Abstract:
A mechanical power-transmission system with chain comprising a mechanical tightener acting on a branch ( 11   a ) of said chain ( 11 ) and comprising in turn a base ( 13 ), a tightening shoe ( 14 ) in dragging contact with the chain ( 11 ) and a sliding wedge ( 16 ) under the thrust of a first spring ( 30 ) in an irreversible manner between the base ( 13 ) and the shoe ( 14 ) to cause progressive withdrawal of the base ( 13 ) and the shoe ( 14 ) in the tightening direction of the chain ( 11 ) characterized in that between said wedge ( 16 ) and said shoe ( 14 ) there is an elastic thrust member ( 17 ) for thrusting the shoe ( 14 ) in the tightening direction.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a chain power-transmission system with mechanical tightener.  
         [0003]     2. State of the Prior Art  
         [0004]     In engineering applications calling for mechanical power transmission through a chain the problem of maintaining the correct chain tightening level over time arises. Indeed, the transmission chain tends to wear in the course of its useful life and to lengthen little by little. In addition, in the presence of mechanical systems which in addition to the changes in transmitted torque call for its reversal (as for example motorcycle primary transmission systems) the taut and slack chain branches are mutually reversed with each torque reversal (for example when changing from the motor pulling phase to the motor braking phase) and in addition chain length changes elastically in a reversible manner dependent on the torque transmitted.  
         [0005]     Under these working conditions of the chain the tightener is required to perform various functions in order to make the chain work efficiently at all times and in particular: 
        it must provide for correctly tightening the normally slack branch of the chain in order to reduce its vibrations, flapping, noise and wear,     it must provide for recovering the irreversible lengthening of the chain due to its natural wear in time,     it must provide for recovering the reversible elastic lengthening of the chain with the changes in tension to which it is subjected in operation, and     it must be capable of opposing movements or shifting which the normally slack branch of the chain would do upon reversal of the torque transmitted while withstanding the high stresses exerted by the chain thereon.        
 
         [0010]     A transmission chain not correctly tightened and excessively slack produces high noise of the system and continuous flapping of the chain and these phenomena can cause considerable distress to users of the machine and considerable shortening of the useful life of the whole transmission system.  
         [0011]     To solve the problem of chain tightening, one of the systems used is the tightener with articulated kinematic parallelogram mechanism. These systems typically comprise a tightening shoe connected to a support base by means of a connecting rod. The shoe is in sliding contact with a branch of the transmission chain and is thrust progressively against it to keep it in tension when the chain wears and becomes gradually slacker. The prior art quadrilateral tightening systems, while allowing recovery of a considerable chain lengthening, are not suited to use with transmission systems calling for reversal of the transmitted torque as for example motorcycle transmission systems. Indeed, in quadrilateral tighteners, the tightening shoe is thrust by a spring towards the chain to tighten it but is not capable of opposing chain movement when a transmitted torque reversal occurs.  
         [0012]     To solve the problems deriving from reversal of the torque transmitted by the chain, systems comprising tightening shoes hinged at one end (traveling circular trajectories) or prismatically guided shoes (traveling rectilinear trajectories) having various nonreturn systems and thrust against the chain with spring or hydraulic action have been designed for example for motor distribution chains. But these systems allow compensation for small chain-lengthenings and opposing rather small torque reversals. They could not for example meet the requirements of a primary transmission for motorcycle use where it is necessary to be able to recover the considerable lengthenings of the chain due to wear, excessive elastic lengthenings thereof with changes in pull, and very high torque reversals when the motor changes from pulling to braking condition.  
         [0013]     The general purpose of the present invention is to remedy the above-mentioned shortcomings by making available a power-transmission chain tightening system usable in mechanical systems calling for reversal of high transmitted torques and occupying little space and ensuring adequate and correct chain tightening level at any motor speed condition even when it is very worn and lengthened.  
       SUMMARY OF THE INVENTION  
       [0014]     In view of this purpose it was sought to provide in accordance with the present invention a mechanical-power transmission-chain system comprising a mechanical tightener acting on a branch of said chain and comprising in turn a base, a tightening shoe in dragging contact with the chain, and a sliding wedge under the thrust of a first spring in an irreversible manner between the base and the shoe to cause progressive withdrawal of the base and the shoe in the chain tightening direction characterized in that between said wedge and said shoe there is an elastic thrust member for thrusting the shoe in the tightening direction. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     To clarify the explanation of the innovative principles of the present invention and its advantages compared with the prior art there is described below with the aid of the annexed drawings a possible embodiment thereof by way of non-limiting example applying said principles. In the drawings:  
         [0016]      FIG. 1  shows a chain transmission system having a mechanical tightener in accordance with the present invention,  
         [0017]      FIG. 2  shows the mechanical tightener of  FIG. 1  in raised position,  
         [0018]      FIG. 3  shows the mechanical tightener of  FIG. 1  with the upper shoe removed,  
         [0019]      FIG. 4  shows a cross section of the mechanical tightener of  FIG. 1 , and  
         [0020]      FIG. 5  shows another cross section of the mechanical tightener of  FIG. 1 .  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]      FIG. 1  shows the housing  10  of the mechanical transmission system, the transmission chain  11  (diagrammatically) and a mechanical tightener  12  in accordance with the present invention. The chain  11  could be for example the primary transmission chain in a motorcycle. In this case, the mechanical torque transmitted by the chain in addition to being very high can reverse its direction. Indeed, every time there is change from the condition of pulling motor to that of braking motor or vice versa there is reversal of the torque transmitted by the chain  11  and consequently the slack branch and the taut branch of the chain are reversed. The mechanical tightener  12  is arranged between an inner wall of the housing  10  and a branch  11   a  (the one normally slack) of the chain  11  in order to keep it at the correct level of tightness during the entire life of the chain  11 .  FIG. 2  shows in detail the mechanical tightener  12 . It includes a supporting base  13 , a tightening shoe  14  and an operating wedge  16 . The supporting base  13  is fastened to the housing  10  of the mechanical transmission system and the tightening shoe  14  is connected to it by means of a parallelogram-type kinematic connection with four connecting rods  15  which allows movement of the shoe  14  while holding constant its orientation in space. Said parallelogram connection is prior art and not further described herein. The supporting base  13  also has a knurled and inclined upper face  19  with an indentation suited to engaging with the lower face of the operating wedge  16  as described below. At one end of the base  13  is hinged the supporting rod  20  by means of the horizontal-axis hinging  21 . The operating wedge  16  has a knurled lower face which engages as above-mentioned the knurled face  19  of the supporting base  13 . The particular inclination of the indentation of the two knurled faces allows rising movement of the operating wedge  16  but not descent. The wedge  16  is traversed by the supporting rod  20  and can run relatively thereto. Between the wedge  16  and a purposeful striker made on the rod  20  is mounted a spring  30  which pushes the wedge  16  upward on the inclined face  19  of the supporting base  13 . The tightening shoe  14  rests on the operating wedge  16  with its lower face in such a manner as to be able to support high stresses. But with its upper face  14   a  it is in dragging contact with the branch  11   a  of the transmission chain  11  and exerts a force thereon to keep it at the correct tightening level. The upper face  14   a  is convex in order to optimize the coupling and reduce wear to a minimum between the shoe and the chain.  
         [0022]     A thrust lever  17  is hinged with horizontal-axis hinging  18  to the operating wedge  16  as shown in  FIG. 3 . Said lever  17  is thrust to withdraw from the wedge  16  by the action of two springs  31  (only one is shown in  FIG. 5 ) installed inside operating wedge  16 . On the wedge  16  is also made a striker  23  imposing an upper limit to the possible rotations of the lever  17  around the hinging  18 . When the springs  31  stretch to withdraw the lever  17  from the wedge  16  the tightening shoe  14  rises from the wedge  16  going to rest with the lever  17 . The elastic force exerted by the springs  31  on the operating wedge  16  is such that the spring  30  can move the wedge  16  to rise on the knurled face  19  of the base  13  only when the springs  31  have reached their maximum possible extension having thrust the lever  17  to the maximum rotation allowed by the striker  23 .  
         [0023]     It should be noted that the inclination of the connecting rods  15  and the inclination of the knurled face  19  of the supporting base  13  are such that the horizontal movements of the tightening shoe  14  and the operating wedge  16  are in the same direction when the wedge rises by one tooth on the knurled face  19  of the supporting base  13 . This stratagem allows keeping an ample support surface between the wedge  16  and the shoe  14  under all operating conditions of the mechanical tightener  12  even when the chain is very worn and the tightening shoe  14  much raised. It should also be noted that the thrust lever  17  is hinged to the wedge  16  in such a manner that its direction of rotation is the same as that of the connecting rod  15  in any situation. The latter stratagem together with the preceding one allows reducing to the minimum the mechanical stresses to which the tightener  12  is subjected in general and the connecting rods  15  in particular. Indeed, sliding friction is reduced to a minimum between the various members of the tightener  12  and the internal tensions which develop at the hinging points of the connecting rod are limited.  
         [0024]     We shall now describe briefly the operation of the mechanical tightener above-mentioned.  
         [0025]     When the transmission chain  11  is new and is not transmitting torque (for example with the motorcycle shut-down) the mechanical tightener  12  is in the lower rest configuration as shown in  FIG. 1 . The chain is kept at the correct level of tightening by the spring  30  which is in maximum compression condition and by the springs  31  and the mechanical tightener  12 .  
         [0026]     When a load is imposed such as to tighten the branch  11   b  and slacken the branch  11   a  (for example starting the motorcycle and accelerating it) the chain lengthens elastically and the mechanical tightener  12  enters into action. Since the elastic force component of the springs  31  in the sliding direction of the wedge  16  is greater than that of the spring  30 , the first member to move between the lever  17  and the wedge  16  is the thrust lever  17 . The latter, rising from the operating wedge  16 , pushes the tightening shoe  14  towards the slack branch  11   a  of the chain  11  which is returned to the correct level of tightening. The sizing of the lever  17  and the angle of maximum rotation granted by the striker  23  are such as to tension the chain  11  without need for irreversible movement of the operating wedge  16  rising on the knurled face  19  of the support base  13 . When the mechanical pull on the chain ceases, the chain shortens elastically and tends to push downward the shoe which can retreat thanks to the sprung movement of the lever  17  without uselessly keeping the chain tightened as would happen if there were only the nonreturn system. But when the pull of the chain is reversed (for example when starting or changing to the braking motor phase) the branch  11   a  tends to be no longer slack so as to push the tightening shoe  14  downward, compress the springs  31  and seek to cause backing of the wedge which however is held in position by the knurled face  19 .  
         [0027]     When in the course of time the chain tends to wear and lengthen, the action of the thrust lever  17  can no longer take the chain to the correct level of tightening under all operation conditions. At this point the operating wedge  16  will trip by one tooth on the knurling but without causing any excessive tightening problem. Once the wedge  16  has tripped and risen one tooth, the thrust lever  17  can again begin to tighten the chain  11  while avoiding further movements of the operating wedge  16  until the next tripping on the knurling.  
         [0028]     In this manner, the wear and resulting lengthening of the chain are compensated for by the slow and irreversible advance of the operating wedge  16  on the knurled face  19  of the support base  13 . Differently, the reversible changes of length due to the rapid and oscillating course of the tightening level due to torque changes is compensated for by the reversible movement of the thrust lever  17 .  
         [0029]     Thus with the present invention a mechanical tightener with limited space occupied and great tightenability is realized. Indeed, the tightener in accordance with the present invention has an external structure quite similar to that of the quadrilateral tighteners which typically occupy little space for their ability to compensate for even considerable chain lengthening. In addition, thanks to the insertion of the thrust lever, the problem of over-tightening which typically appears in those mechanical applications calling for considerable changes in chain pull is solved while reversal of the torques transmitted by the chain is made possible by the irreversible-movement wedge. Accordingly, it will be possible to use this tightener even in these applications.  
         [0030]     Naturally the above description of an embodiment applying the innovative principles of the present invention is given by way of non-limiting example of said principles within the scope of the exclusive right claimed here.