Abstract:
A planetary gear train of an automatic transmission for vehicles may include an input shaft adapted to receive torque of an engine, an output gear adapted to output changed torque, a first planetary gear set having a first sun gear, a first planet carrier, and a first ring gear as rotation elements, a second planetary gear set having a second sun gear, a second planet carrier, and a second ring gear as rotation elements, a third planetary gear set having a third sun gear, a third planet carrier, and a third ring gear as rotation elements, seven rotation shafts connected or selectively connected to one or more rotation elements, other rotation shafts, or a transmission housing, and six friction members interposed selectively at connecting portions of the rotation shafts.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority of Korean Patent Application Number 10-2011-0038971 filed in the Korean Intellectual Property Office on Apr. 26, 2011, the entire contents of which application are incorporated herein for all purposes by this reference. 
     BACKGROUND OF INVENTION 
     1. Field of Invention 
     The present invention relates to an automatic transmission for vehicles. More particularly, the present invention relates to a planetary gear train of an automatic transmission for vehicles which improves power delivery performance and reduces fuel consumption. 
     2. Description of Related Art 
     Typically, a planetary gear train is realized by combining a plurality of planetary gear sets, and the planetary gear train including the plurality of planetary gear sets receives torque from a torque converter and changes and transmits the 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 economical fuel mileage and better performance. For this reason, the planetary gear train that is able to realize more shift speeds is under continuous investigation. 
     Though achieving the same number of speeds, the planetary gear train can have different operating mechanisms due to different connections among rotation elements (i.e., sun gear, planet carrier, and ring gear). 
     In addition, the planetary gear train has different features such as durability, power delivery efficiency, and size depending on the layout thereof. Therefore, designs for a combining structure of a gear train are also under continuous investigation. 
     Currently, four-speed and five-speed automatic transmissions are most often found on the market. However, six-speed automatic transmissions have also been realized for enhancement of performance of power transmission and for enhanced fuel mileage of a vehicle. In addition, eight-speed automatic transmissions and ten-speed automatic transmissions have been developed at a good pace. 
     The information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art. 
     SUMMARY OF INVENTION 
     Various aspects of the present invention are directed to provide a planetary gear train of an automatic transmission for vehicles having advantages of achieving eight forward speeds and two reverse speeds by combining a plurality of planetary gear sets with a plurality of friction members and reducing drag loss by reducing the number of friction members which are not operated at each shift-speed. 
     Exemplary planetary gear trains of an automatic transmission for vehicles according to the present invention may include: an input shaft adapted to receive a torque of an engine; an output gear adapted to output a changed torque; a first planetary gear set having a first sun gear, a first planet carrier, and a first ring gear as rotation elements thereof; a second planetary gear set having a second sun gear, a second planet carrier, and a second ring gear as rotation elements thereof; a third planetary gear set having a third sun gear, a third planet carrier, and a third ring gear as rotation elements thereof; a first rotation shaft connected to the first sun gear and selectively connected to a transmission housing; a second rotation shaft connected to the first planet carrier; a third rotation shaft connected to the first ring gear and directly connected to the input shaft; a fourth rotation shaft connected to the second sun gear and selectively connected to the second rotation shaft; a fifth rotation shaft connected to the third ring gear and directly connected to the output gear; a sixth rotation shaft directly connected to the second planet carrier and the third planet carrier and selectively connected to the input shaft; a seventh rotation shaft directly connected to the second ring gear and the third sun gear and selectively connected to the first rotation shaft or the second rotation shaft; and six friction members interposed selectively at connecting portions of the rotation shafts. 
     The first, second, and third planetary gear sets may be single pinion planetary gear sets. 
     The six friction members may include: a first clutch selectively connecting the second rotation shaft to the fourth rotation shaft; a second clutch selectively connecting the second rotation shaft to the seventh rotation shaft; a third clutch selectively connecting the sixth rotation shaft to the input shaft; a fourth clutch selectively connecting the first rotation shaft to the seventh rotation shaft; a first brake selectively connecting the sixth rotation shaft to the transmission housing; and a second brake selectively connecting the first rotation shaft to the transmission housing. 
     Forward speeds achieved by operating three friction members among the six friction members may include: a first forward speed achieved by operating the first clutch, the fourth clutch, and the first brake; a second forward speed achieved by operating the first clutch, the first brake, and the second brake; a third forward speed achieved by operating the first clutch, the fourth clutch, and the second brake; a fourth forward speed achieved by operating the first clutch, the second clutch, and the second brake; a fifth forward speed achieved by operating the first clutch, the third clutch, and the second brake; a sixth forward speed achieved by operating the first clutch, the second clutch, and the third clutch; a seventh forward speed achieved by operating the second clutch, the third clutch, and the second brake; and an eighth forward speed achieved by operating the third clutch, the fourth clutch, and the second brake. 
     Reverse speeds achieved by operating three friction members among the six friction members may include: a first reverse speed achieved by operating the second clutch, the first brake, and the second brake; and a second reverse speed achieved by operating the second clutch, the fourth clutch, and the first brake. 
     The planetary gear train may further include a one-way clutch disposed in parallel with the first brake, and operated at a normal driving condition. 
     The methods and apparatuses of the present invention have other features and advantages which will be apparent from or are set forth in more detail in the accompanying drawings, which are incorporated herein, and the following Detailed Description, which together serve to explain certain principles of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an exemplary planetary gear train according to the present invention. 
         FIG. 2  is an operational chart of friction members at each shift-speed applied to an exemplary planetary gear train according to the present invention. 
         FIG. 3  is a lever diagram for an exemplary planetary gear train according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to various embodiments of the present invention(s), examples of which are illustrated in the accompanying drawings and described below. While the invention(s) will be described in conjunction with exemplary embodiments, it will be understood that present description is not intended to limit the invention(s) to those exemplary embodiments. On the contrary, the invention(s) is/are intended to cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents and other embodiments, which may be included within the spirit and scope of the invention as defined by the appended claims. 
     Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
     Description of components that are not necessary for explaining the present exemplary embodiment will be omitted, and the same constituent elements are denoted by the same reference numerals in this specification 
     In the detailed description, ordinal numbers are used for distinguishing constituent elements having the same terms, and have no specific meanings or orders 
     Referring to  FIG. 1 , a planetary gear train according to various embodiments of the present invention includes first, second, and third planetary gear sets PG 1 , PG 2 , and PG 3  disposed on the same axis, an input shaft IS, an output gear OG, seven rotation shafts TM 1 , TM 2 , TM 3 , TM 4 , TM 5 , TM 6 , and TM 7  directly or selectively connecting rotation elements of first, second, and third planetary gear sets PG 1 , PG 2 , and PG 3 , six friction members (first, second, third, and fourth clutches C 1 , C 2 , C 3 , and C 4  and first and second brakes B 1  and B 2 ), and a transmission housing H. 
     A torque input from the input shaft IS is changed by the first, second, and third planetary gear sets PG 1 , PG 2 , and PG 3  and is output through the output gear OG. 
     In addition, the planetary gear sets is disposed in a sequence of the third, second, and first planetary gear sets PG 3 , PG 2 , and PG 1  from an engine. 
     The input shaft IS is an input member, and torque from a crankshaft of the engine is changed through a torque converter and is input to the input shaft IS. 
     The output gear OG is an output member and delivers driving torque so as to run driving wheels through a differential apparatus. 
     The first planetary gear set PG 1  is a single pinion planetary gear set, and has a first sun gear S 1 , a first planet carrier PC 1  supporting a first pinion P 1  externally engaged to the first sun gear S 1 , and a first ring gear R 1  internally engaged to the first pinion P 1  as rotation elements thereof. 
     The second planetary gear set PG 2  is a single pinion planetary gear set, and has a second sun gear S 2 , a second planet carrier PC 2  supporting a second pinion P 2  externally engaged to the second sun gear S 2 , and a second ring gear R 2  internally engaged to the second pinion P 2  as rotation elements thereof. 
     The third planetary gear set PG 3  is a single pinion planetary gear set, and has a third sun gear S 3 , a third planet carrier PC 3  supporting a third pinion P 3  externally engaged to the third sun gear S 3 , and a third ring gear R 3  internally engaged to the third pinion P 3  as rotation elements thereof. 
     In addition, three rotation elements of the first planetary gear set PG 1  is operated as independent rotation elements, and the first planetary gear set PG 1  includes first, second, and third rotation shafts TM 1 , TM 2 , and TM 3 . 
     In the second and third planetary gear sets PG 2  and PG 3 , the second and third planet carriers PC 2  and PC 3  are directly connected to each other, and the second ring gear R 2  and the third sun gear S 3  are directly connected to each other such that fourth, fifth, sixth, and seventh rotation shafts TM 4 , TM 5 , TM 6 , and TM 7  are included. 
     The first rotation shaft TM 1  includes the first sun gear S 1 . The second rotation shaft TM 2  includes the first planet carrier PC 1 . The third rotation shaft TM 3  includes the first ring gear R 1  and is directly connected to the input shaft IS so as to be always operated as an input element. The fourth rotation shaft TM 4  includes the second sun gear S 2 . The fifth rotation shaft TM 5  includes the third ring gear R 3  and is directly connected to the output gear OG so as to be always operated as a final output element. The sixth rotation shaft TM 6  includes the second planet carrier PC 2  and the third planet carrier PC 3  directly connected to each other without a friction member. The seventh rotation shaft TM 7  includes the second ring gear R 2  and the third sun gear S 3  directly connected to each other without a friction member. 
     The first clutch C 1  selectively connects the second rotation shaft TM 2  to the fourth rotation shaft TM 4 . The second clutch C 2  selectively connects the second rotation shaft TM 2  to the seventh rotation shaft TM 7 . The third clutch C 3  selectively connects the input shaft IS to the sixth rotation shaft TM 6 . The fourth clutch C 4  selectively connects the first rotation shaft TM 1  to the seventh rotation shaft TM 7 . 
     The first brake B 1  selectively connects the sixth rotation shaft TM 6  to the transmission housing H. The second brake B 2  selectively connects the first rotation shaft TM 1  to the transmission housing H. 
     The first brake B 1  includes a one-way clutch F 1  disposed in parallel thereto, and the one-way clutch F 1  is operated at a normal driving condition instead of the first brake B 1  being operated. 
     A clutch is a friction member selectively connecting one rotation member to another rotation member, and a brake is a friction member connecting a rotation member to a non-rotation member. 
     If the clutch is operated, rotation shafts connected to each other is operated as one rotation member, and if the brake is operated, a rotation shaft connected to the transmission housing H is operated as a non-rotation member. 
     In addition, the friction members comprising of the first, second, third, and fourth clutches C 1 , C 2 , C 3 , and C 4  and the first and second brakes B 1  and B 2  may be conventional multi-plate friction elements of wet type that are operated by hydraulic pressure. However, one will appreciate that other types of friction members can be used. 
       FIG. 2  is an operational chart of friction members at each shift-speed applied to a planetary gear train according to various embodiments of the present invention. It shows that three friction members among six friction members are operated at each shift-speed; and eight forward speeds and two reverse speeds are achieved. 
     Referring to  FIG. 2 , operations of the friction members at each shift-speed are as follows. 
     At a first forward speed D 1 , the first clutch C 1 , the fourth clutch C 4 , and the first brake B 1  are operated. At a second forward speed D 2 , the first clutch C 1 , the first brake B 1 , and the second brake B 2  are operated. At a third forward speed D 3 , the first clutch C 1 , the fourth clutch C 4 , and the second brake B 2  are operated. At a fourth forward speed D 4 , the first clutch C 1 , the second clutch C 2 , and the second brake B 2  are operated. At a fifth forward speed D 5 , the first clutch C 1 , the third clutch C 3 , and the second brake B 2  are operated. At a sixth forward speed D 6 , the first clutch C 1 , the second clutch C 2 , and the third clutch C 3  are operated. At a seventh forward speed D 7 , the second clutch C 2 , the third clutch C 3 , and the second brake B 2  are operated. At an eighth forward speed D 8 , the third clutch C 3 , the fourth clutch C 4 , and the second brake B 2  are operated. 
     At a first reverse speed REV 1 , the second clutch C 2 , the first brake B 1 , and the second brake B 2  are operated. At a second reverse speed REV 2 , the second clutch C 2 , the fourth clutch C 4 , and the first brake B 1  are operated. 
     It is explained that the first clutch C 1 , the fourth clutch C 4 , and the first brake B 1  are operated at the first forward speed D 1 , but the one-way clutch F 1  instead of the first brake B 1  is operated at the normal driving condition. 
       FIG. 3  is a lever diagram for a planetary gear train according to various embodiments of the present invention. 
     Referring to  FIG. 3 , a lower horizontal line represents a rotation speed of “0”, and an upper horizontal line represents a rotation speed of “1.0”, that is, the rotation speed thereof is the same as that of the input shaft IS. 
     Three vertical lines of the first planetary gear set PG 1  sequentially represent the first, second, and third rotation shafts TM 1 , TM 2 , and TM 3  from the left to the right. 
     Herein, distances between the first, second, and third rotation shafts TM 1 , TM 2 , and TM 3  are set according to gear ratios (teeth number of the sun gear/teeth number of the ring gear) of the first planetary gear set PG 1 . 
     Four vertical lines of the second and third planetary gear sets PG 2  and PG 3  sequentially represent the fourth, fifth, sixth, and seventh rotation shafts TM 4 , TM 5 , TM 6 , and TM 7  from the left to the right. 
     Herein, distances between the fourth, fifth, sixth, and seventh rotation shafts TM 4 , TM 5 , TM 6 , and TM 7  are set according to gear ratios (teeth number of the sun gear/teeth number of the ring gear) of the second and third planetary gear sets PG 2  and PG 3 . 
     Referring to  FIG. 2  and  FIG. 3 , shifting processes for each shift-speed in the planetary gear train according to various embodiments will be described. 
     First Forward Speed 
     Referring to  FIG. 2 , the first clutch C 1 , the fourth clutch C 4 , and the first brake B 1  are simultaneously operated at the first forward speed D 1 . 
     As shown in  FIG. 3 , in a state that the rotation speed of the input shaft IS is input to the third rotation shaft TM 3 , the sixth rotation shaft TM 6  is operated as the fixed element by operation of the first brake B 1 . 
     In addition, the first rotation shaft TM 1  is connected to the seventh rotation shaft TM 7  by operation of the fourth clutch C 4 . Thus, the first planetary gear set PG 1  forms a first speed line T 1  and a first reduced speed is output through the second rotation shaft TM 2 . 
     In a state that the rotation speed of the second rotation shaft TM 2  is input to the second and third planetary gear sets PG 2  and PG 3  through the fourth rotation shaft TM 4 , the sixth rotation shaft TM 6  is operated as the fixed element by operation of the first brake B 1  so as to form a first shift line SP 1 . 
     Therefore, the first shift line SP 1  crosses the vertical line of the fifth rotation shaft TM 5  that is the output element such that the first forward speed D 1  is output. 
     Second Forward Speed 
     The fourth clutch C 4  that was operated at the first forward speed D 1  is released and the second brake B 2  is operated at the second forward speed D 2 . 
     As shown in  FIG. 3 , in a state that the rotation speed of the input shaft IS is input to the third rotation shaft TM 3 , the first rotation shaft TM 1  is operated as the fixed element by operation of the second brake B 2 . 
     Thus, the first planetary gear set PG 1  forms a second speed line T 2 , and a second reduced speed is output through the second rotation shaft TM 2 . 
     In a state that the rotation speed of the second rotation shaft TM 2  is input to the second and third planetary gear sets PG 2  and PG 3  through the fourth rotation shaft TM 4  by operation of the first clutch C 1 , the sixth rotation shaft TM 6  is operated as the fixed element by operation of the first brake B 1  so as to form a second shift line SP 2 . 
     Therefore, the second shift line SP 2  crosses the vertical line of the fifth rotation shaft TM 5  that is the output element such that the second forward speed D 2  is output. 
     Third Forward Speed 
     The first brake B 1  that was operated at the second forward speed D 2  is released and the fourth clutch C 4  is operated at the third forward speed D 3 . 
     As shown in  FIG. 3 , in a state that the rotation speed of the input shaft IS is input to the third rotation shaft TM 3 , the first rotation shaft TM 1  is operated as the fixed element by operation of the second brake B 2 . 
     Thus, the first planetary gear set PG 1  forms a second speed line T 2 , and a second reduced speed is output through the second rotation shaft TM 2 . 
     The rotation speed of the second rotation shaft TM 2  is input to the second and third planetary gear sets PG 2  and PG 3  through the fourth rotation shaft TM 4  by operation of the first clutch C 1 . 
     At this state, the seventh rotation shaft TM 7  is connected to the first rotation shaft TM 1  by operation of the fourth clutch C 4 . Therefore, the seventh rotation shaft TM 7  is operated as the fixed element and a third shift line SP 3  is formed. 
     Therefore, the third shift line SP 3  crosses the vertical line of the fifth rotation shaft TM 5  that is the output element such that the third forward speed D 3  is output. 
     Fourth Forward Speed 
     The fourth clutch C 4  that was operated at the third forward speed D 3  is released and the second clutch C 2  is operated at the fourth forward speed D 4 . 
     As shown in  FIG. 3 , in a state that the rotation speed of the input shaft IS is input to the third rotation shaft TM 3 , the first rotation shaft TM 1  is operated as the fixed element by operation of the second brake B 2 . 
     Thus, the first planetary gear set PG 1  forms a second speed line T 2 , and a second reduced speed is output through the second rotation shaft TM 2 . 
     The rotation speed of the second rotation shaft TM 2  is simultaneously input to the fourth rotation shaft TM 4  and the seventh rotation shaft TM 7  by operations of the first clutch C 1  and the second clutch C 2  in the second and third planetary gear sets PG 2  and PG 3 . 
     Therefore, the second and third planetary gear sets PG 2  and PG 3  become direct-coupling state and a fourth shift line SP 4  is formed. Therefore, the fourth shift line SP 4  crosses the vertical line of the fifth rotation shaft TM 5  that is the output element such that the fourth forward speed D 4  is output. 
     Fifth Forward Speed 
     The second clutch C 2  that was operated at the fourth forward speed D 4  is released and the third clutch C 3  is operated at the fifth forward speed D 5 . 
     As shown in  FIG. 3 , in a state that the rotation speed of the input shaft IS is input to the third rotation shaft TM 3 , the first rotation shaft TM 1  is operated as the fixed element by operation of the second brake B 2 . 
     Thus, the first planetary gear set PG 1  forms a second speed line T 2 , and a second reduced speed is output through the second rotation shaft TM 2 . 
     The rotation speed of the second rotation shaft TM 2  is input to the second and third planetary gear sets PG 2  and PG 3  through the fourth rotation shaft TM 4  by operation of the first clutch C 1 . 
     At this state, the rotation speed of the input shaft IS is input to the sixth rotation shaft TM 6  by operation of the third clutch C 3 , and a fifth shift line SP 5  is formed. 
     Therefore, the fifth shift line SP 5  crosses the vertical line of the fifth rotation shaft TM 5  that is the output element such that the fifth forward speed D 5  is output. 
     Sixth Forward Speed 
     The second brake B 2  that was operated at the fifth forward speed D 5  is released and the second clutch C 2  is operated at the sixth forward speed D 6 . 
     Since the first, second, and third clutches C 1 , C 2 , and C 3  are operated, the first, second, and third planetary gear sets PG 1 , PG 2 , and PG 3  become direct-coupling state. 
     Therefore, a third speed line T 3  and a sixth shift line SP 6  are formed and the sixth forward speed D 6  that is the same as the rotation speed of the input shaft IS is output. 
     Seventh Forward Speed 
     The first clutch C 1  that was operated at the sixth forward speed D 6  is released and the second brake B 2  is operated at the seventh forward speed D 7 . 
     As shown in  FIG. 3 , in a state that the rotation speed of the input shaft IS is input to the third rotation shaft TM 3 , the first rotation shaft TM 1  is operated as the fixed element by operation of the second brake B 2 . 
     Thus, the first planetary gear set PG 1  forms a second speed line T 2 , and a second reduced speed is output through the second rotation shaft TM 2 . 
     The rotation speed of the second rotation shaft TM 2  is input to the second and third planetary gear sets PG 2  and PG 3  through the seventh rotation shaft TM 7  by operation of the second clutch C 2 . 
     At this state, the rotation speed of the input shaft IS is input to the sixth rotation shaft TM 6  by operation of the third clutch C 3  and a seventh shift line SP 7  is formed. 
     Therefore, the seventh shift line SP 7  crosses the vertical line of the fifth rotation shaft TM 5  that is the output element such that the seventh forward speed D 7  is output. 
     Eighth Forward Speed 
     The second clutch C 2  that was operated at the seventh forward speed D 7  is released and the fourth clutch C 4  is operated at the eighth forward speed D 8 . 
     As shown in  FIG. 3 , in a state that the rotation speed of the input shaft IS is input to the third rotation shaft TM 3 , the first rotation shaft TM 1  is operated as the fixed element by operation of the second brake B 2 . 
     Thus, the first planetary gear set PG 1  forms a second speed line T 2 , and a second reduced speed is output through the second rotation shaft TM 2 . 
     However, since the first and second clutches C 1  and C 2  are not operated, the rotation speed of the second rotation shaft TM 2  is not delivered to the second and third planetary gear sets PG 2  and PG 3 . 
     In a state that the seventh rotation shaft TM 7  is operated as the fixed element by operations of the fourth clutch C 4  and the second brake B 2 , the rotation speed of the input shaft IS is input to the sixth rotation shaft TM 6  by operation of the third clutch C 3  in the second and third planetary gear sets PG 2  and PG 3 . 
     Therefore, an eighth shift line SP 8  is formed, and the eighth shift line SP 8  crosses the vertical line of the fifth rotation shaft TM 5  that is the output element such that the eighth forward speed D 8  is output. 
     First Reverse Speed 
     The second clutch C 2 , the first brake B 1 , and the second brake B 2  are operated at the first reverse speed REV 1 . 
     As shown in  FIG. 3 , in a state that the rotation speed of the input shaft IS is input to the third rotation shaft TM 3 , the first rotation shaft TM 1  is operated as the fixed element by operation of the second brake B 2 . 
     Thus, the first planetary gear set PG 1  forms a second speed line T 2 , and a second reduced speed is output through the second rotation shaft TM 2 . 
     The rotation speed of the second rotation shaft TM 2  is input to the second and third planetary gear sets PG 2  and PG 3  through the seventh rotation shaft TM 7  by operation of the second clutch C 2 . 
     At this state, the sixth rotation shaft TM 6  is operated as the fixed element by operation of the first brake B 1 , and a first reverse shift line SR 1  is formed. 
     Therefore, the first reverse shift line SR 1  crosses the vertical line of the fifth rotation shaft TM 5  that is the output element such that the first reverse speed REV 1  is output. 
     Second Reverse Speed 
     The second brake B 2  that was operated at the first reverse speed REV 1  is released and the fourth clutch C 4  is operated at the second reverse speed REV 2 . 
     As shown in  FIG. 3 , in a state that the rotation speed of the input shaft IS is input to the third rotation shaft TM 3 , the second and fourth clutches C 2  and C 4  are operated. Therefore, the first planetary gear set PG 1  becomes direct-coupling state. Therefore, the rotation speed of the input shaft IS is delivered to the second and third planetary gear sets PG 2  and PG 3  through the seventh rotation shaft TM 7 . 
     In a state that the rotation speed of the input shaft IS is input to the seventh rotation shaft TM 7 , the sixth rotation shaft TM 6  is operated as the fixed element by operation of the first brake B 1 . Accordingly, the second and third planetary gear sets PG 2  and PG 3  form a second reverse shift line SR 2 . 
     Therefore, the second reverse shift line SR 2  crosses the vertical line of the fifth rotation shaft TM 5  that is the output element such that the second reverse speed REV 2  is output. 
     As described above, eight forward speeds and two reverse speeds are achieved by combining three planetary gear sets PG 1 , PG 2 , and PG 3  with four clutches C 1 , C 2 , C 3 , and C 4  and two brakes B 1  and B 2  according to various embodiments of the present invention. 
     Therefore, power delivery efficiency and fuel economy may be improved and reverse speed performance may be also improved. 
     Since at least three friction members are operated at each shift-speed, the number of the friction members that are not operated can be reduced. Therefore, drag loss may be reduced and power delivery efficiency and fuel economy may be further improved. 
     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.