Abstract:
A power transmission apparatus comprises a case defining a cavity, moveable parts arranged within the cavity, fixed parts arranged within the cavity, the fixed parts including parts moveably supporting the moveable parts, a wall at least partly constituting said case, and a thin wall portion formed in the wall, the thin wall portion being constantly pressed in an outward direction of the cavity by one of the fixed parts, so as to increase its rigidity. Membranous vibration of the case wall may be effectively controlled and the radiation noise from the transmission apparatus can be reduced without increasing the thickness of the transmission case wall or arranging ribs on it.

Description:
BACKGROUND  
       [0001]     The present description relates to a mechanical power transmission apparatus, and in particular relates to reduction of noise radiated from an automotive transmission.  
         [0002]     A mechanical power transmission apparatus, such as an automotive transmission shown in Japanese patent application publication H4-113052, may comprise a case, rotational shafts, gears on the shafts or other moveable parts, and bearing to support the shafts or other fixed parts. The rotational parts inside of the transmission case may generate vibration or noise, such as tooth tapping noise. It may vibrate the wall of the transmission case. This membranous vibration of the case wall may vibrate the surrounding air to generate radiation noise. To prevent the membranous vibration of the case wall, its thickness may be increased. But it may result to weight increase of the transmission case, which may not be acceptable, especially, when the transmission is mounted on a vehicle, due to potential fuel economy impact caused by the weight increase.  
         [0003]     The above Japanese patent application publication describes a method to suppress the membranous vibration of the transmission case without generally thickening the case wall by arranging ribs on the case wall so as to increase its rigidity. However, the arrangement of the ribs on the transmission case may increase overall size of the power transmission apparatus, which may lead to constraint of packaging it into a surrounding system such as an automotive vehicle.  
         [0004]     The inventor herein has recognized the disadvantage of the above method and needs to reduce the noise radiation from the power transmission apparatus while keeping light weight and small size.  
       SUMMARY  
       [0005]     Accordingly, in one aspect of the present invention, there is provided a mechanical power transmission apparatus, such as an automotive transmission, at least part of which is mounted under a floor panel of an automotive vehicle. The power transmission apparatus comprises a case defining a cavity, moveable parts, such as sliding rods, levers, shafts and gears, arranged within the cavity, fixed parts, such as bearing support, arranged within the cavity, the fixed parts including parts moveably supporting the moveable parts, a wall at least partly constituting said case, and a thin wall portion formed in the wall, the thin wall portion being constantly pressed in an outward direction of the cavity by one of the fixed parts, such as a shift rod support member, a resilient member or a damping member.  
         [0006]     In accordance with this apparatus, by the one of the fixed parts pressing the thin wall portion in the outward direction of the cavity, the thin wall portion may be preloaded so as to increase its rigidity. Therefore, the membranous vibration of the case wall may be effectively controlled and the radiation noise from the transmission apparatus can be reduced without increasing the thickness of the transmission case wall or arranging ribs on it. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     The advantages described herein will be more fully understood by reading an example of an embodiment in which the invention is used to advantage, referred to herein as the Detailed Description, with reference to the drawings wherein:  
         [0008]      FIG. 1  is a partially cutout outside view of an automotive transmission laterally mounted on an automotive vehicle in accordance with an embodiment of the present description;  
         [0009]      FIG. 2  is a partial perspective view of the automotive transmission showing a shift operation mechanism;  
         [0010]      FIG. 3  is an exploded view of part of the automotive transmission showing a cover, shift rod support member, a leaf spring member and a damping sheet;  
         [0011]      FIG. 4  is (a) a plane view of the leaf spring member and (b) a vertical sectional view showing the leaf spring member and the surrounding parts; and  
         [0012]      FIG. 5  is a perspective view of the sub-assembled cover and shift rod support member of the automotive transmission in accordance with the embodiment of the present description. 
     
    
     DETAILED DESCRIPTION  
       [0013]     There is shown in  FIG. 1 a  mechanical power transmission apparatus  1  according to an embodiment of the present description. In particular, the mechanical power transmission apparatus  1  in this embodiment is an automotive transmission which is mounted longitudinally on an automotive vehicle under its floor panel  50 , and which can be manually operated with a shift lever  33 . Then it is so called a longitudinally mounted manual transmission having therein an input shaft  35 , an output shaft  36  and a counter shaft  37 , all of which extend in the longitudinal direction of the automotive vehicle. The transmission apparatus  1  comprises a front case  2  and a rear case  102  which are connected with each other by fasteners such as bolts as is well known in the art to provide, in this example, an overall transmission case. The front case  2  connects at its front end to an engine not shown through a clutch housing also not shown as is well known in the art. A propeller shaft not shown emerges from a rear end of the rear case  102 , so as to transmit power from the engine to driving wheels through other part of the drive-train such as differential gear set and drive shafts.  
         [0014]     A gear-train  4  is provided in the front case  2 . The gear-train  4  consists of a plurality of gear sets comprising free rotating gears coaxially and rotatably arranged on either of the input and output shafts  35  and  36  and fixed gears affixed to the counter shaft  37  and meshed with the respective free rotating gears. By moving a synchronizer ring  42  in axial direction to engage an associated dog clutch by means of a gearshift operating mechanism  30  ( FIG. 2 ), either of the free rotating gears may be selected to couple with the input shaft  35  so as to uniformly rotate with it to transmit power through the selected one of the gear sets. By switching the gear sets, the gear ratio can be changed or the gear shifting can be made, as is well known in the art.  
         [0015]     On the upper surface of the front case  2 , there is provided an opening  2   a . A top cover  3  is fastened to the front case  2  with fasteners such as bolts around the opening  2   a  so as to close the opening  2   a . When the transmission apparatus  1  is mounted on the automotive vehicle, there is the floor panel  50  of the vehicle just above the top cover  3 . The gap between the floor panel  50  and the top cover  3  is preferably smaller in order to make more space for a passenger compartment above the floor panel  50  by lowering the height of floor. To make that gap small, the top cover  3  does not have any ribs on its upper or lower top surface which is made flat as well, so that overall wall thickness of its center portion is thinner than that of the surrounding portion to make a thin wall portion  3   c.    
         [0016]     A gearshift operating mechanism  30  shown in  FIG. 5  has a shift rod support member  5  which is attached to the lower surface of the top cover  3  by fastening it at its surrounding portion with fasteners such as bolts. The shift rod support member  5  has shift rod supports  6  and  7  formed integrally with it. The gearshift operating mechanism  30  also comprises four shift rods  8  which are axially moveably supported by the shift rod supports  6  and  7  in cylindrical apertures  6   b  and  7   b  formed therein and shown in  FIGS. 2 and 3 . Referring again to  FIG. 1 , the shift rods  8  are connected with the shift lever  33  through a link mechanism not shown so that a movement of the shift lever  33  by a vehicle driver causes an axial movement of one of the shift rods  8 . The shift rods  8  also have respectively shift forks  40  fixed thereto so that an axial movement of the shift rod  8  causes an axial movement of the synchronizer ring  42  through the shift fork  40  to engage the dog clutch of the gear to be selected so as to accomplish a desired gearshift.  
         [0017]     As shown in  FIGS. 2 and 3 , thickness around the cylindrical apertures  6   b  and  7   b  is substantially constant so as to obtain necessary support strength while avoiding unnecessary weight increase. Accordingly, the upper surfaces of the shift rod supports  6  and  7  are of a wave shape with three valleys  6   a  and  7   a  between the upper surface of the cylindrical apertures  6   b  and  7   b.    
         [0018]     A resilient member, in this example, a leaf spring member  10  is made of resilient metal such as spring steel, and is arranged on the shift rod support member  5  and below the lower surface of the top cover  3  by positioning spring legs  13  of the leaf spring member  10  within the valleys  6   a  and  7   a  of the support member  5 . Further a damping sheet  20  is arranged on the leaf spring member  10  above its spring body  12  and also below the lower surface of the top cover  3 . The damping member  20  may be made of elastomeric material, preferably of rubber.  
         [0019]     As shown in  FIG. 4 ( a ) in more detail, the three spring legs  13  of the leaf spring member  10  extend from each of the front and rear sides of the spring body  12 . The spring legs  13  have S shaped cross sections as shown in  FIG. 4 ( b ) so as to generate resilient force. The legs  13  comprises toes  14  which have concaved upper surfaces as part of the S shaped cross section at their ends, and which fit into the respective valleys  6   a  and  7   a  of the support member  5 .  
         [0020]     The spring body  12  of the leaf spring member  10  has three drilled apertures  11  at bases of the spring legs  13 . On the other hand, as shown in FIGS.  4 ( b ) and  5 , the lower surface  3   b  of the top cover  3  comprises three bosses  3   a  fitting with the respective apertures  11  of the leaf spring member  10  when the shift rod support member  5  is assembled to the top cover  3  as described in more detail below. The damping sheet  20  is shaped to be a rectangular sheet and also comprises three apertures  21  coincident the apertures  11  of the leaf spring  10 , so as to be positioned properly when assembled between the top cover  3  and the leaf spring  10 .  
         [0021]     As shown in  FIG. 4 ( b ), thanks to the S shape, while generating enough resilient force in the direction of the membranous vibration of the thin wall portion  3   c , the leaf spring member  10  may be arranged in a small vertical gap formed by the valleys  6   a  and  7   a  of the shift rod support member  5 , the top cover  3  and the damping sheet  20 . The leaf spring member is vertically compressed mainly by a distortion of its S shaped leg portions  13 , when it is installed in the gap. The resilience of the distortion of the leaf spring member  10  generates spring force to push up the top cover  3  through the rubber sheet  20 .  
         [0022]     Now, function of the transmission apparatus  1 , mainly function of the leaf spring member  10  and the damping sheet  20 , will be described. When a driver operates the shift lever  33  to select a gear, the engine torque transmitted to the input shaft  35  is converted and output from the output shaft  36 . Then in the gear-train  4 , tooth tapping noise and various other vibrations and noise (hereafter referred to vibration and the like) are generated. This vibration and the like is transmitted through the air in the transmission apparatus  1  or the transmission case  2  to the top cover  3  to make a membranous vibration of the top cover  3 , especially of the flat thin wall portion  3   c  in the substantially center portion.  
         [0023]     However on the thin wall portion  3   c  of the top cover  3 , the leaf spring member  10  applies a certain load outwardly of the transmission case  2  through the damping sheet  20 . If the top cover  3  made a membranous vibration, amplitude around its center would be the largest. Since the load by the leaf spring  10  aligns with a direction of the amplitude movement, the membranous vibration of the top cover  3  is effectively controlled. Further, the damping sheet  20  quickly attenuates and absorbs the membranous vibration of the top cover  3 , to enhance the membranous vibration control effect of the top cover  3 .  
         [0024]     If the leaf spring member  10  or the damping sheet  20  were not provided and the large membranous vibration of the top cover  3  were allowed, the membranous vibration could generate radiation noise, which in turn could penetrate the floor panel  50  just above the top cover  3  and could reach to occupants in the vehicle compartment. However, according to the present embodiment, since the membranous vibration of the top cover  3  is effectively controlled, the noise audible in the vehicle compartment from the membranous vibration can be substantially controlled.  
         [0025]     Further, since the leaf spring  10  and the damping sheet  20  are arranged or sandwiched between the shift rod support member  5  and the top cover  3 , it is not necessary to provide additional fasteners to attach either of the leaf spring  10  and the rubber sheet  20 . This small size and light weight construction can generate the force pressing on the top cover  3 .  
         [0026]     Now, assembly process of the top cover  3 , especially assembly process of the leaf spring member  10  and the rubber sheet  20 , will be described.  FIG. 5  shows a vertically reversed state (where a lower surface  3   b  of the top cover  3  is faced up) of the shift operation mechanism  30 . The shift rod support member  5 , the shift rods  8  and the like are assembled to the top cover  3  beforehand (subassembly process). After the subassembly, the top cover  3  is returned from the vertical reversed state and fastened to the transmission case  2  by fastening the periphery of the top cover  3  to the periphery of the case opening  2   a  with fasteners such as bolts. By sub-assembling a part of the gearshift operating mechanism  30  to the top cover  3  beforehand, the transmission assembly  1  can be efficiently assembled.  
         [0027]     In the subassembly process described above, prior to assembling the shift rod support member  5  to the top cover  3 , the leaf spring  10  and the damping sheet  20  are installed. At first, the damping sheet  20  is placed on the top cover  3  in its reversed state while the damping sheet  20  being positioned with its apertures  21  and the bosses  3   a  formed on the lower surface  3   b  of the top cover  3 . Next, the leaf spring member  10  is placed on the damping sheet  20  while being positioned with its apertures  10  and the bosses  3   a . Subsequently, the shift rod support member  5  is attached to the top cover  3  with fasteners such as bolts so that the valleys  6   a  and  7   a  contact the legs  13  of the leaf spring  10  around the toes  14 . As such, the leaf spring  10  and the rubber sheet  20  may be easily and securely installed by placing them on the vertically reversed top cover  3 . After installing the shift rod support member  5  to the top cover  3 , it is possible to have the leaf spring  10  make the deflection to generate some force pressing on the top cover  3 . Consequently, by utilizing the subassembly process described above, the leaf spring  10  and the rubber sheet  20  can be easily arranged between the shift rod support member  5  and the top cover  3 .  
         [0028]     While the embodiment of the present invention has been described, it can be modified without departing from a scope of the present description. For instance, while in the above embodiment the power transmission apparatus  1  was the automotive manual transmission, the embodiment can be applied to various automatic transmissions including a continuously variable transmission (CVT). Also, it is not limited to the so-called longitudinally mounted transmissions, but for example it can be applied to a so-called laterally mounted transmission with input and output shafts  35 ,  36  extending in a lateral direction of a vehicle.  
         [0029]     While the case opening  2   a  is provided on the transmission case  2  at its upper side, it may be provided at a side or bottom, depending on need to thin the case wall and availability of parts which may push the thin wall portion of the case. Further, the case opening  2   a  and the case  2  may not be provided at all, instead, the thin wall portion  3   c  may be directly formed in a proper portion of the case wall  2   b . Also, although the present embodiment is especially suitable when ribs or the like are difficult to arrange on the thin wall portion, it may be applied to an embodiment with ribs or the like on the thin wall portion. If the ribs or the like are arranged on the thin wall portion, further membranous vibration control effect can be achieved in addition to the effect by pressing the thin wall portion.  
         [0030]     The leaf spring  10  may not necessarily be formed a shape having the spring body  12  and the spring legs  13 . Also, although the leaf spring has a benefit that it can generate the pressing force with a simple configuration even in the small gap described above, the thin wall portion of the case  2  or the cover  3  may be pressed by using various other resilient members such as a coil spring, and any load adding mechanism with hydraulic pressure or the like other than the springs may be provided as well. Further, the resilient member itself may be omitted, and for instance, the shift rod support member  5  may be configured to contact directly to the thin wall portion  3   c  or indirectly through the damping sheet  20  and preload it outwardly by setting the dimension of the support member  5  in the direction of the membranous vibration amplitude.  
         [0031]     Although in the above embodiment the leaf spring member  10  is arranged between the top cover  3  and the shift rod support member  5 , it may be directly fastened to the top cover  3 , or be arranged between the transmission case  2  and the top cover  3 . For example, a support portion which supports the toes  14  of the spring leg  13  of the leaf spring  10  may be provided on the transmission case  2  and the leaf spring member  10  as a whole may be provided between the top cover  3  and the transmission case  2  to make the construction of the leaf spring member  10  and its surroundings to be simple, small and light weight.  
         [0032]     The damping sheet  20  may not be needed. Also, while the damping sheet  20  is made of rubber in the above embodiment, the material of the damping sheet is not limited to rubber and it may be anything having rubber like elasticity at a normal temperature. For example, thermoplastic elastomeric resin may be used.  
         [0033]     Further, the resilient member or the leaf spring member  10  itself may be omitted, and for instance, the shift rod support member  5  may be configured to contact directly to the thin wall portion  3   c  or indirectly through the damping sheet  20  and preload it outwardly by setting the dimension of the support member  5  in the direction of the membranous vibration amplitude.