Abstract:
An engine valve drive apparatus. A camshaft rotatably supported by the engine includes a cam for selectively opening and closing a valve. The cam has a cam surface for driving the valve. The cam surface has a profile that varies continuously in the direction of the cam axis. A valve lifter is arranged between the cam and the valve to convey the motion of the cam to the valve. A cam follower is supported on the valve lifter. The cam follower includes a slide surface having a pair of edges. The cam surface is arched outwardly in the direction of the cam axis to prevent the slide surface edges from contacting the cam surface. The curved surface prevents damage to the cam surface and enables smooth sliding between the cam surface and the cam follower. Alternatively, the slide surface of the cam follower may be arched outwardly.

Description:
This is a divisional of application Ser. No. 09/054,551, filed Apr. 3, 1998 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a three-dimensional cam having a surface that varies continuously in the axial direction. More particularly, the present invention relates to a three-dimensional engine valve cam having a profile for controlling the opening and closing of engine valves in accordance with the operating state of the engine. The present invention also pertains to a method for measuring three-dimensional cams, measuring tools for testing profiles of three-dimensional cams, and an apparatus for measuring three-dimensional cams. The present invention also relates to an engine valve drive apparatus employing such three-dimensional cams. 
     FIG. 24 shows a prior art valve drive apparatus that continuously varies the opening and closing timing and lift amount of engine intake valves and engine exhaust valves. Japanese Examined Patent Publication No. 7-45803 and Japanese Unexamined Patent Publication No. 9-32519 describes such apparatus. As shown in FIG. 24, two valves  543 , which are either intake valves or exhaust valves, are provided for a single cylinder of an engine. Each valve  543  is connected to and driven by a three-dimensional cam  540 , which is fixed to a camshaft  542 . The cam  540  has a cam surface  540   a  used to drive the valves  543 . A cam nose, the radius of which changes continuously in the direction of the camshaft axis Y of the camshaft  542 , is defined on the cam surface  540   a . The shifting mechanism  541  shifts the camshaft  542  to displace each cam  540  within a range denoted by D. As the cam  540  shifts, the nose radius of the cam surface  540   a  changes continuously. This varies the lift amount and opening and closing timing of the associated valve  543 . The change in the lift amount (lift control amount) occurs within a range defined between the maximum and minimum values of the cam nose radius. The shifting of the camshaft  542  along the axis Y is controlled so that the maximum lift amount of each valve  543  is small when the engine is in a low speed range and is large when the engine is in a high speed range. This improves engine performance, especially in terms of torque and stability. 
     As shown in FIG. 24, a valve lifter  549  is arranged between each valve  543  and the associated three-dimensional cam  540 . A cam follower seat  544  is defined in the top center surface of each valve lifter  549 . A cam follower  545  is pivotally received in each follower seat  544  so that the valve lifter  549  can follow the cam surface  540   a  of the associated cam  540 . 
     Each cam follower  545  has a flat slide surface  545   a , which slides along the associated cam surface  540 . The shape of the cam follower  545  is shown enlarged in FIGS.  25 ( a ) and  25 ( b ). As shown in FIG.  25 ( a ), the cam follower  545  has a semicircular cross-section. FIG.  25 ( b ) is a side view of the cam follower  545 . 
     As shown in FIG. 26, the cam follower  545  has a first edge  545   b  and a second edge  545   c  that engage the cam surface  540   a . Contact between the cam follower  545  and the cam surface  540   a  occurs between the first edge  545   b  and the second edge  545   c . The first edge  545   b  contacts the cam surface  540   a  where the cam nose radius is smaller than that where the second edge  545   c  contacts the cam surface  540   a.    
     FIG. 27 is a perspective view showing the cam surface  540   a . The uniformly dashed line represents one axial end of the cam  540 , or cam profile  547 , where the cam nose radius is smallest. The long and short dashed line represents the other axial end of the cam  540 , or cam profile  548 , where the cam nose radius is greatest. As apparent from the drawing, the profile of the cam  540  varies continuously in the axial direction. Each elemental line  546  shown in the drawing represents the same angular position on the cam surface  540   a . In other words, the lines  546  represent intersections between the cam surface and planes that include the axis Y. Although the drawing shows a limited number of lines  546 , an infinite number of lines  546  may be defined along the cam surface  540 . Hence, the cam follower  545  comes into linear contact with the cam surface  540   a  along part of each line  540 . 
     As shown in FIG. 26, when the three-dimensional cam  540  shifts along the axis Y, the slide surface  545   a  between the first and second edges  545   a ,  545   b  of the cam follower  545  is in linear contact with and moves relative to the cam surface  540   a . Lubricating oil is removed from the cam surface  540   a  when relative movement takes place between the cam follower  545  and the cam surface  540   a . This occurs especially when the second edge  545   c  scrapes off the lubricating oil from the cam surface  540   a  as the cam follower  545  shifts along the cam surface  540   a  from the smaller radius side to the larger radius side. As a result, lubrication between the second edge  545   c  and the cam surface  540   a  becomes insufficient. This may lead to wear of the second edge  545   c  and the cam surface  540   a.    
     Generally, the small radius side of the cam  540  is used more frequently than the large radius side. Therefore, a difference in wear occurs along the cam surface  540   a  in the axial direction Y. The wear difference causes the cam surface  540   a  to become uneven. An uneven cam surface  540   a  may interfere with the movement of the second edge  545   c  and thus hinder with smooth shifting of the opening and closing timing and lift amount of the associated valve  543 . 
     Additionally, the cam surface  540  is machined with precision so that the surface  540   a  is straight as shown in FIG.  27 . However, tolerances permitted during machining of the cam surface  546  may result in a slight concavity in surface  540   a , as shown in FIG.  28 . In such case, only the first and second edges  545   b ,  545   c  of the cam follower  545  contact the cam surface  540   a . This may cause the first and second edges  545   b ,  545   c  to scratch the cam surface  540   a  during rotation of the cam  545  or cause biased wear of the cam follower  545  at the edges  545   b ,  545   c.    
     When scratches are formed in the cam surface  540   a , the scratches may interfere with axial movement of the three-dimensional cam  540 . This would hinder with smooth varying of the opening and closing timing and lift amount of the associated valve  543 . 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an objective of the present invention to provide a three-dimensional cam and a valve drive apparatus that enable smooth relative movement between the cam surface and the cam follower without damage or wear of the cam surface and cam follower. It is a further objective of the present invention to provide a method and apparatus for measuring the profile of such three-dimensional cam. 
     To achieve the above objectives, the present invention provides a cam mechanism including a cam, a cam follower, and a driven member. The cam rotates about its axis to drive the driven member with the cam follower. The cam mechanism further includes a cam surface defined on the cam to slidably engage the cam follower. The cam surface has a profile that varies continuously in the direction of the cam axis. The cam moves axially and changes the position of the cam surface with respect to the cam follower to vary the behavior of the driven member. A slide surface is defined on the cam follower to slidably engage the cam surface. At least one of the cam surface and the slide surface is convexly arched in the direction of the cam axis. 
     The above cam mechanism is preferably applied to a valve drive apparatus of an automobile engine. 
     In another aspect of the present invention, a cam for driving a driven member with a cam follower is provided. The cam is rotatable about its axis and has a cam surface to slidably engage the cam follower. The cam surface has a profile that varies continuously in the direction of the cam axis and is convexly arched in the direction of the cam axis. 
     In a further aspect of the present invention, a cam follower is provided. The cam follower is arranged between a cam and a driven member to convey the motion of the cam to the driven member. The cam rotates about its axis and has a cam surface to slidably engage the cam follower. The cam surface has a profile that varies continuously in the direction of the cam axis. The cam follower has a slide surface to slidably engage the cam surface. The slide surface has edges. The slide surface is convexly arched in the direction of the cam axis at least at the edges. 
     In a further aspect of the present invention, a measuring tool is provided. The measuring tool is used to measure the profile of a cam surface defined on a cam that rotates about its axis. The measuring tool includes a contact element having a flat measuring surface for contacting the cam surface. A holder supports the contact element pivotally about a pivot axis extending perpendicular to the cam axis. The measuring surface includes the pivot axis and has a portion that constantly contacts the cam surface. The holder moves along a moving axis perpendicular to the pivot axis during rotation of the cam. The position of the holder on the moving axis indicates the radius of the cam surface at a location where the measuring surface contacts the cam surface. 
     In a further aspect of the present invention, an apparatus for measuring the profile of a cam surface defined on a cam that rotates about its axis is provided. The measuring apparatus includes a measuring tool faced toward the cam surface. The measuring tool includes a contact element having a flat measuring surface slidably engaged with the cam surface and a holder for supporting the contact element pivotally about a pivot axis, which extends perpendicular to the cam axis. The measuring surface includes the pivot axis and has a portion that constantly contacts the cam surface. The measuring tool moves along a moving axis during rotation of the cam. The position of the measuring tool along the moving axis indicates the radius of the cam surface at a location where the measuring surface contacts the cam surface. A rotary drive means rotates the cam about its axis to angularly vary the part of the cam surface that the measuring surface contacts. A moving means moves the cam axially to axially vary the part of the cam surface that the measuring surface contacts. A measuring means measures the position of the measuring tool along its moving axis in association with the angular and axial positions of the part of the cam surface that the measuring surface contacts. 
     In a further aspect of the present invention, a method for measuring the profile of a cam surface defined on a cam that rotates about its axis is provided. The measuring method includes the step of facing a measuring tool toward the cam surface. The measuring tool includes a contact element having a flat measuring surface slidably engaged with the cam surface and a holder for supporting the contact element pivotally about a pivot axis extending perpendicular to the cam axis. The measuring surface includes the pivot axis and has a portion that constantly contacts the cam surface. The measuring tool moves along a moving axis during rotation of the cam. The position of the measuring tool along the moving axis indicates the radius of the cam surface at a location where the measuring surface contacts the cam surface. The measuring method further includes the steps of rotating the cam about its axis to angularly vary the part of the cam surface that the measuring surface contacts, moving the cam axially to axially vary the part of the cam surface that the measuring surface contacts, and measuring the position of the measuring tool along its moving axis in association with the angular and axial positions of the part of the cam surface that the measuring surface contacts. 
     In a further aspect of the present invention, a method for measuring the profile of a cam surface defined on a cam that rotates about its axis is provided. The cam surface has a profile that varies continuously in the direction of the cam axis. The cam surface is convexly arched in the direction of the cam axis. The measuring method includes the steps of measuring a physical quantity representing the cam surface radius in association with the angular position and axial position of a measured location on the cam surface, and inspecting the cam by plotting distribution patterns. Each distribution pattern is based on measurement values taken along the cam surface at the same angular position but at different axial positions. The inspection is performed by judging whether each distribution pattern represents a convex cam surface within a predetermined tolerance range to confirm that the cam is satisfactory. 
     Other aspects and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the present invention that are believed to be novel are set forth with particularity in the appended claims. The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
     FIG. 1 is a perspective view showing the cam surface shape of an intake valve cam in a first embodiment according to the present invention; 
     FIG. 2 is a perspective view showing an engine valve drive apparatus used to drive the valve of FIG. 1; 
     FIG. 3 is a graph showing the cam surface shape relative to the axial direction of the intake valve cam of FIG. 1; 
     FIG. 4 is a perspective view of a valve lifter employed in the valve drive apparatus of FIG. 2; 
     FIG.  5 ( a ) is a cross-sectional view of a cam follower of the valve lifter shown in FIG. 4, and FIG.  5 ( b ) is a side view of the cam follower; 
     FIG. 6 is an enlarged cross-sectional view partially showing the valve drive apparatus of FIG. 2; 
     FIG. 7 is a partial enlarged cross-sectional view, as seen in the same direction as FIG. 6, showing contact between the cam surface of the intake valve cam shown in FIG.  1  and the cam follower; 
     FIG. 8 is a block diagram showing a three-dimensional measuring apparatus employed in a second embodiment according to the present invention; 
     FIG. 9 is a perspective view showing a three-dimensional cam profile measuring tool employed in the measuring apparatus of FIG. 8; 
     FIG. 10 is a perspective view showing a contact element of the three-dimensional profile measuring tool of FIG. 9; 
     FIG. 11 is a perspective view showing the contact element of FIG. 10 contacting the intake valve cam; 
     FIGS.  12 ( a ) and  12 ( b ) are flowcharts showing the inspection routine executed by the measuring apparatus of FIG. 8; 
     FIG. 13 is a flowchart showing the measurement routine executed by the measuring apparatus of FIG. 8; 
     FIG. 14 is a graph showing an example of the results obtained by the measuring apparatus of FIG. 8; 
     FIG. 15 is a graph showing an example of data taken by the measuring apparatus of FIG. 8 to inspect the intake valve cam; 
     FIG. 16 is a graph showing an example of data taken by the measuring apparatus of FIG. 8 to inspect the intake valve cam; 
     FIG. 17 is a graph showing an example of data taken by the measuring apparatus of FIG. 8 to inspect the intake valve cam; 
     FIG. 18 is a graph showing an example of data taken by the measuring apparatus of FIG. 8 to inspect the intake valve cam; 
     FIG.  19 ( a ) is a cross-sectional view showing a cam follower employed in a third embodiment according to the present invention, and FIG.  19 ( b ) is a side view showing the cam follower; 
     FIGS.  20 ( a ),  20 ( b ),  20 ( c ) are partially enlarged cross-sectional views showing the relationship between the cam follower of FIG.  19 ( a ) and the cam surface; 
     FIG.  21 ( a ) is an end view showing a cam follower employed in a fourth embodiment according to the present invention, and FIG.  21 ( b ) is a side view showing the cam follower of FIG.  21 ( a ); 
     FIG. 22 is a cross-sectional view showing a cam follower employed in a fifth embodiment according to the present invention; 
     FIG. 23 is a cross-sectional view showing a cam follower employed in a sixth embodiment according to the present invention; 
     FIG. 24 is a cross-sectional view showing a prior art valve drive apparatus; 
     FIG.  25 ( a ) is a cross-sectional view showing a cam follower of the valve drive apparatus of FIG. 24, and FIG.  25 ( b ) is a side view of the cam follower of FIG.  25 ( a ); 
     FIG. 26 is a partially enlarged cross-sectional view showing a state of contact between the cam follower of FIG.  25 ( a ) and the cam surface; 
     FIG. 27 is a perspective view showing the cam surface shape of a three-dimensional cam of the valve drive apparatus of FIG. 24; and 
     FIG. 28 is a partial enlarged view showing a state of contact between the cam surface of the cam of FIG.  27  and the cam follower. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A valve drive apparatus employed in a double overhead cam (DOHC) engine  1  is shown in FIG.  2 . The engine  1  includes cylinders  3  that are each provided with four valves (two intake valves and two exhaust valves). 
     The engine  1  has a cylinder block  2 , which houses the cylinders  3 . A piston  4  is retained in each cylinder  3 . Each piston  4  is connected to a crankshaft  6  by a connecting rod  7 . The crankshaft  6  is supported in a crank case  5  and has an end to which a timing pulley  8  is fixed. 
     A cylinder head  9  is mounted on the cylinder block  2 . An intake valve camshaft  10  is supported in the cylinder head  9  by a plurality of bearings (not shown) so that the camshaft  10  is rotatable and axially movable. Two intake valve cams  11  are formed integrally with the camshaft  10  in correspondence with each cylinder  3 . In the same manner, an exhaust valve camshaft  12  is supported in the cylinder head  9  by a plurality of bearings (not shown) so that the camshaft  12  is rotatable. Two exhaust valve cams  13  are formed integrally with the camshaft  10  in correspondence with each cylinder  3 . 
     The intake valve camshaft  10  has an end to which a timing pulley  14  and a shaft shifting mechanism  15  are connected. The exhaust valve camshaft  12  also has an end to which a timing pulley  16  is fixed. The camshaft timing pulleys  14 ,  16  are connected to the crankshaft timing pulley  8  by a timing belt  17 . Thus, the rotation of the crankshaft  6  rotates the intake valve camshaft  10  and the exhaust valve camshaft  12 . 
     Two intake valves  18  are provided for each cylinder  3 . Each intake valve  18  is connected to one of the associated intake valve cams  11  by a valve lifter  191  or  192 . The valve lifters  191 ,  192  are each slidably retained in a lifter bore (not shown) provided in the cylinder head  9 . 
     Two exhaust valves  20  are provided for each cylinder  3 . Each exhaust valve  20  is connected to one of the associated exhaust valve cams  13  by a valve lifter  21 . Each valve lifter  21  is slidably retained in a lifter bore (not shown) provided in the cylinder head  9 . 
     A combustion chamber  3   a  is defined in each cylinder by the associated piston  4 . Each combustion chamber  3   a  is connected to an intake passage and an exhaust passage (neither shown). Each pair of intake valves  18  is arranged in the intake passage to control the flow of air sent from the intake passage to the associated combustion chamber  3   a . Each pair of exhaust valves  20  is arranged in the exhaust passage to control the flow of exhaust gases from the associated combustion chamber  3   a  to the exhaust passage. The rotation of the intake valve camshaft  10  causes the cams  11  to selectively open and close the intake valves  18  with the associated valve lifter  191 ,  192 . The rotation of the exhaust valve camshaft  13  causes the cams  13  to selectively open and close the exhaust valves  20  with the valve lifters  21 . 
     As shown in the perspective view of FIG. 1, each intake valve cam  11  is a three-dimensional cam and includes a cam surface  11   a . The uniformly dashed line represents one end of the intake valve cam  11  with respect to the camshaft axis A, or a cam profile  47  where the cam nose radius is smallest. The cam profile  47  minimizes the lift amount of the associated intake valve  18 . The long and short dashed line represent the other end of the cam  11 , or a cam profile  48  where the cam nose radius is greatest. The cam profile  48  maximizes the lift amount of the associated intake valve  18 . As apparent from the drawing, the cam profile of the cam  11  varies continuously in the axial direction. Lines  46  shown in the drawing represent the same rotational phase on the cam surface  11   a . That is, each line  46  represents the intersection of the cam surface  11   a  with a plane that contains the axis A. Although the drawing shows a limited number of lines  46 , an infinite number of lines  46  may actually be defined along the cam surface  11   a.    
     As shown in FIG. 3, the cam surface  11   a  of the cam  11  differs from the cam surface  540   a  of the prior art cam  540  shown in FIG. 27 in that the cam surface  540   a  is convex in the axial direction A. The reference line shown in FIG. 3 represents a theoretical linear intersection between the cam surface  11   a  and a plane that includes the axis A. As apparent from the graph, the middle portion of the line  46  representing the cam surface  11   a  is arched outwards. In other words, the cam surface  11   a  is convex. The projecting amount of the line  46  with respect to the reference line is exaggerated in FIG.  3 . The actual projection amount is about 1 μm to 20 μm. 
     As shown in FIG. 4, the valve lifters  191 ,  192 , which are identical to each other, are cylindrical. A guide  23  is provided on the peripheral surface  19   a  of each valve lifter  191 ,  192 . The guide  23  is pressed into or welded into a slot  19   b  extending along the peripheral surface  19   a . An engaging portion (not shown), which may be a groove or the like, is formed in the wall of the associated lifter bore to engage the guide  23  so that rotation of the valve lifter  191 ,  192  in the lifter bore is restricted while axial movement is permitted. 
     Each valve lifter  191 ,  192  has a top surface  19   c  that includes a cam follower seat  24 . A cam follower  25  is tiltably held in each follower seat  24 . FIGS.  5 ( a ) and  5 ( b ) are enlarged views showing the shape of the cam follower  25 . The cam follower  25  has a flat slide surface  25   a , which contacts the cam surface  11   a  of the associated cam  11 , and a cylindrical surface, which is pivotally received in the seat  24 . The long edges of the slide surface  25   a  are first and second edges  25   b ,  25   c , which are continuous with the cylindrical surface. 
     The shaft shifting mechanism  15  shown in FIG. 2 is a known mechanism driven by a hydraulic circuit (not shown) to move the intake valve camshaft  10  and its cams  11  in the axial direction in accordance with the operating conditions of the engine  1  (the conditions include at least the engine speed). As shown in FIG. 6, the shaft shifting mechanism  15  moves the camshaft  10  so that the point of contact between each cam surface  11   a  and the slide surface  25   a  moves between the position where the radius of the cam nose is smallest (refer to the long and short dashed line in FIG. 6) and the position where the cam nose radius is greatest (refer to the solid line in FIG.  6 ). In other words, each cam  11  is displaced within a range denoted by D. The movement of the camshaft  10  varies the lift amount of the intake valves  18  in accordance with the operating conditions of the engine  1 . 
     The middle portion of the cam surface  11   a  of each intake cam  11  is convexly arched from the axial ends of the cam surface  11   a , as shown in FIG.  3 . Thus, the middle portion of the cam surface  11   a  is not recessed regardless of machining tolerances. In other words, tolerances are taken into consideration when designing the cams  11  so that the middle portion of each cam surface  11   a  is higher than the axial ends of the cam surface  11   a . Accordingly, as shown in FIG. 7, only the middle portion of the slide surface  25   a  of each cam follower  25  contacts the cam surface  11   a . Thus, the edges  25   b ,  25   c  of the cam follower  25  do not contact the cam surface  11   a.    
     As a result, the edges  25   b ,  25   c  of the cam follower  25  do not scrape off the lubricating oil film applied to the cam surface  11   a  during axial movement of the associated cam  11 . This maintains sufficient lubrication between the cam surface  11   a  and the cam follower  25 . Thus, smooth relative movement is carried out without causing damage or wear of the cam surface  11   a  and the cam follower  25 . In addition, the cam surface  11   a  is prevented from becoming uneven when wear occurs. Furthermore, scratches, which are formed when the edges  25   b ,  25   c  of the cam follower  25  contact the cam surface  11   a , and biased wear of the edges  25   b ,  25   c  are prevented. Thus, when each cam  11  moves axially, there is no interference between the associated cam follower  25  and scratches or an uneven surface. Accordingly, the lift amount and opening and closing timing of the intake valves  18  are varied smoothly. 
     A second embodiment according to the present invention will now be described with reference to the FIGS. 8 to  18 . The second embodiment pertains to an apparatus for measuring the cam profile of the intake cam  11  of the first embodiment. 
     FIG. 8 is a block diagram showing the structure of a three-dimensional cam profile measuring apparatus  100 . The measuring apparatus  100  includes a control circuit  102 , a rotary drive device  104 , a linear drive device  106 , a scale device  108 , a measuring unit  110 , an external memory  112 , a display device  114 , and a printer  116 . Although not shown in the diagram, the measuring apparatus  100  further includes a host computer and a communication circuit. 
     The control circuit  102  is a computer system that incorporates a central processing unit (CPU), a read only memory (ROM), a random access memory (RAM), an input/output interface, a bus line, an internal memory, and other devices. The CPU executes necessary computations based on programs, which are stored in the ROM, the RAM, the external memory  112 , and other devices, using data sent from the scale device  108  and the measuring unit  110  via the input/output interface. The CPU also stores computation results (data related to the cam profile of the cam surface  11   a  of each intake cam  11 ) in the external memory  112  through the input/output interface, displays the computation results on the display device  114 , and prints out the computation results with the printer  116 . 
     The rotary drive device  104  includes a stepping motor, a servomotor, or the like. The control circuit  102  sends command signals to the rotary drive device  104  to adjust the rotary phase of the intake valve camshaft  10  when measuring cam profiles. 
     The linear drive device  106  is constituted by a linear movement mechanism, which includes a motor associated with a linear solenoid or ball screw. The control circuit  102  sends command signals to the linear drive device  106  to adjust the axial position of the intake valve camshaft  10 . 
     The scale device  108  includes a rotary position sensor and a linear position sensor. The rotary position sensor employs a synchro, a resolver, a rotary encoder, or the like. The linear position sensor employs a potentiometer, a differential transformer, a scale, or the like. The scale device  108  measures the precise rotary phase and axial position of the camshaft  10 , which is rotated by the rotary drive device  104  and moved axially by the linear drive device  106 . Signals corresponding to the measurement results are sent to the control circuit  102 . 
     The measuring unit  110  includes a three-dimensional cam profile measuring tool  120  and a linear position sensor, which employs a potentiometer, a differential transformer, a scale, or the like. The measuring unit  110  has a supporter  110   a  for supporting the measuring tool  120 . The supporter  110   a  permits movement of the measuring tool  120  along a moving axis G (described later) and urges the measuring tool  120  toward the intake valve cam  11 . The measuring unit  110  measures the movement distance of the measuring tool  120  when the measuring tool  120  is in contact with the cam surface  11   a  of the intake valve cam  11 . Signals corresponding to the measurement results are sent to the control circuit  102 . 
     The structure of the profile measuring tool  120  will now be described. As shown in FIG. 9, the measuring tool  120  includes a contact element  122  and a holder  124 , which holds the ends of the contact element  122 . As shown in FIG. 10, the contact element  122  is generally cylindrical and has shafts  126  and  128  projecting from its ends. The contact element  122  shown in FIG. 10 is illustrated upside down with respect to that shown in FIG.  9 . The holder  124  has two arms  130 ,  132  to hold the shafts  126 ,  128  so that the contact element  122  is supported pivotally about its axis F. 
     The middle portion  122   a  of the contact element  122  is cut in half axially along a plane that includes the contact axis F. The contact element  122  is also cut at side to form a plate-like portion as shown in FIG.  10 . The plate-like middle portion  122   a  has a measuring surface  122   b , which includes the axis F. The contact element  122  is made of cemented carbide, and the measuring surface  122   b  is finished with extremely high accuracy. 
     The holder  124  has a base  134  to which the two arms  130 ,  132  are connected. The base  134  is supported by the supporter  110   a  of the measuring unit  110 . The supporter  110   a  holds the base  134  so as to permit movement of the base  134  along the moving axis G, which extends perpendicular to the axis F of the contact element  122 , while preventing rotation of the base  134  about the axis G. As shown in FIG. 11, during profile measurement of each intake valve cam  11 , the measuring surface  122   b  is pressed against the cam surface  11   a  of the cam  11  so that the axis F of the contact element  122  is perpendicular to the axis A of the cam  11 . 
     The profile measurement is executed by the control circuit  102  in accordance with the flowchart shown in FIGS. 12 and 13. To carry out the profile measurement, the camshaft  10  is either manually or automatically set in the measuring apparatus  100 , as shown in FIG.  8 . 
     When starting measurement, the control circuit  102  first performs step S 100  and sets the initial state. That is, the control circuit  102  drives the rotary drive device  104  to arrange the camshaft  10  at an initial rotary phase and drives the linear drive device  106  to arrange the camshaft  10  at an initial axial position to initiate measurement. 
     At step S 110 , the control circuit  102  prepares for interruption of the measurement routine, which is illustrated in FIG.  13 . The measurement routine is executed in an interrupting manner each time the camshaft  10  is rotated by a predetermined angle (e.g., 0.5°). After step S 110 , the control circuit  102  executes the routine of FIG. 13 based on signals sent from the scale device  108  each time the camshaft  10  is rotated by the predetermined angle. 
     When entering the routine of FIG. 13, the control circuit  102  first performs step S 112  and computes the present rotary phase of the camshaft  10  based on the number of interruptions from the initial rotary phase. The control circuit  102  then stores the data related to the present rotary phase in the RAM or the external memory  112 . 
     At step S 114 , the control circuit  102  reads the axial position of the camshaft  10  corresponding to the present rotary phase from signals sent from the scale device  108 . The control circuit  102  then stores the data related to the present axial position in the RAM or the external memory  112  in association with the rotary phase data obtained in step S 112 . 
     At step S 116 , the control circuit  102  computes the height of the cam surface  11   a  of the present subject cam  11  from signals sent from the measuring unit  110 . The control circuit  102  then stores the height in the RAM or external memory  112  in association with the rotary position data, obtained in step S 112 , and the axial position data, obtained in step S 114 . The height of the cam surface  11   a  is represented by either the radial distance between the axis A of the cam  11  and the cam surface  11   a  or by the radial projection amount of the cam surface  11   a  from the radius of the cam base circle. 
     After completing the measurement routine, the control circuit  102  keeps the measurement routine ready until the next interruption cycle. 
     The control circuit  102  proceeds from step S 110  to step S 120  and sends a command signal to the rotary drive device  104  to start the rotation of the camshaft  10 . During rotation of the camshaft  10 , the scale device  108  continuously informs the control circuit  102  of changes in the rotary phase of the camshaft  10 . The control circuit  102  refers to the signals sent from the scale device  108  to execute the measurement routine of FIG.  13  and obtain measurement data each time the camshaft  10  is rotated by the predetermined angle. 
     At step S 130 , the control circuit  102  determines whether or not the camshaft  10  has completed a full rotation, or whether or not the camshaft  10  has been rotated by 360°. If the camshaft  10  has not been rotated by 360°, the control circuit  102  waits until the camshaft  10  is rotated by 360°. Therefore, the height of the cam surface  11   a  is measured repetitively as the measurement routine is carried out each time the camshaft  10  is rotated by the predetermined angle until the camshaft  10  completes a full rotation. The axial position of the camshaft  10  is fixed during rotation. When the camshaft  10  completes a full rotation, the control circuit  102  proceeds to step S 140  and sends a command to the rotary drive device  104  to stop the rotation of the camshaft  10 . 
     At step S 150 , the control circuit  102  determines whether or not the measurement of the present subject cam  11  has been completed. More specifically, the control circuit  102  determines whether or not the measurement of the present subject cam  11  at all predetermined axial measurement positions and all rotary phases for each axial position has been completed. 
     If it is determined that all measurements of the present cam  11  have not been completed, the control circuit  102  proceeds to step S 160 . At step S 160 , the linear drive device  106  moves the camshaft  11  axially to measure a new position on the same cam  11 . The control circuit  102  also drives the rotary drive device  104  to arrange the camshaft  10  at the initial rotary phase so that measurement can be commenced. The control circuit  102  then returns to step S 120  and repetitively performs steps S 120  to S 160  until completing all of the required measurements of the cam  11 . 
     At step S 150 , if it is determined that all measurements of the cam  11  have been completed, the control circuit  102  proceeds to step S 170 , which prohibits interruption of the measurement routine of FIG.  13 . 
     The data obtained during measurement of the subject cam  11  represents the profile of the cam surface  11   a  of the cam  11 . FIG. 14 is a graph showing some of the measurement data. The data for three representative cam profiles are shown in FIG.  14 . The long and short dashed line represents the data taken on the axial end of the cam  11  where the cam nose radius is greatest, or cam profile S 1 . The uniformly dashed line represents the data taken on the other axial end of the cam  11  where the cam nose radius is smallest, or cam profile S 3 . The solid line represents the data taken at the axially middle position of the cam  11 , or cam profile S 2 . In addition to the data of the cam profiles S 1 , S 2 , S 3 , there are actually much more data representing cam profiles of the same cam  11  taken at other axial positions. 
     The control circuit  102  proceeds to step S 180  from step S 170  to evaluate the data of the subject cam  11  and judge whether of not the cam  11  is satisfactory. The control circuit  102  determines whether or not the cam profile height data collected at each predetermined rotary phase by the measuring unit  110  represents a convexly arched cam surface. If it is determined that the cam  11  is convex at each rotary phase, or each angular position, the control circuit  102  judges whether or not the convexity is within a tolerable range. This evaluation is carried out for each measured rotary phase. 
     The evaluation of the cam  11  will be described in detail now. For example, when measuring the height of the cam surface  11   a  at four different positions Pa, Pb, Pc, Pd on the same rotary phase θa, as shown in FIGS. 1 and 14, the measurement values of each position Pa, Pb, Pc, Pd may be plotted as shown in the graph of FIG.  15 . In the graph, the horizontal line T represents a theoretical line located at the same rotary phase as the positions Pa, Pb, Pc, Pd, or rotary phase θa. The theoretical line T corresponds to a straight line inclined with respect to the axis of the cam  11  like the lines  546  of the prior art cam  540  shown in FIG.  27 . The graph of FIG. 15 plots the difference between the measurement value indicating the height of the cam surface  11   a  at each position Pa, Pb, Pc, Pd and the theoretical line T. The range of tolerance is set within a maximum tolerance value, which is set at the positive side of the theoretical line T (or zero), and a minimum tolerance value, which is set at the negative side of the theoretical line (or zero). If the measurement value is on the positive side of the theoretical line T, the corresponding position on the cam surface  11   a  is higher than the theoretical line T. That is, the cam radius is less than that of the line T at that position. If the measurement value is on the negative side of the theoretical line T, the corresponding position on the cam surface  11   a  is lower, or has a smaller radius, than the theoretical line T. 
     As apparent from FIG. 15, positions Pb, Pc, which are located at the middle portion of the cam surface  11   a , are higher than positions Pa, Pd, which are located at the ends of the cam surface  11   a  on the same rotary phase θa. In other words, the cam surface  11   a  is convex so that the middle portion is higher than the ends. Furthermore, the heights of the positions Pa, Pb, Pc, Pd are all included within the tolerance range. 
     In this manner, if the distribution pattern shows that the middle portion of the cam surface  11   a  is convexly arched from the ends at all measured rotary phases and if the height, or radius, of the cam surface is always included within the tolerance range, the control circuit  102  determines that the cam  11  is satisfactory in step S 180 . 
     At step S 190 , the control circuit  102  determines whether or not the subject cam  11  was evaluated as being satisfactory in step S 180 . If the cam  11  was judged as being satisfactory, the control circuit  102  proceeds to step S 200  and determines whether or not the evaluation of all the cams  11  on the camshaft  10  has been finished. If it is determined that there are cams  11  that have not yet been evaluated, the control circuit  200  proceeds to step S 210  and moves the camshaft  10  to initiate measurement of the next cam  11 . More specifically, the control circuit  102  drives the linear drive device  106  to axially move the next cam  11  to the initial measurement position and drives the rotary drive device  104  to rotate the cam  11  to the initial rotary phase. When the cam  11  is positioned, the contact element  122  of the profile measuring tool  120  is in contact with the cam surface  11   a  of the cam  11 . 
     The control circuit  103  then returns to step S 110  shown in FIG.  12 ( a ) and sequentially carries out steps S 110  to S 160  on the subject cam  11 . Steps S 110  to S 210  are repetitively performed as long as the control circuit  102  judges that the subject cam  11  is satisfactory in steps S 180 , S 190  and that all the cams  11  have not yet been measured in step S 200 . 
     The control circuit  102  proceeds to step S 220  when the cam profiles of all of the cams  11  on the camshaft  10  have been measured and when it has been determined that all cams  11  are satisfactory. At step S 220 , the control circuit  102  generates a message that all of the cams  11  of the camshaft  10  have passed the cam surface inspections. For example, the word “satisfactory” together with an inspection number may be displayed on the display device  114  or may be printed out by the printer  116 . The control circuit  102  may also store the inspection result together with the inspection number in the external memory  112 . Furthermore, data related to the inspection result may be transmitted to the host computer, which is connected to the control circuit  102 . 
     If it is determined that any one of the cams  11  has a defective cam surface  11   a , the control circuit  102  proceeds to step S 230  and generates a message notifying of the existence of the defective cam  11 . Examples of defective cams  11  will now be described with reference to the graphs of FIGS. 16 to  18 . In FIG. 16, the distribution pattern of the measurement values taken at different axial positions Pa, Pb, Pc, Pd is inclined with respect to the theoretical line T. The measurement value taken at position Pa, which is located at one end of the cam surface  11   a , is plotted at the positive side of and farthest from the theoretical line T. In FIG. 17, the distribution pattern of the measurement values taken at positions Pa, Pb, Pc, Pd shows that the middle portion of the cam surface  11   a  is recessed from the ends of the cam surface  11   a . In FIG. 18, the distribution pattern of the measurement values taken at positions Pa, Pb, Pc, Pd shows that the middle portion of the cam surface  11   a  is projected from the ends of the cam surface  11   a . However, the measurement values taken at positions Pa, Pc are outside the tolerance range. 
     When the measurement results are as shown in FIGS. 16 to  18 , the control circuit  102  determines that the subject cam  11  is defective in steps S 180 , S 190  and then proceeds to step S 230  to announce the existence of the defective cam  11 . For example, the word “defective” together with an inspection number may be displayed on the display device  114  or may be printed out by the printer  116 . The control circuit  102  may also store the inspection result together with the inspection number in the external memory  112 . Furthermore, data related to the inspection result may be transmitted to the host computer, which is connected to the control circuit  102 . 
     The control circuit  102  terminates the inspection routine after performing either step S 220  or step S 230 . After setting the next camshaft  10  in the measuring apparatus  100 , the inspector pushes a switch, provided in the control circuit  102 , to start measurements. This commences execution of the routines illustrated in FIGS.  12 ( a ),  12 ( b ), and  13 . Thus, the cam profile of each cam  11  in the subject camshaft  10  is measured and inspected. 
     The following are advantages of the measuring apparatus. 
     The profile measuring tool  120  is provided with the contact element  122  and the holder  124 . The measuring tool  120  includes the flat measuring surface  122   b  for contacting the cam surface  11   a . The holder  124  supports the contact element  122  so that the contact element  122  is pivotal about its axis F. Thus, the contact element  122  pivots while following the cam surface  11   a , which is inclined with respect to the axis of the cam  11 . Furthermore, the measuring surface  122   b  includes the axis F. Thus, the measuring surface  122   b  remains in constant contact with the cam surface  11   a  and the axis F is never displaced despite the tilting of the contact element  122 . Accordingly, the cam profile of the entire cam  11  is measured accurately. 
     The cam surface  11   a  is measured accurately especially when the cam surface  11   a  is convex. Therefore, the cam  11  is inspected accurately. This measurement method is effective when inspecting the cam  11  of the first embodiment. Accordingly, the measurement method guarantees that the three-dimensional cams  11  smoothly and accurately vary the opening and closing timing and lift amount of associated valves. 
     The profile measuring tool  120  moves along moving axis G, which is perpendicular to the contact axis F. In addition, the measuring surface  122   b  of the contact element  122  contacts the cam surface  11   a  with the axis F extending perpendicular to the axis A of the cam  11 . The relationship between the cam  11  and the contact element  122  in terms of position is the same as the relationship between the cam  11  and the cam follower  25  of the valve lifter  191 . Accordingly, the profile measurement of the cam  11  is conducted under the same conditions as when the cam  11  is actually employed in the engine  1 . This enhances the reliability of the measurement and inspection results, which are obtained by simulating actual usage conditions. 
     The measurement of the height of the cam surface  11   a  is conducted in association with the rotary phase and axial position of the cam  11 . Thus, the profile of the cam  11  is measured accurately. 
     When judging whether or not each cam  11  is satisfactory, the control circuit  102  determines whether the distribution pattern of the measurement values indicating the cam surface height is included within a tolerance range, which is based on the theoretical line T. The tolerance range does not affect the valve control structure. Thus, the same valve control structure used with the prior art cams  540  may be used with the cams  11 . By using the cams  11 , the shaft shifting mechanism  15  may be controlled in the same manner as in the prior art. Accordingly, the employment of three-dimensional cams  11  selected by the measuring apparatus  100  does not produce additional costs that would be required when changing the control system. 
     A third embodiment according to the present invention will now be described with reference to FIGS.  19 ( a ),  19 ( b ) and  20 . This embodiment relates to an improved cam follower  25  of the valve lifters  191 ,  192  employed in the first embodiment. The cam follower  25  of this embodiment may be used with either the cam  11  of the first embodiment or the cam  540  of the prior art. In this embodiment, the cam follower  25  is applied to a valve drive apparatus employing intake valve cams  311 , which are identical to the prior art cams  540 . The structure of the third embodiment differs from the first embodiment only in the cam follower  25  and the intake valve cam  311 . Thus, parts that are like or identical to corresponding parts in the first embodiment are denoted with the same reference numerals. 
     As shown in FIGS.  19 ( a ) and  19 ( b ), the slide surface  25   a  of each cam follower  25  is convex so that the middle portion is projected in comparison to the long edges. The slide surface  25   a  has a radius of curvature that is 50 to 300 times greater than the width of the cam follower  25 , where the width is measured in the horizontal direction of FIG.  19 ( a ). 
     As shown in FIGS.  20 ( a ), the portion of the cam surface  311  corresponding to the base circle is parallel to the axis of the cam  311 , or cylindrical. The portion of the cam surface  311  corresponding to the cam nose is inclined with respect to the axis of the cam  311 , as shown in FIG.  20 ( b ). Thus, during rotation of the cam  311 , the cam follower  25  is pivoted in its seat  24  in accordance with the inclination of the cam surface  311   a.    
     As shown in FIG.  20 ( a ), a slight clearance exists between the cam surface  311   a  and the slide surface  25   a  of the cam follower  25  when the cam follower  25  faces the portion of the cam surface  311   a  corresponding to the base circle of the cam  311 . The clearance is provided to prevent the portion of the cam surface  311   a  corresponding to the base circle of the cam  311  from opening the associate valve  18  when the cam  311 , the associated valve lifter  191 ,  192 , and the associated valve thermally expand. 
     The cam  311  rotates from the state shown in FIG.  20 ( a ) to the state shown in FIG.  20 ( b ). When the portion of the cam surface  311   a  corresponding to the cam nose faces the cam follower  25 , the cam surface  311   a  comes into contact with the slide surface  25   a . If the slide surface  25   a  is flat, the edge  25   c  of the cam follower  25  would first come into contact with the cam surface  311   a , this may damage the cam surface  311   a . However, in this embodiment, the slide surface  311   a  is convex. Thus, damage to the cam surface  311   a  is prevented since the edge  25   c  does not contact the cam surface  311   a.    
     Furthermore, the convexly arched slide surface  25   a  is in contact with the cam surface  311   a , as shown in FIGS.  20 ( b ) and  20 ( c ). This reduces the force and impact applied to the cam surface  311   a  when the slide surface  25   a  comes into contact with the cam surface  311   a  in comparison to when the edge  25   c  comes into contact with the slide surface  25   a . As a result, damage to and wear of the cam surface  311   a  is prevented. 
     As shown in FIG.  20 ( b ), the cam follower  311   a  pivots in the direction of the arrow when contacting the cam surface  311   a . This faces the slide surface  25   a  of the cam follower  25  toward the cam surface  311   a . In this state, the middle portion of the slide surface  25   a  contacts the cam surface  311   a  and the edges  25   b ,  25   c  of the cam follower  25  do not contact the cam surface  311   a.    
     Accordingly, the same advantages obtained in the first embodiment are obtained in this embodiment by providing the convex slide surface  25   a . More specifically, satisfactory lubrication is maintained between the cam surface  311   a  and the cam follower  25  in the same manner as in the first embodiment. Thus, damage to and wear of the cam surface  311   a  and the cam follower  25  are reduced or eliminated. This maintains smooth relative movement between the cam surface  311   a  and the cam follower  25 . Furthermore, the cam surface  311   a  is prevented from becoming uneven due to wear and is prevented from becoming scratched. Therefore, the cam follower  25  is not interfered with by an uneven surface or scratches when the cam  311  moves axially. Accordingly, the open and closing timing and valve lift amount of the intake valves  18  are varied smoothly. 
     A fourth embodiment according to the present invention will now be described with reference to FIGS.  21 ( a ) and  21 ( b ). In this embodiment, the cam follower  25  of the third embodiment is modified. The cam follower  25  has a slide surface  25   a  that is convexly arched not only in the axial direction of the cam, but also in a direction perpendicular to the axis of the cam. 
     A fifth embodiment according to the present invention will now be described with reference to FIG.  22 . The cam follower  25  of the third embodiment is modified in this embodiment. The cam follower  25  has a slide surface  25   a  provided with a flat middle portion and rounded edges  25   b ,  25   c . In other words, only the edges of the slide surface  25   a  are curved. The radii of curvature R of the edges  25   b ,  25   c  are equal to each other. 
     A sixth embodiment according to the present invention will now be described with reference to FIG.  23 . The cam follower  25  of this embodiment differs from that of the embodiment shown in FIG. 22 in that each edge  25   b ,  25   c  is rounded to define a curved surface having three radii of curvatures R 1 , R 2 , R 3 . In other words, each edge  25   b ,  25   c  includes three portions, each portion having a different radius of curvature R 1 , R 2 , R 3 . In the cam follower  25  of FIG. 22, a ridge line exists between the slide surface  25   a  and the curved surface. However, a ridge line does not exist in the cam follower  25  of FIG.  23 . This guarantees the prevention of damages to the cam surface of the associated cam. 
     It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. More particularly, the present invention may also be embodied as described below. 
     If the shaft shifting mechanism  15  shown in FIG. 2 is provided for the exhaust valve camshaft  12  in addition to or in lieu of that of the intake valve camshaft  10 , the present invention may be applied to cams  13  of the camshaft  12  and the cam followers of the associated valve lifters  21 . 
     The measuring apparatus  100  may be used not only to measure the three-dimensional cam  11  shown in FIG. 1 but also to measure other types of cams. For example, the measuring apparatus  100  may be used to measure a normal cam having a cam surface parallel to the cam axis. Although a slight change may become necessary in the control program, the mechanical structure of the measuring apparatus  100  need not be changed to accommodate different types of cams. 
     In the valve drive apparatus shown in FIG. 6, the intake valve cams  11  are provided integrally with the camshaft  10  and the shaft shifting mechanism  15  axially moves the camshaft  10  together with the cams  11 . However, the camshaft  10  and the cams  11  may be constructed so that the camshaft  10  remains in a fixed position while only the cams  11  move axially. 
     The engine  1  shown in FIG. 2 has four valves for each cylinder. However, the present invention may be applied to an engine that employs more than or less than four valves for each cylinder. 
     In the valve drive apparatus shown in FIG. 2, each valve  11  drives a corresponding valve lifter  191 ,  192 . However, the present invention may be employed in a valve drive apparatus that drives two valve lifters with a single cam  11 . 
     The measuring apparatus  100  shown in FIG. 8 measures the axial position and rotary phase of the camshaft  10  and associates the measured values with the height of the cam surface  11   a . However, the measuring apparatus  100  may be eliminated if the rotary drive device  104  and the linear drive device  106  are driven with high precision. In this case, the command values sent from the control circuit  102  to drive the rotary drive device  104  and the linear drive device  106  are associated with the height of the cam surface  11   a . Such structure also allows accurate measurement of the cam surface. 
     The profile measuring tool  120  shown in FIG. 9 pivotally supports the contact element  122  with the holder  124 . However, the contact element  122  need not be pivotally supported by the holder  124 . For example, the structure supporting the contact element  122  may be replaced by a structure similar to that of the structure supporting the cam follower seat  24  with the associated valve lifter  191 ,  192 . In other words, the holder  124  may have concave recesses similar to that of the cam follower seat  24  to pivotally receive the contact element  122 . 
     When measuring the height of the cam surface  11   a  with the measuring apparatus  100  of FIG. 8, the height of the cam surface  11   a  need not be measured directly. A physical quantity corresponding to the height of the cam surface  11   a  may be measured instead. For example, a predetermined reference point may be defined on the surface of the cam  11  so that the distance from the reference point to the cam surface  11   a  is used as the physical quantity corresponding to the height of the cam surface  11   a . As another option, a contact sensor or a non-contact sensor may be attached to the surface of the cam  11 . In this case, the output signal (e.g., voltage) sent from the sensor is used as the physical quantity corresponding to the height of the cam surface  11   a.    
     Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.