Patent Publication Number: US-8986154-B2

Title: Planetary gear train of automatic transmission for vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority of Korean Patent Application Number 10-2012-0136460 filed Nov. 28, 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 and power delivery performance and reduce fuel consumption. 
     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. 
     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 the number of components as a consequence of achieving eight forward speeds and two reverse speeds by disposing two planetary gear sets separately on a first shaft and a second shaft disposed in parallel and connecting rotation elements of the planetary gear sets through a plurality of externally-meshed gears. 
     In addition, the present invention has been made in an effort to provide a planetary gear train of an automatic transmission for a vehicle having further advantages of enabling of setting optimum gear ratios due to ease of changing gear ratios by using a plurality of externally-meshed gears, and accordingly improving power delivery performance and fuel economy. 
     A planetary gear train of an automatic transmission for a vehicle according to one aspect of the present invention may include: a first shaft receiving torque of an engine; a second shaft disposed in parallel with the first shaft with a predetermined distance; a first planetary gear set disposed on the first shaft, and including a first rotation element selectively connected to the first shaft, a second rotation element selectively connected to the first shaft or a transmission housing, and a third rotation element operated as an output element; a second planetary gear set disposed on the second shaft, and including a fourth rotation element selectively connected to the first shaft through an externally-meshed gear, connected to the first rotation element through an externally-meshed gear, and selectively connected to the transmission housing, a fifth rotation element connected to the third rotation element through an externally-meshed gear and directly connected to an output gear, and a sixth rotation element selectively connected to the first shaft through an externally-meshed gear; four transfer gears connecting the fourth, fifth, and sixth rotation elements with the first shaft and the first and third rotation elements; and frictional elements including clutches selectively connecting the first, second, fourth, and sixth rotation elements with the first shaft and brakes selectively connecting the second and fourth rotation elements with the transmission housing. 
     The first planetary gear set may be a single pinion planetary gear set including a first sun gear being the first rotation element, a first planet carrier being the second rotation element, and a first ring gear being the third rotation element, and the second planetary gear set may be a single pinion planetary gear set including a second sun gear being the fourth rotation element, a second planet carrier being the fifth rotation element, and a second ring gear being the sixth rotation element. 
     The four transfer gears may include: a first transfer gear connecting the first rotation element with the fourth rotation element; a second transfer gear connecting the third rotation element with the fifth rotation element; a third transfer gear connecting the first shaft with the fourth rotation element; and a fourth transfer gear connecting the first shaft with the sixth rotation element. 
     The frictional elements may include: a first clutch disposed between the fourth transfer gear and the sixth rotation element; a second clutch disposed between the third transfer gear and the fourth rotation element; a third clutch disposed between the first shaft and the first rotation element; a fourth clutch disposed between the first shaft and the second rotation element; a first brake disposed between the second rotation element and the transmission housing; and a second brake disposed between the fourth rotation element and the transmission housing. 
     The first clutch and the first brake may be operated at a first forward speed, the first clutch and the second brake may be operated at a second forward speed, the first clutch and the second clutch may be operated at a third forward speed, the first clutch and the third clutch may be operated at a fourth forward speed, the first clutch and the fourth clutch may be operated at a fifth forward speed, the third clutch and the fourth clutch may be operated at a sixth forward speed, the second clutch and the fourth clutch may be operated at a seventh forward speed, the fourth clutch and the second brake may be operated at an eighth forward speed, the second clutch and the first brake may be operated at a first reverse speed, and the third clutch and the first brake may be operated at a second reverse speed. 
     The frictional elements may include: a first clutch disposed between the first shaft and the fourth transfer gear; a second clutch disposed between the third transfer gear and the fourth rotation element; a third clutch disposed between the first shaft and the first rotation element; a fourth clutch disposed between the first shaft and the second rotation element; a first brake disposed between the second rotation element and the transmission housing; and a second brake disposed between the fourth rotation element and the transmission housing. 
     The frictional elements may include: a first clutch disposed between the fourth transfer gear and the sixth rotation element; a second clutch disposed between the first shaft and the third transfer gear; a third clutch disposed between the first shaft and the first rotation element; a fourth clutch disposed between the first shaft and the second rotation element; a first brake disposed between the second rotation element and the transmission housing; and a second brake disposed between the fourth rotation element and the transmission housing. 
     The frictional elements may include: a first clutch disposed between the fourth transfer gear and the sixth rotation element; a second clutch disposed between the third transfer gear and the fourth rotation element; a third clutch disposed between the first shaft and the first rotation element; a fourth clutch disposed between the first shaft and the second rotation element; a first brake disposed between the second rotation element and the transmission housing; and a second brake disposed between the first rotation element and the transmission housing. 
     The first planetary gear set may be a double pinion planetary gear set including a first sun gear being the first rotation element, a first ring gear being the second rotation element, and a first planet carrier being the third rotation element, and the second planetary gear set may be a single pinion planetary gear set including a second sun gear being the fourth rotation element, a second planet carrier being the fifth rotation element, and a second ring gear being the sixth rotation element. 
     A planetary gear train of an automatic transmission for a vehicle according to another aspect of the present invention may include: a first shaft receiving torque of an engine; a second shaft disposed in parallel with the first shaft with a predetermined distance; a first planetary gear set disposed on the first shaft, and including a first rotation element selectively connected to the first shaft, a second rotation element selectively connected to the first shaft or a transmission housing, and a third rotation element; a second planetary gear set disposed on the second shaft, and including a fourth rotation element selectively connected to the first shaft, connected to the first rotation element, and selectively connected to the transmission housing, a fifth rotation element connected to the third rotation element and directly connected to an output gear, and a sixth rotation element selectively connected to the first shaft; a first transfer gear connecting the first rotation element with the fourth rotation element; a second transfer gear connecting the third rotation element with the fifth rotation element; a third transfer gear connecting the first shaft with the fourth rotation element; a fourth transfer gear connecting the first shaft with the sixth rotation element; and frictional elements including clutches selectively connecting the first, second, fourth, and sixth rotation elements with the first shaft and brakes selectively connecting the second and fourth rotation elements with the transmission housing. 
     The first planetary gear set may be a single pinion planetary gear set including a first sun gear being the first rotation element, a first planet carrier being the second rotation element, and a first ring gear being the third rotation element, and the second planetary gear set may be a single pinion planetary gear set including a second sun gear being the fourth rotation element, a second planet carrier being the fifth rotation element, and a second ring gear being the sixth rotation element. 
     The frictional elements may include: a first clutch disposed between the fourth transfer gear and the sixth rotation element; a second clutch disposed between the third transfer gear and the fourth rotation element; a third clutch disposed between the first shaft and the first rotation element; a fourth clutch disposed between the first shaft and the second rotation element; a first brake disposed between the second rotation element and the transmission housing; and a second brake disposed between the fourth rotation element and the transmission housing. 
     The frictional elements may include: a first clutch disposed between the first shaft and the fourth transfer gear; a second clutch disposed between the third transfer gear and the fourth rotation element; a third clutch disposed between the first shaft and the first rotation element; a fourth clutch disposed between the first shaft and the second rotation element; a first brake disposed between the second rotation element and the transmission housing; and a second brake disposed between the fourth rotation element and the transmission housing. 
     The frictional elements may include: a first clutch disposed between the fourth transfer gear and the sixth rotation element; a second clutch disposed between the first shaft and the third transfer gear; a third clutch disposed between the first shaft and the first rotation element; a fourth clutch disposed between the first shaft and the second rotation element; a first brake disposed between the second rotation element and the transmission housing; and a second brake disposed between the fourth rotation element and the transmission housing. 
     The frictional elements may include: a first clutch disposed between the fourth transfer gear and the sixth rotation element; a second clutch disposed between the third transfer gear and the fourth rotation element; a third clutch disposed between the first shaft and the first rotation element; a fourth clutch disposed between the first shaft and the second rotation element; a first brake disposed between the second rotation element and the transmission housing; and a second brake disposed between the first rotation element and the transmission housing. 
     The first planetary gear set may be a double pinion planetary gear set including a first sun gear being the first rotation element, a first ring gear being the second rotation element, and a first planet carrier being the third rotation element, and the second planetary gear set may be a single pinion planetary gear set including a second sun gear being the fourth rotation element, a second planet carrier being the fifth rotation element, and a second ring gear being the sixth rotation element. 
     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 elements at each shift-speed applied to a planetary gear train according to the planetary gear train of  FIG. 1 . 
         FIG. 3  is a lever diagram of a planetary gear train according to the planetary gear train of  FIG. 1 . 
         FIG. 4  is a schematic diagram of an exemplary planetary gear train according to the present invention. 
         FIG. 5  is a schematic diagram of an exemplary planetary gear train according to the present invention. 
         FIG. 6  is a schematic diagram of an exemplary planetary gear train according to the present invention. 
         FIG. 7  is a schematic diagram of an exemplary planetary gear train 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 illustrated 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 includes a first planetary gear set PG 1  disposed on a first shaft IS 1 , a second planetary gear set PG 2  disposed on a second shaft IS 2  disposed in parallel with the first shaft IS 1 , four transfer gears TF 1 , TF 2 , TF 3 , and TF 4 , and frictional elements consisting of four clutches C 1 , C 2 , C 3 , and C 4  and two brakes B 1  and B 2 . 
     Therefore, torque input from the first shaft IS 1  is converted into eight forward speeds and second reverse speeds by cooperation of the first and second planetary gear sets PG 1  and PG 2 , and then is output through an output gear OG. 
     The first shaft IS 1  is an input member, and torque from a crankshaft of the engine is changed through a torque converter and is input to the first shaft IS 1 . 
     The second shaft IS 2  supports the second planetary gear set PG 2  without rotational interference therebetween. 
     The first planetary gear set PG 1  is a single pinion planetary gear set, and includes a first sun gear S 1  being a first rotation element N 1 , a first planet carrier PC 1  rotatably supporting a first pinion P 1  externally meshed with the first sun gear S 1  and being a second rotation element N 2 , and a first ring gear R 1  internally meshed with the first pinion P 1  and being a third rotation element N 3 . 
     The second planetary gear set PG 2  is a single pinion planetary gear set, and includes a second sun gear S 2  being a fourth rotation element N 4 , a second planet carrier PC 2  rotatably supporting a second pinion P 2  externally meshed with the second sun gear S 2  and being a fifth rotation element N 5 , and a second ring gear R 2  internally meshed with the second pinion P 2  and being a sixth rotation element N 6 . 
     Two rotation elements of the first planetary gear set PG 1  are selectively connected to the first shaft IS 1 , three rotation elements of the second planetary gear set PG 2  are connected to the first shaft IS 1  and two rotation elements of the first planetary gear set PG 1  through first, second, third, and fourth transfer gears TF 1 , TF 2 , TF 3 , and TF 4 , and any one rotation element of the second planetary gear set PG 1  is directly connected to the output gear OG. 
     The output gear OG drives a drive shaft including a driving wheel through a final reduction gear and a differential apparatus. 
     The first, second, third, and fourth transfer gears TF 1 , TF 2 , TF 3 , and TF 4  respectively have first, second, third, and fourth transfer drive gears TF 1   a , TF 2   a , TF 3   a , and TF 4   a  and first, second, third, and fourth transfer driven gears TF 1   b , TF 2   b , TF 3   b , and TF 4   b  externally meshed with each other. 
     The first transfer gear TF 1  connects the first rotation element N 1  with the fourth rotation element N 4 . 
     The second transfer gear TF 2  connects the third rotation element N 3  with the fifth rotation element N 5 . 
     The third transfer gear TF 3  connects the first shaft IS 1  with the fourth rotation element N 4 . 
     The fourth transfer gear TF 4  connects the first shaft IS 1  with the sixth rotation element N 6 . 
     Therefore, the rotation elements (including the first shaft IS 1 ) connected with each other through the first, second, third, and fourth transfer gears TF 1 , TF 2 , TF 3 , and TF 4  are rotated in opposite direction to each other according to gear ratios of the first, second, third, and fourth transfer gears TF 1 , TF 2 , TF 3 , and TF 4 . 
     In addition, four clutches C 1 , C 2 , C 3 , and C 4  selectively connecting the selected rotation elements (including the first shaft IS 1 ) and two brakes B 1  and B 2  selectively connecting the selected rotation elements to the transmission housing H are disposed as follows. 
     The first clutch C 1  is disposed between the fourth transfer gear TF 4  and the sixth rotation element N 6 . 
     The second clutch C 2  is disposed between the third transfer gear TF 3  and the fourth rotation element N 4 . 
     The third clutch C 3  is disposed between the first shaft IS 1  and the first rotation element N 1 . 
     The fourth clutch C 4  is disposed between the first shaft IS 1  and the second rotation element N 2 . 
     The first brake B 1  is disposed between the second rotation element N 2  and the transmission housing H. 
     The second brake B 2  is disposed between the fourth rotation element N 4  and the transmission housing H. 
     The frictional elements consisting of the first, second, third, and fourth clutches C 1 , C 2 , C 3 , and C 4  and the first and second brakes B 1  and B 2  are conventional multi-plate friction elements of wet type that are operated by hydraulic pressure. 
       FIG. 2  is an operational chart of friction elements at each shift-speed applied to the planetary gear train of  FIG. 1 . 
     As shown in  FIG. 2 , two frictional elements are operated at each shift-speed in the planetary gear train according to various embodiments of the present invention. 
     The first clutch C 1  and the first brake B 1  are operated at a first forward speed 1ST. 
     The first clutch C 1  and the second brake B 2  are operated at a second forward speed 2ND. 
     The first clutch C 1  and the second clutch C 2  are operated at a third forward speed 3RD. 
     The first clutch C 1  and the third clutch C 3  are operated at a fourth forward speed 4TH. 
     The first clutch C 1  and the fourth clutch C 4  are operated at a fifth forward speed 5TH. 
     The third clutch C 3  and the fourth clutch C 4  are operated at a sixth forward speed 6TH. 
     The second clutch C 2  and the fourth clutch C 4  are operated at a seventh forward speed 7TH. 
     The fourth clutch C 4  and the second brake B 2  are operated at an eighth forward speed 8TH. 
     The second clutch C 2  and the first brake B 1  are operated at a first reverse speed Rev 1 . 
     The third clutch C 3  and the first brake B 1  are operated at a second reverse speed Rev 2 . 
       FIG. 3  is a lever diagram of the planetary gear train of  FIG. 1 , and illustrates shift processes of the planetary gear train according by lever analysis method. 
     Referring to  FIG. 3 , three vertical lines of the first planetary gear set PG 1  are set as the first, second, and third rotation elements N 1 , N 2 , and N 3  from the left to the right, and three vertical lines of the second planetary gear set PG 2  are set as the fourth, fifth, and sixth rotation elements N 4 , N 5 , and N 6  from the left to the right. 
     A middle horizontal line represents a rotation speed of “0”, upper horizontal lines represent positive rotation speeds and lower horizontal lines represent negative rotation speeds. 
     In addition, “−” in  FIG. 3  means that rotational elements are rotated in an opposite direction to the rotation direction of the engine. It is because the first shaft IS 1  and the first and second planetary gear sets PG 1  and PG 2  are externally meshed through the first, second, third, and fourth transfer gears TF 1 , TF 2 , TF 3 , and TF 4  without an idling gear. 
     In addition, distances between the vertical lines of the first and second planetary gear sets PG 1  and PG 2  are set according to gear ratios (teeth number of a sun gear/teeth number of a ring gear). 
     Hereinafter, referring to  FIG. 2  and  FIG. 3 , the shift processes of the planetary gear train according of  FIG. 1  will be described in detail. 
     First Forward Speed 
     Referring to  FIG. 2 , the first clutch C 1  and the first brake B 1  are operated at the first forward speed 1ST. 
     As shown in  FIG. 3 , the first rotation element N 1  is connected to the fourth rotation element N 4  through the first transfer gear TF 1 , and the third rotation element N 3  is connected to the fifth rotation element N 5  through the second transfer gear TF 2 . 
     At this state, a rotation speed of the first shaft IS 1  is converted according to the gear ratio of the fourth transfer gear TF 4  and is then input to the sixth rotation element N 6  by operation of the first clutch C 1 , and the second rotation element N 2  is operated as a fixed element by operation of the first brake B 1 . 
     Therefore, the rotation elements of the first planetary gear set PG 1  form a first speed line T 1  and the rotation elements of the second planetary gear set PG 2  form a first shift line SP 1  by cooperation of the first planetary gear set PG 1  and the second planetary gear set PG 2 . Therefore, D 1  is output through the fifth rotation element N 5  that is the output element. 
     Second Forward Speed 
     The first brake B 1  that was operated at the first forward speed 1ST is released and the second brake B 2  is operated at the second forward speed 2ND. 
     As shown in  FIG. 3 , the first rotation element N 1  is connected to the fourth rotation element N 4  through the first transfer gear TF 1 , and the third rotation element N 3  is connected to the fifth rotation element N 5  through the second transfer gear TF 2 . 
     At this state, the rotation speed of the first shaft IS 1  is converted according to the gear ratio of the fourth transfer gear TF 4  and is then input to the sixth rotation element N 6  by operation of the first clutch C 1 , and the fourth rotation element N 4  is operated as a fixed element by operation of the second brake B 2 . 
     Therefore, the rotation elements of the first planetary gear set PG 1  form a second speed line T 2  and the rotation elements of the second planetary gear set PG 2  form a second shift line SP 2  by cooperation of the first planetary gear set PG 1  and the second planetary gear set PG 2 . Therefore, D 2  is output through the fifth rotation element N 5  that is the output element. 
     Third Forward Speed 
     The second brake B 2  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. 3 , the first rotation element N 1  is connected to the fourth rotation element N 4  through the first transfer gear TF 1 , and the third rotation element N 3  is connected to the fifth rotation element N 5  through the second transfer gear TF 2 . 
     At this state, the rotation speed of the first shaft IS 1  is converted according to the gear ratio of the fourth transfer gear TF 4  and is then input to the sixth rotation element N 6  by operation of the first clutch C 1 . In addition, the rotation speed of the first shaft IS 1  is converted according to the gear ratio of the third transfer gear TF 3  and is then input to the fourth rotation element N 4  by operation of the second clutch C 2 . 
     Therefore, the rotation elements of the first planetary gear set PG 1  form a third speed line T 3  and the rotation elements of the second planetary gear set PG 2  form a third shift line SP 3  by cooperation of the first planetary gear set PG 1  and the second planetary gear set PG 2 . Therefore, D 3  is output through the fifth rotation element N 5  that is the output element. 
     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. 3 , the first rotation element N 1  is connected to the fourth rotation element N 4  through the first transfer gear TF 1 , and the third rotation element N 3  is connected to the fifth rotation element N 5  through the second transfer gear TF 2 . 
     At this state, the rotation speed of the first shaft IS 1  is converted according to the gear ratio of the fourth transfer gear TF 4  and is then input to the sixth rotation element N 6  by operation of the first clutch C 1 . In addition, the rotation speed of the first shaft IS 1  is directly input to the first rotation element N 1  by operation of the third clutch C 3 . 
     Therefore, the rotation elements of the first planetary gear set PG 1  form a fourth speed line T 4  and the rotation elements of the second planetary gear set PG 2  form a fourth shift line SP 4  by cooperation of the first planetary gear set PG 1  and the second planetary gear set PG 2 . Therefore, D 4  is output through the fifth rotation element N 5  that is the output element. 
     Fifth Forward Speed 
     The third clutch C 3  that was operated at the fourth forward speed 4TH is released and the fourth clutch C 4  is operated at the fifth forward speed 5TH. 
     As shown in  FIG. 3 , the first rotation element N 1  is connected to the fourth rotation element N 4  through the first transfer gear TF 1 , and the third rotation element N 3  is connected to the fifth rotation element N 5  through the second transfer gear TF 2 . 
     At this state, the rotation speed of the first shaft IS 1  is converted according to the gear ratio of the fourth transfer gear TF 4  and is then input to the sixth rotation element N 6  by operation of the first clutch C 1 . In addition, the rotation speed of the first shaft IS 1  is directly input to the second rotation element N 2  by operation of the fourth clutch C 4 . 
     Therefore, the rotation elements of the first planetary gear set PG 1  form a fifth speed line T 5  and the rotation elements of the second planetary gear set PG 2  form a fifth shift line SP 5  by cooperation of the first planetary gear set PG 1  and the second planetary gear set PG 2 . Therefore, D 5  is output through the fifth rotation element N 5  that is the output element. 
     Sixth Forward Speed 
     The first clutch C 1  that was operated at the fifth forward speed 5TH is released and the third clutch C 3  is operated at the sixth forward speed 6TH. 
     As shown in  FIG. 3 , the first rotation element N 1  is connected to the fourth rotation element N 4  through the first transfer gear TF 1 , and the third rotation element N 3  is connected to the fifth rotation element N 5  through the second transfer gear TF 2 . 
     At this state, the rotation element of the first shaft IS 1  is directly input to the first rotation element N 1  by operation of the third clutch C 3  and is directly input to the second rotation element N 2  by operation of the fourth clutch C 4 . 
     Therefore, the rotation elements of the first planetary gear set PG 1  form a sixth speed line T 6  and the rotation elements of the second planetary gear set PG 2  form a sixth shift line SP 6  by cooperation of the first planetary gear set PG 1  and the second planetary gear set PG 2 . Therefore, D 6  is output through the fifth rotation element N 5  that is the output element. 
     Seventh Forward Speed 
     The third clutch C 3  that was operated at the sixth forward speed 6TH is released and the second clutch C 2  are operated at the seventh forward speed 7TH. 
     As shown in  FIG. 3 , the first rotation element N 1  is connected to the fourth rotation element N 4  through the first transfer gear TF 1 , and the third rotation element N 3  is connected to the fifth rotation element N 5  through the second transfer gear TF 2 . 
     At this state, the rotation speed of the first shaft IS 1  is converted according to the gear ratio of the third transfer gear TF 3  and is then input to the fourth rotation element N 4  by operation of the second clutch C 2 . In addition, the rotation speed of the first shaft IS 1  is directly input to the second rotation element N 2  by operation of the fourth clutch C 4 . 
     Therefore, the rotation elements of the first planetary gear set PG 1  form a seventh speed line T 7  and the rotation elements of the second planetary gear set PG 2  form a seventh shift line SP 7  by cooperation of the first planetary gear set PG 1  and the second planetary gear set PG 2 . Therefore, D 7  is output through the fifth rotation element N 5  that is the output element. 
     Eighth Forward Speed 
     The second clutch C 2  that was operated at the seventh forward speed 7TH is released and the second brake B 2  is operated at the eighth forward speed 8TH. 
     As shown in  FIG. 3 , the first rotation element N 1  is connected to the fourth rotation element N 4  through the first transfer gear TF 1 , and the third rotation element N 3  is connected to the fifth rotation element N 5  through the second transfer gear TF 2 . 
     At this state, the rotation speed of the first shaft IS 1  is directly input to the second rotation element N 2  by operation of the fourth clutch C 4  and the fourth rotation element N 4  is operated as the fixed element by operation of the second brake B 2 . 
     Therefore, the rotation elements of the first planetary gear set PG 1  form an eighth speed line T 8  and the rotation elements of the second planetary gear set PG 2  form an the eighth shift line SP 8  by cooperation of the first planetary gear set PG 1  and the second planetary gear set PG 2 . Therefore, D 8  is output through the fifth rotation element N 5  that is the output element. 
     First Reverse Speed 
     As shown in  FIG. 2 , the second clutch C 2  and the first brake B 1  are operated at the first reverse speed Rev 1 . 
     As shown in  FIG. 3 , the first rotation element N 1  is connected to the fourth rotation element N 4  through the first transfer gear TF 1 , and the third rotation element N 3  is connected to the fifth rotation element N 5  through the second transfer gear TF 2 . 
     At this state, the rotation speed of the first shaft IS 1  is converted according to the gear ratio of the third transfer gear TF 3  and is then input to the fourth rotation element N 4  by operation of the second clutch C 2 . In addition, the second rotation element N 2  is operated as the fixed element by operation of the first brake B 1 . 
     Therefore, the rotation elements of the first planetary gear set PG 1  form a first reverse speed line Tr 1  and the rotation elements of the second planetary gear set PG 2  form a first reverse shift line RS 1  by cooperation of the first planetary gear set PG 1  and the second planetary gear set PG 2 . Therefore, REV 1  is output through the fifth rotation element N 5  that is the output element. 
     Second Reverse Speeds 
     As shown in  FIG. 2 , the third clutch C 3  and the first brake B 1  are operated at the second reverse speed Rev 2 . 
     As shown in  FIG. 3 , the first rotation element N 1  is connected to the fourth rotation element N 4  through the first transfer gear TF 1 , and the third rotation element N 3  is connected to the fifth rotation element N 5  through the second transfer gear TF 2 . 
     At this state, the rotation speed of the first shaft IS 1  is directly input to the first rotation element N 1  by operation of the third clutch C 3 , and the second rotation element N 2  is operated as the fixed element by operation of the first brake B 1 . 
     Therefore, the rotation elements of the first planetary gear set PG 1  form a second reverse speed line Tr 2  and the rotation elements of the second planetary gear set PG 2  form a second reverse shift line RS 2  by cooperation of the first planetary gear set PG 1  and the second planetary gear set PG 2 . Therefore, REV 2  is output through the fifth rotation element N 5  that is the output element. 
     As described above, the planetary gear train of  FIG. 1  can achieve eight forward speeds and two reverse speeds by combining two planetary gear sets PG 1  and PG 2  being the simple planetary gear sets, four transfer gears TF 1 , TF 2 , TF 3 , and TF 4  being the externally-meshed gears and six frictional elements C 1 , C 2 , C 3 , C 4 , B 1 , and B 2 . 
     In addition, optimum gear ratios may be set due to ease of changing gear ratios by using four transfer gears being externally-meshed gears as well as the planetary gear sets. Since gear ratios can be changed according to target performance, starting performance, power delivery performance and fuel economy may be improved. Therefore, a start-up clutch instead of a torque converter may be used. 
     In addition, reverse speed performance may be improved by achieving two reverse speeds. 
     In addition, two friction elements are operated at each shift-speed and one friction element is released and another friction element is operated so as to shift to a neighboring shift-speed. Therefore, shift control condition is fully satisfied. 
       FIG. 4  is a schematic diagram of a planetary gear train according to various embodiments of the present invention. 
     Referring to  FIG. 4 , the first clutch C 1  is disposed between the fourth transfer gear TF 4  and the sixth rotation element N 6  in the planetary gear train of  FIG. 1 , but the first clutch C 1  is disposed between the first shaft IS 1  and the fourth transfer gear TF 4  in the planetary gear train of  FIG. 4 . 
     Since functions of the planetary gear train of  FIG. 4  are the same as those of the planetary gear train of  FIG. 1  except the position of the first clutch C 1 , detailed description thereof will be omitted. 
       FIG. 5  is a schematic diagram of a planetary gear train according to various embodiments of the present invention. 
     Referring to  FIG. 5 , the second clutch C 2  is disposed between the third transfer gear TF 3  and the fourth rotation element N 4  in the planetary gear train of  FIG. 1 , but the second clutch C 2  is disposed between the first shaft IS 1  and the third transfer gear TF 3  in the planetary gear train of  FIG. 5 . 
     Since functions of the planetary gear train of  FIG. 5  are the same as those of the planetary gear train of  FIG. 1  except the position of the second clutch C 2 , detailed description thereof will be omitted. 
       FIG. 6  is a schematic diagram of a planetary gear train according to various embodiments of the present invention. 
     Referring to  FIG. 6 , the second brake B 2  is disposed between the fourth rotation element N 4  and the transmission housing H in the planetary gear train of  FIG. 1 , but the second brake B 2  is disposed between the first rotation element N 1  and the transmission housing H in the planetary gear train of  FIG. 6 . 
     Since functions of the planetary gear train of  FIG. 6  are the same as those of the planetary gear train of  FIG. 1  except the position of the second brake B 2 , detailed description thereof will be omitted. 
       FIG. 7  is a schematic diagram of a planetary gear train according to various embodiments of the present invention. 
     Referring to  FIG. 7 , the first planetary gear set PG 1  is the single pinion planetary gear set in the planetary gear train of  FIG. 1 , but the first planetary gear set PG 1  is a double pinion planetary gear set in the planetary gear train of  FIG. 7 . 
     Therefore, the second rotation element N 2  is changed from the first planet carrier PC 1  to the first ring gear R 1 , and the third rotation element N 3  is changed from the first ring gear R 1  to the first planet carrier PC 1 . 
     Since functions of the planetary gear train of  FIG. 7  are the same as those of the planetary gear train of  FIG. 1  except the rotation elements consisting of the second and third rotation elements N 2  and N 3 , detailed description thereof will be omitted. 
     Meanwhile, it is illustrated, but is not limited, in the drawings that the second shaft IS 2  merely supports the second planetary gear set PG 2  without rotational interference therebetween. That is, the second shaft IS 2  may be used as an output shaft by directly connecting the fifth rotation element N 5  being the output element and the output gear OG to the second shaft IS 2 . 
     Eight forward speeds and two reverse speeds can be achieved by combining two planetary gear set being the simple planetary gear sets, four transfer gears and six frictional elements. 
     In addition, since two planetary gear sets are disposed separately on the first shaft and the second shaft disposed in parallel with a predetermined distance, 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 four external-meshing gears as well as the planetary gear sets. Since gear ratios can be changed according to target performance, starting performance, power delivery performance, and fuel economy may be improved. Therefore, a start-up clutch instead of a torque converter may be used. 
     In addition, reverse speed performance may be improved by achieving two reverse speeds. 
     In addition, two friction elements are operated at each shift-speed, and one friction element is released and another friction element is operated so as to shift to a neighboring shift-speed. Therefore, shift control condition is fully satisfied. 
     For convenience in explanation and accurate definition in the appended claims, the terms upper or lower, 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.