Abstract:
A gear train of an automatic transmission for a vehicle includes a first planetary gear set being a simple planetary gear set and a second planetary gear set that is formed of two combined simple planetary gear sets. The gear train combines four clutches and two brakes and realizes seven forward speeds. The gear train may enhance transmitting performance and reduce fuel consumption.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority to and the benefit of Korean Patent Application No. 10-2008-0023946, filed on Mar. 14, 2008, the entire contents of which are incorporated herein for all purposes by this reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a gear train of an automatic transmission for a vehicle that realizes a plurality of forward speeds, and in particular, seven forward speeds and reverse. 
     2. Description of Related Art 
     A conventional shift mechanism of an automatic transmission utilizes a combination of a plurality of planetary gear sets. A gear train of such an automatic transmission changes rotating speed and torque received from a torque converter of the automatic transmission and transmits the changed torque to an output shaft. 
     It is well known that when a transmission realizes a greater number of shift speeds, speed ratios of the transmission can be more optimally designed and therefore a vehicle can have better fuel mileage and better performance. For that reason, an automatic transmission that enables more shift speeds is under constant investigation. 
     In addition, with the same number of speeds, features of a gear train such as durability, efficiency in power transmission, and size depend in part on the layout of combined planetary gear sets. Therefore, designs for a combining structure of a gear train are also under constant investigation. 
     A manual transmission that has too many speeds causes inconvenience of excessively frequent shifting operations to a driver. Therefore, the positive features of more shift-speeds are generally more important for automatic transmissions because an automatic transmission automatically controls shifting operations basically with reduced manual operation by a driver. 
     In addition to various developments regarding four and five speed gear trains, six-speed automatic transmissions have recently been developed. 
     The above information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
     BRIEF SUMMARY OF THE INVENTION 
     Various aspects of the present invention are directed to providing a gear train of an automatic transmission for a vehicle that realizes, meaning obtains or achieves, seven forward speeds. A gear train of an automatic transmission for a vehicle according to various aspects of the present invention includes a first planetary gear set being a simple planetary gear set provided with three rotational members, wherein a first rotational member is substantially always operated as a fixed member, a second rotational member forms a first intermediate output pathway where a reduced rotational speed is output, and a third rotational member forms an input pathway that is directly connected with an input shaft. A second planetary gear set may be combined with first and second simple planetary gear sets that have three rotational members respectively. The second planetary gear set may realize fourth, fifth, sixth, and seventh rotational members, wherein the fourth rotational member, which is realized with one rotational member, forms a first intermediate input path by being directly connected with the second rotational member, the fifth rotational member, which is realized with one rotational member, is operated as an output element by being connected with an output shaft, the sixth rotational member, which is realized with two rotational members directly connected with each other, forms a first variable input path by being variably connected with the input shaft, and is selectively operated as a fixed element by being connected with a transmission housing, the seventh rotational member, which is realized with two rotational members variably connected or separated from each other, forms a second intermediate input path by being variably connected with the second rotational member, forms a second variable input path by being variably connected with the input shaft, and is operated as a variable fixed element by being variably connected with the transmission housing. A plurality of friction members including clutches and brakes selectively connect the rotational members with one of the other rotational members, the transmission housing, or the input shaft. 
     The plurality of friction members may include a first clutch disposed for selectively integrally connecting the seventh rotational member; a second clutch disposed in the second intermediate input path; a third clutch disposed in the first variable input path; a fourth clutch disposed in the second variable input path; a one-way clutch and a first brake disposed in parallel between the sixth rotational member and the transmission housing; and a second brake disposed between the seventh rotational member and the transmission housing. 
     The first planetary gear set may be a single pinion planetary gear set, wherein the first rotational member is a first sun gear, the second rotational member is a first planet carrier, and the third second rotational member is a first ring gear; and the second planetary gear set may be a combination of a first simple planetary gear set, which is a single pinion planetary gear set, and a second simple planetary gear set, which is a double pinion planetary gear set, wherein the fourth rotational member is a third sun gear of the second simple planetary gear set, the fifth rotational member is a second ring gear of the first simple planetary gear set, the sixth rotational member is a second planet carrier of the first simple planetary gear set and a third ring gear of the second simple planetary gear set, and the seventh rotational member is a second sun gear of the first simple planetary gear set and a third planet carrier of the second simple planetary gear set. 
     The first clutch and the one-way clutch may be operated in the first forward speed. The first clutch and the second brake may be operated in the second forward speed. The first clutch and the second clutch may be operated in the third forward speed. The first clutch and the fourth clutch may be operated in the fourth forward speed. The first clutch and the third clutch may be operated in the fifth forward speed. The second clutch and the third clutch may be operated in the sixth forward speed. The third clutch and the second brake may be operated in the seventh forward speed. The second clutch and the first brake may be operated in a reverse speed. 
     The third clutch may be disposed between the third ring gear and the input shaft, and the first brake with the one-way clutch may be disposed between the second planet carrier and the transmission housing. 
     The first clutch may be disposed between the second sun gear and the third planet carrier, the second clutch may be disposed between the second rotational member and the second sun gear, and the second brake may be disposed between the second clutch and the second sun gear. 
     The first planetary gear set may be a double pinion planetary gear set, wherein the first rotational member is a first sun gear, the second rotational member is a first ring gear, and the third rotational member is a first planet carrier. The second planetary gear set may be a combination of a first simple planetary gear set, which is a single pinion planetary gear set, and a second simple planetary gear set, which is a double pinion planetary gear set. The fourth rotational member may be a third sun gear of the second simple planetary gear set, the fifth rotational member is a second ring gear of the first simple planetary gear set, the sixth rotational member is a second planet carrier of the first simple planetary gear set and a third ring gear of the second simple planetary gear set, and the seventh rotational member is a second sun gear of the first simple planetary gear set and a third planet carrier of the second simple planetary gear set. 
     The first clutch and the one-way clutch may be operated in the first forward speed. The first clutch and the second brake may be operated in the second forward speed. The first clutch and the second clutch may be operated in the third forward speed. The first clutch and the fourth clutch may be operated in the fourth forward speed. The first clutch and the third clutch may be operated in the fifth forward speed. The second clutch and the third clutch may be operated in the sixth forward speed. The third clutch and the second brake may be operated in the seventh forward speed. The second clutch and the first brake may be operated in a reverse speed. 
     The third clutch may be disposed between the third ring gear and the input shaft, and the first brake with the one-way clutch may be disposed between the second planet carrier and the transmission housing. 
     The first clutch may be disposed between the second sun gear and the third planet carrier. The second clutch may be disposed between the second rotational member and the second sun gear. The second brake may be disposed between the second clutch and the second sun gear. 
     In various embodiments of the present invention, a gear train of an automatic transmission for a vehicle includes a first planetary gear set being a single pinion planetary gear set and including a first sun gear, a first planet carrier, and a first ring gear; a second planetary gear set that is a combination of a first simple planetary gear set, being a single pinion planetary gear set and including a second sun gear, a second planet carrier, and a second ring gear, and a second simple planetary gear set, being a double pinion planetary gear set and including a third sun gear, a third planet carrier, and a third ring gear, wherein the first ring gear is directly connected with an input shaft, the first planet carrier is directly connected with the third sun gear, the second planet carrier is directly connected with the third ring gear, and the second ring gear is directly connected with an output shaft. A first clutch may be disposed between the second sun gear and the third planet carrier. A second clutch may be disposed between the first planet carrier and the second sun gear. A third clutch may be disposed between the input shaft and the third ring gear. A fourth clutch may be disposed between the input shaft and the third planet carrier. A one-way clutch and a first brake may be disposed in parallel between the second planet carrier and a transmission housing. A second brake may be disposed between the second sun gear and the transmission housing. 
     In various embodiments, a gear train of an automatic transmission for a vehicle may include a first planetary gear set being a double pinion planetary gear set and including a first sun gear, a first planet carrier, and first ring gear. A second planetary gear set may be a combination of a first simple planetary gear set, which is a single pinion planetary gear set including a second sun gear, a second planet carrier and a second ring gear, and a second planetary gear set, which is a double pinion planetary gear set including a third sun gear, a third planet carrier and a third ring gear. The first planet carrier may be directly connected with an input shaft. The first ring gear may be directly connected with the third sun gear. The second planet carrier may be directly connected with the third ring gear. The second ring gear, may be directly connected with an output shaft. A first clutch may be disposed between the second sun gear and the third planet carrier. A second clutch may be disposed between the first ring gear and the second sun gear. A third clutch may be disposed between the input shaft and the third ring gear. A fourth clutch may be disposed between the input shaft and the third planet carrier. A one-way clutch and a first brake may be disposed in parallel between the second planet carrier and a transmission housing. A second brake may be disposed between the second sun gear and the transmission housing. 
     The gear train according to various aspects of the present invention may be formed by combining two planetary gear sets, four clutches, and two brakes to realize seven forward speeds. 
     The frictional elements are decentralized so that a hydraulic line may be easily constructed, and only two frictional elements are operated in each shifting so that the volume of a hydraulic pump may be reduced and hydraulic pressure control efficiency may be enhanced. 
     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 of the Invention, 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 gear train according to various aspects of the present invention. 
         FIG. 2  is an operational chart for the exemplary gear train illustrated in  FIG. 1 . 
         FIG. 3  is a lever diagram for the exemplary gear train illustrated in  FIG. 1 . 
         FIG. 4  is a schematic diagram of an exemplary gear train similar to the gear train of  FIG. 1 . 
         FIG. 5  is a lever diagram for the exemplary gear train illustrated in  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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. 
       FIG. 1  is a schematic diagram of a gear train according to various embodiments of the present invention. A gear train includes first and second planetary gear sets PG 1  and PG 2 , four clutches C 1 , C 2 , C 3 , and C 4 , two brakes B 1  and B 2 , and a one-way clutch F. 
     The first planetary gear set PG 1  reduces a rotational speed of an input shaft IS and transmits a reduced speed to the second planetary gear set PG 2 . The second planetary gear set PG 1  receives the reduced speed from the first planetary gear set PG 1  and the rotational speed from the input shaft IS, selectively, and outputs seven forward speeds and one reverse speed through an output shaft OS. 
     For this purpose, the first planetary gear set PG 1  is disposed close to an engine (not shown), and the second planetary gear set PG 2  is sequentially disposed. 
     The input shaft IS is an input member and is a turbine shaft of a torque converter. Torque transmitted from a crankshaft of the engine is supplied to the input shaft IS through the torque converter. The output shaft OS is an output member and torque of the output shaft OS is transmitted to a differential apparatus through an output gear (not shown) and drives a driving wheel. 
     The first planetary gear set PG 1  is a simple and single planetary gear set and includes a first rotational member N 1  of a first sun gear S 1 , a second rotational member N 2  of a first planet carrier PC 1 , and a third rotational member N 3  of a first ring gear R 1 . 
     The first rotational member (N 1 ; the first sun gear S 1 ) is always operated as a fixed element by being directly connected with a transmission housing H. 
     The second rotational member (N 2 ; the first planet carrier PC 1 ) forms a first intermediate output path MOP 1  where reduced rotational speed is output. 
     The third rotational member (N 3 ; the first ring gear R 1 ) is directly connected to the input shaft IS so as to form an input pathway IP. 
     The second planetary gear set PG 2  is formed by combining first and second simple planetary gear sets SPG 1  and SPG 2  and includes fourth, fifth, sixth, and seventh rotational members N 4 , N 5 , N 6 , and N 7 . In various embodiments of the present invention, the first simple planetary gear set SPG 1  is a single pinion planetary gear set and the second simple planetary gear set SPG 2  is a double pinion planetary gear set. 
     The second carrier PC 2  is directly connected with the third ring gear R 3 , and the second sun gear S 2  and the third planet carrier PC 3  are variably connected by the first clutch C 1 . 
     Thus, the fourth rotational member N 4  is the third sun gear S 3 , the fifth rotational member N 5  is the second ring gear R 2 , the sixth rotational member N 6  is the second planet carrier PC 2  and the third ring gear R 3 , and the seventh rotational member N 7  is the second sun gear S 2  and the third planet carrier PC 3 . 
     The fourth rotational member (N 4 ; the third sun gear S 3 ) is directly connected with the second rotational member N 2  and forms a first intermediate input path MIP 1  for receiving the reduced speed from the first planetary gear set PG 1 . 
     The fifth rotational member (N 5 ; the second ring gear R 2 ) is directly connected with the output shaft OS and forms an output path OP. 
     The sixth rotational member (N 6 ; the second planet carrier PC 2  and the third ring gear R 3 ) is variably connected with the input shaft IS by the third clutch C 3 . The sixth rotational member forms a first variable input path VIP 1 . The sixth rotational member is also selectively operated as a input element. Further, the sixth rotational member N 6  is variably connected with the transmission housing H by the first brake B 1  and the one-way clutch F and is selectively operated as a fixed element. 
     The seventh rotational member (N 7 ; the second sun gear S 2  and the third planet carrier PC 3 ) is variably connected with the first planetary gear set PG 1  by interposing the second clutch C 2  and forms a second intermediate input path MIP 2 . Also, the seventh rotational member N 7  is variably connected with the input shaft IS by clutch C 4 , forms a second variable input path VIP 2 , and is selectively operated as an input element. Further, the seventh rotational member N 7  is variably connected with the transmission housing H by interposing the second brake B 2  so as to be selectively operated as a fixed element. 
     The third clutch C 3  of the sixth rotational member N 6  is disposed between the third ring gear R 3  and the input shaft IS, and the first brake B 1  in parallel with the one way clutch F is disposed between the second planet carrier PC 2  and the transmission housing H. 
     The first clutch C 1  is disposed between the second sun gear S 2  and the third planet carrier PC 3 , the second clutch C 2  is disposed between the second rotational member N 2  and the second sun gear S 2 , the fourth clutch C 4  is disposed between the third planet carrier PC 3  and the input shaft IS, and the second brake B 2  is disposed between the second clutch C 2  and the second sun gear S 2 . 
     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  may be enabled as a multi-plate hydraulic pressure friction device that is frictionally engaged by hydraulic pressure. 
     The first clutch C 1  is disposed between the first and second planetary gear sets PG 1  and PG 2 . The first and second brakes B 1  and B 2  and the one-way clutch F may be disposed outside of the first clutch C 1 . 
     The second clutch C 2  is disposed in front of the first planetary gear set PG 1 , and the third and fourth clutches C 3  and C 4  are disposed behind the second planetary gear set PG 2 , so that a decentralized disposition may be achieved. 
     The decentralized disposition may maintain a stable mass center and allow hydraulic lines to be easily constructed for supplying hydraulic pressure to the friction members. 
       FIG. 2  is an operational chart for a gear train according to various embodiments of the present invention, and as shown in  FIG. 2 , the gear train illustrated in  FIG. 1  may shift by operation of two friction members. 
     That is, the first clutch C 1  and the one-way clutch F are operated in the first forward speed, the first clutch C 1  and the second brake B 2  are operated in the second forward speed, and the first clutch C 1  and the second clutch C 2  are operated in the third forward speed. The first clutch C 1  and the fourth clutch C 4  are operated in the fourth forward speed, the first clutch C 1  and the third clutch C 3  are operated in the fifth forward speed, and the second clutch C 2  and the third clutch C 3  are operated in the sixth forward speed. The third clutch C 3  and the second brake B 2  are operated in the seventh forward speed, and the second clutch C 2  and the first brake B 1  are operated in a reverse speed. One will appreciate from the foregoing that the clutch and gear combinations may be modified in accordance with the present invention to provide a plurality of forward speeds. 
       FIG. 3  is a lever diagram of a gear train according to various embodiments of the present invention. In  FIG. 3 , a lower horizontal line represents “0” rotational speed, and an upper horizontal line represents “1.0” rotational speed that is the same as the rotational speed of the input shaft IS. 
     Three vertical lines of the first planetary gear set PG 1  respectively represent the first rotational member N 1  (the first sun gear S 1 ), the second rotational member N 2  (the first planet carrier PC 1 ), and the third rotational member N 3  (the first ring gear R 1 ) sequentially from the left in the drawing, and a distance between them is determined according to a gear ratio (teeth number of sun gear/teeth number of ring gear) of the first simple planetary gear set SPG 1 . 
     Four vertical lines of the second planetary gear set PG 2  respectively represent the fourth rotational member N 4  (the third sun gear S 3 ), the fifth rotational member N 5  (the second ring gear R 2 ), the sixth rotational member N 6  (the second planet carrier PC 2  and the third ring gear R 3 ), and the seventh rotational member N 7  (the second sun gear S 2  and the third planet carrier PC 3 ) sequentially from the left in the drawing, and a distance between them is determined according to a gear ratio (teeth number of sun gear/teeth number of ring gear) of the first and second simple planetary gear sets SPG 1  and SGP 2 . The lever diagram is well known to a person of ordinary skill in the art, and detailed descriptions will be accordingly omitted. 
     Hereinafter, shifting processes in the gear train according to an exemplary embodiment of the present invention will be described in detail. 
     First Forward Speed 
     As shown in  FIG. 2 , the first clutch C 1  and the one-way clutch F are operated at the first forward speed D 1 . 
     In this case, as shown in  FIG. 3 , in a state that the input rotational speed is transmitted to the third rotational member N 3  of the first planetary gear set PG 1 , the first rotational member N 1  is operated as the fixed member. Thus, the reduced rotational speed is generated at the second rotational member N 2 . 
     The reduced rotational speed of the second rotational member N 2  is transmitted to the fourth rotational member N 4  that is directly connected with the second rotational member N 2 . 
     In a state in which the reduced rotational speed is transmitted to the fourth rotational member N 4  of the second planetary gear set PG 2 , the sixth rotational member N 6  is operated as the fixed member by operation of the one-way clutch F. Therefore, a first shift line SP 1  connecting the fourth rotational member N 4  with the sixth rotational member N 6  is formed and the first forward speed D 1  is output to the fifth rotational member N 5 , which is the output member. 
     Second Forward Speed 
     In a state of the first forward speed D 1 , the second brake B 2  is operated to achieve the second forward speed D 2 . 
     In a state in which the reduced rotational speed is transmitted to the fourth rotational member N 4  of the second planetary gear set PG 2 , the seventh rotational member N 7  is operated as the fixed member by an operation of the second brake B 2 . Therefore, a second shift line SP 2  connecting the fourth rotational member N 4  and the seventh rotational member N 7  is formed and the second forward speed D 2  is output to the fifth rotational member N 5 , which is the output member. 
     Third Forward Speed 
     In a state of the second forward speed D 2 , the second brake B 2  is released and the second clutch C 2  is operated to achieve the third forward speed D 3 . 
     The reduced rotation speed of the second rotational member N 2  is transmitted to the fourth rotational member N 4  and to the seventh rotational member N 7  by an operation of the second clutch C 2  and the first clutch C 1 . Thus, the second planetary gear set PG 2  becomes in a lock state and a third shift line SP 3  connecting the fourth rotational member N 4  and the seventh rotational member N 7  is formed, and the third forward speed D 3  is output to the fifth rotational member N 5 , which is the output member. 
     Fourth Forward Speed 
     In a state of the third forward speed D 3 , the second clutch C 2  is released and the fourth clutch C 4  is operated to achieve the fourth forward speed D 4 . 
     The reduced rotation speed of the second rotational member N 2  is transmitted to the fourth rotational member N 4 , and the rotation of the input shaft IS is transmitted to the seventh rotational member N 7  by an operation of the fourth clutch C 4 . Thus, a fourth shift line SP  4  connecting the fourth rotational member N 4  and the seventh rotational member N 7  is formed, and the fourth forward speed D 4  is output to the fifth rotational member N 5 , which is the output member. 
     Fifth Forward Speed 
     In a state of the fourth forward speed D 4 , the fourth clutch C 4  is released and the third clutch C 3  is operated to achieve the fifth forward speed D 5 . 
     The reduced rotation speed of the second rotational member N 2  is transmitted to the fourth rotational member N 4 , and the rotation of the input shaft IS is transmitted to the sixth rotational member N 6  by an operation of the third clutch C 3 . Therefore, a fifth shift line SP 5  connecting the fourth rotational member N 4  and the sixth rotational member N 6  is formed, and the fifth forward speed D 5  is output to the fifth rotational member N 5 , which is the output member. 
     Sixth Forward Speed 
     In a state of the fifth forward speed D 5 , the first clutch C 1  is released and the second clutch C 2  is operated to achieve the sixth forward speed D 6 . 
     The reduced rotation speed of the second rotational member N 2  is transmitted to the seventh rotational member N 7  by an operation of the second clutch C 2 , and the rotation of the input shaft IS is transmitted to the sixth rotational member N 6  by an operation of the third clutch C 3 . Thus, a sixth shift line SP 6  connecting the sixth rotational member N 6  and the seventh rotational member N 7  is formed, and the sixth forward speed D 6  is output to the fifth rotational member N 5 , which is the output member. 
     Seventh Forward Speed 
     In a state of the sixth forward speed D 6 , the second clutch C 2  is released and the second brake B 2  is operated to achieve the seventh forward speed D 7 . 
     The rotation of the input shaft IS is transmitted to the sixth rotational member N 6  by the operation of third clutch C 3 , and the seventh rotational member N 7  is operated as the fixed member by the operation of the second brake B 2 . Therefore, a seventh shift line SP 7  connecting the sixth rotational member N 6  and the seventh rotational member N 7  is formed, and the seventh forward speed D 7  is output to the fifth rotational member N 5 , which is the output member. 
     Reverse Speed 
     The first clutch C 2  and the first brake B 1  are operated in the reverse speed RS. 
     The rotation speed of the input shaft IS is transmitted to the third rotational member N 3  of the first planetary gear set PG 1 , and the first rotational member N 1  is operated as the fixed member. Thus, reduced rotation is output through the second rotational member N 2 . 
     The reduced rotation of the second rotational member N 2  is transmitted to the seventh rotational member N 7  by the operation of the second clutch C 2  and the sixth rotational member N 6  is operated as a fixed element by the operation of the first brake B 1 . Thus, a reverse shift line SR is formed, and the reverse speed RS is output to the fifth rotational member N 5 , which is the output member. 
     In the first forward speed, as shown in  FIG. 3 , in the state that the first clutch C 1  is operated, the fourth rotational member N 4  is operated as an input element and the sixth rotational member N 6  is operated as a fixed element by the operation of the one-way clutch F, and thus, shifting is performed. 
     In a state of the first forward speed D 1 , the second brake B 2  is operated so that the seventh rotational member N 7  is operated as a fixed element, and thus, shifting is performed to achieve the second forward speed D 2 . 
     In a state of the second forward speed D 2 , the second brake B 2  is released and the second clutch C 2  is operated to achieve the third forward speed D 3 . Thus, the reduced rotation of the first planetary gear set PG 1  is transmitted to the fourth rotational member N 4  and the seventh rotational member N 7 , and thus, the second planetary gear set PG 2  becomes or moves to the lock state. 
     In a state of the third forward speed D 3 , the second clutch C 2  is released and the fourth clutch C 4  is operated to achieve the fourth forward speed D 4 . Thus, different rotation speeds are transmitted to the fourth rotational member N 4  and the seventh rotational member N 7 , and shifting is performed to achieve the fourth forward speed D 4 . 
     In a state of the fourth forward speed D 4 , the second clutch C 2  is released and the third clutch C 3  is operated to achieve the fifth forward speed D 5 . Thus, different rotation speeds are transmitted to the fourth rotational member N 4  and the sixth rotational member N 6 , and shifting is performed to achieve the fifth forward speed D 5 . 
     In a state of the fifth forward speed D 5 , the first clutch C 1  is released and the second clutch C 2  is operated to achieve the sixth forward speed D 6 . Thus, a connection of the second sun gear S 2  and the third planet carrier PC 3  is released and different rotation speeds are transmitted to the sixth rotational member N 6  and the seventh rotational member N 7 , and shifting is performed to achieve the sixth forward speed D 6 . 
     In a state of the sixth forward speed D 6 , the second clutch C 2  is released and the second brake B 2  is operated to achieve the seventh forward speed D 7 . Thus, the seventh rotational member N 7  is operated as a fixed, or substantially fixed, element, and shifting is performed to achieve the seventh forward speed D 7 . 
     The second clutch C 2  and the first brake B 1  are operated in the reverse speed RS. A reduced rotation speed is transmitted to the seventh rotational member N 7  and the sixth rotational member N 6  is operated as a fixed element, and thus, shifting is performed to achieve the reverse speed. 
     When the first clutch C 1  is released in the sixth and seventh forward speeds D 6  and D 7 , and the reverse speed shifting are operated, a connection of the second sun gear S 2  and the third planet carrier PC 3  is released. Two rotational members receive rotation speed, and the second planetary gear set PG 2  becomes or moves to the unlock state so that shifting is operated. 
       FIG. 4  is a schematic diagram of a gear train according to various embodiments of the present invention. A scheme of the second planetary gear set PG 2  is the same as that of the above exemplary embodiments while a scheme of the first planetary gear set PG 1  is different thereto. 
     That is, the first planetary gear set PG 1  is a single pinion planetary gear set in the above exemplary embodiment of the present invention; however, the first planetary gear set PG 1  is a double pinion planetary gear set in the present exemplary embodiments of the present invention. 
     The first rotational member N 1  is the first sun gear S 1 , the second rotational member N 2  is the first ring gear R 1 , and the third rotational member N 3  is the first planet carrier PC 1 . 
     The second rotational member N 2  of the first ring gear R 1  forms a first intermediate output path MOP 1 , and the third rotational member N 3  of the first planet carrier PC 1  is operated as an input element. 
     Shifting of the gear train according to the exemplary embodiments of the present invention is the same as that of the above exemplary embodiments of the present invention. 
     That is, friction members are operated in each speed as shown in  FIG. 2 , and the shifting operations of the exemplary embodiment remain the same as those of the above embodiments except for the second and third rotational members N 2  and N 3 , and are therefore not described in further detail. 
     For convenience in explanation and accurate definition in the appended claims, the terms “up” or “upper”, “down” or “lower”, “front” or “rear”, “inside”, 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.