Abstract:
The level of quietness for an automatic transmission case is ensured by preventing vibration by improving the rigidity while suppressing the increase in the mass of a transmission case. In an automatic transmission case in which a valve body is attached to a case wall that covers the transmission mechanism, the rigidity of the automatic transmission case is increased by providing, in the case, an enclosing wall that projects out from the case wall and encloses the periphery of the valve body.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of Invention  
           [0002]    The invention relates to a case for an automatic transmission and, in particular, to a vibration resistant structure thereof.  
           [0003]    2. Description of Related Art  
           [0004]    A case for an automatic transmission is generally formed from a transmission casing that covers the transmission mechanism, a transmission apparatus housing that is fastened to the front of the casing and that covers a hydraulic power transmission apparatus comprising a torque converter or the like, and an extension housing that is mounted to the rear of the transmission casing. The valve body of the hydraulic control apparatus that controls the hydraulic power transmission apparatus and the transmission mechanism of the automatic transmission is usually mounted on the side of the transmission casing. In an automatic transmission for a front engine rear drive (FR) vehicle, in particular, the valve body is normally fixed to the bottom of the transmission casing and protrudes downwards from the casing. The valve body is completely enclosed by an oil pan that covers the area below and surrounding the valve body. A representative example of an automatic transmission case having this type of structure is disclosed in Japanese Patent Laid-Open No. HEI 7-259967.  
           [0005]    Because the size of the outer diameter is severely restricted in an automatic transmission for an FR vehicle in order to ensure sufficient space in the vehicle interior, as is not the case with an automatic transmission for a front engine front drive (FF) vehicle, an automatic transmission for an FR vehicle has a narrow elongated shape in which the size of the outer diameter is small in comparison with the size in the axial direction due to each element used to change gears being aligned in the axial direction. Therefore, it is difficult to secure rigidity in the automatic transmission and when this is mounted in a vehicle and is regarded together with the engine as a vibration system, if the rigidity of this vibration system is low, the vibration system has a resonance point in a comparatively low frequency range near the vibration frequency of the propeller shaft. Accordingly, if this resonance frequency is a frequency that causes a harsh grating sound to be heard in the audible frequency band, then it is not possible to achieve the level of quietness expected in a running vehicle.  
           [0006]    In order to reduce the noise generated from the transmission due to the resonance and ensure an appropriate level of quietness, it is necessary to eliminate the resonance and vibration of the propeller shaft by providing the transmission with a characteristic vibration frequency in a higher frequency region. To achieve this, it is necessary to increase the rigidity. In particular, when a drive apparatus for a four wheel drive vehicle is formed using an automatic transmission for an FR vehicle as the base thereof, in terms of mass, because a transfer having only a slightly smaller mass than the transmission is linked to the rear side of the transmission, it is easy for a vibration system having a long axial length and having a low rigidity joint to be formed between the two large mass portions that are linked to each other. Thus, it is difficult to reduce the noise further without increasing the rigidity of the transmission case to which the transfer is linked.  
           [0007]    Making the case thicker is one simple means of improving the rigidity of a transmission. However, if the case is made thicker, the mass of the case itself is also thereby increased resulting instead in a vibration system with a linking portion having a relatively low rigidity being formed and the desired effect of an improvement in the level of quietness is not obtained.  
           [0008]    Therefore, it is the main aim of the invention to provide a transmission case capable of providing increased rigidity while holding the increase in the mass of the transmission to the absolute minimum in order to ensure that the transmission operates quietly.  
         SUMMARY OF THE INVENTION  
         [0009]    Accordingly, it is an object of the invention to provide an automatic transmission case in which a valve body is attached to a case wall covering a transmission mechanism, wherein an enclosing wall that projects outwards from the case wall enclosing the periphery of the valve body is provided in the case.  
           [0010]    It is effective if the enclosing wall in the above structure is formed higher than the thickness of the valve body, and the opening of the space for containing the valve body that is enclosed by the enclosing wall is covered by an oil pan that is shallower than the thickness of the valve body.  
           [0011]    Moreover, in a structure in which the case comprises a transmission casing that covers the transmission mechanism and a transmission apparatus housing that covers a hydraulic power transmission apparatus, it is also effective if the transmission apparatus housing has a joining portion where it is joined to the transmission casing, the enclosing wall is provided in the transmission casing, and the front portion of the enclosing wall forms a matching portion that matches the joining portion of the transmission apparatus housing; and the transmission apparatus housing is mounted to the transmission casing by matching the joining portion to the front portion of the enclosing wall and fastening the joining portion to the front portion of the enclosing wall using fastening means.  
           [0012]    In this case, it is effective if the joining portion is formed as a joining wall that faces the front portion of the enclosing wall of the transmission casing, and the joining wall is provided with ribs that connect the rear of the joining wall to the transmission apparatus housing.  
           [0013]    Furthermore, it is effective if the valve body is fastened by fastening means to the case wall in the vicinity of the front portion and the rear portion of the enclosing wall.  
           [0014]    Moreover, it is also effective if the rear wall is provided in the case wall in the vicinity of a supporting portion that supports an output shaft of the transmission mechanism.  
           [0015]    Furthermore, it is also effective if the rear wall is provided with ribs that connect the rear of the rear wall with the rear end portion of the transmission casing.  
           [0016]    Moreover, it is also effective if the case comprises a transmission casing that covers the transmission mechanism and a transmission apparatus housing that covers a hydraulic power transmission apparatus, and a rib is provided extending from the front end to the rear end of the transmission casing that is integral with a portion of the case wall other than the portion of the transmission casing where the valve body is mounted.  
           [0017]    According to an aspect of the invention, because the space needed for attaching the valve body is used to enable a case structure to be formed in which the geometrical moment of inertia of the casing is enlarged by forming the enclosing wall that encloses the periphery of the valve body attached to the transmission case projecting out from the transmission case, the rigidity of the case is improved while holding any increase in the mass to a minimum, without causing any increase in the mass to be generated as a result of increasing the thickness of the case wall itself in order to improve the rigidity.  
           [0018]    In a preferred form of the invention, because the space to contain the valve body can be secured using the shallow oil pan that covers the opening of the containing space and using the enclosing wall that is higher than the thickness of the valve body, there is no need to lower the position of the oil pan because the enclosing wall has been made to project out in order to ensure sufficient rigidity, and the minimum ground clearance when the automatic transmission is mounted in a vehicle can be secured in the same way as in a conventional structure.  
           [0019]    In a preferred form of the invention, when the transmission case is formed from a transmission casing that covers the transmission mechanism and a transmission apparatus housing that covers a hydraulic power transmission apparatus in the same way as in a conventional structure, because the transmission casing and the power transmission housing are linked directly by the matching portion formed by the front portion of the enclosing wall on the casing side and the joining portion of the housing, any reduction in the rigidity of the linking portion due to a separate transmission casing being linked to a power transmission housing is controlled.  
           [0020]    In a preferred form of the invention, ribs at the rear of the joining wall of the transmission apparatus housing fastened to the front portion of the enclosing wall of the transmission casing have the function of improving the rigidity of the transmission apparatus housing and, as a result, the rigidity of the entire transmission case including the rigidity of the transmission casing linked to the transmission apparatus housing is improved.  
           [0021]    In a preferred form of the invention, because the valve body is fastened by fastening means to the case wall in the vicinity of the front portion and the rear portion of the enclosing wall that encloses the valve body, the rigidity of the valve body, which conventionally has had a high degree of rigidity, can be used to improve the rigidity of the transmission casing.  
           [0022]    In a preferred form of the invention, by providing the rear wall of the enclosing wall in the vicinity of the case wall supporting the output shaft, the enclosing wall also helps to improve the rigidity of the output shaft supporting portion of the case wall and the rigidity of the output shaft support is also improved  
           [0023]    In a preferred form of the invention, because it is possible to improve the rigidity of the rear portion of the casing using the rigidity of the enclosing wall, the result is that the rigidity of the transmission casing can be improved even further.  
           [0024]    In a preferred form of the invention, it is possible to improve the rigidity of the overall transmission casing by providing a balanced rigidity in response to the improved rigidity of the portion to which the valve body is attached. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]    The invention will be described with reference to the drawings, in which:  
         [0026]    [0026]FIG. 1 is a cross-sectional view in the axial direction of the automatic transmission case according to an embodiment of the invention;  
         [0027]    [0027]FIG. 2 is a bottom view showing the structure of the automatic transmission case, before the oil pan has been attached thereto;  
         [0028]    [0028]FIG. 3 is a perspective view of the entire automatic transmission case as seen from the bottom front; and  
         [0029]    [0029]FIG. 4 is a perspective view of a portion of the automatic transmission case as seen from the bottom rear. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0030]    Hereinafter, an embodiment of the invention will be described with reference to the drawings. FIG. 1 is a cross-sectional view in the axial direction of the transmission case according to an embodiment of the invention. As shown in FIG. 1, the transmission case comprises a transmission casing (in the description given of the embodiment, this will be referred to as the casing)  1  that covers a transmission mechanism (unillustrated); and a converter housing  2  (referred to below as the housing) serving as a power transmission apparatus housing that covers a hydraulic power transmission apparatus (unillustrated) comprising a torque converter, or the like. The valve body  3  that is attached to the case wall of the transmission case, in this embodiment, is attached to the bottom portion of the case wall  10  that is formed by the wall of the casing  1 .  
         [0031]    In accordance with the characteristics of the invention, an enclosing wall  11 , that projects out from the case wall  10  enclosing the periphery of the valve body  3 , is provided in the casing  1 . The enclosing wall  11  projects, or extends, beyond the thickness of the valve body  3 . The opening of the space containing the valve body  3 , that is enclosed by the enclosing wall  11 , is covered by an oil pan  4  that is shallower than the thickness of the valve body  3 . Note that, in the invention, when the thickness of the valve body  3  is referred to, what is meant is the thickness of the portion of the valve body  3  that is contained within the valve body containing space enclosed by the enclosing wall  11 , that is, the valve body lower  3 B of the valve body  3 . By forming the enclosing wall  11 , that encloses the periphery of the valve body  3  attached to the casing  1 , and projects out from the casing  1 , the space needed for attaching the valve body  3  is used to enable a case structure to be formed which permits the geometrical moment of inertia of the casing  1  to be enlarged. In this structure, the rigidity of the casing  1  is improved while limiting any increase in the mass to a minimum, thereby not increasing the mass which would result from increasing the thickness of the case wall  10  itself in order to improve the rigidity. Moreover, by securing the space that contains the valve body  3  using the shallow oil pan  4 , that covers the opening of the valve body containing space, and the enclosing wall  11 , which is higher than the thickness of the valve body  3 , there is no need to lower the position of the oil pan because the enclosing wall  11  projects out to ensure sufficient rigidity. Also, the minimum ground clearance when the automatic transmission is mounted in a vehicle can be secured in the same way as in a conventional structure.  
         [0032]    The housing  2  is provided with a portion where it is joined to the casing  1 . The front portion  11   a  of the enclosing wall  11  provided in the casing  1  forms a matching portion that matches the joining wall  21  of the housing  2 . The housing  2  is attached to the casing  1  by fastening the joining wall  21  to the front portion  11   a  of the enclosing wall  11  using fastening means  5  formed from bolts. By employing this type of structure, the casing  1  and the housing  2  are linked directly by the matching portion formed by the front wall  11   a  of the enclosing wall  11  on the casing side and the joining wall  21  of the housing  2 . Therefore, any reduction in the rigidity of the linking portion due to a separate casing  1  and housing  2  being linked is controlled.  
         [0033]    Furthermore, the joining wall  21  of the housing  2  is formed as a joining wall facing the front wall  11   a  of the enclosing wall  11  of the casing  1  and the joining wall  21  is also provided at the rear side thereof with ribs  22  that are connected to the housing  2 . By employing this structure, the ribs  22 , at the rear of the joining wall  21  of the housing  2  fastened to the front portion  11   a  of the enclosing wall  11  of the casing  1 , have the function of improving the rigidity of the housing  2 . As a result, the rigidity of the entire transmission case, including the rigidity of the casing  1  linked to the housing  2 , is also improved.  
         [0034]    As is shown in FIG. 2, viewing the bottom of the transmission case when the oil pan is removed, the valve body  3  is fastened by fastening means  6 , formed from bolts, to the case wall  10  in the vicinity of the front wall  11   a  and the rear wall  11   b  of the enclosing wall  11 . By employing this structure, the rigidity of the valve body  3 , which has always had a high degree of rigidity, is used to improve the rigidity of the casing  1 .  
         [0035]    The rear wall  11   b  is provided in the vicinity of a supporting portion, or wall,  10   b  (FIG. 4) for an output shaft unillustrated of the transmission mechanism in the case wall  10 . The rear end of the rear wall  11   b  is also provided with ribs  12  that connect the rear end of the casing  1  with the rear wall. By providing the rear wall  11   b  of the enclosing wall  11  in the vicinity of the case wall supporting the output shaft in this way, the enclosing wall  11  also helps to improve the rigidity of the output shaft supporting portion of the case wall  10  and the rigidity of the output shaft support is also improved. Moreover, because it is possible to improve the rigidity of the rear portion of the casing  1  using the rigidity of the enclosing wall  11  which is reinforced by the ribs  12 , the result is that the rigidity of the casing  1  is improved even further.  
         [0036]    The casing  1  is provided with a single rib  13  extending from the front end of the casing  1  to the rear end thereof in a case wall portion, or peripheral wall,  10   a  that is different from the bottom portion where the valve body  3  is provided. This structure is intended to improve the rigidity of the overall casing by providing a balanced rigidity in response to the improved rigidity of the portion to which the valve body  3  is attached.  
         [0037]    More specifically, it can be seen by referring to the perspective view in FIG. 3 of the transmission case as seen from the bottom front thereof, that the casing  1  is formed from a case wall  10  provided with a peripheral wall  10   a  that overall is formed substantially in a cylindrical shape, and a wall  10   b  (FIGS. 1 and 4) in the diametrical direction that shuts off the rear portion of the peripheral wall  10   a  leaving only an opening for the output shaft to pass through. The front end of the peripheral wall  10   a  is formed with a substantial thickness having an outer diameter that is larger at the outer side in the diametrical direction than the inner diameter of the peripheral wall that essentially delineates the inner diameter of the casing  1 . Inside the peripheral wall  10   a  are formed a plurality of fluid passages that communicate with fluid passages of an oil pump body (not shown) mounted on the peripheral wall  10   a  and a shaft that is inserted therein and supported thereby. The thick portion of the front end of the peripheral wall  10   a  and the extended portion  10   b ′ on the bottom side of the wall  10   b  in the diametrical direction of the rear portion together with the side walls that connect both ends of each in the axial direction form a mounting frame for the valve body  3  below the peripheral wall  10   a  form a recess for receiving the valve upper body  3 A of the valve body  3  having a rectangular shape when seen in plan view. The bottom face of this mounting frame forms a joining surface with the valve body  3 . Accordingly, from this positional relationship, the front wall  11   a  and the rear wall  11   b  extend from the joining surface of the valve body  3  and both side walls  11   c  form walls that extend downwards from the peripheral walls  10  that forms the outer diameter portion of the case. Note that the bottom ends of both side walls  11   c  are formed having projecting portions of flanges, that protrude in a transverse direction, as is shown in FIG. 3. These projecting portions are structured so as to serve both as a fastening portion for the mounting bolts of the oil pan  4  and as reinforcement for the enclosing wall  11 . Moreover, a plurality of openings  10   c  used for draining lubricating oil are formed in the portion of the peripheral wall  10   a  positioned within the mounting frame in order to return to the oil pan  4  lubricating oil supplied to each portion of the transmission mechanism from the space enclosed by the peripheral wall of the casing  1 .  
         [0038]    As is shown in FIG. 4 by the view of the rear portion of the casing as seen from the bottom rear, thereof in this embodiment, the rear portion of the case is formed with an adaptor structure in order that it may be linked to the transfer of a four wheel drive vehicle. Because the rigidity of this portion also needs to be improved, the portion between the double wall structure peripheral wall  10   a  and a cylindrical boss portion  10   d , that surrounds the opening in the wall  10   b  in the diametrical direction and supports a bearing that extends in the axial direction, is reinforced with a plurality of ribs  14  radiating outwards. Furthermore, the interior of the space having a trapezoidal cross section that corresponds to the rear portion of the downward extended portion  10   b ′ of the diametrical wall  10   b  is also reinforced by a rib  15  that runs in the vertical direction and by ribs  15  that run in a diagonal direction.  
         [0039]    The housing  2  is formed from a peripheral wall  20  that is open at both the front and rear and is shaped overall like a truncated cylinder. The front side thereof, which is linked to the engine, has a double wall structure, while the rear side, which is linked to the casing  1 , is formed with a wall  23  that serves as a flange used for fastening that projects slightly radially inward. The radially inwardly projecting wall  23  is formed as an end surface that almost exactly matches shape of the opening of the casing  1 . The joining wall  21  is formed as a wall that extends downwards from the radially inwardly projecting wall  23  and the rigidity of the joining wall  21  is reinforced by six ribs  22  that extend from the rear of the joining wall  21  to the front end of the peripheral wall  20 . While the inner ribs have essentially the same height as the height of the joining wall  21  where they are joined to the rear of the joining wall  21 , the ribs  22  at both sides are formed slightly lower at the end joined to the rear of the joining wall  21 . This is to facilitate the job of fastening the side bolts  5  at both sides.  
         [0040]    Next, the valve body  3  is formed from a valve body upper  3 A and a valve body lower  3 B that are formed from die cast material, and a separator that is sandwiched between these two. The valve body lower  3 B is formed larger than the valve body upper  3 A as seen in side view (FIG. 1) and is provided with a fastening portion by which it is fastened to the mounting frame of the casing  1 . As is shown in FIG. 2, the valve body  3  is fastened to the case wall  10  by six front bolts  6  and three rear bolts  6  in the vicinity of the front wall  11   a  and the rear wall  11   b  respectively of the enclosing wall  11  in such a way that the valve body upper  3 A of the valve body  3  is contained inside the mounting frame of the casing  1 , and the lower  3 B is contained inside the enclosing wall  11 . Note that, as shown in FIG. 1, a strainer  7  and solenoids  8  are fitted to the valve body lower  3 B.  
         [0041]    As described above, because the valve body  3  is completely contained inside the enclosing wall  11 , the oil pan  4  covering the opening of the enclosing wall  11  is formed in a shallow dish shape with only the portion that corresponds to the bottom of the intake opening of the strainers  7  fitted to the valve body  3  projecting slightly below the bottommost portion of the enclosing wall  11 , in other words, in a shallow dish shape whose bottommost portion protrudes slightly lower than the portion thereof that is placed against the enclosing wall  11  with the seal inserted between the two. By employing this type of structure, it is possible to secure the space for containing the valve body  3  using the shallow oil pan  4  that covers the opening of the containing space and the enclosing wall  11  that is higher than the thickness of the valve body  3 , namely, in the embodiment, that is thicker than the valve body lower  3 B. As a result, there is no need to lower the position of the oil pan  4  because the enclosing wall  11  has been made to project downwards to secure the required rigidity, and the minimum ground clearance when the automatic transmission is mounted in a vehicle can be secured in the same way as in a conventional structure.  
         [0042]    The casing  1  and the housing  2  that are structured as has been described above are integrally joined together by bolts that are screwed into threaded holes formed in the thick portion of the front end of the peripheral wall  10   a  of the casing  1  via through holes formed in the radially inwardly projecting wall  23  of the housing  2 . The joint between the casing  1  and the housing  2  is reinforced further by bolts  5  being screwed from the joining wall  21  side into the front wall  11   a  of the enclosing wall  11 . The valve body  3  is fixed to the casing  1  using the bottom surface of the mounting frame of the case wall  10  as joining surfaces in the vicinity of the rear wall  11   b  and the front wall  11   a  of the enclosing wall  11 . An integrated transmission case is thus formed by the rigid integration of the casing  1  and housing  2 , whose rigidities have both been reinforced in the manner described above, through the reinforced structural portions thereof being joined together, and by further reinforcing the casing  1  and housing  2  using the rigidity of the valve body  3 .  
         [0043]    A detailed description of the invention has been given above based on one embodiment thereof, however, the invention is not limited to the described embodiment, and may be implemented in various configurations within the range described in the claims. In particular, in the described embodiment, the case rear end portion is formed integrally with an adaptor structure for linking with a transfer for a four-wheel drive vehicle, however, when constructing a normal transmission case for an FR vehicle, it is possible, instead of the adaptor structure, to fasten to the case an extension housing in the form of an extremely short cover.