Abstract:
A power transmission apparatus may include first and second input shafts; first and second clutches; first and second output shafts; first set of input gears fixed on first input shaft; second set of input gears fixed on second input shaft; first set of speed gears rotatable on first output shaft and engaged with first set of input gears or second set of input gears; second set of speed gears rotatable on second output shaft and engaged with first or second set of input gears; first set of synchronizers selectively connecting first set of speed gears to first output shaft; second set of synchronizers selectively connecting second set of speed gears to second output shaft; and reverse speed output unit engaged with one input gear of first and second sets of input gears, and second idle gear engaged with one speed gear of first and second sets of speed gears.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority to and the benefit of Korean Patent Application No. 10-2014-0141155 filed on Oct. 17, 2014, the entire contents of which is incorporated herein for all purposes by this reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a power transmission apparatus for a vehicle using a double clutches. More particularly, the present invention relates to a power transmission apparatus for a vehicle that can reduce length and weight of the power transmission apparatus by minimizing the number of components involved in a reverse speed. 
     Description of Related Art 
     Environmentally-friendly technique of vehicles is very important technique on which survival of future motor industry is dependent. Vehicle makers are focusing on development of environmentally-friendly vehicles so as to meet environment and fuel consumption regulations. 
     Some examples of future vehicle technique are an electric vehicle (EV) and a hybrid electric vehicle (HEV) that use electrical energy, and double clutch transmission (DCT) that improves efficiency and convenience. 
     The DCT includes two clutch devices and a gear train applied to a manual transmission. The DCT selectively transmits torque input from an engine to two input shafts by using two clutches, changes a speed by using the gear train, and outputs the changed torque. 
     Such the DCT is used to realize a compact transmission having more than five forward speeds. Since two clutches and synchronizing devices are controlled by a controller according to the DCT, manual shift maneuver is unnecessary for controlling the DCT. Therefore, the DCT is one type of automated manual transmissions (AMT). 
     The DCT, compared with an automatic transmission using planetary gear sets, has excellent power delivery efficiency and change and addition of components is very easy in the DCT. Therefore, the DCT can meet fuel consumption regulations and multi-speeds efficiency may be improved. 
     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 
     Various aspects of the present invention are directed to providing a power transmission apparatus for a vehicle having advantages of reducing the number of components and a length and improving mountability by transmitting torque of an engine from one input shaft to another input shaft so as to achieve a reverse speed. 
     A power transmission apparatus for a vehicle according to an exemplary embodiment of the present invention may include: a first input shaft selectively connected to a power source through a first clutch; a second input shaft selectively connected to the power source through a second clutch; first and second output shafts disposed in parallel with the first and second input shafts; a first set of input gears fixedly disposed on the first input shaft; a second set of input gears fixedly disposed on the second input shaft; a first set of speed gears rotatably disposed on the first output shaft and engaged with the first set of input gears or the second set of input gears; a second set of speed gears rotatably disposed on the second output shaft and engaged with the first set of input gears or the second set of input gears; a first set of synchronizers selectively connecting the first set of speed gears to the first output shaft; a second set of synchronizers selectively connecting the second set of speed gears to the second output shaft; and a reverse speed output unit including a reverse idle shaft disposed in parallel with the first and second output shafts, a first idle gear engaged with any one input gear of the first and second sets of input gears, and a second idle gear engaged with any one speed gear of the first and second sets of speed gears, wherein one of the first idle gear and the second idle gear is fixedly disposed on the reverse idle shaft and the other of the first idle gear and the second idle gear is rotatably disposed on the reverse idle shaft, and wherein the reverse speed output unit includes a further synchronizer selectively connecting the other of the first idle gear and the second idle gear to the reverse idle shaft. 
     The first set of input gears may include first, second, third, and fourth input gears, and the second set of input gears may include fifth and sixth input gears. 
     The first set of speed gears may include a first speed gear engaged with the second input gear, a second speed gear engaged with the fifth input gear, a fourth speed gear engaged with the sixth input gear, and a fifth speed gear engaged with the fourth input gear, and the second set of speed gears may include a third speed gear engaged with the third input gear, a sixth speed gear engaged with the sixth input gear, and a seventh speed gear engaged with the first input gear. 
     The first set of synchronizers may include a first synchronizer selectively connecting the first speed gear or the fifth speed gear to the first output shaft, and a second synchronizer selectively connecting the second speed gear or the fourth speed gear to the first output shaft, and the second set of synchronizers may include a third synchronizer selectively connecting the third speed gear or the seventh speed gear to the second output shaft, and a fourth synchronizer selectively connecting the sixth speed gear to the second output shaft. 
     In various exemplary embodiments, the first idle gear may be engaged with the first input gear and the second idle gear may be engaged with the second speed gear. 
     The first idle gear may be rotatably disposed on the reverse idle shaft and the second idle gear may be fixedly disposed on the reverse idle shaft. 
     In various exemplary embodiments, the first idle gear may be engaged with the first input gear and the second idle gear may be engaged with the fourth speed gear. 
     The first idle gear may be fixedly disposed on the reverse idle shaft and the second idle gear may be rotatably disposed on the reverse idle shaft. 
     The power transmission apparatus may further include a first output gear fixedly disposed on the first output shaft and outputting torque of the first output shaft; and a second output gear fixedly disposed on the second output shaft and outputting torque of the second output shaft. 
     A power transmission apparatus for a vehicle according to another exemplary embodiment of the present invention may include: a first input shaft selectively connected to a power source through a first clutch and having first, second, third, and fourth input gears fixedly disposed thereon; a second input shaft selectively connected to the power source through a second clutch and having fifth and sixth input gears fixedly disposed thereon; a first output shaft disposed in parallel with the first and second input shafts and having first, second, fourth, and fifth speed gears engaged respectively at least one of the input gears and rotatably disposed on the first output shaft; a second output shaft disposed in parallel with the first and second input shafts and having third, sixth, and seventh speed gears engaged respectively at least one of the input gears and rotatably disposed on the second output shaft; at least one synchronizer selectively connecting the speed gears on the first output shaft to the first output shaft or selectively connecting the speed gears on the second output shaft to the second output shaft; and a reverse speed output unit including a reverse idle shaft disposed in parallel with the first and second output shafts, a first idle gear engaged with any one input gear of the first and second sets of input gears, and a second idle gear engaged with any one speed gear of the first and second sets of speed gears, wherein one of the first idle gear and the second idle gear is fixedly disposed on the reverse idle shaft and the other of the first idle gear and the second idle gear is rotatably disposed on the reverse idle shaft, and wherein the reverse speed output unit includes a further synchronizer selectively connecting the other of the first idle gear and the second idle gear to the reverse idle shaft. 
     The first speed gear may be engaged with the second input gear, the second speed gear may be engaged with the fifth input gear, the third speed gear may be engaged with the third input gear, the fourth speed gear may be engaged with the sixth input gear, the fifth speed gear may be engaged with the fourth input gear, the sixth speed gear may be engaged with the sixth input gear, and the seventh speed gear may be engaged with the first input gear. 
     The at least one synchronizer may include: a first synchronizer selectively connecting the first speed gear or the fifth speed gear to the first output shaft; a second synchronizer selectively connecting the second speed gear or the fourth speed gear to the first output shaft; a third synchronizer selectively connecting the third speed gear or the seventh speed gear to the second output shaft; and a fourth synchronizer selectively connecting the sixth speed gear to the second output shaft. 
     In various exemplary embodiments, the first idle gear may be engaged with the first input gear and the second idle gear may be engaged with the second speed gear. 
     The first idle gear may be rotatably disposed on the reverse idle shaft and the second idle gear may be fixedly disposed on the reverse idle shaft. 
     In various exemplary embodiments, the first idle gear may be engaged with the first input gear and the second idle gear may be engaged with the fourth speed gear. 
     The first idle gear may be fixedly disposed on the reverse idle shaft and the second idle gear may be rotatably disposed on the reverse idle shaft. 
     The power transmission apparatus may further include: a first output gear fixedly disposed on the first output shaft and outputting torque of the first output shaft; and a second output gear fixedly disposed on the second output shaft and outputting torque of the second output shaft. 
     According to exemplary embodiments of the present invention, torque of an engine is transmitted from one input shaft to another input shaft so as to achieve a reverse speed. The number of components may be reduced and a length of a power transmission apparatus may be minimized. 
     In addition, since the number of components is reduced, weight and cost may be reduced and mountability of the power transmission apparatus may be improved. 
     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 a power transmission apparatus for a vehicle according to the various exemplary embodiments of the present invention. 
         FIG. 2  is an operational chart of a power transmitting apparatus for a vehicle according to the various exemplary embodiments of the present invention. 
         FIG. 3  is a schematic diagram for illustrating power delivery path at a reverse speed in a power transmission apparatus for a vehicle according to the various exemplary embodiments of the present invention. 
         FIG. 4  is a schematic diagram of a power transmission apparatus for a vehicle according to the various exemplary embodiments of the present invention. 
         FIG. 5  is a schematic diagram for illustrating power delivery path at a reverse speed in a power transmission apparatus for a vehicle according to the various exemplary embodiments of the present invention. 
     
    
    
     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. 
     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 
     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 the 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. 
     Exemplary embodiments of the present invention will hereinafter 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. 
       FIG. 1  is a schematic diagram of a power transmitting apparatus for a vehicle according to the first exemplary embodiment of the present invention. 
     Referring to  FIG. 1 , a power transmission apparatus according to the first exemplary embodiment of the present invention includes a selective connection unit including first and second clutches CL 1  and CL 2  and selectively transmitting torque of a power source (engine or motor) or not, an input device including first and second input shafts IS 1  and IS 2 , a speed output device changing and outputting torque input from the input device, and a reverse speed output unit. 
     The power source may be an engine in a case of a typical vehicle provided with an internal combustion engine and a manual or an automatic transmission, and may be a motor in a case of a hybrid electric vehicle including an electric vehicle. For better comprehension and ease of description, it is exemplified in the exemplary embodiments of the present invention that the power source is an engine. 
     The first and second clutches CL 1  and CL 2  selectively transmit the torque of the engine ENG to the first and second input shafts IS 1  and IS 2 , respectively. 
     The first clutch C 1  selectively transmits the torque of the engine ENG to the first input shaft IS 1 , and the second clutch CL 2  selectively transmits the torque of the engine ENG to the second input shaft IS 2 . 
     The first and second clutches CL 1  and CL 2  may be conventional multi-plate clutches of wet type, but are not limited to this. The first and second clutches CL 1  and CL 2  may be conventional multi-plate clutches of dry type. The first and second clutches CL 1  and CL 2  may be controlled by a hydraulic control system. 
     The input device includes the first input shaft IS 1  selectively connected to an output side of the engine ENG through the first clutch CL 1 , and the second input shaft IS 2  being a hollow shaft, enclosing the first input shaft IS 1 , and selectively connected to the output side of the engine ENG through the second clutch CL 2 . 
     First, second, third, and fourth input gears G 1 , G 2 , G 3 , and G 4  are fixedly disposed on the first input shaft IS 1 . The first, second, third, and fourth input gears G 1 , G 2 , G 3 , and G 4  are disposed on a rear portion of the first input shaft IS 1  penetrating through the second input shaft IS 2 , and are sequentially disposed from the front to the rear. 
     Fifth and sixth input gears G 5  and G 6  are fixedly disposed on the second input shaft IS 2 . The fifth input gear G 5  is disposed on a front portion of the second input shaft IS 2  and the sixth input gear G 6  is disposed on a rear portion of the second input shaft IS 2 . 
     Therefore, the first input shaft IS 1  and the first, second, third, and fourth input gears G 1 , G 2 , G 3 , and G 4  fixedly disposed thereon are rotated if the first clutch CL 1  is operated, and the second input shaft IS 2  and the fifth and sixth input gears G 5  and G 6  fixedly disposed thereon are rotated if the second clutch CL 2  is operated. 
     The first, second, third, fourth, fifth, and sixth input gears G 1 , G 2 , G 3 , G 4 , G 5 , and G 6  are input gears operating at each gear stage. The first input gear G 1  is operated at a seventh forward speed, the second input gear G 2  is operated at a first forward speed, the third input gear G 3  is operated at a third forward speed, and the fourth input gear G 4  is operated at a fifth forward speed. 
     In addition, the fifth input gear G 5  is operated at a second forward speed, and the sixth input gear G 6  is operated at a fourth forward speed and a sixth forward speed. 
     The input gears G 1 , G 2 , G 3 , and G 4  for achieving odd-numbered gear stages are disposed on the first input shaft IS 1 , and the input gears G 5  and G 6  for achieving even-numbered gear stages are disposed on the second input shaft IS 2 . 
     The speed output device includes first and second output shafts OS 1  and OS 2  disposed in parallel with and apart from the first and second input shafts IS 1  and IS 2 . 
     A first speed gear D 1  and a fifth speed gear D 5  are rotatably disposed on the first output shaft OS 1  and a first synchronizer SL 1  selectively connecting the first speed gear D 1  or the fifth speed gear D 5  to the first output shaft OS 1  is disposed on the first output shaft OS 1 . In addition, a second speed gear D 2  and a fourth speed gear D 4  are rotatably disposed on the first output shaft OS 1  and a second synchronizer SL 2  selectively connecting the second speed gear D 2  or the fourth speed gear D 4  to the first output shaft OS 1  is disposed on the first output shaft OS 1 . A first output gear OG 1  is fixedly disposed on a front portion of the first output shaft OS 1 . 
     The first synchronizer SL 1  is disposed on a rear portion of the first output shaft OS 1  and the second synchronizer SL 2  is disposed on the front portion of the first output shaft OS 1 . 
     The first speed gear D 1  is engaged with the second input gear G 2  and the fifth speed gear D 5  is engaged with the fourth input gear G 4 . 
     The second speed gear D 2  is engaged with the fifth input gear G 5  and the fourth speed gear D 4  is engaged with the sixth input gear G 6 . 
     In addition, the first output gear OG 1  outputs torque transmitted from the first output shaft OS 1  to a differential apparatus DIFF through a final reduction gear FD. 
     In addition, a third speed gear D 3  and a seventh speed gear D 7  are rotatably disposed on the second output shaft OS 2  and a third synchronizer SL 3  selectively connecting the third speed gear D 3  or the seventh speed gear D 7  to the second output shaft OS 2  is disposed on the second output shaft OS 2 . In addition, a sixth speed gear D 6  is rotatably disposed on the second output shaft OS 2  and a fourth synchronizer SL 4  selectively connecting the sixth speed gear D 6  to the second output shaft OS 2  is disposed on the second output shaft OS 2 . A second output gear OG 2  is fixedly disposed on a front portion of the second output shaft OS 2 . 
     The third synchronizer SL 3  is disposed on a rear portion of the second output shaft OS 2  and the fourth synchronizer SL 4  is disposed on a front portion of the second output shaft OS 2 . 
     The third speed gear D 3  is engaged with the third input gear G 3  and the seventh speed gear D 7  is engaged with the first input gear G 1 . 
     The sixth speed gear D 6  is engaged with the sixth input gear G 6 . 
     The second output gear OG 2  outputs torque transmitted from the second output shaft OS 2  to the differential apparatus DIFF through the final reduction gear FD. 
     Herein, the first output gear OG 1  and the second output gear OG 2  are engaged to the final reduction gear FD so as to transmit the torque of the first output shaft OS 1  or second output shaft OS 2  to the final reduction gear FD. The torque transmitted to the final reduction gear FD is transmitted to a driving wheel through the differential apparatus DIFF. 
     The reverse speed output unit includes a reverse idle shaft IDS disposed in parallel with the first and second input shafts IS 1  and IS 2 . 
     A first idle gear IDG 1  and a second idle gear IDG 2  are disposed on the reverse idle shaft IDS. The first idle gear IDG 1  is rotatably disposed on the reverse idle shaft IDS and the second idle gear IDG 2  is fixedly disposed on the reverse idle shaft IDS. 
     In addition, a fifth synchronizer SL 5  selectively connecting the first idle gear IDG 1  to the reverse idle shaft IDS is disposed on the reverse idle shaft IDS. 
     In addition, the first idle gear IDG 1  is engaged with the first input gear G 1  on the first input shaft IS 1  and the second idle gear IDS 2  is engaged with the second speed gear D 2  on the first output shaft OS 1 . 
     Since the first, second, third, fourth, and fifth synchronizers SL 1 -SL 5  are well known to a person of an ordinary skill in the art, detailed description thereof will be omitted. In addition, sleeves SLE 1 , SLE 2 , SLE 3 , SLE 4 , and SLE 5  applied respectively to the first, second, third, fourth, and fifth synchronizers SL 1 -SL 5 , as well known to a person of an ordinary skill in the art, are operated by additional actuators and the actuators are controlled by a transmission control unit. 
     A reference numeral P in  FIG. 1  that is not explained indicates a parking gear. 
       FIG. 2  is an operational chart of a power transmitting apparatus according to the first exemplary embodiment of the present invention. 
     Referring to  FIG. 2 , operation at each gear stage will hereinafter be described. 
     [First Forward Speed] 
     After the first speed gear D 1  and the first output shaft OS 1  are operably connected through the sleeve SLE 1  of the first synchronizer SL 1 , the first clutch CL 1  is operated. Then, shift to the first forward speed is achieved. 
     [Second Forward Speed] 
     In a case that vehicle speed increases at the first forward speed and shift to the second forward speed is necessary, the second speed gear D 2  and the second output shaft OS 2  are operably connected through the sleeve SLE 2  of the second synchronizer SL 2 . After that, the first clutch CL 1  is released and the second clutch CL 2  is operated. Then, the shift to the second forward speed is achieved. 
     After the shift to the second forward speed is completed, the sleeve SLE 1  of the first synchronizer SL 1  is moved to a neutral position. 
     [Third Forward Speed] 
     In a case that the vehicle speed increases at the second forward speed and shift to the third forward speed is necessary, the third speed gear D 3  and the second output shaft OS 2  are operably connected through the sleeve SLE 3  of the third synchronizer SL 3 . After that, the second clutch CL 2  is released and the first clutch CL 1  is operated. Then, the shift to the third forward speed is achieved. 
     After the shift to the third forward speed is completed, the sleeve SLE 2  of the second synchronizer SL 2  is moved to a neutral position. 
     [Fourth Forward Speed] 
     In a case that the vehicle speed increases at the third forward speed and shift to the fourth forward speed is necessary, the fourth speed gear D 4  and the first output shaft OS 1  are operably connected through the sleeve SLE 2  of the second synchronizer SL 2 . After that, the first clutch CL 1  is released and the second clutch CL 2  is operated. Then, the shift to the fourth forward speed is achieved. 
     After the shift to the fourth forward speed is completed, the sleeve SLE 3  of the third synchronizer SL 3  is moved to a neutral position. 
     [Fifth Forward Speed] 
     In a case that the vehicle speed increases at the fourth forward speed and shift to the fifth forward speed is necessary, the fifth speed gear D 5  and the first output shaft OS 1  are operably connected through the sleeve SLE 1  of the first synchronizer SL 1 . After that, the second clutch CL 2  is released and the first clutch CL 1  is operated. Then, the shift to the fifth forward speed is achieved. 
     After the shift to the fifth forward speed is completed, the sleeve SLE 2  of the second synchronizer SL 2  is moved to a neutral position. 
     [Sixth Forward Speed] 
     In a case that the vehicle speed increases at the fifth forward speed and shift to the sixth forward speed is necessary, the sixth speed gear D 6  and the second output shaft OS 2  are operably connected through the sleeve SLE 4  of the fourth synchronizer SL 4 . After that, the first clutch CL 1  is released and the second clutch CL 2  is operated. Then, the shift to the sixth forward speed is achieved. 
     After the shift to the sixth forward speed is completed, the sleeve SLE 1  of the first synchronizer SL 1  is moved to a neutral position. 
     [Seventh Forward Speed] 
     In a case that the vehicle speed increases at the sixth forward speed and shift to the seventh forward speed is necessary, the seventh speed gear D 7  and the second output shaft OS 2  are operably connected through the sleeve SLE 3  of the third synchronizer SL 3 . After that, the second clutch CL 2  is released and the first clutch CL 1  is operated. Then, the shift to the seventh forward speed is achieved. 
     After the shift to the seventh forward speed is completed, the sleeve SLE 4  of the fourth synchronizer SL 4  is moved to a neutral position. 
     [Reverse Speed] 
       FIG. 3  is a schematic diagram for illustrating power delivery path at a reverse speed in a power transmission apparatus for a vehicle according to the first exemplary embodiment of the present invention. 
     Referring to  FIG. 2  and  FIG. 3 , the first speed gear D 1  and the first output shaft OS 1  are operably connected through the sleeve SLE 1  of the first synchronizer SL 1  and the first idle gear IDG 1  and the reverse idle shaft IDS are operably connected through the sleeve SLE 5  of the fifth synchronizer SL 5  for shift to the reverse speed. After that, if the second clutch CL 2  is operated, the shift to the reverse speed is achieved. 
     Therefore, the torque of the engine ENG is sequentially transmitted to the second input shaft IS 2 , the fifth input gear G 5 , the second speed gear D 2 , the second idle gear IDG 2 , the reverse idle shaft IDS, the fifth synchronizer SL 5 , the first idle gear IDG 1 , the first input gear G 1 , the second input gear G 2 , the first speed gear D 1 , the first synchronizer SL 1 , the first output shaft OS 1 , the first output gear OG 1 , the final reduction gear FD, and the differential apparatus DIFF. 
       FIG. 4  is a schematic diagram of a power transmission apparatus for a vehicle according to the second exemplary embodiment of the present invention. 
     Referring to  FIG. 4 , a power transmission apparatus according to the second exemplary embodiment of the present invention is similar to that according to the first exemplary embodiment. Particularly, the power transmission apparatus according to second exemplary embodiment includes the selective connection unit including the first and second clutches CL 1  and CL 2  and selectively transmitting torque of the power source (engine or motor) or not, the input device including the first and second input shafts IS 1  and IS 2 , the speed output device changing and outputting torque input from the input device, and the reverse speed output unit. 
     The first and second clutches CL 1  and CL 2  selectively transmit the torque of the engine ENG to the first and second input shafts IS 1  and IS 2 , respectively. 
     The first clutch C 1  selectively transmits the torque of the engine ENG to the first input shaft IS 1 , and the second clutch CL 2  selectively transmits the torque of the engine ENG to the second input shaft IS 2 . 
     The input device includes the first input shaft IS 1  selectively connected to the output side of the engine ENG through the first clutch CL 1 , and the second input shaft IS 2  being the hollow shaft, enclosing the first input shaft IS 1 , and selectively connected to the output side of the engine ENG through the second clutch CL 2 . 
     The first, second, third, and fourth input gears G 1 , G 2 , G 3 , and G 4  are fixedly disposed on the first input shaft IS 1 . The first, second, third, and fourth input gears G 1 , G 2 , G 3 , and G 4  are disposed on the rear portion of the first input shaft IS 1  penetrating through the second input shaft IS 2 , and are sequentially disposed from the front to the rear. 
     The fifth and sixth input gears G 5  and G 6  are fixedly disposed on the second input shaft IS 2 . The fifth input gear G 5  is disposed on the front portion of the second input shaft IS 2  and the sixth input gear G 6  is disposed on the rear portion of the second input shaft IS 2 . 
     Therefore, the first input shaft IS 1  and the first, second, third, and fourth input gears G 1 , G 2 , G 3 , and G 4  fixedly disposed thereon are rotated if the first clutch CL 1  is operated, and the second input shaft IS 2  and the fifth and sixth input gears G 5  and G 6  fixedly disposed thereon are rotated if the second clutch CL 2  is operated. 
     The first, second, third, fourth, fifth, and sixth input gears G 1 , G 2 , G 3 , G 4 , G 5 , and G 6  are input gears operating at each gear stage. The first input gear G 1  is operated at the seventh forward speed, the second input gear G 2  is operated at the first forward speed, the third input gear G 3  is operated at the third forward speed, and the fourth input gear G 4  is operated at the fifth forward speed. 
     In addition, the fifth input gear G 5  is operated at the second forward speed, and the sixth input gear G 6  is operated at the fourth forward speed and a sixth forward speed. 
     The input gears G 1 , G 2 , G 3 , and G 4  for achieving the odd-numbered gear stages are disposed on the first input shaft IS 1 , and the input gears G 5  and G 6  for achieving the even-numbered gear stages are disposed on the second input shaft IS 2 . 
     The speed output device includes the first and second output shafts OS 1  and OS 2  disposed in parallel with and apart from the first and second input shafts IS 1  and IS 2 . 
     The first speed gear D 1  and the fifth speed gear D 5  are rotatably disposed on the first output shaft OS 1  and the first synchronizer SL 1  selectively connecting the first speed gear D 1  or the fifth speed gear D 5  to the first output shaft OS 1  is disposed on the first output shaft OS 1 . In addition, the second speed gear D 2  and the fourth speed gear D 4  are rotatably disposed on the first output shaft OS 1  and the second synchronizer SL 2  selectively connecting the second speed gear D 2  or the fourth speed gear D 4  to the first output shaft OS 1  is disposed on the first output shaft OS 1 . The first output gear OG 1  is fixedly disposed on the front portion of the first output shaft OS 1 . 
     The first synchronizer SL 1  is disposed on the rear portion of the first output shaft OS 1  and the second synchronizer SL 2  is disposed on the front portion of the first output shaft OS 1 . 
     The first speed gear D 1  is engaged with the second input gear G 2  and the fifth speed gear D 5  is engaged with the fourth input gear G 4 . 
     The second speed gear D 2  is engaged with the fifth input gear G 5  and the fourth speed gear D 4  is engaged with the sixth input gear G 6 . 
     In addition, the first output gear OG 1  outputs torque transmitted from the first output shaft OS 1  to the differential apparatus DIFF through the final reduction gear FD. 
     In addition, the third speed gear D 3  and the seventh speed gear D 7  are rotatably disposed on the second output shaft OS 2  and the third synchronizer SL 3  selectively connecting the third speed gear D 3  or the seventh speed gear D 7  to the second output shaft OS 2  is disposed on the second output shaft OS 2 . In addition, the sixth speed gear D 6  is rotatably disposed on the second output shaft OS 2  and the fourth synchronizer SL 4  selectively connecting the sixth speed gear D 6  to the second output shaft OS 2  is disposed on the second output shaft OS 2 . The second output gear OG 2  is fixedly disposed on the front portion of the second output shaft OS 2 . 
     The third synchronizer SL 3  is disposed on the rear portion of the second output shaft OS 2  and the fourth synchronizer SL 4  is disposed on a front portion of the second output shaft OS 2 . 
     The third speed gear D 3  is engaged with the third input gear G 3  and the seventh speed gear D 7  is engaged with the first input gear G 1 . 
     The sixth speed gear D 6  is engaged with the sixth input gear G 6 . 
     The second output gear OG 2  outputs torque transmitted from the second output shaft OS 2  to the differential apparatus DIFF through the final reduction gear FD. 
     The reverse speed output unit includes the reverse idle shaft IDS disposed in parallel with the first and second input shafts IS 1  and IS 2 . 
     The first idle gear IDG 1  and the second idle gear IDG 2  are disposed on the reverse idle shaft IDS. The first idle gear IDG 1  is rotatably disposed on the reverse idle shaft IDS and the second idle gear IDG 2  is fixedly disposed on the reverse idle shaft IDS. 
     In addition, the fifth synchronizer SL 5  selectively connecting the first idle gear IDG 1  to the reverse idle shaft IDS is disposed on the reverse idle shaft IDS. 
     In addition, the first idle gear IDG 1  is engaged with the first input gear G 1  on the first input shaft IS 1  and the second idle gear IDS 2  is engaged with the second speed gear D 2  on the first output shaft OS 1 . 
     The selective connection unit, the input device, and the speed output device according to the second exemplary embodiment of the present invention are the same as those of the first exemplary embodiment except the reverse speed output unit. 
     Therefore, operations of the first, second, third, fourth, and fifth synchronizers SL 1 , SL 2 , SL 3 , SL 4 , and SL 5  and the first and second clutches CL 1  and CL 2  are the same in the first and second exemplary embodiments. 
       FIG. 5  is a schematic diagram for illustrating power delivery path at a reverse speed in a power transmission apparatus for a vehicle according to the second exemplary embodiment of the present invention. 
     Referring to  FIG. 5 , the first speed gear D 1  and the first output shaft OS 1  are operably connected through the sleeve SLE 1  of the first synchronizer SL 1  and the second idle gear IDG 2  and the reverse idle shaft IDS are operably connected through the sleeve SLE 5  of the fifth synchronizer SL 5  for the shift to the reverse speed in the second exemplary embodiment of the present invention. After that, if the second clutch CL 2  is operated, the shift to the reverse speed is achieved. 
     Therefore, the torque of the engine ENG is sequentially transmitted to the second input shaft IS 2 , the sixth input gear G 6 , the fourth speed gear D 4 , the second idle gear IDG 2 , the fifth synchronizer SL 5 , the reverse idle shaft IDS, the first idle gear IDG 1 , the first input gear G 1 , the second input gear G 2 , the first speed gear D 1 , the first synchronizer SL 1 , the first output shaft OS 1 , the first output gear OG 1 , the final reduction gear FD, and the differential apparatus DIFF. 
     For convenience in explanation and accurate definition in the appended claims, the terms “upper”, “lower”, “inner” and “outer” are used to describe features of the exemplary embodiments with reference to the positions of such features as displayed in the figures. 
     The foregoing descriptions of specific exemplary embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teachings. 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 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.