Patent Publication Number: US-10788120-B2

Title: Automatic transmission

Description:
TECHNICAL FIELD 
     The present disclosure relates to an automatic transmission mounted on a vehicle, and belongs to a technical field of the automatic transmission for vehicles. 
     BACKGROUND OF THE DISCLOSURE 
     It is generally known that automatic transmissions mounted on vehicles may include a hydraulic power transmission device such as a torque converter, coupled to a driving source such as an engine, and a transmission mechanism coupled to the hydraulic power transmission device and provided with a plurality of planetary gear sets (planetary gear mechanisms) and a plurality of friction engaging elements, such as clutches and brakes. The plurality of friction engaging elements are selectively engaged by a hydraulic control to achieve a plurality of gear positions with different gear ratios. 
     In recent years, there is a tendency to eliminate the hydraulic power transmission device because of the desire for an increasing number of gear positions, and a reduction of the weight, etc., of the automatic transmission. In this case, it is possible to realize a smooth start of traveling by carrying out a slip control of at least one of the friction engaging elements which is engaged at a first gear when the vehicle starts traveling, while avoiding an engine stall. 
     When carrying out the slip control of the friction engaging element engaged at the first gear when the vehicle starts traveling, since the brake that causes the hydraulic chamber not to rotate is better in controllability at the time of the engagement than the clutch which causes the hydraulic chamber to rotate, it is possible to carry out slip control of the brake engaged at the first gear when the vehicle starts traveling. 
     In an automatic transmission structured in this way, since the frequency of executing the slip control increases for the brake engaged at the first gear when the vehicle starts traveling, it is necessary to effectively reduce a generation of heat from a friction plate due to the slip control in order to secure necessary durability. 
     In order to reduce the generation of heat from the friction plate, it is possible to increase the quantity of the hydraulic fluid for lubrication supplied to the friction plate to improve the cooling capability. However, in a brake in which a plurality of friction plates are disposed between an inner circumferential surface of the transmission case and an outer circumferential surface of a given rotary member accommodated in the transmission case, there is a possibility that hydraulic fluid for lubrication stagnates near the inner circumferential surface of the transmission case to cause drag between the friction plates that increases the rotational resistance. 
     On the other hand, for example, JP2016-090048A discloses a brake which is engaged at the first gear and is slip-controlled when the vehicle starts traveling. In this brake, a plurality of friction plates are disposed between an outer circumferential surface of a hub member coupled to a transmission case and an inner circumferential surface of a drum member coupled to a given rotary member. 
       FIG. 21  is a cross-sectional view illustrating a brake of a conventional automatic transmission. As illustrated in  FIG. 21 , a brake  200  includes a hub member  202  coupled to a transmission case  201 , a drum member  204  coupled to a given rotary member  203 , a plurality of friction plates  205  disposed between the hub member  202  and the drum member  204 , and a piston  206  which engages the plurality of friction plates  205 . 
     In the brake  200 , a supply oil channel  207  for lubrication extends, as indicated by an arrow  208 , radially inward of the transmission case  201  through the inside of a coupling member  209  coupled to the transmission case  201  and extends from one side in the axial directions to the other side through a spline part  210  formed in an outer circumferential surface of the hub member  202 , to supply hydraulic fluid for lubrication to the plurality of friction plates  205 . 
     The hydraulic fluid for lubrication receives a centrifugal force of the friction plates  205  which are spline-engaged with the drum member  204 , and then, as indicated by an arrow  211 , moves radially outwardly, and is supplied between the friction plates  205 , to cool the friction plates  205  heated by the slip control. Then, the hydraulic fluid for lubrication which moved to an inner circumferential surface of the drum member  204  is prevented from, as indicated by an arrow  212 , moving outward in the axial directions and stagnating by the rotation of the drum member  204 . 
     In the brake disclosed in JP2016-090048A, as illustrated in  FIG. 21 , since the hydraulic fluid for lubrication is moved from one side in the axial directions to the other side through the spline part  210  formed in the outer circumferential surface of the hub member  202 , the hydraulic fluid for lubrication may outflow radially outwardly as indicated by an arrow  213  before reaching the friction plates  205  to cause a reduction in the cooling efficiency of the friction plates  205 . 
     On the other hand, inside the hub member, by forming the supply oil channel for lubrication so as to extend axially, and by providing a supply port and an introduction port in a part where the friction plates are disposed and a part where the coupling member is disposed, respectively, the hydraulic fluid for lubrication can efficiently be supplied to the friction plates to improve the cooling efficiency of the friction plates. 
     In order to reduce the weight, the hub member of the brake may be formed by using an aluminum-based material. However, since a spline part with which the friction plates are spline-engaged needs to secure a given strength for receiving a force inputted from the friction plates when engaging the brake, the tooth length of the spline part may increase to increase the radial dimension. In addition, if a supply oil channel for lubrication is formed in the hub member and the hydraulic fluid for lubrication is supplied from the hub member, the radial dimension of the hub member may further increase. 
     Thus, in an automatic transmission provided with the brake where the plurality of friction plates are disposed between the hub member and the drum member, the automatic transmission needs to be compact in the radial dimension in order to be mounted to a limited internal space of a vehicle. However, if the hub member is provided with a spline part with which the friction plates are spline-engaged, and the supply oil channel for lubrication is formed by using an aluminum-based material, the radial dimension may increase. 
     Similarly, in an automatic transmission provided with a clutch where the plurality of friction plates are disposed between the hub member and the drum member, if the hub member provided with the spline part with which the friction plates are spline-engaged, and the supply oil channel for lubrication is formed by using aluminum-based material, the radial dimension may increase. 
     SUMMARY OF THE DISCLOSURE 
     Therefore, one purpose of the present disclosure is to provide an automatic transmission provided with a friction engaging element where a plurality of friction plates are disposed between a hub member and a drum member, which can supply hydraulic fluid for lubrication from the hub member, while reducing the radial size and the weight. 
     According to one aspect of the present disclosure, an automatic transmission is provided with a friction engaging element having a hub member, a drum member, a plurality of friction plates disposed between the hub member and the drum member, and a piston configured to engage the plurality of friction plates. The hub member includes a first hub member having a cylindrical part provided with a spline part, and is made of a ferrous material, the friction plates being spline-engaged with an outer circumferential part of the spline part, and a second hub member disposed at a side part of the first hub member, having a feed part for lubrication provided with a supply oil channel for lubrication configured to supply hydraulic fluid for lubrication to the friction plates from an inner circumferential side, and is made of an aluminum-based material. A notch for lubrication is formed by circumferentially cutting the cylindrical part of the first hub member corresponding to the feed part for lubrication of the second hub member. The feed part for lubrication supplies the hydraulic fluid for lubrication to the friction plates through the notch for lubrication. 
     According to this configuration, since the first hub member having the cylindrical part with the spline part is made of the ferrous material, the tooth height of the spline part can be reduced as compared with the case where the hub member having the cylindrical part with the spline part and the feed part for lubrication provided with the supply oil channel for lubrication is made of the aluminum-based material, thereby reducing the radial size. 
     Moreover, since the second hub member having the feed part for lubrication with the supply oil channel for lubrication is made of the aluminum-based material, while supplying the hydraulic fluid for lubrication from the hub member side, the weight can be reduced as compared with the case where the hub member having the cylindrical part with the spline part and the feed part for lubrication is made of the ferrous material. 
     Therefore, in the automatic transmission provided with the friction engaging element where the plurality of friction plates are disposed between the hub member and the drum member, the weight and the radial size can be reduced while supplying the hydraulic fluid for lubrication from the hub member side. 
     The friction engaging element may be a brake where the hub member is coupled to a transmission case, and the drum member is coupled to a given rotary member. 
     According to this configuration, the friction engaging element is a brake where the hub member is coupled to the transmission case and the drum member is coupled to the given rotary member. Thus, in the automatic transmission provided with the brake where the plurality of friction plates are disposed between the hub member and the drum member, the weight and the radial size can be reduced while supplying the hydraulic fluid for lubrication from the hub member side. 
     The automatic transmission may include a hydraulic chamber to which hydraulic fluid for biasing the piston is supplied. The second hub member may be provided with a supply oil channel for operation configured to supply the hydraulic fluid to the hydraulic chamber. 
     According to this configuration, the hydraulic chamber to which the hydraulic fluid for biasing the piston is provided, and the supply oil channel for operation which supplies the hydraulic fluid to the hydraulic chamber is provided to the second hub member. Thus, since the supply oil channel for lubrication and the supply oil channel for operation are formed in the second hub member, the supply oil channels of the hydraulic fluid can be simplified, as compared with a case where the supply oil channel for lubrication and the supply oil channel for operation are formed in the first hub member and the second hub member. 
     The automatic transmission may be coupled to a driving source without an intervening hydraulic power transmission device. The friction engaging element may be slip-controlled when a vehicle starts traveling, and is engaged at the first gear. 
     According to this configuration, the automatic transmission is coupled to the driving source without the intervening hydraulic power transmission device, and the friction engaging element is slip-controlled when the vehicle starts traveling and is engaged at the first gear. Thus, in the automatic transmission coupled to the driving source without the intervening hydraulic power transmission device, when the brake is provided, which is slip-controlled when the vehicle starts traveling and is engaged at the first gear, the weight and the radial size can be reduced while supplying the hydraulic fluid for lubrication from the hub member side. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a view schematically illustrating an automatic transmission according to one embodiment of the present disclosure. 
         FIG. 2  is an engagement table of friction engaging elements of the automatic transmission. 
         FIG. 3  is a cross-sectional view of a brake and peripheral parts of the automatic transmission. 
         FIG. 4  is another cross-sectional view of the brake and the peripheral parts of the automatic transmission. 
         FIG. 5  is another cross-sectional view of the brake and the peripheral parts of the automatic transmission. 
         FIG. 6  is another cross-sectional view of the brake and the peripheral parts of the automatic transmission. 
         FIG. 7  is another cross-sectional view of the brake and the peripheral parts of the automatic transmission. 
         FIG. 8  is a perspective view illustrating an assembled state of a hub member, an oil channel forming member, and a piston of the brake. 
         FIG. 9  is a perspective view illustrating an assembled state of the hub member and the oil channel forming member of the brake. 
         FIG. 10  is a perspective view illustrating the hub member of the brake. 
         FIG. 11  is a perspective view illustrating a first hub member. 
         FIG. 12  is a perspective view illustrating a second hub member. 
         FIG. 13  is a perspective view illustrating the piston. 
         FIG. 14  is a cross-sectional view of the hub member, taken along a line Y 14 -Y 14  in  FIG. 5 . 
         FIG. 15  is a perspective view illustrating a biasing unit. 
         FIG. 16  is a cross-sectional view of the biasing unit, taken along a line Y 16 -Y 16  in  FIG. 15 . 
         FIG. 17  is a cross-sectional view illustrating the brake in a released state. 
         FIG. 18  is a cross-sectional view illustrating the brake in an immediately-before-contact state. 
         FIG. 19  is a cross-sectional view illustrating the brake in a zero-clearance state. 
         FIG. 20  is a cross-sectional view illustrating the brake in an engaged state. 
         FIG. 21  is a cross-sectional view illustrating a brake of a conventional automatic transmission. 
     
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     Hereinafter, one embodiment of the present disclosure is described with reference to the accompanying drawings. 
       FIG. 1  is a view schematically illustrating an automatic transmission according to one embodiment of the present disclosure. An automatic transmission  10  is coupled to a driving source, such as an engine, without an intervening hydraulic power transmission device, such as a torque converter. The automatic transmission  10  includes, inside a transmission case  11 , an input shaft  12  coupled to the driving source and disposed at the driving-source side (left side in this figure), and an output shaft  13  disposed at the opposite side from the driving-source side or an anti-driving-source side (right side in the same figure). The automatic transmission  10  is of a longitudinal type for a front-engine rear-drive (FR) vehicle, etc., where the input shaft  12  and the output shaft  13  are disposed coaxially. 
     On the common axes of the input shaft  12  and the output shaft  13 , first, second, third, and fourth planetary gear sets (hereinafter, simply referred to as “first, second, third, and fourth gear sets”) PG 1 , PG 2 , PG 3 , and PG 4  are disposed in this order from the driving-source side. 
     Inside the transmission case  11 , a first clutch CL 1  is disposed at the driving-source side of the first gear set PG 1 , a second clutch CL 2  is disposed at the driving-source side of the first clutch CL 1 , and a third clutch CL 3  is disposed at the driving-source side of the second clutch CL 2 . Moreover, a first brake BR 1  is disposed at the driving-source side of the third clutch CL 3 , and a second brake BR 2  is disposed at the driving-source side of the third gear set PG 3  and at the anti-driving-source side of the second gear set PG 2 . 
     Each of the first, second, third, and fourth gear sets PG 1 , PG 2 , PG 3 , and PG 4  is of a single pinion type in which pinions supported by a carrier directly mesh with a sun gear and a ring gear. The first, second, third, and fourth gear sets PG 1 , PG 2 , PG 3 , and PG 4  each has sun gears S 1 , S 2 , S 3 , and S 4 , ring gears R 1 , R 2 , R 3 , and R 4 , and carriers C 1 , C 2 , C 3 , and C 4  as rotary members, respectively. 
     The first gear set PG 1  is of a double sun gear type in which the sun gear S 1  is axially divided into two. The sun gear S 1  has a first sun gear S 1   a  disposed at the driving-source side, and a second sun gear S 1   b  disposed at the anti-driving-source side. The first and second sun gears S 1   a  and S 1   b  have the same number of teeth, and mesh with the same pinions supported by the carrier C 1 . Thus, the first and the second sun gears S 1   a  and S 1   b  always rotate together. 
     In the automatic transmission  10 , the sun gear S 1  of the first gear set PG 1 , in more detail, the second sun gear S 1   b  is always coupled to the sun gear S 4  of the fourth gear set PG 4 , the ring gear R 1  of the first gear set PG 1  is always coupled to the sun gear S 2  of the second gear set PG 2 , the carrier C 2  of the second gear set PG 2  is always coupled to the carrier C 4  of the fourth gear set PG 4 , and the carrier C 3  of the third gear set PG 3  is always coupled to the ring gear R 4  of the fourth gear set PG 4 . 
     The input shaft  12  is always coupled to the carrier C 1  of the first gear set PG 1  via the first sun gear S 1   a  and the second sun gear S 1   b , and the output shaft  13  is always coupled to the carrier C 4  of the fourth gear set PG 4 . 
     The first clutch CL 1  is disposed between the input shaft  12  and the carrier C 1  of the first gear set PG 1 , and the sun gear S 3  of the third gear set PG 3  to connect and disconnect these gear sets, and the second clutch CL 2  is disposed between the ring gear R 1  of the first gear set PG 1  and the sun gear S 2  of the second gear set PG 2 , and the sun gear S 3  of the third gear set PG 3  to connect and disconnect these gear sets, and the third clutch CL 3  is disposed between the ring gear R 2  of the second gear set PG 2  and the sun gear S 3  of the third gear set PG 3  to connect and disconnect these gear sets. 
     The first brake BR 1  is disposed between the transmission case  11 , and the sun gear S 1  of the first gear set PG 1  (specifically, the first sun gear S 1   a ) to connect and disconnect these gears, and the second brake BR 2  is disposed between the transmission case  11  and the ring gear R 3  of the third gear set PG 3  to connect and disconnect these gears. 
     With the above structure, the automatic transmission  10  forms, as illustrated in  FIG. 2 , first to eighth gear in a D-range or drive range, and a reverse gear in an R-range or reverse range by combining the engaged state of the first clutch CL 1 , the second clutch CL 2 , the third clutch CL 3 , the first brake BR 1 , and the second brake BR 2 . 
     In the automatic transmission  10 , the second brake BR 2  which is engaged at the first gear when the vehicle starts traveling is slip-controlled, and the second brake BR 2  corresponds to a friction engaging element of the automatic transmission according to the present disclosure. Below, this brake BR 2  is described. 
       FIG. 3  is a cross-sectional view of the brake and peripheral parts of the automatic transmission, and  FIG. 4  is another cross-sectional view of the brake and the peripheral parts of the automatic transmission.  FIGS. 5 to 7  are other cross-sectional views of the brake and the peripheral parts of the automatic transmission.  FIGS. 3 to 7  illustrate the cross-section of the brake and peripheral parts of the automatic transmission, taken along lines Y 3 -Y 3 , Y 4 -Y 4 , Y 5 -Y 5 , Y 6 -Y 6 , and Y 7 -Y 7  of  FIG. 8  described later, respectively. 
     As illustrated in  FIGS. 3 to 7 , the brake BR 2  is accommodated in the transmission case  11  formed in a substantially cylindrical shape, and is disposed at the outer circumferential side of a power transfer component  14  which is coupled to the sun gear S 3  of the third gear set PG 3  and is integrally formed with one of a pair of inner and outer rotary members of the first, second, and third clutches CL 1 , CL 2 , and CL 3 . 
     The power transfer component  14  is disposed at the outer circumferential side of a power transfer component  15  which couples the carrier C 2  of the second gear set PG 2  to the carrier C 4  of the fourth gear set PG 4 , and the power transfer component  15  is disposed at the outer circumferential side of a power transfer component  16  which couples the sun gear S 1  of the first gear set PG 1  (specifically, the second sun gear S 1   b ) to the sun gear S 4  of the fourth gear set PG 4 . 
     The brake BR 2  includes a hub member  20  coupled to the transmission case  11 , a drum member  40  disposed at the anti-driving-source side of the hub member  20  and coupled to the ring gear R 3  of the third gear set PG 3 , a plurality of friction plates  50  lined up in the axial directions between the hub member  20  and the drum member  40 , and a piston  60  which is disposed at the anti-driving-source side of the plurality of friction plates  50  and engages the plurality of friction plates  50 . 
     The brake BR 2  has a hydraulic chamber  70  to which hydraulic fluid for biasing the piston  60  is supplied. The hydraulic chamber  70  includes a hydraulic chamber  71  for engagement to which hydraulic fluid for engagement for biasing the piston  60  in an engaging direction is supplied, and a hydraulic chamber  72  for release to which hydraulic fluid for release is supplied. The hydraulic chamber  72  for release is disposed at the opposite side of the hydraulic chamber  71  for engagement with respect to the piston  60 , and biases the piston  60  in a releasing direction. 
     As illustrated in  FIG. 6 , the brake BR 2  has an oil channel forming member  80  which forms a supply oil channel for engagement which supplies the hydraulic fluid to the hydraulic chamber  71  for engagement, and the oil channel forming member  80  is disposed at the anti-driving-source side of the piston  60  and is coupled to the hub member  20  at the anti-driving-source side. 
     The brake BR 2  also has a biasing unit  90  which biases the piston  60 , as illustrated in  FIG. 3 . The biasing unit  90  is provided with a biasing member  91  which biases the piston  60 , and the biasing member  91  includes first springs  92  and second springs  93  as first biasing members and second biasing members, which apply a biasing force to the piston  60  in the engaging direction. 
       FIG. 8  is a perspective view illustrating an assembled state of the hub member, the oil channel forming member, and the piston of the brake,  FIG. 9  is a perspective view illustrating an assembled state of the hub member and the oil channel forming member of the brake,  FIG. 10  is a perspective view illustrating the hub member of the brake,  FIG. 11  is a perspective view illustrating a first hub member,  FIG. 12  is a perspective view illustrating a second hub member,  FIG. 13  is a perspective view illustrating the piston,  FIG. 14  is a cross-sectional view of the hub member taken along a line Y 14 -Y 14  in  FIG. 5 ,  FIG. 15  is a perspective view illustrating the biasing unit, and  FIG. 16  is a cross-sectional views of the biasing unit taken along a line Y 16 -Y 16  in  FIG. 15 . Note that  FIG. 14  also illustrates one of stationary-side friction plates  51  of the friction plates  50 . 
     As illustrated in  FIGS. 3 to 14 , the hub member  20  coupled to the transmission case  11  includes a first hub member  21  with which the friction plates  50  are spline-engaged, and a second hub member  31  which supplies the hydraulic fluid for lubrication to the friction plates  50 . The second hub member  31  is disposed adjacent to the first hub member  21  at the driving-source side. 
     As illustrated in  FIG. 3 , the first hub member  21  includes a vertical wall part  22  which extends in a direction perpendicular to the axial directions of the transmission case  11  and is formed in a substantially annular shape, and a cylindrical part  23  disposed radially inward of the vertical wall part  22  and formed in a substantially cylindrical shape extending to the anti-driving-source side from the vertical wall part  22 . 
     The first hub member  21  has a spline part  24  which is spline formed in an outer circumferential surface of the vertical wall part  22 , and the spline part  24  is coupled to the transmission case  11  by being spline-engaged with a spline part  11   a  formed in the inner circumferential surface of the transmission case  11 . 
     The cylindrical part  23  of the first hub member  21  has a plurality of (e.g., six) spline parts  25  in the circumferential directions, which form a spline in the outer circumferential surface, and the stationary-side friction plates  51  which constitute the friction plates  50  are spline-engaged with the spline parts  25 . The cylindrical part  23  of the first hub member  21  constitutes an inner fixed member coupled to the transmission case  11 . 
     The cylindrical part  23  of the first hub member  21  has a given axial length for the spline parts  25  spline-engaging with the plurality of friction plates  50  also in the released state of the plurality of friction plates  50 , and a part other than the spline parts  25  is formed shorter in the axial directions than the spline parts  25 . 
     As illustrated in  FIG. 5 , the second hub member  31  includes a vertical wall part  32  which extends in a direction perpendicular to the axial directions of the transmission case  11  and is formed in a substantially annular shape, and boss parts  33  for lubrication as a feed part for lubrication which extends to the anti-driving-source side from the vertical wall part  32 , is formed in a substantially cylindrical shape, and supplies the hydraulic fluid for lubrication to the friction plates  50 . 
     As illustrated in  FIG. 3 , the second hub member  31  is fitted at the outer circumferential surface of the vertical wall part  32  into an inner circumferential surface  11   b  of the transmission case  11 , on the driving-source side of the spline part  24  of the first hub member  21 . The second hub member  31  is prevented from being pulled out to the driving-source side by a snap ring  17 , and is coupled to the transmission case  11  by being fixed to the transmission case  11  using a rotation-stop pin  18 . Note that the second hub member  31  may be fixedly press-fitted into the inner circumferential surface  11   b  of the transmission case  11  to be coupled to the transmission case  11 . 
     As illustrated in  FIG. 12 , the second hub member  31  is provided with a plurality of boss parts  33  for lubrication (specifically, five boss parts  33  for lubrication). The five boss parts  33  for lubrication are disposed on substantially the same circumference centering on the axes of the input shaft  12  and the output shaft  13 , but at different positions in the circumferential directions. Each boss part  33  for lubrication is formed with a supply oil channel L 1  for lubrication which supplies the hydraulic fluid for lubrication to the friction plates  50 . 
     As illustrated in  FIG. 4 , in a lower part of the transmission case  11 , a valve body  5  which supplies the hydraulic fluid to the hydraulic chamber  70 , the friction plates  50 , etc. of the brake BR 2  is disposed. The valve body  5  is accommodated in an oil pan (not illustrated) attached to the lower part of the transmission case  11 , and is fixed to the transmission case  11 . The second hub member  31  has a valve body connection  34  for connecting to the valve body  5 , and is formed so that the supply oil channel L 1  for lubrication is connected with the valve body  5  through a case opening  11   c  formed in the transmission case  11 . 
     As illustrated in  FIG. 6 , the second hub member  31  includes a boss part  35  for engagement which extends to the anti-driving-source side from the vertical wall part  32 , is formed in a substantially cylindrical shape, and is provided with a supply oil channel L 2  for engagement to supply the hydraulic fluid to the hydraulic chamber  71  for engagement, and as illustrated in  FIG. 7 , a boss part  36  for release which extends to the anti-driving-source side from the vertical wall part  32 , is formed in a substantially cylindrical shape, and is provided with a supply oil channel L 3  for release to supply the hydraulic fluid to the hydraulic chamber  72  for release. 
     As illustrated in  FIG. 12 , the boss part  35  for engagement and the boss part  36  for release are disposed at different positions on substantially the same circumference centering on the axes of the input shaft  12  and the output shaft  13 , together with the boss parts  33  for lubrication, and are disposed between two of the boss parts  33  for lubrication disposed on a lower side of the transmission case  11 . 
     In the automatic transmission  10 , the supply oil channel L 2  for engagement, the supply oil channel L 3  for release, and the supply oil channel L 1  for lubrication are lined up in the circumferential directions at a lower side of the transmission case  11 , and the second hub member  31  is formed so that the supply oil channel L 2  for engagement, the supply oil channel L 3  for release, and the supply oil channel L 1  for lubrication are connected to the valve body  5 . 
     As illustrated in  FIG. 3 , the second hub member  31  is provided with a first cylindrical part  37 , a second cylindrical part  38 , and a third cylindrical part  39  which extend from the vertical wall part  32  to the anti-driving-source side and are formed in a substantially cylindrical shape. The first cylindrical part  37  is disposed radially inward of the cylindrical part  23  of the first hub member  21  and extends from the radial center side of the vertical wall part  32 , the second cylindrical part  38  extends from the vertical wall part  32  radially inward of the first cylindrical part  37 , and the third cylindrical part  39  extends from the vertical wall part  32  radially inward of the second cylindrical part  38 . The first cylindrical part  37  is disposed on substantially the same circumference as the five boss parts  33  for lubrication, and is provided so as to connect between the boss parts  33  for lubrication except for those located at the lower side of the transmission case  11 . 
     The second cylindrical part  38  is formed longer in the axial directions than the first cylindrical part  37 , and the third cylindrical part  39  is formed longer in the axial directions than the second cylindrical part  38 . The first cylindrical part  37  functions as a stop member which catches a second retainer plate  95  of the biasing unit  90  described later. The second cylindrical part  38  and the third cylindrical part  39  constitute a cylinder  72   a  of the hydraulic chamber  72  for release, together with the vertical wall part  32 . 
     Thus, in the hub member  20  formed in this way, the first hub member  21  is made of material with a higher strength than the second hub member  31 . For example, the first hub member  21  is made of a ferrous material, and the second hub member  31  is made of an aluminum-based material. 
     Since the first hub member  21  which receives the force inputted from the friction plates  50  when engaging the brake BR 2  is made of the material with the higher strength than the second hub member  31 , tooth heights of the spline parts  25  can be reduced, as compared with a case where the first hub member  21  is made of the same material as the second hub member  31 , thereby reducing the radial size. 
     Especially, since the first hub member  21  is made of the ferrous material and the second hub member  31  is made of the aluminum-based material, the tooth heights of the spline parts  25  can be reduced, as compared with the case where the first hub member  21  and the second hub member  31  are made of the aluminum-based material, thereby reducing the radial size. Moreover, the weight can also be reduced as compared with the case where the first hub member  21  and the second hub member  31  are made of the ferrous material. 
     The drum member  40  includes a cylindrical part  41  disposed so as to oppose to the outer circumferential surface of the cylindrical part  23  of the first hub member  21 , extending in the axial directions, and formed in a substantially cylindrical shape, and a vertical wall part  42  extending in a direction perpendicular to the axial directions of the transmission case  11 , to radially inward of the cylindrical part  41  from the anti-driving-source-side of the cylindrical part  41 , is formed in a substantially annular shape. 
     The vertical wall part  42  of the drum member  40  is coupled to the ring gear R 3 . The cylindrical part  41  of the drum member  40  has a spline part  41   a  where spline is formed in an inner circumferential surface thereof, and rotation-side friction plates  52  which constitute the friction plates  50  are spline-engaged with the spline part  41   a . The vertical wall part  42  of the drum member  40  constitutes an outer rotary member coupled to the ring gear R 3  as the rotary member. The stationary-side friction plates  51  and the rotation-side friction plates  52  are disposed alternately in the axial directions. 
     The piston  60  is disposed between the hub member  20  and the drum member  40 , specifically, between the cylindrical part  23  of the first hub member  21  and the cylindrical part  41  of the drum member  40 , and is fitted onto the outer circumferential surface of the third cylindrical part  39  of the second hub member  31  so as to be slidable. The piston  60  is prevented from being pulled out to the anti-driving-source side by a snap ring  19 . 
     The piston  60  is formed annularly, and includes a pressing part  61  which is provided at the outer circumferential side and presses the friction plates  50 , a hydraulic chamber for engagement forming part  62  which is provided at the inner circumferential side and forms the hydraulic chamber  71  for engagement, and a coupling part  63  which couples the pressing part  61  to the hydraulic chamber for engagement forming part  62  and extends radially. 
     The pressing part  61  is disposed at the anti-driving-source side of the friction plates  50 , the hydraulic chamber for engagement forming part  62  is disposed radially inward of the cylindrical part  23  of the first hub member  21 , and the coupling part  63  is provided so as to couple the pressing part  61  to the hydraulic chamber for engagement forming part  62 . The hydraulic chamber for engagement forming part  62  is provided so as to project to the driving-source side from the coupling part  63 . 
     As illustrated in  FIGS. 5 to 7 , the oil channel forming member  80  is disposed at the anti-driving-source side of the piston  60 . The oil channel forming member  80  is fitted at the outer circumferential side of the third cylindrical part  39  of the second hub member  31 , and is coupled to the boss parts  33  and  36  of the second hub member  31 , specifically, to the driving-source side of the boss part  33  for lubrication and the boss part  36  for release. 
     The oil channel forming member  80  includes bonding parts  81  which are provided at the outer circumferential side and are coupled to the anti-driving-source side of the boss parts  33  and  36  of the second hub member  31 , a hydraulic chamber for engagement forming part  82  which is provided to the inner circumferential side, is disposed at the anti-driving-source side of the piston  60 , and forms the hydraulic chamber  71  for engagement, and coupling parts  83  which couple the bonding parts  81  to the hydraulic chamber forming part  82  for engagement and extend radially. 
     As illustrated in  FIG. 3 , the hydraulic chamber for engagement forming part  82  has a given thickness, is formed annularly, and fitted between the third cylindrical part  39  of the second hub member  31  and the outer circumferential surface of the hydraulic chamber for engagement forming part  62  of the piston  60 . The hydraulic chamber  71  for engagement is comprised of the hydraulic chamber for engagement forming part  62  of the piston  60 , the hydraulic chamber for engagement forming part  82  of the oil channel forming member  80 , and the third cylindrical part  39  of the second hub member  31 . 
     As illustrated in  FIGS. 5 and 7 , the bonding parts  81  are formed thinner than the hydraulic chamber for engagement forming part  82 , and overlap with the anti-driving-source side of the hydraulic chamber for engagement forming part  82  in the axial directions. As illustrated in  FIGS. 8 and 9 , the bonding parts  81  are each formed in an arc shape. The oil channel forming member  80  is provided with a plurality of bonding parts  81  in the circumferential directions (in this embodiment, three bonding parts  81 ), which are substantially equally spaced from each other in the circumferential directions. 
     In the bonding parts  81 , bolt insertion holes  81   a  into which fastening bolts  84  as fastening members are inserted, and bolt accommodation holes  81   b  where heads  84   a  of the fastening bolts  84  are accommodated, are provided. The oil channel forming member  80  is coupled to the anti-driving-source side of the boss parts  33  and  36  of the second hub member  31  by threadedly engaging the fastening bolts  84  through the bolt insertion holes  81   a  with threaded holes  33   a  and  36   a  formed at the anti-driving-source side of the boss parts  33  and  36 . As the fastening bolt  84 , a bolt with seal in which an outer circumferential surface of a thread part  84   b  is covered with a sealant is used. 
     Each coupling part  83  of the oil channel forming member  80  has a thickness substantially equal to the bonding parts  81 , and as illustrated in  FIGS. 8 and 9 , it extends radially inward from a center part in the circumferential direction of the bonding part  81  and is coupled to the hydraulic chamber for engagement forming part  82 . 
     As illustrated in  FIG. 9 , the bonding part  81  disposed at the lower side of the transmission case  11  is coupled at both sides in the circumferential direction thereof to the boss parts  33  and  36  of the second hub member  31  by using the two fastening bolts  84 , and the two bonding parts  81  disposed at the upper side of the transmission case  11  is coupled at the center in the circumferential directions to the boss parts  33  of the second hub member  31  by using one fastening bolt  84 . 
     Notches  63   a  for oil channel forming members are formed in the coupling part  63  of the piston  60 , which is formed by cutting the coupling part  63  in substantially the same shape as the bonding parts  81  and the coupling parts  83  of the oil channel forming member  80  so as to correspond to the bonding parts  81  and the coupling parts  83 . The oil channel forming member  80  is disposed within a radial range of the piston  60 , the bonding parts  81  and the coupling parts  83  of the oil channel forming member  80  are fitted into the notches  63   a  for oil channel forming members of the coupling part  63  of the piston  60 , and are disposed so as to overlap with the coupling part  63  of the piston  60  in the axial directions. 
     Thus, by the oil channel forming member  80  and the piston  60  being disposed overlapping each other in the axial directions, the axial dimension can be shortened as compared with the case where the piston  60  extends radially through the anti-driving-source side of the oil channel forming member  80 , thereby reducing the axial size. 
     Notches  63   b  for spline parts are also formed in the coupling part  63  of the piston  60 , which are formed by cutting the coupling part  63  in substantially the same shape as the spline parts  25  of the cylindrical part  23  of the first hub member  21  so as to correspond to the spline parts  25 . The anti-driving-source side of the spline parts  25  of the cylindrical part  23  of the first hub member  21  are fitted into the notches  63   b  for spline parts of the coupling part  63  of the piston  60  so that the cylindrical part  23  of the first hub member  21  overlaps with the piston  60  in the axial directions. 
     Thus, since the anti-driving-source side of the spline parts  25  of the cylindrical part  23  of the first hub member  21  overlaps with the piston  60  in the axial directions, the axial dimension can be shortened as compared with the case where the piston  60  extends radially through the anti-driving-source side of the spline parts  25 , thereby reducing the axial size, while securing the axial length of the spline parts  25  of the cylindrical part  23 . 
     As illustrated in  FIG. 3 , the hydraulic chamber for engagement forming part  62  of the piston  60  includes an outer cylindrical part  62   a  which is fitted onto the outer circumferential side of the hydraulic chamber for engagement forming part  82  of the oil channel forming member  80  and extends axially, a hydraulic pressure for engagement receiving part  62   b  which extends radially inward from the driving-source side part of the outer cylindrical part  62   a , and an inner cylindrical part  62   c  which extends to the anti-driving-source side from the radially inward part of the hydraulic pressure for engagement receiving part  62   b , is fitted onto the third cylindrical part  39  of the second hub member  31 , and extends axially. 
     In the automatic transmission  10 , the hydraulic chamber  70  is disposed radially inward of the cylindrical part  23  of the first hub member  21  and the boss parts  33 ,  35 , and  36  of the second hub member  31 , and the hydraulic chamber  71  for engagement and the hydraulic chamber  72  for release are disposed radially inward of the cylindrical part  23  of the first hub member  21  and the boss parts  33 ,  35 , and  36  of the second hub member  31 . 
     As described above, the hydraulic chamber  71  for engagement is comprised of the hydraulic chamber for engagement forming part  62  of the piston  60 , the hydraulic chamber for engagement forming part  82  of the oil channel forming member  80 , and the third cylindrical part  39  of the second hub member  31 . The inner cylindrical part  62   c  of the piston  60  is prevented from being pulled out to the anti-driving-source side by the snap ring  19 . 
     As illustrated in  FIG. 3 , the hydraulic chamber  72  for release is comprised of a bulged part  62   d  of the piston  60  and the cylinder  72   a  of the second hub member  31 . The bulged part  62   d  where the radially inward part of the hydraulic pressure for engagement receiving part  62   b  of the piston  60  is bulged to the driving-source side in a substantially channel shape in the cross section, is slidably fitted into the cylinder  72   a  of the second hub member  31  through seal members  73  and  74 . 
     In the automatic transmission  10 , the hydraulic chamber  72  for release is formed smaller in the outer diameter than the hydraulic chamber  71  for engagement, and a biasing force receiving member  100  which is coupled to the piston  60  and receives a biasing force by the biasing member  91  of the biasing unit  90  is disposed at the outer circumferential side of the hydraulic chamber  72  for release. 
     As illustrated in  FIG. 3 , the biasing force receiving member  100  is formed annularly, and includes a radially extended part  101  extending radially between the cylindrical part  23  of the first hub member  21  and the second cylindrical part  38  of the second hub member  31 , and an axially extended part  102  extending axially from the radially inward part to the anti-driving-source part of the radially extended part  101 . 
     The biasing force receiving member  100  is coupled to the piston  60  by the anti-driving-source side of the axially extended part  102  being coupled to a radially outward part of the bulged part  62   d  of the hydraulic pressure for engagement receiving part  62   b  of the piston  60 . The biasing unit  90  is attached to a part between the biasing force receiving member  100  (specifically, the radially extended part  101 ) and the oil channel forming member  80 . 
     As illustrated in  FIGS. 15 and 16 , the biasing unit  90  includes the first springs  92  and the second springs  93  which extend axially, a first retainer plate  94  which holds ends of the first springs  92  and the second springs  93  on the anti-driving-source side which are one end parts of the first springs  92  and the second springs  93 , and the second retainer plate  95  which is disposed so as to be separated from the first retainer plate  94  in the axial directions and holds ends of the first springs  92  on the driving-source side which are the other end parts of the first springs  92 . 
     The first retainer plate  94  is formed annularly and is provided with first spring guide parts  94   a  and second spring guide parts  94   b  which project to the driving-source side and formed in a cylindrical shape, to which the first springs  92  and the second springs  93  are mounted, respectively. The first springs  92  and the second springs  93  are disposed so as to radially overlap each other, but at different positions in the circumferential directions. In the automatic transmission  10 , two first springs  92  are disposed at both sides of the six second springs  93 , respectively. 
     The second retainer plate  95  is formed substantially symmetrical to the first retainer plate  94  in the axial directions. The second retainer plate  95  is provided with first spring guide parts  95   a  which project to the anti-driving-source side and is formed in a cylindrical shape, to which the first springs  92  are mounted. The second retainer plate  95  is provided with insertion holes  95   b  through which the second springs  93  are inserted so that the other ends of the second springs  93  on the driving-source side can project to the opposite side of the first retainer plate. 
     The first springs  92  have a larger biasing force than the second springs  93 . The first springs  92  and the second springs  93  are coil springs, and the first springs  92  are large coil springs with a coil diameter larger than the second springs  93 . The second springs  93  have a longer free length than the first springs  92 , and are held by the first retainer plate  94  so that the other end parts of the second springs  93  can project from the second retainer plate  95  to the opposite side of the first retainer plate. 
     As illustrated in  FIG. 3 , the biasing unit  90  is attached to the transmission case  11  by the first retainer plate  94  being supported on the driving-source side at both sides in the circumferential directions of the bonding parts  81  of the oil channel forming member  80 , and the second retainer plate  95  being supported at the anti-driving-source side of the radially extended part  101  of the biasing force receiving member  100 . 
     The radially extended part  101  of the biasing force receiving member  100  has a radial dimension which is smaller than the outer diameter of the second retainer plate  95  and substantially equal to the second springs  93  so that it supports the second retainer plate  95 , and supports the other end parts of the second springs  93  inserted into the insertion holes  95   b  of the second retainer plate  95 . 
     The inner circumferential surface of the cylindrical part  23  of the first hub member  21  is formed radially larger than the first retainer plate  94  and the second retainer plate  95 , and the biasing unit  90  is disposed at the inner circumferential side of the cylindrical part  23 . In the biasing unit  90 , notches  94   c  and  95   c  are formed in the first retainer plate  94  and the second retainer plate  95 , respectively, corresponding to the boss parts  33 ,  35 , and  36  of the second hub member  31 . 
     The first cylindrical part  37  of the second hub member  31  is formed radially larger than the radially extended part  101  of the biasing force receiving member  100  so that, when the second retainer plate  95  receives the biasing force of the biasing member  91  at an end surface on the anti-driving-source side and is moved to the driving-source side, the first cylindrical part  37  catches the second retainer plate  95 . The first cylindrical part  37  of the second hub member  31  functions as a stop member which catches or stops the second retainer plate  95 . 
     The first cylindrical part  37  of the second hub member  31  is set, when the second retainer plate  95  supported by the biasing force receiving member  100  contacts thereto, the piston  60  is located at an immediately-before-contact position where the piston  60  is about to contact the plurality of friction plates  50 . The immediately-before-contact position of the piston  60  is suitably set between a released position where the plurality of friction plates  50  are in the released state and a zero-clearance state where the clearance is zero. 
     When the second retainer plate  95  contacts the biasing force receiving member  100 , the biasing force receiving member  100  receives the biasing force in the engaging direction only from the second springs  93 . The piston  60  is set to be located at the zero-clearance position when the second springs  93  reach their free lengths. 
     Thus, the biasing unit  90  is structured so that the first springs  92  cause the biasing force to act on the piston  60  in the engaging direction from the released position to the immediately-before-contact position through the biasing force receiving member  100 , and the second springs  93  cause the biasing force to act on the piston  60  in the engaging direction from the released position to the zero-clearance position through the biasing force receiving member  100 . 
     Then, when hydraulic pressure for engagement is supplied to the hydraulic chamber  71  for engagement while the piston  60  is in the zero-clearance position, the piston  60  pushes the plurality of friction plates  50  to move them to an engaged position where the plurality of friction plates  50  become in the engaged state where the plurality of friction plates  50  become impossible to relatively rotate, by being pinched between holding parts  26  which project to the anti-driving-source side from the vertical wall part  22  of the first hub member  21 , and the piston  60 . 
     On the other hand, when the oil pressure for engagement is discharged from the hydraulic chamber  71  for engagement and oil pressure for release is then supplied to the hydraulic chamber  72  for release, while the piston  60  is in the engaged position, the piston  60  is biased in the releasing direction and moved to the zero-clearance position. 
     The piston  60  is further biased in the releasing direction while resisting the second springs  93 , and reaches the immediately-before-contact position. Then, the piston  60  is biased in the releasing direction while resisting the first springs  92  and the second springs  93 , and reaches the released position. 
     Next, the supply oil channels which supply the hydraulic fluid to the brake BR 2  is described. The supply oil channel L 2  for engagement which supplies the hydraulic fluid for engagement to the hydraulic chamber  71  for engagement of the brake BR 2  is formed in the second hub member  31  and the oil channel forming member  80 . The supply oil channel L 3  for release which supplies the hydraulic fluid for release to the hydraulic chamber  72  for release of the brake BR 2  and the supply oil channel L 1  for lubrication which supplies the hydraulic fluid for lubrication to the friction plates  50 , are formed in the second hub member  31 . The supply oil channel L 2  for engagement and the supply oil channel L 3  for release constitute a supply oil channel for operation which supplies the hydraulic fluid to the hydraulic chamber  70 . 
     As illustrated in  FIG. 6 , the supply oil channel L 2  for engagement is comprised of a radial oil channel  111  which is formed in the vertical wall part  32  of the second hub member  31 , and extends radially, an axial oil channel  112  which is formed in the boss part  35  for engagement, extends axially, and is coupled to the radial oil channel  111 , axial oil channels  113  which are formed in the bonding parts  81  of the oil channel forming member  80 , extend axially, and are coupled to the axial oil channel  112 , radial oil channels  114  which are formed in the bonding parts  81 , the coupling parts  83 , and the hydraulic chamber for engagement forming part  82  of the oil channel forming member  80 , extend radially, and are coupled to the axial oil channels  113 , and axial oil channels  115  which are formed in the hydraulic chamber for engagement forming part  82  of the oil channel forming member  80 , extend axially, are connected to the radial oil channels  114 , and open to the hydraulic chamber  71  for engagement. 
     The second hub member  31  is formed so as to connect the supply oil channel L 2  for engagement to the valve body  5 . The radial oil channel  111  of the second hub member  31  is formed in the vertical wall part  32  of the second hub member  31 , opens to a lower surface of the valve body connection  34 , and is connected to the valve body  5 . The valve body  5  supplies the hydraulic fluid for engagement to the hydraulic chamber  71  for engagement through the supply oil channel L 2  for engagement to supply a given oil pressure for engagement. 
     The radial oil channels  114  of the oil channel forming member  80  are formed so as to extend radially inward from the outer circumferential surface of the bonding parts  81  of the oil channel forming member  80 , and block members  85  which block openings of the radial oil channels  114  are attached to the outer circumferential surfaces of the bonding parts  81 . 
     As illustrated in  FIG. 7 , the supply oil channel L 3  for release is comprised of a radial oil channel  121  which is formed in the vertical wall part  32  of the second hub member  31 , extends radially, and opens to the hydraulic chamber  72  for release, and an axial oil channel  122  which is formed in the boss part  36  for release, extends axially, and is coupled to the radial oil channel  121 . 
     The second hub member  31  is formed so as to connect the supply oil channel L 3  for release to the valve body  5 . The radial oil channel  121  of the second hub member  31  is formed in the vertical wall part  32  of the second hub member  31 , opens to the lower surface of the valve body connection  34 , and is connected to the valve body  5 . The valve body  5  supplies the hydraulic fluid for release to the hydraulic chamber  72  for release through the supply oil channel L 3  for release to supply a given hydraulic pressure for release. 
     The axial oil channel  122  of the second hub member  31  is formed so as to extend axially to the driving-source side from an end surface of the boss part  36  for release on the anti-driving-source side, and the fastening bolt  84  is attached as a block member which blocks an opening of the axial oil channel  122 , to the end surface of the boss part  36  for release on the anti-driving-source side. The threaded hole  36   a  is formed in the opening of the axial oil channel  122  at the anti-driving-source side of the boss part  36  for release. 
     As illustrated in  FIGS. 4 and 5 , the supply oil channel L 1  for lubrication is comprised of a radial oil channel  131  which is formed in the vertical wall part  32  of the second hub member  31  and extends radially, a circumferential oil channel  132  which is formed in the vertical wall part  32  of the second hub member  31 , extends in the circumferential directions in an arc shape, and is connected to the radial oil channel  131 , axial oil channels  133  which are formed in the boss parts  33  for lubrication, extend axially, and are coupled to the circumferential oil channel  132 , and supply ports  134  which are formed in the boss parts  33  for lubrication, extend radially, are connected to the axial oil channels  133 , and open to the outer circumferential surfaces of the boss parts  35  for engagement. 
     As illustrated in  FIG. 12 , in the automatic transmission  10 , the radial oil channels  111 ,  121 , and  131  which constitute the supply oil channel L 2  for engagement, the supply oil channel L 3  for release, and the supply oil channel L 1  for lubrication, respectively, are lined up in the circumferential directions at the lower side of the transmission case  11 . The circumferential oil channel  132  which constitutes the supply oil channel L 1  for lubrication is connected to the radial oil channel  131 , extends in the circumferential directions to the radial oil channel  121  side in an arc shape, and extends to the opposite side of the radial oil channel  111  from the radial oil channel  121 . 
     Each of the five boss parts  33  for lubrication includes the axial oil channels  133  which are connected to the circumferential oil channel  132  and extend axially, and the supply ports  134  which extend radially outward from the axial oil channels  133  to the outer circumferential surface of the boss part  33  for lubrication and supply the hydraulic fluid for lubrication to the friction plates  50 . The plurality of supply ports  134  are lined up in the axial directions. 
     The second hub member  31  is formed so as to connect the supply oil channel L 1  for lubrication to the valve body  5 . The radial oil channel  131  of the second hub member  31  is formed in the vertical wall part  32  of the second hub member  31 , opens to the lower surface of the valve body connection  34 , and is connected to the valve body  5 . The valve body  5  can supply the hydraulic fluid for lubrication to the plurality of friction plates  50  through the supply oil channel L 1  for lubrication. 
     In the brake BR 2 , the hydraulic fluid for lubrication is supplied to the friction plates  50  from the outer circumferential surfaces of the boss parts  33  for lubrication of the second hub member  31  to cool the friction plates  50 . It is prevented that the hydraulic fluid for lubrication supplied to the friction plates  50  moves to the inner circumferential surface of the cylindrical part  41  of the drum member  40 , moves in the axial directions by the rotation of the cylindrical part  41  of the drum member  40 , and stagnates there. 
     The two axial oil channels  133  of the second hub member  31  are formed so as to extend axially to the driving-source side from the end surfaces of the boss parts  33  for lubrication on the anti-driving-source side as illustrated in  FIGS. 4 and 12 , and block members  29  which block the openings of the axial oil channels  133  are attached to the end surfaces of the boss parts  33  for lubrication on the anti-driving-source side. 
     As illustrated in  FIG. 5 , three axial oil channels  133 ′ of the second hub member  31  are formed so as to extend axially to the driving-source side from the end surfaces of the boss parts  33  for lubrication on the anti-driving-source side, and the fastening bolts  84  which block the openings of the axial oil channels  133 ′ as block members are attached to the end surfaces of the boss parts  33  for lubrication on the anti-driving-source side. The threaded holes  33   a  are formed in the openings of the axial oil channels  133 ″ at the anti-driving-source side of the boss parts  33  for lubrication. 
     As illustrated in  FIG. 10 , notches  27  for lubrication are formed by cutting the cylindrical part  23  of the first hub member  21  in the circumferential directions corresponding to the boss parts  33  for lubrication of the second hub member  31 . The boss parts  33  for lubrication of the second hub member  31  are disposed corresponding to the notches  27  for lubrication of the cylindrical part  23  of the first hub member  21 , and can supply the hydraulic fluid for lubrication to the friction plates  50  through the notches  27  for lubrication. 
     As illustrated in  FIG. 14 , the supply ports  134  of the boss parts  33  for lubrication incline to the downstream side in the rotating direction of the friction plates  50  (a direction indicated by an arrow  50   a ) as they go radially outward from the axial oil channels  133 . Each supply port  134  inclines with respect to the radial direction of the second hub member  31  (a direction indicated by an arrow  31   a ) passing through a center axis  33   b  of the boss part  33  for lubrication, at a given angle θ 1  to the downstream side in the rotating direction of the friction plates  50 , as indicated by an arrow  134   a . The angle θ 1  is set larger than 0° and smaller than 90°, and is preferably set within an angle range between 30° and 45°. 
     Thus, since the hydraulic fluid for lubrication is supplied from the supply ports  134  to the downstream side in the rotating direction of the friction plates  50 , a shear resistance of the hydraulic fluid, such as lubricating oil, can be reduced as compared with a case where the hydraulic fluid for lubrication is supplied toward a direction perpendicular to the rotating direction of the friction plates  50 , thereby reducing the drag between the friction plates caused by the hydraulic fluid for lubrication. 
     Next, operation of the brake BR 2  structured in this way is described.  FIG. 17  is a cross-sectional view illustrating the brake in the released state,  FIG. 18  is a cross-sectional view illustrating the brake in the immediately-before-contact state,  FIG. 19  is a cross-sectional view illustrating the brake in the zero-clearance state, and  FIG. 20  is a cross-sectional view illustrating the brake in the engaged state.  FIGS. 17 to 20  illustrate enlarged views of a substantial part of the brake BR 2  in  FIG. 3 . 
     In  FIG. 17 , as the hydraulic pressure for engagement is discharged from the hydraulic chamber  71  for engagement and the hydraulic pressure for release is supplied to the hydraulic chamber  72  for release, the first springs  92  and the second springs  93  are compressed through the biasing force receiving member  100 , and the piston  60  is moved in the releasing direction (i.e., to the anti-driving-source side).  FIG. 17  illustrates the released state of the brake BR 2  where the piston  60  is located at the released position where the plurality of friction plates  50  are in the released state. 
     When engaging the brake BR 2 , the hydraulic pressure for release is discharged from the hydraulic chamber  72  for release in the released state illustrated in  FIG. 17 . As illustrated in  FIG. 18 , the piston  60  receives the biasing force of the first springs  92  and the second springs  93  through the biasing force receiving member  100  and is moved in the engaging direction (i.e., the driving-source side) until the second retainer plate  95  contacts the first cylindrical part  37  of the second hub member  31 . The piston  60  then reaches the immediately-before-contact position where the piston  60  is about to contact the plurality of friction plates  50 , to make the brake BR 2  in the immediately-before-contact state. 
     In this immediately-before-contact state illustrated in  FIG. 18 , when the second retainer plate  95  contacts the first cylindrical part  37  of the second hub member  31  as illustrated in  FIG. 19 , the piston  60  receives the biasing force of the second springs  93  through the biasing force receiving member  100  and is moved in the engaging direction until the second springs  93  reach their free lengths. Thus, the piston  60  is located at the zero-clearance position at which the piston  60  becomes in the zero-clearance state where the piston  60  contacts or substantially contacts the friction plates  50  without pressing the plurality of friction plates  50 , thereby the brake BR 2  becoming in the zero-clearance state. 
     Then, when the hydraulic pressure for engagement is supplied to the hydraulic chamber  71  for engagement in the zero-clearance state illustrated in  FIG. 19 , as illustrated in  FIG. 20 , the piston  60  is biased and moved in the engaging direction by the oil pressure for engagement supplied to the hydraulic chamber  71  for engagement, and the piston  60  is located at the engaged position where the piston  60  pushes the plurality of friction plates  50  and the relative rotation of the plurality of friction plates  50  becomes impossible, thereby the brake BR 2  becoming in the engaged state. 
     On the other hand, when releasing the brake BR 2 , the hydraulic pressure for engagement is discharged from the hydraulic chamber  71  for engagement in the engaged state illustrated in  FIG. 20 , and the hydraulic pressure for release is supplied to the hydraulic chamber  72  for release, and then, the piston  60  is biased and moved in the releasing direction (i.e., the anti-driving-source side) by the hydraulic pressure for release supplied to the hydraulic chamber  72  for release, and the piston  60  becomes in the released state illustrated in  FIG. 17  via the zero-clearance state illustrated in  FIG. 19  and the immediately-before-contact state illustrated in  FIG. 18 . 
     In the brake BR 2 , the piston  60  can be moved with a sufficient response by the first springs  92  and the second springs  93  from the released position to the immediately-before-contact position, and is then moved with sufficient accuracy by the second springs  93  from the immediately-before-contact position to the zero-clearance position. 
     As described above, the brake BR 2  is slip-controlled when the vehicle starts traveling. When engaging the brake BR 2 , after the hydraulic pressure lower than the oil pressure for engagement is supplied to the hydraulic chamber  71  for engagement to make the plurality of friction plates  50  into the slipping state, the hydraulic pressure for engagement is supplied to the hydraulic chamber  71  for engagement to engage the plurality of friction plates  50 . On the other hand, when releasing the brake BR 2 , after the hydraulic pressure lower than the hydraulic pressure for release is supplied to the hydraulic chamber  72  for release to make the plurality of friction plates  50  into the slipping state, the hydraulic pressure for release is supplied to the hydraulic chamber  72  for release to release or disengage the plurality of friction plates  50 . 
     When engaging and releasing the brake BR 2 , the hydraulic fluid for lubrication is supplied to the plurality of friction plates  50  through the supply oil channel L 1  for lubrication, and when the slip control of the brake BR 2  is carried out, the hydraulic fluid for lubrication is supplied to the plurality of friction plates  50  through the supply oil channel L 1  for lubrication. 
     In this embodiment, in the brake BR 2  in which the plurality of friction plates  50  are disposed between the hub member  20  coupled to the transmission case  11  and the drum member  40  coupled to the ring gear R 3 , the hub member  20  includes the first hub member  21  having the cylindrical part  23  provided with the spline parts  25 , and is made of ferrous material, and the second hub member  31  having the feed part  33  for lubrication provided with the supply oil channel L 1  for lubrication, and is made of the aluminum-based material. The feed part  33  for lubrication of the second hub member  31  supplies the hydraulic fluid for lubrication to the friction plates  50  through the notch  27  for lubrication formed in the cylindrical part  23  of the first hub member  21 . 
     Similar to this, for the other brake and clutch in which a plurality of friction plates are disposed between a hub member and a drum member, the hub member may include a first hub member having a cylindrical part provided with spline parts, and is made of the ferrous material, and a second hub member having a feed part for lubrication provided with a supply oil channel for lubrication, and is made of the aluminum-based material, and the feed part for lubrication of the second hub member may supply the hydraulic fluid for lubrication to the friction plates through a notch for lubrication formed in the cylindrical part of the first hub member. 
     Thus, the automatic transmission  10  according to this embodiment includes the second brake BR 2  where the plurality of friction plates  50  are disposed between the hub member  20  and the drum member  40 . The hub member  20  includes the first hub member  21  having the cylindrical part  23  with the spline parts  25  with which the friction plates  50  are spline-engaged and made of ferrous material, and the second hub member  31  having the feed part  33  for lubrication with the supply oil channel L 1  for lubrication and made of aluminum-based material. The notch  27  for lubrication is formed in the cylindrical part  23  of the first hub member  21  and the feed part  33  for lubrication of the second hub member  31  supplies the hydraulic fluid for lubrication to the friction plates  50  through the notch  27  for lubrication. 
     Thus, since the first hub member  21  having the cylindrical part  23  with the spline parts  25  is made of ferrous material, the tooth height of the spline parts  25  can be reduced as compared with the case where the hub member  20  having the cylindrical part  23  with the spline parts  25  and the feed part  33  for lubrication provided with the supply oil channel L 1  for lubrication is made of the aluminum-based material, thereby reducing the radial size. 
     Moreover, since the second hub member  31  having the feed part  33  for lubrication with the supply oil channel L 1  for lubrication is made of the aluminum-based material, while supplying the hydraulic fluid for lubrication from the hub member side, the weight can be reduced as compared with the case where the hub member  20  having the cylindrical part  23  with the spline parts  25  and the feed part  33  for lubrication is made of the ferrous material. 
     Therefore, in the automatic transmission  10  provided with the second brake BR 2  where the plurality of friction plates  50  are disposed between the hub member  20  and the drum member  40 , the weight and the radial size can be reduced while supplying the hydraulic fluid for lubrication from the hub member side. 
     Moreover, the friction engaging element is the second brake BR 2  where the hub member  20  is coupled to the transmission case  11  and the drum member  40  is coupled to the ring gear R 3 . Thus, in the automatic transmission  10  provided with the brake BR 2  where the plurality of friction plates  50  are disposed between the hub member  20  and the drum member  40 , the weight and the radial size can be reduced while supplying the hydraulic fluid for lubrication from the hub member side. 
     Moreover, the hydraulic chamber  70  to which the hydraulic fluid for biasing the piston  60  is provided, and the supply oil channels L 2  and L 3  for operation which supply the hydraulic fluid to the hydraulic chamber  70  are provided to the second hub member  31 . Thus, since the supply oil channel L 1  for lubrication and the supply oil channels L 2  and L 3  for operation are formed in the second hub member  31 , the supply oil channels of the hydraulic fluid can be simplified, as compared with a case where the supply oil channel L 1  for lubrication and the supply oil channels L 2  and L 3  for operation are formed in the first hub member  21  and the second hub member  31 . 
     Moreover, the automatic transmission  10  is coupled to the driving source without the intervening hydraulic power transmission device, and the second brake BR 2  is slip-controlled when the vehicle starts traveling and is engaged at the first gear. Thus, in the automatic transmission  10  coupled to the driving source without the intervening hydraulic power transmission device, when the brake BR 2  is provided, which is slip-controlled when the vehicle starts traveling and engaged at the first gear, the weight and the radial size can be reduced while supplying the hydraulic fluid for lubrication from the hub member side. 
     The present disclosure is not intended to be limited to the illustrated embodiment, and various improvements and design changes are possible without departing from the subject matter of the present disclosure. 
     As described above, according to the present disclosure, in the automatic transmission provided with the friction engaging element where the plurality of friction plates are disposed between the hub member and the drum member, since the weight and the radial size can be reduced while supplying the hydraulic fluid for lubrication from the hub member side, the automatic transmission may suitably be used in the manufacturing field of this kind of automatic transmissions or vehicles to which the automatic transmission is mounted. 
     It should be understood that the embodiments herein are illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within metes and bounds of the claims, or equivalence of such metes and bounds thereof, are therefore intended to be embraced by the claims. 
     DESCRIPTION OF REFERENCE CHARACTERS 
     
         
         
           
               10  Automatic Transmission 
               20  Hub Member 
               21  First Hub Member 
               23  Cylindrical Part of First Hub Member 
               25  Spline Part 
               27  Notch for Lubrication 
               31  Second Hub Member 
               33  Boss Part for Lubrication 
               40  Drum Member 
               50  Friction Plate 
               60  Piston 
               70  Hydraulic Chamber 
             BR 2  Second Brake 
             L 1  Supply Oil Channel for Lubrication 
             L 2  Supply Oil Channel for Engagement 
             L 3  Supply Oil Channel for Release