Abstract:
A power train for an automatic transmission having at realizes seven forward speeds and two reverse speeds may include: a first planetary gear set, a second planetary gear set, a third planetary gear, an input shaft, an output gear, and a transmission case. Utilizing arrangements of these components as described herein, such an automatic transmission may be provided with increased durability and more compact size.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to and the benefit of Korean Patent Application No. 10-2006-0058907 filed in the Korean Intellectual Property Office on Jun. 28, 2006, the entire contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    (a) Field of the Invention 
         [0003]    The present invention relates to a power train for an automatic transmission. More particularly, the present invention relates to a power train for an automatic transmission that realizes seven forward speeds and two reverse speeds. 
         [0004]    (b) Description of the Related Art 
         [0005]    A typical shift mechanism of an automatic transmission utilizes a combination of a plurality of planetary gear sets. A power train of such an automatic transmission that includes the plurality of planetary gear sets changes rotating speed and torque received from a torque converter of the automatic transmission, and accordingly changes and transmits the changed torque to an output shaft. 
         [0006]    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. 
         [0007]    In addition, with the same number of speeds, features of a power train such as durability, efficiency in power transmission, and size depend a lot on the layout of combined planetary gear sets. Therefore, designs for a combining structure of a power train are also under constant investigation. 
         [0008]    A manual transmission that has too many speeds causes an inconvenience of excessively frequent shifting operations to a driver. Therefore, the positive features of more shift-speeds are more important for automatic transmissions because an automatic transmission automatically controls shifting operations basically without needing any manual operation. 
         [0009]    The above information disclosed in this Background section is only for enhancement of understanding of the 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. 
       SUMMARY OF THE INVENTION 
       [0010]    Embodiments of the present invention provide a power train for an automatic transmission having advantages of enhancing durability of the automatic transmission by reducing a speed ratio of each operational element. Embodiments of the present invention also provide a power train for an automatic transmission having further advantages of providing a compact arrangement of clutches. 
         [0011]    An exemplary power train for an automatic transmission according to an embodiment of the present invention may include: a first planetary gear set having operational elements of a first sun gear, a first ring gear, and a first planet carrier; a second planetary gear set having operational elements of a second sun gear, a second ring gear, and a second planet carrier; a third planetary gear set having operational elements of a third sun gear, a third ring gear, and a third planet carrier; an input shaft; an output gear; and a transmission case, wherein the second planet carrier is fixedly connected to the third ring gear, the third planet carrier is fixedly connected to the second ring gear, the first sun gear is always stationary by being fixedly connected to the transmission case, the first ring gear always acts as an input element by being fixedly connected to the input shaft, at least one of the fixedly connected third planet carrier and the second ring gear always acts as an output element by being fixedly connected to the output gear, the second sun gear is variably connected to the first ring gear via a first clutch, the third sun gear is variably connected to the first planet carrier via a second clutch, the second sun gear is variably connected to the first planet carrier via a third clutch, the second planet carrier is variably connected to the first ring gear via a fourth clutch, the second planet carrier is variably connected to the transmission case via a first brake and is subjected to a stopping operation of the first brake, and the third sun gear is variably connected to the transmission case via a second brake and is subjected to a stopping operation of the second brake. 
         [0012]    The first, second, and third planetary gear sets may be disposed in a sequence of the first planetary gear set, the second planetary gear set, and the third planetary gear set. 
         [0013]    The first, second, third, and fourth clutches may be disposed between the first planetary gear set and the second and third planetary gear sets. 
         [0014]    The output gear may be disposed between the first brake and the second brake. 
         [0015]    The output gear may be disposed on an opposite side to the input shaft with reference to the first planetary gear set. 
         [0016]    In addition, an exemplary power train for an automatic transmission according to an embodiment of the present invention may further include a first one-way clutch disposed in series with the second brake. 
         [0017]    An exemplary power train for an automatic transmission according to an embodiment of the present invention may further include a second one-way clutch disposed in parallel with the first brake. 
         [0018]    The third clutch, the second brake, and the first one-way clutch may be operated in a first forward speed, the second clutch and the second one-way clutch may be operated in a second forward speed, the fourth clutch, the second brake, and the first one-way clutch may be operated in a third forward speed, the second and third clutches may be operated in a fourth forward speed, the second and fourth clutches may be operated in a fifth forward speed, the first and fourth clutches may be operated in a sixth forward speed, the third and fourth clutches may be operated in a seventh forward speed, the third clutch and the first brake may be operated in a first reverse speed, and the first clutch and the first brake may be operated in a second reverse speed. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a schematic diagram of a power train for an automatic transmission according to an exemplary embodiment of the present invention. 
           [0020]      FIG. 2  is an operational chart for a power train for an automatic transmission according to an exemplary embodiment of the present invention. 
           [0021]      FIG. 3  is a speed diagram according to a lever analysis method for showing shifting processes from a first forward speed to a seventh forward speed in a power train for an automatic transmission according to an embodiment of the present invention. 
           [0022]      FIG. 4  is a speed diagram according to a lever analysis method for showing a shifting process from a first reverse speed to a second reverse speed in a power train for an automatic transmission according to an embodiment of the present invention. 
           [0023]      FIG. 5  is a schematic diagram of a power train for an automatic transmission according to another exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0024]    Exemplary embodiments of the present invention will hereinafter be described in detail with reference to the accompanying drawings. 
         [0025]    As shown in  FIG. 1  and  FIG. 5 , exemplary power trains for an automatic transmission according to the embodiments of the present invention include three planetary gear sets of first, second, and third planetary gear sets PG 1 , PG 2 , and PG 3 . 
         [0026]    The first planetary gear set PG 1  is a single pinion planetary gear set, and includes a first sun gear S 1 , a first planet carrier PC 1 , and a first ring gear R 1  as operational elements thereof. A first pinion gear P 1 , being engaged with both the first ring gear R 1  and the first sun gear S 1 , is connected to and carried by the first planet carrier PC 1 . 
         [0027]    The second planetary gear set PG 2  is a single pinion planetary gear set, and includes a second sun gear S 2 , a second planet carrier PC 2 , and a second ring gear R 2  as operational elements thereof. A second pinion gear P 2 , being engaged with both the second ring gear R 1  and the second sun gear S 2 , is connected to and carried by the second planet carrier PC 2 . 
         [0028]    The third planetary gear set PG 3  is a single pinion planetary gear set, and includes a third sun gear S 3 , a third planet carrier PC 3 , and a third ring gear R 3  as operational elements thereof. A third pinion gear P 3 , being engaged with both the third ring gear R 3  and the third sun gear S 3 , is connected to and carried by the third planet carrier PC 3 . 
         [0029]    In addition, as shown in  FIG. 1  and  FIG. 5 , exemplary power trains for an automatic transmission according to the embodiments of the present invention include an input shaft  100  for receiving torque from an engine (not shown), an output gear  200  for outputting torque from the power train, and a transmission case  300 . 
         [0030]    The first sun gear S 1  is preferably always stationary by being fixedly connected to the transmission case  300 . 
         [0031]    The first ring gear R 1  preferably always acts as an input element by being fixedly connected to the input shaft  100 . 
         [0032]    Therefore, the first planetary gear set PG 1  receives torque from the input shaft  100  and may transmit the torque to the second and third planetary gear sets PG 2  and PG 3  with a 1:1 speed ratio through the first ring gear R 1 . 
         [0033]    Alternatively, the first planetary gear set PG 1  receives the torque from the input shaft  100  and may transmit the torque to the second and third planetary gear sets PG 2  and PG 3  with a reduced speed ratio through the first planet carrier PC 1 . 
         [0034]    The second planet carrier PC 2  is fixedly connected to the third ring gear R 3 . 
         [0035]    The third planet carrier PC 3  is fixedly connected to the second ring gear R 2 . 
         [0036]    The second sun gear S 2  is variably connected to the first ring gear R 1  via a first clutch C 1 . 
         [0037]    The third sun gear S 3  is variably connected to the first planet carrier PC 1  via a second clutch C 2 . 
         [0038]    The second sun gear S 2  is also variably connected to the first planet carrier PC 1  via a third clutch C 3 . 
         [0039]    The second planet carrier PC 2  is variably connected to the first ring gear R 1  via a fourth clutch C 4 . 
         [0040]    At least one of the fixedly connected third planet carrier PC 3  and the second ring gear R 2  is preferably always stationary by being fixedly connected to the output gear  200 . 
         [0041]    The second planet carrier PC 2  is variably connected to the transmission case  300  via a first brake B 1  and is subjected to a stopping operation of the first brake B 1 . 
         [0042]    The third sun gear S 3  is variably connected to the transmission case  300  via a second brake B 2  and is subjected to a stopping operation of the second brake B 2 . 
         [0043]    In addition, as shown in  FIG. 1 , a first one-way clutch OWC 1  may be disposed in series with the second brake B 2 , and may be disposed between the third sun gear S 3  and the transmission case  300 . 
         [0044]    As shown in  FIG. 1  and  FIG. 5 , a second one-way clutch OWC 2  may be disposed in parallel with the first brake B 1 , and may be disposed between the second planet carrier PC 2  and the transmission case  300 . 
         [0045]    Meanwhile, as shown in  FIG. 1  and  FIG. 5 , the first, second, and third planetary gear sets PG 1 , PG 2 , and PG 3  are disposed in a sequence of the first planetary gear set PG 1 , the second planetary gear set PG 2 , and the third planetary gear set PG 3 . 
         [0046]    The first, second, third, and fourth clutches C 1 , C 2 , C 3 , and C 4  are disposed between the first planetary gear set PG 1  and the second and third planetary gear sets PG 2  and PG 3 . In addition, the output gear  200  is disposed between the first brake B 1  and the second brake B 2 . Therefore, a compact power train may be designed by optimizing a clutch arrangement. 
         [0047]    The output gear  200  is disposed on an opposite side to the input shaft  100  with reference to the first planetary gear set PG 1 . This shows that exemplary power trains for an automatic transmission according to the embodiments of present invention are applicable to a front-engine and rear-drive (FR) type engine, it is to be understood that the scope of the present invention is not limited to the disclosed embodiments or engine type. The output gear  200  may be disposed on the same side as the input shaft  100 , and this can be easily obtained by a person of an ordinary skill in the art based on the teachings herein. 
         [0048]    As shown in  FIG. 2 , according to exemplary embodiments of the present invention, brakes and clutches are operated as follows: The third clutch C 3  is operated in a neutral state. The third clutch C 3 , the second brake B 2 , and the first one-way clutch OWC 1  are operated in a first forward speed D 1 . The second clutch C 2  and the second one-way clutch OWC 2  are operated in a second forward speed D 2 . The fourth clutch C 4 , the second brake B 2 , and the first one-way clutch OWC 1  are operated in a third forward speed D 3 . The second and third clutches C 2  and C 3  are operated in a fourth forward speed D 4 . The second and fourth clutches C 2  and C 4  are operated in a fifth forward speed D 5 . The first and fourth clutches C 1  and C 4  are operated in a sixth forward speed D 6 . The third and fourth clutches C 3  and C 4  are operated in a seventh forward speed D 7 . 
         [0049]    In addition, according to the exemplary embodiments of the present invention, the third clutch C 3  and the first brake B 1  are operated in a first reverse speed REV.  1 , and the first clutch C 1  and the first brake B 1  are operated in a second reverse speed REV.  2 . 
         [0050]    Since the second one-way clutch OWC 2  is operated in the second forward speed for smooth shifting, the first brake B 1  may not be operated in the second forward speed. However, both the first brake B 1  and the second one-way clutch OWC 2  may be operated together. 
         [0051]    Hereinafter, shifting processes of the exemplary power trains for an automatic transmission according to the embodiments of the present invention will be described. 
         [0052]    In a shifting process from the first forward speed D 1  to the second forward speed D 2 , the third clutch C 3  is released and the second clutch C 2  is operated. In this case, the second one-way clutch OWC 2  is automatically operated without an additional control. 
         [0053]    In a shifting process from the second forward speed D 2  to the third forward speed D 3 , the second clutch C 2  is released and the fourth clutch C 4  and the second brake B 2  are operated. In this case, the first one-way clutch OWC 1  is automatically operated without an additional control and the second brake B 2  need not be controlled. 
         [0054]    In a shifting process from the third forward speed D 3  to the fourth forward speed D 4 , the fourth clutch C 4  is released and the second and third clutches C 2  and C 3  are operated. In this case, the second brake B 2  and the first one-way clutch OWC 1  are automatically released without an additional control. 
         [0055]    In a shifting process from the fourth forward speed D 4  to the fifth forward speed D 5 , the third clutch C 3  is released and the fourth clutch C 4  is operated. 
         [0056]    In a shifting process from the fifth forward speed D 5  to the sixth forward speed D 6 , the second clutch C 2  is released and the first clutch C 1  is operated. 
         [0057]    In a shifting process from the sixth forward speed D 6  to the seventh forward speed D 7 , the first clutch C 1  is released and the third clutch C 3  is operated. 
         [0058]    In a shifting process from the neutral state to the first reverse speed REV.  1 , the first brake B 1  is operated since the third clutch C 3  is operated in the neutral state. 
         [0059]    In a shifting process from the first reverse speed REV.  1  to the second reverse speed REV.  2 , the third clutch C 3  is released and the first clutch C 1  is operated. 
         [0060]    In a skip down shifting process from the fourth forward speed D 4  to the second forward speed D 2 , the third clutch C 3  is released. In this case, the second one-way clutch OWC 2  is automatically operated. 
         [0061]    In a skip down shifting process from the fifth forward speed D 5  to the third forward speed D 3 , the second clutch C 2  is released. In this case, the second brake B 2  need not to be controlled since the first one-way clutch OWC 1  is automatically operated. 
         [0062]    In a skip down shifting process from the fifth forward speed D 5  to the second forward speed D 2 , the second one-way clutch OWC 2  is operated without an additional control when the fourth clutch C 4  is released. 
         [0063]    In a skip down shifting process from the sixth forward speed D 6  to the third forward speed D 3 , the first clutch C 1  is released and the second brake B 2  is operated. In this case, the second brake B 2  need not to be controlled since the first one-way clutch OWC 1  is automatically operated. 
         [0064]    In addition, in a skip down shifting process from the seventh forward speed D 7  to the fifth forward speed D 5 , the third clutch C 3  is released and the second clutch C 2  is operated. 
         [0065]    As shown in  FIG. 3  and  FIG. 4 , the exemplary power trains for an automatic transmission according to the embodiments of the present invention include three single pinion planetary gear sets. The second planet carrier PC 2  is fixedly connected to the third ring gear R 3 , and the third planet carrier PC 3  is fixedly connected to the second ring gear R 2 . Thus, operational elements of the exemplary power trains for an automatic transmission according to the embodiments of the present invention are represented as seven nodes in the lever diagram. 
         [0066]    Accordingly, the first sun gear S 1  is set to a first node N 1 , the first planet carrier PC 1  is set to a second node N 2 , the first ring gear R 1  is set to a third node N 3 , the second sun gear S 2  is set to a fourth node N 4 , the second planet carrier PC 2  and the third ring gear R 3  are set to a fifth node N 5 , the second ring gear R 2  and the third planet carrier PC 3  are set to a sixth node N 6 , and the third sun gear S 3  is set to a seventh node N 7 . 
         [0067]    As described above, the first node N 1  of the first sun gear S 1  is always stationary by being fixedly connected to the transmission case  300 , and the third node N 3  of the first ring gear R 1  always acts as the input element by being fixedly connected to the input shaft  100 . 
         [0068]    In addition, the second sun gear S 2  is variably connected to the first ring gear R 1  and the first planet carrier PC 1  via the first clutch C 1  and the third clutch C 3 , respectively. The third sun gear S 3  is variably connected to the first planet carrier PC 1  via the second clutch C 2 , and the second planet carrier PC 2  is variably connected to the first ring gear R 1  via the fourth clutch C 4 . 
         [0069]    Therefore, a rotation speed of the engine is delivered to the fourth node N 4 , the fifth node N 5 , and the seventh node N 7  with a 1:1 speed ratio or a reduced speed ratio through the input shaft  100  according to operations of the first, second, third, and fourth clutches C 1 , C 2 , C 3 , and C 4 . 
         [0070]    In addition, the second planet carrier PC 2  is variably connected to the transmission case  300  via the first brake B 1 . Therefore, the fifth node N 5  of the second planet carrier PC 2  may be stopped by an operation of the first brake B 1 . 
         [0071]    In addition, the third sun gear S 3  is variably connected to the transmission case  300  via the second brake B 2 . Therefore, the seventh node N 7  of the third sun gear S 3  may be stopped by an operation of the second brake B 2 . 
         [0072]    Hereinafter, formation of each speed by the power trains for an automatic transmission according to the embodiments of the present invention will be described, with reference to  FIG. 3  and  FIG. 4 . 
         [0073]    In the first forward speed D 1 , the fourth node N 4  rotates with a reduced rotating speed since the third clutch C 3  is operated. In addition, the seventh node N 7  is stationary since the first one-way clutch OWC 1  is operated. Therefore, the first forward speed D 1  is achieved at the sixth node N 6  that is the output element. 
         [0074]    In the second forward speed D 2 , the seventh node N 7  rotates with the reduced rotating speed since the second clutch C 2  is operated. In addition, the fifth node N 5  is stationary since the second one-way clutch OWC 2  is operated. Therefore, the second forward speed D 2  is achieved at the sixth node N 6  that is the output element. 
         [0075]    In the third forward speed D 3 , the fifth node N 5  rotates with the same rotating speed as the input shaft  100  since the fourth clutch C 4  is operated. In addition, the seventh node N 7  is stationary since the first one-way clutch OWC 1  is operated. Therefore, the third forward speed D 3  is achieved at the sixth node N 6  that is the output element. 
         [0076]    In the fourth forward speed D 4 , the fourth node N 4  and the seventh node N 7  rotate respectively with the reduced rotating speed since the second clutch C 2  and the third clutch C 3  are operated. Therefore, the fourth forward speed D 4  is achieved at the sixth node N 6  that is the output element. 
         [0077]    In the fifth forward speed D 5 , the seventh node N 7  rotates with the reduced rotating speed since the second clutch C 2  is operated, and the fifth node N 5  rotates with the same rotating speed as the input shaft  100  since the fourth clutch C 4  is operated. Therefore, the fifth forward speed D 5  is achieved at the sixth node N 6  that is the output element. 
         [0078]    In the sixth forward speed D 6 , the fourth node N 4  and the fifth node N 5  rotate respectively with the same rotating speed as the input shaft  100  since the first clutch C 1  and the fourth clutch C 4  are operated. Therefore, the sixth forward speed D 6  is achieved at the sixth node N 6  that is the output element. 
         [0079]    In the seventh forward speed D 7 , the fourth node N 4  rotates with the reduced speed since the third clutch C 3  is operated, and the fifth node N 5  rotates with the same rotating speed as the input shaft  100  since the fourth clutch C 4  is operated. Therefore, the seventh forward speed D 7  is achieved at the sixth node N 6  that is the output element. 
         [0080]    In the first reverse speed REV.  1 , the fourth node N 4  rotates with the reduced speed since the third clutch C 3  is operated. In addition, the fifth node N 5  is stationary since the first brake B 1  is operated. Therefore, the first reverse speed REV.  1  is achieved at the sixth node N 6  that is the output element. 
         [0081]    In the second reverse speed REV.  2 , the fifth node N 4  rotates with the same rotating speed as the input shaft  100  since the first clutch C 1  is operated. In addition, the fifth node N 5  is stationary since the first brake B 1  is operated. Therefore, the second reverse speed REV.  2  is achieved at the sixth node N 6  that is the output element. 
         [0082]    As described above, the speed line for each planetary gear set may be easily obtained by a person of an ordinary skill in the art based on the teachings herein. 
         [0083]    According to exemplary embodiments of the present invention, the seven forward speeds and two reverse speeds may be realized by utilizing three planetary gear sets, four clutches, and two brakes. 
         [0084]    In addition, a compact automatic transmission may be designed by disposing the four clutches between the first planetary gear set and the second and third planetary gear sets according to the exemplary embodiments of the present invention. 
         [0085]    In addition, a durability of the automatic transmission may be enhanced by reducing a rotating speed of operational elements that are often used under acceleration. 
         [0086]    While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.