Abstract:
A gear train for an automatic transmission includes first, second and third planetary gear sets. The first planetary gear set includes a first element variably connected to an input shaft, a second element variably connected to the input shaft and the first element, and a third element connected to a transfer shaft to transmit power to the transfer shaft. The second planetary gear set includes a fourth element fixedly connected to the first element while being variably connected to the input shaft, a fifth element variably connected to the input shaft while being variably connected to a combination of the first and fourth elements, and a sixth element connected to the transfer shaft to transmit power to the transfer shaft. The third planetary gear set includes a seventh element fixedly connected to the combination of the first and fourth elements, an eighth element variably connected to the transmission housing, and a ninth element fixedly connected to the third element while being connected to the transfer shaft to transmit power thereto. A plurality of friction elements are provided to realize the variable connections.

Description:
BACKGROUND OF THE INVENTION 
     (a) Field of the Invention 
     The present invention relates to a gear train for an automatic transmission used in vehicles. More particularly, the invention relates to a gear train for a 6-forward speed and 1-reverse speed automatic transmission. 
     (b) Description of the Related Art 
     Generally, automatic transmission systems for vehicles are provided with a transmission control unit (“TCU”) which automatically controls shift ratios according to changes in a running condition of the vehicle. 
     The typical TCU controls a plurality of friction elements provided in a gear train to either operative or inoperative states to select one of the three essential elements of the planetary gear set (a sun gear, a ring gear and a planet carrier) to be an input element, one to be a reaction element, and one to be an output element, thereby controlling an output number of revolutions. 
     Particularly, a gear train that can realize a 5-forward speed and a 1-reverse speed comprises a plurality of heavy and large-sized clutches and brakes and a plurality of inoperative friction elements, resulting in deterioration of power and space efficiencies. 
     SUMMARY OF THE INVENTION 
     Therefore, the present invention has been made in an effort to solve the above problems. 
     It is an object of the present invention to provide a gear train for an automatic transmission, which is compact in size and light in weight, while providing improved power efficiency. 
     To achieve the above object, the present invention provides a gear train for an automatic transmission comprising a first planetary gear set comprising a first element variably connected to an input shaft, a second element variably connected to the input shaft and the first element, and a third element connected to a transfer shaft to transmit power to the transfer shaft; a second planetary gear set comprising a fourth element fixedly connected to the first element while being variably connected to the input shaft, a fifth element variably connected to the input shaft while being variably connected to a combination of the first and fourth elements, and a sixth element connected to the transfer shaft to transmit power to the transfer shaft; a third planetary gear set comprising a seventh element fixedly connected to the combination of the first and fourth elements, an eighth element variably connected to the transmission housing, and a ninth element fixedly connected to the third element while being connected to the transfer shaft to transmit power thereto; and friction means for realizing the variable connections. 
     Preferably, the first planetary gear set is a single pinion planetary gear set, the second planetary gear set is a double pinion planetary gear set, and the third planetary gear set is a single pinion planetary gear set. 
     Preferably, the first element is a first sun gear, the second element is a first ring gear, the third element is a first planet carrier, the fourth element is a second sun gear, the fifth element is a second planet carrier, the sixth element is a second ring gear, the seventh element is a third sun gear, the eighth element is a third planet carrier, and the ninth element is a third ring gear. 
     The friction means comprises a first clutch interposed between the input shaft and the second carrier, a second clutch interposed between the input shaft and the first planet carrier, a third clutch interposed between the input shaft and the combination of the first and second sun gears, a first brake interposed between the third planet carrier and the transmission housing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention, and, together with the description, serve to explain the principles of the invention: 
     FIG. 1 is a schematic diagram of a gear train for an automatic transmission according to a preferred embodiment of the present invention; 
     FIG. 2 is an operational chart of friction elements in each shift range according to a preferred embodiment of the present invention; 
     FIG. 3 is a view for illustrating shift ratio of a first speed in a drive “D” range according to a preferred embodiment of the present invention through a lever analogy method; 
     FIG. 4 is a view for illustrating a shift ratio of a second speed in a drive “D ” range according to a preferred embodiment of the present invention through a lever analogy method; 
     FIG. 5 is a view for illustrating a shift ratio of a third speed in a drive “D” range according to a preferred embodiment of the present invention through a lever analogy method; 
     FlG.  6  is a view for illustrating a shift ratio of a fourth speed in a drive “D” range according to a preferred embodiment of the present invention through a lever analogy method; 
     FIG. 7 is a view for illustrating a shift ratio of a fifth speed in a drive “D” range according to a preferred embodiment of the present invention through a lever analogy method; and 
     FIG. 8 is a view for illustrating a shift ratio of a reverse speed according to a preferred embodiment of the present invention through a lever analogy method. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A preferred embodiment of the present invention will now be described in detail with reference to the accompanying drawings. 
     FIG. 1 shows a schematic diagram of a gear train according to a preferred embodiment of the present invention. 
     The inventive gear train comprises first, second and third planetary gear sets PG 1 , PG 2  and PG 3 . that are disposed around an input shaft  1 . The first planetary gear set PG 1  is a single pinion type, which comprises a sun gear S 1  variably connected to the input shaft  1 , a ring gear Rl variably connected to the input shaft  1 , and a planet carrier Ca 1  connected to a transfer shaft  5  to transmit power thereto. The sun gear S 1  is also variably connected to the ring gear R 1 . 
     The second planetary gear set PG 2  is a double pinion type, which comprises a sun gear S 2  fixedly connected to the sun gear S 1  of the first planetary gear set PG 1  while being variably connected to the input shaft  1 , a planet carrier Ca 2  variably connected to the input shaft  1  while being variably connected to the combination of the sun gears S 1  and S 2 , and a ring gear R 2  connected to the transfer shaft  5  to transmit power thereto. 
     The third planetary gear set PG 3  is a single pinion type, which comprises a sun gear S 3  fixedly connected to the combination of the sun gears S 1  and S 2 , a planet carrier Ca 3  variably connected to the transmission housing  3 , and a ring gear fixedly connected to the planet carrier Ca 1  of the first planetary gear set PG 1  while being connected to the transfer shaft  5  to transmit power thereto. 
     For the above described variable connection, a first clutch C 1  is interposed between the input shaft  1  and the planet carrier Ca 2  of the second planetary gear set PG 2 , a second clutch C 2  is interposed between the input shaft  1  and the planet carrier Ca 1 , and a third clutch is interposed between the input shaft  1  and the combination of the sun gears S 1  and S 2  of the first and second planetary gear sets PG 1  and PG 2 . In addition, a first brake B 1  is interposed between the planet carrier Ca 3  of the third planetary gear set PG 3  and the transmission housing  3 . 
     As a result of the above, through the selective operation of the first, second and third clutches C 1 , C 2  and C 3  and the first brake B 1 , 5-forward speed and 1-reverse speed are realized and transmitted to the transfer shaft  5 . The selective operation of the friction elements is controlled by the TCU. First and second transfer drive gears  7  and  9  respectively fixed on the ring gear R 2  of the second planetary gear set PG 2  and the combination of the planet carrier Ca 1  of the first planetary gear set PG 1  and the ring gear R 3  of the third planetary gear set PG 3  are engaged with first and second transfer driven gears  11  and  13  of the transfer shaft  5 , respectively. 
     That is, the friction elements are operated in each speed as shown in the friction elements operation chart of FIG.  2 . The shift process will be explained hereinafter using the operation chart of FIG.  2  and the lever analogy diagrams of FIGS. 3,  4 ,  5 ,  6 ,  7  and  8 , in which the first, second, and third planetary gear sets PG 1 , PG 2  and PG 3  are represented by first to sixth levers L 1  to L 6 . 
     First to sixth nodes N 1  to N 6  located on each lever denotes each element of the first, second and third planetary gear sets PG 1 , PG 2  and PG 3 . That is, the first node N 1  denotes the planet carrier Ca 2  of the second planetary gear set PG 2 , the second node N 2  denotes the ring gear R 1  of the first planetary gear set PG 1 , the third node N 3  denotes the combination of the ring gear Ca 1  of the first planetary gear set PG 1  and the ring gear R 3  of the third planetary gear set PG 3 , and the fourth node N 4  indicates the ring gear R 2  of the second planetary gear set PG 2 . The fifth node N 5  indicates the planet carrier Ca 3  of the third planetary gear set PG 3  and the sixth node N 6  denotes the combination of the sun gears S 1 , S 2  and S 3  of the first, second and third planetary gear sets PG 1 , PG 2  and PG 3 . 
     FIRST-FORWARD SPEED 
     A speed ratio of the first-forward speed will be explained hereinafter with reference to FIG.  3 . 
     In the first-forward speed, since the first clutch C 1  and the first brake B 1  are controlled to operate by the TCU as shown in FIG. 2, in the first lever L 1  of FIG. 3, the third and fourth nodes N 3  and N 4  become the output elements, the fifth node N 5  becomes the reacting element, and the first node N 1  operates as the input element. 
     It is assumed that an output speed of the third node N 3  is fixed at “1,” and an output speed of the fourth node N 4  is lower than the output speed “1” of the third node N 3 . 
     Accordingly, an extended line connecting the output speed “1,”outputted from the output node N 3 , to the reacting node N 5  becomes a first-forward speed line I 1 . 
     Therefore, a line vertically connecting the input node N 1  to the first speed line I 1  becomes a first input speed line D 1 . The first input speed line D 1  is higher than the output speed “1.” 
     Accordingly, it is noted that an output number of rotations becomes much smaller than an input number of rotations, and a reduction in speed is realized through a first speed shift ratio. 
     SECOND-FORWARD SPEED 
     In the above first-forward speed state, if vehicle speed and throttle opening are increased, the TCU disengages the first clutch C 1  and operates the second clutch C 2  as shown in the operation chart of FIG.  2 . Accordingly, in the second lever L 2  shown in FIG. 4, the input element is changed from the first node N 1  to the second node N 2  while the third and fourth nodes N 3  and N 4  become the output elements and the fifth node N 5  becomes the reacting element. 
     It is still assumed that an output speed of the third node N 3  is fixed at “1,” and an output speed of the fourth node N 4  is lower than the output speed “1” of the third node N 3 . 
     Therefore, an extended line connecting the output speed, outputted from the fourth node N 4 , to the reacting node N 5  becomes the second-forward speed line I 2 . 
     Accordingly, a line vertically connecting the input node N 2  to the second-forward speed line I 2  becomes the second input speed line D 2 . 
     The second input speed line D 2  is higher than the output speed “1,”but lower than the first input speed line D 1 . 
     Accordingly, it is noted that an output number of rotations in the second speed becomes smaller than an input number of rotations, but higher than that in the first speed. 
     THIRD-FORWARD SPEED 
     In the above third speed state, if vehicle speed and throttle opening are increased, the TCU disengages the first brake B 1 , operating the first clutch C 1 . Accordingly, in the third lever L 3  shown in FIG. 5, the third and fourth nodes N 3  and N 4  operate as the output elements, and the first and second node N 1  and N 2  become the input elements. 
     It is still assumed that an output speed of the third node N 3  is fixed at “1,” and an output speed of the fourth node N 4  is lower than the output speed “1” of the third node N 3 . 
     It is further assumed that inputs speeds D 3  of the first and second nodes N 1  and N 2  are higher than “1,” but lower than that in the second speed. 
     Therefore, an extended line connecting the output speed “1,” outputted from the output node N 3 , to the input speed of the node N 2  becomes a first third-forward speed line  13 ′ and an extended line connecting the output speed of the output node N 4  to the input speed of the node N 2  becomes a second third-forward speed line I 3 .″ 
     Accordingly, it can be noted that, in the third-forward speed, the planet carrier Ca 3  of the fifth node N 5  and the combination of the sun gears S 1 , S 2  and S 3  rotates at the lower speed that other elements. 
     FOURTH-FORWARD SPEED 
     In the above third speed state, if vehicle speed and throttle opening are increased, the TCU disengages the first clutch C 1  and operates the third clutch C 3 . Accordingly, in the fourth lever L 4  shown in FIG. 6, the third and fourth nodes N 3  and N 4  operate as the output elements, and the second and sixth nodes N 2  and N 6  become the input elements. 
     It is still assumed that an output speed of the third node N 3  is fixed at “1,” and an output speed of the fourth node N 4  is lower than the output speed “1” of the third node N 3 . 
     It is further assumed that input speeds D 4  of the second and sixth nodes N 2  and N 6  are the same as the output speed “1” of the output node N 3 . That is, in the fourth speed, an input number of rotations is the same as an output number of rotations. 
     Accordingly, an extended line connecting the output speed “1” of the output node N 3  to the input speed of the second node N 2  becomes a first fourth-forward speed line  14 ′, and an extended line connecting the output speed of the output node N 4  to the input speed of the sixth node N 2  becomes a second fourth-forward speed line  14 ″. 
     FIFTH-FORWARD SPEED 
     In the above fourth-forward speed state, if vehicle speed and throttle opening are increased, the TCU disengages the second clutch C and operates the first clutch C 1 . Accordingly, in the fifth lever L 5  shown in FIG. 7, the third and fourth nodes N 3  and N 4  operate as the output elements, and the first and sixth nodes N 1  and N 6  become the input elements. 
     It is still assumed that an output speed of the third node N 3  is fixed at “1,” and an output speed of the fourth node N 4  is lower than the output speed “1” of the third node N 3 . 
     It is further assumed that input speeds D 5  of the second and sixth nodes N 2  and N 6  are the same as the output speed of the fourth speed. That is, in the sixth speed, an overdrive is achieved, in which an output number of rotations is higher than an input number of rotations. 
     Accordingly, an extended line connecting the output speed “1” of the output node N 3  to the input speed of the input node N 6  becomes a first fifth-forward speed line I 5 ′, and an extended line connecting the output speed of the output node N 4  to the output speed of the input speed of the input node N 1  becomes a second fifth-forward speed line  15 ″. 
     REVERSE SPEED 
     If the driver changes the selector lever to a reverse R range, the TCU controls the third clutch C 3  and the first brake B 1  to operate as shown in FIG.  2 . Accordingly, in the sixth lever L 6  shown in FIG. 7, the third and fourth nodes N 3  and N 4  operate as the output elements, and the sixth node N 6  operates as the input element. The fifth node N 5  functions as the reacting element. 
     It is still assumed that an output speed of the third node N 3  is fixed at “1,” and an output speed of the fourth node N 4  is lower than the output speed “1” of the third node N 3 . 
     Accordingly, an extended line connecting the output speed “1” of the output node N 3  to the reacting node N 5  becomes a reverse speed line IR 1 . 
     Therefore, a line vertically connecting the input node N 6  to the reverse speed line IR 1  becomes an actual reverse input speed line r 1 , realizing reverse shifting. That is, it can be noted that, the input of the reverse speed is opposite to the output. 
     Although preferred embodiments of the present invention have been described in detail hereinabove, it should be clearly understood that many variations and/or modifications of the basic inventive concepts herein taught which may appear to those skilled in the present art will still fall within the spirit and scope of the present invention, as defined in the appended claims.