Abstract:
A gear train in an automatic transmission for vehicles may include: a first planetary gear set outputting reduced and same rotational speeds through two pathways by using torque received from an input shaft through one input pathway; a second planetary gear set outputting reversed and same outputs by using the reduced rotational speed from the first planetary gear set, and outputting an increased input when the reduced and same rotational speeds are simultaneous inputted from the first planetary gear set; and a third planetary gear set outputting six forward speeds and one reverse speed by using the outputs from the second planetary gear set and torque from the input shaft.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to and the benefit of Korean Patent Application No. 10-2008-0015511 filed in the Korean Intellectual Property Office on Feb. 20, 2008, the entire contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    (a) Field of the Invention 
         [0003]    The present invention relates to a gear train of an automatic transmission for vehicles that achieves six forward speeds by using three planetary gear sets, four clutches, two brakes, and one one-way clutch. 
         [0004]    (b) Description of the Related Art 
         [0005]    A typical shift mechanism of an automatic transmission utilizes a combination of a plurality of planetary gear sets. A power train of such an automatic transmission that includes the plurality of planetary gear sets changes rotational speed and torque received from a torque converter of the automatic transmission, and accordingly transmits the changed torque to an output shaft. 
         [0006]    It is well known that when a transmission realizes a greater number of shift speeds, speed ratios of the transmission can be more optimally designed, and therefore a vehicle can have economic fuel mileage and better performance. For that reason, an automatic transmission that is able to realize more shift speeds is under continuous investigation. 
         [0007]    In addition, with the same number of speeds, features of a power train, such as durability, efficiency in power transmission, and size, depend a lot on the layout of the combined planetary gear sets. Therefore, designs for a combining structure of a power train are also under continuous investigation. 
         [0008]    A manual transmission that has too many speeds causes inconvenience to a driver. Therefore, the advantageous features of having more shift-speeds are more important in an automatic transmission because an automatic transmission automatically controls the shifting operations. 
         [0009]    Currently, four-speed and five-speed automatic transmissions are most often found on the market. However, six-speed automatic transmissions have also been realized for enhancement of performance of power transmission and for enhanced fuel mileage of a vehicle. 
         [0010]    The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
       SUMMARY OF THE INVENTION 
       [0011]    Embodiments of the present invention provide a gear train of an automatic transmission for vehicles having advantages of improving power delivery performance and fuel mileage as a consequence of six forward speeds and one reverse speed being achieved by using three simple planetary gear sets, four clutches, two brakes, and one one-way clutch. 
         [0012]    A gear train of an automatic transmission for vehicles according to an exemplary embodiment of the present invention may include: a first planetary gear set being a simple planetary gear set provided with three rotational members, wherein a first rotational member is always operated as a fixed member, a second rotational member reduces a rotational speed of an input shaft to a reduced rotational speed and forms a first intermediate output pathway where the reduced rotational speed is outputted, and a third rotational member is directly connected to the input shaft and forms a second intermediate output pathway where a rotational speed that is the same as the rotational speed of the input shaft is outputted; a second planetary gear set being a simple planetary gear set provided with three rotational members, wherein a fourth rotational member is directly connected to the second rotational member, a fifth rotational member is selectively connected to the third rotational member and is operated as a variable fixed member, and a sixth rotational member forms a third intermediate output pathway; and a third planetary gear set being a simple planetary gear set provided with three rotational members, wherein a seventh rotational member is directly connected to the sixth rotational member, an eighth rotational member is operated as a final output member, and a ninth rotational member is selectively connected to the input shaft IS or the seventh rotational member. 
         [0013]    The first planetary gear set may be a double pinion planetary gear set, where the first rotational member is a first sun gear, the second rotational member is a first ring gear R 1 , and the third rotational member is a first planet carrier; the second planetary gear set may be a single pinion planetary gear set where the fourth rotational member is a second sun gear, the fifth rotational member is a second planet carrier, and the sixth rotational member is a second ring gear; and the third planetary gear set may be a single pinion planetary gear set where the seventh rotational member is a third ring gear, the eighth rotational member is a third planet carrier, and the ninth rotational member is a third sun gear. 
         [0014]    The first, second, and third planetary gear sets may be combined by using four clutches, two brakes, and one one-way clutch, wherein the first rotational member is directly connected to a transmission housing, the third rotational member is directly connected to the input shaft, the second rotational member is directly connected to the fourth rotational member, the sixth rotational member is directly connected to the seventh rotational member, and the eighth rotational member is directly connected to an output shaft, and wherein the input shaft is selectively connected to the ninth rotational member by interposing a first clutch, the fifth rotational member is selectively connected to the sixth rotational member by interposing a second clutch, the seventh rotational member is selectively connected to the ninth rotational member by interposing a third clutch, the third rotational member is selectively connected to the fifth rotational member by interposing a fourth clutch, the fifth rotational member is selectively connected to the transmission housing by interposing a first brake and a one-way clutch disposed in parallel with each other, and the sixth rotational member is selectively connected to the transmission housing by interposing a second brake. 
         [0015]    The first clutch may be disposed in a rear side of the third planetary gear set, the second and third clutches may be disposed between the second and third planetary gear sets, the fourth clutch may be disposed between the first and second planetary gear sets, and the one-way clutch and the first and second brakes may be disposed at an exterior side of the second planetary gear set, wherein the second clutch may be disposed close to the second planetary gear set and the third clutch may be disposed close to the third planetary gear set. 
         [0016]    The first clutch and the one-way clutch may be operated at a first forward speed, the first clutch and the second brake may be operated at a second forward speed, the first clutch and the second clutch may be operated at a third forward speed, the first clutch and the third clutch may be operated at a fourth forward speed, the first clutch and the fourth clutch may be operated at a fifth forward speed, the third clutch and the fourth clutch may be operated at a sixth forward speed, and the third clutch and the first brake may be operated at a reverse speed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a schematic diagram of a gear train according to an exemplary embodiment of the present invention. 
           [0018]      FIG. 2  is an operational chart for a gear train according to an exemplary embodiment of the present invention. 
           [0019]      FIG. 3  is a lever diagram showing shifting processes from a first forward speed to a sixth forward speed and one reverse speed in a gear train according to an exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0020]    Hereinafter, an exemplary embodiment of the present invention will be described in detail with reference to the accompanying drawings. 
         [0021]      FIG. 1  is a schematic diagram of a gear train according to an exemplary embodiment of the present invention. As shown in  FIG. 1 , a gear train according to an exemplary embodiment of the present invention includes first, second, and third planetary gear sets PG 1 , PG 2 , and PG 3  disposed concentrically, clutch means provided with four clutches C 1 , C 2 , C 3 , and C 4 , brake means provided with two brakes B 1  and B 2 , and one one-way clutch F. 
         [0022]    Accordingly, an input rotational speed transmitted from an input shaft IS is changed by passing through the first, second, and third planetary gear sets PG 1 , PG 2 , and PG 3 , and a changed rotational speed is outputted through an output shaft OS. The first, second, and third planetary gear sets PG 1 , PG 2 , and PG 3  are disposed in a sequence of the first planetary gear set PG 1 , the second planetary gear set PG 2 , and the third planetary gear set PG 3  from an engine. 
         [0023]    The input shaft IS is an input member and is a turbine shaft of a torque converter. Torque transmitted from a crankshaft of the engine is supplied to the input shaft IS through the torque converter. The output shaft OS is an output member, and torque of the output shaft OS is transmitted to a differential apparatus through an output gear (not shown) and drives a driving wheel. 
         [0024]    The first planetary gear set PG 1  is a double pinion planetary gear set, and includes a first rotational member N 1  of a first sun gear S 1 , a second rotational member N 2  of a first ring gear R 1 , and a third rotational member N 3  of a first planet carrier PC 1 . 
         [0025]    In addition, the first rotational member N 1  (the first sun gear S 1 ) is directly connected to a transmission housing H and is always operated as a fixed member, the second rotational member N 2  (the first ring gear R 1 ) forms a first intermediate output pathway MOP 1  where a reduced rotational speed is outputted, and the third rotational member N 3  (the first planet carrier PC 1 ) is directly connected to the input shaft IS so as to form an input pathway IP and form a second intermediate output pathway MOP 2  where a rotational speed that is the same as an input rotational speed is outputted. 
         [0026]    Accordingly, the input rotational speed transmitted from the input shaft IS through the input pathway IP is changed to reduced and same rotational speeds. The reduced rotational speed is outputted through the first intermediate output pathway MOP 1 , and the same rotational speed is outputted through the second intermediate output pathway MOP 2 . 
         [0027]    The second planetary gear set PG 2  is a single pinion planetary gear set, and includes a fourth rotational member N 4  of a second sun gear S 2 , a fifth rotational member N 5  of a second planet carrier PC 2 , and a sixth rotational member N 6  of a second ring gear R 2 . 
         [0028]    In addition, the fourth rotational member N 4  (the second sun gear S 2 ) forms a first intermediate input pathway MIP 1  directly connected to the first intermediate output pathway MOP 1 , the fifth rotational member N 5  (the second planet carrier PC 2 ) forms a second intermediate input pathway MIP 2  selectively connected to the second intermediate output pathway MOP 2  by fourth clutch C 4  and is selectively connected to the transmission housing H by a first brake B 1  so as to be operated as a selective fixed member, and the sixth rotational member N 6  (the second ring gear R 2 ) forms a third intermediate output pathway MOP 3  where the reduced and same rotational speeds transmitted respectively from the first and second intermediate input pathways MIP 1  and MIP 2  are selectively changed into reverse, same, and increased outputs and the reverse, same, and increased outputs are outputted, and is selectively connected to the transmission housing H by a second brake B 2  so as to be operated as a selective fixed member. 
         [0029]    The fifth rotational member N 5  (the second planet carrier PC 2 ) and the sixth rotational member N 6  (the second ring gear R 2 ) are selectively connected to each other such that the second planetary gear set PG 2  may selectively become a lock state. 
         [0030]    Accordingly, the reverse and the same outputs are outputted through the third intermediate output pathway MOP 3  by using the reduced rotational speed of the first intermediate output pathway MOP 1  received through the first intermediate input pathway MIP 1 , and the increased output is outputted through the third intermediate output pathway MOP 3  by using the reduced rotational speed of the first intermediate output pathway MOP 1  and the same rotational speed of the second intermediate output pathway MOP 2  received respectively through the first intermediate input pathway MIP 1  and the second intermediate input pathway MIP 2 . 
         [0031]    The third planetary gear set PG 3  is a single pinion planetary gear set, and includes a seventh rotational member N 7  of a third ring gear R 3 , an eighth rotational member N 8  of a third planet carrier PC 3 , and a ninth rotational member N 9  of a third sun gear S 3 . 
         [0032]    In addition, the seventh rotational member N 7  (the third ring gear R 3 ) forms a third intermediate input pathway MIP 3  so as to receive the outputs from the third intermediate output pathway MOP 3 , the eighth rotational member N 8  (the third planet carrier PC 3 ) forms a final output pathway OP so as to always be operated as a final output member, and the ninth rotational member N 9  (the third sun gear S 3 ) is selectively connected to the input shaft IS by a first clutch C 1  so as to form a variable input pathway AIP. 
         [0033]    The seventh rotational member N 7  (the third ring gear R 3 ) and the ninth rotational member N 9  (the third sun gear S 3 ) are selectively connected to each other by a third clutch C 3  such that the third planetary gear set PG 3  may selectively become a lock state. 
         [0034]    The input rotational speed of the input shaft IS received through the variable input pathway AIP, and the outputs received through the third intermediate input pathway MIP 3  is changed into six forward speeds and one reverse speed, and the six forward speeds and the one reverse speed are outputted through the final output pathway OP. 
         [0035]    The gear train according to an exemplary embodiment of the present invention is formed by combining the first planetary gear set PG 1  provided with the first, second, and third rotational members N 1 , N 2 , and N 3 , the second planetary gear set PG 2  provided with the fourth, fifth, and sixth rotational members N 4 , N 5 , and N 6 , and the third planetary gear set PG 3  provided with the seventh, eighth, and ninth rotational members N 7 , N 8 , and N 9 . 
         [0036]    The first rotational member N 1  is directly connected to the transmission housing H, the third rotational member N 3  is directly connected to the input shaft IS, the second rotational member N 2  and the fourth rotational member N 4  are directly connected to each other, the sixth rotational member N 6  and the seventh rotational member N 7  are directly connected to each other, and the eighth rotational member N 8  is directly connected to the output shaft OS. 
         [0037]    In addition, the input shaft IS is selectively connected to the ninth rotational member N 9  by interposing a first clutch C 1 , the fifth rotational member N 5  is selectively connected to the sixth rotational member N 6  by interposing a second clutch C 2 , the seventh rotational member N 7  is selectively connected to the ninth rotational member N 9  by interposing a third clutch C 3 , the third rotational member N 3  is selectively connected to the fifth rotational member N 5  by interposing a fourth clutch C 4 , the fifth rotational member N 5  is selectively connected to the transmission housing H by interposing a first brake B 1  and a one-way clutch F disposed in parallel with each other, and the sixth rotational member N 6  is selectively connected to the transmission housing H by interposing a second brake B 2 . 
         [0038]    The first, second, third, and fourth clutches C 1 , C 2 , C 3 , and C 4  and the first and second brakes B 1  and B 2  may include a plurality of disks that are engaged by frictional force applied by hydraulic pressure. 
         [0039]    The first clutch C 1  is disposed at a rear side of the third planetary gear set PG 3 , the second and third clutches C 2  and C 3  are disposed between the second and third planetary gear sets PG 2  and PG 3  wherein the second clutch C 2  is closer to the second planetary gear set PG 2  and the third clutch C 3  is closer to the third planetary gear set PG 3 , and the fourth clutch C 4  is disposed between the first and second planetary gear sets PG 1  and PG 2 . 
         [0040]    In addition, the first and second brakes B 1  and B 2  are disposed at an exterior side of the second planetary gear set PG 2 , and the one-way clutch F, disposed in parallel with the first brake B 1 , is disposed between the first and second brakes B 1  and B 2 . 
         [0041]    If the frictional members are dispersedly disposed, mass center may be stable, and it may be easy to form hydraulic lines through which hydraulic pressure is supplied to the frictional members. 
         [0042]      FIG. 2  is an operational chart for a gear train according to an exemplary embodiment of the present invention. As shown in  FIG. 2 , each speed is achieved by operations of two frictional members according to the present invention. 
         [0043]    That is, the first clutch C 1  and the one-way clutch F are operated at a first forward speed D 1 , the first clutch C 1  and the second brake B 2  are operated at a second forward speed D 2 , the first clutch C 1  and the second clutch C 2  are operated at a third forward speed D 3 , the first clutch C 1  and the third clutch C 3  are operated at a fourth forward speed D 4 , the first clutch C 1  and the fourth clutch C 4  are operated at a fifth forward speed D 5 , the third clutch C 3  and the fourth clutch C 4  are operated at a sixth forward speed D 6 , and the third clutch C 3  and the first brake B 1  are operated at a reverse speed REV. 
         [0044]      FIG. 3  is a lever diagram showing shifting processes from a first forward speed to a sixth forward speed and one reverse speed in a gear train according to an exemplary embodiment of the present invention. In  FIG. 3 , a lower horizontal line represents “0” rotational speed, and an upper horizontal line represents “1.0” rotational speed that is the same as the rotational speed of the input shaft IS. 
         [0045]    Three vertical lines of the first planetary gear set PG 1  respectively represent the first rotational member N 1  (the first sun gear S 1 ), the second rotational member N 2  (the first ring gear R 1 ), and the third rotational member N 3  (the first planet carrier PC 1 ) sequentially from the left in the drawing, and a distance between them is determined according to a gear ratio (teeth number of sun gear/teeth number of ring gear) of the first planetary gear set PG 1 . 
         [0046]    Three vertical lines of the second planetary gear set PG 2  respectively represent the fourth rotational member N 4  (the second sun gear S 2 ), the fifth rotational member N 5  (the second planet carrier PC 2 ), and the sixth rotational member N 6  (the second ring gear R 2 ) sequentially from the left in the drawing, and a distance between them is determined according to a gear ratio (teeth number of sun gear/teeth number of ring gear) of the second planetary gear set PG 2 . 
         [0047]    Three vertical lines of the third planetary gear set PG 3  respectively represent the seventh rotational member N 7  (the third ring gear R 3 ), the eighth rotational member N 8  (the third planet carrier PC 3 ) connected to output shaft, and the ninth rotational member N 9  (the third sun gear S 3 ) sequentially from the left in the drawing, and a distance between them is determined according to a gear ratio (teeth number of sun gear/teeth number of ring gear) of the third planetary gear set PG 3 . The lever diagram is well known to a person of ordinary skill in the art, and detailed descriptions will be accordingly omitted. 
         [0048]    Hereinafter, shifting processes in the gear train according to the exemplary embodiment of the present invention will be described in detail. 
       [First Forward Speed] 
       [0049]    As shown in  FIG. 2 , the first clutch C 1  and the one-way clutch F are operated at the first forward speed D 1 . 
         [0050]    In this case, as shown in  FIG. 3 , in a state that the input rotational speed is transmitted to the third rotational member N 3  of the first planetary gear set PG 1  through the input pathway IP, the first rotational member N 1  is operated as the fixed member. Thus, the reduced rotational speed is generated at the second rotational member N 2 , forming the first intermediate output pathway MOP 1  and is transmitted to the fourth rotational member N 4  of the second planetary gear set PG 2  through the first intermediate input pathway MIP 1 . 
         [0051]    In a state in which the reduced rotational speed is transmitted to the fourth rotational member N 4  of the second planetary gear set PG 2 , the fifth rotational member N 5  is operated as the fixed member by an operation of the one-way clutch F. The reverse output of the sixth rotational member N 6  is transmitted to the seventh rotational member N 7  of the third planetary gear set PG 3  through the third intermediate output pathway MOP 3  and the third intermediate input pathway MIP 3 . 
         [0052]    In a state in which the input rotational speed is transmitted to the ninth rotational member N 9  of the third planetary gear set PG 3  through the variable input pathway AIP by an operation of the first clutch C 1 , the reverse output is transmitted to the seventh rotational member N 7 . Therefore, a first shift line SP 1  connecting the seventh rotational member N 7  with the ninth rotational member N 9  is formed and the first forward speed D 1  is outputted to the eighth rotational member N 8  that is the output member. 
       [Second Forward Speed] 
       [0053]    In a state of the first forward speed D 1 , the one-way clutch F is not operated but the second brake B 2  is operated at the second forward speed D 2 . 
         [0054]    In this case, the reduced rotational speed of the second rotational member N 2  is transmitted to the fourth rotational member N 4  of the second planetary gear set PG 2  through the first intermediate output pathway MOP 1  and the first intermediate input pathway MIP 1 , and is the same as the first forward speed D 1 . In addition, the sixth rotational member N 6  is operated as the fixed member by an operation of the second brake B 2 . 
         [0055]    The seventh rotational member N 7  of the third planetary gear set PG 3  that is directly connected to the sixth rotational member N 6  through the third intermediate input pathway MIP 3  is operated as the fixed member, and the input rotational speed is transmitted to the ninth rotational member N 9  through the variable input pathway AIP by an operation of the first clutch C 1 . Therefore, a second shift line SP 2  connecting the seventh rotational member N 7  and the ninth rotational member N 9  is formed and the second forward speed D 2  is outputted to the eighth rotational member N 8  that is the output member. 
         [0056]    That is, the first and second planetary gear sets PG 1  and PG 2  are not related to a shift to the second forward speed D 2 , but torque transmitted to the ninth rotational member N 9  of the third planetary gear set PG 3  is reduced and the shift to the second forward speed D 2  is achieved. 
       [Third Forward Speed] 
       [0057]    As shown in  FIG. 2 , in a state of the second forward speed D 2 , the second brake B 2  is released and the second clutch C 2  is operated at the third forward speed D 3 . 
         [0058]    In this case, the reduced rotational speed of the second rotational member N 2  is transmitted to the fourth rotational member N 4  of the second planetary gear set PG 2  through the first intermediate output pathway MOP 1  and the first intermediate input pathway MIP 1 . At this time, the second planetary gear set PG 2  becomes the lock state by an operation of the second clutch C 2 , and the output that is the same as the reduced rotational speed is transmitted to the seventh rotational member N 7  of the third planetary gear set PG 3  through the third intermediate input pathway MIP 3 . 
         [0059]    In a state in which the input rotational speed of input shaft IS is transmitted to the ninth rotational member N 9  of the third planetary gear set PG 3  through the variable input pathway AIP, the reduced rotational speed is transmitted to the seventh rotational member N 7  through the third intermediate input pathway MIP 3 . Therefore, a third shift line SP 3  connecting the seventh rotational member N 7  with the ninth rotational member N 9  is formed and the third forward speed D 3  is outputted to the eighth rotational member NS that is the output member. 
       [Fourth Forward Speed] 
       [0060]    As shown in  FIG. 2 , in a state of the third forward speed D 3 , the second clutch C 2  is released and the third clutch C 3  is operated at the fourth forward speed D 4 . 
         [0061]    In this case, the input rotational speed is transmitted to the ninth rotational member N 9  through the variable input pathway AIP by an operation of the first clutch C 1 , and the third planetary gear set PG 3  becomes the lock state by an operation of the third clutch C 3 . Thus, a fourth shift line SP 4  is formed and the fourth forward speed D 4  is outputted to the eighth rotational member N 8  that is the output member. 
         [0062]    That is, because of the lock state of the third planetary gear set PG 3 , the input rotational speed is directly outputted and the shift to the fourth forward speed D 4  is achieved. 
       [Fifth Forward Speed] 
       [0063]    As shown in  FIG. 2 , in a state of the fourth forward speed D 4 , the third clutch C 3  is released and the fourth clutch C 4  is operated at the fifth forward speed D 5 . 
         [0064]    In this case, the reduced rotational speed of the second rotational member N 2  is transmitted to the fourth rotational member N 4  of the second planetary gear set PG 2  through the first intermediate output pathway MOP 1  and the first intermediate input pathway MIP 1 , and the same rotational speed of the third rotational member N 3  is transmitted to the fifth rotational member N 5  of the second planetary gear set PG 2  through the second intermediate output pathway MOP 2  and the second intermediate input pathway MIP 2  by an operation of the fourth clutch C 4 . 
         [0065]    At this time, the increased output is generated at the sixth rotational member N 6  by the reduced and same rotational speeds input respectively to the fourth rotational member N 4  and the fifth rotational member N 5  of the second planetary gear set PG 2 . The increased output is transmitted to the seventh rotational member N 7  of the third planetary gear set PG 3  through the third intermediate output pathway MOP 3  and the third intermediate input pathway MIP 3 . In addition, the input rotational speed of the input shaft IS is transmitted to the ninth rotational member N 9  of the third planetary gear set PG 3  through the variable input pathway AIP by an operation of the first clutch C 1 . Therefore, a fifth shift line SP 5  connecting the seventh rotational member N 7  with the ninth rotational member N 9  is formed and the fifth forward speed D 5  is outputted to the eighth rotational member N 8  that is the output member. 
         [0066]    That is, the reduced and same rotational speeds of the first planetary gear set PG 1  are transmitted to the second planetary gear set PG 2  through the first and second intermediate output pathways MOP 1  and MOP 2 , and the second planetary gear set PG 2  generates the increased output. After that, the fifth forward speed D 5  is achieved by the increased output and the input rotational speed transmitted to the third planetary gear set PG 3 . 
       [Sixth Forward Speed] 
       [0067]    As shown in  FIG. 2 , in a state of the fifth forward speed D 5 , the first clutch C 1  is released and the third clutch C 3  is operated at the sixth forward speed D 6 . 
         [0068]    In this case, the reduced rotational speed of the second rotational member N 2  is transmitted to the fourth rotational member N 4  of the second planetary gear set PG 2  through the first intermediate output pathway MOP 1  and the first intermediate input pathway MIP 1 , and the same rotational speed of the third rotational member N 3  is transmitted to the fifth rotational member N 5  of the second planetary gear set PG 2  through the second intermediate output pathway MOP 2  and the second intermediate input pathway MIP 2  by an operation of the fourth clutch C 4 . 
         [0069]    At this time, the increased output is generated at the sixth rotational member N 6  by the reduced and same rotational speeds input respectively to the fourth rotational member N 4  and the fifth rotational member N 5  of the second planetary gear set PG 2 . The increased output is transmitted to the seventh rotational member N 7  of the third planetary gear set PG 3  through the third intermediate output pathway MOP 3  and the third intermediate input pathway MIP 3 . 
         [0070]    In addition, the third planetary gear set PG 3  becomes the lock state by an operation of the third clutch C 3 . Therefore, a sixth shift line SP 6  is formed and the sixth forward speed D 6  is outputted to the eighth rotational member NS that is the output member. 
         [0071]    That is, the reduced and same rotational speeds of the first planetary gear set PG 1  are transmitted to the second planetary gear set PG 2  through the first and second intermediate output pathways MOP 1  and MOP 2 , and the second planetary gear set PG 2  generates the increased output. After that, the increased output is transmitted to the third planetary gear set PG 3 . Because of the lock state of the third planetary gear set PG 3 , the increased output is directly outputted and the shift to the sixth forward speed D 6  is achieved. 
       [Reverse Speed] 
       [0072]    As shown in  FIG. 2 , the third clutch C 3  and the first brake B 1  are operated at the reverse speed REV. 
         [0073]    In this case, as shown in  FIG. 3 , in a state in which the input rotational speed is transmitted to the third rotational member N 3  of the first planetary gear set PG 1  through the input pathway IP, the first rotational member N 1  is operated as the fixed member. Thus, the reduced rotational speed is generated at the second rotational member N 2  and is transmitted to the fourth rotational member N 4  of the second planetary gear set PG 2  through the first intermediate output pathway MOP 1  and the first intermediate input pathway MIP 1 . 
         [0074]    In a state in which the reduced rotational speed is transmitted to the fourth rotational member N 4  of the second planetary gear set PG 2 , the fifth rotational member N 5  is operated as the fixed member by an operation of the first brake B 1 . The reverse output is generated at the sixth rotational member N 6  and is transmitted to the seventh rotational member N 7  of the third planetary gear set PG 3  through the third output pathway MOP 3  and the third intermediate input pathway MIP 3 . 
         [0075]    In addition, a reverse shift line SR is formed as a consequence of the third planetary gear set PG 3  becoming the lock state by an operation of the third clutch C 3 , and the reverse speed REV is outputted to the eighth rotational member N 8  that is the output member. 
         [0076]    If teeth numbers of the first sun gear S 1 , the first ring gear R 1 , the second sun gear S 2 , the second ring gear R 2 , the third sun gear S 3 , and the third ring gear R 3  are 46, 91, 49, 86, 64, and 101, respectively, according to the gear train of an exemplary embodiment of the present invention, shift ratios of the first, second, third, fourth, fifth, and sixth forward speeds D 1 , D 2 , D 3 , D 4 , D 5 , and D 6  and the reverse speed REV are 4.642, 2.578, 1.448, 1.000, 0.850, 0.776, and 3.549 respectively. 
         [0077]    As described above, power delivery performance and fuel mileage may be improved as a consequence of achieving six forward speeds and one reverse speed by using three simple planetary gear sets, four clutches, two brakes, and one one-way clutch. 
         [0078]    In addition, hydraulic lines may be simplified by dispersedly disposing frictional members. 
         [0079]    In addition, capacity of a hydraulic pump may be decreased and control efficiency of hydraulic pressure may be improved as a consequence of a shift to a target speed being achieved by operating only two frictional members. 
         [0080]    While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.