Abstract:
The present invention provides an eight speed automatic transmission for motor vehicles having three planetary gear sets, four friction clutches, a friction brake, a band brake and an optional overrunning clutch. The eight speed transmission according to the present invention exhibits good gear ratio progression and high efficiency due to its excellent gear mesh efficiency, low spin losses and low pinion speeds.

Description:
FIELD 
     The present disclosure relates to an automatic transmission and more particularly to a high efficiency eight speed automatic transmission for motor vehicles. 
     BACKGROUND 
     The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art. 
     A typical modern multiple speed automatic transmission includes a combination of planetary gear assemblies and selectively engaged clutches which achieve a plurality of forward speeds or gears ratios and reverse. 
     Whereas three and four speed automatic transmissions were once commonplace, considered suitable and provided sufficient flexibility and performance, the industry and consumer preference is moving to six, seven and eight speed automatic transmissions. 
     In such transmissions, various elements of a plurality of planetary gear assemblies are connected by permanent couplings, selectively connected by clutches and selectively grounded by brakes. Specific combinations of the clutches and brakes are engaged or activated to provide a sequence of numerically related gear ratios and thus speed and torque ratios. 
     Because they so closely match the power and torque curves of the engine to the imposed load and speed of the vehicle, such six, seven and eight speed transmissions provide significant performance enhancements and reduced fuel consumption. 
     The present invention is directed to an eight speed automatic transmission having a good ratio progression, excellent gear mesh efficiency, low spin losses and low pinion speeds. 
     SUMMARY 
     The present invention provides an eight speed automatic transmission for motor vehicles having three planetary gear sets, four friction clutches, a friction brake and a band brake. The eight speed transmission according to the present invention exhibits high efficiency due to its excellent gear mesh efficiency, low spin losses and low pinion speeds. To improve the quality of the first gear to second gear shift, a freewheeling or overrunning clutch may also be included in the transmission. 
     Thus it is an object of the present invention to provide an eight speed automatic transmission. 
     It is a further object of the present invention to provide a high efficiency eight speed automatic transmission. 
     It is a still further object of the present invention to provide a high efficiency eight speed automatic transmission having a plurality of planetary gear assemblies, friction clutches and brakes. 
     It is a still further object of the present invention to provide a high efficiency eight speed automatic transmission having three planetary gear assemblies, four friction clutches, a friction brake and a band brake. 
     It is a still further object of the present invention to provide a high efficiency eight speed automatic transmission having three planetary gear assemblies, four friction clutches, a friction brake, a band brake and, optionally, a one way or overrunning clutch. 
     It is a still further object of the present invention to provide an eight speed automatic transmission having good gear mesh efficiency, low spin losses and low pinion speeds. 
     Further objects, advantages and areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       DRAWINGS 
       The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
         FIG. 1  is a lever diagram of an eight speed automatic transmission according to the present invention; 
         FIG. 2  is a diagrammatic view of an embodiment of an eight speed automatic transmission according to the present invention; and 
         FIG. 3  is a truth table presenting the state of engagement of the various torque transmitting clutches and brakes in each of the available forward and reverse speeds or gear ratios of the automatic transmission illustrated in  FIGS. 1 and 2 . 
     
    
    
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. 
     With reference now to  FIG. 1 , a multiple, i.e., eight, speed automatic transmission  10  according to the present invention is illustrated in a lever diagram. A lever diagram is a schematic representation of the components of an automatic transmission wherein components of planetary gear assemblies are represented by nodes and the planetary gear assemblies themselves are represented by bold vertical bars. The relative lengths of the vertical bars between nodes of a given planetary gear assembly represent the ratios between the components. Mechanical couplings or interconnections between the nodes of various planetary gear assemblies are represented by horizontal lines and torque transmitting devices such as friction clutches and brakes are represented by interleaved or nested fingers. Further explanation of the format, purpose and use of lever diagrams can be found in SAE Paper No. 810102 entitled “The Lever Analogy: A New Tool in Transmission Analysis” by Benford and Leising which is fully incorporated herein by reference. 
     The multiple speed automatic transmission  10  includes an input shaft or member  12  which is driven by the output of a torque converter (not illustrated), a first planetary gear assembly  14  having three nodes: a first node  14 A, a second node  14 B and a third node  14 C; a second planetary gear assembly  16  having three nodes: a first node  16 A, a second node  16 B and a third node  16 C and a third planetary gear assembly  18  having a first node  18 A, a second node  18 B and third node  18 C. Drive torque from the transmission  10  is delivered by an output shaft or member  20  to a final drive assembly (not illustrated). 
     The first node  18 A of the third planetary gear assembly  18  is coupled to and driven by the input shaft  12 . The third node  14 C of the first planetary gear assembly  14  is coupled to the first node  16 A of the second planetary gear assembly  16 . The first node  14 A of the first planetary gear assembly  14  is coupled to ground such as a housing  22  of the transmission  10 . The third node  16 C if the second planetary gear assembly  16  is coupled to and drives the output shaft  20 . 
     It should be noted that the inventor is familiar with the convention of characterizing a torque controlling or selectively transmitting device disposed between two rotatable members as a clutch whereas such a device disposed between one rotatable member and one fixed or stationary member is characterized as a brake. Within the transmission art, however, such torque controlling or transmitting devices are collectively referred to or characterized as clutches. Accordingly, in the following description, such devices will be referred to or characterized as clutches and if their application and use involves a stationary member to which a rotating member is selectively connected and grounded, the word “brake” will appear in parentheses following the word “clutch.” 
     A first friction clutch (brake)  30  selectively connects the third node  18 C of the third planetary gear set  18  to ground such as the transmission housing  22 . A second friction clutch  32  selectively connects the second node  14 B of the first planetary gear assembly  14  to the third node  18 C of the third planetary gear assembly  18 . A third friction clutch  34  selectively connects the third node  14 C of the first planetary gear assembly  14  to the second node  18 B of the third planetary gear assembly  18 . A fourth friction clutch  36  selectively connects the second node  16 B of the second planetary gear assembly  16  to the second node  18 B of the third planetary gear assembly  18 . A fifth friction clutch  38  selectively connects the second node  16 B of the second planetary gear assembly  16  to the first node  18 A of the third planetary gear assembly  18 . A first band clutch (brake)  40  selectively connects the second node  16 B of the second planetary gear assembly  16  to ground such as the transmission housing  22 . 
     An optional first freewheeling or overrunning clutch (brake)  42  couples the second node  16 B of the second planetary gear assembly  16  to ground such as the transmission housing  22  upon rotation relative to ground in one direction and allows the second node  16 B of the second planetary gear assembly  16  to freewheel or overrun upon rotation relative to ground, i.e., the transmission housing  22 , in the opposite direction. 
     Referring now to  FIG. 2 , a stick diagram presents a schematic layout of the eight speed automatic transmission  10  according to the present invention. In  FIG. 2 , the numbering from the lever diagram of  FIG. 1  has been carried over. Thus, the clutches, brakes and couplings are correspondingly presented whereas the nodes of the planetary gear assemblies now appear as components of planetary gear assemblies such as sun gears, planet gear carriers and ring gears. 
     The input shaft or member  12  is continuously connected to and driven by a turbine of a torque converter (not illustrated) which, in turn, is driven by the output of an engine (also not illustrated). The input shaft  12  or an extension thereof is coupled to a third sun gear  18 A of the third simple planetary gear assembly  18  and one side of the fifth friction clutch  38 . A first shaft, quill or intermediate member  50  couples a third ring gear  18 C of the third simple planetary gear assembly  18  and the ungrounded side of the first friction clutch (brake)  30  to one side of the second friction clutch  32 . The other side of the second friction clutch  32  is coupled by a second shaft, quill or intermediate member  52  to a first planet gear carrier  14 B of the first simple planetary gear assembly  14 . Rotatably disposed in the first planet gear carrier  14 B of the first simple planetary gear assembly  14  are a plurality of first planet gears  14 D, one of which is illustrated in  FIG. 2 , which are in constant mesh with the first sun gear  14 A and the first ring gear  14 C of the first simple planetary gear assembly  14 . A third shaft, quill or intermediate member  54  couples the third planet gear carrier  18 B of the third simple planetary gear assembly  18  to one side of the third friction clutch  34  and one side of the fourth friction clutch  36 . Rotatably disposed in the third planet gear carrier  18 B of the third simple planetary gear assembly  18  are a third plurality of planet gears  18 D, one of which is illustrated in  FIG. 2 , which are in constant mesh with the third sun gear  18 A and the third ring gear  18 C of the third simple planetary gear assembly  18 . 
     A fourth shaft, quill or intermediate member  56  couples the other side of the third friction clutch  34  to a first ring gear  14 C of the first simple planetary gear assembly  14  and a second sun gear  16 A of the second simple planetary gear assembly  16 . A fifth shaft, quill or intermediate member  58  couples the other side of the fourth friction clutch  36  and the other side of the fifth friction clutch  38  to a second planet gear carrier  16 B of the second simple planetary gear assembly  16 . Rotatably disposed in the second planet gear carrier  16 B of the second simple planetary gear assembly  16  are a plurality of second planet gears  16 D, one of which is illustrated in  FIG. 2 , which are in constant mesh with the second sun gear  16 A and a second ring gear  16 C of the second simple planetary gear assembly  16 . 
     A sixth shaft, quill or intermediate member  60  couples a first sun gear  14 A of the first simple planetary gear assembly  14  and one side of the optional first freewheeling or overrunning clutch (brake)  42  to ground, such as the housing  22  of the automatic transmission  10 . It should be understood that the first freewheeling or overrunning clutch (brake)  42  is optional and may be included in the automatic transmission  10  when the smoothest possible shift between first and second gears is desired. A seventh shaft, quill or intermediate member  62  couples the second planet gear carrier  16 B of the second simple planetary gear assembly  16  (and the fourth and fifth friction clutches  36  and  38  through the fifth shaft, quill or intermediate member  58 ) and the other side of the first freewheeling or overrunning clutch (brake)  42  with one side of the first band clutch (brake)  40 . The other side of the first band clutch (brake)  40  is coupled to ground such as the housing  22  of the automatic transmission  10 . The output shaft or member  20  is coupled to and driven by the second ring gear  16 C of the second planetary gear assembly  16  and, as noted above, is coupled to and drives a final drive assembly such as a prop shaft, differential and axles (all not illustrated). 
     Referring now to  FIG. 3 , a truth table is presented which sets forth the various combinations of torque transmitting devices, i.e., clutches and brakes, that are engaged or activated to achieve the various forward and reverse gear states or ratios. Actual numerical gear ratios of the various gear states are also presented although it should be appreciated that these numerical gear ratios are presented by way of illustration and example only and that they may be adjusted over significant ranges to accommodate various applications and operational criteria of the automatic transmission  10 . 
     In the truth table of  FIG. 3 , an “X” means a torque transmitting device is activated or engaged and is carrying torque, an “O” means a torque transmitting device is activated or engaged but is not carrying torque, a “G” means the torque transmitting device is a garage shift element, is activated or engaged and is carrying torque and a “C” means a torque transmitting device is activated or engaged only in a manual range or setting of the automatic transmission  10  for coast (engine) braking. 
     In reverse gear, the second friction clutch  32  and the first band clutch (brake)  40  are engaged or activated and the third friction clutch  34  is engaged or activated as a garage shift element and is also carrying torque. 
     In neutral, both the second friction clutch  32  and the first band clutch (brake)  40  are engaged or activated but neither is carrying torque. 
     In first gear, the second friction clutch  32  is engaged or activated, the fourth friction clutch  36  is engaged or activated as a garage shift element and the first freewheeling or overrunning clutch (brake)  42  is engaged (locked) and connects the seventh shaft, quill or intermediate member  62 , the second planet gear carrier  16 B of the second simple planetary gear assembly  16  and associated components to ground. Additionally, the first band clutch (brake)  40  may be engaged or activated in the manual range for coast (engine) braking, if desired. 
     In second gear, if it is achieved in direct sequence from or following first gear, the first band clutch (brake)  40 , if engaged in first gear, is disengaged or deactivated and the first freewheeling or overrunning clutch (brake)  42  is released or disengaged due to a change in the relative direction of rotation between the elements. As noted above, the first freewheeling or overrunning clutch (brake)  42  is an optional component in the automatic transmission  10  which is utilized to improve the quality of the first to second gear shift. The second friction clutch  32  and the fourth friction clutch  36  are maintained in their activated or engaged states and the first friction clutch (brake)  30  is activated or engaged. 
     In third gear, if it is achieved in direct sequence from or following second gear, the second friction clutch  32  and the fourth friction clutch  36  are maintained in their activated or engaged states, the first friction clutch (brake)  30  is deactivated or disengaged and the third friction clutch  34  is activated or engaged. 
     In fourth gear, if it is achieved in direct sequence from or following third gear, the second friction clutch  32  and the fourth friction clutch  36  are maintained in their activated or engaged states, the third friction clutch  34  is deactivated or disengaged and the fifth friction clutch  38  is activated or engaged. 
     In fifth gear, if it is achieved in direct sequence from or following fourth gear, the fourth friction clutch  36  and the fifth friction clutch  38  are maintained in their activated or engaged states, the second friction clutch  32  is deactivated or disengaged and the third friction clutch  34  is activated or engaged. 
     Sixth, seventh and eighth gears are achieved in a similar manner according to the activation or engagement sequence and states set forth in  FIG. 3 . 
     With regard to the foregoing described operation, several features and operational considerations should be noted. First of all, the incorporation of the first freewheeling or overrunning clutch (brake)  42  provides significantly improved first to second gear shifts, especially during hard acceleration. Without the freewheeling clutch  42 , the smoothness of the first to second gear shift is primarily dependent upon simultaneous release of one clutch and engagement of another which can be an engineering and operational challenge. The freewheeling clutch  42  obviates this situation by simply releasing certain elements when a desired relative rotational condition is achieved. Second of all, the foregoing description assumes that all clutches and brakes not specifically referenced in a given gear state are inactive or disengaged. Furthermore, and as noted, the description also assumes that during gear shifts between at least adjacent gear states, a clutch or brake that is activated or engaged in both gear states will remain activated or engaged during the shift. Finally, the foregoing description assumes that downshifts follow essentially the opposite sequence of shifts and that power on skip shifts, e.g., from first to third gear or eight to sixth gear, are possible. 
     The description of the invention is merely exemplary in nature and variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.