Abstract:
A spur gear camshaft drive for an internal combustion engine having a crankshaft timing gear, a camshaft gear and at least one intermediate gear arranged between the timing gear and camshaft gear is disclosed. At least one of the gears of the camshaft drive is provided with a hub and a gear ring configured in such a way that the gear ring can be turned against the hub up to a predetermined angle. A readjusting element is provided between the hub and gear ring to exert a readjusting force onto the hub and gear ring depending on the torsion exerted.

Description:
RELATED APPLICATION 
     This application is a continuation of PCT/EP99/08674 filed Nov. 11, 1999 claiming priority from Germany patent application number 198 53 634.8 filed on Nov. 20, 1998, which International application was published by the International Bureau in German on Jun. 2, 2000, from which priority is claimed. 
    
    
     BACKGROUND OF INVENTION 
     The invention relates to a spur gear camshaft drive for an internal combustion engine including a crankshaft timing gear and a camshaft gear and at least one intermediate gear arranged between the timing gear and the camshaft gear, where the gears of the spur gear camshaft drive are each provided with a hub and a gear ring. 
     In internal combustion engines having a camshaft drive which consists of a plurality of gears between a crankshaft and a camshaft, a variety of influences result in vibration stresses on the camshaft drive. For example, in a diesel internal combustion engine, a relatively strong but short-lived excitation of the crankshaft takes place during an operating stroke in a cylinder, while virtually no torque acts on the crankshaft in periods between operating strokes. This results in relatively irregular, non-uniform movement of the crankshaft with a comparatively jerky vibratory motion. Tolerances in components of the camshaft drive lead to corresponding clearance irregularities between meshing teeth of the gears, which also results in vibrations of the camshaft. In addition, such clearance irregularities are further intensified because steel gears or camshaft drive components made of steel are frequently used, whereas the crank housing is made of aluminum. Because the materials have different temperature expansion coefficients, the distance between centers of respective gears of the camshaft drive during operation of the internal combustion engine is variable, causing additional clearance irregularities. 
     These clearance irregularities in the camshaft drive have three kinds of undesirable effects. First, synchronization between the crankshaft and camshaft is hampered, since relative motions between camshaft and crankshaft are possible. In internal combustion engines having an integrated pump-nozzle system, in which the camshaft directly determines injection times, the synchronization problems can result in displacements of injection times and, hence, interference with optimal operation of the internal combustion engine. Second, the irregular motion of the crankshaft is transmitted through the camshaft drive into the camshaft as vibration leading to inaccurate injection and valve actuation times. Third, the clearance irregularities lead to increased noise, which may irritate passengers or a vehicle operator in a vehicle compartment. 
     In addition, an especially great clearance irregularity between a camshaft gear and the last gear of the camshaft drive meshing with the crankshaft timing gear is often present when multiple gears are used between the crankshaft and the camshaft, because tolerances and inaccuracies of distances between centers accumulate correspondingly. 
     The above-mentioned vibrations are further intensified by forces acting on the camshaft against its direction of rotation, such as during the actuation of a fuel pump of a pump-nozzle element, where a cam of the camshaft has to overcome a great resistance. 
     Because of all the disadvantages mentioned above, a camshaft drive by means of gears or spur gears is traditionally not used in passenger cars. A camshaft drive by spur gears is used only when there is a small space between crankshaft and camshaft, i.e., in a bottom-lying camshaft. For reducing clearance irregularities, helical gears are used so that operation is as quiet as possible. Despite this, such spur gear or gear camshaft drives have thus far failed to achieve the comfort, quiet operation and accuracy of a camshaft drive with toothed belt wheels or chain wheels with a roller chain. 
     Self-study program No. 197, “Der 1,8 1 Dieselmotor im LT &#39;97,” published by Volkswagen AG, Wolfsburg, of August 1997, discloses a spur gear camshaft drive wherein, between an intermediate gear of the camshaft and an intermediate gear of the drive for the camshaft, there is provided a spacer fork in such a way that a specified gear tooth clearance is obtained. There, the intermediate gear of the camshaft is arranged on an adjusting-bearing lever which, for assembly, is capable of swiveling about the axis of rotation of the camshaft gear. For assembly, the spacer fork is linked swiveling to the other intermediate gear. After assembly, the spacer fork is tightened, so that the axis of rotation of the intermediate gear of the camshaft is fixed. 
     The object of the present invention is to provide an improved spur gear camshaft drive wherein the aforementioned disadvantages are overcome so that vibrations in the spur gear camshaft are reduced as rapidly as possible. In particular, feedback of vibrations from the camshaft into the spur gear camshaft drive is largely prevented. 
     SUMMARY OF THE INVENTION 
     According to one preferred embodiment of the present invention, there is provided a camshaft gear, a crankshaft timing gear, and at least one intermediate gear, where at least one of the aforementioned is configured in such a way that, if its gear ring is turned against its respective hub at a predetermined angle, a readjusting device provided between the hub and the gear ring will exert a readjusting force against said hub and said gear ring, depending on the torsion exerted. This has the advantage that vibrations between the gear ring and the hub are reduced, without affecting synchronization between crankshaft and camshaft of the internal combustion engine. Accordingly, induced torque peaks on the camshaft and on the injection system drive can be reduced. 
     In a preferred embodiment, cooperating stops limiting torsion are provided on the hub and the gear ring. 
     In a further preferred embodiment, the readjusting device is designed in such a way that, at a torque of 80 Nm or more, the stops engage one another. 
     In an especially preferred embodiment, the readjusting device comprises at least one spring, in particular a flat spring, where the springs are designed for example U-shaped and are arranged spaced at equal distances apart, particularly in the peripheral direction, between hub and gear ring. For this purpose, there are provided recesses at equal distances apart in the peripheral direction accommodating the springs in the hub and an annular cover is provided for holding the springs in the recesses and for closing off the recesses in the hub. The cover may be bolted on. 
     In a further preferred embodiment, for optimal damping of especially high-frequency vibrations, elevations between two springs are provided on an inner periphery of the gear ring, in such a way that the springs exert a readjusting force on the elevations, upon turning of gear ring and hub against one another, as soon as a stop of the hub moves out of a mid-position between two adjacent stops on the gear ring. 
     In a further especially preferred embodiment, there is arranged between the crankshaft timing gear and the camshaft gear at least one equalizing intermediate gear, whose axis of rotation is freely movable with regard to the crank housing. In this manner, the movable equalizing intermediate gear compensates for varying spacing of axes of rotation between two adjacent gears, by a corresponding relative motion of the axis of the equalizing intermediate gear, so that no clearance irregularity appears even in case of varying spacing of the axes of the adjacent gears. This allows control times to be maintained more accurately by the camshaft and provides quieter operation of the spur gear camshaft drive. 
     In a preferred embodiment, the axis of rotation of the equalizing intermediate gear is held capable of swiveling by at least two brackets, each of which is supported capable of swiveling at ends facing the axis of the equalizing intermediate gear on fixed axes of gears adjacent to the equalizing intermediate gear. An equalization of clearance directly on the camshaft is obtained, in that the equalizing intermediate gear is arranged between the camshaft gear and a last intermediate gear of the spur gear camshaft drive. Because the entire clearance of the spur gear camshaft drive accumulates at this point, maximum clearance equalization is thereby obtained. 
     In another preferred embodiment, a transmission ratio of the crankshaft to the camshaft of 2:1 is advantageously obtained by ensuring that the ratio of the number of gear teeth between the equalizing intermediate gear and an adjacent intermediate gear is appropriately selected. Here, the adjacent intermediate gear is, for example, a crankshaft-side intermediate gear. 
     The invention is described in detail below by reference to the accompanying drawings of systems, which illustrate preferred embodiments of the present invention and serve to explain the principles of the present invention. The drawings are incorporated and constitute part of this disclosure. The scope of the invention is pointed out in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a preferred embodiment of a spur gear camshaft drive according to the present invention, in front view. 
     FIG. 2 shows a sectional view along line I—I of FIG.  1 . 
     FIG. 3 shows an exploded view of the embodiment illustrated in FIG.  1 . 
     FIG. 4 shows an exploded view of the embodiment illustrated in FIG. 1 in the region of the camshaft gear. 
     FIG. 5 shows an alternative embodiment of a camshaft gear in rear view. 
     FIG. 6 is a sectional view along line A—A of FIG.  5 . 
     FIG. 7 is a sectional view along line B—B of FIG.  6 . 
     FIG. 8 is an exploded view of the embodiment illustrated in FIG.  5 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiment represented in FIGS. 1 to  4  of a spur gear camshaft drive according to the invention in a crank housing  10  of an internal combustion engine, comprises a crankshaft timing gear  12 , a first intermediate gear  14 , a second intermediate gear  16 , an equalizing intermediate gear  18  and a camshaft gear  20 . 
     The second intermediate gear  16  meshes with a drive gear  22  for a water pump, not shown. The crankshaft timing gear  12  in addition meshes with a drive gear  24  for an oil pump, not shown, as well as with a third intermediate gear  26 . The third intermediate gear  26  in turn meshes with a fourth intermediate gear  28 , which meshes with a gear  30  to drive a generator, not shown, and a drive gear  32  of a hydraulic pump, not shown. The second intermediate gear  16  is connected, fixed against rotation, with another gear ring  34 , while the second intermediate gear  16  meshes with the first intermediate gear  14  and the additional gear ring  34  meshes with the equalizing intermediate gear  18 . At the same time, a tooth number of the additional gear ring  34  and a tooth number of the equalizing intermediate gear  18  are selected in such a way that a transmission ratio of the crankshaft timing gear  12  to the camshaft gear  20  of 2:1 is obtained. 
     An axis of rotation  36  of the equalizing intermediate gear  18  is freely movable with regard to the crank housing  10  and a cylinder head and is predetermined only with regard to the axes of rotation  42 ,  44  of the second intermediate gear  16  and camshaft gear  20  by two pairs of brackets  38  and  40 . One side of the brackets  38  keeps the axis  36  of the equalizing intermediate gear  18  rotatable, while an opposite side of the brackets  38  is supported on the axis  42  of the second intermediate gear  16 . One side of the brackets  40  likewise keeps the axis  36  of the equalizing intermediate gear  18  rotatable, while an opposite side of the brackets  40  is supported in the cylinder head housing via the axis of rotation  44  of the camshaft gear  20 . 
     In this way, a spacing between the axis of rotation  36  and the axes  42  and  44  is established. When there is a varying distance between the axes  42  and  44 , the brackets  38  and  40  can enclose a varying angle to one another, so that the equalizing intermediate gear  18  always meshes tightly and substantially without clearance irregularities with the gear ring  34  as well as with the camshaft gear  20 . The brackets  38  and  40  thereby form a joint with the articulated shaft on the axis of rotation  36  of the equalizing intermediate gear  18 , which, corresponding to varying distances between the axes of rotation  42  and  44 , expands comparatively due to thermal expansion. Wobbling of the equalizing intermediate gear  18  is prevented by means of plate springs  46 . 
     FIG. 4 shows a region of the spur gear camshaft drive, covered in FIG. 1, in the region of the camshaft gear  20 . The bracket pair  40  is supported in the cylinder head housing via the shaft  44  of the camshaft gear  20 , whereas each of the ends  49  of the brackets  40  facing the camshaft gear  20  hold the shaft  36  of the equalizing intermediate gear  18 , not shown in FIG.  4 . The camshaft arrangement, which in addition comprises a connecting disk  52 , is bolted together by means of a bolt  48  and a sleeve  50 . 
     As can be seen in FIG. 4, the camshaft gear  20  comprises a gear ring  56  and a hub  54 , which are designed capable of turning against one another. For this purpose, there are provided in the hub  54 , in corresponding recesses not visible in FIG. 4, U-shaped spring elements  58 , only one of which, for reasons of clarity, is represented in FIG.  4 . These spring elements  58  are arranged uniformly distributed in the hub in the peripheral direction. A holding element  60  is capable of being slipped onto the hub  54  on the spring side and holds the spring elements  58  in the recesses of the hub  54 . 
     The spring elements  58  are supported by the recesses of the hub  54  at one end and the other side of the spring ends project into regions at an inner periphery of the gear ring  56 , in which elevations  62  are provided. The arrangement is designed in such a way that the gear ring  56  can turn against the hub  54 , while the elevations  62 , upon turning, strike the ends of the spring elements  58 . In this way, the spring elements  58  exert a readjusting force on the elevations  62  and hence on the gear ring  56 . 
     For limiting the turning angle between gear ring  56  and hub  54 , stops  64 , which cooperate with corresponding stops, not visible in FIG. 4, on the hub  54 , are provided on the gear ring  56 . The stops  64  of the gear ring  56 , as well as the stops of the hub  54 , are spaced uniformly apart in the peripheral direction and are arranged staggered. If a torque acting on the hub exceeds a value predetermined by the design of the spring elements  58 , for example if the torque exceeds 80 Nm, the elevations  62  of the gear ring  56  strike the elevations of the hub  54 , so that direct transmission of the torque takes place. In this way, a direct force-locking connection of the spur gear camshaft drive is ensured in phases in which the camshaft actuates valves or a fuel injection pump of a pump-nozzle element of the internal combustion engine. After actuation of a fuel-injection pump of a pump-nozzle element by the camshaft, a counterforce acting on the camshaft suddenly subsides, so that the camshaft enters into secondary vibration actions. These are damped by reciprocal turning of the gear ring  56  and hub  54  under the effect of the spring element  58  and rapidly reduced, and thus are eliminated from the spur gear camshaft drive and are not transmitted to the crankshaft. 
     FIGS. 5 to  8  show an alternative embodiment according to the present invention utilizing a camshaft gear  120  with a gear ring  122  and a hub  124 , which, under the effect of spring elements  126  (FIG.  7 ), are capable of being turned against one another in a damped vibration. The spring elements  126  are designed U-shaped and are arranged in recesses  128  (FIG. 7) in such a way that respective ends of the spring elements  126  radially overlap elevations  130  provided on an inner periphery of the gear ring  122  (FIG.  8 ). In this manner the spring elements  126 , upon a twist of the hub  124  against the gear ring  122 , exert a readjusting force on the elevations  130  and hence on the gear ring  122 . Here, stops  132  on the hub  124  and stops  134  on the gear ring  122  are designed staggered, and cooperate in such a way that the angle of twist between hub  124  and gear ring  122  is limited (FIG.  8 ). 
     For closing off the recesses  128  and for holding the spring element  126  located in the latter, an annular cover  136  is provided which is capable of being fastened to the hub  124  by means of bolts  138 . The mode of operation of this alternative embodiment corresponds to the mode of operation of the preceding embodiment, as explained above with reference to FIG.  4 . 
     According to the invention, the arrangement corresponds to a mechanical low-pass filter. High-frequency vibrations with relatively low torque are absorbed, low-frequency vibrations with high torque, as for example in the extreme case of a force transmission for actuation of a pump-nozzle element with zero frequency, are undamped and transmitted directly. 
     Although in the preceding examples, ability to turn between gear ring  56  and  122  and hub  54  and  124  is described only for the camshaft gear  20  and  120 , in alternative embodiments, it is also possible to provide any other desired gear of the spur gear camshaft drive with such functionality. In particular, it is possible to provide a plurality or all gears of the spur gear camshaft drive with this turning function and so to make them pulse-absorbing. Various other modifications and alterations to the described embodiments will be apparent to those skilled in the art in view of the teachings herein. It will thus be fully appreciated that those skilled in the art will be able to devise numerous systems and methods which, although not explicitly shown or described, embody the principles of the invention and are thus within the spirit and scope of the invention. As noted the scope of the invention is defined only by the appended claims.