Abstract:
A gear train of an automatic transmission for vehicles has the advantages of simplifying structures of the automatic transmission and improving power delivery performance and fuel economy as a consequence of realizing at least nine forward speeds and one reverse speed by combining three simple planetary gear sets with four clutches and three brakes.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to and the benefit of Korean Patent Application No. 10-2010-0113007 filed Nov. 12, 2010, the entire contents of which application is incorporated herein for all purposes by this reference. 
       BACKGROUND OF INVENTION 
       [0002]    1. Field of Invention 
         [0003]    The present invention relates to a gear train of an automatic transmission for vehicles which realizes at least nine forward speeds and one reverse speed by combining three simple planetary gear sets with four clutches and three brakes. 
         [0004]    2. Description of Related Art 
         [0005]    A typical shift mechanism of an automatic transmission utilizes a combination of a plurality of planetary gear sets. A gear 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]    In such an automatic transmission, a gear train is realized by combining a plurality of planetary gear sets, and the gear train including the plurality of planetary gear sets receives torque from a torque converter and changes and transmits the torque to an output shaft. 
         [0007]    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 economical fuel mileage and better performance. For that reason, an automatic transmission that is able to realize more shift speeds is under continuous investigation. 
         [0008]    In addition, with the same number of speeds, features of a gear train, such as durability, efficiency in power transmission, and size, substantially depend on the layout of the combined planetary gear sets. Therefore, designs for a combining structure of a gear train are also under continuous investigation. 
         [0009]    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 
         [0010]    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. In addition, seven-speed automatic transmissions and eight-speed automatic transmissions have been developed at a good pace. 
         [0011]    The information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art. 
       SUMMARY OF INVENTION 
       [0012]    Various aspects of the present invention provide for a gear train of an automatic transmission for vehicles having advantages of simplifying structures of the automatic transmission and improving power delivery performance and fuel economy as a consequence of realizing at least nine forward speeds and one reverse speed by combining three simple planetary gear sets with four clutches and three brakes. 
         [0013]    One aspect of the present invention is directed to a gear train of an automatic transmission for vehicles that may include a first rotation element having a first member of the first planetary gear set directly connected to an input shaft so as to be always operated as an input element, a second rotation element having a second member of the first planetary gear set forming a first intermediate output path through which a reduced rotation speed is output and operated as a selective fixed element, a third rotation element having a third member of the first planetary gear set forming a second intermediate output path through which an inverse rotation speed is output and operated as a selective fixed element, a fourth rotation element having a first member of the second planetary gear set connected selectively to the first and second intermediate output paths so as to form a first variable input path and operated as a selective fixed element, a fifth rotation element having a second member of the second planetary gear set and a first member of the third planetary gear set selectively connected to an input shaft so as to form a second variable input path and operated as a selective fixed element, a sixth rotation element having a second member of the third planetary gear set connected to an output gear so as to form a final output path, a seventh rotation element having a third member of the second planetary gear set and a third member of the third planetary gear set selectively connected to the input shaft so as to form a third variable input path, and friction members having a plurality of clutches selectively connecting each rotation element to the input shaft or another rotation element and a plurality of brakes selectively connecting each rotation element to a transmission housing. 
         [0014]    The friction member may include first, second, third, and fourth clutches and first, second, and third brakes, wherein the first clutch selectively connects the third rotation element to the fourth rotation element, the second clutch selectively connects the input shaft to the seventh rotation element, the third clutch selectively connects the second rotation element to the fourth rotation element, the fourth clutch selectively connects the input shaft to the fifth rotation element, the first brake selectively connects the second rotation element to the transmission housing, the second brake selectively connects the fifth rotation element to the transmission housing, and the third brake selectively connects the third rotation element to the transmission housing. 
         [0015]    The second brake may be provided with a one-way clutch disposed in parallel therewith. 
         [0016]    The first clutch and the first and second brakes may be operated at a first forward speed, the second clutch and the first and second brakes may be operated at a second forward speed, the first and second clutches and the first brake may be operated at a third forward speed, the first and second clutches and the third brake may be operated at a fourth forward speed, the second and third clutches and the third brake may be operated at a fifth forward speed, the second and fourth clutches and the third brake may be operated at a sixth forward speed, the third and fourth clutches and the third brake may be operated at a seventh forward speed, the first and fourth clutches and the third brake may be operated at an eighth forward speed, the first and fourth clutches and the first brake may be operated at a ninth forward speed, the third clutch and the second and third brakes may be operated at a reverse speed. 
         [0017]    The first member may be a first sun gear, the second member may be a first planet carrier, and the third member may be a first ring gear in the first planetary gear set. 
         [0018]    The first member may be a second ring gear, the second member may be a second planet carrier, and the third member may be a second sun gear in the second planetary gear set. 
         [0019]    The first member may be a third ring gear, the second member may be a third planet carrier, and the third member may be a third sun gear. 
         [0020]    The methods and apparatuses of the present invention have other features and advantages which will be apparent from or are set forth in more detail in the accompanying drawings, which are incorporated herein, and the following Detailed Description, which together serve to explain certain principles of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a schematic diagram of an exemplary gear train according to the present invention. 
           [0022]      FIG. 2  is an operational chart for an exemplary gear train according to the present invention. 
           [0023]      FIG. 3  is a lever diagram for an exemplary gear train according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    Reference will now be made in detail to various embodiments of the present invention(s), examples of which are illustrated in the accompanying drawings and described below. While the invention(s) will be described in conjunction with exemplary embodiments, it will be understood that present description is not intended to limit the invention(s) to those exemplary embodiments. On the contrary, the invention(s) is/are intended to cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents and other embodiments, which may be included within the spirit and scope of the invention as defined by the appended claims. 
         [0025]    Description of components that are not necessary for explaining the present invention will be omitted, and the same constituent elements are denoted by the same reference numerals in this specification. 
         [0026]    In the detailed description, ordinal numbers are used for distinguishing constituent elements having the same terms, and have no specific meanings. 
         [0027]      FIG. 1  is a schematic diagram of a gear train according to various embodiments of the present invention. A gear train according to various embodiments of the present invention includes first, second, and third planetary gear sets PG 1 , PG 2 , and PG 3  disposed on the same axis, clutch means consisting of four clutches C 1 , C 2 , C 3 , and C 4 , and brake means consisting of three brakes B 1 , B 2 , and B 3 . 
         [0028]    A rotation speed input from the input shaft IS is changed by the first, second, and third planetary gear sets PG 1 , PG 2 , and PG 3  and is output through the output gear OG. At this time, the first planetary gear set PG 1  is disposed at the rearmost, and the second and third planetary gear sets PG 2  and PG 3  are sequentially dispose to the front. 
         [0029]    The input shaft IS is an input member and denotes a turbine shaft of a torque converter. Torque transmitted from a crankshaft of the engine is converted by the torque converter and is input to the gear train through the input shaft IS. The output shaft OS is an output member and is connected to a well-known differential apparatus so as to transmit an output of the gear train to driving wheels. 
         [0030]    The first planetary gear set PG 1  is a single pinion planetary gear set, and includes three rotation elements consisting of a sun gear, a planet carrier, and a ring gear. For better comprehension and ease of description, the sun gear is indicated by the first sun gear S 1 , the planet carrier is indicated by the first planet carrier PC 1 , and the ring gear is indicated by the first ring gear R 1 . 
         [0031]    The first sun gear S 1  is directly connected to the input shaft IS so as to be operated as a first rotation element N 1  forming an input path IP, the first planet carrier PC 1  is selectively connected to a transmission housing H and is operated as a second rotation element N 2  forming a first intermediate output path MOP 1 , and the first ring gear R 1  is selectively connected to the transmission housing H and is operated as a third rotation element N 3  forming a second intermediate output path MOP 2 . 
         [0032]    Accordingly, in a state that a rotation speed of the input shaft IS is always input to the first rotation element N 1 , an inverse rotation speed is output through the second intermediate output path MOP 2  if the second rotation element N 2  is operated as a fixed element, and a reduced rotation speed is output through the first intermediate output path MOP 1  if the third rotation element N 3  is operated as a fixed element. 
         [0033]    That is, the first planetary gear set PG 1  selectively outputs the reduced rotation speed and the inverse rotation speed. 
         [0034]    The second planetary gear set PG 2  is a single pinion planetary gear set and includes three rotation elements consisting of a sun gear, a planet carrier, and a ring gear. For better comprehension and ease of description, the sun gear is indicated by a second sun gear S 2 , the planet carrier is indicated by a second planet carrier PC 2 , and the ring gear is indicated by a second ring gear R 2 . 
         [0035]    The third planetary gear set PG 3  is a single pinion planetary gear set and includes three rotation elements consisting of a sun gear, a planet carrier, and a ring gear. For better comprehension and ease of description, the sun gear is indicated by a third sun gear S 3 , the planet carrier is indicated by a third planet carrier PC 3 , and the ring gear is indicated by a third ring gear R 3 . 
         [0036]    The second sun gear S 2  is directly connected to the third sun gear S 3  and the second planet carrier PC 2  is directly connected to the third ring gear R 3  such that the second and third planetary gear sets PG 2  and PG 3  forms one compound planetary gear set and includes four rotation elements. 
         [0037]    Accordingly, the second ring gear R 2  is operated as a fourth rotation element N 4 , the second planet carrier PC 2  and the third ring gear R 3  are operated as a fifth rotation element N 5 , the third planet carrier PC 3  is operated as a sixth rotation element N 6 , and the second and third sun gears S 2  and S 3  are operated as a seventh rotation element N 7 . 
         [0038]    In addition, the second ring gear R 2  of the fourth rotation element N 4  is selectively connected to the first intermediate output path MOP 1  of the second rotation element N 2  and to the second intermediate output path MOP 2  of the third rotation element N 3  so as to form a first variable input path VIP 1  selectively receiving the reduced rotation speed and the inverse rotation speed. 
         [0039]    The second planet carrier PC 2  and the third ring gear R 3  of the fifth rotation element N 5  are selectively connected to the input shaft IS so as to form a second variable input path VIP 2  and are selectively connected to the transmission housing H so as to be operated as a selective fixed element. 
         [0040]    The third planet carrier PC 3  of the sixth rotation element N 6  is directly connected to the output gear OG which is an output member so as to form a final output path OP. 
         [0041]    The second and third sun gears S 2  and S 3  of the seventh rotation element N 7  are selectively connected to the input shaft IS so as to form a third variable input path VIP 3 . 
         [0042]    Friction members such as first, second, third, and fourth clutches C 1 , C 2 , C 3 , and C 4  and first, second, and third brakes B 1 , B 2 , and B 3  are used for connection between the rotation elements, connection between each rotation element and the input shaft IS, and connection between each rotation element and the transmission housing H. 
         [0043]    The first clutch C 1  is disposed between the third rotation element N 3  and the fourth rotation element N 4 , the second clutch C 2  is disposed between the input shaft IS and the seventh rotation element N 7 , the third clutch C 3  is disposed between the second rotation element N 2  and the fourth rotation element N 4 , the fourth clutch C 4  is disposed between the input shaft IS and the fifth rotation element N 5 , the first brake B 1  is disposed between the second rotation element N 2  and the transmission housing H, the second brake B 2  is disposed between the fifth rotation element N 5  and the transmission housing H, and the third brake B 3  is disposed between the third rotation element N 3  and the transmission housing H. 
         [0044]    In addition, the second brake B 2  and a one-way clutch F 1  are disposed in parallel according to various embodiments of the present invention. Since the one-way clutch F 1  is operated instead of the second brake B 2  at a normal first forward speed D 1 , shift shock may be prevented when upshift to a second forward speed D 2 . 
         [0045]    If the one-way clutch F 1  is omitted, the second brake B 2  must be operated at the first forward speed D 1 . 
         [0046]    The first and third brakes B 1  and B 3  are disposed at an external circumferential portion of the first planetary gear set PG 1 , the first and third clutches C 1  and C 3  are disposed between the first and second planetary gear sets PG 1  and PG 2 , the second brake B 2  including the one-way clutch F 1  is disposed at an external circumferential portion of the second planetary gear set PG 2  or between the second and third planetary gear sets PG 2  and PG 3 , and the second and fourth clutches C 2  and C 4  are disposed at a front portion of the third planetary gear set PG 3 . 
         [0047]    If the friction members are dispersed as described above, formation of hydraulic lines for supplying hydraulic pressure to such friction members may be simplified, and weight balance in the automatic transmission may be enhanced. 
         [0048]    Friction members consisting of the first, second, third, and fourth clutches C 1 , C 2 , C 3 , and C 4  and the first, second, and third brakes B 1 , B 2 , and B 3  are conventional multi-plate friction elements of wet type that are operated by hydraulic pressure. 
         [0049]      FIG. 2  is an operational chart for a gear according to various embodiments of the present invention. 
         [0050]    According to various embodiments of the present invention, three friction members are operated at each shift-speed. 
         [0051]    That is, the first clutch C 1  and the first and second brakes B 1  and B 2  are operated at the first forward speed D 1 , the second clutch C 2  and the first and second brakes B 1  and B 2  are operated at the second forward speed D 2 , the first and second clutches C 1  and C 2  and the first brake B 1  are operated at a third forward speed D 3 , the first and second clutches C 1  and C 2  and the third brake B 3  are operated at a fourth forward speed D 4 , the second and third clutches C 2  and C 3  and the third brake B 3  are operated at a fifth forward speed D 5 , the second and fourth clutches C 2  and C 4  and the third brake B 3  are operated at a sixth forward speed D 6 , the third and fourth clutches C 3  and C 4  and the third brake B 3  are operated at a seventh forward speed D 7 , the first and fourth clutches C 1  and C 4  and the third brake B 3  are operated at an eighth forward speed D 8 , the first and fourth clutches C 1  and C 4  and the first brake B 1  are operated at a ninth forward speed D 9 , and the third clutch C 3  and the second and third brakes B 2  and B 3  are operated at a reverse speed REV. 
         [0052]    At the first and second forward speeds D 1  and D 2 , the one-way clutch F 1  may be operated instead of the second brake B 2 . The second brake B 2  is not operated at a normal forward driving, and the second brake B 2  is operated at L and  2  ranges at which large driving torque is necessary. 
         [0053]      FIG. 3  is a lever diagram for a gear train according to various embodiments of the present invention. In the drawings, a lower horizontal line represents a rotational speed is “0”, and an upper horizontal line represents a rotational speed is “1.0”, that is, the rotational speed thereof is the same as that of the input shaft IS. 
         [0054]    Three vertical lines of the first planetary gear set PG 1  sequentially represent the first sun gear S 1  being the first rotation element N 1 , the first planet carrier PC 1  being the second rotation element N 2 , and the first ring gear R 3  being the third rotation element N 3  from the left to the right, and distances therebetween are set according to a gear ratio (teeth number of the sun gear/teeth number of the ring gear) of the first planetary gear set PG 1 . 
         [0055]    Four vertical lines of the second and third planetary gear sets PG 2  and PG 3  sequentially represent the second ring gear R 2  being the fourth rotation element N 4 , the second planet carrier PC 2  and the third ring gear R 3  being the fifth rotation element N 5 , the third planet carrier PC 3  being the sixth rotation element N 6 , the second and third sun gears S 2  and S 3  being the seventh rotation element N 7 , and distances therebetween are set according to gear ratios (teeth number of the sun gear/teeth number of the ring gear) of the second and third planetary gear sets PG 2  and PG 3 . 
         [0056]    Position of each rotation element in the lever diagram is well known to a person of an ordinary skill in the art who designs a gear train, and thus detailed description will be omitted. 
         [0057]    First Forward Speed 
         [0058]    As shown in  FIG. 2 , the first clutch C 1  and the first and second brakes B 1  and B 2  are operated at the first forward speed D 1 . 
         [0059]    Accordingly, in a state that the rotation speed of the input shaft IS is input to the first rotation element N 1  forming the input path IP, the second rotation element N 2  is operated as the fixed element by operation of the first brake B 1 . Therefore, the rotation elements of the first planetary gear set PG 1  form a first forward speed line T 1  and the inverse rotation speed is output through the third rotation element N 3  forming the second intermediate output path MOP 2 . 
         [0060]    The inverse rotation speed of the second intermediate output path MOP 2  is input to the fourth rotation element N 4  through the first intermediate input path VIP 1  by operation of the first clutch C 1 , and the fifth rotation element N 5  is operated as the fixed element by operation of the second brake B 2 . Therefore, the rotation elements of the second and third planetary gear sets PG 2  and PG 3  form a first shift line SP 1 , and the first shift line SP 1  crosses the vertical line of the sixth rotation element N 6  that is the output element so as to output the first forward speed D 1 . 
         [0061]    Second Forward Speed 
         [0062]    The first clutch C 1  which was operated at the first forward speed D 1  is released and the second clutch C 2  is operated at the second forward speed D 2 . 
         [0063]    The rotation speed of the input shaft IS is input to the seventh rotation element N 7  through the third variable input path VIP 3  by operation of the second clutch C 2 , and the fifth rotation element N 5  is operated as the fixed element by operation of the second brake B 2 . Therefore, the rotation elements of the second and third planetary gear sets PG 2  and PG 3  form a second shift line SP 2 , and the second shift line SP 2  crosses the vertical line of the sixth rotation element N 6  that is the output element so as to output the second forward speed D 2 . 
         [0064]    At this time, the rotation speed of the input shaft IS is input to the first planetary gear set PG 1  through the input path IP, but it does not affect on shifting because the first and third clutches C 1  and C 3  connected to the second and third planetary gear sets PG 2  and PG 3  are not operated. 
         [0065]    In addition, it is exemplified that the second brake B 2  is operated at the first and second forward speeds D 1  and D 2 , but the one-way clutch F 1  performs function of the second brake B 2  even though the second brake B 2  is not operated. If the one-way clutch F 1  is operated at the second forward speed D 2 , shift feel may be improved when upshift to the third forward speed D 3 . 
         [0066]    Third Forward Speed 
         [0067]    As shown in  FIG. 2 , the second brake B 2  which was operated at the second forward speed D 2  is released and the first clutch C 1  is operated at the third forward speed D 3 . 
         [0068]    Accordingly, in a state that the rotation speed of the input shaft IS is input to the first rotation element N 1  forming the input path IP, the second rotation element N 2  is operated as the fixed element by operation of the first brake B 1  such that the rotation elements of the first planetary gear set PG 1  form the first forward speed line T 1 . Therefore, the inverse rotation speed is output through the third rotation element N 3  forming the second intermediate output path MOP 2 . 
         [0069]    At this state, the rotation speed of the input shaft IS is input to the seventh rotation element N 7  through the third variable input path VIP 3  by operation of the second clutch C 2  and the inverse rotation speed is input to the fourth rotation element N 3  by operation of the first clutch C 1 . Therefore, the rotation elements of the second and third planetary gear sets PG 2  and PG 3  form a third shift line SP 3 , and the third shift line SP 3  crosses the vertical line of the sixth rotation element N 6  that is the output element so as to output the third forward speed D 3 . 
         [0070]    Fourth Forward Speed 
         [0071]    As shown in  FIG. 2 , the first brake B 1  which was operated at the third forward speed D 3  is released and the third brake B 3  is operated at the fourth forward speed D 4 . 
         [0072]    Accordingly, in a state that the rotation speed of the input shaft IS is input to the first rotation element N 1  forming the input path IP, the third rotation element N 3  is operated as the fixed element by operation of the third brake B 3 . Therefore, the rotation elements of the first planetary gear set PG 1  form a second forward speed line T 2  and the fourth rotation element N 4  is operated as the fixed element by operation of the first clutch C 1 . 
         [0073]    At this state, the rotation speed of the input shaft IS is input to the seventh rotation element N 7  through the third variable input path VIP 3  by operation of the second clutch C 2 . Therefore, the rotation elements of the second and third planetary gear sets PG 2  and PG 3  form a fourth shift line SP 4 , and the fourth shift line SP 4  crosses the vertical line of the sixth rotation element N 6  that is the output element so as to output the fourth forward speed D 4 . 
         [0074]    Fifth Forward Speed 
         [0075]    As shown in  FIG. 2 , the first clutch C 1  which was operated at the fourth forward speed D 4  is released and the third clutch C 3  is operated at the fifth forward speed D 5 . 
         [0076]    Accordingly, in a state that the rotation speed of the input shaft IS is input to the first rotation element N 1  forming the input path IP, the third rotation element N 3  is operated as the fixed element by operation of the third brake B 3 . Therefore, the rotation elements of the first planetary gear set PG 1  form the second forward speed line T 2  and the reduced rotation speed is output through the first intermediate output path MOP 1  of the second rotation element N 2 . 
         [0077]    At this state, the reduced rotation speed is input to the fourth rotation element N 4  through the first variable input path VIP 1  by operation of the third clutch C 3 , and the rotation speed of the input shaft IS is input to the seventh rotation element N 7  through the third variable input path VIP 3  by operation of the second clutch C 2 . Therefore, the rotation elements of the second and third planetary gear sets PG 2  and PG 3  form a fifth shift line SP 5 , and the fifth shift line SP 5  crosses the vertical line of the sixth rotation element N 6  that is the output element so as to output the fifth forward speed D 5 . 
         [0078]    Sixth Forward Speed 
         [0079]    As shown in  FIG. 2 , the third clutch C 3  which was operated at the fifth forward speed D 5  is released and the fourth clutch C 4  is operated at the sixth forward speed D 6 . 
         [0080]    Accordingly, the rotation speed of the input shaft IS is input to the fifth rotation element N 5  and the seventh rotation element N 7  through the third and fourth variable input paths VIP 3  and VIP 4  by operation of the second and fourth clutches C 2  and C 4  respectively, and the second and third planetary gear sets PG 2  and PG 3  become direct-coupling state. Therefore, the rotation elements of the second and third planetary gear sets PG 2  and PG 3  form a sixth shift line SP 6 , and the sixth shift line SP 6  crosses the vertical line of the sixth rotation element N 6  that is the output element so as to output the sixth forward speed D 6 . 
         [0081]    At this time, the rotation speed of the input shaft IS is input to the first planetary gear set PG 1  through the input path IP and the reduced rotation speed is output through the first intermediate output path MOP 1  by operation of the third brake B 3 . However, it does not affect on shifting because the third clutch C 3  is not operated. 
         [0082]    Seventh Forward Speed 
         [0083]    As shown in  FIG. 2 , the second clutch C 2  which was operated at the sixth forward speed D 6  is released and the third clutch C 3  is operated at the seventh forward speed D 7 . 
         [0084]    Accordingly, in a state that the rotation speed of the input shaft IS is input to the first rotation element N 1  forming the input path IP, the third rotation element N 3  is operated as the fixed element by operation of the third brake B 3 . Therefore, the rotation elements of the first planetary gear set PG 1  form the second forward speed line T 2  and the reduced rotation speed is output through the first intermediate output path MOP 1  of the second rotation element N 2 . 
         [0085]    At this state, the reduced rotation speed is input to the fourth rotation element N 4  through the first variable input path VIP 1  by operation of the third clutch C 3 , and the rotation speed of the input shaft IS is input to the fifth rotation element N 5  through the second variable input path VIP 2  by operation of the fourth clutch C 4 . Therefore, the rotation elements of the second and third planetary gear sets PG 2  and PG 3  form a seventh shift line SP 7 , and the seventh shift line SP 7  crosses the vertical line of the sixth rotation element N 6  that is the output element so as to output the seventh forward speed D 7 . 
         [0086]    Eighth Forward Speed 
         [0087]    As shown in  FIG. 2 , the third clutch C 3  which was operated at the seventh forward speed D 7  is released and the first clutch C 1  is operated at the eighth forward speed D 8 . 
         [0088]    Accordingly, in a state that the rotation speed of the input shaft IS is input to the first rotation element N 1  forming the input path IP, the third rotation element N 3  is operated as the fixed element by operation of the third brake B 3 . Therefore, the rotation elements of the first planetary gear set PG 1  form the second forward speed line T 2  and the fourth rotation element N 4  is operated as the fixed element by operation of the first clutch C 1 . 
         [0089]    At this state, the rotation speed of the input shaft IS is input to the fifth rotation element N 5  through the second variable input path VIP 2  by operation of the fourth clutch C 4 . Therefore, the rotation elements of the second and third planetary gear sets PG 2  and PG 3  form an eighth shift line SP 8 , and the eighth shift line SP 8  crosses the vertical line of the sixth rotation element N 6  that is the output element so as to output the eighth forward speed D 8 . 
         [0090]    Ninth Forward Speed 
         [0091]    As shown in  FIG. 2 , the third brake B 3  which was operated at the eighth forward speed D 8  is released and the first brake B 1  is operated at the ninth forward speed D 9 . 
         [0092]    Accordingly, in a state that the rotation speed of the input shaft IS is input to the first rotation element N 1  forming the input path IP, the second rotation element N 2  is operated as the fixed element by operation of the first brake B 1 . Therefore, the rotation elements of the first planetary gear set PG 1  form the first forward speed line T 1  and the inverse rotation speed is output through the third rotation element N 3  forming the second intermediate output path MOP 2 . 
         [0093]    The inverse rotation speed of the second intermediate output path MOP 2  is input to the fourth rotation element N 4  through the first intermediate input path VIP 1  by operation of the first clutch C 1 , and the rotation speed of the input shaft IS is input to the fifth rotation element N 5  by operation of the fourth clutch C 4 . Therefore, the rotation elements of the second and third planetary gear sets PG 2  and PG 3  form a ninth shift line SP 9 , and the ninth shift line SP 9  crosses the vertical line of the sixth rotation element N 6  that is the output element so as to output the ninth forward speed D 9 . 
         [0094]    Reverse Speed 
         [0095]    As shown in  FIG. 2 , the third clutch C 3  and the second and third brakes B 2  and B 3  are operated at the reverse speed REV. 
         [0096]    Accordingly, in a state that the rotation speed of the input shaft IS is input to the first rotation element N 1  forming the input path IP, the third rotation element N 3  is operated as the fixed element by operation of the third brake B 3 . Therefore, the rotation elements of the first planetary gear set PG 1  form the second forward speed line T 2  and the reduced rotation speed is output through the first intermediate output path MOP 1  of the second rotation element N 2 . 
         [0097]    In a state that the reduced rotation speed is input to the fourth rotation element N 4  through the first variable input path VIP 1  by operation of the third clutch C 3 , the fifth rotation element N 5  is operated as the fixed element by operation of the second brake B 2 . Therefore, the rotation elements of the second and third planetary gear sets PG 2  and PG 3  form a reverse shift line RS, and the reverse shift line RS crosses the vertical line of the sixth rotation element N 6  that is the output element so as to output the reverse speed REV. 
         [0098]    As described above, nine forward speeds and one reverse speed are achieved by combining three simple planetary gear sets with four clutches and three brakes and operating three frictional elements at each shift-speed. Therefore, power delivery performance and fuel economy may be improved. 
         [0099]    Since the friction members including a plurality of clutches and brakes are dispersedly disposed, formation of hydraulic lines for supplying hydraulic pressure thereto may be simplified and weight balance in an automatic transmission may be enhanced. 
         [0100]    For convenience in explanation and accurate definition in the appended claims, the terms upper or lower, front or rear, inside or outside, and etc. are used to describe features of the exemplary embodiments with reference to the positions of such features as displayed in the figures. 
         [0101]    The foregoing descriptions of specific exemplary embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teachings. The exemplary embodiments were chosen and described in order to explain certain principles of the invention and their practical application, to thereby enable others skilled in the art to make and utilize various exemplary embodiments of the present invention, as well as various alternatives and modifications thereof. It is intended that the scope of the invention be defined by the Claims appended hereto and their equivalents.