Abstract:
A planetary gear train of an automatic transmission may include a first planetary gear set including a first sun gear, a first planet carrier, and a first ring gear, a second planetary gear set including a second sun gear receiving an inverse rotation speed from the first ring gear, a second planet carrier, and a second ring gear selectively connected to the first planet carrier or the first ring gear, a third planetary gear set including a third sun gear selectively connected to the first planet carrier, the first ring gear, or the second ring gear, a third planet carrier, a third ring gear selectively connected to the first sun gear, three transfer gears engaging three rotation elements of the first planetary gear set with selected rotation elements, and five frictional elements selectively interconnecting the rotation elements or selectively connecting the rotation elements to a transmission housing.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority to Korean Patent Application No. 10-2012-0119958 filed on Oct. 26, 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. 
     In an aspect of the present invention, a planetary gear train of an automatic transmission for a vehicle may include 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 rotation elements thereof a first sun gear selectively operated as an output element or a fixed element, a first planet carrier directly connected to the first shaft and continuously operated as an input element, and a first ring gear operated as an output element, a second planetary gear set disposed on the second shalt, and including as rotation elements thereof a second sun gear receiving an inverse rotation speed from the first ring gear, a second planet carrier directly connected to the second shaft, and a second ring gear selectively connected to the first planet carrier or the first ring gear, a third planetary gear set disposed on the second shaft, and including as rotation elements thereof a third sun gear selectively connected to the first planet carrier, the first ring gear, or the second ring gear, a third planet carrier directly connected to the second shaft and an output gear, a third ring gear selectively connected to the first sun gear, three transfer gears engaging three rotation elements of the first planetary gear set with selected rotation elements among the rotation elements of the second and third planetary gear sets, and five frictional elements selectively interconnecting the rotation elements of the first, second, and third planetary gear sets or selectively connecting the rotation elements to a transmission housing. 
     Each of the first, second, and third planetary gear sets is a single pinion planetary gear set. 
     The three transfer gears may include a first transfer gear including a first transfer drive gear connected to the first ring gear and a first transfer driven gear connected to the second sun gear, a second transfer gear including a second transfer drive gear selectively connected to the first planet carrier or the first ring gear and a second transfer driven gear connected to the third sun gear, and a third transfer gear including a third transfer drive gear selectively connected to the first sun gear and a third transfer driven gear connected to the third ring gear. 
     The second transfer driven gear is selectively connected to the second ring gear. 
     The five frictional elements may include a first brake mounted between the first sun gear and the transmission housing, a first clutch mounted between the first sun gear and the third transfer drive gear, a second clutch mounted between the first planet carrier and the second transfer drive gear, a third clutch mounted between the second ring gear and the second transfer driven gear, and a fourth clutch mounted between the first ring gear and the second transfer drive gear. 
     The third clutch is mounted between the second ring gear and the third sun gear. 
     The fourth clutch is mounted between the first transfer drive gear and the second transfer drive gear. 
     A first forward speed is achieved by operating the first brake and the first and second clutches, a second forward speed is achieved by operating the first brake and the first and fourth clutches, a third forward speed is achieved by operating the first, second, and fourth clutches, a fourth forward speed is achieved by operating the first, third, and fourth clutches, a fifth forward speed is achieved by operating the first, second, and third clutches, a sixth forward speed is achieved by operating the second, third, and fourth clutches, a seventh forward speed is achieved by operating the first brake and the second and third clutches, an eighth forward speed is achieved by operating the first brake and the third and fourth clutches, and a reverse speed is achieved by operating the first brake and the first and third clutches. 
     In another aspect of the present invention, a planetary gear train of an automatic transmission for a vehicle, may include a first shaft receiving torque of an engine, a second shaft mounted in parallel with the first shaft, a first planetary gear set mounted on the first shaft, and including a rotation elements thereof a first sun gear selectively operated as an output element or a fixed element, a first planet carrier directly connected to the first shaft and continuously operated as an input element, and a first ring gear operated as an output element, a second planetary gear set mounted on the second shaft, and including as rotation elements thereof a second sun gear receiving an inverse rotation speed from the first ring gear, a second planet carrier directly connected to the second shaft, and a second ring gear selectively connected to the first planet carrier or the first ring gear, a third planetary gear set mounted on the second shaft, and including as rotation elements thereof a third sun gear selectively connected to the first planet carrier, the first ring gear, or the second ring gear, a third planet carrier directly connected to the second shaft and an output gear, a third ring gear selectively connected to the first sun gear, a first transfer gear including a first transfer drive gear connected to the first ring gear and a first transfer driven gear connected to the second sun gear, a second transfer gear including a second transfer drive gear selectively connected to the first planet carrier or the first ring gear and a second transfer driven gear connected to the third sun gear, a third transfer gear including a third transfer drive gear selectively connected to the first sun gear and a third transfer driven gear connected to the third ring gear, a first brake mounted between the first sun gear and the transmission housing, a first clutch mounted between the first sun gear and the third transfer drive gear, a second clutch mounted between the first planet carrier and the second transfer drive gear, a third clutch mounted between the second ring gear and the second transfer driven gear, and a fourth clutch mounted between the first ring gear and the second transfer drive gear. 
     Each of the first, second, and third planetary gear sets is a single pinion planetary gear set. 
     The third clutch is mounted between the second ring gear and the third sun gear. 
     A first forward speed is achieved by operating the first brake and the first and second clutches, a second forward speed is achieved by operating the first brake and the first and fourth clutches, a third forward speed is achieved by operating the first, second, and fourth clutches, a fourth forward speed is achieved by operating the first, third, and fourth clutches, a fifth forward speed is achieved by operating the first, second, and third clutches, a sixth forward speed is achieved by operating the second, third, and fourth clutches, a seventh forward speed is achieved by operating the first brake and the second and third clutches, an eighth forward speed is achieved by operating the first brake and the third and fourth clutches, and a reverse speed is achieved by operating the first brake and the first and third 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 . 
     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 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  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 planet carrier PC 1  is directly connected to the first shaft IS 1  so as to be always operated as an input element, the second planet carrier PC 2  and the third planet carrier PC 3  are connected through the second shaft IS 2 , and the third planet carrier PC 3  is directly connected to an output gear OG so as to be always operated as an output element. 
     The first, second, and third transfer gears TF 1 , TF 2 , and TF 3  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  directly connected to the second sun gear S 2 . 
     The second transfer gear TF 2  includes the second transfer drive gear TF 2   a  selectively connected to the first planet carrier PC 1  or the first ring gear R 1  and the second transfer driven gear TF 2   b  directly connected to the third sun gear S 3 . 
     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 . 
     The first shaft IS 1  or rotation elements of the first planetary gear set PG 1  and rotation elements of the second planetary gear set PG 2  or the third planetary gear set PG 3  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 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 drive gear TF 3   a.    
     The second clutch C 2  is disposed between the second planet carrier PC 2  and the second transfer drive gear TF 2   a.    
     The third clutch C 3  is disposed between the second ring gear R 2  and the second transfer driven gear TF 2   b.    
     The fourth clutch C 4  is disposed between the first ring gear R 1  and the second transfer drive gear TF 2   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 according to an exemplary embodiment of the present invention. 
     A first forward speed 1ST is achieved by operating the first brake S 1  and the first and second clutches C 1  and C 2 . 
     A second forward speed 2ND is achieved by operating the first brake B 1  and the first and fourth clutches C 1  and C 4 . 
     A third forward speed 3RD is achieved by operating the first, second, and fourth clutches C 1 , C 2 , and C 4 . 
     A fourth forward speed 4TH is achieved by operating the first, third, and fourth clutches C 1 , C 3 , and C 4 . 
     A fifth forward speed 5TH is achieved by operating the first, second, and third clutches C 1 , C 2 , and C 3 . 
     A sixth forward speed 6TH is achieved by operating the second, third, and fourth clutches C 2 , C 3 , and C 4 . 
     A seventh forward speed 7TH is achieved by operating the first brake B 1  and the second and third clutches C 2  and C 3 . 
     An eighth forward speed 8TH is achieved by operating the first brake B 1  and the third and fourth clutches C 3  and C 4 . 
     A reverse speed REV is achieved by operating the first brake B 1  and the first and third clutches C 1  and C 3 . 
       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 sun gear S 1 , the first planet carrier PC 1 , and the first ring gear R 1  from the left to the right, three vertical lines of the second planetary gear set PG 2  are set as the second sun gear S 2 , the second planet carrier PC 2 , and the second ring gear R 2  from the left to the right, and three vertical lines of the third planetary gear set PG 3  are set as the third sun gear S 3 , the third planet carrier PC 3 , and the third ring gear R 3  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 first planetary gear set PG 1  are externally meshed with the second planetary gear set PG 2  and the third planetary gear set PG 3  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 second clutches C 1  and C 2  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 first planet carrier PC 1 , and the first sun gear S 1  and the third ring gear R 3  are operated as fixed elements by operation of the first brake B 1  and the first clutch C 1 . 
     The rotation speed of the first ring gear R 1  is increased according to the gear ratio of the first transfer gear TF 1  and is then input to the second sun gear S 2  as an inverse rotation speed, and the rotation speed of the first planet carrier PC 1  is reduced according to the gear ratio of the second transfer gear TF 2  and is then input to the third sun gear S 3  as an inverse rotation speed by operation of the second clutch C 2 . 
     Therefore, the rotation elements of the third planetary gear set PG 3  form a first shift line SP 1  such that D 1  is output through the third planet carrier PC 3 . 
     [Second Forward Speed] 
     The second clutch C 2  that was operated at the first forward speed 1ST is released and the fourth clutch C 4  is operated at the second forward speed 2ND. 
     As shown in  FIG. 3B , the rotation speed of the first shaft  151  is input to the first planet carrier PC 1 , and the first sun gear S 1  and the third ring gear R 3  are operated as fixed elements by operation of the first brake B 1  and the first clutch C 1 . 
     The rotation speed of the first ring gear R 1  is increased according to the gear ratio of the first transfer gear TF 1  and is then input to the second sun gear S 2  as an inverse rotation speed, and the rotation speed of the first ring gear R 1  is reduced according to the gear ratio of the second transfer gear TF 2  and is then input to the third sun gear S 3  as an inverse rotation speed by operation of the fourth clutch C 4 . 
     Therefore, the rotation elements of the third planetary gear set PG 3  form a second shift line SP 2  such that D 2  is output through the third planet carrier PC 3 . 
     [Third Forward Speed] 
     The first brake B 1  that was operated at the second forward speed 2ND is released and the second clutch C 2  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 first planet carrier PC 1 , and the first planetary gear set PG 1  becomes a direct-coupling state by operation of the second clutch C 2  and the fourth clutch C 4 . 
     The rotation speed of the first ring gear R 1  is reduced according to the gear ratio of the second transfer gear TF 2  and is then input to the third sun gear S 3  as an inverse rotation speed by operation of the fourth clutch C 4 , and the rotation speed of the first sun gear S 1  is reduced according to the gear ratio of the third transfer gear TF 3  and is then input to the third ring gear R 3  as an inverse rotation speed. 
     Therefore, the rotation elements of the third planetary gear set PG 3  form a third shift line SP 3  such that D 3  is output through the third planet carrier PC 3 . 
     [Fourth Forward Speed] 
     The second clutch C 2  that was operated at the third forward speed 3RD is released and the third clutch C 3  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 first planet carrier PC 1 , the first ring gear R 1  is connected to the second sun gear S 2  through the first transfer gear TF 1 , the second ring gear R 2  is connected to the third sun gear S 3  through the second transfer gear TF 2  by operation of the third and fourth clutches C 3  and C 4 , and the first sun gear S 1  is connected to the third ring gear R 3  through the third transfer gear TF 3  by operation of the first clutch C 1 . 
     Therefore, the rotation elements of the third planetary gear set PG 3  form a fourth shift line SP 4  by operation of the first, second, and third planetary gear sets PC 1 , PC 2 , and PC 3  such that D 4  is output through the third planet carrier PC 3 . 
     [Fifth Forward Speed] 
     The fourth clutch C 4  that was operated at the fourth forward speed 4TH is released and the second clutch C 2  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 first planet carrier PC 1 , the first ring gear R 1  is connected to the second sun gear S 2  through the first transfer gear TF 1 , the first planet carrier PC 1  is connected to the second ring gear R 2  and the third sun gear S 3  through the second transfer gear TF 2  by operation of the second and third clutches C 2  and C 3 , and the first sun gear S 1  is connected to the third ring gear R 3  through the third transfer gear TF 3  by operation of the first clutch C 1 . 
     Therefore, the rotation elements of the third planetary gear set PG 3  form a fifth shift line SP 5  by operation of the first, second, and third planetary gear sets PC 1 , PC 2 , and PC 3  such that D 5  is output through the third planet carrier PC 3 . 
     [Sixth Forward Speed] 
     The first clutch C 1  that was operated at the fifth forward speed 5TH is released and the fourth clutch C 4  is operated at the sixth forward speed 6TH. 
     As shown in  FIG. 3F , the rotation speed of the first shaft IS 1  is input to the first planet carrier PC 1 , and the first planetary gear set PG 1  becomes the direct-coupling state by operation of the second clutch C 2  and the fourth clutch C 4 . 
     The rotation speed of the first ring gear R 1  is increased according to the gear ratio of the first transfer gear TF 1  and is then input to the second sun gear R 2  as an inverse rotation speed, and the rotation speed of the first ring gear R 1  is reduced according to the gear ratio of the second transfer gear TF 2  and is then input to the second ring gear R 2  and the third sun gear S 3  as an inverse rotation speed by operation of the third and fourth clutches C 3  and C 4 . 
     Therefore, the rotation elements of the third planetary gear set PG 3  form a sixth shift line SP 6  such that D 6  is output through the third planet carrier PC 3 . 
     [Seventh Forward Speed] 
     The fourth clutch C 4  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 first planet carrier PC 1 , and the first sun gear S 1  is operated as a fixed element by operation of the first brake B 1 . 
     The rotation speed of the first ring gear R 1  is increased according to the gear ratio of the first transfer gear TF 1  and is then input to the second sun gear S 2  as an inverse rotation speed, and the rotation speed of the first planet carrier PC 1  is reduced according to the gear ratio of the second transfer gear TF 2  and is then input to the second ring gear R 2  and the third sun gear S 3  as an inverse rotation speed by operation of the second and third clutches C 2  and C 3 . 
     Therefore, the rotation elements of the third planetary gear set PG 3  form a seventh shift line SP 7  such that D 7  is output through the third planet carrier PC 3 . 
     [Eighth Forward Speed] 
     The second clutch C 2  that was operated at the seventh forward speed 7TH is released and the fourth clutch C 4  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 first planet carrier PC 1 , and the first sun gear S 1  is operated as a fixed element by operation of the first brake B 1 . 
     The rotation speed of the first ring gear R 1  is increased according to the gear ratio of the first transfer gear TF 1  and is then input to the second sun gear S 2  as an inverse rotation speed, and the rotation speed of the first ring gear R 1  is reduced according to the gear ratio of the second transfer gear TF 2  and is then input to the second ring gear R 2  and the third sun gear S 3  as an inverse rotation speed by operation of the third and fourth clutches C 3  and C 4 . 
     Therefore, the rotation elements of the third planetary gear set PG 3  form an eighth shift line SP 8  such that D 8  is output through the third planet carrier PC 3 . 
     [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 first planet carrier PC 1 , and the first sun gear S 1  and the third ring gear R 3  are operated as fixed elements by operation of the first brake B 1  and the first clutch C 1 . 
     The rotation speed of the first ring gear R 1  is increased according to the gear ratio of the first transfer gear TF 1  and is then input to the second sun gear S 2  as an inverse rotation speed, and the second ring gear R 2  is connected to the third sun gear S 3  by operation of the third clutch C 3 . 
     Therefore, the rotation elements of the third planetary gear set PG 3  form a reverse shift line RS such that REV is output through the third planet carrier PC 3 . 
     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”, “up,” “down,” “upper”, “lower,” “upwards,” “downwards”, “front”, “rear”, “back”, “inside”, “outside”, “inwardly,” “outwardly,” “interior”, “exterior”, “inner,” “outer”, “forwards” and “backwards” 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.