Abstract:
Various aspects of the present invention are directed to provide a gear train of an automatic transmission for vehicles having advantages of simplifying structures of the automatic transmission and improving power delivery performance and fuel economy as a consequence of realizing at least eight forward speeds and one reverse speed by combining two simple planetary gear sets and a double sun gear planetary gear set having two sun gears and a center member disposed between the sun gears and connected to a planet carrier with four clutches and two brakes.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application claims priority to Korean Patent Application No. 10-2010-0063162 filed in the Korean Intellectual Property Office on Jun. 30, 2010, the entire contents of which is incorporated herein for all purposes by this reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a gear train of an automatic transmission for vehicles which realizes at least eight forward speeds and one reverse speed by combining two simple planetary gear sets and a double sun gear planetary gear set having two sun gears and a center member disposed between the sun gears and connected to a planet carrier with four clutches and two brakes. 
         [0004]    2. Description of Related Art 
         [0005]    A typical shift mechanism of an automatic transmission utilizes a combination of a plurality of planetary gear sets. A gear train of such an automatic transmission that includes the plurality of planetary gear sets changes rotational speed and torque received from a torque converter of the automatic transmission, and accordingly 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 economical fuel mileage and better performance. For that reason, an automatic transmission that is able to realize more shift speeds is under continuous investigation. 
         [0007]    In addition, with the same number of speeds, features of a gear train, such as durability, efficiency in power transmission, and size, substantially depend on the layout of the combined planetary gear sets. Therefore, designs for a combining structure of a gear train are also under continuous investigation. 
         [0008]    A manual transmission that has too many speeds causes inconvenience to a driver. Therefore, the advantageous features of having more shift-speeds are more important in an automatic transmission because an automatic transmission automatically controls the shifting operations. 
         [0009]    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, seven-speed automatic transmissions and eight-speed automatic transmissions have been developed at a good pace. 
         [0010]    The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art. 
       BRIEF SUMMARY OF THE INVENTION 
       [0011]    Various aspects of the present invention are directed to provide a gear train of an automatic transmission for vehicles having advantages of simplifying structures of the automatic transmission and improving power delivery performance and fuel economy as a consequence of realizing at least eight forward speeds and one reverse speed by combining two simple planetary gear sets and a double sun gear planetary gear set having two sun gears and a center member disposed between the sun gears and connected to a planet carrier with four clutches and two brakes. 
         [0012]    In an aspect of the present invention, the gear train of an automatic transmission for vehicles may achieve eight forward speeds and one reverse speed by combining first, second, and third planetary gear sets with friction members including four clutches and two brakes, wherein the first planetary gear set may include a first rotation element always operated as a fixed element, a second rotation element always outputting a reduced rotation speed, and a third rotation element connected to an input shaft and operated as an input element, wherein the second and third planetary gear sets may be connected by first and second connecting members so as to be operated as a compound planetary gear set having four rotation elements, wherein the third planetary gear set may be a double sun gear planetary gear set provided with two sun gears and a center member disposed between the two sun gears and connected to a third planet carrier, and wherein the second and third planetary gear sets may include a fourth rotation element selectively connected to the input shaft and the second rotation element and selectively operated as a fixed element, a fifth rotation element selectively connected to the input shaft and selectively operated as a fixed element, a sixth rotation element connected to an output gear and always operated as an output element, and a seventh rotation element selectively connected to the second rotation element. 
         [0013]    The first planetary gear set may be a double pinion planetary gear set having a first sun gear, a first planet carrier, and a first ring gear as rotation elements thereof, the second planetary gear set may be a single pinion planetary gear set having a second sun gear, a second planet carrier, and a second ring gear as rotation elements thereof, and the third planetary gear set may be the double sun gear planetary gear set having third and fourth sun gears, a third planet carrier, and a third ring gear as rotation elements thereof. 
         [0014]    The first connecting member directly connects the second sun gear with the third sun gear, and the second connecting member directly connects the second carrier and the third ring gear. 
         [0015]    The first rotation element may be the first sun gear, the second rotation element may be the first ring gear, the third rotation element may be the first planet carrier, the fourth rotation element may be the second, third, and fourth sun gears, the fifth rotation element may be the third planet carrier, the sixth rotation element may be the second planet carrier and the third ring gear, and the seventh rotation element may be the second ring gear. 
         [0016]    The friction members may include a first clutch selectively connecting the first ring gear with the second ring gear, a second clutch selectively connecting the first ring gear with the second sun gear, a third clutch selectively connecting the input shaft with the center member, a fourth clutch selectively connecting the input shaft with the second sun gear, a first brake selectively connecting the fourth sun gear with a transmission housing, and a second brake selectively connecting the third planet carrier with the transmission housing. 
         [0017]    The first clutch and the second brake may be operated at a first forward speed, the first clutch and the first brake may be operated at a second forward speed, the first and second clutches may be operated at a third forward speed, the first and fourth clutches may be operated at a fourth forward speed, the first and third clutches may be operated at a fifth forward speed, the third and fourth clutches may be operated at a sixth forward speed, the second and third clutches may be operated at a seventh forward speed, the third clutch and the first brake may be operated at an eighth forward speed, and the second clutch and the second brake may be operated at a reverse speed. 
         [0018]    In another aspect of the present invention, the gear train of an automatic transmission for vehicles may include a first planetary gear set being a double pinion planetary gear set, and having a first sun gear, a first planet carrier, and a first ring gear, a second planetary gear set being a single pinion planetary gear set, and having a second sun gear, a second planet carrier, and a second ring gear, a third planetary gear set being a double sun gear planetary gear set, and having third and fourth sun gears, a third planet carrier, and a third ring gear, and four clutches and two brakes selectively combining the first, second, and third planetary gear sets, wherein an input shaft may be directly connected to the first planet carrier, the second and third sun gears may be directly connected to each other through a first connecting member, the second planet carrier and the third ring gear may be directly connected to each other, the first sun gear may be directly connected to a transmission housing, and the third ring gear may be directly connected to an output gear, and wherein a first clutch selectively connects the first ring gear with the second ring gear, a second clutch selectively connects the first ring gear with the second sun gear, a third clutch selectively connects a center member disposed between the third and fourth sun gears and connected to the third planet carrier with the input shaft, a fourth clutch selectively connects the input shaft with the second sun gear, a first brake selectively connects the fourth sun gear with the transmission housing, and a second brake selectively connects the third planet carrier with the transmission housing. 
         [0019]    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 
         [0020]      FIG. 1  is a schematic diagram of a gear train according to an exemplary embodiment of the present invention. 
           [0021]      FIG. 2  is a schematic diagram showing connections of rotation elements and friction members in a gear train according to an exemplary embodiment of the present invention. 
           [0022]      FIG. 3  is an operational chart for a gear according to an exemplary embodiment of the present invention. 
           [0023]      FIG. 4  is a lever diagram for a gear train according to an exemplary embodiment of the present invention. 
       
    
    
       [0024]    It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various features illustrative of the basic principles of the invention. The specific design features of the present invention as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes will be determined in part by the particular intended application and use environment. 
         [0025]    In the figures, reference numbers refer to the same or equivalent parts of the present invention throughout the several figures of the drawing. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    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. 
         [0027]    Description of components that are not necessary for explaining the present invention will be omitted, and the same constituent elements are denoted by the same reference numerals in this specification. 
         [0028]    In the detailed description, ordinal numbers are used for distinguishing constituent elements having the same terms, and have no specific meanings. 
         [0029]      FIG. 1  is a schematic diagram of a gear train according to an exemplary embodiment of the present invention. PG 1  represents a first planetary gear set, PG 2  represents a second planetary gear set, PG 3  represents a third planetary gear set, C 1  represents a first clutch, C 2  represents a second clutch, C 3  represents a third clutch, C 4  represents a fourth clutch, B 1  represents a first brake, B 2  represents a second brake, IS represents an input shaft forming an input member, and OG represents an output gear forming an output member in the drawings. 
         [0030]    The first, second, and third planetary gear sets PG 1 , PG 2 , and PG 3  are disposed on the same axis (input shaft). A rotation speed 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. The first planetary gear set PG 1  is disposed close to an engine, and the second and third planetary gear sets PG 2  and PG 3  are sequentially disposed at the rear of the first planetary gear set PG 1 . 
         [0031]    The input shaft IS is an input member and denotes a turbine shaft of a torque converter. Torque transmitted from a crankshaft of the engine is converted by the torque converter and is input to the gear train through the input shaft IS. The output shaft OS is an output member and is connected to a well-known differential apparatus so as to transmit an output of the gear train to driving wheels. 
         [0032]    The first planetary gear set PG 1  is a double pinion planetary gear set, and includes a first sun gear  51 , a first ring gear R 1 , and a first planet carrier PC 1  supporting a first pinion P 1  engaged with both gears S 1  and R 1 . 
         [0033]    The second planetary gear set PG 2  is a single pinion planetary gear set, and includes a second sun gear S 2 , a second ring gear R 2 , and a second planet carrier PC 2  supporting a second pinion P 2  engaged with both gears S 2  and R 2 . 
         [0034]    The third planetary gear set PG 3  is a double sun gear planetary gear set, and includes a third sun gear S 3  and a fourth sun gear S 4 , a third pinion P 3  engaged with the third and the fourth sun gears S 3  and S 4 , a third planet carrier PC 3  supporting the third pinion P 3 , a center member CM disposed between the third and fourth sun gears S 3  and S 4  and connected to the third planet carrier PC 3 , and a third ring gear R 3  engaged with the third pinion P 3 . 
         [0035]    The center member CM is coupled to the third planet carrier PC 3  and rotates integrally with the third planet carrier PC 3 . 
         [0036]    In addition, the second planetary gear set PG 2  and the third planetary gear set PG 3  are connected to each other by first and second connecting members M 1  and M 2 . The first connecting member M 1  connects the second sun gear S 2  with the third sun gear S 3 , and the second connecting member M 2  connects the second planet carrier PC 2  with the third ring gear R 3  such that the second planetary gear set PG 2  and the third planetary gear set PG 3  are operated as one compound planetary gear set. 
         [0037]    In addition, the first, second, and third planetary gear sets PG 1 , PG 2 , and PG 3  are combined by four clutches C 1 , C 2 , C 3 , and C 4  and two brakes B 1  and B 2 . 
         [0038]    The first planet carrier PC 1  is directly connected to the input shaft IS so as to be always operated as an input element, the first sun gear  51  is connected to a transmission housing H so as to be always operated as a fixed element, and the second planet carrier PC 2  and the third ring gear R 3  are directly connected to the output gear OG so as to be always operated as an output element. 
         [0039]    In addition, the first clutch C 1  selectively connects the first ring gear R 1  with the second ring gear R 2 , the second clutch C 2  selectively connects the first ring gear R 1  with the second sun gear S 2 , the third clutch C 3  selectively connects the input shaft IS with the center member CM, and the fourth clutch C 4  selectively connects the input shaft IS with the second sun gear S 2 . 
         [0040]    In addition, the first brake B 1  selectively connects the fourth sun gear S 4  with the transmission housing H, and the second brake B 2  selectively connects the third planet carrier PC 3  with the transmission housing H. 
         [0041]    Connections of the rotation elements and the friction members may be well understood referring to  FIG. 2 . 
         [0042]    Meanwhile, the first and second clutches C 1  and C 2  are disposed at an external circumference of the second planetary gear set PG 2 , the first and second brakes B 1  and B 2  are disposed at the rear of the third planetary gear set PG 3 , and the third and fourth clutches C 3  and C 4  are disposed at the rear of the first and second brakes B 1  and B 2 . 
         [0043]    If the friction elements are dispersed as described above, formation of hydraulic lines for supplying hydraulic pressure to such friction elements may be simplified, and weight balance in the automatic transmission may be enhanced. 
         [0044]      FIG. 3  is an operational chart for a gear train according to an exemplary embodiment of the present invention. That is,  FIG. 3  shows which clutches and brakes are operated at each shift-speed. As shown in  FIG. 3 , two friction members are operated at each shift-speed according to an exemplary embodiment of the present invention. 
         [0045]    That is, the first clutch C 1  and the second brake B 2  are operated at a first forward speed D 1 , the first clutch C 1  and the first brake B 1  are operated at a second forward speed D 2 , the first and second clutches C 1  and C 2  are operated at a third forward speed D 3 , the first and fourth clutches C 1  and C 4  are operated at a fourth forward speed D 4 , the first and third clutches C 1  and C 3  are operated at a fifth forward speed D 5 , the third and fourth clutches C 3  and C 4  are operated at a sixth forward speed D 6 , the second and third clutches C 2  and C 3  are operated at a seventh forward speed D 7 , the third clutch C 3  and the first brake B 1  are operated at an eighth forward speed D 8 , and the second clutch C 2  and the second brake B 2  are operated at a reverse speed REV. 
         [0046]      FIG. 4  is a lever diagram for a gear train according to an exemplary embodiment of the present invention. In the drawings, a lower horizontal line represents a rotational speed is “0”, and an upper horizontal line represents a rotational speed is “1.0”, that is, the rotational speed thereof is the same as that of the input shaft IS. 
         [0047]    Three vertical lines of the first planetary gear set PG 1  sequentially represent the first sun gear S 1  being a first rotation element N 1 , the first ring gear R 1  being a second rotation element N 2 , and the first planet carrier PC 1  being a third rotation element N 3  from the left to the right, and distances therebetween are set according to a gear ratio (teeth number of the sun gear/teeth number of the ring gear) of the first planetary gear set PG 1 . 
         [0048]    Four vertical lines of the second and third planetary gear sets PG 2  and PG 3  sequentially represent the second, third, and fourth sun gears S 2 , S 3 , and S 4  being a fourth rotation element N 4 , the third planet carrier PC 3  being a fifth rotation element N 5 , the second planet carrier PC 2  and the third ring gear R 3  being a sixth rotation element N 6 , the second ring gear R 2  being a seventh rotation element N 7 , and distances therebetween 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 . 
         [0049]    Position of each rotation element in the lever diagram is well known to a person of an ordinary skill in the art who designs a gear train, and thus detailed description will be omitted. 
         [0050]    [First Forward Speed] 
         [0051]    As shown in  FIG. 3 , the first clutch C 1  and the second brake B 2  are operated at the first forward speed D 1 . 
         [0052]    Accordingly, in a state that a rotation speed of the input shaft IS is input to the third rotation element N 3 , the first rotation element N 1  is operated as the fixed element. Therefore, the rotation elements of the first planetary gear set PG 1  forms a first shift line T 1  and the reduced rotation speed is output through the second rotation element N 2 . 
         [0053]    In a state that the reduced rotation speed is delivered to the second planetary gear set PG 2  through the seventh rotation element N 7  by an operation of the first clutch C 1 , the fifth rotation element N 5  is operated as the fixed element by an operation of the second brake B 2 . Therefore, a first shift line SP 1  is formed as shown in  FIG. 4 . Therefore, the first shift line SP 1  crosses the vertical line of the sixth rotation element N 6  that is the output element and the first forward speed D 1  is output. 
         [0054]    [Second Forward Speed] 
         [0055]    The second brake B 2  which was operated at the first forward speed D 1  is released and the first brake B 1  is operated at the second forward speed D 2 . 
         [0056]    In a state that the reduced rotation speed is input to the seventh rotation element N 7  such as the first forward speed D 1 , the fourth rotation element N 4  is operated as the fixed element by an operation of the first brake B 1 . Therefore, a second shift line SP 2  is formed as shown in  FIG. 4 . Therefore, the second shift line SP 2  crosses the vertical line of the sixth rotation element N 6  that is the output element and the second forward speed D 2  is output. 
         [0057]    [Third Forward Speed] 
         [0058]    As shown in  FIG. 3 , the first brake B 1  that was operated at the second forward speed D 2  is released and the second clutch C 2  is operated at the third forward speed D 3 . 
         [0059]    In a state that the reduced rotation speed is input to the seventh rotation element N 7  such as the second forward speed D 2 , the reduced rotation speed is also input to the fourth rotation element N 4  by an operation of the second clutch C 2 . As shown in  FIG. 4 , the second and third planetary gear sets PG 2  and PG 3  become direct-coupling state and a third shift line SP 3  is formed. Therefore, the third shift line SP 3  crosses the vertical line of the sixth rotation element N 6  that is the output element and the third forward speed D 3  is output. 
         [0060]    [Fourth Forward Speed] 
         [0061]    As shown in  FIG. 3 , the second clutch C 2  that was operated at the third forward speed D 3  is released and the fourth clutch C 4  is operated at the fourth forward speed D 4 . 
         [0062]    In a state that the reduced rotation speed is input to the seventh rotation element N 7  such as the third forward speed D 3 , a rotation speed of the third rotation element N 3  (the same as that of the input shaft) is input to the fourth rotation element N 4  by an operation of the fourth clutch C 4 . As shown in  FIG. 4 , a fourth shift line SP 4  is formed. Therefore, the fourth shift line SP 4  crosses the vertical line of the sixth rotation element N 6  that is the output element and the fourth forward speed D 4  is output. 
         [0063]    [Fifth Forward Speed] 
         [0064]    As shown in  FIG. 3 , the fourth clutch C 4  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 . 
         [0065]    In a state that the reduced rotation speed is input to the seventh rotation element N 7  such as the fourth forward speed D 4 , the rotation speed of the input shaft IS is input to the third planet carrier PC 3  forming the fifth rotation element N 5  through the center member CM by an operation of the third clutch C 3 . As shown in  FIG. 4 , a fifth shift line SP 5  is formed. Therefore, the fifth shift line SP 5  crosses the vertical line of the sixth rotation element N 6  that is the output element and the fifth forward speed D 5  is output. 
         [0066]    [Sixth Forward Speed] 
         [0067]    As shown in  FIG. 3 , the first clutch C 1  that was operated at the fifth forward speed D 5  is released and the fourth clutch C 4  is operated at the sixth forward speed D 6 . 
         [0068]    In a state that the rotation speed of the input shaft IS is input respectively to the fifth and fourth rotation elements N 5  and N 4  by the operations of the third and fourth clutches C 3  and C 4 . As shown in  FIG. 4 , the second and third planetary gear sets PG 2  and PG 3  become the direct-coupling state and a sixth shift line SP 6  is formed. Therefore, the sixth shift line SP 6  crosses the vertical line of the sixth rotation element N 6  that is the output element and the sixth forward speed D 6  is output. 
         [0069]    [Seventh Forward Speed] 
         [0070]    As shown in  FIG. 3 , the fourth clutch C 4  that was operated at the sixth forward speed D 6  is released and the second clutch C 2  is operated at the seventh forward speed D 7 . 
         [0071]    In a state that the rotation speed of the input shaft IS is input to the fifth rotation element N 5  by the operation of the third clutch C 3 , the rotation speed of the second rotation element N 2  is input to the fourth rotation element N 4  by the operation of the second clutch C 2 . As shown in  FIG. 4 , a seventh shift line SP 7  is formed. Therefore, the seventh shift line SP 7  crosses the vertical line of the sixth rotation element N 6  that is the output element and the seventh forward speed D 7  is output. 
         [0072]    [Eighth Forward Speed] 
         [0073]    As shown in  FIG. 3 , the second clutch C 2  that was operated at the seventh forward speed D 7  is released and the first brake B 1  is operated at the eighth forward speed D 8 . 
         [0074]    In a state that the rotation speed of the input shaft IS is input to the fifth rotation element N 5  such as the seventh forward speed D 7 , the fourth rotation element N 4  is operated as the fixed element by the operation of the first brake B 1 . As shown in  FIG. 4 , an eighth shift line SP 8  is formed. Therefore, the eighth shift line SP 8  crosses the vertical line of the sixth rotation element N 6  that is the output element and the eighth forward speed D 8  is output. 
         [0075]    [Reverse Speed] 
         [0076]    As shown in  FIG. 3 , the second clutch C 2  and the second brake B 2  are operated at the reverse speed REV. 
         [0077]    In a state that the rotation speed of the second rotation element N 2  is input to the fourth rotation element N 4  by the operation of the second clutch C 2 , the fifth rotation element N 5  is operated as the fixed element by the operation of the second brake B 2 . As shown in  FIG. 4 , a reverse shift line RS is formed. Therefore, the reverse shift line RS crosses the vertical line of the sixth rotation element N 6  that is the output element and the reverse speed REV is output. 
         [0078]    As described above, eight forward speeds and one reverse speed are achieved by combining two simple planetary gear sets and a double sun gear planetary gear set having two sun gears and a center member disposed between the sun gears and connected to a planet carrier with four clutches and two brakes with four clutches and two brakes. Therefore, power delivery performance and fuel economy may be improved, and particularly, performance in reverse speeds may be improved according to an exemplary embodiment of the present invention 
         [0079]    Since the frictional members are dispersedly disposed, formation of hydraulic lines may be simplified, weight balance in an automatic transmission may be enhanced. 
         [0080]    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.