Patent Publication Number: US-9897176-B2

Title: Planetary gear train of automatic transmission for vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority to Korean Patent Application No. 10-2016-0013649, filed Feb. 3, 2016, the entire contents of which is incorporated herein for all purposes by this reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an automatic transmission for a vehicle. 
     Description of Related Art 
     Recent increases in oil prices are triggering stiff competition in enhancing fuel consumption of a vehicle. 
     In this sense, research on an automatic transmission has been performed to simultaneously provide better drivability and fuel consumption by achieving more shift stages. 
     However, in order to achieve more shift stages for an automatic transmission, the number of parts is typically increased, which may deteriorate installability and/or power flow efficiency and may increase production cost, and weight. 
     Therefore, in order to maximally enhance fuel consumption of an automatic transmission having more shift stages, it is important for better efficiency to be derived by a smaller number of parts. 
     In this respect, an eight-speed automatic transmission has been recently introduced, and a planetary gear train for an automatic transmission enabling more shift stages is under investigation. 
     An automatic transmission of eight or more shift-stages typically includes three to four planetary gear sets and five to six control elements (frictional elements), and may easily become lengthy, thereby deteriorating installability. 
     In this regard, disposing planetary gear sets in parallel or employing dog clutches instead of wet-type control elements has been attempted. However, such an arrangement is not widely applicable, and using dog clutches may easily deteriorate shift-feel. 
     In addition, a recent eight-speed automatic transmission typically shows a gear ratio span in a level of 6.5 to 7.5, which requires improvement for better fuel efficiency. 
     Thus, shortening a length of an automatic transmission without deteriorating performance will be beneficial. 
     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 realizing at least nine forward speeds and at least one reverse speed by a combination of four planetary gear sets, two external gears and six control elements, thereby providing improvement of power delivery performance and fuel consumption and improving installability by reducing a length. 
     In addition, various aspects of the present invention are directed to providing a planetary gear train of an automatic transmission for a vehicle having a wide available range of varying gear teeth of transfer gears enabling easy obtaining of optimum gear ratios for respective vehicles, thereby improving power delivery performance and fuel consumption. 
     According to various aspects of the present invention, a planetary gear train of an automatic transmission for a vehicle may include a first planetary gear set including first, second, and third rotation elements, a second planetary gear set including fourth, fifth, and sixth rotation elements, a third planetary gear set including seventh, eighth, and ninth rotation elements, a fourth planetary gear set including tenth, eleventh, and twelfth rotation elements, an input shaft mounted with the first, second, and third planetary gear sets at an external circumference of the input shaft, an output shaft disposed in parallel with the input shaft and mounted with the fourth planetary gear set on an external circumference of the output shaft, a first shaft connected with the first rotation element and directly connected with the input shaft, a second shaft interconnecting the second rotation element and the sixth rotation element, a third shaft connected with the third rotation element, a fourth shaft connected with the fourth rotation element, a fifth shaft interconnecting the fifth rotation element and the eighth rotation element, a sixth shaft connected with the seventh rotation element, a seventh shaft connected with the ninth rotation element, an eighth shaft connected with the tenth rotation element, and gear-meshed with one shaft among the fourth and seventh shafts, a ninth shaft connected with the eleventh rotation element and directly connected with the output shaft, a tenth shaft connected with the twelfth rotation element, and gear-meshed with a remaining shaft among the fourth and seventh shafts that is not externally gear-meshed with the eighth shaft, and transfer gears, each gear-meshed with at least one of the shafts. 
     The second shaft, the third shaft, and the fourth shaft may each be selectively connectable with a transmission housing, the fifth shaft may be selectively connectable with the input shaft, and the sixth shaft may be selectively connectable with each of the input shaft and the third shaft. 
     The first planetary gear may include a first sun gear as the first rotation element, a first ring gear as the second rotation element, and a first planet carrier as the third rotation element, the second planetary gear set may include a second sun gear as the fourth rotation element, a second planet carrier as the fifth rotation element, and a second ring gear as the sixth rotation element, the third planetary gear set may include a third sun gear as the seventh rotation element, a third planet carrier as the eighth rotation element, and a third ring gear as the ninth rotation element, and the fourth planetary gear set may include a fourth sun gear as the tenth rotation element, a fourth planet carrier as the eleventh rotation element, and a fourth ring gear as the twelfth rotation element. 
     The eighth shaft may be gear-meshed with the fourth shaft and selectively connectable with the transmission housing, and the tenth shaft may be gear-meshed with the seventh shaft. 
     The transfer gears may include a first transfer gear disposed between the fourth shaft and the eighth shaft, and a second transfer gear disposed between the seventh shaft and the tenth shaft. 
     The planetary gear train may further include a first clutch disposed between the fifth shaft and the input shaft, a second clutch disposed between the sixth shaft and the input shaft, a third clutch disposed between the third shaft and the sixth shaft, a first brake disposed between the second shaft and the transmission housing, a second brake disposed between the fourth shaft and the transmission housing, and a third brake disposed between the third shaft and the transmission housing. 
     The planetary gear train may further include a first clutch disposed between the fifth shaft and the input shaft, a second clutch disposed between the sixth shaft and the input shaft, a third clutch disposed between the third shaft and the sixth shaft, a first brake disposed between the second shaft and the transmission housing, a second brake disposed between the eighth shaft and the transmission housing, and a third brake disposed between the third shaft and the transmission housing. 
     The eighth shaft may be gear-meshed with the seventh shaft, and the tenth shaft may be gear-meshed with the fourth shaft and selectively connectable with the transmission housing. 
     The transfer gears may include a first transfer gear disposed between the fourth shaft and the tenth shaft, and a second transfer gear disposed between the seventh shaft and the eighth shaft. 
     The planetary gear train may further include a first clutch disposed between the fifth shaft and the input shaft, a second clutch disposed between the sixth shaft and the input shaft, a third clutch disposed between the third shaft and the sixth shaft, a first brake disposed between the second shaft and the transmission housing, a second brake disposed between the fourth shaft and the transmission housing, and a third brake disposed between the third shaft and the transmission housing. 
     According to various aspects of the present invention, a planetary gear train of an automatic transmission for a vehicle may include a first planetary gear set including first, second, and third rotation elements, a second planetary gear set including fourth, fifth, and sixth rotation elements, a third planetary gear set including seventh, eighth, and ninth rotation elements, a fourth planetary gear set including tenth, eleventh, and twelfth rotation elements, an input shaft mounted with the first, second, and third planetary gear sets at an external circumference of the input shaft, and an output shaft disposed in parallel with the input shaft and mounted with the fourth planetary gear set on an external circumference of the output shaft, in which the first rotation element may be directly connected with the input shaft, the second and sixth rotation elements are directly connected together and selectively connectable with a transmission housing, the third rotation element may be selectively connectable with the transmission housing, the fourth rotation element may be gear-meshed with one rotation element among the tenth and twelfth rotation elements, and selectively acts as a fixed element, the fifth and eighth rotation elements may be directly connected together and selectively connectable with the input shaft, the seventh rotation element may be selectively connectable with the input shaft and the third rotation element, the ninth rotation element may be gear-meshed with a remaining rotation element among the tenth and twelfth rotation elements that is not gear-meshed with the fourth rotation element, and the eleventh rotation element may be directly connected with the output shaft. 
     The fourth rotation element may be gear-meshed with the tenth rotation element through a first transfer gear, and the seventh rotation element may be gear-meshed with the twelfth rotation element through a second transfer gear. 
     The planetary gear train may further include a first clutch selectively connecting the eighth rotation element and the input shaft, a second clutch selectively connecting the seventh rotation element and the input shaft, a third clutch selectively connecting the third rotation element and the seventh rotation element, a first brake selectively connecting the second rotation element with the transmission housing, a second brake selectively connecting the fourth rotation element with the transmission housing, and a third brake selectively connecting the third rotation element with the transmission housing. 
     The planetary gear train may further include a first clutch selectively connecting the eighth rotation element and the input shaft, a second clutch selectively connecting the seventh rotation element and the input shaft, a third clutch selectively connecting the third rotation element and the seventh rotation element, a first brake selectively connecting the second rotation element with the transmission housing, a second brake selectively connecting the tenth rotation element with the transmission housing, and a third brake selectively connecting the third rotation element with the transmission housing. 
     The fourth rotation element may be gear-meshed with the twelfth rotation element through the first transfer gear, and the seventh rotation element may be gear-meshed with the tenth rotation element through the second transfer gear. 
     According to a planetary gear train of various embodiments of the present invention, planetary gear sets are dividedly arranged on input and output shafts disposed in parallel, thereby reducing a length and improving installability. 
     According to a planetary gear train of various embodiments of the present invention, at least eight forward speeds and at least one reverse speed are realized by employing two transfer gears in addition to a combination of planetary gear sets, thereby providing a wide range of varying gear teeth so as to easily achieve optimum gear ratio and to easily comply with required performance for respective vehicles. 
     In addition, according to a planetary gear train of various embodiments of the present invention, a gear ratio span of more than 9.6 is achieved while realizing at least nine forward speeds and at least one reverse speed, thereby maximizing an engine driving efficiency. 
     Furthermore, the linearity of step ratios of shift stages is secured while multi-staging the shift stages with high efficiency, thereby making it possible to improve drivability such as acceleration before and after a shift, an engine speed rhythmic sense, and the like. 
     It is understood that the term “vehicle” or “vehicular” or other similar terms as used herein is inclusive of motor vehicles in general such as passenger automobiles including sports utility vehicles (SUV), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, and the like, and includes hybrid vehicles, electric vehicles, plug-in hybrid electric vehicles, hydrogen-powered vehicles and other alternative fuel vehicles (e.g., fuel derived from resources other than petroleum). As referred to herein, a hybrid vehicle is a vehicle that has two or more sources of power, for example, both gasoline-powered and electric-powered vehicles. 
     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 various embodiments of the present invention. 
         FIG. 2  is an operational chart for respective control elements at respective shift-stages in a planetary gear train according to various embodiments of the present invention. 
         FIG. 3  is a schematic diagram of a planetary gear train according to various embodiments of the present invention. 
         FIG. 4  is a schematic diagram of a planetary gear train according to various embodiments 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. 
     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. 
       FIG. 1  is a schematic diagram of a planetary gear train according to a first exemplary embodiment of the present invention. 
     Referring to  FIG. 1 , a planetary gear train according to a first exemplary embodiment of the present invention includes an input shaft IS, an output shaft OS, first, second, third, and fourth planetary gear sets PG 1 , PG 2 , PG 3 , and PG 4 , two transfer gears TF 1  and TF 2  and control elements of three clutches C 1 , C 2 , and C 3  and three brakes B 1 , B 2 , and B 3 . 
     The input shaft IS is an input member and the torque from a crankshaft of an engine is input into the input shaft IS, after being torque-converted through a torque converter. 
     The output shaft OS is an output member, and, arranged in parallel with the input shaft IS, outputs a shifted driving torque to a drive shaft through a differential apparatus. 
     The first, second, and third planetary gear sets PG 1 , PG 2 , and PG 3  are arranged at external circumference of the input shaft IS and form a main shifting portion. The fourth planetary gear set PG 4  is arranged at external circumference of the output shaft OS disposed in parallel with the input shaft IS and forms an auxiliary shifting portion. 
     The first planetary gear set PG 1  is a double pinion planetary gear set, and includes a first sun gear S 1 , a first ring gear R 1  internally gear-meshed with the first pinion P 1  externally gear-meshed with the first sun gear S 1 , and a first planet carrier PC 1  supporting the first pinion P 1 . The first sun gear S 1  acts as a first rotation element N 1 , the first ring gear R 1  acts as a second rotation element N 2 , and the first planet carrier PC 1  acts as 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 , a second planet carrier PC 2  that supports second pinion P 2  externally engaged with the second sun gear S 2 , and a second ring gear R 2  that is internally engaged with the second pinion P 2 . The second sun gear S 2  acts as a fourth rotation element N 4 , the second planet carrier PC 2  acts as a fifth rotation element N 5 , and the second ring gear R 2  acts as a sixth rotation element N 6 . 
     The third planetary gear set PG 3  is a single pinion planetary gear set, and includes a third sun gear S 3 , a third planet carrier PC 3  that supports third pinion P 3  externally engaged with the third sun gear S 3 , and a third ring gear R 3  that is internally engaged with the third pinion P 3 . The third sun gear S 3  acts as a seventh rotation element N 7 , the third planet carrier PC 3  acts as an eighth rotation element N 8 , and the third ring gear R 3  acts as a ninth rotation element N 9 . 
     The fourth planetary gear set PG 4  is a single pinion planetary gear set, and includes a fourth sun gear S 4 , a fourth planet carrier PC 4  that supports fourth pinion P 4  externally engaged with the fourth sun gear S 4 , and a fourth ring gear R 4  that is internally engaged with the fourth pinion P 4 . The fourth sun gear S 4  acts as a tenth rotation element N 10 , the fourth planet carrier PC 4  acts as a eleventh rotation element N 11 , and the fourth ring gear R 4  acts as a twelfth rotation element N 12 . 
     In the arrangement of the first, second, and third planetary gear sets PG 1 , PG 2 , and PG 3 , the second rotation element N 2  is directly connected with the sixth rotation element N 6 , the fifth rotation element N 5  is directly connected with the eighth rotation element N 8 , by seven shafts TM 1  to TM 7 . 
     Three shafts TM 8  to TM 10  are connected to the fourth planetary gear set PG 4 . 
     The ten shafts TM 1  to TM 10  are hereinafter described in detail. 
     The first shaft TM 1  is connected with the first rotation element N 1  (first sun gear S 1 ), and directly connected with the input shaft IS, thereby continuously acting as an input element. 
     The second shaft TM 2  interconnects the second rotation element N 2  (first ring gear R 1 ) and the sixth rotation element N 6  (second ring gear R 2 ), and is selectively connectable with the transmission housing H, thereby selectively acting as a fixed element. 
     The third shaft TM 3  is connected with the third rotation element N 3  (first planet carrier PC 1 ), and selectively connectable with the transmission housing H, thereby selectively acting as a fixed element. 
     The fourth shaft TM 4  is connected with the fourth rotation element N 4  (second sun gear S 2 ), and selectively connectable with the transmission housing H, thereby selectively acting as a fixed element. 
     The fifth shaft TM 5  interconnects the fifth rotation element N 5  (second planet carrier PC 2 ) and the eighth rotation element N 8  (third planet carrier PC 3 ), and is selectively connectable with the input shaft IS, thereby selectively acting as an input element. 
     The sixth shaft TM 6  is connected with the seventh rotation element N 7  (third sun gear S 3 ), selectively connectable with the input shaft IS thereby selectively acting as an input element, and selectively connectable with the third shaft N 3 . 
     The seventh shaft TM 7  is connected with ninth rotation element N 9  (third ring gear R 3 ). 
     The eighth shaft TM 8  is connected with the tenth rotation element N 10  (fourth sun gear S 4 ), and externally gear-meshed with the fourth shaft TM 4 . 
     The ninth shaft TM 9  is connected with the eleventh rotation element N 11  (fourth planet carrier PC 4 ), and directly connected with the output shaft OS thereby continuously acting as an output element. 
     The tenth shaft TM 10  is connected with the twelfth rotation element N 12  (fourth ring gear R 4 ), and externally gear-meshed with the seventh shaft TM 7 . 
     The two transfer gears TF 1  and TF 2  deliver a shifted torque of the main shifting portion having the first, second, and third planetary gear sets PG 1 , PG 2 , and PG 3  to the auxiliary shifting portion having the fourth planetary gear set PG 4 , in a reverse rotation. 
     The first transfer gear TF 1  includes a first transfer gear TF 1   a  directly connected with the fourth shaft TM 4  and a first transfer gear TF 1   b  directly connected with the eighth shaft TM 8 , and externally gear-meshes the fourth shaft TM 4  and the eighth shaft TM 8 . 
     The second transfer gear TF 2  includes a second transfer gear TF 2   a  directly connected with the seventh shaft TM 7  and a second transfer gear TF 2   b  directly connected with the tenth shaft TM 10 , and externally gear-meshes the seventh shaft TM 7  and the tenth shaft TM 10 . 
     As a result, respective shafts connected by the first and second transfer gears TF 1  and TF 2  rotate in opposite directions, and the gear ratios of the first and second transfer gears TF 1  and TF 2  may be preset in consideration of required speed ratio of the transmission. 
     Control elements of the three clutches C 1 , C 2 , and C 3  and the three brakes B 1 , B 2 , and B 3  are arranged as follows. 
     The first clutch C 1  is arranged between the input shaft IS and the fifth shaft TM 5 , such that the input shaft IS and the fifth shaft TM 5  may selectively become integral. 
     The second clutch C 2  is arranged between the input shaft IS and the sixth shaft TM 6 , such that the input shaft IS and the sixth shaft TM 6  may selectively become integral. 
     The third clutch C 3  is arranged between the third shaft TM 3  and the sixth shaft TM 6 , such that the third shaft TM 3  and the sixth shaft TM 6  may selectively become integral. 
     The first brake B 1  is arranged between the second shaft TM 2  and the transmission housing H, such that the second shaft TM 2  may selectively act as a fixed element. 
     The second brake B 2  is arranged between the fourth shaft TM 4  and the transmission housing H, such that the fourth shaft TM 4  may selectively act as a fixed element. 
     The third brake B 3  is arranged between the third shaft TM 3  and the transmission housing H, such that the third shaft TM 3  may selectively act as a fixed element. 
     The control elements of the first, second, and third clutch C 1 , C 2 , and C 3  and the first, second, third brake B 1 , B 2 , and B 3  may be realized as multi-plate hydraulic pressure friction devices that are frictionally engaged by hydraulic pressure. 
       FIG. 2  is an operational chart for respective control elements at respective shift-stages in a planetary gear train according to the first exemplary embodiment of the present invention. 
     Referring to  FIG. 2 , a planetary gear train according to an exemplary embodiment of the present invention realizes shifting by operating three control elements among the three clutches C 1 , C 2 , and C 3  and the three brakes B 1 , B 2 , and B 3 . 
     [The Forward First Speed] 
     In the forward first speed shift-stage D 1 , the third clutch C 3  and the first and second brakes B 1  and B 2  are operated. 
     As a result, the third shaft TM 3  is connected with the sixth shaft TM 6  by the operation of the third clutch C 3 . In this state, the torque of the input shaft IS is input to the first shaft TM 1 , and the second shaft TM 2  and the fourth shaft TM 4  act as fixed elements by the operation of the first brake B 1  and the second brake B 2 , thereby realizing the forward first speed by cooperative operation of respective shafts and outputting a shifted torque to the output shaft OS connected with the ninth shaft TM 9 . 
     [The Forward Second Speed] 
     In the forward second speed shift-stage D 2 , the third clutch C 3  and the second and third brakes B 2  and B 3  are operated. 
     As a result, the third shaft TM 3  is connected with the sixth shaft TM 6  by the operation of the third clutch C 3 . In this state, the torque of the input shaft IS is input to the first shaft TM 1 , and the fourth shaft TM 3  and the third shaft TM 3  act as fixed elements by the operation of the second brake B 2  and the third brake B 3 , thereby realizing the forward second speed by cooperative operation of respective shafts and outputting a shifted torque to the output shaft OS connected with the ninth shaft TM 9 . 
     [The Forward Third Speed] 
     In the forward third speed shift-stage D 3 , the second and third clutches C 2  and C 3  and the second brake B 2  are operated. 
     As a result, the sixth shaft TM 6  is connected with the input shaft IS by the operation of the second clutch C 2 , and the third shaft TM 3  is connected with the sixth shaft TM 6  by the operation of the third clutch C 3 . In this state, the torque of the input shaft IS is input to the first, third shaft TM 1  and TM 3 , and the fourth shaft TM 4  acts as a fixed element by the operation of the second brake B 2 , thereby realizing the forward third speed by cooperative operation of respective shafts and outputting a shifted torque to the output shaft OS connected with the ninth shaft TM 9 . 
     [The Forward Fourth Speed] 
     In the forward fourth speed shift-stage D 4 , the first and third clutches C 1  and C 3  and the second brake B 2  are operated. 
     As a result, the fifth shaft TM 5  is connected with the input shaft IS by the operation of the first clutch C 1 , and the third shaft TM 3  is connected with the sixth shaft TM 6  by the operation of the third clutch C 3 . In this state, the torque of the input shaft IS is input to the first, fifth shaft TM 1  and TM 5 , and the fourth shaft TM 4  acts as a fixed element by the operation of the second brake B 2 , thereby realizing the forward fourth speed by cooperative operation of respective shafts and outputting a shifted torque to the output shaft OS connected with the ninth shaft TM 9 . 
     [The Forward Fifth Speed] 
     In the forward fifth speed shift-stage D 5 , the first and second clutch C 1  and C 2  and the second brake B 2  are operated. 
     As a result, the fifth shaft TM 5  is connected with the input shaft IS by the operation of the first clutch C 1 , and the sixth shaft TM 6  is connected with the input shaft IS by the operation of the second clutch C 2 . In this state, the torque of the input shaft IS is input to the first, fifth, sixth shaft TM 1 , TM 5 , and TM 6 , and the fourth shaft TM 4  acts as a fixed element by the operation of the second brake B 2 , thereby realizing the forward fifth speed by cooperative operation of respective shafts and outputting a shifted torque to the output shaft OS connected with the ninth shaft TM 9 . 
     [The Forward Sixth Speed] 
     In the forward sixth speed shift-stage D 6 , the first, second, and third clutches C 1 , C 2 , and C 3  are operated. 
     As a result, the fifth shaft TM 5  is connected with the input shaft IS by the operation of the first clutch C 1 , the sixth shaft TM 6  is connected with the input shaft IS by the operation of the second clutch C 2 , and the third shaft TM 3  is connected with the sixth shaft TM 6  by the operation of the third clutch C 3 . In this state, the torque of the input shaft IS is input to the first, fifth, and sixth shafts TM 1 , TM 5 , and TM 6 , thereby realizing the forward sixth speed by cooperative operation of respective shafts and outputting a shifted torque to the output shaft OS connected with the eighth shaft TM 8 .
         [the forward seventh speed]       

     In the forward seventh speed shift-stage D 7 , the first and second clutch C 1  and C 2  and the third brake B 3  are operated. 
     As a result, the fifth shaft TM 5  is connected with the input shaft IS by the operation of the first clutch C 1 , and the sixth shaft TM 6  is connected with the input shaft IS by the operation of the second clutch C 2 . In this state, the torque of the input shaft IS is input to the first, fifth, sixth shaft TM 1 , TM 5 , and TM 6 , and the third shaft TM 3  acts as a fixed element by the operation of the third brake B 3 , thereby realizing the forward seventh speed by cooperative operation of respective shafts and outputting a shifted torque to the output shaft OS connected with the ninth shaft TM 9 . 
     [The Forward Eighth Speed] 
     In the forward eighth speed shift-stage D 8 , the first and third clutches C 1  and C 3  and the third brake B 3  are operated. 
     As a result, the fifth shaft TM 5  is connected with the input shaft IS by the operation of the first clutch C 1 , and the third shaft TM 3  is connected with the sixth shaft TM 6  by the operation of the third clutch C 3 . In this state, the torque of the input shaft IS is input to the first, fifth shaft TM 1  and TM 5 , and the third shaft TM 3  acts as a fixed element by the operation of the third brake B 3 , thereby realizing the forward eighth speed by cooperative operation of respective shafts and outputting a shifted torque to the output shaft OS connected with the ninth shaft TM 9 . 
     [The Forward Ninth Speed] 
     In the forward ninth speed shift-stage D 9 , the first and third clutches C 1  and C 3  and the first brake B 1  are operated. 
     As a result, the fifth shaft TM 5  is connected with the input shaft IS by the operation of the first clutch C 1 , and the third shaft TM 3  is connected with the sixth shaft TM 6  by the operation of the third clutch C 3 . In this state, the torque of the input shaft IS is input to the first, fifth shaft TM 1  and TM 5 , and the second shaft TM 2  acts as a fixed element by the operation of the first brake B 1 , thereby realizing the forward ninth speed by cooperative operation of respective shafts and outputting a shifted torque to the output shaft OS connected with the ninth shaft TM 9 . 
     [The Reverse Speed] 
     In the reverse speed REV, the second clutch C 2  and the first and second brakes B 1  and B 2  are operated. 
     As a result, the sixth shaft TM 6  is connected with the input shaft IS by the operation of the second clutch C 2 . In this state, the torque of the input shaft IS is input to the first, sixth shaft TM 1  and TM 6 , and the second shaft TM 2  and the fourth shaft TM 4  act as fixed elements by the operation of the first brake B 1  and the second brake B 2 , thereby realizing the reverse speed by cooperative operation of respective shafts and outputting a shifted torque to the output shaft OS connected with the ninth shaft TM 9 . 
       FIG. 2  shows gear ratios calculated under the condition that the gear ratio of the first ring gear R 1 /the first sun gear S 1  is 2.20, the gear ratio of the second ring gear R 2 /the second sun gear S 2  is 2.11, the gear ratio of the third ring gear R 3 /the third sun gear S 3  is 3.42, the gear ratio of the fourth ring gear R 4 /the fourth sun gear S 4  is 3.83, the gear ratio of the first transfer driven gear TF 1   b /the first transfer drive gear TF 1   a  is 1.00, and the gear ratio of the second transfer driven gear TF 2   b /the second transfer drive gear TF 2   a  is 1.00. 
       FIG. 3  is a schematic diagram of a planetary gear train according to a second exemplary embodiment of the present invention. 
     In a planetary gear train according to the first exemplary embodiment of the present invention, the second brake B 2  is arranged between the fourth shaft TM 4  and the transmission housing H, as shown in  FIG. 1 . However, referring to  FIG. 3 , the second brake B 2  is arranged between the eighth shaft TM 8  and the transmission housing H according to a planetary gear train according to the second exemplary embodiment. 
     That is, in the planetary gear train according to the first exemplary embodiment, the fourth shaft TM 4  acts as a fixed element by the operation of the second brake B 2 , and the eighth shaft TM 8  externally gear-meshed with the fourth shaft TM 4  simultaneously acts as a fixed element. In comparison, in a planetary gear train according to the second exemplary embodiment, the eighth shaft TM 8  acts as a fixed element by the operation of the second brake B 2 , and the fourth shaft TM 4  externally gear-meshed with the eighth shaft TM 8  simultaneously acts as a fixed element. Thus, only a location of the second brake B 2  differs between the first and second embodiment, and operation and function remains the same, which is therefore not described in further detail. 
       FIG. 4  is a schematic diagram of a planetary gear train according to a third exemplary embodiment of the present invention. 
     In a planetary gear train according to the first exemplary embodiment of the present invention, the fourth shaft TM 4  is externally gear-meshed with the eighth shaft TM 8  through the first transfer gear TF 1 , and the seventh shaft TM 7  is externally gear-meshed with the tenth shaft TM 10  through second transfer gear TF 2 , as shown in  FIG. 1 . However, referring to  FIG. 4 , the fourth shaft TM 4  is externally gear-meshed with the tenth shaft TM 10  through the first transfer gear TF 1 , and the seventh shaft TM 7  is externally gear-meshed with the eighth shaft TM 8  through the second transfer gear TF 2 , according to the third exemplary embodiment. 
     Thus, the third exemplary embodiment differs from the first exemplary embodiment in that rotation speeds outputted through the fourth shaft TM 4  and the seventh shaft TM 7  may be different. However, it may be easily understood that only speed ratios of the auxiliary shifting portion and the operation and shift-pattern remains the same, which is therefore not described in further detail. 
     As described above, according to a planetary gear train of various embodiments of the present invention, at least nine forward speeds and at least one reverse speed are realized by a combination of four planetary gear sets, two transfer gears, and six control elements, thereby providing improvement of power delivery performance and fuel consumption and improving installability by shortening the length of an automatic transmission. 
     In addition, according to a planetary gear train of various embodiments of the present invention, two transfer gears of external gears arranged on the output shaft OS are employed in addition to three planetary gear sets, and thus, gear teeth may be widely varied so as to easily achieve optimum gear ratio and to easily comply with required performance for respective vehicles. 
     Furthermore, according to a planetary gear train of various embodiments of the present invention, a gear ratio span of more than 9.6 is achieved while realizing at least nine forward speeds and at least one reverse speed, thereby maximizing an engine driving efficiency. 
     Additionally, the linearity of step ratios of shift stages is secured while multi-staging the shift stages with high efficiency, thereby making it possible to improve drivability such as acceleration before and after a shift, an engine speed rhythmic sense, and the like. 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. 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.