Abstract:
A planetary gear train including: a first planetary gear set on a first shaft including a first sun gear operated as an output or fixed element, a first planet carrier operable as an output element, and a first ring gear operated as an input element; a second planetary gear set on a second shaft including a second sun gear connected to the first planet carrier, a second planet carrier, and a second ring gear engaging the first planet carrier and the first ring gear; a third planetary gear set on the second shaft including a third sun gear connected to the second ring gear, a third planet carrier engaging the second planet carrier and connected to an output gear, and a third ring gear engaging the first sun gear; three transfer gears; and frictional elements interconnecting the first, second, and third planetary gear sets and/or connecting a transmission housing.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority of Korean Patent Application Number 10-2012-0124112 filed Nov. 5, 2012, the entire contents of which application is incorporated herein for all purposes by this reference. 
     BACKGROUND OF INVENTION 
     1. Field of Invention 
     The present invention relates to an automatic transmission for a vehicle. More particularly, the present invention relates to a planetary gear train of an automatic transmission for a vehicle that can improve mountability by reducing a length thereof and reduce fuel consumption by improving power delivery performance. 
     2. Description of Related Art 
     Recently, vehicle makers direct all their strength to improve fuel economy due to worldwide high oil prices and strengthen of exhaust gas regulations. 
     Improvement of fuel economy may be achieved by multi-shift mechanism realizing greater number of shift speeds in an automatic transmission. Typically, a planetary gear train is realized by combining a plurality of planetary gear sets and friction elements. 
     It is well known that when a planetary gear train realizes a greater number of shift speeds, speed ratios of the planetary gear train can be more optimally designed, and therefore a vehicle can have economical fuel mileage and better performance. For that reason, the planetary gear train that is able to realize more shift speeds is under continuous investigation. 
     a different operating mechanism according to a connection between rotation elements (i.e., sun gear, planet carrier, and ring gear). In addition, the planetary gear train has different features such a durability, power delivery efficiency, and size depend on the layout thereof. Therefore, designs for a combining structure of a gear train are also under continuous investigation. 
     If the number of shift-speeds, however, increases, the number of components in the automatic transmission also increases. Therefore, mountability, cost, weight and power delivery efficiency may be deteriorated. 
     Particularly, since the planetary gear train having a number of components is hard to be mounted in a front wheel drive vehicle, researches for minimizing the number of components have been developed. 
     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. 
     BRIEF SUMMARY 
     Various aspects of the present invention provide for a planetary gear train of an automatic transmission for a vehicle having advantages of improving mountability by shortening a length thereof and reducing fuel consumption by improving power delivery performance as a consequence of achieving eight forward speeds and one reverse speed having excellent operating condition of frictional elements and step ratios by combining three planetary gear sets separately disposed on a first shaft and a second shaft, three externally-meshing gears, and five frictional elements. 
     Various aspects of the present invention provide for a planetary gear train of an automatic transmission including: a first shaft receiving torque of an engine; a second shaft disposed in parallel with the first shaft; a first planetary gear set disposed on the first shaft, and including a first sun gear operated as an output element or a fixed element, a first planet carrier selectively operated as an output element, and a first ring gear directly connected to the first shaft and always operated as an input element as rotation elements thereof; a second planetary gear set disposed on the second shaft, and including a second sun gear connected to the first planet carrier through an externally-meshed gear, a second planet carrier, and a second ring gear selectively connected to the first planet carrier and the first ring gear through externally-meshed gears as rotation elements thereof; a third planetary gear set disposed on the second shaft, and including a third sun gear connected to the second ring gear, a third planet carrier selectively connected to the second planet carrier and directly connected to an output gear so as to be operated as an output element, and a third ring gear selectively connected to the first sun gear through an externally-meshed gear as rotation elements thereof; three transfer gears forming the externally-meshed gears; and frictional elements selectively interconnecting the rotation elements of the first, second, and third planetary gear sets or selectively connecting the rotation element to a transmission housing. 
     The first planetary gear set may be a double pinion planetary gear set, and the second and third planetary gear sets may be single pinion planetary gear sets. 
     The three transfer gears may include: a first transfer gear connecting the first ring gear to the second ring gear; a second transfer gear connecting the first planet carrier directly to the second sun gear and selectively to the second ring gear; and a third transfer gear selectively connecting the first sun gear to the third ring gear. 
     The frictional elements may include: a first brake disposed between the first sun gear and the transmission housing; a first clutch disposed between the first sun gear and the third transfer gear; a second clutch disposed between the second transfer gear and the second ring gear; a third clutch disposed between the second planet carrier and the third planet carrier; and a fourth clutch disposed between the first transfer gear and the second ring gear. 
     The first brake and the first and fourth clutches may be operated at a first forward speed, the first brake and the first and second clutches may be operated at a second forward speed, the first, second, and fourth clutches may be operated at a third forward speed, the first, second, and third clutches may be operated at a fourth forward speed, the first, third, and fourth clutches may be operated at a fifth forward speed, the second, third, and fourth clutches may be operated at a sixth forward speed, the first brake and the third and fourth clutches may be operated at a seventh forward speed, the first brake and the second and third clutches may be operated at an eighth forward speed, and the first brake and the first and third clutches may be operated at a reverse speed. 
     Various aspects of the present invention provide for a planetary gear train of an automatic transmission for a vehicle including: a first shaft receiving torque of an engine; a second shaft disposed in parallel with the first shaft; a first planetary gear set disposed on the first shaft, and including a first sun gear operated as an output element or a fixed element, a first planet carrier selectively operated as an output element, and a first ring gear directly connected to the first shaft and always operated as an input element; a second planetary gear set disposed on the second shaft, and including a second sun gear connected to the first planet carrier, a second planet carrier, and a second ring gear selectively connected to the first planet carrier and the first ring gear; a third planetary gear set disposed on the second shaft, and including a third sun gear connected to the second ring gear, a third planet carrier selectively connected to the second planet carrier and directly connected to an output gear so as to be operated as an output element, and a third ring gear selectively connected to the first sun gear; a first transfer gear connecting the first ring gear to the second ring gear; a second transfer gear connecting the first planet carrier directly to the second sun gear and selectively to the second ring gear; a third transfer gear selectively connecting the first sun gear to the third ring gear; a first brake disposed between the first sun gear and the transmission housing; a first clutch disposed between the first sun gear and the third transfer gear; a second clutch disposed between the second transfer gear and the second ring gear; a third clutch disposed between the second planet carrier and the third planet carrier; and a fourth clutch disposed between the first transfer gear and the second ring gear. 
     The first planetary gear set may be a double pinion planetary gear set, and the second and third planetary gear sets may be single pinion planetary gear sets. 
     The first brake and the first and fourth clutches may be operated at a first forward speed, the first brake and the first and second clutches may be operated at a second forward speed, the first, second, and fourth clutches may be operated at a third forward speed, the first, second, and third clutches may be operated at a fourth forward speed, the first, third, and fourth clutches may be operated at a fifth forward speed, the second, third, and fourth clutches may be operated at a sixth forward speed, the first brake and the third and fourth clutches may be operated at a seventh forward speed, the first brake and the second and third clutches may be operated at an eighth forward speed, and the first brake and the first and third clutches may be operated at a reverse speed. 
     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 
         FIG. 1  is a schematic diagram of an exemplary planetary gear train according to the present invention. 
         FIG. 2  is an operational chart of friction members at each shift-speed applied to an exemplary planetary gear train according to the present invention. 
         FIG. 3A  is a lever diagram of an exemplary planetary gear train at the first forward speed according to the present invention. 
         FIG. 3B  is a lever diagram of an exemplary planetary gear train at the second forward speed according to the present invention. 
         FIG. 3C  is a lever diagram of an exemplary planetary gear train at the third forward speed according to the present invention. 
         FIG. 3D  is a lever diagram of an exemplary planetary gear train at the fourth forward speed according to the present invention. 
         FIG. 3E  is a lever diagram of an exemplary planetary gear train at the fifth forward speed according to the present invention. 
         FIG. 3F  is a lever diagram of an exemplary planetary gear train at the sixth forward speed according to the present invention. 
         FIG. 3G  is a lever diagram of an exemplary planetary gear train at the seventh forward speed according to the present invention. 
         FIG. 3H  is a lever diagram of an exemplary planetary gear train at the eighth forward speed according to the present invention. 
         FIG. 3I  is a lever diagram of an exemplary planetary gear train at a reverse speed according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     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. 
     Description of components that are not necessary for explaining the various embodiments will be omitted, and the same constituent elements are denoted by the same reference numerals in this specification. 
     In the detailed description, ordinal numbers are used for distinguishing constituent elements having the same terms, and have no specific meanings. 
       FIG. 1  is a schematic diagram of a planetary gear train according to various embodiments of the present invention. 
     Referring to  FIG. 1 , a planetary 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 , five frictional elements B 1 , C 1 , C 2 , C 3 , and C 4 , and three transfer gears TF 1 , TF 2 , and TF 3 . 
     The first planetary gear set PG 1  is disposed on a first shaft IS 1 , and the second and third planetary gear sets PG 2  and PG 3  are disposed on a second shaft IS 2  disposed apart from and in parallel with the first shaft IS 1 . 
     Therefore, torque input from the first shaft IS 1  is transmitted to the second and third planetary gear sets PG 2  and PG 3  through the first planetary gear set PG 1 , is converted into eight forward speeds and one reverse speed by operations of the first, second, and third planetary gear sets PG 1 , PG 2 , and PG 3 , and is then output through an output gear OG. 
     The first planetary gear set PG 1  is a double pinion planetary gear set, and having a first sun gear S 1 , a first ring gear R 1 , and a first planet carrier PC 1  rotatably supporting a first pinion P 1  engaged with the first sun gear S 1  and the first ring gear R 1  as rotation elements thereof. 
     The second planetary gear set PG 2  is a single pinion planetary gear set, and having a second sun gear S 2 , a second ring gear R 2 , and a second planet carrier PC 2  rotatably supporting a second pinion P 2  engaged with the second sun gear S 2  and the second ring gear R 2  as rotation elements thereof. 
     The third planetary gear set PG 3  is a single pinion planetary gear set, and having a third sun gear S 3 , a third ring gear R 3 , and a third planet carrier PC 3  rotatably supporting a third pinion P 3  engaged with the third sun gear S 3  and the third ring gear R 3  as rotation elements thereof. 
     The first ring gear R 1  is directly connected to the first shaft IS 1  and is always operated as an input element. 
     In addition, the second ring gear R 2  is connected to the third sun gear S 3  through the second shaft IS 2 , the second planet carrier PC 2  is selectively connected to the third planet carrier PC 3 , and the third planet carrier PC 3  is directly connected to an output gear OG and is always operated as an output element. 
     It is illustrated, but is not limited, that the second ring gear R 2  is connected to the third sun gear S 3  through the second shaft IS 2 . That is, the second ring gear R 2  may be connected to the third sun gear S 3  through a rotating member disposed on an external circumferential portion of the second shaft IS 2  without rotational interference between the rotating member and the second shaft IS 2 . 
     In addition, the first ring gear R 1  as well as the first shaft IS 1  is selectively connected to the second ring gear R 2  through an externally-meshed gear, the first sun gear S 1  is selectively connected to the third ring gear R 3  through an externally-meshed gear and is selectively connected to a transmission housing H, and the first planet carrier PC 1  is connected to the second sun gear S 2  and is selectively connected to the second ring gear R 2  through an externally-meshed gear. 
     The first, second, and third transfer gears TF 1 , TF 2 , and TF 3 , being the externally-meshed gears, may be helical gears and respectively have first, second, and third transfer drive gears TF 1   a , TF 2   a , and TF 3   a  and first, second, and third transfer driven gears TF 1   b , TF 2   b , and TF 3   b  externally meshed with each other. 
     The first transfer gear TF 1  includes the first transfer drive gear TF 1   a  directly connected to the first ring gear R 1  and the first transfer driven gear TF 1   b  selectively connected to the second ring gear R 2 . 
     Therefore, the first transfer gear TF 1  selectively connects the first ring gear R 1  to the second ring gear R 2 . 
     The second transfer gear TF 2  includes the second transfer drive gear TF 2   a  directly connected to the first planet carrier PC 1  and the second transfer driven gear TF 2   b  directly connected to the second sun gear S 2  and selectively connected to the second ring gear R 2 . 
     Therefore, the second transfer gear TF 2  connects the first planet carrier PC 1  directly to the second sun gear S 2  and selectively to the second ring gear R 2 . 
     The third transfer gear TF 3  includes the third transfer drive gear TF 3   a  selectively connected to the first sun gear S 1  and the third transfer driven gear TF 3   b  directly connected to the third ring gear R 3 . 
     Therefore, the third transfer gear TF 3  selectively connects the first sun gear S 1  to the third ring gear R 3 . 
     The rotation elements connected to each other by the first, second, and third transfer gears TF 1 , TF 2 , and TF 3  are rotated in opposite direction to each other. Gear ratios of the first, second, and third transfer gears TF 1 , TF 2 , and TF 3  are set according to speed ratios demanded at shift-speeds. 
     Arrangements of the frictional elements B 1 , C 1 , C 2 , C 3 , and C 4  will be described in detail. 
     The first brake B 1  is disposed between the first sun gear S 1  and the transmission housing H. 
     The first clutch C 1  is disposed between the first sun gear S 1  and the third transfer gear TF 3 . 
     The second clutch C 2  is disposed between the second transfer gear TF 2  and the second ring gear R 2 . 
     The third clutch C 3  is disposed between the second planet carrier PC 2  and the third planet carrier PC 3 . 
     The fourth clutch C 4  is disposed between the first transfer gear TF 1  and the second ring gear R 2 . 
     The frictional elements consisting of the first, second, third, and fourth clutches C 1 , C 2 , C 3 , and C 4  and the first brake B 1  are conventional multi-plate friction elements of wet type that are operated by hydraulic pressure. 
       FIG. 2  is an operational chart of friction members at each shift-speed applied to a planetary gear train according to various embodiments of the present invention. 
     As shown in  FIG. 2 , three frictional elements are operated at each shift-speed in the planetary gear train according to various embodiments of the present invention. 
     The first brake B 1  and the first and fourth clutches C 1  and C 4  are operated at a first forward speed 1 ST . 
     The first brake B 1  and the first and second clutches C 1  and C 2  are operated at a second forward speed 2 ND . 
     The first, second, and fourth clutches C 1 , C 2 , and C 4  are operated at a third forward speed 3 RD . 
     The first, second, and third clutches C 1 , C 2 , and C 3  are operated at a fourth forward speed 4 TH . 
     The first, third, and fourth clutches C 1 , C 3 , and C 4  are operated at a fifth forward speed 5 TH . 
     The second, third, and fourth clutches C 2 , C 3 , and C 4  are operated at a sixth forward speed 6 TH . 
     The first brake B 1  and the third and fourth clutches C 3  and C 4  are operated at a seventh forward speed 7 TH . 
     The first brake B 1  and the second and third clutches C 2  and C 3  are operated at an eighth forward speed 8 TH . 
     The first brake B 1  and the first and third clutches C 1  and C 3  are operated at a reverse speed Rev. 
       FIG. 3A  to  FIG. 3I  are lever diagrams of the planetary gear train at each shift-speed according to various embodiments of the present invention, and illustrate shift processes of the planetary gear train according to the various embodiments of the present invention by lever analysis method. 
     Referring to  FIG. 3A  to  FIG. 3I , three vertical lines of the first planetary gear set PG 1  are set as the first sun gear S 1  being a first rotation element N 1 , the first ring gear R 1  being a second rotation element N 2 , and the first planet carrier PC 1  being a third rotation element N 3  from the left to the right. 
     In addition, the second and third planetary gear sets PG 2  and PG 3  are operated as a selective compound planetary gear set according to operation of the third clutch C 3 . Four vertical lines of the second and third planetary gear sets PG 2  and PG 3  are set as the second sun gear S 2  being a fourth rotation element N 4 , the third ring gear R 3  being a fifth rotation element N 5 , the third planet carrier PC 3  or the second planet carrier PC 2  and the third planet carrier PC 3  being a sixth rotation element N 6 , and the second ring gear R 2  and the third sun gear S 3  being a seventh rotation element N 7  from the left to the right. 
     Since the third clutch C 3  is not operated from the first forward speed to the third forward speed, the third planet carrier PC 3  is set as the sixth rotation element N 6 . Since the third clutch C 3  is operated from the fourth forward speed to the eighth forward speed and at the reverse speed, however, the second and third planet carriers PC 2  and PC 3  are set as the sixth rotation element N 6 . 
     In addition, a middle horizontal line represents a rotation speed of “0”, an upper horizontal line represents a rotation speed of “1.0”, and a lower horizontal line represents a rotation speed of “−1.0”. 
     “−” means that rotational elements is rotated in an opposite direction of a rotational direction of the engine. It is because the rotation elements are externally meshed with each other through the first, second, and third transfer gears TF 1 , TF 2 , and TF 3  without an idling gear. 
     In addition, the rotation speed of “1.0” represents the same rotational speed as the first shaft IS  1  which is an input shaft. Distances between the vertical lines of the first, second, and third planetary gear sets PG 1 , PG 2 , and PG 3  are set according to each gear ratio (teeth number of a sun gear/teeth number of a ring gear). 
     Hereinafter, referring to  FIG. 2  and  FIG. 3A  to  FIG. 3I , the shift processes of the planetary gear train according to various embodiments of the present invention will be described in detail. 
     First Forward Speed 
     Referring to  FIG. 2 , the first brake B 1  and the first and fourth clutches C 1  and C 4  are operated at the first forward speed 1 ST . 
     As shown in  FIG. 3A , a rotation speed of the first shaft IS 1  is input to the second rotation element N 2 , and the first rotation element N 1  and the fifth rotation element N 5  are operated as fixed elements by operation of the first brake B 1  and the first clutch C 1 . 
     Therefore, a rotation speed of the third rotation element N 3  is decreased according to the gear ratio of the second transfer gear TF 2  and is then input to the fourth rotation element N 4  as an inverse rotation speed, and the rotation speed of the first shaft IS 1  is converted according to the gear ratio of the first transfer gear TF 1  by operation of the fourth clutch C 4  and is then input to the seventh rotation element N 7  as an inverse rotation speed. 
     Therefore, the rotation elements of the third planetary gear set PG 3  form a first shift line SP 1  and D 1  is output through the sixth rotation element N 6  that is the output element. 
     At this time, the rotation elements of the second planetary gear set PG 2  form a thick dotted line T, but it does not have any effect on shifting. 
     Second Forward Speed 
     The fourth clutch C 4  that was operated at the first forward speed 1 ST  is released and the second clutch C 2  is operated at the second forward speed 2 ND . 
     As shown in  FIG. 3B , the rotation speed of the first shaft IS 1  is input to the second rotation element N 2 , and the first rotation element N 1  and the fifth rotation element N 5  are operated as fixed elements by operation of the first brake B 1  and the first clutch C 1 . 
     Therefore, the rotation speed of the third rotation element N 3  is decreased according to the gear ratio of the second transfer gear TF 2  and is then input to the fourth rotation element N 4  as an inverse rotation speed, and the second planetary gear set PG 2  becomes a direct-coupling state by operation of the second clutch C 2 . 
     Therefore, the rotation elements of the third planetary gear set PG 3  form a second shift line SP 2  and D 2  is output through the sixth rotation element N 6  that is the output element. 
     At this time, the rotation elements of the second planetary gear set PG 2  form a thick dotted line T, but it does not have any effect on shifting. 
     Third Forward Speed 
     The first brake B 1  that was operated at the second forward speed 2 ND  is released and the fourth clutch C 4  is operated at the third forward speed 3 RD . 
     As shown in  FIG. 3C , the rotation speed of the first shaft IS 1  is input to the second rotation element N 2 , the third rotation element N 3  is connected to the fourth rotation element N 4  through the second transfer gear TF 2 , the first rotation element N 1  is connected to the fifth rotation element N 5  through the third transfer gear TF 3  by operation of the first clutch C 1 , and the second rotation element N 2  is connected to the seventh rotation element N 7  through the first transfer gear TF 1  by operation of the fourth clutch C 4 . 
     Therefore, the rotation elements of the third planetary gear set PG 3  form a third shift line SP 3  and D 3  is output through the sixth rotation element N 6  that is the output element. 
     At this time, the rotation elements of the second planetary gear set PG 2  form a thick dotted line T, but it does not have any effect on shifting. 
     Fourth Forward Speed 
     The fourth clutch C 4  that was operated at the third forward speed 3 RD  is released and the third clutch C 3  is operated at the fourth forward speed 4 TH . 
     As shown in  FIG. 3D , the rotation speed of the first shaft IS 1  is input to the second rotation element N 2 , the third rotation element N 3  is connected to the fourth rotation element N 4  through the second transfer gear TF 2 , the first rotation element N 1  is connected to the fifth rotation element N 5  through the third transfer gear TF 3  by operation of the first clutch C 1 , and the third rotation element N 3  is connected to the seventh rotation element N 7  through the second transfer gear TF 2  by operation of the second clutch C 2 . 
     Therefore, the second and third planetary gear sets PC 2  and PC 3  become direct-coupling state, the rotation elements of the second and third planetary gear sets PC 2  and PC 3  form a fourth shift line SP 4 , and D 4  is output through the sixth rotation element N 6  that is the output element. 
     Fifth Forward Speed 
     The second clutch C 2  that was operated at the fourth forward speed 4 TH  is released and the fourth clutch C 4  is operated at the fifth forward speed 5 TH . 
     As shown in  FIG. 3E , the rotation speed of the first shaft IS 1  is input to the second rotation element N 2  and is input to the seventh rotation element N 7  through the first transfer gear TF 1  by operation of the fourth clutch C 4 . 
     In addition, the third rotation element N 3  is connected to the fourth rotation element N 4  through the second transfer gear TF 2 , and the first rotation element N 1  is connected to the fifth rotation element N 5  through the third transfer gear TF 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 fifth shift line SP 5  and D 5  is output through the sixth rotation element N 6  that is the output element. 
     Sixth Forward Speed 
     The first clutch C 1  that was operated at the fifth forward speed 5 TH  is released and the second clutch C 2  is operated at the sixth forward speed 6 TH . 
     As shown in  FIG. 3F , the rotation speed of the first shaft IS 1  is input to the second rotation element N 2  and is input to the seventh rotation element N 7  through the first transfer gear TF 1  by operation of the fourth clutch C 4 . 
     In addition, the third rotation element N 3  is connected to the fourth rotation element N 4  through the second transfer gear TF 2  and is connected to the seventh rotation element N 7  by operation of the second clutch C 2 . 
     Therefore, the second and third planetary gear sets PC 2  and PC 3  become direct-coupling state, the rotation elements of the second and third planetary gear sets PC 2  and PC 3  form a sixth shift line SP 6 , and D 6  is output through the sixth rotation element N 6  that is the output element. 
     Seventh Forward Speed 
     The second clutch C 2  that was operated at the sixth forward speed 6 TH  is released and the first brake B 1  is operated at the seventh forward speed 7 TH . 
     As shown in  FIG. 3G , the rotation speed of the first shaft IS 1  is input to the second rotation element N 2  and is input to the seventh rotation element N 7  through the first transfer gear TF 1  by operation of the fourth clutch C 4 , and the first rotation element N 1  is operated as a fixed element by operation of the first brake B 1 . 
     In addition, the third rotation element N 3  is connected to the fourth rotation element N 4  through the second transfer gear TF 2 . 
     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 D 7  is output through the sixth rotation element N 6  that is the output element. 
     Eighth Forward Speed 
     The fourth clutch C 4  that was operated at the seventh forward speed 7 TH  is released and the second clutch C 2  is operated at the eighth forward speed 8 TH . 
     As shown in  FIG. 3H , the rotation speed of the first shaft IS 1  is input to the second rotation element N 2 , and the first rotation element N 1  is operated as a fixed element by operation of the first brake B 1 . 
     In addition, the third rotation element N 3  is connected to the fourth rotation element N 4  through the second transfer gear TF 2  and is connected to the seventh rotation element N 7  through the second transfer gear TF 2  by operation of the second clutch C 2 . 
     Therefore, the second and third planetary gear sets PC 2  and PC 3  become direct-coupling state, the rotation elements of the second and third planetary gear sets PC 2  and PC 3  form an eighth shift line SP 8 , and D 8  is output through the sixth rotation element N 6  that is the output element. 
     Reverse Speed 
     As shown in  FIG. 2 , the first brake B 1  and the first and third clutches C 1  and C 3  are operated at the reverse speed Rev. 
     As shown in  FIG. 3I , the rotation speed of the first shaft IS 1  is input to the second rotation element N 2 , and the first rotation element N 1  and the fifth rotation element N 5  are operated as fixed elements by operation of the first brake B 1  and the first clutch C 1 . 
     In addition, the third rotation element N 3  is connected to the fourth rotation element N 4  through the second transfer gear TF 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 REV is output through the sixth rotation element N 6  that is the output element. 
     Since three planetary gear sets are separately disposed on the first shaft and the second shaft disposed apart from and in parallel with each other in the planetary gear train according to various embodiments of the present invention, a length thereof may be reduced and mountability may be improved. 
     In addition, optimum gear ratios may be set due to ease of changing gear ratios by using three external-meshing gears as well as the planetary gear sets. Since gear ratios can be changed according to target performance, starting performance may be improved. Therefore, a start-up clutch instead of a torque converter may be used. 
     Since three frictional elements are operated at each shift-speed, non-operated frictional element may be minimized and drag torque may be reduced. In addition, fuel consumption may be reduced by increasing power delivery efficiency. 
     In addition, since torque load of each frictional element can be reduced, compact design is possible. 
     For convenience in explanation and accurate definition in the appended claims, the ten is upper or lower, front, and etc. are used to describe features of the exemplary embodiments with reference to the positions of such features as displayed in the figures. 
     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.