Patent Publication Number: US-2010122678-A1

Title: Valve driving apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit under 35 U.S.C. §119 of Korean Patent Application No. 10-2008-0113562, filed Nov. 14, 2008, which is hereby incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a valve driving apparatus and, more particularly, to a valve driving apparatus that is capable of continuously adjusting lift amounts of valves opening and closing a combustion chamber of an internal combustion engine with a double-cam unit to provide optimum fuel economy and output. 
     2. Description of the Related Art 
     In general, an internal combustion engine of vehicles includes a combustion chamber in which fuel is burned to generate power. The combustion chamber is provided with a valve train, which includes intake/exhaust valves controlling the flow of intake and exhaust gases and is associated with a crankshaft to open and close the combustion chamber. 
     In a typical valve train, the valve is open in a constant lift amount by a cam having a predetermined configuration, so that the amount of intake or exhaust gas is restricted to a constant amount. Thus, with a design contemplated for low speed driving conditions, time and degree of valve opening are insufficient in high-speed drive conditions, and with a design contemplated for the high speed drive conditions, a reverse phenomenon occurs in the low speed drive conditions. 
     More specifically, since a general internal combustion engine tuned for high speed driving is set to have a high valve lift value, it exhibits good performance under high-speed conditions, but is very disadvantageous in terms of idle stability and low-speed torque characteristics under low speed conditions. On the contrary, the engine tuned for low speed driving provides good idle stability and low-speed torque under low-speed conditions but exhibits limited performance under high-speed conditions. However, a variable valve lift technique varies the lift amount of the valve corresponding to the high-speed and low-speed conditions, thereby providing merits both under low-speed and high-speed conditions. 
     Accordingly, it has been attempted in recent years to develop a technique for increasing a charging efficiency in addition to a multi-valve technique to enhance fuel economy and output. As a result, various techniques have been developed, for example a variable induction system (VIS), a variable valve timing (VVT) technique, and a variable valve lift (VVL) technique. The variable induction system changes a length or cross-sectional area of an intake manifold in accordance with suction resistance of air that varies in accordance with an engine speed. In the variable valve timing technique and the variable valve lift technique, the time and degree of opening the valve are adjusted in accordance with an engine speed to control overlap timing, thereby controlling the cylinder charging amount and the remaining gas amount. 
     In a conventional variable valve lift-type valve driving apparatus, however, since the lift amount of the valve cannot be varied and time of opening or closing the intake or exhaust valve is fixed in a constant state, the amount of intake or exhaust air cannot be adjusted, so that optimum fuel economy and output cannot be obtained corresponding to an engine operation state. Therefore, there is a need to provide a valve driving apparatus that overcomes such problems. 
     BRIEF SUMMARY 
     The present invention is conceived to solve the above problems of the related art, and an aspect of the invention is to provide a valve driving apparatus that is capable of precisely adjusting lift amounts of valves periodically opening and closing a combustion chamber of an internal combustion engine with a double-cam unit associated therewith in order to achieve optimum fuel economy and output. 
     According to an aspect of the invention, a valve driving apparatus includes: at least one swing arm having one side corresponding to an upper side of a valve opening and closing a combustion chamber of an internal combustion engine; and a cam unit periodically pivoting the swing arm. Here, the swing arm performs a swing motion on the other side thereof by external manipulation to periodically compress the valve. 
     The cam unit may include: a pivot member acting as a reference axis of the cam unit; a web member formed on the pivot member; a high rotary shaft forcibly rotated by an external force; a high lift cam formed on the high rotary shaft and including a high eccentric rod eccentrically rotated during rotation of the high rotary shaft; and a sub-cam unit connected to the web member and simultaneously operating one or more swing arms through the high eccentric rod during rotation of the high lift cam. 
     The sub-cam unit may include: a low rotary shaft rotatably connected to the web member; a cam body formed on the low rotary shaft; a follow arm formed on the cam body to be rotated in a direction of the swing arm by periodic compression of the high eccentric rod; a low lift cam formed on the low rotary shaft and including a low eccentric rod to periodically compress one side of each of the swing arms at the same time during rotation of the follow arm; and an elastic member elastically supporting the cam body. 
     The cam body may include the same number of low lift cams at opposite sides of the follow arm. The follow arm may be provided with a rotatable follow roller at a portion thereof where the follower arm directly contacts the high lift cam. The low lift cam may include a contact surface formed to support a center of each of the swing arm. The swing arm may include a rotatable swing roller disposed at a portion thereof contacting the contact surface. 
     The follow arm and the low lift cam may be extended from the cam body in different directions from each other. 
     The web member may be rotatably connected to the link shaft, and the link shaft may be connected to an actuation unit. 
     The actuation unit may include: a connecting rod connected to the link shaft; a link pin linked to the connecting rod; a screw member linked to the link pin and linearly reciprocating in a perpendicular direction to an axis of the link shaft to reciprocate the link shaft on the arcuate track; and a power transmission member formed around a circumference of the screw member and reciprocating the screw member in a forward and rearward direction when the power transmission member is forcibly rotated by an external force. 
     The actuation unit may include: an actuator having a receiving groove that receives the link shaft therein; and a screw member screwed to the actuator and linearly reciprocating the actuator in a forward and rearward direction to guide reciprocation of the link shaft on an arcuate track when the screw member is forcibly rotated by an external force. 
     The elastic member may be supported at one side thereof by the link shaft and at the other side thereof by the cam body. The sub-cam unit may be received in a housing, and the sub-cam unit and the housing may include oil passages connected to each other between the sub-cam unit and the housing to receive a lubricant from outside through the oil passages. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features and advantages of the invention will become apparent from the following detailed description given in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a side section view of a valve driving apparatus mounted in a housing in accordance with one embodiment of the present invention; 
         FIGS. 2 and 3  are perspective views of connection between components of the valve driving apparatus in accordance with the embodiment of the present invention; 
         FIG. 4  is an exploded perspective view of the valve driving apparatus in accordance with the embodiment of the present invention; 
         FIGS. 5 to 8  are operational diagrams of the valve driving apparatus in accordance with the embodiment of the present invention; 
         FIG. 9  is a perspective view of a valve driving apparatus in accordance with another embodiment of the present invention; and 
         FIG. 10  shows oil passages formed in a valve driving apparatus in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the invention will now be described in detail with reference to the accompanying drawings. It should be noted that the drawings are not to precise scale and may be exaggerated in thickness of lines or sizes of components for descriptive convenience and clarity only. Furthermore, the terms as used herein are defined by taking functions of the invention into account and can be changed according to the custom or intention of users or operators. Therefore, definition of the terms should be made according to the overall disclosures set forth herein. 
     Referring to  FIGS. 1 to 4 , a valve driving apparatus according to one embodiment of the invention includes swing arms  100  and a cam unit  300 , which can variably control lift amounts of valves  20  opening and closing a combustion chamber  10  in an internal combustion engine. 
     Each of the swing arms  100  has one side corresponding to an upper side of the valve  20 , which opens and closes the combustion chamber  10 , and performs a swing motion on the other side thereof by external manipulation to directly compress the valve  20  in a periodic manner. The swing arm  100  generates a constant profile of oscillation to allow the valve  20  to reciprocate in an up-down direction in a constant period. Accordingly, as the swing arm  100  rotates about the other side thereof to allow the one side thereof to periodically compress an upper side of the valve  20 , the valve  20  can be open or closed, that is, reciprocate in the up-down direction. 
     Here, the swing arm  100  is provided at the one side thereof with the valve  20  to face a lower surface of the one side in the longitudinal direction of the swing arm  100 , and at the other side thereof with a stationary support member  200  to face a lower surface of the other side thereof. Thus, the other side of the swing arm  100  is supported by a support member  200  such that the swing arm  100  can rotate about the other side thereof. 
     As such, the lower surface of the one side of the swing arm  100  comes into contact with the valve  20  and the lower surface of the other side of the swing arm  100  is in contact with the support member  200 , so that the support member  200  continues to exert an upward force to the lower surface of the other side of the swing arm  100 . Accordingly, the swing arm  100  is capable of rotating about the other side center P 1  supported by the support member  200 . In other words, an imaginary center line L 1  connecting the other side center P 1  with one side center P 2  of the swing arm  100  is reciprocatively pivoted about the other side center P 1  in a predetermined range of pivot angles. 
     Here, the support member  200  is securely mounted on a housing  30 . A common structure may be used to mount the support member  200  on the housing  30 . In addition, since the swing arm  100  is supportively connected to the support member  200  and the valve  20 , the swing arm  100  is not easily vibrated or displaced. Here, the housing  30  is formed at an upper side of the combustion chamber  10 . Various modifications may be made to the swing arm  100  and the support member  200 . 
     As described above, the swing arm  100  is rotated about the other side thereof by an external force. Here, the cam unit  300  serves to periodically pivot the swing arm  100 . The cam unit  300  includes a pivot member  310 , web members  320 , a high rotary shaft  330 , a high lift cam  340 , and a sub-cam unit  350 . 
     The pivot member  310  acts as a reference axis of the cam unit  300 . Thus, the pivot member  310  is secured inside the housing  30 . Here, a single pivot member  310  may be provided to one inner side of the housing  30 , but two pivot members  310  may be provided to opposite inner sides of the housing  30  to firmly support the web members  320 , respectively. In this case, each of the web members  320  is rotatably connected to the corresponding pivot member  310 . In other words, each of the pivot members  310  is rotatably fitted into the corresponding web member  320 , which is rotated about the pivot member  310 . It should be understood that, with the pivot members  310  rotatably inserted into the housing  30 , the web members  320  may be integrally formed with the pivot members  310 . In this embodiment, the pivot member  310  is shown as having a shaft shape, but various modifications may be made to the pivot member  310 . 
     As such, the web member  320  is connected to the corresponding pivot member  310 . In other words, the pivot member  310  is rotatably inserted into a lower side of the web member  320 . Thus, the web member  320  can be rotated about the pivot member  310 . It should be understood that with the web members  320  integrally formed with the corresponding pivot members  310  to rotate together, the pivot members  310  may be rotatably mounted on the housing  30 . In this embodiment, the web member  320  is shown as having a plate shape, but various modifications may be made to the web member  320 . 
     The high rotary shaft  330  is forcibly rotated by an external force. Particularly, the high rotary shaft  330  is supported by the housing  30 . That is, opposite ends of the high rotary shaft  330  are rotatably supported on the opposite sides of the housing  30 , respectively. With this configuration, the high rotary shaft  330  can be firmly supported. 
     The high rotary shaft  330  is provided with the high lift cam  340 . The high lift cam  340  has a high eccentric rod  342  extending from one side thereof. With this configuration, the high lift cam  340  eccentrically rotates. In other words, at any time when the high lift cam  340  is rotated about its rotational axis P 3  to position the high eccentric rod  342  extending from one side thereof at a predetermined location on the circumference thereof, the high eccentric rod  342  compresses the swing arms  100  and follow arms  353  of the cam unit  300  described below, so that the valves  20  opens and closes the combustion chamber  10  while reciprocating in the up-down direction. 
     Here, the high lift cam  340  rotates in the same direction and at the same speed as those of the high rotary shaft  330 . The high lift cam  340  may be separably coupled to the high rotary shaft  330  or may be integrally formed therewith by molding. The separable coupling of the high lift cam  340  to the high rotary shaft  330  may be achieved by bolts and the like, particularly, by diffusion bonding. 
     On the other hand, the sub-cam unit  350  is connected to the web members  320 . The sub-cam unit  350  serves to simultaneously operate one or more swing arms  100  in cooperation with the high eccentric rod  342  during rotation of the high lift cam  340 . 
     The sub-cam unit  350  includes a low rotary shaft  351 , a cam body  352 , the follow arms  353 , low lift cams  355 , and elastic members  358 . 
     The low rotary shaft  351  is secured to the web members  320  by press-fitting or the like. Here, although the low rotary shaft  351  may be rotatably linked to the web members  320 , this configuration requires change of an oil passage  410  for supply of an oil to the cam body  352  and can cause rigidity deterioration of the housing  30 . 
     Here, since a pair of web members  320  is provided to the cam unit  300  in this embodiment, the opposite ends of the low rotary shaft  351  support the corresponding web members  320 , respectively. The low rotary shaft  351  is located below the high rotary shaft  330  inside the housing  30 . 
     The cam body  352  is formed around the circumference of the low rotary shaft  351 . The cam body  352  serves to support the follow arms  353  and the low lift cams  355 . Here, the cam body  352  may be integrally formed with the low rotary shaft  351  or may be separably coupled thereto. Particularly, the cam body  352  may be formed in a ring shape to be inserted and rotatably coupled to the low rotary shaft  351 . With this configuration, when the cam body  352  is rotated, the low rotary shaft  351  is rotated in the same direction and at the same speed as the cam body  352 . The cam body  352  may have a variety of shapes. It should be noted that all of the follow arms  353  and the low lift cams  355  may be integrally formed with the low rotary shaft  351  using a mold. 
     Each of the follow arms  353  formed on the cam body  352  is reciprocatively pivoted in the direction of the swing arm  100  by periodic compression of the high eccentric rod  342 . Here, a rotatable follow roller  354  may be provided to the follow arms  353  to minimize friction on contact points between the follow arms  353  and the high eccentric rod  342 . Here, the sub-cam unit  350  may include a pair of follow arms  353  facing each other such that the follow roller  354  is rotatably interposed between the follow arms  353 . It should be noted that the follow roller  354  may be rotatably mounted inside the follow arms  353  to protrude from upper and lower sides of the follow arms  353 . Additionally, a central axis P 6  of the follow roller  354  rotates along a circular track around a central axis P 4  of the low rotary shaft  351 . 
     The low lift cams  355  formed on the cam body  352  serve to periodically compress the corresponding sides of the swing arms  100  at the same time during rotation of the follow arms  353 . At this time, each of the low lift cams  355  has a low eccentric rod  356  extending from one side thereof. With this configuration, the low lift cams  355  can be eccentrically rotated. 
     In other words, at any time when each of the low lift cams  355  is rotated about its rotational axis P 4  to position the low eccentric rod  356  extending from one side thereof at a predetermined location on the circumference thereof, the low eccentric rod  356  repetitiously compresses the corresponding swing arm  100 , so that the valve  20  opens and closes the combustion chamber  10  while reciprocating in the up-down direction. 
     The follow arms  353  and the low lift cams  355  may be integrally formed with the cam body  352  and extend from the cam body  352  in different directions from each other. Particularly, the low eccentric rod  356  of the low lift cam  355  may be located lower than the follow roller  354  of the follow arm  353  on the cam body  352 . This configuration serves to allow the low eccentric rod  356  to compress the one side of the corresponding swing arm  100  while being minimally pivoted when the follow roller  354  is rotated by compression of the high eccentric rod  342 . 
     The cam body  352  may have the same number of low lift cams  355  at opposite sides of the follower arms  353 . In other words, a plurality of low lift cams  355  is formed on the cam body  352  to compress the corresponding swing arms  100 . At this time, the same number of low lift cams  355  may be formed at the opposite sides of the follower arms  353  to provide at least one pair of low lift cams  355 . If the cam body  352  has a different number of low lift cams  355  at the opposite sides of the follower arms  353 , slightly different rotational moments can be generated to cause unbalanced behavior of the swing arms  100 . For descriptive convenience, the cam body  352  is shown as having a single low lift cam  355  at either side of the follower arms  353  to have a pair of low lift cams  355 . 
     On the other hand, the cam body  352  is resiliently supported by the elastic members  358 . This serves to maintain the follow roller  354  and the high lift cam  340  in close contact with each other during operation and to prevent the inertial force of the cam body  352  from being transferred to the swing arms  100 . 
     If the high lift cam  340  is separated from the follow roller  354  during operation, various problems such as noise and valve jumping can occur. Noise generation can cause fatigue failure resulting from repetitious impact and the higher the speed with which such a phenomenon occurs, the greater the inertial force of the cam body  352 . As a result, as soon as the high lift cam  340  of the high rotary shaft  330  pushes the follow roller  354  of the low lift shaft  351 , a degree of opening the valve  20  is increased over a desired degree by the inertial force of the cam body  352 . Accordingly, it is desirable that the elastic members  358  firmly support the cam body  352 . 
     More specifically, each of the elastic members  358  is coupled to the low rotary shaft  351  to be supported at one side thereof by a link shaft  360  described below and at the other side thereof by the cam body  352 . It should be understood that the opposite sides of the elastic member  358  may be supported at different positions. In this embodiment, the elastic member  358  may be a torsion spring. It should be understood that the invention is not limited thereto and a variety of mechanical elements may be used as the elastic member  358  as long as they can perform the function of the elastic member in this embodiment. Further, a pair of elastic members  358  may be provided to opposite sides of the low rotary shaft  351  to resiliently support the opposite sides of the cam body  352  with the same force. 
     With this configuration, each of the elastic members  358  is supported at one side thereof by the link shaft  360  to elastically support the cam body  352 . 
     The low lift cam  355  may be formed with a contact surface  357  to support the center of the swing arm  100 . The contact surface  357  serves to compress an upper side of the swing arm  100  in order to maintain the swing arm  100  in a state of being supported by the bottom surface of the housing  30 . When the low eccentric rod  356  of the low lift cam  355  compresses the one side of the swing arm  100 , the contact surface  357  may be brought into slight contact with the center of the swing arm  100  or may be slightly separated from the swing arm  100 . This is for the purpose of minimizing friction between the contact surface  357  and the swing arm  100 . Here, a swing roller  110  is rotatably mounted on an inner center of the swing arm  100  and exposed upward to directly contact the contact surface  357 . The swing roller  110  has a central axis P 5  at the center of the center line L 1  connecting the other side center P 1  with the one side center P 2  of the swing arm  100 . 
     On the other hand, the link shaft  360  rotatably connected to the web members  320  is connected to an actuation unit  370 . The actuation unit  370  serves to adjust the compression of the low lift cam  355  on the one side of each of the swing arms  100  depending on a slanted degree of the follower arm  353  which varies by movement of the link shaft  360  along an arcuate track about the pivot member  310 . 
     For example, the actuation unit  370  includes a connecting rod  371 , a link pin  372 , a screw member  373 , and a power transmission member  374 . 
     The connecting rod  371  is connected to the link shaft  360  to directly push the link shaft  360  upon application of an external pushing force of linear reciprocation. Since the link shaft  360  is connected to the connecting rod  371 , it moves along with the connecting rod  371  and is provided to the web members  320 , which are rotatably mounted on the pivot member  310 . As a result, the web members  320  are capable of reciprocating along an arcuate track together with the link shaft  360  about the pivot member  310 . The connecting rod  371  may be integrally formed with the link shaft  360  or may be separably mounted on the link shaft  360 . Further, various modifications may be made to the connecting rod  371 . 
     Additionally, the link pin  372  is linked to the connecting rod  371 , and the screw member  373  is linked to the link pin  372  and linearly reciprocates perpendicular to an axis of the link shaft  360  to induce reciprocation of the link shaft  360  on the arcuate track. The power transmission member  374  is formed on the circumference of the screw member  373  and reciprocates the screw member  373  in a forward and rearward direction while being forcibly rotated by an external force. 
     Namely, when the power transmission member  374  is rotated by the external force, the screw member  373  is linearly moved forward and rearward by a rotational force of the power transmission member  374  and the connecting rod  371  is reciprocated by the external force in the forward and rearward direction from the link pin  372  to thereby control the link shaft  360  to reciprocate along the arcuate track. 
     The screw member  373  may include, but is not limited to, a ball screw as well as a variety of mechanical elements. Further, the power transmission member  374  may be a nut which is capable of being rotated by an external force. The power transmission member  374  is connected to a power generator  374   a  such as a motor and the like. The power generator  374   a  may be fastened to the power transmission member  374  by a bolt. The power generator  374   a  serves to restrict the power transmission member  374  so as not to move in the axial direction of the actuation unit  370  during movement or stoppage of the power transmission member  374 . 
       FIGS. 5 to 8  are operational diagrams of the valve driving apparatus in accordance with the embodiment of the invention. 
     Referring to  FIG. 5 , the actuation unit  370  is operated such that the imaginary line L 2  connecting the central axis P 5  of the swing roller  110  to the central axis P 4  of the low rotary shaft  351  is perpendicular to the center line L 1  connecting the other side center P 1  of the swing arm  100  to the one side center P 2  thereof, in a state wherein the swing arm  100  is not compressed by the low lift cam  355 , so that the swing arm  100  is not pivoted and the valve  20  closes the combustion chamber  10 . 
     The contact surface  357  of the low lift cam  355  is in slight contact with the swing roller  110  to thereby support the upper side of the swing arm  100  in the downward direction. 
     In addition, the high lift cam  340  contacts the follow roller  354  between the follow arms  353 , with the high eccentric rod  342  of the high lift cam  340  directed opposite the follow roller  354 . 
     Descriptions of other components are the same as those described in the above embodiment and will be omitted herein. 
       FIG. 6  shows that the high rotary shaft  330  is rotated by an external force in a state wherein the power transmission member  374  is stopped and the imaginary line L 2  is perpendicular to the center line L 1  connecting the other side center P 1  of the swing arm  100  to the one side center P 2  thereof. 
     When the high rotary shaft  330  is rotated by the external force, the high lift cam  340  is rotated together with the high rotary shaft  330 , so that the high eccentric rod  342  compresses the follow roller  354  of the follow arm  353  for a predetermined duration. 
     When the high eccentric rod  342  compresses the follow roller  354 , the low lift cam  355  is rotated in the same direction as the cam body  352  as soon as the cam body  352  is rotated. Then, the low eccentric rod  356  compresses one side of the corresponding swing arm  100 . Accordingly, the valve  20  opens (or closes) the combustion chamber  10 . 
     Particularly, when the follow roller  354  is compressed by a portion of the high eccentric rod  342  corresponding to an end of the longest axis (r) from the central axis P 3  of the high rotary shaft  330 , the valve  20  is completely lowered to a predetermined distance D 1 , thereby maximally opening the combustion chamber  10  within a preset range. Here, as the high eccentric rod  342  continues to compress the follow roller  354  in a predetermined round section, the valve  20  opens (or closes) the combustion chamber  10  by a pushing force of the swing arm  100 . 
     Descriptions of other components are the same as those described in the above embodiment and will be omitted herein. 
     In  FIG. 7 , the power transmission member  374  is rotated to allow the connecting rod  371  and the web member  320  to rotate along an arcuate track about the pivot member  310 . 
     In other words,  FIG. 7  shows that the imaginary line L 2  is slanted to the center line L 1  in a state wherein the swing arm  100  is not compressed by the low lift cam  355  so that the swing arm  100  is not pivoted and the valve  20  closes the combustion chamber  10 . 
     When the screw member  373  is linearly retracted, the web member  320  and the low rotary shaft  351  are moved a predetermined distance “A” along an arcuate track in the counterclockwise direction. 
     Here, the central axis P 6  of the follow roller  354  is moved the same distance as the predetermined distance “A”, by which the web member  320  and the low rotary shaft  351  are moved along the arcuate track in the counterclockwise direction, so that the follow arm  353  is lifted a predetermined height in the counterclockwise direction about the central axis P 4  of the low rotary shaft  351 . 
     The contact surface  357  of the low lift cam  355  is in slight contact with the swing roller  110  to thereby support the upper side of the swing arm  100  in the downward direction. Thus, preferably, the swig roller  110  has a diameter so as to continue to contact the contact surface  357  moving along the arcuate track in an initial closed state of the valve  20 . 
     In addition, the high lift cam  340  contacts the follow roller  354  of the follow arm  353 , with the high eccentric rod  342  directed opposite the follow roller  354 . 
     Descriptions of other components are the same as those described in the above embodiment and will be omitted herein. 
       FIG. 8  shows that the high rotary shaft  330  is rotated by an external force in a state wherein the power transmission member  374  is stopped and the imaginary line L 2  is slanted to the center line L 1 . 
     When the high rotary shaft  330  is rotated by the external force, the high lift cam  340  is rotated together with the high rotary shaft  330 , so that the high eccentric rod  342  compresses the follow roller  354  of the follow arm  354  for a predetermined duration. 
     When the high eccentric rod  342  compresses the follow roller  354 , the low lift cam  355  is rotated in the same direction as the cam body  352  as soon as the cam body  352  is rotated. Then, the low eccentric rod  356  compresses one side of the corresponding swing arm  100 . As a result, the valve  20  opens (or closes) the combustion chamber  10 . 
     Then, when the follow roller  354  is compressed by a portion of the high eccentric rod  342  corresponding to the end of the longest axis (r) from the central axis P 3  of the high rotary shaft  330 , the valve  20  is completely lowered to a predetermined distance D 2 , thereby maximally opening the combustion chamber  10  within a preset range. Here, when the high eccentric rod  342  continues to compress the follow roller  354  in a predetermined round section, the valve  20  opens (or closes) the combustion chamber  10  by a pushing force of the swing arm  100 . 
     The lowered distance D 2  of the valve  20  with the center line L 1  slanted to the imaginary line L 2  is longer than the lowered distance D 1  of the valve  20  with the center line L 1  perpendicular to the imaginary line L 2 . This is attributed to the fact that, as the follow roller  354  is lifted a predetermined height with respect to the central axis P 4  of the low rotary shaft  351  while being moved by the predetermined distance D 1  along the arcuate track, the valve  20  is subjected to a greater pushing amount of the high eccentric rod  342  than in the other case. 
     Descriptions of other components are the same as those described in the above embodiment and will be omitted herein. 
       FIG. 9  is a perspective view of a valve driving apparatus including a drive unit in accordance with another embodiment of the present invention. 
     In this embodiment, a drive unit  370  of the valve driving apparatus includes an actuator  375  and a screw member  378 . 
     The actuator  375  has a receiving groove  376  which receives a link shaft  360  therein. Here, since the link shaft  360  is moved along an arcuate track, the receiving groove  376  may have an elongated shape in the up-down direction or may have a hole shape which is open toward a lower side of the actuator  375 . 
     Here, a plurality of actuators  375  may be formed on the link shaft  360  to actuate simultaneously, but only a single actuator  375  is shown as being connected to the link shaft  360  for convenience of description in this embodiment. Various modifications may be made to the actuator  375 . 
     A screw member  378  is axially inserted into the actuator  375  in a perpendicular direction to an axial direction of the link shaft  360 . Thus, when the screw member  378  is rotated by an external force, the actuator  375  is linearly reciprocated forward and rearward in the axial direction of the screw member  378 . The screw member  378  may be a ball screw. The structure wherein the screw member  378  is rotated by an external force and the structure wherein the screw member  378  is rotatably positioned may be realized in various ways. 
     When the actuator  375  is linearly reciprocated in the forward and rearward direction, the link shaft  360  and web members  320  are reciprocated along an arcuate track about a pivot member  310 . 
     The principle in variation of the lowered distance of the valves according to forward and rearward movement of the actuator  375  is the same as in the above embodiment. 
     Descriptions of other components are the same as those described in the above embodiment and will be omitted herein. 
       FIG. 10  shows oil passages  410  formed to minimize friction between respective components of the valve driving apparatus when a lubricant is supplied to the respective components through the housing  30 . 
     The housing  30  is formed at one side thereof with an inlet  420  through which the housing  30  receives a lubricant. The housing  30  has an oil passage  410  defined from the inlet  420  to a portion thereof on which the support member  200  is mounted. The support member  200  can continue to support a lower surface of the other side of the swing arm  100  by a pushing force of the lubricant supplied through the oil passage  410 . 
     The housing  30  is formed with another oil passage  410  from the inlet  420  to the pivot member. Then, the pivot member  310  guides the lubricant into the web members  320 , which in turn guide the lubricant into the low rotary shaft  351 . Then, the lubricant is supplied from the low rotary shaft  351  to the cam body  352  and is then discharged through an outlet  430  formed in the cam body  352  to flow towards contact portions between the high lift cam  340  and the follow rollers  354 . Thus, the oil passage  410  is defined in the pivot member  310 , the web members  320 , the low rotary shaft  351 , and the cam body  352 . 
     It should be understood that the oil passage  410  may be formed along various paths so as to flow to various contact portions between all of the components. 
     As apparent from the above description, according to one embodiment of the invention, the valve driving apparatus is capable of precisely adjusting lift amounts of valves periodically opening and closing the combustion chamber through adjustment of a rolling contact area and a cam separation of a double-cam unit cooperatively connected thereto, thereby realizing optimum fuel economy and output. Further, according to another embodiment of the invention, a high rotary shaft having an upper side cam of the double-cam unit mounted thereon is not directly connected to a low rotary shaft having a lower side cam of the double-cam unit, thereby allowing the separation between the cams of the double-cam unit to be easily adjusted. Moreover, according to a further embodiment of the invention, a pair of web members connected to opposite ends of the low rotary shaft is simultaneously pivoted in the same direction and at the same speed to allow the low rotary shaft to move along an arcuate track while maintaining its axial direction, so that a plurality of valves can be controlled to have the same lift amount. 
     In understanding the scope of the invention, the use of articles “a,” “an” and “the” in the context of describing the invention, especially in the context of the embodiments, are to be construed to cover both the singular and the plural, unless otherwise indicated or clearly contradicted by context. 
     Although some embodiments have been provided to illustrate the invention in conjunction with the drawings, it will be apparent to those skilled in the art that the embodiments are given by way of illustration only, and that various modifications, changes, substitutions, and equivalent embodiments can be made without departing from the spirit and scope of the invention. The scope of the invention should be limited only by the accompanying claims.