Patent Publication Number: US-8523536-B2

Title: Axial piston pump, and power transmission device with axial piston pump

Description:
INCORPORATION BY REFERENCE 
     The disclosure of Japanese Patent Application No. 2007-225090 filed on Aug. 31, 2007, including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an axial piston pump capable of reciprocating a piston provided in a cylinder chamber in an axial direction of a drive shaft by using cam device capable of rotating integrally with the drive shaft. The invention also relates to a power transmission device having the axial piston pump. 
     2. Description of the Related Art 
     There is a conventional multi-stroke type axial piston pump, which has cam members having cam surfaces facing in an axial direction of a drive shaft and rotating integrally with the drive shaft, and in which roller rolling on the cam surfaces are supported to pistons reciprocating in the axial direction (see Japanese Patent Application Publication No. 2006-233972 (JP-A-2006-233972)). 
     The shape of each cam surface of the cam member of the pump disclosed in JP-A-2006-233972 is constant, and the pump capacity cannot be changed due to a constant stroke quantity of the pistons. Therefore, the pump disclosed in JP-A-2006-233972 is not suitable for changing the pump capacity depending on the situation. 
     However, when such a pump is incorporated in an automatic transmission of a vehicle such as an automobile, and the input side and the output side of a power transmission path are connected to a drive shaft and a driven shaft of the pump, respectively, to drive the pump by means of a rotational difference between the input side and the output side, the flow rate of oil suctioned by the pump increases and thereby the suction resistance of the oil increases due to a significant rotational difference between the input side and the output side upon startup from rest, which might impede the rollers from following the cam surface. Therefore, it is desired to change this configuration in accordance with the situation of the pump capacity and prevent the increase of the flow rate of the oil suctioned by the pump. 
     SUMMARY OF THE INVENTION 
     Therefore, this invention provides an axial piston pump capable of changing the pump capacity, and a power transmission device for a vehicle which has this pump. 
     Therefore, according to an aspect of this invention, an axial piston pump that generates hydraulic pressure by means of rotational power input from a drive shaft is provided. This axial piston pump has: a cylinder body that forms a cylinder chamber extending in an axial direction of the drive shaft and rotates integrally with a driven shaft; a piston that is inserted into the cylinder chamber and reciprocates in the axial direction of the drive shaft in the cylinder chamber; and a cam device. This cam device rotates integrally with the drive shaft and has: a fixed cam member that has a cam surface capable of coming into contact with a cam follower coupled to the piston and is capable of rotating integrally with the drive shaft, with movement of the fixed cam member in the axial direction being restricted; and a movable cam member that has a cam surface capable of coming into contact with the cam follower and is capable of rotating integrally with the drive shaft, with movement of the movable cam member in the axial direction being allowed, an irregularity difference in the axial direction of the cam surface of the fixed cam member and an irregularity difference in the axial direction of the cam surface of the movable cam member being different from each other. 
     According to this axial piston pump, the stroke quantity of the piston can be changed by separately using the fixed cam member and the movable cam member that have different irregularity differences on the respective cam surfaces. Accordingly the pump capacity can be changed depending on the situation. Since the fixed cam member is restricted in moving in the axial direction, a stroke of the piston corresponding to the cam surface of the fixed cam member can be secured even when the movable cam member can no longer move for any reason. 
     Also, according to another aspect of the invention, a power transmission device that is provided within a power transmission path extending from a power source for traveling of a vehicle to a drive wheel is provided. This power transmission device has: a drive shaft to which one of an output side and an input side of the power transmission path is connected; a driven shaft that is disposed coaxially with the driven shaft and to which the other one of the output side and the input side of the power transmission path is connected; a cam device that rotates integrally with the drive shaft; a cylinder body that forms therein a cylinder chamber extending in the axial direction of the drive shaft and integrally rotates with the driven shaft; a piston that is inserted into the cylinder chamber and reciprocates; an axial piston pump that is capable of reciprocating the piston with respect to the axial direction by means of the cam device and discharging fluid suctioned into the cylinder chamber from the cylinder chamber. The cam device has: a fixed cam member that has a cam surface capable of coming into contact with a cam follower coupled to the piston and is capable of rotating integrally with the drive shaft, with movement of the fixed cam member in the axial direction being restricted; a movable cam member that has a cam surface capable of coming into contact with the cam follower and is capable of rotating integrally with the drive shaft, with movement of the movable cam member in the axial direction being allowed; and a cam effecting device that uses the fluid discharged from the cylinder chamber to change over between a restrained state where the movable cam member is restrained to an effective position with respect to the axial direction, in which the cam follower can follow the cam surface of the movable cam member, and a release state where the restraint of the movable cam member to the effective position is released. The axial piston pump is characterized in that an irregularity difference in the axial direction of the cam surface of the fixed cam member is smaller than an irregularity difference in the axial direction of the cam surface of the movable cam member. 
     According to this power transmission device, since the axial piston pump is interposed between the output side and input side of the power transmission path, the pump can be driven by the rotational difference between the input side and the output side to suction or discharge the oil. The cam device provided in this pump has the fixed cam member and the movable cam member that have different irregularity differences on the respective cam surfaces so that these cam members can be used separately depending on the traveling condition of the vehicle and the condition of the power source for traveling. In such a circumstance as the startup of the vehicle, where the rotational difference between the input side and the output side is significant, the flow rate of the oil suctioned by the pump can be prevented from increasing by reducing the pump capacity, whereby followability of the cam follower relative to the cam surface can be secured. At the time of steady traveling, the rotational difference between the input side and the output side can be reduced by increasing the pump capacity, preventing the energy loss in the pump. Furthermore, even in the case where the cam effecting device cannot readily obtain the hydraulic pressure to be used immediately after starting up the power source, the fixed cam member having a small irregularity difference on the cam surface thereof is made effective automatically. When it is difficult to obtain the hydraulic pressure, the rotational difference between the input side and the output side is significant when the vehicle is stopped. Therefore, making the fixed cam member having a small irregularity difference on the cam surface thereof effective can secure the followability of the cam follower relative to the cam surface even in this kind of situation. 
     As described above, according to this invention, the stroke quantity of the piston can be changed by separately using the fixed cam member and the movable cam member that have different irregularity differences on the respective cam surfaces. As a result, the pump capacity can be changed according to the situation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features, advantages, and technical and industrial significance of this invention will be described in the following detailed description of example embodiments of the invention with reference to the accompanying drawings, in which like numerals denote like elements, and wherein: 
         FIG. 1  is a skeleton diagram showing simplified power transmission path and other elements of a vehicle which is provided with a power transmission device incorporated with a pump related to an embodiment of the invention; 
         FIG. 2  is a vertical cross-sectional view showing a substantial part of the pump of  FIG. 1 ; 
         FIG. 3  is an explanatory diagram taken along a direction of arrow III shown in  FIG. 2 ; 
         FIG. 4  is a vertical cross-sectional view showing an element of the pump relating to a flow of lubricant oil, the element being shown in  FIG. 2 ; 
         FIG. 5  is a horizontal cross-sectional view showing a cross section taken along line V-V of  FIG. 4 ; 
         FIG. 6  is a horizontal cross-sectional view showing a cross section taken along line VI-VI of  FIG. 4 ; 
         FIG. 7  is a horizontal cross-sectional view showing a cross section taken along line VII-VII of  FIG. 4 ; 
         FIG. 8  is a horizontal cross-sectional view showing a cross section taken along line VIII-VIII of  FIG. 4 ; 
         FIG. 9  is a horizontal cross-sectional view showing a cross section taken along line IX-IX of  FIG. 4 ; 
         FIG. 10  is a horizontal cross-sectional view showing a cross section taken along line X-X of  FIG. 4 ; 
         FIG. 11  is a horizontal cross-sectional view showing a cross section taken along line XI-XI of  FIG. 4 ; and 
         FIG. 12  is a horizontal cross-sectional view showing a cross section taken along line XII-XII of  FIG. 4 ; 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Example embodiments of the present invention will be described in greater detail below with reference to the accompanying drawings. 
       FIG. 1  is a skeleton diagram showing simplified power transmission path and other elements of a vehicle which is provided with a power transmission device incorporated with an axial piston pump related to an embodiment of the invention. A vehicle  1  is provided with an internal combustion engine  2  as its power source for traveling. An output torque of the internal combustion engine  2  is input to a power transmission device  4  accommodated in a casing  3  and then transmitted to a drive wheel  12  after gear change and other various operations are performed. The power transmission device  4  is configured such that a torque transmitted to an input shaft  6  via a damper mechanism  5  is transmitted to the drive wheel  12  via a pump  7 , forward/reverse change-over device  8 , continuously variable transmission  9 , transmission device  10  and final reduction gear  11 . The vehicle  1  is provided with an electronic control unit (ECU)  110  functioning as a computer for controlling the entire vehicle  1 , and a hydraulic control device  120  for controlling hydraulic pressure element of the power transmission device  4  on the basis of an output signal from the ECU  110 . 
     The pump  7  functions as both an oil pump function serving as a hydraulic pressure source, and a power transmission function serving as a starting device of the vehicle  1 . The pump  7  is configured as a multi-stroke type axial piston pump which is capable of reciprocating a piston  14  with respect to a direction of axis Ax 1  of the input shaft  6  by means of a cam unit  13  serving as cam means capable of rotating integrally with the input shaft  6  serving as a drive shaft, and reciprocating the piston  14  at least twice at each rotation of the cam unit  13 . The rotation of the piston  14  is transmitted to a hollow connecting drum  15  that is coaxially provided outside the input shaft  6 . 
     The forward/reverse change-over device  8  is interposed between the connecting drum  15  and a primary shaft  16  of the continuously variable transmission  9  and changes over the rotation direction of the primary shaft  16  between a normal rotation direction and a reverse rotation direction. The forward/reverse change-over device  8  has a planetary gear mechanism  17 . The planetary gear mechanism  17  has a sun gear  17   a  that integrally rotates with the primary shaft  16 , a ring gear  17   b  that is provided coaxially with the sun gear  17   a , a pinion  17   c  that is meshed with these gears  17   a ,  17   b , and a carrier  17   d  that holds the pinion  17   c  around the sun gear  17   a  so that the pinion  17   c  can rotate and revolve around the sun gear  17   a . The forward/reverse change-over device  8  further has a clutch  20  that connects the sun gear  17   a  and the ring gear  17   b  to each other or releases the connection, and a braking device  21  that inhibits rotation of the carrier  17   d  and releases the inhibition of the rotation. The forward/reverse change-over device  8  changes over the rotation direction of the primary shaft  16  to the normal rotation direction by connecting the sun gear  17   a  and the ring gear  17   b  to each other by the clutch  20 , with the braking device  21  allowing the carrier  17   d  to rotate, and changes over the rotation direction of the primary shaft  16  to the reverse rotation direction by releasing the connection between the sun gear  17   a  and the ring gear  17   b  by the clutch  20 , with the braking device  21  inhibiting the rotation of the carrier  17   d.    
     The continuously variable transmission  9  is configured as a conventional continuously variable transmission that uses a belt. The continuously variable transmission  9  changes the groove width of a primary pulley  23  that rotates integrally with the primary shaft  16  and the groove width of a secondary pulley  25  that orates integrally with a secondary shaft  24  connected to the transmission device  10  to change the winding diameter of a belt  26  wound between the pulleys  23 ,  25 . Consequently, the rotational speed ratio between the primary shaft  16  and the secondary shaft  24  can be changed continuously. The rotation that is output from the continuously variable transmission  9  is decelerated by the transmission device  10  and thereafter by the final reduction gear  11 , and then output to a drive shaft  27  coupled to the drive wheel  12 . 
     Next, the pump  7  shown in  FIG. 1  is described in detail with reference to  FIGS. 2 to 12 .  FIG. 2  is a vertical cross-sectional view showing a substantial part of the pump  7 . Note that  FIG. 2  illustrates a cross section of the characterizing parts of elements of the pump  7 , wherein the positions of movable elements of the pump  7  differ between the upper half and the lower half of the diagram with respect to the direction of the axis Ax 1  because these movable elements are shown in one diagram. 
     As shown in  FIG. 2 , the pump  7  has a pump housing  30  that accommodates elements such as the cam unit  13  and the piston  14 . In the pump housing  30  the input shaft  6  and the connecting drum  15  are supported coaxially so as to be able to rotate freely. The input shaft  6  and the connecting drum  15  are joined coaxially to each other with a bearing  31  interposed therebetween, so as to be rotatable relative to each other as shown on the right side of  FIG. 2 . An interposed member  32  is spline-coupled to the outer periphery of the connecting drum  15  and mounted on this connecting drum  15  so as to be rotatable integrally therewith. This interposed member  32  is supported rotatably to an opening  30   a  of the pump housing  30  via a bearing  33 . The input shaft  6  is configured as a stepped shaft the outer diameter of which increases in a stepwise fashion toward the left-hand side of  FIG. 2 , and an oil hole  35  that extends in the direction of the axis Ax 1  (called “axial direction” hereinafter) and is opened leftward is formed in the center of the input shaft  6 . A guide piece  36  in the form of a stepped shaft for guiding oil to a predetermined position is coaxially fitted in the oil hole  35 . Note that the oil is supplied, as lubricant oil, between the input shaft  6  and the connecting drum  15  by supply paths  101 . The supply paths  101  are configured by both a supply pipe  100  inserted into the center of the guide piece  36  and the oil hole  35  of the input shaft  6 . The oil that is supplied as lubricant oil is led to each part of the power transmission device  4 . 
     The cam unit  13  is provided on the outer periphery of the input shaft  6  so as to be rotatable integrally with the input shaft  6 . The piston  14  which is driven by the cam unit  13  is inserted into a cylinder chamber  41  of a cylinder body  40  so as to be reciprocable, the cylinder body  40  being disposed coaxially with the input shaft  6 . Between the cam unit  13  and the cylinder body  40 , a rotary valve  47  for changing over between suction and discharge of the oil from and to the cylinder chamber  41  is mounted on the outer periphery of the input shaft  6 . A bearing  43  that bears the radial load is interposed between the cylinder body  40  and the input shaft  6 . A collar  44  which projects up to a part of a side surface of the cylinder  40  is mounted on the input shaft  6 , and a bearing  45  which bears the axial load is interposed between the collar  44  and the side surface of the cylinder body  40 . The cylinder body  40  is made rotatable relative to the input shaft  6  by means of these bearings  43 ,  45 . The cylinder body  40  has a projecting part  46  that projects from the side surface of the cylinder body  40  to the right-hand side of  FIG. 2 . This projecting part  46  is spline-coupled to the interposed member  32  rotating integrally with the connecting drum  15 . Therefore, the cylinder body  40  can rotate integrally with the connecting drum  15  while being to rotate relative to the input shaft  6 . 
       FIG. 3  is an explanatory diagram taken along a direction of arrow III shown in  FIG. 2 . As shown in  FIG. 2  and  FIG. 3 , the cam unit  13  has: a fixed cam member  51  which has a cam surface  52  capable of coming into contact with a roller  50  serving as a cam follower coupled rotatably to the piston  14  and is restricted in moving in the axial direction; a first movable cam member  53  which has a cam surface  54  capable of coming into contact with the roller  50  and is capable of moving in the axial direction; a second movable cam member  55  which has a cam surface  56  capable coming into contact with the roller  50  and is capable of moving in the axial direction; and a moving device  57  which is capable of moving the two cam members  53 ,  55  separately to predetermined positions in the axial direction and restraining these cam members  53 ,  55  to these positions. Also, an urging member  58 , such as a coil spring, for urging the roller  50  to the cam surfaces  52 ,  54 ,  55  is provided within the cylinder chamber  41  in order to cause the roller  50  to follow each of the cam members  51 ,  53 ,  55 . These cam members  51 ,  53 ,  55  are disposed coaxially, with the fixed cam member  51  being disposed on the innermost side, the second movable cam member  55  on the outermost side, and the first movable cam member  53  therebetween. 
     As shown in  FIG. 2 , the fixed cam member  51  is spline-coupled to the outer periphery of the input shaft  6  so as to be unrotatable relative to the input shaft  6 , and rotates integrally with the input shaft  6 . A projecting part  6   a  of the input shaft  6 , which projects radially outward, restricts the fixed cam member  51  in moving to be apart from the piston  14  in the axial direction, i.e., toward the left-hand side of  FIG. 2 . The fixed cam member  51  is also press-fitted onto the outer periphery of the input shaft  6  so that it is restricted in moving toward the right-hand side of  FIG. 2  as well. Note that the fixed cam member  51  may be restricted in moving in the axial direction by providing the input shaft  6  with a snap ring or a collar. The first movable cam member  53  is spline-coupled to the fixed cam member  51  in a state in which the first movable cam member  53  is allowed to move in the axial direction, so as to be able to rotate integrally with the input shaft  6 . Also, the second movable cam member  55  is spline-coupled to the first movable cam member  53  in a state in which the second movable cam member  55  is allowed to move in the axial direction, so as to be able to rotate integrally with the input shaft  6 . 
     As shown in  FIG. 3 , an irregularity difference with respect to the axial direction of the cam surface  52  of the fixed cam member  51 , that is, a lifted amount L 1 , is smaller than lifted amounts L 2 , L 3  of the other cam members  53 ,  55 . In addition, the lifted amount L 2  of the first movable cam member  53  is smaller than the lifted amount L 3  of the second movable cam member  55 . Therefore, a relationship of L 1 &lt;L 2 &lt;L 3  is established among these lift amounts L 1  to L 3 . The stroke quantity of the piston  14  can be changed, accordingly, by appropriately selecting a cam member from these three cam members  51 ,  53 ,  55  to push the roller  50  (piston  14 ) in. In other words, the capacity of the pump  7  can be changed. 
     In order to push the roller  50  in by means of the cam member or, in other words, in order to make the cam member effective, this specific cam member needs to be restrained to a predetermined position with respect to the axial direction. In this regard, since the fixed cam member  51  is restricted in moving in the axial direction, the fixed cam member  51  is automatically made effective by not restraining the other movable cam members  53 ,  55  to their positions (see the section below the axis Ax 1  shown in  FIG. 2 ). 
     The moving range of the first movable cam member  53  is set so that it can move between a position on a virtual line where the apex  54   a  of the cam surface  54  is on a position P 2  of the apex  52   a  of the cam surface  52  of the fixed cam member  51  or recedes from the position P 2 , and a position on a solid line where the lowermost part  54   b  of the cam surface  54  is on a position P 1  of the lowermost part  52   b  of the cam surface  52  or moves forward of the position P 1 . In the moving range of the first movable cam member  53 , the receding movement thereof is restricted by a stopper  61  which is provided coaxially with the input shaft  6  so as not to be movable in the axial direction, while the forward movement of the first movable cam member  53  is restricted by the fixed cam member  51 , as shown in  FIG. 2 . The moving range of the second movable cam member  55  is also set, as shown in  FIG. 3 , so that it can move between a position on a virtual line where the apex  56   a  of the cam surface  56  is on the position P 2  or recedes from the position P 2 , and a position on the solid line where the lowermost part  56   b  of the cam surface  56  is on the position P 1  or moves forward of the position P 1 . In the moving range of the second movable cam member  55 , the receding movement thereof is restricted by a stopper  62  which is provided coaxially with the input shaft  6  so as not to be movable in the axial direction, while the forward movement of the second movable cam member  55  is restricted by a stopper  63  which is provided coaxially with the input shaft  6  between the stopper  61  and the stopper  63  so as not to be movable in the axial direction, as shown in  FIG. 2 . These stoppers  61  to  63  are held between the projecting part  6   a  of the input shaft  6  and a collar  64  mounted on the input shaft  6 , and thus are inhibited from moving in the axial direction. 
     The moving device  57  is operated using hydraulic pressure and has: a first control chamber  71  for moving and restraining the first movable cam member  53  to a position shown by a solid line in  FIG. 3 ; a second control chamber  72  for moving and restraining the second movable cam member  55  to a position shown in  FIG. 3 ; and an oil pressure regulator  73  (see  FIG. 1 ) for regulating hydraulic pressure (pressure) of oil guided to each of the control chambers  71 ,  72  as working oil. Here, the oil corresponds to the fluid associated with this invention and the oil pressure regulator  73  to a pressure regulator associated with this invention. The first control chamber  71  is provided in a region surrounded by the first movable cam member  53 , the stopper  61  and the input shaft  6 . The second control chamber  72  is provided in a region surrounded by the second movable cam member  55 , the stopper  62  and the stopper  63 . As shown in  FIG. 1 , the oil pressure regulator  73  as a part of the components of the hydraulic control device  120  controlling hydraulic pressure of each part of the power transmission device  4 . Appropriate operation of the oil pressure regulator  73  provided in the hydraulic control device  120  allows individual adjustment of the hydraulic pressure of the oil guided to each of the control chambers  71 ,  72 . A flow of oil of the moving device  57  having the oil pressure regulator  73  is described hereinafter along with the description of a flow of oil suctioned and discharged by the pump  7 . 
       FIG. 4  is a vertical cross-sectional view showing an element of the pump  7  relating to a flow of the oil, the element being shown in  FIG. 2 .  FIGS. 5 to 12  are horizontal cross-sectional views showing cross sections taken along line V-V, line VI-VI, line VII-VII, line VIII-VIII, line IX-IX, line X-X, line XI-XI, and line XII-XII of  FIG. 4 , respectively. Note that the flow of the oil is shown by the arrowed lines in these drawings. 
     As shown in  FIGS. 4 and 5 , in the cylinder body  40  twelve cylinder chambers  41  are formed at equal intervals in a circumferential direction, and each of the cylinder chambers  41  is provided with the piston  14 . Oil paths  81  are formed in the cylinder body  40 . Each of the oil paths  81  has an opening  81   a  communicated with each cylinder chamber  41  and opened in the axial direction. As shown in  FIG. 4  and  FIGS. 6 to 9 , ten suction ports  82  and ten discharge ports  83  are formed alternately at equal intervals along the circumferential direction in the rotary valve  47 . In this embodiment, each of the cam surfaces  52 ,  54 ,  56  has ten concave parts and convex parts, the numbers of which correspond to the numbers of the suction ports  82  and discharge ports  83 . Each of the suction ports  82  has an opening  82   a  opened in the axial direction and an opening  82   b  opened in the radial direction. Each of the discharge ports  83  also has an opening  83   a  opened in the axial direction and an opening  83   b  opened in the radial direction. The opening  82   a  of the suction port  82  and the opening  83   a  of the discharge port  83  are disposed in the same position as the opening  81   a  of the oil path  81  of the cylinder body  40  with respect to the radial direction so as to be communicated with the opening  81   a.  As is clear from the  FIGS. 4 ,  7  and  9 , the opening  82   b  of the suction port  82  and the opening  83   b  of the discharge port are disposed in different position with respect to the axial direction. Specifically, the opening  82   b  of the suction port  82  is provided in a position where the opening port  82   b  can be communicated with suction paths  84  formed in the guide piece  36  and the input shaft  6 , while the opening  83   b  of the discharge port  83  is provided in a position where the opening port  83   b  can be communicated with discharge paths  85  formed in the input shaft  6  and the guide piece  36 . 
     Because the suction ports  82  of the rotary valve  47  are communicated with the suction path  84  and the discharge ports  83  are communicated with the discharge paths  85  as described above, when the cylinder body  40  rotates relative to the rotary valve  47  in accordance with a rotational difference between the cylinder body  40  and the cam unit  13 , the ports that are communicated with the openings  81   a  of the oil path  81  of the cylinder body  40  are sequentially changed over between the suction ports  82  and the discharge ports  83 . Therefore, the oil is guided to the cylinder chambers  41  through the suction paths  84  and the suction ports  82  when the cylinder chambers  41  is in a suction stroke, and the oil of the cylinder chambers  41  is discharged through the discharge ports  83  and the discharge paths  85  when the cylinder chambers  41  are in the discharge stroke. 
     Next, a flow of the oil in the moving device  57  is described. As shown in  FIG. 4  and  FIGS. 10 to 12 , the moving device  57  is further provided with a first introduction path  91  for guiding the lubricant oil to the first control chamber  71 , and a second introduction path  92  for guiding the lubricant oil to the second control chamber  72 . As shown in  FIGS. 4 and 11 , the first introduction path  91  has a vertical path  91   a  that is formed in the guide piece  36  and extends in the axial direction, and a horizontal path  91   b  that extends in the radial direction and is communicated with the vertical path  91   a  and the first control chamber  71 . The vertical path  91   a  is opened at a left end of the guide piece  36  and communicated with a first control path  93  formed on an inner surface of the pump housing  30 . The horizontal path  91   b  is formed in the guide piece  36  and the input shaft  6 . On the other hand, the second introduction path  92  has a horizontal path  92   a  that is formed in the guide piece  36  and extends in the axial direction, and a horizontal path  92   b  that extends in the radial direction and is communicated with the horizontal path  92   a  and the second control chamber  72 , as shown in  FIGS. 4 and 12 . The horizontal path  92   a  is opened at the left end of the guide piece  36  and communicated with a second control path  94  formed on the inner surface of the pump housing  30 . Note that the opened positions of the vertical path  92   a  of the second introduction path  92  and of the vertical path  91   a  of the first introduction path  91  at the left end of the guide piece  36  are different with respect to the circumferential direction, and these vertical paths  91   a ,  92   a  are communicated with the first and second control paths  93 ,  94  while being sealed by sealing means such as an O-ring. As a result, the vertical path  91   a  of the first introduction path  91  is communicated only with the first control path  93 , and the vertical path  92   a  of the second introduction path  92  is communicated only with the second control path  94 . 
     As shown in  FIGS. 1 and 4 , the oil pressure regulator  73  has a first control valve  96  and second control valve  97  for independently regulating the hydraulic pressure of the first control path  93  and the hydraulic pressure of the second control path  94 . The first control valve  96  is capable of changing over between a state that allows the communication between the first control path  93  and the discharge paths  85  and a state that opens the first control path  93  to an oil pan  115  ( FIG. 1 ). The second control valve  97  is capable of changing over between a state that allows the communication between the second control path  94  and the discharge paths  85  and a state that opens the second control path  94  to the oil pan. Therefore, then the first control path  93  is communicated with the oil paths  85  by the first control valve  96 , the hydraulic pressure of the first control path  93  increases and the first introduction path  91  and the first control chamber  71  become filled with the oil. As a result, the capacity of the first control chamber  71  increases and the first movable cam member  53  is restrained to an effective position (see the section above the axis Ax 1  shown in  FIGS. 2 and 3 ). This state corresponds to a restrained state associated with the invention. When, on the other hand, the first control path  93  is opened to the oil pan  115  by the first control valve  96 , the hydraulic pressure of the first control path  93  decreases. As a result, the hydraulic pressure of the first control chamber  71  decreases and the first movable cam member  53  restrained to the effective position thereof is released (see the section below the axis Ax 1  shown in  FIGS. 2 and 3 ). This state corresponds to a release state associated with the invention. In the second control path  94  as well, when the second control path  94  and the discharge paths  85  are communicated with each other by the second control valve  97 , the hydraulic pressure of the second control path  94  increases and the second introduction path  92  and the second control chamber  72  become filled with the oil. As a result, the capacity of the second control chamber  72  increases and the second movable cam member  55  is restrained to the effective position (see the section above the axis Ax 1  shown in  FIGS. 2 and 3 ). When, on the other hand, the second control path  94  is opened to the oil pan  115  by the second control valve  97 , the hydraulic pressure of the second control path  94  decreases. As a result, the hydraulic pressure of the second control chamber  72  decreases and the second movable cam member  55  restrained to the effective position thereof is released (see the section below the axis Ax 1  shown in  FIGS. 2 and 3 ). 
     Therefore, by opening the first control path  93  and the second control path  94  to the oil pan  115  by means of the first control valve  96  and the second control valve  97 , the fixed cam member  51  shown in  FIGS. 2 and 3  is made effective. Moreover, by allowing the first control path  93  and the discharge paths  85  to be communicated with each other by means of the first control valve  96  and opening the second control path  94  to the oil pan  115  by means of the second control valve  97 , the first movable cam member  53  is made effective. In addition, by opening the first control path  93  to the oil pan  115  by means of the first control valve  96  and allowing the second control path  94  and the discharge paths  85  to be communicated with each other by means of the second control valve  97 , the second movable cam member  55  is made effective. Note in the embodiment shown in  FIG. 3  that the position of the lowermost part  56   b  of the cam surface  56  of the restrained second movable cam member  55  is set at the position same as or forward of the position of the lowermost part  54   b  of the cam surface  54  of the restrained first movable cam member  53 . For this reason, by allowing the first control path  93  and the second control path  94  to be communicated with the discharge paths  85  by means of the first control valve  96  and the second control valve  97 , the second movable cam member  55  can be made effective. Therefore, for example, by allowing the second control path  94  and the discharge paths  85  to be communicated with each other by means of the second control valve  97  while keeping the first movable cam member  53  effective, the second movable cam member  55  is made effective. As a result, it becomes possible to readily control the transition of changing over the operation of making these movable cam members  53 ,  55  effective. 
     When changing over between the cams to be made effective, the piston  14  is inhibited from stroking along the cam surfaces of at least two cams in the course of the changing over. Therefore, the fixed cam member  51 , the first movable cam member  53  and the second movable cam member  55  shown in  FIG. 2  are configured such that the axial rigidities of the movable cam members  53 ,  55  are lower than the axial rigidity of the fixed cam member  51 . Axial rigidity means the degree of change in the dimensions of the cam members  51 ,  53 ,  55  in the axial direction, the change being caused by the load of the piston  14 . Specifically, the movable cam members  53 ,  55  illustrated in the embodiment are configured such that the degree of change in the dimension of each of the movable cam members  53 ,  55  is greater than that of the fixed cam member  51 . More specifically, the cam members  51 ,  53 ,  55  are configured as follows. 
     As shown in  FIG. 2 , the first movable cam member  53  has a load bearing part  53   a  that bears the loads of the piston  14  and of the first control chamber  71 . The load bearing part  53   a  is made of a material having a Young&#39;s modulus lower than that of a load bearing part  51   a  of the fixed cam member  51 . The load bearing part  51   a  bears the loads of the piston  14  and of the projecting part  6   a  of the input shaft  6  (bearing reaction force). Also, the first movable cam member  53  is configured such that the axial thickness of the load bearing part  53   a  is made thicker than the axial thickness of the load bearing part  51   a  of the fixed cam member  51 . Moreover, the first movable cam member  53  is configured such that the moment arm of the load bearing part  53   a  is made longer than the moment arm of the load bearing part  51   a  of the fixed cam member  51 . The moment arm of the load bearing part  53   a  is equivalent to the distance in the radial direction between the first control chamber  71  and the cam surface  54 , while the moment arm of the load bearing part  51   a  is equivalent to the distance in the radial direction between the projecting part  6   a  and the cam surface  52 . In this way, the first movable cam member  53  is configured to have rigidity lower than that of the fixed cam member  51 . Note that, as another embodiment, at least one of the means for reducing the Young&#39;s modulus, reducing the thickness and increasing the moment arm of the material of the first movable cam member  53  in relation to the fixed cam member  51  can be performed on the first movable cam member  53  to reduce the rigidity of the first movable cam member  53  lower than the rigidity of the fixed cam member  51 . 
     The second movable cam member  55  has a load bearing part  55   a  for bearing the loads of the piston  14  and of the second control chamber  72 . The load bearing part  55   a  is made of a material having a Young&#39;s modulus lower than that of the load bearing part  51   a  of the fixed cam member  51 . Therefore, the second movable cam member  55  is configured to have rigidity lower than that of the fixed cam member  51 . Note that, as with the case described above, at least one of the means for reducing the axial thickness of the load bearing part  55   a  more than the axial thickness of the load bearing part  51   a  and increasing the moment arm of the load bearing part  55   a  more than the moment arm of the load bearing part  51   a  can be performed on the second movable cam member  55  to reduce the rigidity of the second movable cam member  55  lower than the rigidity of the fixed cam member  51 . 
     Because the piston  14  is inhibited from stroking along the cam surfaces of at least two cams in the course of changing over between the cams to be made effective, fluctuation of the hydraulic pressure of the cylinder  41  can be prevented. 
     Since the first control chamber  71  and the second control chamber  72  are configured to rotate integrally with the input shaft  6  as shown in  FIG. 2 , rotation of the input shaft  6  generates centrifugal force in the oil of the first control chamber  71  and the second control chamber  72 , generating centrifugal hydraulic pressure. Therefore, the moving device  57  further has a first canceling chamber  75  and second canceling chamber  76  for preventing the first movable cam member  53  and the second movable cam member  55  from being moved by this centrifugal hydraulic pressure against a control command. The oil is supplied to the first canceling chamber  75  and the second canceling chamber  76  by the guide piece  36  and a canceling path  99  formed on the input shaft  6  as shown in  FIGS. 4 and 10 . 
     Returning to  FIG. 1 , control of each part of the power transmission device  4  is now described. The power transmission device  4  is controlled by the ECU  110  and the hydraulic control device  120 . Various parameters that reflect the operational state of the internal combustion engine  2  and the traveling condition of the vehicle  1  are input to the ECU  110 . For example, rotational speed of the internal combustion engine  2  is input from a crank angle sensor  111  and traveling speed of the vehicle  1  is input from a vehicle speed sensor  112 . Based on these parameters, the ECU  110  outputs a signal for controlling the internal combustion engine  2  and a signal for controlling the hydraulic control device  120 . In addition to the oil pressure regulator  73  having the first control valve  96  and the second control valve  97 , the hydraulic control device  120  is further provided with a flow regulating valve  113  and the like as described hereinafter. The hydraulic control device  120  controls these valves based on the output signals from the ECU  110  and thereby control the operations of the pump  7  of the power transmission device  4 , the forward/reverse change-over device  8 , and the continuously variable transmission  9 . 
     With respect to the operational control of the pump  7 , the hydraulic control device  120  controls the first control valve  96  and the second control valve  97  shown in  FIG. 4  on the basis of the output signals from the ECU  110 , and thereby selects a cam member suitable to the situation. For example, by controlling the first control valve  96  and the second control valve  97  depending on the load of the internal combustion engine  2  while the vehicle  1  is traveling, the fixed cam member  51 , the first movable cam member  53  and the second movable cam member  55  are used separately. As a result, the capacity of the pump  7  can be changed according to the operational state of the internal combustion engine  2  and the traveling condition of the vehicle  1  and the loss of energy in the pump  7  can be reduced. Also, due to a significant difference between the rotational speed of the input shaft  6  coupled to the internal combustion engine  2  and the rotational speed of the connecting drum  15  coupled to the drive wheel  12  when the vehicle  1  is started (rotational difference), the flow rate of the oil suctioned into the cylinder chamber  41  increases and accordingly the suction resistance of the oil increases, which easily impedes the roller  50  from following the cam surface. Even in such a situation, the flow rate of the oil can be prevented from increases and followability of the roller  50  relative to the cam surface can be secured, by making the fixed cam member  51  having a small lifted amount effective. Moreover, when it is difficult to obtain sufficient hydraulic pressure immediately after starting up the engine, the rotational difference between the input shaft  6  and the connecting drum  15  is significant when the vehicle is stopped. However, in the case where the hydraulic pressure is not supplied to the first control chamber  71  and the second control chamber  72 , the fixed cam member  51  with a small lifted amount is made effective automatically. Consequently, the followability of the roller  50  relative to the cam surface can be secured even in such a situation. 
     As shown in  FIG. 1 , the discharge path  85  of the pump  7  is provided with the regulating valve  113  for regulating the flow rate of the oil to be discharged from the pump  7 . Upon startup of the vehicle  1 , the flow rate regulating valve  113  is operated to regulate the flow rate of the oil discharged from the pump  7  so that the rotational speed of the output side of the pump  7 , i.e., the connecting drum  15 , can be controlled. In this manner, the pump  7  can be caused to function as a starting device. 
     The forward/reverse change-over device  8  and the continuously variable transmission  9  are controlled in the same manner as in the related art. Specifically, with regard to the control of the forward/reverse change-over device  8 , the ECU  110  detects a forward or reverse request based on a signal from a shift position sensor (not shown) for detecting the position of the shift lever of the vehicle  1 , and controls the clutch  20  and the braking device  21  to realize the request. With regard to the control of the continuously variable transmission  9 , the ECU  110  controls the groove widths of the primary pulley  23  and secondary pulley  25  so as to obtain an appropriate transmission gear ratio proportionate to the rotational speed of the internal combustion engine  2  and the vehicle speed of the vehicle  1 . 
     The invention is not limited to the embodiment described above, and thus various types of modifications are possible within the scope of the invention. The power transmission device is not the only subject of application of the pump according to the embodiment of the invention. Therefore, the pump according to the invention may be used for various purposes. Although the cam unit  13  is provided on the input side and the cylinder body  40  (piston  14 ) on the output side in the embodiment described above, the invention can be implemented in an embodiment in which the cam unit  13  is provided on the output side and the cylinder body  40  (piston  14 ). 
     In addition, the two movable cam members  53 ,  55  were described as an example of the movable cam members according to the invention, but there is no limit on the number of the movable cam members. Therefore, the invention can be implemented as a pump having one or three or more movable cam members. 
     While the invention has been described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the exemplary embodiments or constructions. To the contrary, the invention is intended to cover various modifications and equivalent arrangements. In addition, while the various elements of the exemplary embodiments are shown in various combinations and configurations, which are exemplary, other combinations and configurations, including more, less or only a single elements, are also within the spirit and scope of the invention.