BACKGROUND OF THE INVENTION 
     (a) Field of the Invention 
     The present invention relates to a timing chain system, and in particular, to an improved sprocket assembly for an automotive timing chain system capable of reducing noise. 
     (b) Description of the Related Art 
     As well known, internal combustion engines employ poppet valves that are operated by a camshaft that is driven by a crankshaft, with a suitable timing mechanism, and timing chains are used to transmit power and motion between the camshaft and the crankshaft. These chains are formed of ranks of links interleaved with adjacent ranks of links and having aligned apertures for receiving pivot pins to join the ranks and provide articulation of the chain as it passes around the driving and driven sprockets. 
     FIG. 1 shows a conventional internal combustion engine embodying a timing chain and sprocket system. 
     As shown in FIG. 1, a driving sprocket  51  on the crankshaft  60  and the driven sprocket  52  on the camshaft  61  are connected to each other by a timing chain  53 . The sprockets  51  and  52  have a plurality of teeth  62  equally spaced around the circumference of the driving and driven sprockets  51  and  52 , and the timing chain  53  consists of pairs of inside links  54  alternating with pairs of flanking guide links  65  joined by pivot pins  55  insulated by rollers  56  such that the chain  53  engages the driving and driven sprockets  51  and  52 . 
     The tension of the timing chain  53  is adjusted by a damper  58  and a tension device  59  mounted to an engine block. 
     In this configuration, the engine torque is transmitted to the drive sprocket  52  via the crankshaft  60  and consequently to the driven sprocket  52  on the camshaft  61  by the chain  53  such that the camshaft  61  rotates so as to operate a valve system. 
     FIG. 2 is an enlarged view of a camshaft sprocket engaged with the chain. As shown in FIG. 2, the chain  53  wraps on the sprocket  52  in a way that the pivot pins  55  having rollers  56  are inserted between the teeth  62  such that the teeth on the sprocket act to transfer a load from the chain  53 . 
     In this timing chain system, however, the friction surface between the rollers  56  insulating the pivot pins  55  and the driven sprocket  52  is large so backlash is generated, and in particular there exists a gap “C” between the pivot pin  55  and the roller  56  such that vibration noise is caused by engine resonance. Also, the rollers  56  generate noise when they contact the sprocket  52 . 
     SUMMARY OF THE INVENTION 
     The present invention has been made in an effort to solve the above problems of the prior art. 
     It is an object of the present invention to provide an improved sprocket for a timing chain system allowing a reduction in backlash between the chain and the sprocket by integrally forming an auxiliary sprocket with the driving and driven sprockets, resulting in a reduction of noise. 
     It is another object of the present invention to provide an improved sprocket for a timing chain system capable of preventing vibration noise from between the pivot pin and the roller from being generated by integrally forming an auxiliary sprocket with the driving and driven sprockets. 
     To achieve the above object, a sprocket assembly for an automotive timing chain system comprises a main sprocket, an auxiliary sprocket mounted onto one side of the main sprocket for assisting the main sprocket, and an elastic member elastically coupling the auxiliary sprocket to the main sprocket for generating an elastic force between the main and auxiliary sprockets. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention, and, together with the description, serve to explain the principles of the invention: 
     FIG. 1 is a front view showing an engine equipped with a prior art timing chain system; 
     FIG. 2 is a partially enlarged view showing a driven sprocket engaged with a chain of the timing chain system of FIG. 1; 
     FIG. 3 is an exploded perspective view showing a sprocket according to the preferred embodiment of the present invention; 
     FIG. 4 is an assembled perspective view showing the sprocket of FIG.  3 ; 
     FIG. 5 is a cross sectional view cut along the line V—V of FIG. 4; and 
     FIG. 6 is an operational view showing a driven sprocket engaged with the chain according to the preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A preferred embodiment of the present invention will be described hereinafter with reference to the accompanying drawings. 
     FIG.  3  and FIG. 4 respectively show an exploded and an assembled driven sprocket according to the preferred embodiment of the present invention. FIG. 5 is a cross sectional view cut along the line V—V of FIG.  4 . Since the sprocket assembly of the present invention can be adapted to the conventional chain system, the prior art reference numerals will be used for the same elements. 
     As shown in FIG.  3  and FIG. 4, the sprocket assembly  1  comprises a main sprocket  2 , an auxiliary sprocket  4  mounted on one side of the main sprocket  2 , and a ring spring  6  installed between the main sprocket  2  and the auxiliary sprocket  4  for elastically connecting the main and the auxiliary sprockets  2  and  4 . 
     The main sprocket  2  is provided with a hub  7  that extends on either side of a central portion thereof. The hub is formed having first and second extenders  8  and  9  in which an exterior diameter of the first extender  8  is smaller than that of the second extender  9 , and an opening for fixedly receiving a camshaft (not shown).  4 ) 
     The auxiliary sprocket  4  is provided with a flanged opening at its central portion so as to be mounted on the first extender  8  of the hub  7 , and it has the same tooth profile as the main sprocket  2 . 
     In addition, the main and auxiliary sprockets  2  and  4  are each provided with a pin  16  fixed on the surface facing the other sprocket. 
     The ring spring  6  is designed as an open ring with pin holes  12  and  14  in free ends thereof such that the ring spring  6  is installed between the main sprocket  2  and the auxiliary sprocket  4  by inserting the respective pins  16  into the pin holes  12  and  14 . 
     After the auxiliary sprocket  4  is coupled with the main sprocket  2 , a fixing member  10  is screwed on the extender  8  and a fixing ring  18  is then fixed thereon. 
     The sprockets  2  and  4  can turn relative to each other by as much as one sprocket tooth when not engaged with the chain, such that the ring spring  6  connecting the main and auxiliary sprocket  2  and  4  to each other extends so as to cause an elastic force therebetween when they are engaged with the chain. 
     The operation of the sprocket assembly according to the preferred embodiment of the present invention will be described hereinafter with reference to FIG.  6 . 
     FIG. 6 is an operational view showing a driven sprocket engaged with the chain. 
     As shown in FIG. 6, the main and auxiliary sprockets  2  and  4  are rotationally coupled to each other by means of the ring spring  6  (see FIG.  5 ), and while engaged with the chain  53  with a one sprocket tooth rotational preload the auxiliary sprocket  4  rotates relative to the main sprocket  2  such that the main and auxiliary sprockets  2  and  4  generate a displacement to each other by the elastic force of the ring spring  6  to the limitation of two neighboring pivot pins  55  of the chain  53 . Accordingly, a main sprocket tooth flank  11   a  and the auxiliary tooth flank  13   a  contact opposite sides of the rollers  56  of the chain  53  such that the rollers  56  are fitted between the main sprocket teeth  11  and the auxiliary sprocket teeth  13  of FIG.  3 . 
     That is, the chain  53  starts engaging with the sprocket assembly  1  by the back flank of the auxiliary sprocket tooth  13  contacting the roller  56  of the chain  53  at the engaging portion P 1 , and it disengages from the sprocket assembly  1  by the front flank of the main sprocket tooth  11  breaking contact with the roller  56  of the chain at the disengaging portion P 2 . 
     As a result, the vibration caused by the gap C between the pivot pin  55  and the roller  56  surrounding it is absorbed by the sprocket assembly  1  while fitting the rollers  56  to the main and auxiliary sprocket teeth  11  and  13 . 
     Also, at a completely engaged portion P 3 , backlash between the main sprocket  2  and the roller  56  is avoided because the auxiliary sprocket tooth  13  supports the roller  56  such that the freeplay between the roller  56  and the sprocket assembly  1  is absorbed and metallic noise caused by the freeplay is obviated. At the same time, the vibration noise caused by the gap C between the roller  56  and the pivot pin  55  is also prevented because the auxiliary sprocket tooth  13  pushes the roller against the pivot pin  55 . 
     Although the sprocket assembly of the present invention has been described in use on the camshaft, it can be adapted to a driving sprocket on a crankshaft as well. 
     As described above, in the sprocket assembly according to the preferred embodiment of the present invention, the auxiliary sprocket is elastically connected to the main sprocket by means of the ring spring, which allows the main and auxiliary sprockets to turn relative to each other such that the displacement between the main sprocket tooth and the auxiliary sprocket tooth caused by the elastic force of the ring spring avoids generation of backlash, and results in avoidance of freeplay noise generated when the roller contacts the sprocket tooth, particularly the tooth flank. Furthermore, since the auxiliary sprocket tooth pushes the roller against the pivot pin, vibration noise caused by the gap C between the roller and the chain is also prevented. 
     While this invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
A sprocket assembly for an automotive timing chain system includes a main sprocket, an auxiliary sprocket mounted onto one side of the main sprocket for assisting the main sprocket, and an elastic member elastically coupling the auxiliary sprocket to the main sprocket for generating an elastic force between the main and auxiliary sprockets.