TECHNICAL FIELD  
       [0001]     The present invention relates to active gear damping mechanisms.  
       BACKGROUND OF THE INVENTION  
       [0002]     Intermeshing gears may sometimes produce a noise or gear rattle during transient relative rotational speed changes between a drive and a driven gear. One example where this may occur is within a manual shift or countershaft transmission. A countershaft transmission has an input shaft, a countershaft, and an output shaft. The input shaft and the countershaft are interconnected by meshing gears (head gear set). The countershaft and the output shaft are interconnected by a plurality of meshing gears (speed gears) that are selectively connectible to one of the shafts through synchronizer clutch arrangements. Thus, a plurality of gear meshes are present between the input shaft and the output shaft. The speed ratio between the input shaft and the output shaft is controlled by the meshing speed gears. The speed ratio between the input shaft and the output shaft is changed by interchanging the synchronizers that control the connection of the speed gears to their respective shafts. The head gear set and the active speed gear set have a lash condition. Under some operating conditions, the lash condition of the head gear set and the active speed gear set can reverse resulting in a gear rattle caused by the lash reversal.  
         [0003]     Gear rattle may occur as a transient lash condition during transient drive events such as throttle “tip in”, throttle “tip out”, and rapid clutch disengagement. As is well known, the clutch is disengaged and re-engaged for each ratio interchange and during stopping and launching of the vehicle. Additionally, a countershaft transmission may exhibit gear rattle under steady state drive events, such as when the vehicle is traversing a hill in gear. The gear rattle, in this case, is caused by engine generated torque oscillations within the driveline.  
         [0004]     Modern vehicular drivelines may have a number of additional components that may also include meshing gear sets that may be subject to gear rattle. These may include transaxles, transfer cases, and differentials.  
         [0005]     Attempts have been made to attenuate gear rattle. These include various bearing designs, component designs, and gear designs to name a few. Each of these attempts may result in increased drag on the shafts to which the gear is mounted, which may be continuously present. This inherent drag may reduce the mechanical efficiency of the system.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention provides a system and method to actively damp gears thereby reducing the occurrence of gear rattle as a transient lash condition. The active nature of the present invention will allow the drag within the bearing to which the gear mounts to be selectively increased, thereby only increasing the system drag at critical operating conditions. The ability to selectively increase drag may translate into increased mechanical system efficiencies, fuel economy, and component life over traditional means of gear rattle attenuation.  
         [0007]     Accordingly, the present invention provides a hydraulic active damping system having a drive gear and a driven gear in meshing relation with the drive gear. The driven gear and the drive gear have a transient lash condition. A fluid supply structure is also provided. At least one of the drive gear and the driven gear is mounted on an enclosed bearing, where the enclosed bearing is selectively pressurizable by the fluid supply structure to vary frictional loss within the enclosed bearing in response to whether the transient lash condition is present or absent.  
         [0008]     The present invention may include a hydraulic pump operable to selectively deliver pressurized fluid to the enclosed bearing via the supply structure. An electric motor may be provided to drive the hydraulic pump. The electric motor may receive control signals from an electronic control unit. The present invention may also provide a fluid return structure operable to evacuate air and/or fluid from the enclosed bearing. Additionally, the hydraulic active damping system of the present invention may include a flow restrictor in one or both of the fluid supply structure and the fluid return structure. The flow restrictors are operable to provide fluid flow control within the structures.  
         [0009]     The present invention also provides a method of actively damping at least one gear subject to a transient lash condition by mounting the gear on an enclosed bearing capable of being selectively pressurized with fluid. Then, the enclosed bearing is pressurized with fluid to increase frictional loss within the enclosed bearing when transient lash condition is present. Subsequently, the enclosed bearing is de-pressurized when the transient lash condition is absent  
         [0010]     The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a schematic diagram of the hydraulic active damping system of the present invention illustrating the various elements of the system;  
         [0012]      FIG. 2  is a fragmentary sectional schematic view of a powertrain illustrating an exemplary embodiment of the present invention;  
         [0013]      FIG. 3  is a diagrammatic representation of gears within the countershaft transmission illustrating gear tooth mesh; and  
         [0014]      FIG. 4  is a sectional perspective view of a portion of the powertrain shown in  FIG. 2  illustrating the incorporation of the present invention within a countershaft transmission. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]      FIG. 1  is an exemplary schematic diagram of a hydraulic active damping system  10  of the present invention. The hydraulic device  12  is a sealed or enclosed specialized roller bearing  14  having an inner race  16  axially disposed about a shaft  18 . Circumscribing the inner race  16  is an outer race  20 . A void or enclosure  22  is defined by both the inner race  16  and the outer race  20  and a plurality of roller elements  24  disposed therein. The first axial side  28  and the second axial side  30  of the specialized roller bearing  14  are sealed by a first seal  32  and second seal  34 . Mounted with respect to the outer race  20  is a gear  36  that may be in meshing contact with a gear  38 .  
         [0016]     The hydraulic device  12  further includes a fluid supply port  40  operable to form a passage through which pressurized fluid may flow from the fluid supply structure  42  into the volume of void  22  not occupied by the plurality of roller elements  24 . The location of the fluid supply port  40  will be dictated by the component design and may be located within the inner race  16 , first seal  32 , second seal  34 , or outer race  20 , as shown. The fluid supply structure  42  may include cast in place or drilled passages, or external lines.  
         [0017]     Additionally, the hydraulic device  12  includes a fluid return port  44  operable to form a passage through which pressurized fluid may flow from the volume of void or enclosure  22  not occupied by the plurality of roller elements  24  to the fluid return structure  46 . The location of the fluid return port  44  will be dictated by the component design and may be located in the inner race  16 , first seal  32 , outer race  20 , or second seal  34 , as shown. The fluid return structure  46  may be cast in place, drilled passages, or external lines. The fluid return port  44  and fluid return structure  46  cooperate to evacuate air and/or fluid that may be trapped within the enclosed or specialized roller bearing  14  thereby enabling complete filling upon pressurization of the hydraulic device  12 . Additionally the fluid return port  44  will provide an opening through which fluid may be scavenged from the bearing upon activation of the hydraulic active damping system  10 .  
         [0018]     A supply flow restrictor  48  and return flow restrictor  50  cooperate to control the fluid flow and pressure within the hydraulic device  12 . The supply flow restrictor  48  and return flow restrictor  50  may be a type of valve known in the art of hydraulic controls or may simply be an appropriately sized orifice.  
         [0019]     Fluid within the fluid return structure  46  feeds the suction side of a hydraulic pump  52 . The pressure side of the hydraulic pump  52  feeds pressurized fluid to the fluid supply structure  42 . The hydraulic pump  52  is operated by an electrical motor  54 , which is in electrical communication with the electronic control module  56 . The electronic control module  56  is operable to start and stop the electric motor  54  thereby providing pressurized fluid to the fluid supply structure  42 . Various inputs  58  are input to the electronic control module  56 , and may include vehicle operating conditions such as engine speed, vehicle speed, etc. The electronic control module may be included in the electronic control unit  124 , shown in  FIG. 2 , or may be separate.  
         [0020]     A tapered roller bearing  14  is shown in  FIG. 1 ; however, those skilled in the art will comprehend that other types of bearings such as ball, straight roller, and needle may be used within the enclosure  22  of the hydraulic device  12  while remaining within the scope of that which is claimed.  
         [0021]      FIG. 2  is a fragmentary sectional schematic view of a powertrain  70  illustrating an exemplary embodiment of the present invention. A powertrain  70  has an engine  72  and a countershaft transmission  74 . The countershaft transmission  74  includes a manually actuated clutch assembly  76 , an input shaft  78 , a countershaft  80 , and an output shaft  18  disposed within a housing  84 . The input shaft  78  is coaxially aligned with the output shaft  18  and the countershaft  80  is rotatably supported within the housing  84  in a parallel relation with both the input shaft  78  and the output shaft  18 .  
         [0022]     The engine  72  has a throttle control  86  and the clutch assembly  76  has a clutch control  88 . Both of the controls  86  and  88  are manually operated by the operator. The clutch assembly  76  includes a friction element  90  that is urged into and out of engagement with an engine flywheel  92  by actuation of the clutch control  88  and a diaphragm spring  94 . Upon engagement of the clutch  76 , the engine  72  will couple with the input shaft  78  and rotate with a common rotational speed.  
         [0023]     The input shaft has a head gear  36  drivingly connected thereto and meshing with a head gear  38  that is drivingly connected with the countershaft  80  such that the countershaft  80  will rotate whenever the input shaft  78  is rotating. The countershaft  80  has a plurality of speed or ratio gears  102 ,  104 ,  106 , and  108  drivingly connected therewith and meshing with respective speed or ratio gears  110 ,  112 ,  114  and  116  that are disposed on the output shaft  18 . A reverse idler  118  is rotatably mounted on an idler shaft, not shown, and is in meshing relation with a ratio gear  120  on countershaft  80  and a ratio gear  122  on output shaft  18 . Each of the ratio gears  110 ,  112 ,  114 ,  116 , and  122  are selectively, individually connectable with the output shaft  18  by respective synchronizers, not shown, of conventional design. A hydraulic device  12  that may be selectively pressurized with fluid is positioned between the head gear  36  and the output shaft  18 .  
         [0024]     When the operator wishes to change the speed ratio between the input shaft  78  and the output shaft  18 , the throttle control  86  is released and clutch mechanism is  88  is actuated by the operator. The operator then manually, through a conventional shift control linkage not shown, manipulates the synchronizers to release one gear set and engage another. This operation is well known in the art. In addition, during vehicle deceleration, the operator releases the throttle control  86  to permit a reduction in engine speed thereby slowing the vehicle. This throttle release is also known as “tip out”.  
         [0025]     The hydraulic device  12  is operable to increase the frictional drag between the input shaft  78  and the output shaft  18  when the hydraulic device  12  is pressurized. The output shaft  18  is rotatably supported on the input shaft  78  by the specialized or enclosed roller bearing  14 , shown in  FIG. 4 , of the hydraulic device  12 . Upon pressurization of the hydraulic device  12 , changes in relative motion between the input shaft  78 , the countershaft  80 , and the output shaft  18  are restrained due to the frictional drag caused by the attritional volume of pressurized fluid within-the hydraulic device  12 . Therefore, the drag torque and direction remain essentially unchanged such that the tooth contact between the torque carrying gear members is undisturbed. In other words, the gears on the input shaft  78 , the countershaft  80 , and the output shaft  18  are constrained from moving into their lash zones. The frictional drag within the hydraulic device  12  may be controlled such that the drag occurs only when significant changes in gear lash might be present. Therefore, the efficiency of the powertrain is not significantly affected.  
         [0026]     Significant changes in the gear lash can occur during various operating conditions. If the clutch is rapidly disengaged, the torque carrying ratio gear set and the head gear set change from a forward driven mesh to a reverse driven mesh. This results in noise or rattle in the clutch, the splines, and the gear meshes. Another situation wherein the gear lash might change is upon a sudden actuation or release of the throttle, which results in a rapid change in engine speed and therefore the speed of the input shaft  78 . Additionally, a countershaft transmission  74  may exhibit gear rattle under steady state drive events, such as when the vehicle is traversing a hill in gear. The gear rattle, in this case, is caused by engine generated torque oscillations within the driveline. The pressurization of the hydraulic device  12  under this operating condition may also prevent gear noise, due to gear lash changes. In each of these and many other operating conditions, the electronic control unit  124  may anticipate the gear lash change, and selectively pressurize the hydraulic device  12  to prevent the noise that might otherwise occur. In each of the operating conditions that result in gear rattle or clutch clunk, the input shaft  78  undergoes a rapid acceleration. The control scheme may be programmed to ignore acceleration levels that occur within the normal operating range of the powertrain  70 .  
         [0027]      FIG. 3  is a representation of the meshing relation between the head gear  36  on the input shaft  78 , the head gear  38  on the countershaft  80 , and the ratio gears on the countershaft  80  and the output shaft  18 . The ratio gears shown in  FIG. 3  are only representative of the ratio gears shown in  FIG. 2 , and the output shaft  18  is shown rotated out of alignment with the input shaft  78  for clarity. The arrows A and B represent the direction of drag torque imposed by the hydraulic device  12  when pressurized.  
         [0028]      FIG. 4  is a sectional perspective view of a portion of the powertrain  70  shown in  FIG. 2  illustrating the incorporation of the hydraulic device  12  within the countershaft transmission  74 . As mentioned above, the output shaft  18  is rotatably supported on the input shaft  78  by the enclosed or specialized roller bearing  14  of the hydraulic device  12 . The enclosed or specialized roller bearing  14  has an inner race  16  and an outer race  20  with a plurality of roller elements  24  disposed therebetween. The axial ends of the specialized bearing  14  are sealed with a first seal  32  and a second seal, not shown. When additional damping of the head gear  36  is required, pressurized fluid will move through a fluid supply structure  42  to a fluid supply port  40  defined within the inner race  16 . The pressurized fluid will increase frictional losses within the specialized bearing  14 . Simultaneously any air and/or fluid within the enclosed or specialized bearing  14  will pass through the fluid return port  44  defined by the outer race  20  and into the fluid return structure  46 .  
         [0029]     When damping of the head gear  36  is no longer required, the pressurized fluid flow within the fluid supply structure  42  will be discontinued. The fluid remaining within the enclosed or specialized bearing  14  will be scavenged by a hydraulic pump, by introducing suction to the enclosed or specialized bearing  14  through the fluid return structure  46  and the fluid return port  44 .  
         [0030]     While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.
The present invention provides a hydraulic active damping system to damp gears, thereby reducing the occurrence of gear rattle or noise. The active nature of the present invention will allow the drag within the bearing to which the gear mounts to be selectively increased, by pressurizing an enclosed or specialized bearing with fluid during critical events. The pressure within the specialized bearing is subsequently decreased, thereby only increasing the system drag at critical operating conditions. The present invention also provides a method of damping gears by introducing pressurized fluid into a specialized bearing and subsequently reducing the fluid pressure therein.