Abstract:
A planetary gear train of an automatic transmission includes: a first shaft receiving engine torque; a second shaft parallel with the first shaft and selectively inversely receiving rotation of the first shaft; a first planetary gear set disposed on the first shaft and including a first, second and a third rotation paths; a compound planetary gear set formed by combining a second planetary gear set and a third planetary gear set, and including a fourth, fifth, sixth and seventh rotation paths; three transfer gears interposed between the rotation shafts, and configured to transfer an inverse rotation speed to the second shaft and the rotation paths of the compound planetary gear set; and five frictional elements interconnecting the rotation paths or selectively connecting the rotation paths to a transmission housing.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority to Korean Patent Application No. 10-2012-0116830 filed on Oct. 19, 2012, the entire contents of which is incorporated herein for all purposes by this reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the 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 
     Typically, a planetary gear train is realized by combining a plurality of planetary gear sets and friction members. 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. 
     Though achieving the same number of speeds, the planetary gear train has 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. 
     In addition, the planetary gear train realizes a plurality of shift-speeds. However, another friction member must be operated after one friction member is released so as to shift to a neighboring shift-speed from a view of shift control. In addition, a step ratio between the neighboring shift-speeds should be controlled to be suitable according to the planetary gear train. 
     The information disclosed in this Background of the Invention 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 are directed to providing 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. 
     A planetary gear train of an automatic transmission for a vehicle according to one or more exemplary embodiments of the present invention may include a first shaft receiving torque of an engine, a second shaft disposed in parallel with the first shaft and selectively receiving a rotation speed of the first shaft as an inverse rotation speed, a first planetary gear set disposed on the first shaft and including a first rotation path selectively operated as a fixed element or an output element, a second rotation path directly connected to the first shaft and operated as an input element, and a third rotation path selectively operated as an output element, a compound planetary gear set formed by combining a second planetary gear set and a third planetary gear set, and including a fourth rotation path selectively connected to the third rotation shaft, a fifth rotation path selectively connected to the first rotation shaft, a sixth rotation path connected to an output gear so as to be continuously operated as an output element, and a seventh rotation path directly connected to the second shaft, three transfer gears interposed between the rotation shafts, and configured to transfer an inverse rotation speed to the second shaft and the rotation paths of the compound planetary gear set, and five frictional elements interconnecting the rotation paths or selectively connecting the rotation paths to a transmission housing. 
     The first planetary gear set may be a single pinion planetary gear set having a first sun gear, a first planet carrier, and a first ring gear, the second planetary gear set may be a single pinion planetary gear set having a second sun gear, a second planet carrier, and a second ring gear, and the third planetary gear set may be a single pinion planetary gear set having a third sun gear, a third planet carrier, and a third ring gear. 
     The first rotation path may include the first sun gear, the second rotation path may include the first planet carrier, the third rotation path may include the first ring gear, the fourth rotation path may include the second sun gear, the fifth rotation path may include the second planet carrier and the third ring gear, the sixth rotation path may include the second ring gear and the third planet carrier, and the seventh rotation path may include the third sun gear. 
     The three transfer gears may include a first transfer gear including a first transfer drive gear selectively connected to the first shaft and a first transfer driven gear connected to the second shaft, a second transfer gear including a second transfer drive gear selectively connected to the third rotation path and a second transfer driven gear connected to the fourth rotation shaft, and a third transfer gear including a third transfer drive gear selectively connected to the first rotation path and a third transfer driven gear connected to the fifth rotation shaft. 
     The five frictional elements may include a first clutch mounted between the first shaft and the first transfer drive gear, a second clutch mounted between the third rotation path and the first transfer drive gear, a third clutch mounted between the third rotation path and the second transfer drive gear, a fourth clutch mounted between the first rotation path and the third transfer drive gear, and a first brake mounted between the first rotation path and the transmission housing. 
     A first forward speed may be achieved by operating the first brake and the first and fourth clutches, a second forward speed may be achieved by operating the first brake and the second and fourth clutches, a third forward speed may be achieved by operating the first, second, and fourth clutches, a fourth forward speed may be achieved by operating the second, third, and fourth clutches, a fifth forward speed may be achieved by operating the first, third, and fourth clutches, a sixth forward speed may be achieved by operating the first, second, and third clutches, a seventh forward speed may be achieved by operating the first brake and the first and third clutches, an eighth forward speed may be achieved by operating the first brake and the second and third clutches, and a reverse speed may be achieved by operating the first brake and the third and fourth clutches. 
     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 a planetary gear train according to an exemplary embodiment of the present invention. 
         FIG. 2  is an operational chart of friction members at each shift-speed applied to a planetary gear train according to an exemplary embodiment of the present invention. 
         FIG. 3A  is a lever diagram of a planetary gear train at the first forward speed according to an exemplary embodiment of the present invention. 
         FIG. 3B  is a lever diagram of a planetary gear train at the second forward speed according to an exemplary embodiment of the present invention. 
         FIG. 3C  is a lever diagram of a planetary gear train at the third forward speed according to an exemplary embodiment of the present invention. 
         FIG. 3D  is a lever diagram of a planetary gear train at the fourth forward speed according to an exemplary embodiment of the present invention. 
         FIG. 3E  is a lever diagram of a planetary gear train at the fifth forward speed according to an exemplary embodiment of the present invention. 
         FIG. 3F  is a lever diagram of a planetary gear train at the sixth forward speed according to an exemplary embodiment of the present invention. 
         FIG. 3G  is a lever diagram of a planetary gear train at the seventh forward speed according to an exemplary embodiment of the present invention. 
         FIG. 3H  is a lever diagram of a planetary gear train at the eighth forward speed according to an exemplary embodiment of the present invention. 
         FIG. 3I  is a lever diagram of a planetary gear train at a reverse speed according to an exemplary embodiment of the present invention. 
     
    
    
     It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various features illustrative of the basic principles of the invention. The specific design features of the present invention as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes will be determined in part by the particular intended application and use environment. 
     In the figures, reference numbers refer to the same or equivalent parts of the present invention throughout the several figures of the drawing. 
     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 the 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. 
     An exemplary embodiment of the present invention will hereinafter be described in detail with reference to the accompanying drawings. 
     Description of components that are not necessary for explaining the present exemplary embodiment 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 an exemplary embodiment of the present invention. 
     Referring to  FIG. 1 , a planetary 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 , 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 . 
     The first shaft IS 1  is an input member and supports the first planetary gear set PG 1 , and torque transmitted from an engine is transmitted to the first planetary gear set PG 1 . 
     The second shaft IS 2  supports the second and third planetary gear sets PG 2  and PG 3  and transmits torque selectively transmitted from the first shaft IS 1  and the first and second planetary gear sets PG 1  and PG 2  to the third planetary gear set PG 3 . 
     Therefore, torque input from the first shaft IS 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 single 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 . 
     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 . 
     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 . 
     The first planetary gear set PG 1  is operated independently, and the second and third planetary gear sets PG 2  and PG 3  are operated as a compound planetary gear set CPG. 
     Therefore, the first planetary gear set PG 1  includes three rotation paths N 1 , N 2 , and N 3 . 
     The first rotation path N 1  includes the first sun gear S 1 , is variably connected to a transmission housing H so as to be operated as a selective fixed element, and is operated as a selective output element. 
     The second rotation path N 2  includes the first planet carrier PC 1  and is directly connected to the first shaft IS 1  so as to be operated as an input element. 
     The third rotation path N 3  includes the first ring gear R 1  and is operated as a selective output element. 
     In addition, since the second ring gear R 2  is directly connected to the third planet carrier PC 3  and the third ring gear R 3  is directly connected to the second planet carrier PC 2 , the second and third planetary gear sets PG 2  and PG 3  include four rotation paths N 4 , N 5 , N 6 , and N 7 . 
     The fourth rotation path N 4  includes the second sun gear S 2  and selectively receives torque of the third rotation path N 3 . 
     The fifth rotation path N 5  includes the second planet carrier PC 2  and the third ring gear R 3 , and selectively receives torque of the first rotation path N 1 . 
     The sixth rotation path N 6  includes the second ring gear R 2  and the third planet carrier PC 3 , and is connected to the output gear OG so as to be continuously operated as an output element. 
     The seventh rotation path N 7  includes the third sun gear S 3 , and is directly connected to the second shaft IS 2  so as to receive torque selectively from the first shaft IS 1  and the third rotation path N 3 . 
     In addition, the rotation paths N 1 -N 7  are combined by first, second, and third transfer gears TF 1 , TF 2 , and TF 3  and frictional elements including a first brake B 1  and first, second, third, and fourth clutches C 1 , C 2 , C 3 , and C 4 . 
     The first, second, and third transfer gears TF 1 , TF 2 , and TF 3  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  connects the first shaft IS 1  to the second shaft IS 2 . 
     The second transfer gear TF 2  connects the third rotation path N 3  to the fourth rotation path N 4 . 
     The third transfer gear TF 3  connects the first rotation path N 1  to the fifth rotation path N 5 . 
     Therefore, rotation paths connected by the first, second, and third transfer gears TF 1 , TF 2 , and TF 3  are rotated in opposite directions to each other. Gear ratios of the first, second, and third transfer drive gears TF 1   a , TF 2   a , and TF 3   a  and the first, second, and third transfer driven gears TF 1   b , TF 2   b , and TF 3   b  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 mounted between the first rotation path N 1  and the transmission housing H. 
     The first clutch C 1  is mounted between the first shaft IS 1  and the first transfer drive gear TF 1   a.    
     The second clutch C 2  is mounted between the third rotation path N 3  and the first transfer drive gear TF 1   a.    
     The third clutch C 3  is mounted between the third rotation path N 3  and the second transfer drive gear TF 2   a.    
     The fourth clutch C 4  is mounted between the first rotation path N 1  and the third transfer drive gear TF 3   a.    
     The frictional elements including 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 an exemplary embodiment of the present invention. 
     As shown in  FIG. 2 , three frictional elements are operated at each shift-speed in the planetary gear train an exemplary embodiment of the present invention. 
     The first forward speed 1ST is achieved by operating the first brake B 1  and the first and fourth clutches C 1  and C 4 . 
     The second forward speed 2ND is achieved by operating the first brake B 1  and the second and fourth clutches C 2  and C 4 . 
     The third forward speed 3RD is achieved by operating the first, second, and fourth clutches C 1 , C 2 , and C 4 . 
     The fourth forward speed 4TH is achieved by operating the second, third, and fourth clutches C 2 , C 3 , and C 4 . 
     The fifth forward speed 5TH is achieved by operating the first, third, and fourth clutches C 1 , C 3 , and C 4 . 
     The sixth forward speed 6TH is achieved by operating the first, second, and third clutches C 1 , C 2 , and C 3 . 
     The seventh forward speed 7TH is achieved by operating the first brake B 1  and the first and third clutches C 1  and C 3 . 
     The eighth forward speed 8TH is achieved by operating the first brake B 1  and the second and third clutches C 2  and C 3 . 
     The reverse speed REV is achieved by operating the first brake B 1  and the third and fourth clutches C 3  and C 4 . 
       FIG. 3A  to  FIG. 3I  are lever diagrams of the planetary gear train at each shift-speed according to the first exemplary embodiment of the present invention, and illustrate shift processes of the planetary gear train according to the first exemplary embodiment 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 rotation path N 1 , the second rotation path N 2 , and the third rotation path N 3  from the left to the right, and four vertical lines of the compound planetary gear set CPG are set as the fourth rotation path N 4 , the fifth rotation path N 5 , the sixth rotation path N 6 , and the seventh rotation path N 7  from the left to the right. 
     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 first shaft IS 1  and the rotation paths of the first planetary gear set PG 1  are meshed externally with the second shaft IS 2  and the rotation paths of the compound planetary gear set CPG 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 an exemplary embodiment 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 1ST. 
     As shown in  FIG. 3A , the rotation speed of the first shaft IS 1  is input to the second rotation path N 2 , and a rotation speed of the first shaft IS 1  is changed according to the gear ratio of the first transfer gear TF 1  by operation of the first clutch C 1  and is then input to the seventh rotation path N 7  as an inverse rotation speed. 
     At this state, the first rotation path N 1  and the fifth rotation path N 5  are operated as the fixed elements by operation of the first brake B 1  and the fourth clutch C 4 . Therefore, the rotation paths of the compound planetary gear set CPG form a first shift line SP 1  such that D 1  is output through the sixth rotation path N 6 . 
     [Second Forward Speed] 
     The first clutch C 1  that was operated at the first forward speed 1ST is released and the second clutch C 2  is operated at the second forward speed 2ND. 
     As shown in  FIG. 3B , the rotation speed of the first shaft IS 1  is input to the second rotation path N 2 , and a rotation speed of the third rotation path N 3  is changed according to the gear ratio of the first transfer gear TF 1  and is then input to the seventh rotation path N 7  as an inverse rotation speed by operation of the second clutch C 2 . 
     At this state, the first rotation path N 1  and the fifth rotation path N 5  are operated as fixed elements by operation of the first brake B 1  and the fourth clutch C 4 . Therefore, the rotation paths of the compound planetary gear set CPG form a second shift line SP 2  such that D 2  is output through the sixth rotation path N 6 . 
     [Third Forward Speed] 
     The first brake B 1  that was operated at the second forward speed 2ND is released and the first clutch C 1  is operated at the third forward speed 3RD. 
     As shown in  FIG. 3C , the rotation speed of the first shaft IS 1  is input to the second rotation path N 2  and the first planetary gear set PG 1  becomes a direct-coupling state by operation of the first and second clutches C 1  and C 2 . 
     Therefore, the rotation speed of the third rotation path N 3  is changed according to the gear ratio of the first transfer gear TF 1  and is then input to the seventh rotation path N 7  as an inverse rotation speed, and the rotation speed of the first rotation path N 1  is changed according to the gear ratio of the third transfer gear TF 3  and is then input to the fifth rotation path N 5  as an inverse rotation speed. Therefore, the rotation paths of the compound planetary gear set CPG form a third shift line SP 3  such that D 3  is output through the sixth rotation path N 6 . 
     [Fourth Forward Speed] 
     The first clutch C 1  that was operated at the third forward speed 3RD is released and the second clutch C 2  is operated at the fourth forward speed 4TH. 
     As shown in  FIG. 3D , the rotation speed of the first shaft IS 1  is input to the second rotation path N 2 , the rotation speed of the third rotation path N 3  is changed according to the gear ratio of the first transfer gear TF 1  and is then input to the seventh rotation path N 7  as an inverse rotation speed by operation of the second clutch C 2 , and the rotation speed of the third rotation path N 3  is changed according to the gear ratio of the second transfer gear TF 2  and is then input to the fourth rotation path N 4  as an inverse rotation speed by operation of the third clutch C 3 . 
     Therefore, the rotation paths of the compound planetary gear set CPG form a fourth shift line SP 4  such that D 4  is output through the sixth rotation path N 6 . 
     [Fifth Forward Speed] 
     The second clutch C 2  that was operated at the fourth forward speed 4TH is released and the first clutch C 1  is operated at the fifth forward speed 5TH. 
     As shown in  FIG. 3E , the rotation speed of the first shaft IS 1  is input to the second rotation path N 2 , the rotation speed of the first shaft IS 1  is changed according to the gear ratio of the first transfer gear TF 1  and is then input to the seventh rotation path N 7  as an inverse rotation speed by operation of the first clutch C 1 , and the rotation speed of the third rotation path N 3  is changed according to the gear ratio of the second transfer gear TF 2  and is then input to the fourth rotation path N 4  as an inverse rotation speed by operation of the third clutch C 3 . 
     Therefore, the rotation paths of the compound planetary gear set CPG form a fifth shift line SP 5  such that D 5  is output through the sixth rotation path N 6 . 
     [Sixth Forward Speed] 
     The fourth clutch C 4  that was operated at the fifth forward speed 5TH is released and the second clutch C 2  is operated at the sixth forward speed 6TH. 
     As shown in  FIG. 3F , the first planetary gear set PG 1  becomes the direct-coupling state by operation of the first and second clutches C 1  and C 2 . 
     At this state, the rotation speed of the first shaft IS 1  is changed according to the gear ratio of the first transfer gear TF 1  and is then input to the seventh rotation path N 7  as an inverse rotation speed by operation of the first clutch C 1 , and the rotation speed of the third rotation path N 3  is changed according to the gear ratio of the second transfer gear TF 2  and is then input to the fourth rotation path N 4  as an inverse rotation speed by operation of the third clutch C 3 . 
     Therefore, the rotation paths of the compound planetary gear set CPG form a sixth shift line SP 6  such that D 6  is output through the sixth rotation path N 6 . 
     [Seventh Forward Speed] 
     The second clutch C 2  that was operated at the sixth forward speed 6TH is released and the first brake B 1  is operated at the seventh forward speed 7TH. 
     As shown in  FIG. 3G , the rotation speed of the first shaft IS 1  is input to the second rotation path N 2 , and the first rotation path N 1  is operated as a fixed element by operation of the first brake B 1 . 
     In addition, the rotation speed of the first shaft IS 1  is changed according to the gear ratio of the first transfer gear TF 1  and is then input to the seventh rotation path N 7  as an inverse rotation speed by operation of the first clutch C 1 , and the rotation speed of the third rotation path N 3  is changed according to the gear ratio of the second transfer gear TF 2  and is then input to the fourth rotation path N 4  as an inverse rotation speed by operation of the third clutch C 3 . 
     Therefore, the rotation paths of the compound planetary gear set CPG form a seventh shift line SP 7  such that D 7  is output through the sixth rotation path N 6 . 
     [Eighth Forward Speed] 
     The first clutch C 1  that was operated at the seventh forward speed 7TH is released and the second clutch C 2  is operated at the eighth forward speed 8TH. 
     As shown in  FIG. 3H , the rotation speed of the first shaft IS 1  is input to the second rotation path N 2 , and the first rotation path N 1  is operated as a fixed element by operation of the first brake B 1 . 
     In addition, the rotation speed of the third rotation path N 3  is changed according to the gear ratio of the first transfer gear TF 1  and is then input to the seventh rotation path N 7  as an inverse rotation speed by operation of the second clutch C 2 , and the rotation speed of the third rotation path N 3  is changed according to the gear ratio of the second transfer gear TF 2  and is then input to the fourth rotation path N 4  as an inverse rotation speed by operation of the third clutch C 3 . 
     Therefore, the rotation paths of the compound planetary gear set CPG form an eighth shift line SP 8  such that D 8  is output through the sixth rotation path N 6 . 
     [Reverse Speed] 
     As shown in  FIG. 2 , the first brake B 1  and the third and fourth clutches C 3  and C 4  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 path N 2 , the first rotation path N 1  and the fifth rotation path N 5  are operated as fixed elements by operation of the first brake B 1  and the fourth clutch C 4 . 
     In addition, the rotation speed of the third rotation path N 3  is changed according to the gear ratio of the second transfer gear TF 2  and is then input to the fourth rotation path N 4  as an inverse rotation speed by operation of the third clutch C 3 . Therefore, the rotation paths of the compound planetary gear set CPG form a reverse shift line RS such that REV is output through the sixth rotation path N 6 . 
     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 an exemplary embodiment 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 terms “upper”, “lower”, “inner” and “outer” 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. 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. 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.