Abstract:
A single pinion type planetary gear unit and a double sun gear type planetary gear units are coaxially arranged on different positions of a given axis of an input shaft. First and second brakes are coaxially arranged on different positions of the given axis. The first brake selectively fixes a carrier of the double sun gear type planetary gear unit and the second brake selectively fixes an outer one of the sun gears of the double sun gear type planetary gear unit. First and second clutches are coaxially arranged on different positions of the given axis to transmit a speed changed rotation to rotational elements of the two planetary gear unit to provide an associated automatic transmission with a desired speed. The first clutch transmits the speed changed rotation to the ring gear of the single pinion type planetary gear unit, and the second clutch transmits the speed changed rotation to the sun gear of the single pinion type planetary gear unit. The first and second clutches are coaxially arranged in the vicinity of the single pinion type planetary gear unit having the second clutch positioned in a space defined between the first clutch and the input shaft.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates in general to automatic transmissions for wheeled motor vehicles, and more particularly a speed change mechanism of the automatic transmissions, which comprises two planetary gear units which are coaxially arranged, two clutches which are arranged to selectively transmit a speed-changed input rotation to rotational elements of the mechanism and brakes which are arranged to selectively fix other rotational elements of the mechanism for allowing the transmission to assume a desired speed or position. 
   2. Field of the Related Art 
   Nowadays, for improving fuel consumption and drivability of the motor vehicles, the speed change mechanisms of automatic transmissions have a tendency to increase the number of speeds, viz., speed positions. One of them is shown in Laid-open Japanese Patent Application (Tokkai) 2000-55152, which comprises a planetary gear unit that mainly reduces the speed of rotation transmitted from the engine, a speed change mechanism that is arranged behind the planetary gear unit, two direct clutches that are capable of directly transmitting the rotation of the engine to rotational elements of the speed change mechanism and brakes that are capable of fixing the rotational elements of the speed change mechanism, for allowing the transmission to have a plurality of speeds. 
   The speed change mechanism of the above-mentioned application employs a Ravigneawx type planetary gear unit. That is, by selectively engaging or disengaging the clutches and brakes in a controlled manner, the rotational elements of the speed change mechanism are arranged to constitute a power transmitting path for a desired speed. 
   However, due to its inherent construction, usage of the Ravigneawx type planetary gear unit as the speed change mechanism causes the mechanism to have a bulky construction, that is, a bulky construction whose size is enlarged in a radial direction. That is, for transmitting the speed reduced input rotation (viz., speed changed rotation) from the speed reducing planetary gear unit to the two sun gears of the Ravigneawx type planetary gear unit through the above-mentioned two clutches, there is a need of providing two connecting members, one being for connecting one of the clutches to one of the sun gears and the other being for connecting the other clutch to the other sun gear. However, due to inevitability in layout, the two connecting members are compelled to have axially extending portions which are concentrically overlapped on each other, which causes enlargement of the speed change mechanism in a radial direction. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of the present invention to provide a speed change mechanism of an automatic transmission, which can provide the transmission with a plurality of speeds without enlarging the radial size thereof. 
   That is, according to the present invention, there is provided a speed change mechanism of an automatic transmission wherein a rotation from one clutch is transmitted to a sun gear of the speed change mechanism through one connecting member, a rotation from the other clutch is transmitted to a ring gear of the speed change mechanism through another connecting member, and these two connecting members are arranged at the same side of the speed change mechanism, and wherein the two connecting members have no portions that are concentrically overlapped on each other. 
   According to a first aspect the present invention, there is provided a speed change mechanism of an automatic transmission, which comprises a single pinion type planetary gear unit concentrically disposed on a position of a given axis and includes a sun gear, pinions meshed with the sun gear, a ring gear meshed with the pinions and a carrier rotatably carrying the pinions; a double sun gear type planetary gear unit concentrically disposed on the other position of the given axis and includes two sun gears, pinions meshed with the two sun gears, a ring gear meshed with the pinions and a carrier rotatably carrying the pinions; first and second brakes concentrically arranged on different positions of the given axis, the first brake being arranged to selectively fix the carrier of the double sun gear type planetary gear unit and the second brake being arranged to selectively fix one of the two sun gears of the double sun gear type planetary gear unit; and first and second clutches coaxially arranged on different positions of the given axis to transmit a speed changed rotation to rotational elements of the two planetary gear units to provide the automatic transmission with a desired speed, wherein the first clutch is arranged to transmit the speed changed rotation to the ring gear of the single pinion type planetary gear unit, and the second clutch is arranged to transmit the speed changed rotation to the sun gear of the single pinion type planetary gear unit; and wherein the first clutch is concentrically arranged around the single pinion type planetary gear unit, and the second clutch is coaxially arranged beside the first clutch at a position that is away from the double sun gear type planetary gear unit as compared with the single pinion type planetary gear unit. 
   According to a second aspect of the present invention, there is provided a speed change mechanism of an automatic transmission having an input shaft that is rotatable about an axis, the speed change mechanism comprising a single pinion type planetary gear unit concentrically disposed around a position of the axis near the input shaft and includes a sun gear, pinions meshed with the sun gear, a ring gear meshed with the pinions and a carrier rotatably carrying the pinions; a double sun gear type planetary gear unit concentrically disposed around the other position of the axis away from the input shaft and includes two sun gears, pinions meshed with the two sun gears, a ring gear meshed with the pinions and a carrier rotatably carrying the pinions, the two sun gears being an inner sun gear directed toward the input shaft and an outer sun gear directed away from the input shaft; first and second brakes coaxially arranged around different positions of the axis, the first brake selectively fixing the carrier of the double sun gear type planetary gear unit and the second brake selectively fixing the outer sun gear of the double sun gear type planetary gear unit; and first and second clutches coaxially arranged around different positions of the axis, the first clutch selectively transmitting a speed changed rotation to the ring gear of the single pinion type planetary gear unit, and the second clutch selectively transmitting a speed changed rotation to the sun gear of the single pinion type planetary gear unit, wherein the first and second clutches are arranged in the vicinity of the single pinion type planetary gear unit having the second clutch positioned in a space defined between the first clutch and the input shaft. 
   According to a third aspect of the present invention, there is provided a speed change mechanism of an automatic transmission having an input shaft that is rotatable about an axis, the speed change mechanism comprising a single pinion type planetary gear unit concentrically disposed around a position of the axis near the input shaft and includes a sun gear, pinions meshed with the sun gear, a ring gear meshed with the pinions and a carrier rotatably carrying the pinions; a double sun gear type planetary gear unit concentrically disposed around the other position of the axis away from the input shaft and includes two sun gears, pinions meshed with the two sun gears, a ring gear meshed with the pinions and a carrier rotatably carrying the pinions, the two sun gears being an inner sun gear directed toward the input shaft and an outer sun gear directed away from the input shaft; first and second brakes coaxially arranged around different positions of the axis, the first brake selectively fixing the carrier of the double sun gear type planetary gear unit and the second brake selectively fixing the outer sun gear of the double sun gear type planetary gear unit; first and second clutches coaxially arranged around different positions of the axis; a first connecting member through which a speed changed rotation is transmitted to the ring gear of the single pinion type planetary gear unit from the first clutch; and a second connecting member through which a speed change is transmitted to the sun gear of the single pinion type planetary gear unit from the second clutch, wherein the first and second connecting members have no portions which are concentrically overlapped around the axis. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a schematic view of a speed change mechanism of an automatic transmission, which is an embodiment of the present invention; 
       FIG. 2  is a table showing a relationship between ON/OFF condition of each friction element and speeds, which is established in the speed change mechanism of the present invention; 
       FIGS. 3  to  8  are views similar to  FIG. 1 , but showing torque transmitting paths established in first, second, third, fourth, fifth and sixth speeds of the speed change mechanism of the present invention, respectively; 
       FIG. 9  is a view similar to  FIG. 1 , but showing a torque transmitting path established in a reverse position of the speed change mechanism of the present invention; 
       FIG. 10  is a schematic view of the speed change mechanism of the present invention; and 
       FIG. 11  is an enlarged view of a part of the view of  FIG. 10 , which is essential in the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In the following, the embodiment of the present invention will be described in detail with reference to the accompanying drawings. 
     FIG. 1  shows schematically a speed change mechanism of an automatic transmission, which is the embodiment of the present invention. 
   In the drawing, denoted by references G 1 , G 2  and G 3  are first, second and third planetary gear units which are coaxially arranged in order. Denoted by references M 1  and M 2  are first and second connecting structures, denoted by references C 1 , C 2  and C 3  are first, second and third clutches, and denoted by references B 1  and B 2  are first and second brakes. Denoted by references “Input” and “Output” are input and output portions, viz., an input shaft  1  and an output gear  2 , respectively. 
   In the embodiment, the first and second planetary gear units G 1  and G 2  are of a single pinion type, and the third planetary gear unit G 3  is of a double sun gear type. 
   The first planetary gear unit G 1  is arranged to constitute a speed reduction mechanism and the second and third planetary gear units G 2  and G 3  are arranged to constitute a speed change mechanism which is arranged behind the speed reduction mechanism. 
   The first planetary gear unit G 1  comprises a constantly fixed first sun gear S 1 , a first ring gear R 1 , first pinions P 1  meshed with both the gears S 1  and R 1 , and a first carrier PC 1  rotatably holding the first pinions P 1 . 
   The second planetary gear unit G 2  comprises a sun gear S 2 , a second ring gear R 2 , second pinions P 2  meshed with both the gears S 2  and R 2 , and a second carrier PC 2  rotatably holding the second pinions P 2 . 
   The third planetary gear unit G 3  comprises a third sun gear S 3  located near the input portion “Input”, a fourth sun gear S 4  located a way from the input portion “Input”, third pinions P 3  meshed with both the third and fourth sun gears S 3  and S 4 , a third carrier PC 3  rotatably holding the third pinions P 3 , and a third ring gear R 3  meshed with the third pinions P 3 . 
   Although the third and fourth sun gears S 3  and S 4  are arranged coaxially, it is not always necessary for them to have the same number of teeth. 
   To the third carrier PC 3 , there is connected a center member CM which extends radially inward between the third and fourth sun gears S 3  and S 4 . Furthermore, to the third carrier PC 3 , there is connected an outer member OM which extends radially outward. 
   As will be described in detail hereinafter, the outer member OM has a unique arrangement. 
   The center member CM is arranged on a pitch circle of the third pinions P 3  and has portions each extending radially inward between neighboring two of the third pinions P 3 . As shown, the portions of the center member CM are integral with one part of the clutch C 3 . 
   The input portion “Input” is the input shaft  1 , and the input shaft  1  is connected to the first ring gear R 1 . Although not shown in the drawing, the input shaft  1  is driven by an engine of the vehicle through a torque converter. Thus, the power of the engine is fed to the first ring gear R 1  through the torque converter and the input shaft  1 . 
   The output portion “Output” is the output gear  2 . The output gear  2  is connected with both the second carrier PC 2  and the third ring gear R 3  through the second connecting structure M 2 . Although not shown in the drawing, the output gear  2  is engaged with a final gear unit, and thus the output rotation of the output gear  2  is transmitted to driven road wheels of the vehicle through the final gear unit and a differential gear unit. 
   The first connecting structure M 1  is a member to integrally connect the second sun gear S 2  and the third sun gear S 3 . 
   As shown, the first sun gear S 1  of the first planetary gear unit G 1  is fixed to a transmission case  3  and thus it is constantly fixed. The first carrier PC 1  is connectable to the second ring gear R 2  through the first clutch C 1 , and furthermore, the first carrier PC 1  is connectable to the second sun gear S 2  through the second clutch C 2 . 
   The center member CM of the third carrier PC 3  is connectable to the input shaft  1  through the third clutch C 3 , and thus the third clutch C 3  can constitute a direct clutch that directly transmits the input rotation to a speed change mechanism that includes the second and third planetary gear units G 2  and G 3 . 
   The outer member OM of the third carrier PC 3  of the third planetary gear unit G 3  is connectable to the transmission case  3  through the first brake B 1 , and thus the third carrier PC 3  is fixable. The fourth sun gear S 4  is connectable to the transmission case  3  through the second brake B 2 , and thus the fourth sun gear S 4  is fixable. 
   When the above-mentioned clutches C 1 , C 2  and C 3  and brakes B 1  and B 2  are engaged or disengaged in a manner as is indicated by the table of  FIG. 2 , six forward speeds and one reverse position can be selectively assumed by the speed change mechanism. Although not shown in the drawing, the frictional elements, viz., clutches C 1 , C 2  and C 3  and brakes B 1  and B 2 , are driven or controlled by a control valve unit (not shown). The control valve unit may be of a hydraulic type, electronic type or a combination of these types. 
   In the following, operation of the speed change mechanism will be described with reference to  FIGS. 2  to  9  of the drawings. 
     FIG. 2  is a table depicting the engaged/disengaged condition of each frictional element with respect to a speed (or reverse) assumed by the transmission (viz., speed change mechanism),  FIGS. 3  to  8  are drawings showing torque transmitting paths established in first to sixth speeds of the speed change mechanism respectively and  FIG. 9  is a drawing showing the torque transmitting path established in the reverse position of the speed change mechanism. 
   In  FIGS. 3  to  9 , the torque transmitting paths are illustrated by thicker lines and the elements that participate in the torque transmission are hatched. 
   First Speed 
   As is seen from the table of  FIG. 2 , in the first speed of the speed change mechanism, the first clutch C 1  and the first brake B 1  are engaged. 
   As is seen from  FIG. 3 , in the first speed, due to the engagement of the first clutch C 1 , a reduced speed rotation of the first planetary gear unit G 1  is inputted to the second ring gear R 2  of the second planetary gear unit G 2 . In addition, due to the engagement of the first brake B 1  inducing fixing of the third carrier PC 3 , rotation of the third ring gear R 3  causes a reduced speed rotation of the third sun gear S 3  in an opposite direction. The rotation of the third sun gear S 3  is transmitted to the second sun gear S 2  of the second planetary gear unit G 2  through the first connecting structure M 1 . 
   Accordingly, at the second planetary gear unit G 2 , there are inputted a reduced speed rotation in a normal direction from the second ring gear R 2  and a reduced speed rotation in an opposite direction from the second sun gear S 2 , so that a rotation whose speed is much reduced as compared with the rotation of the second ring gear R 2  is transmitted to the output gear  2  through the second carrier PC 2  and second connecting structure M 2 . 
   The torque transmitting path established in this first speed is illustrated by thicker lines. That is, the first clutch C 1 , the first brake B 1 , the first and second connecting structures M 1  and M 2 , the outer member OM, the first and second planetary gear units G 1  and G 2  and the third planetary gear unit G 3  except the fourth sun gear S 4  are applied with a certain torque. 
   That is, in the first speed, the first planetary gear unit G 1  and the second and third planetary gear units G 2  and G 3  which constitute the speed change mechanism behind the first planetary gear unit G 1  participate in the torque transmission. 
   Second Speed 
   As is seen from the table of  FIG. 2 , in the second speed, the first clutch C 1  and the second brake B 2  are engaged. Thus, if the second speed would take place following the first speed, the first brake B 1  is released and the second brake B 2  is engaged. 
   As is seen from  FIG. 4 , in the second speed, due to the engagement of the first clutch C 1 , a reduced speed rotation of the first planetary gear unit G 1  is inputted to the second ring gear R 2  of the second planetary gear unit G 2 . In addition, due to the engagement of the second brake B 2 , the fourth sun gear S 4  is fixed to the transmission case  3  and thus the third sun gear S 3  connected to the third pinions P 3  is fixed, and the second sun gear  52  connected to the third sun gear S 3  through the first connecting structure M 1  is fixed to the transmission case  3 . 
   Accordingly, at the second planetary gear unit G 2 , there is inputted a reduced speed rotation in a normal direction from the second ring gear R 2  inducing fixing of the second sun gear S 2 , so that a rotation whose speed is much reduced as compared with the rotation of the second ring gear R 2  (but higher than the rotation in the first speed) is transmitted to the output gear  2  through the second carrier PC 2  and the second connecting structure M 2 . 
   The torque transmitting path established in this second speed is illustrated by thicker lines. That is, the first clutch C 1 , the second brake B 2 , the first and second connecting structures M 1  and M 2 , and the first and second planetary gear units G 1  and G 2  are applied with a certain torque. 
   However, in the third planetary gear unit G 3 , due to turning of the third ring gear R 3 , the third pinions P 3  is turned around both the third and fourth sun gears S 3  and S 4  which are fixed to the transmission case  3 . Thus, even when serving as a rotation element, the third planetary gear unit G 3  does not participate in the torque transmission. 
   Third Speed 
   As is seen from the table of  FIG. 2 , in the third speed, the first and second clutches C 1  and C 2  are engaged. Thus, if the third speed would take place following the second speed, the second brake B 2  is released and the second clutch C 2  is engaged. 
   As is seen from  FIG. 5 , in the third speed, due to the engagement of the first clutch C 1 , a reduced speed rotation of the first planetary gear unit G 1  is inputted to the second ring gear R 2 , and at the same time, due to the engagement of the second clutch C 2 , the reduced speed rotation is inputted to the second sun gear S 2 . 
   Accordingly, at the second planetary gear unit G 2 , there are inputted the same reduced speed rotations from the second ring gear R 2  and the second sun gear S 2 , so that a reduced speed rotation (which is the same as that of the first planetary gear unit G 1 ) is outputted from the second carrier PC 2  which rotates integrally with the gears R 2  and S 2  to the output gear  2  through the second connecting structure M 2 . 
   The torque transmitting path established in this third speed is illustrated by thicker lines. That is, the first clutch C 1 , the second clutch C 2 , the second connecting structure M 2 , and the first and second planetary gear units G 1  and G 2  are applied with a certain torque. 
   It is to be noted that like in the second speed, the third planetary gear unit G 3  does not participate in the torque transmission. 
   Fourth Speed 
   As is seen from the table of  FIG. 2 , in the fourth speed, the first and third clutches C 1  and C 3  are engaged. Thus, if the fourth speed would take place following the third speed, the second clutch C 2  is released and the third clutch C 3  is engaged. 
   As is seen from  FIG. 6 , in the fourth speed, due to the engagement of the first clutch C 1 , a reduced speed rotation of the first planetary gear unit G 1  is inputted to the second ring gear R 2 , and at the same time, due to the engagement of the third clutch C 3 , an input rotation of the input shaft  1  is inputted to the third carrier PC 3  through the center member CM. 
   Accordingly, the rotation speed of the third sun gear S 3  becomes higher than that of the third ring gear R 3 , and the rotation of the third sun gear S 3  is transmitted to the second sun gear S 2  through the first connecting structure M 1 . 
   Thus, at the second planetary gear unit G 2 , there are inputted a reduced speed rotation from the second ring gear R 2  and an increased speed rotation from the second sun gear S 2 , and thus a rotation whose speed is higher than the reduced speed rotation of the second ring gear R 2  (but lower than that of the input shaft  1 ) is outputted to the output gear  2  from the second carrier PC 2  through the second connecting structure M 2 . 
   The torque transmitting path established in this fourth speed is illustrated by thicker lines. That is, the first clutch C 1 , the third clutch C 3 , the first and second connecting structures M 1  and M 2 , the center member CM, the first and second planetary gear units G 1  and G 2  and the third planetary gear unit G 3  except the fourth sun gear S 4  are applied with a certain torque. 
   Fifth Speed 
   As is seen from the table of  FIG. 2 , in the fifth speed, the second and third clutches C 2  and C 3  are engaged. Thus, if the fifth speed would take place following the fourth speed, the first clutch C 1  is released and the second clutch C 2  is engaged. 
   As is seen from  FIG. 7 , in the fifth speed, due to the engagement of the second clutch C 2 , a reduced speed rotation of the first planetary gear unit G 1  is inputted to the third sun gear S 3  through the second sun gear S 2  and the first connecting structure M 1 , and at the same time, due to the engagement of the third clutch C 3 , an input rotation of the input shaft  1  is inputted to the third carrier PC 3  through the center member CM. 
   Accordingly, at the third planetary gear unit G 3 , the input rotation is applied to the third carrier PC 3 , and the reduced speed rotation of the first planetary gear unit G 1  is inputted to the third sun gear S 3 , so that a rotation whose speed is higher than that of the input rotation is outputted to the output gear  2  through the third ring gear R 3  and the second connecting structure M 2 . 
   The torque transmitting path established in this fifth speed is illustrated by thicker lines. That is, the second clutch C 2 , the third clutch C 3 , the first and second connecting structures M 1  and M 2 , the center member CM, the first planetary gear unit G 1  and the third planetary gear unit G 3  except the fourth sun gear S 4  are applied with a certain torque. 
   Sixth Speed 
   As is seen from the table of  FIG. 2 , in the sixth speed, the third clutch C 3  and the second brake B 2  are engaged. Thus, if the sixth speed would take place following the fifth speed, the second clutch C 2  is released and the second brake B 2  is engaged. 
   As is seen from  FIG. 8 , in the sixth speed, due to the engagement of the third clutch C 3 , the input rotation of the input shaft  1  is inputted to the third carrier PC 3  through the center member CM of the third planetary gear unit G 3 , and at the same time, due to the engagement of the second brake B 2 , the fourth sun gear S 4  of the third planetary gear unit G 3  is fixed to the transmission case  3 . 
   Accordingly, at the third planetary gear unit G 3 , there is inputted the input rotation into the third carrier PC 3 , so that because of the fixing of the fourth sun gear S 4 , a rotation whose speed is higher than that of the input rotation is outputted to the output gear  2  from the third ring gear R 3  through the second connecting structure M 2 . 
   The torque transmitting path established in this sixth speed is illustrated by thicker lines. That is, the third clutch C 3 , the second brake B 2 , the second connecting structure M 2  and the third planetary gear unit G 3  except the third sun gear S 3  are applied with a certain torque. 
   Reverse 
   As is seen from the table of  FIG. 2 , in the reverse, the second clutch C 2  and the first brake B 1  are engaged. 
   As is seen from  FIG. 9 , in the reverse position, due to the engagement of the second clutch C 2 , a reduced speed rotation of the first planetary gear unit G 1  is inputted to the third sun gear S 3  through the second sun gear S 2  and the first connecting structure M 1 , and at the same time, due to the engagement of the first brake B 1 , the third carrier PC 3  is fixed to the transmission case  3 . 
   Accordingly, at the third planetary gear unit G 3 , there is inputted a reduced speed rotation in a normal direction to the third sun gear S 3 , so that because of the fixing of the third carrier PC 3 , a reduced speed rotation in a reverse direction is outputted from the third ring gear R 3  to the output gear  2  through the second connecting structure M 2 . 
   The torque transmitting path established in this reverse position is illustrated by thicker lines. That is, the second clutch C 2 , the first brake B 1 , the first and second connecting structures M 1  and M 2 , the outer member OM, the first planetary gear unit G 1  and the third planetary gear unit G 3  except the fourth sun gear S 4  are applied with a certain torque. 
   Referring to  FIGS. 10 and 11 , particularly  FIG. 10 , there is shown in a sectional manner the speed change mechanism of automatic transmission, to which the present invention is practically applied.  FIG. 11  is an enlarged view of an essential part of the speed change mechanism of the invention. 
   In the following, the detailed construction of the speed change mechanism of the present invention will be described with reference to the drawings. 
   It is to be noted that  FIGS. 10 and 11  are drawings with input and output portions of the speed change mechanism shown at right and left portions of the drawings, which is reversed to case of  FIGS. 1 and 3  to  9 . 
   For ease of understanding, various directional terms, such as, right, left, upper, lower, rightward, etc., will be used in the following description. However, such terms are to be understood with respect to only the drawing or drawings on which a corresponding part or portion is shown. 
   As is seen from the drawings, within the transmission case  3 , there are coaxially installed the input shaft  1  and an intermediate shaft  4 . These two shafts  1  and  4  are coaxially rotatable to each other. A front opening of the transmission case  3  is covered with a pump case that comprises a pump housing  5  and a pump cover  6 . The pump case has the input shaft  1  passed therethrough. Bearings (no numerals) are arranged between the pump housing and the input shaft  1  for bearing the input shaft  1 . A rightward leading end of the input shaft  1  is connected to an output element of a torque converter (not shown) that is driven an engine (not shown). 
   Within the pump case, there are arranged pump elements to constitute an oil pump that is directly driven by the engine through a pump drive shaft  51 . A left end of the intermediate shaft  4  is rotatably supported by an rear end lid  7  of the transmission case  3 . 
   In an axially middle portion of the transmission case  3 , there is provided an intermediate wall  8  around which the output gear  2  is rotatably supported. In a center opening of the intermediate wall  8 , there is arranged a hollow shaft  9  through which mutually engaged portions of the input shaft  1  and the intermediate shaft  4  are rotatably held. 
   As is seen from  FIG. 10 , in a front open space of the transmission case  3  that is defined between the oil pump case ( 5 ,  6 ) and the intermediate wall  8 , there is arranged the first planetary gear unit G 1 . The third clutch C 3  is arranged to surround the first planetary gear unit G 1 . 
   In the first planetary gear unit G 1 , the first sun gear Si is immovably held on a center boss portion  6   a  of the pump cover  6  through a serration connection. With this, the first sun gear S 1  can serve as a reaction force holder. The first ring gear R 1  serving as a rotation input member is mounted to a peripheral portion of a flange  10  which extends radially outward from the input shaft  1 . 
   A clutch drum  11  extends radially outward from a front end of the intermediate shaft  4  and surrounds the first ring gear R 1 . 
   In an annular clearance defined between the clutch drum  11  and the first ring gear R 1 , there is arranged the third clutch C 3 . The third clutch C 3  comprises a clutch pack  12  that includes alternatively put first and second groups of clutch plates, the first group effecting a spline-engagement with an inner surface of the clutch drum  11  and the second group effecting the spline-engagement with an outer surface of the first ring gear R 1 . As shown, the third clutch C 3  is arranged around the first planetary gear unit G 1 . The first ring gear R 1  can serve as a clutch hub of the third clutch C 3 . 
   A clutch piston  13  of the third clutch C 3  is located at a position opposite to the oil pump case ( 5 ,  6 ) with respect to the first planetary gear unit G 1 . Thus, the clutch piston  13  is coupled with both an end wall  11   a  of the clutch drum  11  and a front end of the intermediate shaft  4  which face the first planetary gear unit G 1 . 
   Between the third clutch piston  13  and the end wall  11   a  of the clutch drum  11 , there is defined a chamber  52  which is connected to the control valve unit (not shown) through a third clutch working oil passage which comprises an oil passage  14   a  formed in the pump cover  6 , an oil passage  14   b  formed in the input shaft  1  and an oil passage  14   c  formed in the intermediate shaft  4 . A connecting portion between the oil passages  14   a  and  14   b  is sealed from the surrounding by seal rings  54   a  that are disposed between the input shaft  1  and the pump cover  6 , and a connecting portion between the oil passages  14   b  and  14   c  is sealed from the surrounding by seal rings  54   b  that are disposed between the input shaft  1  and the intermediate shaft  4 . 
   Accordingly, when receiving a pressurized working oil from the control valve unit through the oil passages  14   a ,  14   b  and  14   c , the third clutch piston  13  is moved rightward in the drawing establishing an engaged condition of the third clutch C 3 . 
   A drum-shaped connecting member  53  extends radially outward from the front end of the hollow shaft  9  in a manner to cover the third clutch C 3 . The connecting member  53  has a front end connected to the first carrier PC 1 . 
   As has been mentioned hereinabove, the first carrier PC 1  serves as a rotation output member of the first planetary gear unit G 1 . 
   In a space defined between the intermediate wall  8  and the rear end lid  7 , there are installed the speed change mechanism including the second and third planetary gear units G 2  and G 3 , the first and second clutches C 1  and C 2  and the first and second brakes B 1  and B 2  in such a manner as is shown by  FIGS. 10 and 11 . 
   The second and third planetary gear units G 2  and G 3  are coaxially disposed on the intermediate shaft  4 , and the second planetary gear unit G 2  is positioned closer to the input shaft  1  than the third planetary gear unit G 3  is. 
   The second sun gear S 2  of the second planetary gear unit G 2  and the third sun gear S 3  of the third planetary gear unit G 3  are connected by the first connecting structure M 1  and rotatably mounted on the intermediate shaft  4 . 
   A clutch drum  15  extends radially outward from a middle portion of the hollow shaft  9  and extends axially rearward to cover the second ring gear R 2 . 
   In an annular clearance defined between the clutch drum  15  and the second ring gear R 2 , there is arranged the first clutch C 1 . The first clutch C 1  comprises a clutch pack  16  that includes alternatively put first and second groups of clutch plates, the first group effecting a spline-engagement with an inner surface of the clutch drum  15  and the second group effecting the spline-engagement with an outer surface of the second ring gear R 2 . As shown, the first clutch C 1  is arranged around the second planetary gear unit G 2 . The second ring gear R 2  can serve as a clutch hub of the first clutch C 1 . 
   As is described hereinabove, the second clutch C 2  is positioned closer to the input shaft  1  than the first clutch C 1  is. The first clutch C 1  is arranged around the second planetary gear unit G 2 . That is, the second clutch C 2  is positioned away from the third planetary gear unit G 3  as compared with the first clutch C 1 . A clutch hub  17  extends radially outward from a rear end of the second sun gear S 2 , which has an annular plate portion. 
   In annular clearance defined between the annular plate portion of the clutch hub  17  and the clutch drum  15 , there is arranged the second clutch C 2 . The second clutch C 2  comprises a clutch pack  18  that includes alternatively put first and second groups of clutch plates, the first group effecting a spline-engagement with an inner surface of the clutch drum  15  and the second group effecting a spline-engagement with an outer surface of the annular plate portion of the clutch hub  17 . 
   A clutch piston  19  of the first clutch C 1  and a clutch piston of the second clutch C 2  constitute a so-called double piston structure in which the clutch piston  20  is coaxially movable within the clutch piston  19 , and the two pistons  19  and  20  are positioned closer to the second clutch C 2 , that is, away from the first clutch C 1 . With this arrangement, the clutch piston  20  is coupled with an end wall of the clutch drum  15 , the end wall facing the second planetary gear unit G 2 . 
   Upon receiving a pressurized working oil from the control valve unit through respective oil passages  21  formed in the intermediate wall  8  and the hollow shaft  9 , the two clutch pistons  19  and  20  are coaxially moved separately. That is, when these pistons  19  and  20  are moved leftward in the drawing, the first and second clutches C 1  and C 2  assume their engaged condition. 
   As has been mentioned hereinabove, the third planetary gear unit G 3  is of double sun gear type. A face width of the third ring gear R 3  is made smaller than that of each of the third pinions P 3 , so that the third ring gear R 3  is meshed with the third pinions P 3  at a position near the second planetary gear unit G 2 . Thus, the second connecting structure M 2  through which the third ring gear R 3  is connected to the second carrier PC 2  of the second planetary gear unit G 2  can be shortened. 
   To the peripheral portion of the third ring gear R 3 , there is connected an end of a cylindrical connecting member  22  that surrounds the clutch drum  15  of the first and second clutches C 1  and C 2 . The other end of the cylindrical connecting member  22  is connected to the output gear  2  to rotate therewith. 
   To the third carrier PC 3  of the third planetary gear unit G 3 , there is provided the center member CM which extends radially inward from the third carrier PC 3  between the third and fourth sun gears S 3  and S 4 , and to an axially middle parts of the third pinions P 3 , there is provided the outer member OM which extends radially outward from the third pinions P 3  along an end surface of the third ring gear R 3 . 
   The center member CM is connected with the intermediate shaft  4 , so that the third carrier PC 3  is connected to the clutch drum  11  of the third clutch C 3  through the center member CM and the intermediate shaft  4 . 
   The outer member OM is provided at its peripheral portion with a brake hub  23 . The brake hub  23  is arranged around the cylindrical connecting member  22  and extends forward to the intermediate wall  8 . 
   In an annular clearance defined between a front end portion of the brake hub  23  and the transmission case  3 , there is arranged the first brake B 1 . The first brake B 1  comprises a clutch pack  24  that includes alternatively put first and second groups of clutch plates, the first group effecting a spline-engagement with an inner surface of the transmission case  3  and the second group effecting the spline-engagement with an outer surface of the front end portion of the brake hub  23 . Behind the brake pack  24 , there is arranged a brake piston  25 . That is, when the brake piston  25  is moved rightward in the drawing, the first brake B 1  assumes its engaged condition. 
   A brake hub  26  is arranged to surround a rear end portion of the brake hub  23 . The brake hub  23  has a rear end wall  26   a  that extends radially inward along the back portion of the third planetary gear unit G 3 . The rear end wall  26   a  is secured at its inward end to the fourth sun gear S 4  of the third planetary gear unit G 3 . 
   In an annular clearance defined between the brake hub  26  and the transmission case  3 , there is arranged the second brake B 2 . The second brake B 2  comprises a clutch pack  27  that includes alternatively put first and second groups of clutch plates, the first group effecting a spline-engagement with an inner surface of the transmission case  3  and the second group effecting the spline-engagement with an outer surface of the brake hub  26 . Behind the brake pack  17 , there is arranged a brake piston  28 . When the brake piston  28  is moved rightward in the drawing, the second brake B 2  assumes its engaged condition. 
   As is described hereinabove, the first and second brakes B 1  and B 2  are arranged around the first and second clutches C 1  and C 2 . But, the first brake B 1  is positioned closer to the input shaft  1  (or the first planetary gear unit G 1 ) than the second brake B 2  is, and these first and second brakes Be and B 2  are positioned closer to the second planetary gear unit G 2  than the third planetary gear unit G 3 . 
   Between a front end of the brake hub  23  of the first brake B 1  and the transmission case  3 , there is arranged a one-way clutch OWC. When, with the first brake B 1  kept released, the one-way clutch OWC suppresses rotation of the third carrier PC 3  in a direction, the forward first speed is obtained. 
   In case of the first brake effected by the operation of the one-way clutch OWC, engine brake is not obtained because the one-way clutch OWC permits a reversed rotation of the third carrier PC 3  under engine brake. Accordingly, upon need of the engine brake, the first brake B 1  is engaged for suppressing the reversed rotation of the third carrier PC 3 . 
   As is seen from  FIG. 10 , within the transmission case  3 , there is arranged a countershaft  29  which extends in parallel with the input and intermediate shafts  1  and  4  and is rotatable about its axis. The countershaft  29  has both a counter gear  30  and a final drive pinion  31  integrally formed thereon. The counter gear  30  is meshed with the output gear  2 , and the final drive pinion  31  is meshed with an input gear of a differential gear device (not shown) for drive road wheels. 
   As has been mentioned hereinabove, in the speed change mechanism of the invention, the second planetary gear unit G 2  is of a single pinion type and the third planetary gear unit G 3  is of a double sun gear type. These units G 2  and G 3  constitute the speed change mechanism that is arranged behind the first planetary gear unit G 1  of single pinion type. In the two clutches C 1  and C 2 , the first clutch C 1  can transmit a reduced speed rotation from the first planetary gear unit G 1  to the second ring gear R 2  of the second planetary gear unit G 2 , and the second clutch C 2  can transmit a reduced speed rotation from the first planetary gear unit G 1  to the second sun gear S 2  of the second planetary gear unit G 2 . Furthermore, the first clutch C 1  is located near the second planetary gear unit G 2  (single pinion type) and the second clutch C 2  is located beside the first clutch C 1  at a position that is away from the third planetary gear unit G 3  (double sun gear type) as compared with the second planetary gear unit G 2  (single pinion type). 
   Accordingly, the speed change mechanism of the present invention has the following advantages. 
   That is, as is seen from  FIGS. 1 and 11 , the input rotation from the second clutch C 2  is transmitted to the second sun gear S 2  of the second planetary gear unit G 2  through the clutch hub  17  (or  200 , see  FIG. 1 ) and the input rotation from the first clutch C 1  is transmitted to the second ring gear R 2  of the same planetary gear unit G 2  through a part ( 100 , see  FIG. 1 ) of the second ring gear R 2 . 
   Thus, as is seen from  FIGS. 10 and 11 , even when the clutch hub  17  (or  200 ) and the part  100  of the second ring gear R 2  are arranged at the same side of the second planetary gear unit G 2 , such members  17  and  100  have no portions concentrically overlapped, which suppresses enlargement of the speed change mechanism in a radial direction. 
   Both the first and second clutches C 1  and C 2  are located close to the second planetary gear unit G 2 . This arrangement brings about non-necessity of arranging clutches in an annular space defined around the third planetary gear unit G 3  (double sun gear type) that is positioned away from the first planetary gear unit G 1  (single pinion type). Accordingly, as is seen from  FIGS. 10 and 11 , the annular space can be used for neatly receiving a rear end (viz., rear end lid  7 ) of the transmission case  3 , which brings about a reduction in axial length of the speed change mechanism, that is, downsizing of the transmission case  3 . 
   For achieving the above-mentioned advantages, the speed change mechanism of the invention has the following arrangements. 
   The third carrier PC 3  of the third planetary gear unit G 3  is arranged to selectively transmit the input rotation without changing the speed and arranged fixable by the first brake B 1 . The fourth sun gear S 4  of the third planetary gear unit G 3  that is positioned away from the second planetary gear unit G 2  is arranged fixable by the second brake B 2 . The third sun gear S 3  of the third planetary gear unit G 3  that is positioned near the second planetary gear unit G 2  is connected to the second sun gear S 2  of the second planetary gear unit G 2  to constitute a first unit, so that a speed changed input rotation is transmitted to the first unit from the second clutch C 2 . A speed changed input rotation from the first clutch C 1  is transmittable to the second ring gear R 2  of the second planetary gear unit G 2 . The second carrier PC 2  of the second planetary gear unit G 2  and the third ring gear R 3  of the third planetary gear unit G 3  are connected to constitute a second unit, so that the output rotation of the transmission is outputted from the second unit through the second gear  2 . 
   In the speed change mechanism of the invention, the first clutch C 1  is arranged around the second planetary gear unit G 2  and the second ring gear R 2  of the second planetary gear unit G 2  is arranged to serve as the clutch hub of the first clutch C 1 . Accordingly, there is no need of providing a connecting member between the first clutch C 1  and the second ring gear R 2  of the second planetary gear unit G 2 , which brings about a compact and low cost production of the speed change mechanism. 
   The clutch hub  17  of the second clutch C 2  is arranged at the position that is near the second planetary gear unit G 2  and far from the third planetary gear unit G 3 , and the clutch hub  17  is connected to the second sun gear S 2  of the second planetary gear unit G 2 . Thus, the connecting member extending between the second clutch C 2  and the second sun gear S 2  of the second planetary gear unit G 2  can have a reduced length, which brings about an assured strength of the connecting member as well as a smaller and low cost production of the speed change mechanism. 
   Furthermore, in the present invention, the first and second clutches C 1  and C 2  are constructed to have the common clutch drum  15  to which the reduced speed rotation from the first planetary gear unit G 1  is transmitted. Accordingly, the number of parts is reduced and thus the cost of the speed change mechanism is reduced. 
   The entire contents of Japanese Patent Application 2002-207290 (filed Jul. 16, 2002) are incorporated herein by reference. 
   Although the invention has been described above with reference to the embodiment of the invention, the invention is not limited to such embodiment as described above. Various modifications and variations of such embodiment may be carried out by those skilled in the art, in light of the above description.