Abstract:
A six-speed automatic transmission for a vehicle includes a transmission housing, an input member, an output member, three planetary gearsets for changing a ratio of torque between the input member and the output member, clutch assemblies to selectively couple the input member to predetermined members of the planetary gearsets, and brake assemblies to selectively couple predetermined members of the planetary gearsets to the transmission housing. The transmission achieves an improved gear ratio spacing and, correlatively, enhanced vehicle drivability, performance, and fuel economy, by respectively permanently coupling the planetary carrier of the first gearset with the sun gear of the second gearset, the annulus gears of the first and third gearsets with the planetary carrier of the second gearset, and the output member with both the annulus gear of the second gearset and the planetary carrier of the third gearset.

Description:
FIELD OF THE INVENTION 
     The invention relates generally to an automatic transmission for a motor vehicle, such as an automobile, incorporating three planetary gear sets. 
     BACKGROUND OF THE INVENTION 
     A conventional automatic transmission includes a hydrodynamic torque converter to transfer engine torque from an engine crankshaft to a rotatable input member of the transmission through fluid-flow forces. The transmission also includes frictional units, typically referred to as clutches, which couple the rotatable input member to one or more members of the planetary gearsets. Other frictional units, typically referred to as brakes, hold one or more members of the planetary gearsets stationary during the flow of power. Such transmissions also typically provide for one or more planetary gearsets in order to provide various ratios of torque and to ensure that the available torque and the respective tractive power demand are matched to each other. 
     One disadvantage of such known automatic transmissions is that it is more difficult with good planetary gearset design to provide an overdrive ratio of 0.75 or greater while yet achieving a gear ratio spacing in the transmission&#39;s lower gears (numerically higher gear ratios) that enhances vehicle drivability, performance, and fuel economy. 
     SUMMARY OF THE INVENTION 
     Under the invention, an automatic transmission for a vehicle includes a transmission housing, an input member, an output member, three planetary gearsets for changing a ratio of torque between the input member and the output member, a plurality of clutch assemblies to selectively couple the input member to predetermined members of the planetary gearsets, and a plurality of brake assemblies to selectively couple predetermined members of the planetary gearsets to the transmission housing. In accordance with an aspect of the invention, the planetary carrier of the first planetary gearset is permanently coupled for rotation with the sun gear of the second planetary gearset, the annulus gears of the first and third planetary gearsets are each permanently coupled for rotation with the planetary carrier of the second planetary gearset, and the output member is permanently coupled for rotation with each of the annulus gear of the second planetary gearset and the planetary carrier of the third planetary gearset. 
     In accordance with another aspect of the invention, in a preferred embodiment, a third clutch assembly selectively couples the input member to the sun gear of the third planetary gearset to achieve at least one underdrive gear ratio. And, in accordance with another aspect of the invention, in the preferred embodiment, a second clutch assembly selectively couples the input member to the planetary carrier of the second planetary gearset to achieve at least one overdrive gear ratio. In accordance with yet another aspect of the invention, in the preferred embodiment, a first clutch assembly selectively couples the input member to the sun gear of the first planetary gearset, and the output member rotates in a direction opposite to that of the input member, whereby a reverse gear of the transmission is achieved. 
     In a six-speed embodiment of the automatic transmission of the invention, the transmission employs an arrangement of interconnected planetary gearsets to achieve an improved ratio spacing of small to medium magnitude, especially between second and third gears. The six-speed transmission of the invention further advantageously provides an improved overdrive ratio spacing between fifth and sixth gears. 
     Other objects, features, and advantages of the present invention will be readily appreciated upon a review of the subsequent description of the preferred embodiment and the appended claims, taken in conjunction with the accompanying Drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional schematic view of an exemplary automatic transmission in accordance with the invention; and 
         FIG. 2  is a chart of gears, gear ratio, and clutches and brakes engaged for the six-speed automatic transmission of  FIG. 1  when used as a six-speed overdrive transmission. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , a cross-sectional schematic view of an exemplary six-speed automatic transmission  10  in accordance with the invention includes a transmission housing  12  for enclosing a torque converter assembly (not shown), an input member  14 , an output member  16 , a multi-clutch and brake assembly  18 , and a gear assembly  20 . The transmission housing  12  is stationary relative to the rotatable input member  14 , output member  16 , and gear assembly  20 . Power is transmitted from a rotating crankshaft (not shown) of an engine (not shown) to the torque converter assembly, in turn, to the rotatable input member  14 . 
     The multi-clutch and brake assembly  18  allows predetermined gears within the gear assembly  20  to be selectively engaged and disengaged from either the input member  14  or the transmission housing  12 . Near the input side of the transmission  10 , the multi-clutch and brake assembly  18  includes an underdrive clutch assembly  22  (applied in first, second, third and fourth gears), an overdrive clutch assembly  24  (applied in fourth, fifth, and sixth gears), and a reverse clutch assembly  26  (applied in reverse gear). Near the output side of the transmission  10 , the multi-clutch and brake assembly  18  includes a three/five brake assembly  28  (applied in third and fifth gears), a two/six brake assembly  30  (applied in second and sixth gears), and a low/reverse brake assembly  32  (applied in first and reverse gears) and. The multi-clutch and brake assembly  18  also includes an input retainer  34  splined to the input member  14 . 
     The underdrive clutch assembly  22  includes a plurality of axially-spaced annular plates  22   a  and a plurality of axially spaced annular discs  22   b  which are alternated between the plates  22   a . When the underdrive clutch assembly  22  is not applied, plates  22   a  and discs  22   b  are free to move or rotate relative to each other. The plates  22   a  are mounted to the input retainer  34  and the discs  22   b  are mounted to an underdrive hub  36 . The underdrive hub  36  is operatively connected to the gear assembly  20 . 
     The overdrive clutch assembly  24  includes a plurality of axially-spaced annular plates  24   a  and a plurality of axially spaced annular discs  24   b . The plates  24   a  are mounted to the input retainer  34  and the discs  24   b  are mounted to an overdrive hub  38  operatively connected to the gear assembly  20 . 
     The reverse clutch assembly  26  includes a plurality of axially-spaced annular plates  26   a  and a plurality of axially spaced annular discs  26   b . The plates  26   a  are mounted to the input retainer  34  and the discs  26   b  are mounted to a reverse hub  40  operatively connected to the gear assembly  20 . It should be appreciated that the clutch assemblies  22 ,  24 , and  26  are applied by fluid actuating devices (not shown). 
     The three/five brake assembly  28  includes a plurality of axially spaced annular plates  28   a  and a plurality of axially spaced annular discs  28   b . The plates  28   a  are mounted to the transmission housing  12  and the discs  28   b  are mounted to the clutch hub  40 . 
     The two/six brake assembly  30  includes a plurality of axially-spaced annular plates  30   a  and a plurality of axially spaced annular discs  30   b . The plates  30   a  are mounted to the transmission housing  12  and the discs  30   b  are mounted to the gear assembly  20  as described below. 
     The low/reverse brake assembly  32  includes a plurality of axially-spaced annular plates  32   a  and a plurality of axially-spaced annular discs  32   b . The plates  32   a  are mounted to the transmission housing  12  and the discs  32   b  are mounted to the gear assembly  20  as described below. It will be appreciated that the brake assemblies  28 ,  30  and  32  are applied by fluid actuating devices (not shown). 
     A freewheel clutch  42  rotationally coupled to the discs  32   b  ensures that the discs  32   b  rotate in only one direction relative to the plates  32   a.    
     The gear assembly  20  includes a front or first planetary gearset  44 , an axially-spaced intermediate or second planetary gearset  46 , and an axially-spaced rear or third planetary gearset  48 . The first planetary gearset  44  includes a first sun gear  50  coupled to the reverse hub  40 . The first planetary gearset  44  also includes a first planetary carrier  52  supporting a plurality of circumferentially-spaced first planet gears  54  in engagement with the first sun gear  50 . The first planetary carrier  52  is rotationally coupled to the discs  30   b  of the two/six brake assembly  30 . The first planetary gearset  44  further includes a first annulus gear  56  disposed about the first planetary carrier  52  which engages the first planet gears  54 . The first annulus gear  56  is rotationally coupled to the discs  32   b  of the low/reverse brake assembly  32 . By way of example only, in the exemplary embodiment, the ratio of the number of teeth on the first annulus gear  56  to the number of teeth on the first sun gear  50  is 1.67. 
     The second planetary gearset  46  includes a second sun gear  58  rotationally coupled to the first planetary carrier  52  of the first planetary gear set  44 . The second planetary gearset  46  also includes a second planetary carrier  60  supporting a plurality of circumferentially-spaced second planet gears  62  for engagement with the second sun gear  58 . The second planetary carrier  60  is rotationally coupled to both the overdrive hub  38  and the discs  32   b  of the low/reverse brake assembly  32 , and the first and second annulus gears  56 , 72  of the first and third planetary gearsets  44 , 48 . The second planetary gearset  46  further includes a second annulus gear  64  disposed about the second planetary carrier  60  and engaging the second planet gears  62 . The second annulus gear  64  is connected to the third planetary gearset  48  as described below. By way of example only, in the exemplary embodiment, the ratio of the number of teeth on the second annulus gear  64  to the number of teeth on the second sun gear  58  is 1.67. 
     The third planetary gearset  48  includes a third sun gear  66  rotationally coupled to the underdrive hub  36 . The third planetary gearset  48  also includes a third planetary carrier  68  supporting a plurality of circumstantially-spaced planet gears  70  for engagement with the third sun gear  66 . The third planetary carrier  68  is rotationally coupled to both the second annulus gear  64  and the output member  16  of the transmission  10 . The third planetary gearset  48  further includes a third annulus gear  72  disposed about the third planetary carrier  68  and engaging the third planet gears  70 . By way of example only, in the exemplary embodiment, the ratio of the number of teeth on the third annulus gear  72  to the third sun gear  66  is 2.23. 
     Referring again to  FIG. 1 , in operation, upon application of the underdrive clutch assembly  22 , frictional forces generated between the plates  22   a  and discs  22   b  cause the input retainer  34  to drive the underdrive hub  36  and, with it, the third sun gear  66  of the third planetary gearset  48 . Upon application of the overdrive clutch assembly  24 , frictional forces generated between the plates  24   a  and discs  24   b  cause the input retainer  34  to drive the overdrive hub  38  and, with it, the second planetary carrier  60  and the supported second planets  62  of the second planetary gearset  46 , as well as the first and third annulus gears  56 , 72  of the first and third planetary gearsets  44 , 48 . And, upon application of the reverse clutch assembly  26 , frictional forces generated between the plates  26   a  and discs  26   b  cause the input retainer  34  to drive the reverse hub  40  and the first sun gear  50  of the first planetary gearset  44 . 
     Similarly, upon application of the three/five brake assembly  28 , frictional forces generated between the plates  28   a  and discs  28   b  cause the first sun gear  50  of the first planetary gearset  44  to be grounded to the transmission housing  12 . Upon application of the two/six brake assembly  30 , frictional forces generated between the plates  30   a  and discs  30   b  cause both the first planetary carrier  52  of the first planetary gearset  44  and the second sun gear  58  of the second planetary gearset  46  to be grounded to the transmission housing  12 . And, upon application of the low/reverse brake assembly  32 , frictional forces generated between the plates  30   a  and discs  30   b  of the low/reverse brake assembly  30  cause the first and third annulus gears  56 , 72  of the first and third planetary gearsets  44 , 48 , as well as the second planetary carrier  60  of the second planetary gearset  46 , to be grounded to the transmission housing  12 . 
     Referring to  FIGS. 1 and 2 , when the transmission  10  is to be operated in “first gear,” the underdrive clutch assembly  22  and low/reverse brake assembly  32  are applied, causing the third sun gear  66  to be driven by the input shaft  14  via the underdrive hub  36  while the third annulus gear  72  is held stationary (initially through application of the low/reverse clutch assembly  32  and thereafter by the freewheel clutch  42 ). As a result, the rotating third sun gear  66  drives the third planetary carrier  68  via the third planet gears  70 . Because the output member  16  is coupled to the third planetary carrier  68 , the rotation of the third planetary carrier  68  causes the output member  16  to rotate, with a reducing gear ratio of 3.23. 
     When the transmission  10  is to be operated in “second gear,” the underdrive clutch assembly  22  and the two/six brake assembly  30  are applied, causing the third sun gear  66  to be driven by the input shaft  14  (via the underdrive hub  36 ) while the second sun gear  58  is held stationary (via the grounding of the rotationally-coupled first planetary carrier  52 ). As a result, the rotating second planetary carrier  60  drives the second annulus gear  64  via the second planet gears  62 . Because the output member  16  is coupled to the second annulus gear  64  (via the rotationally-coupled third planetary carrier  68 ), the rotation of the second planetary carrier  60  causes the second annulus gear  64  to rotate, with a reducing gear ratio of 1.84. 
     When the transmission  10  is to be operated in “third gear,” the underdrive clutch assembly  22  and the three/five brake assembly  28  are applied, causing the third sun gear  66  to rotate and the first sun gear  50  to be held stationary. As a result, rotation of the third sun gear  66  causes each of the third planetary carrier  68  and second annulus gear  64 , as well as each of the third annulus gear  72 , the second planetary carrier  60 , and the first annulus gear  56  to rotate, whereupon the first planetary carrier  52  further rotates with the second sun gear  58 . As a result, the output member  16  will be caused to rotate, with a reducing gear ratio of 1.41. 
     When the transmission  10  is to be operated in “fourth gear,” the underdrive and overdrive clutch assemblies  22 ,  24  are applied, causing the third sun gear  66  and the second planetary carrier  60  (and, thus, the third annulus gear  72 ) to rotate at the same speed. As a result, the third planetary carrier  68  and the output member  16  are caused to rotate at the same speed, thereby providing a gear ratio of 1.00. 
     When the transmission  10  is to be operated in “fifth gear,” the overdrive clutch assembly  24  and the three/five brake assembly  28  are applied, causing each of the second planetary carrier  60 , the first annulus gear  56 , and the third annulus gear  72  to rotate, and causing the first sun gear  50  to be held stationary. As a result, the first planetary carrier  52  rotates with the rotationally-coupled second sun gear  58 , thereby causing the second annulus gear  64 , third planetary carrier  68 , and the output member  16  to rotate together faster than in fourth gear, and, in the exemplary embodiment, producing an overdrive gear ratio of 0.82. 
     When the transmission  10  is to be operated in “sixth gear,” the overdrive clutch assembly  24  and the two/six brake assembly  30  are applied, causing the second planetary carrier  60  to rotate, and causing the first planetary carrier  52  and the second sun gear  58  to be held stationary. As a result, the second annulus gear  64 , the third planetary carrier  68 , and the output member  16  rotate together faster than in fifth gear, to produce a further overdrive gear ratio of 0.63. 
     When the transmission  10  is desired to operate in “reverse gear,” the reverse clutch assembly  26  and low/reverse brake assembly  32  are applied, causing the first sun gear  50  to rotate and each of the first annulus gear  56 , the second planetary carrier  62 , and the third annulus gear  72  to be held stationary. As a result, the first sun gear  50  causes the first planetary carrier  52  to rotate. The rotation of the first planetary carrier  52  causes the second sun gear  58  to rotate which, in turn, causes rotation of the second annulus gear  64  and the output member  16  in a direction opposite to the other gear positions. This arrangement of the gear assembly  20  produces a gear ratio of −4.44. 
     Accordingly, the six-speed automatic transmission  10  has a unique arrangement of the planetary gearsets  44 , 46 , 48  to advantageously achieve a 0.82 overdrive ratio in fifth gear and a 0.63 overdrive ratio in sixth gear. The unique arrangement of the planetary gearsets  46 ,  48  and  50  further beneficially results in ratio steps of small to medium magnitude, from first gear through sixth gear, to improve vehicle fuel efficiency and enhance vehicle drivability. 
     While the above description constitutes the preferred embodiment, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the proper scope and fair meaning of the subjoined claims. By way of example only, while the reverse clutch  26  of the preferred embodiment  10  operates to selectively couple the input member  14  to the first sun gear  50  of the first planetary gearset  44 , the invention contemplates the alternative possibility of driving the planetary carrier  52  of the first planetary gearset  44  upon application of the reverse clutch  26  (when accompanied by such other changes, for example, in gearset ratios as necessary to obtain the desired transmission performance). Similarly, while the overdrive clutch  24  of the preferred embodiment operates to selectively couple the input member  14  to the planetary carrier  60  of the second planetary gearset  44  (along with each of the permanently coupled first and third annulus gears  56 , 72  of the first and third planetary gearsets  44 , 48 ), the invention contemplates the alternative possibility of driving the second sun gear  58  of the second planetary gearset  52  upon application of the overdrive clutch  24  (again, when accompanied by such other changes, for example, in gearset ratios as necessary to obtain the desired transmission performance).