Abstract:
A planetary gear train of an automatic transmission includes an input shaft, an output shaft, and first to fourth planetary gear sets respectively including first to third, fourth to sixth, seventh to ninth, and tenth to twelfth rotating elements, wherein the input shaft and the second rotating element, the output shaft and the eleventh rotating element, the first and sixth rotating elements, the first and eighth rotating elements, the seventh and eleventh rotating elements, and the ninth and tenth rotating elements are fixedly interconnected, and the output shaft is selectively connected with the third rotating element, and achieves ten forward speeds and one reverse speed by selectively operating three control elements among seven control elements.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority to Korean Patent Application Number 10-2015-0084256 filed Jun. 15, 2015, 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, and more particularly, to a planetary gear train of an automatic transmission for a vehicle which implements forward at least 10-speed and increases an entire gear ratio by a minimum configuration to improve power transmission performance and fuel efficiency and secure linearity of step ratios between transmission stages. 
     Description of Related Art 
     Recently, increasing oil prices have caused vehicle manufacturers all over the world to rush into infinite competition. Particularly in the case of engines, manufacturers have been pursuing efforts to reduce the weight and improve fuel efficiency of vehicles by reducing engine size, etc. 
     As a result, research into reduction of weight and enhancement of fuel efficiency through downsizing has been conducted in the case of an engine and research for simultaneously securing operability and fuel efficiency competitiveness through multiple speed stages has been conducted in the case of an automatic transmission. 
     However, in the automatic transmission, as the number of transmission speed stages increases, the number of internal components also increases, and as a result, the automatic transmission may be difficult to mount, the manufacturing cost and weight may be increased, and power transmission efficiency may be deteriorated. 
     Accordingly, development of a planetary gear train which may bring about maximum efficiency with a small number of components may be important in order to increase a fuel efficiency enhancement effect through the multiple speed stages. 
     In this aspect, in recent years, 8 and 9-speed automated transmissions tend to be implemented and the research and development of a planetary gear train capable of implementing more transmission steps has also been actively conducted. 
     However, improvement effect of fuel efficiency is not so good in case of 8-speed automated transmission, because an entire gear ratio is maintained in a level of 6.5˜7.5. 
     Therefore, development of at least 9-speed high efficiency automatic transmission is required in that operation efficiency of an engine and drivability of a vehicle are deteriorated because linearity of step ratios between transmission stages can&#39;t be secured in case of enabling an entire gear ratio of an 8-speed automated transmission to be a level more than or equal to 9.0. 
     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 which implements forward at least 10-speed and reverse 1-speed transmission steps with a minimum configuration, improves power transmission efficiency and fuel efficiency through increasing an entire gear ratio, and secures linearity of step ratios between transmission stages. 
     According to various aspects of the present invention, a planetary gear train of an automatic transmission for a vehicle may include an input shaft receiving rotary power of an engine, an output shaft outputting the rotary power with rotary speed changed, a first planetary gear set including a first, a second, and a third rotating element, a second planetary gear set including a fourth, a fifth, and a sixth rotating element, a third planetary gear set including a seventh, an eighth, and a ninth rotating element, a fourth planetary gear set including a tenth, a eleventh, and a twelfth rotating element, and seven control elements disposed between one of the rotating elements and another of the rotating elements or the input shaft, between one of the rotating elements and the output shaft, or between one of the rotating elements and a transmission housing, in which the input shaft may be directly connected with the second rotating element, the output shaft may be directly connected with the eleventh rotating element, the first rotating element may be directly connected with the sixth rotating element, the first rotating element may be directly connected with the eighth rotating element, the seventh rotating element may be directly connected with the eleventh rotating element, the ninth rotating element may be directly connected with the tenth rotating element, and the output shaft may be selectively connected with the third rotating element, and the planetary gear train may implement transmission steps of at least ten forward speeds and at least one reverse speed while three control elements operate among the seven control elements. 
     The fourth rotating element may be selectively connected with the transmission housing, the fifth rotating element may be selectively connected with the transmission housing, the twelfth rotating element may be selectively connected with the transmission housing, the input shaft may be selectively connected with the fourth rotating element and the fifth rotating element respectively, and the ninth rotating element may be selectively connected with the fifth rotating element. 
     The seven control elements may include a first clutch selectively connecting the output shaft and the third rotating element, a second clutch selectively connecting the input shaft and the fourth rotating element, a third clutch selectively connecting the input shaft and the fifth rotating element, a fourth clutch selectively connecting the ninth rotating element and the fifth rotating element, a first brake selectively connecting the fourth rotating element and the transmission housing, a second brake selectively connecting the fifth rotating element and the transmission housing, and a third brake selectively connecting the twelfth rotating element and the transmission housing. 
     The first, the second, and the third rotating element of the first planetary gear set may be a sun gear, a planetary carrier, and a ring gear respectively, the fourth, the fifth, and the sixth rotating element of the second planetary gear set are a sun gear, a planetary carrier, a ring gear respectively, the seventh, the eighth, and the ninth rotating element of the third planetary gear set are a sun gear, a planetary carrier, and a ring gear respectively, and the tenth, the eleventh, and the twelfth rotating element of the fourth planetary gear set are a sun gear, a planetary carrier, and a ring gear respectively. 
     According to various aspects of the present invention, a planetary gear train of an automatic transmission for a vehicle may include an input shaft receiving rotary power of an engine, an output shaft outputting the rotary power with rotary speed changed, a first planetary gear set including a first, a second, and a third rotating element, a second planetary gear set including a fourth, a fifth, and a sixth rotating element, a third planetary gear set including a seventh, an eighth, and a ninth rotating element, a fourth planetary gear set including a tenth, a eleventh, and a twelfth rotating element, seven control elements disposed between one of the rotating elements and another of the rotating elements or the input shaft, between one of the rotating elements and the output shaft, or between one of the rotating elements and a transmission housing, a first rotating shaft including the first rotating element, the sixth rotating element, and the eighth rotating element, a second rotating shaft including the second rotating element and directly connected with the input shaft, a third rotating shaft including the third rotating element, a fourth rotating shaft including the fourth rotating element and selectively connected with the second rotating shaft and the transmission housing, a fifth rotating shaft including the fifth rotating element and selectively connected with the second rotating shaft, a sixth rotating shaft including the seventh rotating element and the eleventh rotating element, directly connected with the output shaft, and selectively connected with the third rotating shaft, a seventh rotating shaft including the ninth rotating element and the tenth rotating element and selectively connected with the fifth rotating shaft, and an eighth rotating shaft including the twelfth rotating element and selectively connected with the transmission housing. 
     The first planetary gear set may be a single-pinion planetary gear set, the first rotating element may be a first sun gear, the second rotating element may be a first planetary carrier, and the third rotating element may be a first ring gear, the second planetary gear set may be a single-pinion planetary gear set, the fourth rotating element may be a second sun gear, the fifth rotating element may be a second planetary carrier, and the sixth rotating element may be a second ring gear, the third planetary gear set may be a single-pinion planetary gear set, the seventh rotating element may be a third sun gear, the eighth rotating element may be a third planetary carrier, and the ninth rotating element may be a third ring gear, and the fourth planetary gear set may be a single-pinion planetary gear set, the tenth rotating element may be a fourth sun gear, the eleventh rotating element may be a fourth planetary carrier, and the twelfth rotating element may be a fourth ring gear. 
     The four planetary gear sets may be disposed in an order of the second, the first, the third, and the fourth planetary gear set starting from an engine side. 
     Transmission stages implemented by selectively operating the six control elements may include a first forward transmission stage implemented by simultaneously operating the second and the fourth clutches and the third brake, a second forward transmission stage implemented by simultaneously operating the third and the fourth clutches and the third brake, a third forward transmission stage implemented by simultaneously operating the second and the third clutches and the third brake, a fourth forward transmission stage implemented by simultaneously operating the third clutches and the first and the third brakes, a fifth forward transmission stage implemented by simultaneously operating the first and the third clutches and the third brake, a sixth forward transmission stage implemented by simultaneously operating the first and the third clutches and the first brake, a seventh forward transmission stage implemented by simultaneously operating the first, the third, and the fourth clutches, an eighth forward transmission stage implemented by simultaneously operating the first and the fourth clutches and the second brake, a ninth forward transmission stage implemented by simultaneously operating the first clutch and the first and the second brakes, a tenth forward transmission stage implemented by simultaneously operating the first and the second clutches and the second brake, and a reverse transmission stage implemented by simultaneously operating the second clutch and the second and the third brakes. 
     According to various aspects of the present invention, a planetary gear train of an automatic transmission for a vehicle may include an input shaft receiving rotary power of an engine, an output shaft outputting the rotary power with rotary speed changed, a first planetary gear set including a first, a second, and a third rotating element, a second planetary gear set including a fourth, a fifth, and a sixth rotating element, a third planetary gear set including a seventh, an eighth, and a ninth rotating element, a fourth planetary gear set including a tenth, a eleventh, and a twelfth rotating element, a first rotating shaft including the first rotating element, the sixth rotating element, and the eighth rotating element, a second rotating shaft including the second rotating element and directly connected with the input shaft, a third rotating shaft including the third rotating element, a fourth rotating shaft including the fourth rotating element and selectively connected with the second rotating shaft and a transmission housing, a fifth rotating shaft including the fifth rotating element and selectively connected with the second rotating shaft, a sixth rotating shaft including the seventh rotating element and the eleventh rotating element, directly connected with the output shaft, and selectively connected with the third rotating shaft, a seventh rotating shaft including the ninth rotating element and the tenth rotating element and selectively connected with the fifth rotating shaft, an eighth rotating shaft including the twelfth rotating element and selectively connected with the transmission housing, a first clutch selectively connecting the third rotating shaft and the sixth rotating shaft, a second clutch selectively connecting the second rotating shaft and the fourth rotating shaft, a third clutch selectively connecting the second rotating shaft and the fifth rotating shaft, a fourth clutch selectively connecting the fifth rotating shaft and the seventh rotating shaft, a first brake selectively connecting the fourth rotating shaft with the transmission housing, a second brake selectively connecting the fifth rotating shaft with the transmission housing, and a third brake selectively connecting the eighth rotating shaft with the transmission housing. 
     According to various aspects of the present invention, a planetary gear train of an automatic transmission for a vehicle may include an input shaft receiving rotary power of an engine, an output shaft outputting the rotary power with rotary speed changed, a first planetary gear set, which may be a single-pinion planetary gear set, including a first sun gear, a first planetary carrier, and a first ring gear, a second planetary gear set, which may be a single-pinion planetary gear set, including a second sun gear, a second planetary carrier, and a second ring gear, a third planetary gear set, which may be a single-pinion planetary gear set, including a third sun gear, a third planetary carrier, and a third ring gear, a fourth planetary gear set, which may be a single-pinion planetary gear set, including a fourth sun gear, a fourth planetary carrier, and a fourth ring gear, a first rotating shaft including the first sun gear, the second ring gear, and the third planetary carrier, a second rotating shaft including the first planetary carrier and directly connected with the input shaft, a third rotating shaft including the first ring gear, a fourth rotating shaft including the second sun gear and selectively connected with the second rotating shaft and a transmission housing, a fifth rotating shaft including the second planetary carrier and selectively connected with the second rotating shaft, a sixth rotating shaft including the third sun gear and the fourth planetary carrier, directly connected with the output shaft, and selectively connected with the third rotating shaft, a seventh rotating shaft including the third ring gear and the fourth sun gear and selectively connected with the fifth rotating shaft, an eighth rotating shaft including the fourth ring gear and selectively connected with the transmission housing, and seven control elements disposed at portions selectively connecting the rotating shafts respectively or selectively connecting the rotating shafts and the transmission housing respectively. 
     The planetary gear train may further include a first clutch selectively connecting the first ring gear and the third sun gear, a second clutch selectively connecting the first planetary carrier and the second sun gear, a third clutch selectively connecting the first planetary carrier and the second planetary carrier, a fourth clutch selectively connecting the third ring gear and the second planetary carrier, a first brake selectively connecting the second sun gear with the transmission housing, a second brake selectively connecting the second planetary carrier with the transmission housing, and a third brake selectively connecting the fourth ring gear with the transmission housing. 
     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 an exemplary planetary gear train according to the present invention. 
         FIG. 2  is an operation table for each of transmission steps of respective control elements applied to an exemplary planetary gear train according to 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 an exemplary planetary gear train according to the present invention. 
     Referring to  FIG. 1 , the planetary gear train according to various embodiments of the present invention includes a first, a second, a third, and a fourth planetary gear set PG 1 , PG 2 , PG 3 , and PG 4  disposed on a same axis line, an input shaft IS, an output shaft OS, eight rotating shafts TM 1  to TM 8  that connect respective rotating elements of the first, second, third, and fourth planetary gear sets PG 1 , PG 2 , PG 3 , and PG 4  to each other, seven control elements C 1  to C 4  and B 1  to B 3 , and a transmission housing H. 
     Further, rotary power input from the input shaft IS is transmitted by an inter-complementation operation of the first, second, third, and fourth planetary gear sets PG 1 , PG 2 , PG 3 , and PG 4  to be output through the output shaft OS with rotary speed changed. 
     The respective simple planetary gear sets are disposed in an order of the second, the first, the third, and the fourth planetary gear set PG 2 , PG 1 , PG 3 , and PG 4  starting from an engine side. 
     The input shaft IS is an input member and rotary power from a crankshaft of an engine is torque-converted through a torque converter to be input into the input shaft IS. 
     The output shaft OS as an output member is disposed on a same axis line as the input shaft IS and transmits transmitted driving power to a driving shaft through a differential. 
     The first planetary gear set PG 1 , which is a single-pinion planetary gear set, includes a first sun gear S 1  which is a first rotating element N 1 , a first planetary carrier PC 1  which is a second rotating element N 2  and rotatably supports a first pinion P 1  which engages externally with the first sun gear S 1 , and a first ring gear R 1  which is a third rotating element N 3  and engages internally with the first pinion P 1  as rotating elements. 
     The second planetary gear set PG 2 , which is a single-pinion planetary gear set, includes a second sun gear S 2  which is a fourth rotating element N 4 , a second planetary carrier PC 2  which is a fifth rotating element N 5  and rotatably supports a second pinion P 2  which engages externally with the second sun gear S 2 , and a second ring gear R 2  which is a sixth rotating element N 6  and engages internally with the second pinion P 2 . 
     The third planetary gear set PG 3 , which is a single-pinion planetary gear set, includes a third sun gear S 3  which is a seventh rotating element N 7 , a third planetary carrier PC 3  which is an eighth rotating element N 8  and rotatably supports a third pinion P 3  which engages externally with the third sun gear S 3 , and a third ring gear R 3  which is a ninth rotating element N 9  and engages internally with the third pinion P 3 . 
     The fourth planetary gear set PG 4 , which is a single-pinion planetary gear set, includes a fourth sun gear S 4  which is a tenth rotating element N 10 , a fourth planetary carrier PC 4  which is a eleventh rotating element N 11  and rotatably supports a fourth pinion P 4  which engages externally with the fourth sun gear S 4 , and a fourth ring gear R 4  which is a twelfth rotating element N 12  and engages internally with the fourth pinion P 4 . 
     In the first, second, third, fourth planetary gear sets PG 1 , PG 2 , PG 3 , and PG 4 , the first rotating element N 1  is directly connected with the sixth rotating element N 6  and the eighth rotating element N 8 , the seventh rotating element N 7  is directly connected with the eleventh rotating element N 11 , and the ninth rotating element N 9  is directly connected with the tenth rotating element N 10 , and the planetary gear sets have a total of eight rotating shafts TM 1  to TM 8 . 
     Configurations of the eight rotating shafts TM 1  to TM 8  will be described below. 
     The first rotating shaft TM 1  includes the first rotating element N 1  (the first sun gear S 1 ), the sixth rotating element N 6  (the second ring gear R 2 ), and the eighth rotating element N 8  (the third planetary carrier PC 3 ). 
     The second rotating shaft TM 2  includes the second rotating element N 2  (the first planetary carrier PC 1 ) and is directly connected with the input shaft IS. 
     The third rotating shaft TM 3  includes the third rotating element N 3  (the first ring gear R 1 ). 
     The fourth rotating shaft TM 4  includes the fourth rotating element N 4  (the second sun gear S 2 ) and is selectively connected with the second rotating shaft TM 2  and a transmission housing H. 
     The fifth rotating shaft TM 5  includes the fifth rotating element N 5  (the second planetary carrier PC 2 ) and is selectively connected with the second rotating shaft TM 2 . 
     The sixth rotating shaft TM 6  includes the seventh rotating element N 7  (the third sun gear S 3 ) and the eleventh rotating element N 11  (the fourth planetary carrier PC 4 ) and is selectively connected with the third rotating shaft TM 3 . 
     The seventh rotating shaft TM 7  includes the ninth rotating element N 9  (the third ring gear R 3 ) and the tenth rotating element N 10  (the fourth sun gear S 4 ) and is selectively connected with the fifth rotating shaft TM 5 . 
     The eighth rotating shaft TM 8  includes the twelfth rotating element N 12  (the fourth ring gear R 4 ) and is selectively connected with the transmission housing H. 
     In addition, four clutches C 1 , C 2 , C 3 , and C 4  which are control elements are disposed at portions selectively connecting the rotating shafts respectively, among the rotating shafts TM 1  to TM 8 . 
     Further, three brakes B 1  to B 2  which are other control elements are disposed at portions selectively connecting the transmission housing H and the rotating shafts respectively, among the rotating shafts TM 1  to TM 8 . 
     Layout positions of the seven control elements C 1  to C 4  and B 1  to B 3  will be described below. 
     The first clutch C 1  is interposed between the third rotating shaft TM 3  and the sixth rotating shaft TM 6  and operates such that the third rotating shaft TM 3  and the sixth rotating shaft TM 6  selectively operate as one body. 
     The second clutch C 2  is interposed between the second rotating shaft TM 2  and the fourth rotating shaft TM 4  and operates such that the second rotating shaft TM 2  and the fourth rotating shaft TM 4  selectively operate as one body. 
     The third clutch C 3  is interposed between the second rotating shaft TM 2  and the fifth rotating shaft TM 5  and operates such that the second rotating shaft TM 2  and the fifth rotating shaft TM 5  selectively operate as one body. 
     The fourth clutch C 4  is interposed between the fifth rotating shaft TM 5  and the seventh rotating shaft TM 7  and operates such that the fifth rotating shaft TM 5  and the seventh rotating shaft TM 7  selectively operate as one body. 
     The first brake B 1  is interposed between the fourth rotating shaft TM 4  and the transmission housing H and operates such that the fourth rotating shaft TM 4  can selectively operate as a fixing element. 
     The second brake B 2  is interposed between the fifth rotating shaft TM 5  and the transmission housing H and operates such that the fifth rotating shaft TM 4  can selectively operate as a fixing element. 
     The third brake B 3  is interposed between the eighth rotating shaft TM 8  and the transmission housing H and operates such that the eighth rotating shaft TM 8  can selectively operate as a fixing element. 
     The respective control elements constituted by the first, the second, the third, and the fourth clutch C 1 , C 2 , C 3 , and C 4  and the first, the second, and the third brake B 1 , B 2 , and B 3  as described above may be configured by multiple-disk hydraulic friction joining units which are friction-joined by hydraulic pressure. 
       FIG. 2  is an operation table for each of transmission steps of respective control elements applied to an exemplary planetary gear train according to the present invention. 
     As illustrated in  FIG. 2 , a transmission is performed while three control elements operate in the respective transmission steps, in an exemplary planetary gear train according to the present invention. 
     In a first forward transmission step D 1 , the second and the fourth clutch C 2  and C 4  and the third brake B 3  simultaneously operate. 
     As a result, an input is made into the second rotating shaft TM 2  in a state of the second rotating shaft TM 2  and the fourth rotating shaft TM 4  being connected by an operation of the second clutch C 2  and the fifth rotating shaft TM 5  and the seventh rotating shaft TM 7  being connected by an operation of the fourth clutch C 4  and the first forward transmission step D 1  is performed and an output is output through the sixth rotating shaft TM 6  while the eighth rotating shaft TM 8  operates as a fixing element by an operation of the third brake B 3 . 
     In a second forward transmission step D 2 , the third and the fourth clutch C 3  and C 4  and the third brake B 3  simultaneously operate. 
     As a result, an input is made into the second input shaft TM 2  in a state of the fifth rotating shaft TM 5  being connected with the second rotating shaft TM 2  and the seventh rotating shaft TM 7  by an operation of the third and the fourth clutch C 3  and C 4  and the second forward transmission step D 2  is performed and an output is output through the sixth rotating shaft TM 6  while the eighth rotating shaft TM 8  operates as a fixing element by an operation of the third brake B 3 . 
     In a third forward transmission step D 3 , the second and the third clutch C 2  and C 3  and the third brake B 3  simultaneously operate. 
     As a result, an input is made into the second input shaft TM 2  in a state of the second rotating shaft TM 2  being connected with the fourth rotating shaft TM 4  and the fifth rotating shaft TM 5  by an operation of the second clutch C 2  and the third clutch C 3  and the third forward transmission step D 3  is performed and an output is output through the sixth rotating shaft TM 6  while the eighth rotating shaft TM 8  operates as a fixing element by an operation of the third brake B 3 . 
     In a fourth forward transmission step D 4 , the third clutch C 3  and the first and the third brake B 1  and B 3  simultaneously operate. 
     As a result, an input is made into the second input shaft TM 2  in a state of the second rotating shaft TM 2  and the fifth rotating shaft TM 5  being connected by an operation of the third clutch C 3  and the fourth forward transmission step D 4  is performed and an output is output through the sixth rotating shaft TM 6  while the fourth rotating shaft TM 4  and the eighth rotating shaft TM 8  operate as fixing elements by an operation of the first brake B 1  and the third brake B 3 . 
     In a fifth forward transmission step D 5 , the first and the third clutch C 1  and C 3  and the third brake B 3  simultaneously operate. 
     As a result, an input is made into the second input shaft TM 2  in a state of the third rotating shaft TM 3  and the sixth rotating shaft TM 6  being connected by an operation of the first clutch C 1  and the second rotating shaft TM 2  and the fifth rotating shaft TM 5  being connected by an operation of the third clutch C 3  and the fifth forward transmission step D 5  is performed and an output is output through the sixth rotating shaft TM 6  while the eighth rotating shaft TM 8  operates as a fixing element by an operation of the third brake B 3 . 
     In a sixth forward transmission step D 6 , the first and the third clutch C 1  and C 3  and the first brake B 1  simultaneously operate. 
     As a result, an input is made into the second input shaft TM 2  in a state of the third rotating shaft TM 3  and the sixth rotating shaft TM 6  being connected by an operation of the first clutch C 1  and the second rotating shaft TM 2  and the fifth rotating shaft TM 5  being connected by an operation of the third clutch C 3  and the sixth forward transmission step D 6  is performed and an output is output through the sixth rotating shaft TM 6  while the fourth rotating shaft TM 4  operates as a fixing element by an operation of the first brake B 1 . 
     In a seventh forward transmission step D 7 , the first, the third, and the fourth clutch C 1 , C 3 , and C 4  simultaneously operate. 
     As a result, an input is made into the second input shaft TM 2  in a state of the third rotating shaft TM 3  and the sixth rotating shaft TM 6  being connected by an operation of the first clutch C 1  and the fifth rotating shaft TM 5  being connected with the second rotating shaft TM 2  and the seventh rotating shaft TM 7  by an operation of the third and the fourth clutch C 3  and C 4  and the seventh forward transmission step D 7  in which rotary power received by the second rotating shaft TM 2  is intactly output through the sixth rotating shaft TM 6 , is performed while all of the first, second, third, and fourth planetary gear sets PG 1 , PG 2 , PG 3 , and PG 4  are serially connected. 
     In an eighth forward transmission step D 8 , the first and the fourth clutch C 1  and C 4  and the second brake B 2  simultaneously operate. 
     As a result, an input is made into second input shaft TM 2  in a state of the third rotating shaft TM 3  and the sixth rotating shaft TM 6  being connected by an operation of the first clutch C 1  and the fifth rotating shaft TM 5  and the seventh rotating shaft TM 7  being connected by an operation of the fourth clutch C 4  and the eighth forward transmission step D 8  is performed and an output is output through the sixth rotating shaft TM 6  while the fifth rotating shaft TM 5  and the seventh rotating shaft TM 7  operate as fixing elements by an operation of the fourth clutch C 4  and the first brake B 2 . 
     In a ninth forward transmission step D 9 , the first clutch C 1  and the first and the second brake B 1  and B 2  simultaneously operate. 
     As a result, an input is made into the second input shaft TM 2  in a state of the third rotating shaft TM 3  and the sixth rotating shaft TM 6  being connected by an operation of the first clutch C 1  and the ninth forward transmission step D 9  is performed and an output is output through the sixth rotating shaft TM 6  while the fourth rotating shaft TM 4  and the fifth rotating shaft TM 5  are fixed by an operation of the first and the second brake B 1  and B 2 , the entire second planetary gear set PG 2  is fixed, and thereby the first rotating shaft TM 1  connected therewith operates as a fixing element. 
     In a tenth forward transmission step D 10 , the first and the second clutch C 1  and C 2  and the second brake B 2  simultaneously operate. 
     As a result, an input is made into the second rotating shaft TM 2  in a state of the third rotating shaft TM 3  and the sixth rotating shaft TM 6  being connected by an operation of the first clutch C 1  and the second rotating shaft TM 2  and the fourth rotating shaft TM 4  being connected by an operation of the second clutch C 2  and the tenth forward transmission step D 10  is performed and an output is output through the sixth rotating shaft TM 6  while the fifth rotating shaft TM 5  operates as a fixing element by an operation of the second brake B 2 . 
     In a reverse transmission step REV, the second clutch C 2  and the second and the third brake B 2  and B 3  simultaneously operate. 
     As a result, an input is made into the second input shaft TM 2  in a state of the second rotating shaft TM 2  and the fourth rotating shaft TM 4  being connected by an operation of the second clutch C 2  and the reverse transmission step REV is performed and an output is output through the sixth rotating shaft TM 6  while the fifth and the eighth rotating shaft TM 5  and TM 8  operate as fixing elements by an operation of the second and the third brake B 2  and B 3 . 
     As described above, in the planetary gear train according to various embodiments of the present invention, four planetary gear sets PG 1 , PG 2 , PG 3 , and PG 4  may implement the forward at least 10-speed and reverse 1-speed transmission steps through the operation-control of four clutches C 1 , C 2 , C 3 , and C 4  and three brakes B 1 , B 2 , and B 3 . 
     Further, drivability such as acceleration before and after a speed change and rhythmic feeling of engine speed can be improved because linearity of step ratios between transmission stages is secured even though step ratios between forward 6/7, 7/8, 8/9, and 9/10 transmission steps are less than 1.2. 
     In addition, operation efficiency of an engine can be maximized because an entire gear ratio is secured to be equal to or greater than 9.0. 
     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.