Abstract:
The present invention relates to a frictional engagement apparatus comprising a first frictional engagement unit ( 42 ), a second frictional engagement unit ( 44 ) and being disposed axially spaced from the first frictional engaging unit, and one case ( 40 ) for supporting the first and the second frictional engagement unit non-rotatably circumferentially but shiftably axially relative thereto. One retaining ring ( 51 ) is fitted to the case to be located between the first and second frictional engagement unit, the retaining ring being abutted to the nearest first friction plate and/or the second friction plate to stop an approaching shift of the first frictional engagement unit and/or the second frictional engagement unit. At a portion of the retaining ring remote from the case a recess ( 51   a ) is formed for allowing an elastic deformation in a direction decreasing thickness thereof, when the first and second frictional engagement units approach to each other.

Description:
[0001]     The present application is based on Japanese Patent Application No. 2005-294174 filed on Oct. 6, 2005, the content of which is incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a frictional engagement apparatus mounted, for example, in an automatic transmission for vehicle. In particular, it relates to the frictional engagement apparatus of small-size and light-weight.  
         [0004]     2. Related Art  
         [0005]     Conventionally, an automatic transmission for vehicle provided with plural planetary gear apparatuses and plural frictional engagement apparatuses has been known and practically used. In this automatic transmission for vehicle, plural rotary elements constructed through connection of the plural planetary gears are suitably engaged or released by the plural frictional engagement apparatuses to establish plural speed change ratios. With recent multi speed change ratios of the automatic transmission for vehicle, more planetary gear apparatuses and more frictional engagement apparatuses are mounted on the automatic transmission for vehicle. For example, the Patent Document 1 (Japanese Patent Application Laid-open No. 9-53691) discloses two clutch apparatuses as the frictional engagement apparatuses.  
         [0006]     On the other hand, the more planetary gear apparatuses and the more frictional engagement apparatuses cause problems of large-size and heavy-weight of the automatic transmission for vehicle. To overcome such the problem, some techniques including the above Patent Document 1 for realizing the small-size and the light-weight have been developed.  
         [0007]      FIG. 6  shows a conventional automatic transmission for vehicle  100  disclosed in the above Patent Document 1. The automatic transmission for vehicle  100  includes a clutch drum  102  and two sets of frictional engagement units  104  and  108 . In detail, the clutch drum  102  has a drum shape and is commonly used for the two sets of frictional engagement units  104  and  108 . Both the first frictional engagement unit  104  and the second frictional engagement unit  108  are disposed at an inner periphery of the clutch drum  102  and respectively include plural friction plates arranged axially.  
         [0008]     In the first frictional engagement unit  104 , plural friction plates  105  held by the clutch drum  102  and plural friction plates  107  held by a clutch hub  106  are alternately arranged. Similarly, in the second frictional engagement unit  108 , plural friction plates  109  held by the clutch drum  102  and plural friction plates  111  held by a ring R are alternately arranged.  
         [0009]     On an inner surface of the clutch drum  102 , between the first frictional engagement unit  104  and the second frictional engagement unit  108 , a stop ring  112  is held so as not to be shifted axially and not to be rotated circumferentially relative to the clutch drum  102 . Commonly using the stop ring  112  for both the first frictional engagement unit  104  and the second frictional engagement unit  108  can shorten an axial length of the automatic transmission for vehicle  100 .  
         [0010]     In the frictional engagement apparatus of the Patent Document 1, as shown in  FIG. 7 , a first piston  114  disposed behind the first frictional engagement unit  104  pushes it. Thus, the friction plates  105  and the friction plates  107  are brought into contacted state, and the stop ring  112  is deformed toward the second frictional engagement unit  108 . The deformation of the second frictional engagement unit  108  causes contact between the friction plates  109  and the friction plates  111 , so that these friction plates  109  and  111  may be deteriorated in durability thereof. For this reason, improvement for avoiding such unexpected contact of the friction plates  109  and  111  has been required.  
         [0011]     On the other hand, when a piston  116  pushes the second frictional engagement unit  108 , the stop ring  112  deforms toward the first frictional engagement unit  104  to deteriorate durability of the friction plates  105  and the friction plates  107 .  
       SUMMARY OF THE INVENTION  
       [0012]     The present invention is made in view of the above circumstances and has an object to provide a frictional engagement apparatus which can shorten an axial length thereof and in turn an axial length of an automatic transmission, without deteriorating a function thereof in turn a function of the automatic transmission.  
         [0013]     For achieving the above object, the frictional engagement apparatus of the present invention (i) comprising a first frictional engagement unit including plural first friction plates, a second frictional engagement unit including plural second friction plates and being disposed axially spaced from the first frictional engaging unit, and one case for supporting the first frictional engagement unit and the second frictional engagement unit non-rotatable circumferentially but shiftable axially relative thereto, (ii) wherein one retaining ring is fitted to the case to be located between the first frictional engagement unit an the second frictional engagement unit, the retaining ring being abutted to the nearest first friction plate and/or the nearest second friction plate to stop an approaching shift of the first frictional engagement unit and/or the second frictional engagement unit, and (iii) at a portion of the retaining ring remote from the case a recess is formed for allowing an elastic deformation in a direction decreasing thickness thereof, when the first frictional engagement unit and the second frictional engagement unit approach to each other.  
         [0014]     According to the frictional engagement apparatus according to the invention recited in the claim  1 , even when the retaining ring is deformed in the direction decreasing thickness thereof by being pressed with one of the first and second frictional engagement units, deformation of the retaining ring is absorbed within the recess portion. As a result, influence to the other of the first and second frictional engagement units is prevented.  
         [0015]     As regard the case, it may be a rotatable clutch drum of hollow cylindrical shape mounted in an automatic transmission for vehicle, and on an inner peripheral surface thereof the first frictional engagement unit and the second frictional engagement unit are supported. By supporting the retaining ring on the inner peripheral surface of the clutch drum, the recess is formed on the inner peripheral surface thereof. The clutch drum has a hollow cylindrical portion on which plural concave grooves spaced circumferentially, extended axially and concaved radially outwardly are formed, and each of the first friction plates and each of the second friction plates have convex portions at an outer periphery thereof to be engaged with the concave grooves. The hollow cylindrical portion is provided with a support groove for supporting the retaining ring therein.  
         [0016]     The retaining ring has C-shape in a non-fitted state, and is reduced in a diameter upon fitting to the hollow cylindrical portion. The recess formed on an inner peripheral surface of the retaining ring has a predetermined width and depth, and a pair of ring portions are formed at axially both sides thereof. Here, shape and size of the recess and the paired ring portions are so selected that deformation of one of the first ring portions and the second ring portions does not affect the other of them.  
         [0017]     As regard the first frictional engagement unit and the second frictional engagement unit, a radially outer part of the first friction plate nearest to the retaining ring, and a radially outer part of the second friction plate nearest to the retaining ring are abutted to the retaining ring. Further, the frictional engagement apparatus can include, in addition to the first and second frictional engagement units, the case and the retaining ring, a first piston and a second piston respectively pressing the first frictional engagement unit and the second frictional engagement unit toward the retaining ring. The first piston and the second piston can be operated independently, to thereby operate only the first frictional engagement unit, only the second frictional engagement unit, or both the first and second frictional engagement unit. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  is a conceptual view of an automatic transmission for vehicle to which the present invention is applied;  
         [0019]      FIG. 2  is a diagram showing engagement of clutches and brakes for establishing each of speed change ratios for the automatic transmission for vehicle shown in  FIG. 1 ;  
         [0020]      FIG. 3  is a longitudinal sectional view showing construction the automatic transmission for vehicle shown in  FIG. 1  and shows one embodiment of the present invention;  
         [0021]      FIG. 4  is a lateral sectional view of a hollow cylindrical portion and the like in  FIG. 1 ;  
         [0022]      FIG. 5  is a view for explaining an operation when a frictional engagement unit shown in  FIG. 3  is pressed;  
         [0023]      FIG. 6  is a longitudinal sectional view showing construction of a conventional automatic transmission for vehicle; and  
         [0024]      FIG. 7  is a view for explaining an operation when a frictional engagement unit shown in  FIG. 6  is pressed. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0025]     Hereinafter, an embodiment of the present invention will be explained in detail with reference to attached drawings.  
         [0026]      FIG. 1  schematically shows a construction of a driving apparatus for vehicle  10  to which the present invention is applied. The driving apparatus  10  is mounted onto, e.g., an FF (front-engine, front-drive) type vehicle, and includes an engine  12  as a driving source. An output of the engine  12  constructed by an internal combustion engine is transmitted to right and left driving wheels via a torque converter  14  functioning as a hydraulic type transmitting device, an automatic transmission for vehicle  16 , a differential gear unit (not shown), and a pair of axles. Here, the automatic transmission for vehicle  16  and the torque converter  14  have a construction substantially symmetrical with respect to a horizontal centerline, and therefore a lower half thereof is not omitted in  FIG. 1 .  
         [0027]     The torque converter  14  includes a pump impeller  14   p  connected to a crankshaft of the engine  12 , a turbine impeller  14   t  connected to an input shaft  32  of the automatic transmission for vehicle  16 , and a stator impeller  14   s  connected to a housing case  36  via a one-way clutch (not shown) transmitting the rotation in one direction but does not transmit the rotation in other direction. The torque converter  14  amplifies the torque produced by the engine  12  and transmits the amplified torque to the automatic transmission for vehicle  16 . Between the pump impeller  14   p  and the turbine impeller  14   t,  a lockup clutch  38  is provided for allowing the pump impeller  14   p  and the turbine impeller  14   t  to be rotated integrally by an engagement therebetween.  
         [0028]     The automatic transmission for vehicle  16  includes a first transmission portion  24  and a second transmission portion  30 . The first transmission portion  24  is mainly constructed by a single-pinion type first planetary gear set  22 . The second transmission portion  30  is mainly constructed by a single-pinion type second planetary gear set  26  and a double-pinion type third planetary gear set  28 , and is coaxial with the first transmission portion  24 .  
         [0029]     The automatic transmission for vehicle  16  speed changes rotation of an input shaft  32  disposed upstream of the first planetary gear set  22 , and output the speed changed rotation from an output gear  34  disposed downstream of the third planetary gear set  28 . Here, the input shaft  32  corresponding to a claimed input member is a turbine shaft rotated integrally with the turbine impeller  14   t.  The output gear  34  corresponding to a claimed output member meshes with the differential gear unit via a counter shaft or directly to drive and rotate the left and right driving wheels.  
         [0030]     The first planetary gear set  22  constructing the first transmission portion  24  includes three rotary elements, i.e., a sun gear S 1 , a carrier CA 1  and a ring gear R 1 . The sun gear S 1  is connected to the input shaft  32  to be driven and rotated thereby, and the ring gear R 1  is selectively connected via a third brake B 3  to a housing case  36  which is a non-rotary member, the carrier CA 1  functioning as an intermediate output member is rotated at a speed lower than that of the input shaft  32 .  
         [0031]     The second planetary gear set  26  and the third planetary gear set  28  constructing the second transmission portion  30  are partially connected to each other to provide four rotary elements RM 1 , RM 2 , RM 3  and RM 4 . Specifically, a sun gear S 3  of the third planetary gear set  28  provides the first rotary element RM 1 ; respective ring gears R 2  and R 3  of the second and third planetary gear sets  26  and  28  are connected to each other to provide the second rotary element RM 2 ; respective carriers CA 2  and CA 3  of the second and third planetary gear sets  26  and  28  are connected to each other to provide the third rotary element RM 3 ; and a sun gear S 2  of the second planetary gear set  26  provides the fourth rotary element RM 4 .  
         [0032]     Thus, the second and third planetary gear sets  26  and  28  provide a Ravigneaux-type planetary gear train in which the respective ring gears R 2  and R 3  of the second and third planetary gear sets  26  and  28  are constituted integrally with each other; the respective carriers CA 2  and CA 3  of the second and third planetary gear sets  26  and  28  are constituted by a common member; and the pinion gears of the first planetary gear set  26  also function as the second pinion gears of the third planetary gear set  28 .  
         [0033]     The first rotary element RM 1  (sun gear S 3 ) is selectively connected by the first brake B 1  to the housing case  36  to be inhibited from being rotated relative thereto, and the second rotary element RM 2  (ring gears R 2  and R 3 ) is selectively connected by the second brake B 2  to the housing case  36  to be inhibited from being rotated relative thereto. The fourth rotary element RM 4  (sun gear S 2 ) is selectively connected by the first clutch C 1  to the input shaft  32  to be rotated integrally therewith, and the second rotary element RM 2  (ring gears R 2  and R 3 ) is selectively connected by the second clutch C 2  to the input shaft  32  to be rotated integrally therewith.  
         [0034]     The first rotary element RM 1  (sun gear S 3 ) is integrally connected to the carrier CA 1  of the first planetary gear set  22  functioning as the intermediate output member to be rotated integrally therewith thereby outputting the rotation, and the third rotary element RM 3  (carriers CA 2  and CA 3 ) is integrally connected to the output gear  34  to be rotated integrally therewith thereby outputting the rotation. The first to third brakes B 1  to B 3 , and the first and second clutches C 1  and C 2  are multiple disc hydraulic type frictional engagement apparatus which are engaged and released by hydraulic cylinders.  
         [0035]      FIG. 2  shows an operation table representing a relationship between the speed change ratios of the automatic transmission for vehicle  16 , and a plurality of combinations of respective operating states of the first and second clutches C 1  and C 2 , and the first, second and third brakes B 1 , B 2  and B 3  to establish those speed change ratios. In the operation table, symbol “O” indicates an engaged state of each of the clutches C 1  and C 2 , and each of the brakes B 1  to B 3 . No symbol (blank) indicates an disengaged state of each clutch and each brake.  
         [0036]     In the automatic transmission for vehicle  16  of this embodiment, any two of the two clutches C 1  and C 2 , and three brakes B 1  to B 3  are engaged with each other to establish the multiple speed change ratio type transmission having six forward speed change ratios and one rearward speed change ratio. For example, the first clutch C 1  and the second brake B 2  are engaged to establish the forward 1st speed change ratio.  
         [0037]      FIG. 3  is a partial sectional view showing a detail structure of the second speed change portion  30  of the automatic transmission for vehicle  16  to which the present invention is applied.  FIG. 4  is a lateral sectional view of the hollow cylindrical portion  40   b  and the like. More specifically, a left half of  FIG. 4  is the sectional view at the position offset from the snap ring  51 , and a right half thereof is the sectional view at the position including the snap ring  51 .  
         [0038]     In  FIGS. 3 and 4 , the automatic transmission for vehicle  16  includes, in radially inside of a drum  40  of which one axial end is closed, a first frictional element  42  and a second frictional engagement unit  44 . The first frictional element  42  is positioned at the opened side, while the second frictional engagement unit  44  is positioned at the closed side. The drum  40  is connected to the input shaft  32  shown in  FIG. 1  to be always rotated integral therewith, and corresponds to the claimed clutch drum. The drum  40  is comprised of a bottom plate portion  40   a  of substantially disc shape, and a hollow cylindrical portion  40   b  connected to an outer periphery thereof.  
         [0039]     As shown in  FIGS. 3 and 4 , the hollow cylindrical portion  40   b  is provided with plural concave grooves  41  each protruding radial inwardly. The plural concave grooves  41  are spaced circumferentially by a predetermined pitch. Each concave groove  41  has a bottom part  41   a  and a pair of incline parts  41   b  continuing to both sides thereof, and extends over full length of the hollow cylindrical portion  40   b.  Between the adjacent concave grooves  41 , a convex bead  41   c  convexed radially inwardly and extending axially is formed to function as a spline tooth. The plural friction plates  46   b  construct the second frictional engagement unit  44  positioned at the open side of the hollow cylindrical portion  40   b,  while the plural friction plates  46   a  construct the first frictional engagement unit  42  positioned at the open side thereof. Protrusions (convex portion)  46   c  formed at an outer periphery of the friction plate  46   a  and the like engage with the convex grooves  41   c.  Thus, the friction plates  46   a  and the friction plate  46   b  can be shifted axially relative to the hollow cylindrical portion  40   b,  but can not be rotated circumferentially relative thereto.  
         [0040]     The first frictional engagement unit  42  corresponding to the first clutch C 1  in  FIG. 1  includes plural friction plate  46   a  and plural friction plates  50   a.  The friction plates  46   a  are spline-engaged with the hollow cylindrical portion  40   b,  while the friction plates  50   a  positioned between the adjacent friction plates  46   a  are spline-engaged with an outer peripheral surface of a clutch hub  48  which transmits the rotation to the sun gear S 2  of the second planetary gear set  26 .  
         [0041]     The second frictional engagement unit  44  corresponding to the second clutch C 2  in  FIG. 1  includes plural friction plate  46   b  and plural friction plates  50   b.  The friction plates  46   b  are spline-engaged with the hollow cylindrical portion  40   b,  while the friction plates  50   b  positioned between the adjacent friction plates  46   b  are spline-engaged with an outer peripheral surface of the ring gear R. The ring gear R corresponds to the ring gears R 2  and R 3  of the second planetary gear set  26  and the third frictional engagement unit  28  commonly used.  
         [0042]     Between the first frictional engagement unit  42  and the second frictional engagement unit  44  in the axial direction of the hollow cylindrical portion  40   b,  a snap ring  51  is held thereby. The snap ring  51  is made of a spring steel and has a C shape in a normal (non-fitted) state, and a rectangular shape in cross section. By elastically deforming such that both opposed ends approach, diameter of the snap ring  51  decreases. The snap ring  51  is fitted into a circumferential groove  53  formed in convex bead  41   c  and the incline parts  41   b  of the hollow cylindrical portion  40   b  not to be shifted axially. To one end surface and other end surface of the snap ring  51 , an inner end surface of the friction plate  46   a  and that of the friction plate  46   b  can be abutted respectively. Thus, the snap ring  51  can prohibit the first frictional engagement unit  42  and the second frictional engagement unit  44  from axially shifting to approach to each other beyond a predetermined axial position.  
         [0043]     On an inner peripheral surface of the snap ring  51 , a C-shape groove  51   a  of a predetermined width and depth is formed. The snap ring  51  corresponds to a claimed stop ring, while the groove  51   a  corresponds to a claimed recess. Formation of the groove  51   a  provides a pair of rings  51   b  and  51   c  protruding radially inwardly and are deformable axially. To one ring  51   b  the first frictional engagement unit  42  abuts, and to the other ring  51   c  the second frictional engage element  44  abuts. The snap ring  51  has a U-shape cross section in which a radially outer part is continuous while an radially inner part is interrupted by the groove  51   a.    
         [0044]     Thus, the snap ring  51  is elastically deforms, when depressing force is applied by the depressing portions  52   a  or  64   c,  in the radially inner part far remote from the hollow cylindrical portion  40   b  to reduce the thickness thereof. In detail, one of the paired rings  51   b  and  51   c  elastically deforms toward the other thereof.  
         [0045]     Between the bottom plate  40   a  portion of the drum  40  and the clutch hub  48 , a first clutch piston  52  near to the bottom plate  40   a  and a spring support plate  54  near to the spring support plate  54  are disposed. The first clutch piston  52  corresponding to a claimed first piston is axially shiftable, and has at a radially outer periphery a press portion  52   a  for pressing the first frictional engagement unit  42 . It presses the first frictional engagement unit  42  axially toward the snap ring  51  to bring the friction plates  46   a  and  50   a  engaged state with each other.  
         [0046]     The spring support member  54  is abutted at an inner periphery thereof to a snap ring (not shown) which can not be displaced axially, so that an outer periphery can be displace axially. Between the spring support plate  54  and the first clutch piston  52 , a return spring  56  is interposed to bias the first clutch piston  52  toward the bottom plate  40   a  of the drum  40 .  
         [0047]     A chamber formed between the drum  40  and the first clutch piston  52  is kept in an oil-tight state by an oil seal  58  interposed therebetween. This oil-tight chamber functions as a first oil chamber  60  for rendering an impulsive force to shift the first clutch piston  52  axially. A chamber formed between the first clutch piston  52  and the spring support plate  54  is kept in an oil-tight state by an oil seal  62  interposed therebetween. This chamber functions as so-called centrifugal hydraulic pressure cancel chamber.  
         [0048]     A second clutch piston  64  is disposed so as to cover the drum  40 . The second clutch piston  64  corresponding to a claimed second piston includes a bottom plate portion  64   a,  and a hollow cylindrical portion  64   b.  An inner periphery of the bottom plate portion  64   a  of hollow disc shape is fitted on an outer periphery of a base member  66  via a seal ring to be shiftable axially. The hollow cylindrical portion  64   b  of substantially hollow cylindrical shape is connected to an outer periphery of the bottom plate portion  64   a.    
         [0049]     At axially one open end of the hollow cylindrical portion  64   b  a press portion  64   c  is formed to protrude radially inwardly and axially. The press portion  64   c  press the second frictional engagement unit  44  toward the snap ring  51 , so that the friction plates  46   b  and  50   b  are brought engaged state with each other. On an inner peripheral surface at other closed axial end of the hollow cylindrical portion  64   b,  a snap ring  67  protruding radially inwardly is fitted. The snap ring  67  has an end surface to which an end surface of the outer periphery of the bottom plate portion  64   a  is abutted.  
         [0050]     On an inner peripheral surface of the hollow cylindrical portion  64   b,  plural protrusions  64   d  having a longitudinal shape circumferentially are protruded radially inwardly such that one end surface thereof is abutted to an end surface of the outer periphery of the bottom plate portion  64   a.  In this way, the outer periphery of the bottom plate portion  64   a  is fixed non-shiftably in the axial direction relative to the hollow cylindrical portion  64   b.  The outer peripheral surface of the hollow cylindrical portion  40   b  of the drum  40  is engaged with the protrusion  64   d,  whereby the drum  40  and the second clutch piston  64  are rotated integrally.  
         [0051]     A chamber formed between the bottom plate portion  64   a  of the second clutch piston  64  and the bottom plate  40   a  of the drum  40  is kept in an oil-tight state by a seal ring  68  interposed therebetween. This oil-tight chamber functions as a second oil chamber  70  for rendering an impulsive force to shift the second clutch piston  64  axially.  
         [0052]     At a side of the bottom plate portion  64   a  opposite to the second oil chamber  70 , a spring support member  72  is abutted at an inner periphery thereof to a snap ring (not shown) non-shiftably in the axial direction, so that the outer periphery can be displaced axially. Between the spring support plate  72  and the bottom plate portion  64   a,  a return spring  74  is interposed to bias the second clutch piston  64  toward the drum  40 .  
         [0053]     A chamber formed between the spring support  72  and the bottom plate portion  64   a  is kept in an oil-tight state by a seal ring  76  interposed therebetween. This chamber functions as so-called centrifugal hydraulic pressure cancel chamber.  
         [0054]     Next, an operation of the automatic transmission for vehicle  16  thus constructed will be explained. In FIGS.  1  to  3 , upon supplying of an operating oil from an operating oil supplying source (not shown) to the first oil chamber  60 , the first clutch piston  52  shifts axially toward the clutch hub  48 . The press portion  52   a  presses the first frictional engagement unit  42  to thereby bring the first clutch C 1  into engaged state.  
         [0055]     On the other hand, upon supplying of an operating oil to the second oil chamber  70 , the second clutch piston  64  shifts axially toward the spring support plate  72 . The press portion  64   a  presses the second frictional engagement unit  44  to thereby bring the second clutch C 2  into engaged state. Here, on account of independent oil supplying to the first clutch piston  52  and the second clutch piston  64  from separated oil sources, the first and second clutch pistons  52  and  54  can be operated independently to establish the fourth speed state shown in  FIG. 2 , for example.  
         [0056]      FIG. 5  is a view for explaining an operated state of the first frictional engagement unit  42  and the second frictional engagement unit  44 , when the press portion  52   a  of the first clutch piston  52  presses the first frictional engagement unit  42 . Being presses by the press portion  52   a,  the friction plates  46   a  and the friction plates  50   a  of the first frictional engagement unit  42  inclines relative to a direction perpendicular to the axis. With inclination of the friction plate  46   a , the ring portion  51   b  elastically deforms in the axial direction of the snap ring  51  to reduce thickness thereof.  
         [0057]     This elastic deformation of the ring portion  51   b  is permitted by the C-shape groove  51   a,  that is, the ring portion  51   b  deforms axially within the space defined by the groove  51   a.  For this reason, the elastic deformation of the ring portion  51   b  does not affect the ring portion  51   c  at the side of the second frictional engagement unit  44 . In this way, even when the first frictional engagement unit  42  is pressed, affect thereof is not transmitted to the second frictional engagement unit  44 . Incidentally, when the press portion  64   c  of the second friction piston  64  presses the second frictional engagement unit  44 , an influence thereof is not transmitted to the first frictional engagement units  42 .  
         [0058]     According to the embodiment described above, following advantages can be obtained. Firstly, between the first frictional engagement unit  42  and the second frictional engagement unit  44 , the snap ring  51  is disposed to commonly function as the stopper, and the snap ring  51  is provided with the groove  51   a  in the inner periphery thereof. As a result, even when the ring portion  51   b  or the ring portion  51   c  is deformed by pressing of the first frictional engagement unit  42  or the second frictional engagement unit  44 , such the deformation is absorbed by the groove  51   a.  Thus, the second frictional engagement unit  44  is not affected by deformation of the ring portion  51   b,  and the first frictional engagement unit  42  is not affected by deformation of the ring portion  51   c.    
         [0059]     In addition, using one snap ring  51  commonly as the stopper for the first frictional engagement unit  42  and the second frictional engagement unit  44  is effective to shorten the axial length of the automatic transmission for vehicle  16 , compared to a case where two snap rings are respectively provided for the first and second frictional engagement units  42  and  44 . Further, due to small thickness (axial dimension), the snap ring  51  disposed between the first frictional engagement unit  42  and the second frictional engagement unit  44  does not practically increase the axial dimension. In addition, the C-shape snap ring  51  having the rectangular cross section is light in weight.  
         [0060]     In addition, the first clutch piston  52  pressing the first frictional engagement unit  42  and the second clutch piston  64  pressing the second frictional engagement unit  44  are operated independently from each other. As a result, only the first frictional engagement unit  42 , only the second frictional engagement unit  44 , and both the first and second frictional engagement units  42  and  44  can be engaged, to establish various speed change state shown in  FIG. 2 , if needed.  
         [0061]     In the foregoing, one embodiment of the present invention has been explained with reference to the attached drawings. However, noted is that the present invention can be embodied in other various modes. For example, the above-described embodiment employs the first and second frictional engagement units  42  and  44 , and the snap ring  51  interposed therebetween, all of which are arranged in the radially inner space of the clutch drum  40 . However, the first and second frictional engagement units  42  and  44 , and the snap ring  51  interposed can be arranged in the radially outer space of the clutch hub  48 . In this case, the snap ring  51  is fitted to an outer peripheral surface of the clutch hub  48 , and the recess portion  51   a  is formed on an outer peripheral surface thereof.  
         [0062]     In addition, the snap ring  51  constructed by single member in the above-described embodiment can be constructed by plural members. In detail, between a pair of C-shape snap rings, a disc shape spacer ring is interposed to form the recess functioning in the same manner as the above-described groove  51   a.  Further, the snap ring  51  made of the spring steel in the above-described embodiment can be made of material other than the spring steel which is excellent in elastic deforming property.  
         [0063]     In addition to the above, the present invention can be carried out in various modes which are modified along knowledge of the skilled person in this field.