Abstract:
The invention relates to a device for converting a rotational displacement into a displacement back and forth. Said device comprises at least one control cam region, provided with a driven cam element, having an eccentric control surface, and a cam follower element that can be displaced or pivoted by the cam element. The cam element is rotatably mounted in a flexible encircling element which is displaceably connected to the cam follower element on a plane that is perpendicular to the rotational axis of the cam element. The encircling length of the flexible encircling element and the peripheral length of the control cam region are configured to correspond and can be modified.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation, under 35 U.S.C. §120, of international applications PCT/AT03/00050, filed Feb. 17, 2003, and PCT/AT02/00096, filed Mar. 28, 2002, and of copending patent application Ser. No. 10/213,625, filed Aug. 6, 2002; the application further claims the foreign priority, under 35 U.S.C. §119, of Austrian patent application AT 1728/2002, filed Nov. 15, 2002. 

   BACKGROUND OF THE INVENTION 
   FIELD OF THE INVENTION 
   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, having a support shaft and having at least one control cam region, in each control cam region a rotatably driven cam element having an eccentric control surface and a cam follower element which can be displaced or pivoted by the cam element, in particular a valve tappet or the like, being provided, the cam element being arranged rotatably in a flexible enclosing element which is connected to the cam follower element. 
   One essential sphere of use for devices of this type is in the construction of internal combustion engines, in particular for motor vehicles. Since customary valves of internal combustion engines require for their closing restoring springs which have to apply considerable forces, constrained guides which require weaker restoring springs or render them unnecessary have also already been proposed. Particular embodiments of a constrained guide of this type can be gathered, for example, from DE 37 00 715 A or FR 28 17 908 A, in which the cam element is surrounded in each case in a loosely fitting manner by a flexible enclosing element which is connected to the valve actuating element. The cam element therefore revolves in the enclosing element. 
   Different variants of this type of constrained guide are described in WO 01/12958 A and WO 01/12959 A. The enclosing element surrounds the circumference of the cam element without significant play, and so it is matched to the shape of the cam; and the cam element can rotate in the enclosing element on account of the structure of the latter. Since the enclosing element, which is connected to the valve adjusting element, cannot rotate together with the cam element, the migration of the cam region about the axis of rotation of the cam element is converted into a lifting or reciprocating movement of the cam follower element, which is mounted displaceably or pivotably in the cylinder head. The cam follower element does not execute any movement as long as the region in which the enclosing element is connected to the cam follower element rests on the base circle region of the rotating cam element, and is then moved away from the axis of rotation of the cam element in the radial direction and finally is returned again, while the cam region of the cam element moves past the region in which the enclosing element is connected to the cam follower element. 
   A multiplicity of different constructions have been disclosed under the term “variable valve control”, by means of which the opening and the closing time and also the stroke of the valve can be changed in order to improve the power, the exhaust behavior, the fuel consumption, the torque, etc. of an internal combustion engine. In comparison to a non-adjustable valve control with fixed values, the filling of a cylinder is improved if the valve is opened later and closed earlier at low rotational speeds and is opened earlier and closed later at higher rotational speeds, and if the stroke is varied. It is therefore possible, by means of adjusting the valve control as a function in particular of load and/or rotational speed, to optimize the exhaust behavior, the torque, the engine power, etc. The variable valve controls usually change the position of the actuating surface of the cam follower element relative to the eccentric valve control surface or to the axis of the support shaft (WO 98/26161) by rotation, displacement or enlargement of the cam element. 
   In the case of the constrained guides which are mentioned at the beginning and in which the cam element is surrounded in a manner essentially free from play by a flexible enclosing element and the enclosing element is connected to the cam follower element, a variable control by rotation, displacement or enlargement of the enclosed cam element is not possible. 
   WO 01/12959 which has been mentioned has already described a type of variable control, the enclosing element exclusively being changeable in a reversible manner there. The non-changeable, rotating cam element produces tensile forces, which rise as a function of the rotational speed, at the point of connection to the cam follower element, with the result that the enclosing element, which bears against the circumference of the cam element in a manner free from play at a low rotational speed, lifts off further and further from the circumference as the rotational speed rises, and, as a result, adopts positions which correspond to cam elements having relatively large circumferential lengths. The stroke of the cam follower element is therefore increased as a function of the rotational speed. 
   Two possibilities are described: in a first variant, the enclosing element has at least one subregion which is elastic in length to a limited extent and, in the second variant, the enclosing element is not elastic, but is longer than the enclosing element, and the excess length is accommodated in a protuberance, with an elastic change of the size of the protuberance being provided. 
   SUMMARY OF THE INVENTION 
   The invention has now set itself the object of, in the case of a device of the type mentioned at the beginning, making a variable control possible despite the enclosing element surrounding the circumference in a manner essentially free from play. 
   According to the invention, this is achieved by the enclosing length of the enclosing element and the circumferential length of the cam element being changeable in a corresponding manner. In this case, changeable in a corresponding manner is understood to mean that the enclosing element always surrounds the cam element in a manner essentially free from play, with it being insignificant which of the two lengths serves as the correcting variable for the second length. It is possible for both the circumferential length of the cam element to be changeable in a variable manner corresponding to the length of the enclosing element and also for the enclosing length of the enclosing element to be designed in a manner such that it can be matched to the particular circumferential length of the cam element. 
   The active adjustment of the cam element permits an enlarged adjustment region, since a zero stroke can be selected as the starting point, in which stroke the parts of the cam element are joined to one another within a circular circumferential contour. The zero stroke is of importance, for example, if it is intended to be possible to be able to take individual valves of an internal combustion engine out of operation. 
   The cam element can preferably move on the support shaft out and in in a radial plane without changing a part which surrounds the normal-position bearing surface of the control cam region. 
   In one variant, the part which surrounds the normal-position bearing surface is provided on the support shaft and rotates together with the latter. The support shaft is hollow and accommodates a control shaft on which a control surface, which brings about at least the pushing-out of the cam element, is provided in the control cam region. 
   In a first variant, the control shaft can be displaced longitudinally in the support shaft and can have an oblique surface which interacts with a corresponding oblique surface on the cam element. In a further variant, the control shaft can be arranged in a rotatable manner in the support shaft and can have a spiral control surface which interacts with an inwardly protruding web or the like of the cam element. 
   A third variant, in which the control shaft can likewise be rotated in the support shaft, makes provision for the cam element to be arranged in a manner such that it can pivot about an axis parallel to the axis of rotation of the support shaft and to be provided with a guide surface which interacts with a crank pin of the control shaft. If the cam element is pivoted, an asymmetrical change in the cam profile takes place. Different opening and closing properties are additionally produced in the case of a valve timing gear. 
   The support shaft may itself be used as a control shaft if it is arranged in a longitudinally displaceable manner in at least two bearing elements and has an oblique surface which interacts with an oblique surface of the cam element, the cam element being held, for example by the two bearing elements, in a manner such that it cannot be displaced axially. 
   In all of the previously described cases, the movement of the cam element taking place in the radial plane may also be guided in a constrained manner, with the result that the cam element is retracted again into the support shaft by the actuating mechanism. 
   The frictional ratios can be substantially improved if the support shaft and the cam element have channels for feeding a friction-reducing medium to the eccentric control surface. 
   In a preferred, first variant, an enclosing element which can be changed in the enclosing length is formed from an extension-resistant material and has a circumferential length corresponding to the maximum circumferential length of the cam element, the difference between the enclosing length and the circumferential length of the enclosing element being arranged in at least one variable inward protrusion or protuberance. The formation of a variable inward protrusion or protuberance is possible, since the enclosing element is prevented from rotating on account of its connection to the cam follower element, and so the inward protrusion or protuberance can be provided in a stationary manner at each suitable point, in terms of clearance, around or in the cam element. The inward protrusion or protuberance can be elastically flexible, and in particular can be acted upon by a spring or the like, or can be acted upon in an adjustable manner by means of a hydraulic element or the like. 
   In a further variant, provision can be made for the protuberance to be provided with an elastically flexible constriction. 
   During each enlargement of the cam circumference, which brings about a change in the valve stroke, part of the length of the enclosing element that is deposited or stored in the variant is removed, and during each reduction in size, is returned back into the protuberance, so that the enclosing length is always matched to the length of the cam circumference. 
   In a further variant, the enclosing element can have parts of different materials, at least one material being elastically extensible. The extensible parts render the protuberance, which is provided in the case of extension-resistant materials, superfluous. 
   One preferred variant of an extension-resistant enclosing element or of a combination of extension-resistant and extensible parts is realized by means of a multi-link element, in particular by means of a chain which has lateral plates connected by pivot pins and, if appropriate, rollers. 
   In an extension-resistant chain, provision is preferably made for the spring bringing about the protuberance to act between two non-consecutive pivot pins of the chain, with a pivot pin situated in-between being skipped. If the spring is a tension spring, then the pivot pin situated in-between is pressed upward by the cam circumference and forms the protuberance, the plates being raised obliquely on both sides. 
   A further variant makes provision for the spring to be formed by a spring sheet-metal strip which is guided over the pivot pin which has been skipped and engages with its ends under the two pivot pins. Two protuberances are formed here, since each of the two pivot pins which are engaged under are pressed up by the cam circumference. 
   In an extendable chain, the plates are elastically flexible and are preferably shaped from a spring wire, plastic, rubber or the like to form a frame-like element which encloses two pivot pins and is prestressed with the effect of shortening the distance between the pivot pins. The plate is therefore stretched when the length of the cam circumference is enlarged, and shortened when it is reduced in size. The difference in length which can be obtained is small if each plate of the closed chain is of elastically flexible design. 
   If the plates are not formed from resilient material, a compression spring element, for example of rubber, can spread the mutually opposite sides in each plate apart, thus providing the change in length of the plates even in the case of a flexible, non-elastic material. 
   The stretched position can in each case constitute an extension limit, so that the maximum stroke length is not exceeded, even if rotational-speed-induced additional tensile forces from the cam follower element, which is coupled to the enclosing element, become effective. 
   In a further variant, provision is made for the enclosing element to consist of an elastically extensible material, an extension limit preferably also being assigned to this enclosing element. For example, the enclosing element may be a band of a textile-bonded surface material, in particular a woven fabric or the like which is produced in a circular working technique and is extensible, with threads which are woven in or are additionally extension-resistant and the length of which corresponds to the length of the maximum cam circumference being provided. 
   If the band consists of extension-resistant threads or fibers, then the difference in length, as mentioned at the beginning, is stored in at least one spring-actuated protuberance. 
   Since the enclosing element is exposed by the reciprocating cam follower element in particular to relatively high tensile forces when the push-off acceleration is braked, that part of the enclosing element which lies opposite the connecting region is pressed fixedly onto the circumference of the cam element. Conversely, that part of the enclosing element which includes the connecting region is exposed to correspondingly high compressive forces shortly before it returns into the starting position, since the restoring acceleration is braked, and said part 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 in order to feed in the lubricating medium. For example, in conventional cylinder heads there is a pressure of 2 to 5 bar, and at least 10 times the pressure would have to be applied in order to press the enclosing element away from the circumference and to let the medium emerge. (The values of this example refer to oil lubrications). Only partial lubricant films are produced, and a mixed friction occurs, the coefficient of friction of which is not smaller than 0.1. 
   Since the flexible enclosing element is prevented from rotating, in a further variant, the frictional ratios can be improved once again if the flexible enclosing element surrounds the eccentric control surface of the driven cam element and a non-driven normal-position bearing surface for the cam follower element. A non-driven bearing surface is understood in particular to mean a cylindrical bearing surface which is fixed on the device, for example on a bearing element of the support shaft. This makes it possible, depending on the shape of the cam, to reduce the contact surface, which produces a substantial part of the friction, between the cam element and the enclosing element in length by at least one third, and, in the case of conventional shapes of cam, even by up to two thirds. Since the cam element is additionally also narrower than the enclosing element—an in particular annular end region of a bearing element adjoins the cam element axially at least on one side, preferably on both sides—the contact surface producing the friction is also narrower than in the variants mentioned at the beginning. 
   However, the non-driven bearing surface may also be formed on a ring or the like mounted rotatably, for example, on the bearing element, so that a minimal rotation to and fro of the bearing surface is possible, said rotation arising because of the slightly alternating and changing geometrical ratios between the point at which the enclosing element is connected to the cam element and the migrating control surface. 
   Further friction-reducing measures may include the arrangement of rolling bearings between each bearing element and the support shaft and/or the cam element, and/or the arrangement of a rotatably mounted roller in the eccentric control surface of the cam element and/or the formation of channels for feeding a friction-reducing medium, in particular lubricating oil, to contact surfaces producing the friction. 
   In the abovementioned cases in which high tensile or compressive forces occur, said forces are transmitted directly to the bearing elements by means of the design according to the invention, with the result that the sliding or rolling bearings between the bearing elements and the support shaft are relieved of load. To relieve the mounting of the cam follower element from load, in a further preferred variant, provision is made for that end of the cam follower element which is connected to the enclosing element to be guided in a guide fixed on the device. 
   The reduction in size of the friction-producing contact surfaces furthermore reduces the quantity of heat which is produced and the removal thereof is facilitated if the vertical 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 need for lubricant. 
   A small restoring force acting on the cam follower element may be advantageous. In one preferred variant, in which the cam follower element is coupled to 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 fixed on the device by means of an elastic element. To produce the restoring force, use can be made, for example, of a leg spring or the like which is supported at one end on the bearing pin and at the other end on the bearing element or the like. One preferred variant makes provision for the bearing pin to have at least one exposed end region, and for an elastically flexible band of steel, rubber or the like to be guided around the exposed end region and the bearing element. 
   The invention is described in greater detail below with reference to the figures of the attached drawings, in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a perspective view of parts of a first variant of a valve timing gear, 
       FIGS. 2 to 4  show longitudinal sections through the first variant,  FIG. 2  showing a zero stroke position,  FIG. 3  showing a normal stroke position and  FIG. 4  showing a maximum stroke position, 
       FIGS. 5 and 6  show the zero stroke and maximum stroke position of a second variant, 
       FIGS. 7 and 8  show the zero stroke and maximum stroke position of a third variant, 
       FIG. 9  shows oblique views of the support shaft and of the cam element in three different positions of a fourth variant, 
       FIG. 10  shows a longitudinal section similar to  FIG. 4  through the variant according to  FIG. 9 , 
       FIGS. 11 and 12  show longitudinal sections through a fifth variant of the device according to the invention, the cam follower element bearing in each case against the bearing surface, 
       FIGS. 13 and 14  show sections along the line XIII—XIII from  FIG. 11  and the line XIV—XIV from  FIG. 12 , 
       FIGS. 15 and 16  show schematic side views of a sixth variant with a cam element guided in a constrained manner by means of a crank drive, in two different positions, 
       FIG. 17  shows a control shaft of a seventh variant in an oblique view, 
       FIGS. 18 and 19  show longitudinal sections through the seventh variant, 
       FIG. 20  shows a section through the line XX—XX from  FIG. 18 , 
       FIG. 21  shows parts of an eighth variant in an oblique view, 
       FIG. 22  shows an end view of the eighth variant in the maximum stroke position, 
       FIG. 23  shows a schematic end view of the eighth variant in the normal stroke position, 
       FIG. 24  shows a schematic oblique view of the protuberance spring from  FIG. 23 , 
       FIGS. 25 to 27  show schematic end views of a ninth, tenth and eleventh variant, in each case in the normal stroke position, 
       FIG. 28  shows a schematic end view of a twelfth variant in the normal stroke position, 
       FIG. 29  shows a first design of a link plate of the twelfth variant, 
       FIGS. 30 and 31  show a second design of the link plate in the normal stroke and in the maximum stroke position, 
       FIG. 32  shows a schematic illustration of a thirteenth variant, 
       FIG. 33  shows parts of a fourteenth variant in an oblique view, and 
       FIG. 34  shows an oblique view of the fourteenth variant in the normal stroke position. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A device according to the invention for converting a rotational movement into a reciprocating, rectilinear or pivoting movement comprises a driven support shaft  1  on which in each control cam region  2  a cam element  71  having an eccentric control surface  4  is fixed in a manner not shown specifically. The eccentric control surface  4  enables a cam follower element  10 , which is held in a manner bearing against it, to move to and fro 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 control of internal combustion engines. However, such devices may also be used, for example, in cam controls of machine tools, in special gears or the like, the cam follower element  10 , which forms a valve tappet in the exemplary embodiments shown, being designed in accordance with the use. 
   The drawings each show just one valve timing gear for a valve, a valve timing gear which is used for an internal combustion engine of a motor vehicle having, on a driven support shaft  1 , the number of cam elements  71  required for the valves. 
   Each control cam region  2  comprises a base circle surface  3 ,  53  and an eccentric end surface  4  which is provided on a cam element  71  and is surrounded by an essentially adjacent enclosing element  6 , so that the cam element  71  can be driven in the enclosing element  6  around the axis of rotation  8  with continuous, pulsating deformation of the enclosing element  6 . The cross-sectional shape of the enclosing element  6  is illustrated in the figures in a manner matched in each case to the cam element  71 , since the valve time gear here is shown in an exploded illustration, whereas it has the shape of a collapsed oval or the like and it is in the form of a loose individual element. The enclosing element  6  is prevented from rotating by the connection to a cam follower element  10  which, in the case of a valve tappet, is mounted in a manner such that it can be displaced in a guide sleeve  41  of the cylinder block  80  and, in the event of a tilting or drag lever, is mounted pivotably in a pivot bearing. The enclosing element  6  is connected to the cam follower element  10  in a manner such that it can tilt or pivot about an axis  15 , so that, during the passage of the cam element  71  through the connecting region of the cam follower element  10 , a pivoting of the enclosing element  6  relative to the cam follower element  10  is made possible. This is necessary, since the guide sleeve  41  of the valve stem  11  does not permit any lateral deflection of the valve stem  11 , and the valve stem  11  has to be aligned radially with the axis of rotation  8 . The rotation of the cam element  71  leads to an oscillating movement of the enclosing element  6  which, however, owing to its connection to the cam follower element  10 , cannot rotate, but rather, while the cam element  71  is rotating continuously, is raised from the base circle surface  3 . In the process, the cam follower element  10  is transferred from bearing against the base circle surface  3 , in which it is at the shortest distance from the axis of rotation  8 , into a position the maximum distance away from the axis of rotation  8  when the greatest amount of the eccentric control surface  4  of the cam element  71  is effective, and, on further rotation, is drawn back into the normal position again. In the tappet valves shown, this up and down movement constitutes the valve stroke, the length of the stroke being settable in particular as a function of the rotational speed and/or load, as explained further below. 
   In all of the variants, the guide sleeve  41  and the valve stem  11  are illustrated in a radial alignment with the axis of rotation  8 . However, a lateral offset, from which a distance results between the axis of rotation  8  and the axis of the guide sleeve  41 , is readily possible. A variant of this type has opening and closing properties which are asymmetrical in relation to a symmetrical cam element  71 , which may be advantageous in certain uses. 
   In order, in spite of the constrained control, to achieve an enlargement of the valve stroke, in particular as a function of the rotational speed, the enclosing element  6  can either be prestressed to be elastically extensible and to contract, or can have a maximum circumferential length, the particular excess, in the case of a small valve stroke, being stored in a “store”, for example in the form of a protuberance. 
   An elastically extensible enclosing element  6  may be, for example, a band which is produced in a textile circular working technique and which is preferably assigned an extension limit by threads of fibers consisting of Kevlar, glass, carbon, high modulus polyethylene, polyester, boron or aramid or similar fibers which are essentially constant in length, or combinations of these fibers, which threads extend in the circumferential direction of the cam element, being provided in the extensible material or parallel to it. The elastic extension may be selected to be linear, progressive or degressive by, for example, threads having different extension properties which are effective at the same time or one after another being incorporated. Particularly suitable materials for an enclosing element having at least elastically extensible subregions have a modulus of elasticity of between 1 and 4,000 N/mm 2 . Gummy materials have low moduli of elasticity and are preferably provided with an extension limit. Materials, such as plastics, having higher moduli of elasticity, in particular between 600 and 2000 N/mm 2 , preferably between 800 and 1200 N/mm 2 , generally do not need any extension limit but one may, of course, be provided. 
   In  FIGS. 1 to 14 , the enclosing element  6  is formed by an elastic band of this type. The control cam region  2  comprises a cam part  70  which is connected in a rotationally fixed manner to the support shaft  1 , and a cam element  71 . The cam part  70  has a cylindrical basic shape with the base circle surface  3  and a central aperture  75  comprising less than half of the circumference, with two cam regions  43  having a circular outer contour remaining on both sides. According to  FIGS. 1 to 4 , a control rod  64  which rotates at the same time as the support shaft  1  can be displaced axially therein and has, in the region of each cam element  71 , a control section  66  having an axially rising oblique surface  67 . The cam element  71  of the cam element  2  bears against the oblique surface  67 , which is exposed in the aperture  75 , said cam element likewise having an oblique surface  72  and the eccentric control surface  4  is provided on it and it is guided between the lateral cam regions  43  in the aperture  75 .  FIGS. 2 to 4  show various positions. In  FIG. 2 , the control rod  64  is displaced so far to the left that the cam element  71  reaches its deepest position in which it lies within the circular outer contour of the cam part  70 . In this position, the rotation of the cam element  71  also does not cause a valve stroke. The cam follower element  10 , which is guided in the guide sleeve  41  of the cylinder block  80  or the like, remains in the closed position, and the enclosing element  6  is not extended. An associated cylinder of the internal combustion engine is therefore not in operation. 
   If the control rod  64  is displaced to the right (arrow  81 ) by an actuating mechanism (not shown), then, in a position according to  FIG. 3 , a normal stroke position is provided, as is favorable, for example, for the idling speed or low rotation speed range, with the cam element  71  having being pushed out radially. The enclosing element  6 , which is elastic in this variant, is extended by a certain extent in the direction of the valve stem  11  partly by the cam element  71  being pushed out and partly by the tensile forces, which are also in effect in the idling speed, the increase in distance between the axis rotation  8  and the holder  12  corresponding to the pushing-out of the cam element  71 . Without an expansion of the enclosing element  6  that is brought about actively by the displacement of the control rod  64 , the transition from the zero stroke position of  FIG. 2  into the normal stroke position according to  FIG. 3  is not possible, since the cam follower element  10 , which is at rest in  FIG. 2 , does not exert on the enclosing element  6  any tensile forces resulting from the rotation of the support shaft  1 . A further displacement of the control rod  64  in the direction of the arrow  81  transfers the cam element  71  into the maximum stroke position of  FIG. 4 , in which the uppermost region of the oblique surface  76  is reached. The enclosing element  6  is extended to its maximum length and contracts again when the control rod  64  is displaced back. 
   In the variant according to  FIGS. 5 and 6 , a rotatable control shaft  65  is provided for the radial movement of the cam element  71  in the support shaft  1 , the control region of which control shaft is formed by a crank-pin-like element  68  having an eccentric control surface  69  rising spirally. The sequence of adjustment of the cam element  71 , when the control shaft  65  is rotated in the direction of the arrow  82 , can be seen from comparing the two  FIGS. 5 and 6 . In the pushed-out position according to  FIG. 6 , the cam element  71  is held by the element  68  of the control shaft  65 . If the control shaft  65  is rotated back counterclockwise in the support shaft  1 , then the cam element  71 , which bears against the spiral control surface  69 , migrates inward again until the zero stroke position according to  FIG. 5  is reached. In this, the cam element  71  is situated within the cylindrical outer surface of the cam part  70 , with the result that the contracted enclosing element  6  fits around the cam part  70 . 
   Owing to the elasticity of the enclosing element  6 , it may be advantageous if it contains in the transverse direction, i.e. in the axial direction of the support shaft  1 , stiffenings, for example in the form of reinforcing ribs  63 , which have, for example, pins inserted or bonded into them. The transverse stiffenings prevent unsupported parts of the enclosing element  6  from being pulled into clearances  73  in the control cam region  2 , the clearances arising from the intermeshing of the cam part  70  and the cam element  71 . 
   The variants according to  FIGS. 7 and 8  show a cam element  71  which bears the eccentric control surface  4  and is mounted on the cam part  70  in a manner such that it can pivot about an axis  77  parallel to the axis of rotation  8 . The pivoting takes place via a crank pin  76  of the support shaft  75  which pivots the cam element  71  upwards (arrow  83 ) from the zero stroke position according to  FIG. 7  into the maximum stroke position according to  FIG. 8 , with the enclosing element  6  being extended. The eccentric control surface  4  of the cam element  71 , which surface is situated in  FIG. 7  within the circular circumferential contour of the cylindrical cam part  70 , gives the control cam region  2  an asymmetrical shape, with the result that there are different opening and closing properties of the valve for each size of the stroke. In this case, selection of the curved shape of the cam element  71  preferably provides a pivoted-out position in which the control cam region  2  is symmetrical ( FIG. 8 ). However, this does not have to be the maximum stroke position. 
   In the previous variants of  FIGS. 1 to 8 , the control cam region  2  exclusively comprises elements rotating at the same time about the axis of rotation  8 . In the following variants according to  FIGS. 9 to 16 , not all of the elements rotate about the axis of rotation  8 ; on the contrary, the base circle region  3  is provided on an element which does not rotate at the same time and which therefore has a fixed normal-position bearing surface  53  for the cam follower element  10 . 
     FIGS. 9 and 10  show variants in which the bearing elements  16  are shown as pipe lengths having ring-like end regions  17  which are fixed, for example, in securing means fixed on the housing, or are provided with corresponding fastening parts. The cam element  71  has the eccentric control surface  4  whose axial extent around the two annular end regions  17  of the bearing elements  16  is shorter than the width of the enclosing element  6 . The enclosing element  6  therefore surrounds part of the cylindrical circumferential surface of the two end regions  17  and the eccentric control surface  4  of the cam element  71 . Since only the eccentric control surface  4  has to slide on the inner surface of the enclosing element  6 , the friction-producing contact surface is smaller than half the inner surface of the enclosing element  6 . As mentioned, the latter is connected via its holder  12  to the cam follower element  10  in an articulated manner, so that no friction occurs between the enclosing element  6  and the cylindrical outer surface, serving as the bearing surface  53 , of the two end regions  17  which, as parts of the bearing elements  16 , are fixed on the housing. The axis of articulation  15  runs parallel to the axis of rotation  8  of the support shaft  1 . The rotation of the cam element  71  leads in turn to an oscillating movement of the enclosing element  6  which, owing to its connection to the cam follower element  10 , is raised continually all the way round by the outer surface of the end region  17 . In the process, the cam follower element  10  is transferred by the bearing surface  53 , in which the cam follower element  10  is at the shortest distance from the axis of rotation  8  and which forms part of the outer surface of the end region  17 , into a position the maximum distance away from the axis of rotation  8  when the maximum extent of the eccentric control surface  4  of the cam element  71  is effective, and, on further rotation, is retracted again into the normal position. In the case of the valve timing gear, the closed position is therefore the normal position and the position the maximum distance away is the open position of the valve disk  13 . 
   As  FIGS. 9 and 14  show, one central, hub-like region  14  of the support shaft  1  is mounted rotatably preferably on both sides in a bearing element  16 , per control cam region  2 . The region  14  has a cutout  5  which is provided with an oblique surface  7 , which rises in the longitudinal direction, and lateral parallel flattened sections. The cam element  71  is provided with an approximately U-shaped cutout on the side lying opposite the eccentric control surface  4  and is guided in a manner such that it can be pushed out and in vertically on the parallel flattened sections. 
   According to the variant shown in  FIGS. 11 to 14 , an extended bearing pin  48 , on which a cam follower element  10  is mounted rotatably on both sides, is inserted into the holder  12  of the enclosing element  6 . The ends  61  of the bearing pin  48  protrude in each case through a slot  87  in securing means  85  and are pressed against the bearing elements  16  by a band of rubber, a clamp of spring steel or another elastic element  31 . The element  31  is prevented from slipping laterally by a collar. The elastic elements  31  are extended by the cam element  71  during the downwards movement of the cam follower elements  10 , i.e. during the opening of the valves, and produce a force which assists the return and may be advantageous in many uses. Substantially stronger restoring springs engaging directly on the cam follower elements  10  are rendered superfluous by the constrained guidance of the enclosing element  6 . Instead of the band shown, other spring devices, for example leg springs, but also piston/cylinder units or the like which can be acted upon, may also be provided.  FIG. 14  clearly shows that the cam element  71 , which does not protrude in a deepest position over the circumference of the bearing element  16 , is raised, when the support shaft  1  is displaced to the left, by the oblique surface  7 , which rises in a wedge-shaped manner, and is transferred into the position in which it is extended to the maximum (shown in  FIG. 13 ). 
   The elastic enclosing element  6  can make the elastic elements  31 , shown in  FIGS. 11 and 12 , unnecessary, since it likewise exerts a restoring force on the bearing pin  48 . Owing to the elasticity of the enclosing element  6 , it may be advantageous if, in the transverse direction, i.e. in the axial direction of the support shaft  1 , it contains stiffenings, for example in the form of reinforcing ribs  63 , which have pins which can be inserted or bonded in. The transverse stiffenings prevent unsupported parts of the enclosing element  6  from being pulled in in the region of the cam element  71 . 
     FIGS. 15 and 16  show a variant in which the cam element  71  is extended and retracted under constrained guidance. A control shaft  94  in the interior of the support shaft  1  has a slot  95  in which a guide rod  98  is mounted rotatably on a bearing pin  96 . The second end of the guide rod  98  is arranged on a bearing pin  97  which is mounted in the interior of the cam element  71 , the cam element  71  being of approximately U-shaped design and being arranged in a manner such that it can be pushed out and in in a guide of the support shaft  1  or in a guide sleeve arranged on the support shaft  1 . The constrained guide therefore constitutes a crank mechanism which can be rotated over an angle of approximately 120°.  FIG. 15  shows a partial stroke and  FIG. 16  shows the full stroke of the cam element  71 . 
   In the variants according to  FIGS. 11 to 16 , the enclosing element  6  forms on both sides a rectilinear bridging of the transition region between the non-rotatable bearing surface  53  and the eccentric control surface  4 , the region changing when the stroke changes. 
   The enclosing element  6  used in  FIGS. 17 to 27  and  FIGS. 32 to 34  is extension-resistant, with the result that the particular difference in length between the enclosing length of the control cam region  2  and a circumferential length corresponding at least to the maximum stroke has to be stored. 
     FIGS. 17 to 21  show a variant similar to  FIGS. 1 to 4  with a control rod  64  which can be displaced longitudinally in the support shaft  1  and has, per control cam region  2 , a control section  66  which is flattened on both sides and has an obliquely rising longitudinal slot  60 . The support shaft  1  is provided in each control cam region  2  with two circumferential ribs  22  between which a guide groove for the cam element  71  is formed, and between which the support shaft  1  is trimmed in some regions. In the apertures  75 , which are formed in a manner similar to  FIG. 1 , the cam element  71 , which is of approximately U-shape design, can be moved out of the support shaft  1  and into it, it sliding on the flattened control section  66  in the manner of a rider. A pin  79  passing through the oblique longitudinal slot  60  brings about the constrained guidance of the cam element  71  when the control rod  64  is displaced in both directions. In the central region, the pin  79  is arranged in a slider  62  which slides along the oblique surfaces  67  of the longitudinal slot  60 . 
   In the variants according to  FIGS. 17 to 31 , the enclosing element  6  is formed in each case by means of a multi-link element, in particular by means of a chain  21 , the pivot pins  23  of which connect lateral plates  24  which also bring about the axial guidance on the cam element  71 , which engages between the plates  24 . In the variants according to  FIGS. 17 to 27 , the chain  21  constitutes an extension-resistant enclosing element  6  which is protruded to match it to the enclosing length. In this case, one or two pivot pins  23  or rollers  25  can be raised from the circumference of the cam, the plates  24  being raised in each case in a roof-like manner by spring action, as is apparent from the different exemplary embodiments of  FIGS. 23 to 27 . 
   The height of the circumferential ribs  22  on the support shaft  1  is selected at least in such a manner that the pivot pins  23  of the rollers  25  bear against the circumferential ribs  22 , and the plates  24  engage over the circumferential rib  22  on the outside, so that the chain  21  is secured axially in the control cam region  2 . 
   As  FIGS. 17 to 19  show, the support shaft  1  can have inlet holes  19  in the bearing regions  9  for feeding a lubricating medium to the cam element and to the chain links. The associated section of the control rod  64  has a circumferential groove  88 , the length of which corresponds at least to the displacement length of the control rod  64 , and from which overflow channels  89  lead into the flattened control sections  66 . The lubricating medium entering through the holes  19  passes via the circumferential groove  88 , the overflow channels  89  and the control section  66  on the one hand into the longitudinal slot  60 , to the oblique surfaces  67  thereof, and on the other hand through the apertures  75  between the circumferential ribs  22  and along the outer surfaces of the cam element  71  to the individual chain links which are supplied uniformly with the lubricating medium by the cam element  71  revolving in the chain  21 . 
   As mentioned above, in the case of an extension-resistant enclosing element  6 , it is necessary to store the particular excess lengths which, in the case of a chain  21 , can take place for example, by making the chain protrude in a suitable region of the circumference. In  FIGS. 21 to 24 , two spring strips  35  are provided for this, said strips running over a central pivot pin  23  and engaging under the two adjoining pivot pins  23 . The spring strips  35  are prestressed in such a manner that they raise the two pivot pins  23  engaged under when the cam element  71  is pulled in.  FIG. 22  shows the maximum stroke position in which the chain  21  bears all the way around, and  FIG. 23  shows a normal stroke position. 
   Since, as already mentioned a number of times, the enclosing element  6  does not rotate at the same time, it can also be fitted, as  FIG. 25  shows, in a positionally fixed manner on the cylinder block  80  or the like via an actively controllable, hydraulic piston/cylinder unit  39 .  FIG. 26  shows triangular pivot plates  26 , a tension spring  40  which acts upon two protuberances  34  being inserted between the raised ends. In  FIG. 27 , the tension spring  40  is arranged between a pivot pin  23  and the next but one pivot pin  23 , so that a protuberance  34  is formed between these two pivot pins and the central pivot pin  23  is raised. In both variants, the tension spring is stressed further when the cam element  71  is extended into the maximum stroke position. Instead of the tension springs  40 , it is also possible in these variants to use hydraulic piston/cylinder units which can be acted upon. In particular in the variant according to  FIG. 27 , the length of the chain can be matched to the smallest circumferential length of the control cam region  2  by means of a piston/cylinder unit, it also being possible for a piston/cylinder unit of this type to be replaced by a chain link or a pair of plates. 
   As an alternative, in order to be able to adjust the chain  21  in length, at least one pivot pin  23  can be provided with an eccentric region (not shown), so that the rotation of the pivot pin changes the distance from the next pivot pin  23 . 
     FIGS. 28 to 31  show variants in which the enclosing element  6  is formed as a length-changeable chain  21 , the change in length being possible in the plates  24 . Of course, pivot pins having adjusting eccentric regions may also be used in this variant. 
     FIG. 29  shows a frame-like plate  24  of a flexible material, which loops around the two pivot pins  23 . A spring  32  which spreads the longitudinal sides apart is arranged between the two longitudinal sides. The plates  24  can therefore be stretched during the extension of the circumference, with the result that the chain  21  becomes longer, and becomes shorter when the cam element  71  is pulled in. The spring  32  may be designed as desired, and formed not only by the compression spring (shown schematically), but also by a rubber cushion or the like. 
     FIGS. 30 and 31  show a frame-like plate  24  of a resilient material, for example spring wire or the like, which is prestressed to shorten the distance between the pivot pins  23  ( FIG. 30 ). When the cam element  71  is transferred into the maximum stroke position, each plate  24  is stretched into the end position shown in  FIG. 31 . The plates  24  may also be cast into a gummy material or vulcanized onto it. 
     FIGS. 17 to 20  and  23  to  31  in each case show rollers  25  which are mounted on the pivot pins  23 . The rollers may be produced, for example, from a highly wear-resistant, low-friction ceramic, for example of silicon nitride (Si 3 N 4 ). Instead of the rollers  25 , sliding bodies may also be provided. 
   In the variant according to  FIGS. 21 and 22 , a pivot pin is extended and forms the bearing pin  48  for the fork-shaped holder  12  of the cam follower element  10 , into which the valve stem  11 , which is provided with a thread  28 , is screwed in an adjustable manner and is fastened by the counter nut  27 . In the schematic illustration according to  FIGS. 23 and 27 , the holder  12  is formed by a triangular axis of articulation, in which the cam follower element is coupled to an additional bearing pin  48 . 
   The enclosing element shown in  FIGS. 32 to 34  is again extension-resistant and is designed as a band or the like. 
     FIG. 32  shows a schematic possibility for storing the band by the excess length being guided in the form of a protuberance  34  via two deflecting rollers  36 , which are parallel to each other and are kept at a distance, and via a deflecting roller  38  which is pressed outward, for example by means of a spring  30 , a hydraulic piston/cylinder or the like. When the circumferential length is enlarged by the cam element  71  being extended or pivoted, part of the enclosing element  6  is pulled in from the protuberance  34 , as a result of which the spring  30  is more strongly compressed. On return into the normal stroke or even zero stroke position, the spring  30  presses the deflecting roller  38  further outward again. 
     FIGS. 33 and 34  show a variant in which the protuberance  34  has a constriction  37  of the enclosing element  6 , which constriction is of elastically flexible design. The solid lines of the enclosing element  6  show the normal stroke position. The maximum stroke position is shown by dashed lines with the constriction  37  expanded, the eyelet  47  which holds the holder  12  being spaced further away from the axis of rotation  8  in this position. 
   The insert  54  which is inserted into the protuberance  34  has a latching or threaded hole  57  into which that end of the valve stem  11  which is latchable or is provided with a thread  28  can be inserted or screwed. In the latter case, a counter nut  27  serves to set or fix the length of the cam follower element  10 . The forces acting when the cam part  71  is pushed out expand the constriction  37 , the regions of which that are in contact with each other being moved away from each other. The constriction  37  is brought about by two clamping jaws  49  which can be braced against each other by means of springs  33 . The two clamping jaws  49  may also be of identical design, with the result that one connecting screw in each case is inserted into a clamping jaw  49 . If appropriate, the prestressing of the springs  33  may also be settable. 
   Instead of the clamping jaws  49 , a latchable, elastically expandable constricting device is also conceivable by, for example, two identically designed parts which are provided with latching hooks and latching openings being clipped to each other. 
   In order to minimize tolerances, it is advantageous, in particular in the production of the valve timing gears for internal combustion engines, if the cam shaft is ground after assembly. In this case, grinding dust penetrates all of the cavities and has to be removed. The dismantling and reassembly after grinding is, on the one hand, very complex and, on the other hand, leads again to small inaccuracies. Use may be made here of the lubricant feed paths described with reference to  FIGS. 17 to 20  in order to prevent the grinding dust from penetrating the interior. This takes place by the cam shaft being rinsed during the grinding process with a liquid under pressure, the liquid entering via the inlet holes  19  and emerging again at all outlet possibilities present. In particular, the grinding liquid used for the grinding is suitable for this.