  and which has a sprocket wheel  3  configured in such a way that it is integral with it. A gear  6  is connected to the camshaft  1  by means of a screw  4  and a disk  5 , which gear  6  is configured as a thin-walled cylinder terminated at one end by a rigid disk  7 . The screw  4  secures the disk  7  of the gear  6  non rotatably relative to the disk  5  and the camshaft  1 . In the area  8 , the sleeve  2  is configured as a circular, internally toothed gear that engages the gear  6 . By its inner side, the gear  6  props on the outer ring  9  of a roller bearing  10  which is configured as a ball bearing. The inner ring  11  of the roller bearing  10  has got an elliptical shape so that the gear  6  only engages the internally toothed area  8  of the sleeve  2  by two points facing each other in circumferential direction. The number of teeth of the gear  6  is smaller by two than the number of teeth of the internally toothed area  8  that constitutes a hollow gear. The inner ring  11  of the roller bearing  10  is rigidly connected in an inner rotor  12 , which is essentially shaped like a tube. On the opposite end, the rotor  12  is borne on the sleeve  2  via a ball bearing  13 . On its outer periphery, the inner rotor  12  carries a winding  14  interacting with a winding  15  configured on the inner periphery of the sleeve  2 , which constitutes the outer rotor. The windings  14  and  15  are further connected by a ribbon cable  22  that transmits electric current from the sleeve  2  onto the inner rotor  12 . The ribbon cable  22  has got length enough to bridge the potential area of rotation of the rotors  2 ,  12  running in opposite directions. An electronic control unit, which is not illustrated in FIG. 1, may be provided to change for example the frequency of the current transmitted to the winding  14  in order to change accordingly the speed of the inner rotor  12  relative to the sleeve  2 . 
     The electric power for energizing the windings  14  and  15  is obtained by a stationary coil arrangement  16 , which cooperates with a coil arrangement  17  arranged on the sleeve  2 . In order for the current transmission across the air gap  18  to be independent of the speed of the sleeve  2 , the coil arrangements are wound in circumferential direction. Sheet iron  19  is used to reinforce the electromagnetic field. 
     The sleeve  2  is rotatably borne opposite the housing  20 , which has been only hinted at, and the whole device is closed by a cover  21 . Thanks to the tube-like configuration of the rotor  12 , the complete device except for its cover  21  may be fastened by only screwing in the screw  4  on the camshaft  1 . 
     Operation of the device of the present invention now will be described more thoroughly. If the adjusting angle of the camshaft  1  cannot be changed, it is actually not necessary to energize the electric motor, since the set of gears  23  consisting of the gears  6 ,  8  is self-locking. In order to secure the position, a current may however be transmitted via the coil arrangements  16 ,  17 , which keeps the windings  14 ,  15  in a stable position relative to one another. In this case, drive gear  3  and camshaft  1  rotate at the same speed. If the phase angle of the camshaft  1  can be adjusted, a motor-actuated control feeds the coil arrangement  16  with an appropriate voltage that is transmitted to the coil arrangement  17  on the sleeve  2 . The current induced therein supplies the winding  15  on the sleeve  2  and, via the ribbon cable  22  and the control unit, the winding  14  on the inner rotor  12 . In the simplest case, the control unit may be configured as a rectifier circuit that feeds the windings  14  with a constant direct current so that the magnetic polarity existing on the periphery of the outer rotor  2  is independent of the frequency of the alternative current transmitted through the coil arrangements  16 ,  17 . An alternating current however is applied on the outer winding  15  and produces a rotating electromagnetic field effecting a rotation of the inner rotor  12  relative to the sleeve  2 . Since the gear  6  has two teeth less than the hollow gear  8 , one complete revolution of the inner rotor  12  causes the camshaft  1  to rotate relative to the sleeve  2  to the extent of two teeth. Therefore, the torque that has to be produced on the electric motor only constitutes a small fraction of the actually required adjusting torque. 
     The variant of FIG. 1A widely corresponds to the one of FIG.  1 . The analogous parts are referred to with the same reference numerals and are not described again in the following. In this variant of an embodiment, screws  25  are screwed in the sleeve  2  in radial direction, said screws engaging into ring segment shaped recesses  40  of the camshaft  1 . The allowable rotating angle of the camshaft  1  is thus defined. This variant also differs from the afore described one by its cover  21  that is pulled over the sprocket wheel  3  and directly fixed to the housing  20 . The drive chain  3   a  is illustrated in FIG.  1 A. FIG. 1A also shows the control unit  41  which is supplied by the coil arrangement  17 . The winding  15  on the sleeve  2  is directly energized via the control unit  41 , whereas the winding  14  on the rotor  12  is fed via the ribbon cable  22 . In this variant, the control unit  41  may be formed in such a manner that the control pulses transmitted by pulse-width modulation are used together with the actual driving power for producing two alternating currents in order to energize the windings  14  and  15 . At the same frequency, no rotation takes place and the adjusting angle of the camshaft  1  remains the same. A leading or a lagging movement of the camshaft  1  may be occasioned by an appropriate difference in frequency. Thanks to a Hall detector or the like, which is not illustrated in the drawings herein, it is possible to get some information about the instant adjusting angle of the camshaft  1 . Power supply occurs via connections  28  that communicate with the coil arrangement  16 . 
     FIG. 2 shows details of a Harmonic Drive in an axonometric exploded view. The inner ring  11  of the roller bearing  10  is elliptic with a slight eccentricity. The outer ring  9  is directly supported by the inner side of a flexible gear  6 . This gear  6  meshes by two opposite points with a rigid, internally toothed gear  8 , which has got a circular shape. 
     FIGS. 3A, B, C and D show the mode of operation of this Harmonic Drive. In the position shown in FIG. 3B, the inner ring  11  is rotated 90° clockwise relative to the position shown in FIG.  3 A. The FIG. 3C shows a further rotation by 90° and FIG. 3D one complete revolution by 360°. For the sake of clarity, an arrow  11   a  was introduced into the FIG. 3A, B, C and D. The number of teeth of the flexible gear  6  is smaller by two than the number of teeth of the internally toothed gear  8 . A small difference in angular velocity between gear  6  and gear  8  arises out of it. As may be seen in the Figures, the sign  6   a  that alludes to the gear  6  is moved slowly counterclockwise while the inner ring  11  is turning. As a whole, the rotating angle corresponds to the central angle of two teeth of the gear  6 . 
     The variant of FIG. 4 only differs from the variant of FIG. 1 by having the winding  14  of the inner rotor  12  supplied via a separate coil arrangement  27  that interacts with another, stationary coil arrangement  26 . Thus, intensity of current, frequency and phase position of the currents in the windings  14  and  15  may be adjusted irrespective of one another. Control may thus be structured with particular degrees of freedom. 
     It is noted that in cases in which the adjusting torque is small, permanent magnets may be arranged on the inner rotor  12 , said permanent magnets cooperating with a winding  15  on the sleeve  2 . In this way, one pair of coil arrangements feeding the winding  15  on the sleeve  2  may suffice and it is no longer necessary to provide a ribbon cable like in the variant in FIG. 1 or another coil arrangement like in the embodiment of FIG.  2 . 
     In order to ensure emergency operation in case of failure of the adjusting device, a projection  30  is provided in the sleeve  2 , said projection engaging a peripheral groove of the camshaft  1  so that adjustment is possible in an allowable range only. 
     In the variant of FIG. 5, the coil arrangements  36 ,  37  are arranged in such a way that their front sides are conspiring. The drive gear  3  is configured as a pulley having two engagement surfaces  3   a  and  3   b , one of them serving to receive a toothed belt (not shown) driving said pulley via a crankshaft, whereas the other is provided to drive another camshaft (not shown). Furthermore, a recess  38  having the shape of a circular segment is provided in the camshaft  1 , a mating projection communicating with the drive gear  3  engaging said recess in order to limit the motion of rotation. The remaining structure is similar to the one of the afore described variant. 
     FIG. 6 shows schematically another variant of the invention. A motor winding  44  is rigidly connected to the sleeve  2 , which is rotatable relative to the camshaft  1  and which has an integrated drive gear  3 , so that the sleeve  2  constitutes the rotor of the electric motor. Another rotor of this electric motor is constituted by a short-circuit rotor  45  that communicates with the camshaft  1  via a Harmonic Drive  56  which only is diagrammatically hinted at in FIG.  1 . The Harmonic Drive may be embodied as shown in the FIGS. 2 and 3A,  3 B,  3 C and  3 D or it may be configured as a friction gear of mainly the same structure. The Harmonic Drive  56  has a big transmission ratio of for example 50:1 so that the electric motor only has to provide one fifty est of the adjusting torque of the camshaft  1 . In order to limit the holding load to nearly zero when stationary, a double free-wheel  57  is provided between the short-circuit rotor  45  and the sleeve  2 , said double free-wheel effecting that the drive torque of the camshaft  1  does not produce any torque on the electric motor, so that, when stationary, the electric motor does not have to provide any torque. Further more, a first coil arrangement  58  configured as a generator winding is connected to the rotor, said first coil arrangement also communicating electrically with the motor winding  44 . A stationary coil arrangement  59  induces in the coil arrangement  58  a current that serves to move the short-circuit rotor  45  relative to the motor winding  44 . The speed of the short-circuit rotor  45  relative to the motor winding  44  may be controlled via the current supply  60  of the stationary coil arrangement  59 . Since only an exciting current has to be transmitted to the connection  60 , the electrical supply system of the motor vehicle is hardly loaded by the adjusting device. When the first coil arrangement  58  has got twelve or eighteen poles for example and the motor winding  44  has got six poles, an electrical transmission ratio is created that guarantees a fast adjusting motion of the camshaft even at low speed. 
     In FIG. 7, the stationary coil arrangement  59  representing the field winding, the first coil arrangement  58  representing the winding of the generator, the motor winding  4 , which is connected mechanically and electrically to the first coil arrangement, and the short-circuit rotor  45  are depicted. In the illustrated variant, the different component parts are placed radially into each other. Any other geometric configuration like for example an axial arrangement with a disk rotor or an arrangement having the electric motor outside the generator may be chosen, when constructional circumstances so require. The short-circuit rotor  45  also may have a conical shape in order to optimize the torque behavior. 
     FIG. 8 shows schematically how the device according to the invention is built in. A housing  70  covers the camshaft  1 , a cover  71  receives the stationary coil arrangement  59 . A shaft seal  82  seals the adjusting device relative to the cylinder head. 
     The gear of FIG. 9 consists of a motor shaft  100  provided on its end with a bearing surface  101  on which a roller bearing  102  configured as a ball bearing is wedged up. The roller bearing  102  has an inner ring  103  whose outer periphery is elliptical. The flexible outer ring  105  gets its elliptical shape by the rollers  104 . Since the outer periphery of the outer ring  105  is slightly beveled, the ring as a whole adopts the shape of an elliptical cone. A flexible engaging part  106  sits close to the outer ring  105 . The second plane of action  107  is arranged on the outer periphery of the engaging part  106 , said second engaging part engaging a first engaging part  108  arranged on the inner periphery of a first engaging part  109 . The first plane of action  109  has got the shape of a circular cone. An output shaft  110  is communicating with the engaging part  106 . A thin-walled inner ring of uniform thickness that is fixed on an elliptical bearing surface may be used instead of an elliptical inner ring  103 , so that an elliptical circumferential surface is obtained by resilient deformation. 
     The way of operation of the device according to the invention is explained more thoroughly in the following. In the position illustrated in FIG. 10, the planes of action  107  and  108  are in contact in the areas  111  and  112 , which are opposite to one another. When the motor shaft  100  rotates, these areas  111  and  112  creep along the first plane of action  108  until they reach their original position. The second plane of action  107  thereby rolls off on the first plane of action  108 . Since the circumference U 1 , of the first plane of action  108  is slightly bigger than the circumference U 2  of the second plane of action  107 , the engaging part  106  rotates slightly in a direction opposite the direction of rotation of the motor shaft  100 . The transmission ratio i, which is defined by the speed of the motor shaft over the speed of the output shaft, corresponds to the reciprocal value of the eccentricity ε, when the latter is defined according to the following equation: 
     
       
         =(U 1 −U 2 )/U 1   
       
     
     Depending on the material chosen, values of i=1/ may be achieved in a range of 100 through 300 and more. Generally speaking, the harder the substances used for the planes of action  107  and  108 , the bigger the transmission ratio may become. 
     FIG. 11 shows a set of gears according to the invention that is part of an adjusting device for the camshaft of an internal combustion engine, which has not been illustrated in detail. A frictional wheel  206 , configured as a thin-walled cylinder and constituting the flexible engaging part, is fixed to the camshaft  201  by means of a screw  204 . A sleeve  202  having a sprocket wheel  203  integrated to it is rotatable relative to the camshaft  201 . A roller bearing  210  is given an elliptical shape so that the frictional wheel  206  engages an engaging area of the sleeve  202  by only two points located opposite each other in circumferential direction. The circumference of the frictional wheel  206  is slightly smaller than the circumference of the plane of action  208 , which is arranged on a ring  220  that may be displaced in axial direction relative to the sleeve  202  and that is configured as a hollow gear. The inner ring of the roller bearing  210  is rigidly connected in a drive element  212  communicating with an adjusting motor (not shown). 
     To make sure that the required pressure of the frictional wheel  206  acting against the ring  220  is permanent, a spring  221  is provided that prestresses the ring  220  relative to the sleeve  202 . 
     In FIG. 11A, an exploded view shows the structure of the above mentioned set of gears. Although the ring  220  is axially slidable in guides  219  on the sprocket wheel  203 , it is non rotatably borne. The drive element  212  is connected to a support  214  for the inner ring of the roller bearing  210  via a ring  213 . The connection is secured by a disk  215  and by a Seeger circlip ring. 
     The present invention permits creation of an electric motor driven adjustment of the camshaft of an internal combustion engine that has got a particularly simple design and that is particularly robust and long-lived due to the fact that it avoids any slip contacts.