Patent Publication Number: US-2022221079-A1

Title: Flow channel switching valve

Description:
TECHNICAL FIELD 
     The present invention relates to a flow channel switching valve that switches connection of flow channels by rotating a valve member. 
     BACKGROUND ART 
     An example of a conventional flow channel switching valve is disclosed in Patent Literature 1. The flow channel switching valve includes a ball valve member and a valve case. The ball valve member has an inflow path and an outflow path. The valve case has a valve chamber in which the ball valve member is rotatably housed, an inlet flow channel communicating with the valve chamber, and a plurality of outlet flow channels communicating with the valve chamber. A valve stem is coupled to the ball valve member, the ball valve member is rotated as the valve stem rotates, and hence the inlet flow channel selectively communicates with one of the plurality of outlet flow channels. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2010-223418 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     In such a flow channel switching valve, the valve stem is combined with a rotation angle sensor, and the rotational position of the valve member is detected based on the output of the rotation angle sensor. However, there has been a possibility that the output of the rotation angle sensor with respect to the rotational position of the valve member includes an error due to accumulation of the tolerances of the valve member, the valve stem, the rotation angle sensor, and so forth. Thus, there has been room for improvement in the precision of the rotational position of the valve member. 
     Accordingly, an object of the present invention is to provide a flow channel switching valve capable of effectively improving the precision of the rotational position of a valve member. 
     Solution to Problem 
     To attain the above-described object, a flow channel switching valve according to an aspect of the present invention is a flow channel switching valve including a valve body provided with a valve chamber and a plurality of flow channels that communicate with the valve chamber, a valve member that is rotatably housed in the valve chamber and that switches connection of the flow channels in accordance with a rotational position, a valve stem that is attached to the valve member along a rotation axis of the valve member, and a drive unit that rotates the valve member via the valve stem. The flow channel switching valve includes a rotation angle output shaft that is press-fitted into an attachment hole provided in an end surface of the valve stem; and a rotation angle detection unit that detects a rotation angle of the rotation angle output shaft around the rotation axis. The rotation angle output shaft has a small-diameter portion, a medium-diameter portion, and a large-diameter portion that are sequentially connected in a direction of the rotation axis, and is supported rotatably around the rotation axis in the attachment hole in an inserted state before being press-fitted into the attachment hole. The attachment hole is provided with a press-fit portion into which one portion of the small-diameter portion, the medium-diameter portion, and the large-diameter portion is press-fitted. 
     According to the present invention, the rotation angle output shaft has the small-diameter portion, the medium-diameter portion, and the large-diameter portion that are sequentially connected in the direction of the rotation axis. The rotation angle output shaft is supported rotatably around the rotation axis in the attachment hole in the inserted state before being press-fitted into the attachment hole. The attachment hole provided in the valve stem is provided with the press-fit portion into which the one portion of the small-diameter portion, the medium-diameter portion, and the large-diameter portion of the rotation angle output shaft is press-fitted. With this configuration, in the state in which the rotation angle output shaft has been inserted into the attachment hole, the rotation angle output shaft can be rotated around the rotation axis to perform positioning, and can be press-fitted and fixed after the positioning. Accordingly, it is possible to further reduce an error of the output of the rotation angle detection unit due to the tolerances of the valve member, the valve stem, and the rotation angle detection unit. Thus, it is possible to effectively improve the precision of the rotational position of the valve member. 
     In the present invention, it is preferable that the valve stem be press-fitted into a through hole provided in a gear of the drive unit, and the press-fit portion is provided at a position shifted from the gear in the direction of the rotation axis. With this configuration, it is possible to avoid the influence of deformation of the valve stem (attachment hole) caused by press-fitting of the gear. Accordingly, it is possible to prevent the rotation angle output shaft from being press-fitted in an inclined manner. Thus, it is possible to effectively improve the precision of the rotational position of the valve member. 
     In the present invention, it is preferable that the attachment hole have a guide portion with which at least one of other portions of the small-diameter portion, the medium-diameter portion, and the large-diameter portion except the portion that is press-fitted into the press-fit portion comes into contact slidably in an axial direction and a circumferential direction. With this configuration, the other portion comes into contact with the guide portion to guide the movement of the rotation angle output shaft. Thus, it is possible to more effectively prevent the rotation angle output shaft from being press-fitted in an inclined manner with respect to the valve stem. 
     In the present invention, it is preferable that the guide portion be provided to be shifted from the gear in the direction of the rotation axis. With this configuration, it is possible to avoid the influence of deformation of the valve stem (attachment hole) caused by press-fitting of the gear. Accordingly, it is possible to further effectively prevent the rotation angle output shaft from being press-fitted in an inclined manner. 
     In the present invention, it is preferable that the guide portion be provided to be shifted in the direction of the rotation axis from a bearing portion that rotatably supports the valve stem. With this configuration, it is possible to avoid the influence of deformation of the valve stem caused by press-fitting of the rotation angle output shaft. Thus, it is possible to prevent the rotation of the valve stem from being hindered and to ensure smooth rotation of the valve member. 
     In the present invention, it is preferable that the attachment hole be provided with the press-fit portion into which the medium-diameter portion is press-fitted, and two guide portions with which the small-diameter portion and the large-diameter portion come into contact slidably in an axial direction and a circumferential direction. With this configuration, the small-diameter portion and the large-diameter portion come into contact with the two guide portions to guide the movement of the rotation angle output shaft. Thus, it is possible to more effectively prevent the rotation angle output shaft from being press-fitted in an inclined manner with respect to the valve stem. 
     Advantageous Effects of Invention 
     According to the present invention, it is possible to effectively improve the precision of the rotational position of the valve member. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view including a partial section of a flow channel switching valve according to a first embodiment of the present invention. 
         FIG. 2  is a sectional view taken along line A-A of  FIG. 1 . 
         FIG. 3  is a six sided view of a ball valve member included in the flow channel switching valve of  FIG. 1 . 
         FIG. 4  is a view for explaining a method of assembling the flow channel switching valve of  FIG. 1  and illustrates a state in which a gear is about to be attached after a valve stem is attached to the valve member. 
         FIG. 5  is a view for explaining the method of assembling the flow channel switching valve of  FIG. 1  and illustrates a state in which a potentiometer shaft is about to be inserted into an attachment hole of the valve stem. 
         FIG. 6  is a view for explaining the method of assembling the flow channel switching valve of  FIG. 1  and illustrates a state in which a potentiometer base is about to be fixed to a valve body. 
         FIG. 7  is a view for explaining the method of assembling the flow channel switching valve of  FIG. 1  and illustrates a state in which a potentiometer is about to be attached to the potentiometer base. 
         FIG. 8  is a view for explaining the method of assembling the flow channel switching valve of  FIG. 1  and illustrates a state in which the position of the potentiometer shaft is adjusted after the potentiometer is attached to the potentiometer base. 
         FIG. 9  is a view for explaining the method of assembling the flow channel switching valve of  FIG. 1  and illustrates a state in which the potentiometer shaft is press-fitted into the attachment hole of the valve stem. 
         FIG. 10  is a view illustrating a flow channel switching valve according to a second embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
     A configuration of a flow channel switching valve according to a first embodiment of the present invention will be described below with reference to  FIGS. 1 to 3 . 
       FIG. 1  is a perspective view including a partial section of the flow channel switching valve according to the first embodiment of the present invention.  FIG. 2  is a sectional view taken along line A-A of  FIG. 1 .  FIG. 3  is a six sided view of a ball valve member included in the flow channel switching valve of  FIG. 1 . In the following description, “upper, lower, left, and right” are used to indicate the relative positional relationship between respective components in each of the drawings, but do not indicate the absolute positional relationship. In each of the drawings, a left-right direction is defined as an X-axis direction, a front-back direction is defined as a Y-axis direction, and an up-down direction is defined as a Z-axis direction. The X axis, the Y axis, and the Z axis are orthogonal to one another. 
     As illustrated in  FIGS. 1 and 2 , a flow channel switching valve  1  of the present embodiment includes a valve body  10 , a ball valve member  20 , seat members  30 ,  30 , sealing members  31 ,  31 , a drive unit  40 , and a valve stem  50 . The flow channel switching valve  1  also includes a potentiometer shaft  60  that is a rotation angle output shaft, a potentiometer base  70  that is a base body, and a potentiometer  80  that is a rotation angle detection unit. 
     The valve body  10  is made of synthetic resin and formed in a substantially cubic box shape. A substantially L-shaped first flow channel  11  is provided in a left wall portion  10   a  of the valve body  10 . A straight second flow channel  12  is provided in a front wall portion  10   b  of the valve body  10 . A substantially L-shaped third flow channel  13  is provided in a right wall portion  10   c  of the valve body  10 . The third flow channel is plane-symmetrical to the first flow channel  11 . An opening  11   a  of the first flow channel  11 , an opening  12   a  of the second flow channel  12 , and an opening  13   a  of the third flow channel  13  face the front direction (the forward direction from the paper in  FIG. 1 , downward in  FIG. 2 ). The first flow channel  11 , the second flow channel  12 , and the third flow channel  13  communicate with a valve chamber  14  provided in the valve body  10 . As the flow channels that communicate with the valve chamber  14 , two or four or more flow channels may be provided. 
     The ball valve member  20  is formed in a hollow ball-like shape (spherical shape). The ball valve member  20  is made of, for example, metal or synthetic resin. The ball valve member  20  is housed in the valve chamber  14 . The ball valve member  20  is rotatably supported by the seat members  30 ,  30  (described later). At the rotational position illustrated in  FIG. 2 , the ball valve member  20  has a first opening  21  open toward the left side, a second opening  22  open toward the front side, and a third opening  23  open toward the right side. A switching flow path  25  that connects the first opening  21 , the second opening  22 , and the third opening  23  to one another is provided inside the ball valve member  20 . The switching flow path  25  is formed in a substantially T-like shape in plan view. Alternatively, the ball valve member  20  may have, for example, only the first opening  21  and the second opening  22 , and may be provided with a switching flow path  25  that is substantially L-shaped in plan view and connects the first opening  21  and the second opening  22  to each other at the rotational position illustrated in  FIG. 2 . Although the ball valve member  20  is used as a valve member in the present embodiment, a columnar valve member may be used. 
     The switching flow path  25  is configured to switch the connection of the first flow channel  11 , the second flow channel  12 , and the third flow channel  13  in accordance with the rotational position. Specifically, the switching flow path  25  connects the first flow channel  11 , the second flow channel  12 , and the third flow channel  13  to one another when the ball valve member  20  is at the rotational position illustrated in  FIG. 2 . The switching flow path  25  connects the first flow channel  11  and the second flow channel  12  to each other when the ball valve member  20  is at a rotational position rotated 90 degrees clockwise in plan view from the rotational position illustrated in  FIG. 2 . The switching flow path  25  connects the second flow channel  12  and the third flow channel  13  to each other when the ball valve member  20  is at a rotational position rotated 90 degrees counterclockwise in plan view from the rotational position illustrated in  FIG. 2 . 
     A valve stem insertion hole  24  into which the valve stem  50  (described later) is inserted is provided in an upper portion of the ball valve member  20 . The valve stem insertion hole  24  is formed such that the ball valve member  20  rotates around an axis L that is a rotation axis of the ball valve member  20  as the valve stem  50  rotates. Specifically, the valve stem insertion hole  24  is formed in the same shape as the sectional shape (cross-sectional shape) in the direction orthogonal to the axial direction of an angular columnar portion  52  of the valve stem  50 . In the present embodiment, the valve stem insertion hole  24  is formed in a regular hexagonal shape ( FIG. 3 ). 
     The seat members  30 ,  30  are made of, for example, synthetic resin such as polytetrafluoroethylene (PTFE) and are formed in an annular shape. The seat members  30 ,  30  form a pair. The seat members  30 ,  30  are housed in the valve chamber  14 . The seat members  30 ,  30  are disposed to face each other with a gap in the X-axis direction. The seat members  30 ,  30  rotatably support the ball valve member  20  in the valve chamber  14  with the ball valve member  20  interposed therebetween. 
     The sealing members  31 ,  31  each are, for example, an O-ring made of an elastic material such as a rubber material. The sealing member  31  is disposed to be sandwiched in a compressed state between one of the seat members  30  and the left wall portion  10   a  of the valve body  10 . The sealing member  31  is disposed to be sandwiched in a compressed state between the other of the seat members  30  and the right wall portion  10   c  of the valve body  10 . In the present embodiment, the sealing member  31  is mounted in an annular groove  30   a  provided in the seat member  30 . Part of the sealing member  31  protrudes from the annular groove  30   a . The sealing members  31 ,  31  seal gaps between the valve body  10  and the ball valve member  20  together with the seat members  30 ,  30 . Alternatively, the sealing members  31 ,  31  may be omitted, and seat members  30 ,  30  made of an elastic material such as a rubber material and also having a function of a sealing member may be employed. 
     The drive unit  40  includes a drive mechanism, a lower case  43 , and an upper case (not illustrated). The drive mechanism includes a combination of a motor (not illustrated) and a speed reducer that includes a gear  41 . The lower case  43  and the upper case are made of resin and in which the drive mechanism is housed. The upper case is attached to the lower case  43  by an attachment structure such as a screwing structure or a snap-fit structure. The drive unit  40  rotates the ball valve member  20  around the axis L via the valve stem  50  (described later). 
     The lower case  43  integrally has a circular tubular bearing portion  45  at the center of a bottom wall  43   a . The valve stem  50  is inserted into the bearing portion  45 . The bearing portion  45  rotatably supports the valve stem  50 . An inner peripheral wall portion  43   b  having a quadrangular tubular shape is provided on the bottom wall  43   a  of the lower case  43 . The inner peripheral wall portion  43   b  is disposed inside the valve body  10  and combined with an upper end portion of the valve body  10 . The inner peripheral wall portion  43   b  and the valve body  10  are joined (in the present embodiment, ultrasonically welded). Alternatively, the lower case  43  and the valve body  10  may be assembled with each other by a screwing structure or the like. 
     The valve stem  50  is made of synthetic resin and is formed in a columnar shape extending straight as a whole. The valve stem  50  includes a circular columnar portion  51  and the angular columnar portion  52  coaxially connected to a lower end of the circular columnar portion  51 . The valve stem  50  is disposed along the axis L. 
     The circular columnar portion  51  has an annular stopper portion  53  provided at a lower end portion of the circular columnar portion  51  and protruding radially outward. The stopper portion  53  is formed to have an outer diameter larger than the inner diameter of the bearing portion  45 . 
     Moreover, the circular columnar portion  51  has a groove provided in the lower end portion over the entire circumference at a position above the stopper portion  53 . An annular O-ring  54  made of a rubber material or the like is fitted to the groove. The circular columnar portion  51  is inserted into the bearing portion  45 . The circular columnar portion  51  is rotatably supported by the bearing portion  45 . The outer diameter of the circular columnar portion  51  is slightly smaller than the inner diameter of the bearing portion  45 . When the circular columnar portion  51  is inserted into the bearing portion  45 , the O-ring  54  seals a gap between the valve stem  50  and the bearing portion  45 . The sealing with the O-ring  54  prevents the fluid in the valve chamber  14  from leaking to the outside. 
     The gear  41  of the drive mechanism of the drive unit  40  is press-fitted onto an upper end portion of the circular columnar portion  51 . Specifically, the circular columnar portion  51  is press-fitted into a through hole  41   a  provided at the center of the gear  41 . Part of the circular columnar portion  51  protrudes from an upper surface  41   b  of the gear  41 . Moreover, the upper end portion of the circular columnar portion  51  is provided with a planar portion  51   a  for preventing the press-fitted gear  41  from slipping. The gear  41  may be attached to the valve stem  50  by a method other than press-fitting. 
     The angular columnar portion  52  is formed in a columnar shape having a regular hexagonal cross-sectional shape. The angular columnar portion  52  is inserted into the valve stem insertion hole  24  of the ball valve member  20 , and is attached to the ball valve member  20  along the axis L. The axis L of the ball valve member  20  serves as a rotation axis of the valve stem  50 . The valve stem  50  is rotated around the axis L as the gear  41  rotates. The valve stem insertion hole  24  is formed in the same regular hexagonal shape as the cross-sectional shape of the angular columnar portion  52 . Thus, the valve stem insertion hole  24  and the angular columnar portion  52  are fitted to each other. The ball valve member  20  is rotated around the axis L as the valve stem  50  rotates. The angular columnar portion  52  is formed to have an outer diameter smaller than that of the stopper portion  53 . 
     The angular columnar portion  52  may have a polygonal columnar shape such as a triangular columnar shape or a quadrangular columnar shape, or a columnar shape having a D-shaped section in which part of the side surface of a circular column is formed as a plane, other than the regular hexagonal shape. In this case, the valve stem insertion hole  24  is also formed in the same shape as the cross-sectional shape of the angular columnar portion  52 . 
     Moreover, an attachment hole  55  is provided at the center of an upward facing end surface  51   b  of the circular columnar portion  51 . The attachment hole  55  has a substantially circular columnar inner space along the axis L. The attachment hole  55  has an attachment hole lower portion  56  and an attachment hole upper portion  57  coaxially connected to the upper side of the attachment hole lower portion  56  ( FIG. 9 ). The attachment hole lower portion  56  and the attachment hole upper portion  57  have circular cross-sectional shapes. The attachment hole  55  is formed such that the diameter of the attachment hole lower portion  56  is smaller than the diameter of the attachment hole upper portion  57 . A step portion  58  is provided between the attachment hole lower portion  56  and the attachment hole upper portion  57 . 
     The attachment hole lower portion  56  is provided to be shifted from the gear  41  in the direction of the axis L. In the present embodiment, the attachment hole lower portion  56  is provided to be disposed on the radially inner side of the bearing portion  45 . An upper end portion  57   a  of the attachment hole upper portion  57  is located above the upper surface  41   b  of the gear  41 . That is, the upper end portion  57   a  is a portion provided to be shifted from the bearing portion  45  and the gear  41  in the direction of the axis L. 
     The upper end portion  57   a  of the attachment hole upper portion  57  is a press-fit portion into which a large-diameter portion  62  of the potentiometer shaft  60  (described later) is press-fitted. The attachment hole lower portion  56  is a guide portion with which a small-diameter portion  64  comes into contact slidably in the axial direction (the direction of the axis L that is also the insertion direction of the potentiometer shaft  60  into the attachment hole  55 ) and the circumferential direction (the direction around the axis L). 
     The potentiometer shaft  60  is made of metal such as stainless steel or brass, or synthetic resin such as polyphenylene sulfide (PPS). The potentiometer shaft  60  is a separate component from the valve stem  50 . The potentiometer shaft  60  is press-fitted into the attachment hole  55  of the valve stem  50 . The potentiometer shaft  60  is fixedly attached coaxially to the valve stem  50  by press-fitting. The potentiometer shaft  60  includes a fitting shaft portion  61 , the large-diameter portion  62 , a medium-diameter portion  63 , and the small-diameter portion  64  sequentially from the upper side to the lower side in the axial direction of the potentiometer shaft  60 . In other words, the small-diameter portion  64 , the medium-diameter portion  63 , and the large-diameter portion  62  are sequentially connected in the direction of the axis L from the ball valve member  20  side toward the gear  41  side. 
     The fitting shaft portion  61  is formed in a columnar shape (so-called D-cut shape) having a D-shaped section in which part of the side surface of a circular column is formed as a plane. The fitting shaft portion  61  is provided at one end portion of the potentiometer shaft  60 , and is fitted to a rotor  81  of the potentiometer  80  (described later). At least the distal end of the fitting shaft portion  61  protrudes from the attachment hole  55 . The large-diameter portion  62  is formed in a circular columnar shape. The large-diameter portion  62  has a diameter larger than the diameter of the attachment hole upper portion  57  of the attachment hole  55 . The medium-diameter portion  63  is formed in a circular columnar shape. The medium-diameter portion  63  has a diameter smaller than the diameter of the large-diameter portion  62  and the diameter of the attachment hole upper portion  57  and larger than the diameter of the small-diameter portion  64 . The medium-diameter portion  63  connects the large-diameter portion  62  and the small-diameter portion  64 . The small-diameter portion  64  is formed in a circular columnar shape. The small-diameter portion  64  has the same diameter (including substantially the same diameter) as the diameter of the attachment hole lower portion  56  of the attachment hole  55 . 
     The potentiometer base  70  is made of synthetic resin. The potentiometer base  70  integrally has a base body portion  71  and a meter attachment portion  72 . The base body portion  71  is formed in a substantially flat plate shape. The base body portion  71  is fixed to bosses  43   c ,  43   c  protruding upward from the bottom wall  43   a  of the lower case  43  by screws  78 ,  78 . The meter attachment portion  72  has a disk-shaped bottom wall portion  72   a  and a peripheral wall portion  72   b  standing upward from the peripheral edge of the bottom wall portion  72   a . A recess  72   c  is provided at the center of the bottom wall portion  72   a . The recess  72   c  is a portion to which the potentiometer  80  (described later) is attached. Moreover, a through hole  72   d  is provided in the recess  72   c . The fitting shaft portion  61  of the potentiometer shaft  60  is passed through the through hole  72   d . The diameter of the through hole  72   d  is larger than the diameter of the fitting shaft portion  61 . 
     The potentiometer  80  is a rotation angle sensor for detecting a rotation angle. The potentiometer  80  has the disk-shaped rotor  81  and a meter body portion  82 . The meter body portion  82  rotatably supports the rotor  81 . The meter body portion  82  is a signal output unit that outputs a signal (voltage) corresponding to the rotation angle of the rotor  81 . A fitting hole  81   a  having a D-like shape in plan view is provided at the center of the rotor  81 . The fitting shaft portion  61  of the potentiometer shaft  60  passes through the fitting hole  81   a . The fitting shaft portion  61  is fitted to the fitting hole  81   a  so that the rotor  81  rotates together with the fitting shaft portion  61 . Rotation of the fitting shaft portion  61  makes the rotor  81  rotating. Thus, the potentiometer  80  detects the rotation angle of the potentiometer shaft  60  around the axis L. 
     In the flow channel switching valve  1 , the rotation of the motor of the drive unit  40  is output to the valve stem  50  through the gear  41 . The valve stem  50  is rotated around the axis L. The ball valve member  20  is rotated around the axis L as the valve stem  50  rotates, and is positioned at each rotational position. Thus, the connection of the flow channels corresponding to the rotational position is provided. Moreover, the potentiometer shaft  60  is rotated around the axis L together with the valve stem  50 . A signal corresponding to the rotation angle of the potentiometer shaft  60  is output from the potentiometer  80 . The rotational position of the ball valve member  20  can be monitored based on the signal output from the potentiometer  80 . 
     Next, an example of a method of assembling the flow channel switching valve  1  of the present embodiment will be described with reference to  FIGS. 4 to 9 . 
       FIGS. 4 to 9  are views for explaining a method of assembling the flow channel switching valve of  FIG. 1 , and are, sequentially, (1) a perspective view illustrating a state in which a gear is about to be attached after a valve stem is attached to a valve member, (2) a perspective view illustrating a state in which a potentiometer shaft is about to be inserted into an attachment hole of the valve stem, (3) a perspective view illustrating a state in which a potentiometer base is about to be fixed to a valve body, (4) a perspective view illustrating a state in which a potentiometer is about to be attached to the potentiometer base, (5) a perspective view illustrating a state in which the position of the potentiometer shaft is adjusted after the potentiometer is attached to the potentiometer base, and (6) a sectional view illustrating a state in which the potentiometer shaft is press-fitted into the attachment hole of the valve stem.  FIG. 9A  illustrates a state in which the potentiometer shaft has been inserted into the attachment hole of the valve stem (inserted state before being press-fitted), and  FIG. 9B  illustrates a state in which the potentiometer shaft has been press-fitted into and fixed to the attachment hole of the valve stem. 
     First, the ball valve member  20 , the seat members  30 ,  30 , and the sealing members  31 ,  31  are housed in the valve chamber  14  of the valve body  10 . Then, a jig (not illustrated) is inserted from the second flow channel  12  to position the ball valve member  20 . The angular columnar portion  52  of the valve stem  50  is inserted into the valve stem insertion hole  24  to attach the valve stem  50  to the ball valve member  20 . In this state, the valve stem  50  is disposed along the axis L (Z-axis direction). Then, the valve body  10  and the lower case  43  are assembled with each other while the circular columnar portion  51  of the valve stem  50  is inserted into the bearing portion  45 . Ultrasonic waves are applied to the lower case  43  to ultrasonically weld the lower case  43  to the valve body  10 . 
     Next, as illustrated in  FIG. 4 , the circular columnar portion  51  of the valve stem  50  is press-fitted into the gear  41 , and other components (not illustrated) constituting the drive mechanism are assembled with the lower case  43 . 
     Next, as illustrated in  FIG. 5 , the potentiometer shaft  60  is inserted into the attachment hole  55  of the valve stem  50 . Specifically, the potentiometer shaft  60  is inserted into the attachment hole  55  from the small-diameter portion  64 , and the medium-diameter portion  63  is inserted into the attachment hole  55  following the small-diameter portion  64 . The diameter of the small-diameter portion  64  of the potentiometer shaft  60  is the same as the diameter of the attachment hole lower portion  56 . The outer peripheral surface of the small-diameter portion  64  comes into contact with the inner peripheral surface of the attachment hole lower portion  56  slidably in the insertion direction and the circumferential direction. As described above, since the small-diameter portion  64  and the attachment hole lower portion  56  are provided, it is possible to prevent the potentiometer shaft  60  from being inserted in an inclined manner with respect to the valve stem  50 . The potentiometer shaft  60  is guided coaxially with the valve stem  50  along the axis L. Then, the potentiometer shaft  60  is further inserted until the large-diameter portion  62  abuts against the end surface  51   b  of the valve stem  50 . At this time, the potentiometer shaft  60  is in the inserted state before being press-fitted. The fitting shaft portion  61  and the large-diameter portion  62  of the potentiometer shaft  60  protrude from the attachment hole  55 . 
     Next, as illustrated in  FIG. 6 , the fitting shaft portion  61  of the potentiometer shaft  60  is passed through the through hole  72   d  provided in the meter attachment portion  72  of the potentiometer base  70 . The potentiometer base  70  is disposed on the bosses  43   c ,  43   c  of the lower case  43 . Then, the screws  78 ,  78  are screwed into the bosses  43   c ,  43   c  to fix the potentiometer base  70  to the lower case  43 . The lower case  43  is joined to the valve body  10  by ultrasonic welding. Thus, the potentiometer base  70  is fixed with respect to the valve body  10 . 
     Next, as illustrated in  FIG. 7 , the fitting shaft portion  61  of the potentiometer shaft  60  is passed through the fitting hole  81   a  provided in the rotor  81  of the potentiometer  80 , and the fitting shaft portion  61  is fitted to the fitting hole  81   a . The potentiometer  80  is disposed in the recess  72   c  of the meter attachment portion  72  of the potentiometer base  70 . The potentiometer  80  is attached and fixed by soldering or the like. Alternatively, the potentiometer base  70  may be fixed to the lower case  43  after the potentiometer  80  is attached to the potentiometer base  70 . 
     Next, the potentiometer shaft  60  is positioned around the axis L. As illustrated in  FIGS. 8 and 9A , the potentiometer shaft  60  has been fitted to the fitting hole  81   a  of the rotor  81 . Positioning of the potentiometer shaft  60  around the axis L is performed by rotating the potentiometer shaft  60  around the axis L in the attachment hole  55  so that a correct signal is output from the potentiometer  80  with respect to the rotational position of the ball valve member  20 . After the positioning of the potentiometer shaft  60  is completed, as illustrated in  FIG. 9B , the potentiometer shaft  60  is further inserted and pushed downward. In this manner, the large-diameter portion  62  of the potentiometer shaft  60  is press-fitted into the attachment hole upper portion  57  of the attachment hole  55 . The potentiometer shaft  60  is fixed to the valve stem  50  in the state in which the potentiometer shaft  60  has been positioned. Specifically, the large-diameter portion  62  is press-fitted into the upper end portion  57   a  of the attachment hole upper portion  57 . Thus, the potentiometer shaft  60  is fixed to the upper end portion  57   a  by press-fitting. 
     Finally, the upper case (not illustrated) is attached to the lower case  43  to complete the flow channel switching valve  1 . 
     As described above, according to the flow channel switching valve  1  of the present embodiment, the potentiometer shaft  60  has the small-diameter portion  64 , the medium-diameter portion  63 , and the large-diameter portion  62  that are sequentially connected in the direction of the axis L from the ball valve member  20  side. The potentiometer shaft  60  is supported rotatably around the axis L in the attachment hole  55  in the inserted state before being press-fitted into the attachment hole  55 . The attachment hole  55  provided in the valve stem  50  is provided with the upper end portion  57   a  of the attachment hole upper portion  57  serving as the press-fit portion into which the large-diameter portion  62  of the potentiometer shaft  60  is press-fitted. With this configuration, in the flow channel switching valve  1 , in the state in which the potentiometer shaft  60  has been inserted into the attachment hole  55 , the potentiometer shaft  60  can be rotated around the axis L to perform positioning, and can be press-fitted and fixed after the positioning. Accordingly, it is possible to further reduce an error of the output of the potentiometer  80  due to the tolerances of the ball valve member  20 , the valve stem  50 , and the potentiometer shaft  60 . Thus, it is possible to effectively improve the precision of the rotational position of the ball valve member  20 . 
     Moreover, the potentiometer shaft  60  is press-fitted into the upper end portion  57   a  of the attachment hole  55  of the valve stem  50 , the upper end portion  57   a  being shifted from the bearing portion  45  and the gear  41  in the direction of the axis L. Thus, in the flow channel switching valve  1 , it is possible to avoid the influence of deformation of the valve stem  50  caused by press-fitting of the potentiometer shaft  60 . For example, it is possible to avoid excessive compression of the O-ring  54  or contact of the valve stem  50  with the bearing portion  45  due to deformation of the valve stem  50  such that the diameter thereof increases. Thus, it is possible to prevent the rotation of the valve stem  50  from being hindered and to ensure smooth rotation of the ball valve member  20 . Moreover, it is possible to avoid the influence of deformation of the valve stem  50  (attachment hole  55 ) caused by press-fitting of the gear  41 . Accordingly, it is possible to prevent the potentiometer shaft  60  from being press-fitted in an inclined manner. Thus, it is possible to effectively improve the precision of the rotational position of the ball valve member  20 . 
     Moreover, the attachment hole  55  has the attachment hole lower portion  56  with which the small-diameter portion  64  of the potentiometer shaft  60  comes into contact slidably in the axial direction (the direction of the axis L) and the circumferential direction (the direction around the axis L). The attachment hole lower portion  56  is provided to be shifted from the gear  41  in the direction of the axis L. With this configuration, in the flow channel switching valve  1 , it is possible to avoid the influence of deformation of the valve stem  50  (attachment hole  55 ) caused by press-fitting of the gear  41 . Accordingly, it is possible to prevent the potentiometer shaft  60  from being press-fitted in an inclined manner, and it is possible to effectively improve the precision of the rotational position of the ball valve member  20 . 
     In the flow channel switching valve  1  of the present embodiment, the small-diameter portion  64  provided near the lower end of the potentiometer shaft  60  is guided by the attachment hole lower portion  56 , and the large-diameter portion  62  provided near the upper end of the potentiometer shaft  60  is press-fitted into the upper end portion  57   a  of the attachment hole upper portion  57 . However, the present invention is not limited thereto. That is, the attachment hole  55  (the attachment hole lower portion  56  and the attachment hole upper portion  57 ) of the valve stem  50  and the potentiometer shaft  60  may have any configurations as long as the configurations do not impair the object of the present invention. 
     Second Embodiment 
     A configuration of a flow channel switching valve according to a second embodiment of the present invention will be described below with reference to  FIG. 10 . 
       FIG. 10  is a view illustrating the flow channel switching valve according to the second embodiment of the present invention.  FIG. 10  is a sectional view illustrating a state in which a potentiometer shaft is press-fitted into an attachment hole of a valve stem.  FIG. 10A  illustrates a state in which the potentiometer shaft has been inserted into the attachment hole of the valve stem (inserted state before being press-fitted), and  FIG. 10B  illustrates a state in which the potentiometer shaft has been press-fitted into and fixed to the attachment hole of the valve stem. 
     A flow channel switching valve  2  of the present embodiment has the same configuration as that of the flow channel switching valve  1  of the above-described first embodiment except that the flow channel switching valve  2  includes a potentiometer shaft  60 A having a configuration different from that of the potentiometer shaft  60  instead of the potentiometer shaft  60 . In the flow channel switching valve  2 , the same components as those of the flow channel switching valve  1  described above are denoted by the same reference signs, and the description thereof will be omitted. 
     The potentiometer shaft  60 A includes a fitting shaft portion  61 , a large-diameter portion  62 A, a medium-diameter portion  63 A, and a small-diameter portion  64 A sequentially from the upper side to the lower side in the axial direction thereof. In other words, the small-diameter portion  64 A, the medium-diameter portion  63 A, and the large-diameter portion  62 A are sequentially connected in the direction of the axis L from the ball valve member  20  side toward the gear  41  side. 
     The large-diameter portion  62 A is formed in a circular columnar shape. The large-diameter portion  62 A has the same diameter (including substantially the same diameter) as the diameter of the attachment hole upper portion  57  of the attachment hole  55 . The medium-diameter portion  63 A is formed in a circular columnar shape. The medium-diameter portion  63 A has a diameter smaller than the diameter of the large-diameter portion  62 A and the diameter of the attachment hole upper portion  57  and larger than the diameter of the small-diameter portion  64 A. The medium-diameter portion  63 A connects the large-diameter portion  62 A and the small-diameter portion  64 A. The small-diameter portion  64 A is formed in a circular columnar shape. The small-diameter portion  64 A has the same diameter (including substantially the same diameter) as the diameter of the attachment hole lower portion  56  of the attachment hole  55 . The diameter of the medium-diameter portion  63 A is slightly larger than the diameter of the small-diameter portion  64 A (that is, the diameter of the attachment hole lower portion  56 ). In this embodiment, the diameter of the medium-diameter portion  63 A is larger than the diameter of the small-diameter portion  64 A by about 0.04 to about 0.06 mm. Accordingly, the medium-diameter portion  63 A is press-fitted into an upper end portion  56   a  of the attachment hole lower portion  56 . 
     In assembling the flow channel switching valve  2 , as illustrated in  FIG. 10A , the potentiometer shaft  60 A is inserted into the attachment hole  55  of the valve stem  50 . The diameter of the small-diameter portion  64 A of the potentiometer shaft  60 A is the same as the diameter of the attachment hole lower portion  56  serving as the guide portion. The outer peripheral surface of the small-diameter portion  64 A comes into contact with the inner peripheral surface of the attachment hole lower portion  56  slidably in the insertion direction and the circumferential direction. Moreover, the diameter of the large-diameter portion  62 A of the potentiometer shaft  60 A is the same as the diameter of the attachment hole upper portion  57  serving as the guide portion. The outer peripheral surface of the large-diameter portion  62 A comes into contact with the inner peripheral surface of the attachment hole upper portion  57  slidably in the insertion direction and the circumferential direction. As described above, since the small-diameter portion  64 A and the attachment hole lower portion  56 , and the large-diameter portion  62 A and the attachment hole upper portion  57  are provided, it is possible to prevent the potentiometer shaft  60 A from being inserted in an inclined manner with respect to the valve stem  50 , and the potentiometer shaft  60 A is guided along the axis L coaxially with the valve stem  50 . Then, as illustrated in  FIG. 10B , when the potentiometer shaft  60 A is further inserted, the medium-diameter portion  63 A is press-fitted into the upper end portion  56   a  of the attachment hole lower portion  56  serving as a press-fit portion. Thus, the potentiometer shaft  60 A is fixed to the valve stem  50 . The upper end portion  56   a  functions as a guide portion and a press-fit portion. 
     The flow channel switching valve  2  of the second embodiment also has the functions and effects similar to or the same as those of the flow channel switching valve  1  of the first embodiment described above. 
     In particular, in the flow channel switching valve  2 , the upper end portion  56   a  of the attachment hole lower portion  56  serving as the press-fit portion into which the medium-diameter portion  63 A is press-fitted is provided in the attachment hole  55 . The attachment hole  55  is provided with the attachment hole lower portion  56  and the attachment hole upper portion  57  as two guide portions with which the small-diameter portion  64 A and the large-diameter portion  62 A come into contact slidably in the axial direction and the circumferential direction. With this configuration, in the flow channel switching valve  2 , the small-diameter portion  64 A and the large-diameter portion  62 A come into contact with the two guide portions to guide the movement of the potentiometer shaft  60 A. Thus, it is possible to more effectively prevent the potentiometer shaft  60 A from being press-fitted into the valve stem  50  in an inclined manner. 
     Although the embodiments of the present invention have been described above, the present invention is not limited to these embodiments. Those skilled in the art may appropriately perform addition, deletion, or design change of a component with respect to the above-described embodiments, or may appropriately combine features of the embodiments. The modified or combined embodiments are included in the scope of the present invention as long as not impairing the gist of the present invention. 
     REFERENCE SIGNS LIST 
       1 ,  2  . . . flow channel switching valve,  10  . . . valve body,  10   a  . . . left wall portion,  10   b  . . . front wall portion,  10   c  . . . right wall portion,  11  . . . first flow channel,  12  . . . second flow channel,  13  . . . third flow channel,  11   a ,  12   a ,  13   a  . . . opening,  14  . . . valve chamber,  20  . . . ball valve member,  21  . . . first opening,  22  . . . second opening,  23  . . . third opening,  24  . . . valve stem insertion hole,  25  . . . switching flow path,  30  . . . seat member,  30   a  . . . annular groove,  31  . . . sealing member,  40  . . . drive unit,  41  . . . gear,  41   a  . . . through hole,  41   b  . . . upper surface,  43  . . . lower case,  43   a  . . . bottom wall,  43   b  . . . inner peripheral wall portion,  45  . . . bearing portion,  50  . . . valve stem,  51  . . . circular columnar portion,  51   a  . . . planar portion,  51   b  . . . end surface,  52  . . . angular columnar portion,  53  . . . stopper portion,  54  . . . O-ring,  55  . . . attachment hole,  56  . . . attachment hole lower portion,  56   a  . . . upper end portion,  57  . . . attachment hole upper portion,  57   a  . . . upper end portion,  60  . . . potentiometer shaft,  61  . . . fitting shaft portion,  62  . . . large-diameter portion,  63  . . . medium-diameter portion,  64  . . . small-diameter portion,  70  . . . potentiometer base,  71  . . . base body portion,  72  . . . meter attachment portion,  72   a  . . . bottom wall portion,  72   b  . . . peripheral wall portion,  72   c  . . . recess,  72   d  . . . through hole,  80  . . . potentiometer,  81  . . . rotor,  81   a  . . . fitting hole,  82  . . . meter body portion