Abstract:
A transmission clutch cooling system includes a clutch housing defined between a drive hub of a drive member and a clutch hub of a driven member and a clutch assembly housed in the clutch housing. The clutch hub includes a hub deck having a plurality of bores therethrough, an inner annulus extending axially from the hub deck, and an outer annulus extending axially from the hub deck, the outer annulus including a plurality of radial orifices therethrough and a lip flange extending from a distal end towards the inner annulus. The clutch assembly includes a plurality of clutch plates secured to the drive hub and a plurality of friction plates secured to the driven member. A piston assembly is provided for engaging or disengaging the plurality of friction plates against the plurality of clutch plates to cause or release integrated rotation of the drive member and the driven member.

Description:
TECHNICAL FIELD 
     The disclosure relates to transmission clutches and, more particularly, to a passive clutch cooling system to reduce drag loss between the interleaved friction discs and the clutch plates of a rotating clutch. 
     BACKGROUND 
     Rotational clutches are frequently used as one of the mechanisms for engaging or disengaging the various gear components of a transmission in order to establish different gear ratios between an input member and an output member. A conventional rotational clutch assembly typically includes a set of clutch plates and a set of friction discs, sometimes referred to as a clutch pack, interleaved between one another in a clutch housing. When the clutch assembly is disengaged, the clutch plates and friction discs normally turn past one another without contact. However, when the corresponding components of a particular clutch, i.e., a drive member and a driven member, are to be engaged during a particular gear range, for example, a hydraulically actuated or spring-loaded piston forces the clutch plates and friction discs together. Friction surfaces on the clutch plates and the friction discs interact until the drive member and the driven member of the clutch assembly rotate in unison without slip. 
     In operation, a great deal of thermal energy is generated during the engagement and disengagement of the clutch plates and the friction discs, as well as during the period of full engagement, when the kinetic energy generated by the engaged clutch pack is also translated into a large amount of thermal energy. This thermal energy must be dissipated to prevent damage to the various components of the clutch assembly, particularly the frictional surfaces of the clutch plates and the friction discs. A continuous supply of a coolant, such as transmission fluid, is typically supplied to the clutch housing to serve this purpose. In a rotational clutch assembly, the transmission fluid may be supplied to an inside diameter portion of the engaged clutch plates and the friction discs and allowed to flow by centrifugal force across the plate surfaces to an outside diameter portion. The hot transmission fluid is then directed away from the clutch assembly to pass through a heat exchange process for transfer and release of the thermal energy absorbed into the transmission fluid. 
     When the rotational clutch assembly is not engaged, the clutch plates and the friction discs simply rotate past one another without contact. During this period of disengagement, the amount of thermal energy that must be dissipated is minimal. Furthermore, simply maintaining a continuous flow of transmission fluid to the clutch pack during disengagement may also result in significant inefficiencies. For example, depending on the relative speed of the rotating drive member with respect to the disengaged, driven member, drag losses may be generated as a result of shear experienced by the transmission fluid between the clutch plates and the friction discs. The shear increases proportionally with the amount of transmission fluid provided to the clutch pack during disengagement. Thus, particularly in gears where the relative rotational speed differential between the clutch plates and the friction discs is highest, it is desirable to limit the flow of coolant to the clutch pack. 
     Various clutch cooling systems have been proposed to address controlling the flow of coolant to the clutch pack during engagement and disengagement. For example, U.S. Pat. No. 5,988,335 describes actively controlling the flow to the clutch with a diverter valve and a sensor arrangement to sense the gear ratio of the transmission and divert flow from the clutch assembly in response to the transmission being in a selected gear ratio. U.S. Pat. No. 6,244,407 proposes a more passive system that does not rely on a sensor actuated valve. Rather, an outer ring is mounted onto the piston used to actuate engagement of the clutch pack. The outer ring has an orifice provided therein for allowing a flow of coolant therethrough. The outer ring is movable between a first position wherein the orifice is closed and the drive and driven members are disconnected and a second position where the orifice is open to allow the flow of pressurized fluid through the orifice to the clutch pack when dictated by movement of the piston to engage the clutch plates and the friction discs. Other types of “slider valve” arrangements are common in the industry, wherein the piston moves the slider valve in a direction to uncover an orifice for increasing coolant flow to the clutch pack during engagement. Typically, a spring, for example, may be employed to close the slider valve over the orifice when the clutch disengages. 
     As described above, conventional clutch cooling systems can often be complex and/or require the addition of various components to provide a variable flow of coolant to the clutch pack. The increased complexity of these designs may add to the cost of manufacture, assembly, and maintenance of the transmission and creates additional opportunities for failure during operation. As such, a clutch cooling system is needed that eliminates the requirement for additional components while taking advantage of the natural operational characteristics of rotational clutch assemblies. 
     SUMMARY 
     The foregoing needs are met, to a great extent, by aspects of the present disclosure, wherein a transmission clutch cooling system includes a drive member including a drive hub situated about an axis of rotation, a driven member including a clutch hub concentrically situated about the axis of rotation, wherein the clutch hub includes a hub deck having a plurality of bores therethrough, an inner annulus extending axially from the hub deck, and an outer annulus extending axially from the hub deck, the outer annulus including a plurality of radial orifices therethrough and a lip flange extending from a distal end towards the inner annulus. A clutch housing is defined between the drive hub and the clutch hub, and a clutch assembly is housed in the clutch housing that includes a plurality of clutch plates secured to the drive hub to rotate with the drive member, a plurality of friction plates secured to the driven member to rotate with the clutch hub, and a piston assembly for engaging or disengaging the plurality of friction plates against the plurality of clutch plates to cause or release integrated rotation of the drive member and the driven member. 
     In accordance with other aspects of the present disclosure, a clutch hub includes a hub deck having a plurality of bores therethrough, an inner annulus extending axially from the hub deck, and an outer annulus extending axially from the hub deck, the outer annulus including a plurality of orifices therethrough and a lip flange extending from a distal end towards the inner annulus. 
     In accordance with yet other aspects of the present disclosure, a method of cooling a rotational clutch includes supplying a flow of coolant to an annular clutch hub having axial bores and radial holes, the radial holes being in fluid communication with a clutch pack, increasing the flow of coolant to the clutch pack through the radial holes during engagement of the clutch pack, and diverting a portion of the flow away from the clutch pack through the axial bores when the clutch pack is disengaged. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagrammatic illustration of a machine including a multi speed transmission, in accordance with aspects of the present disclosure; 
         FIG. 2  is a schematic illustration of a transmission, in accordance with aspects of the present disclosure; 
         FIG. 3  is a partial cross-sectional side view of a clutch cooling system, in accordance with aspects of the present disclosure; 
         FIG. 4  is a chart illustrating rotational relative speed of a drive member and a driven member for a given set of gear ratios, in accordance with aspects of the present disclosure; 
         FIG. 5  is a bar graph illustrating estimated power loss in a clutch assembly for a given engine speed over a range of gears when transmission fluid flow to the clutch assembly is not restricted, in accordance with aspects of the present disclosure; 
         FIG. 6  is an axial view of a clutch hub component of a clutch cooling system, in accordance with aspects of the present disclosure; 
         FIG. 7  is a partial cross-sectional side view illustrates aspects of a clutch cooling system, in accordance with aspects of the present disclosure; and 
         FIG. 8  is a partial cross-sectional side view illustrating aspects of a clutch cooling system, in accordance with aspects of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The disclosure will now be described with reference to the drawing figures, in which like reference numerals refer to like parts throughout. 
     Referring to  FIG. 1 , a multi-speed transmission  10  may be included in a machine  12 . An input member  14  may connect the transmission  10  to a prime mover  16  by a torque converter  18 , and an output member  20  may connect the transmission  10  to one or more traction devices  22 . Although the machine  12  is shown as a truck, it may be any type of machine that may benefit from the use of a multi speed transmission. The prime mover  16  may be of any type that outputs power in a form usable by the multi-speed transmission  10 . For example, the prime mover  16  may be an internal combustion engine (such as a diesel engine, a gasoline engine, a turbine engine or a natural gas engine), an electric motor, or other device capable of generating a power output. The traction devices  22  may be any type of traction devices, such as, for example, wheels as shown in  FIG. 1 , tracks, belts, or any combinations thereof. 
     As shown in the schematic illustration of  FIG. 2 , the multi-speed transmission  10  may be a planetary transmission having a series of annular components rotatably supported and aligned about a rotational axis  24 , the schematic illustrating aspects of the transmission on one side of the axis  24  only. Torque may be supplied to the input member  14  by the prime mover  16  through the torque converter  18 , for example. At least one, and often a plurality of gear sets, may be interconnected between the input member  14  and the output member  20 . As shown in  FIG. 2 , the multi-speed transmission  10  may have four interconnected planetary gear sets,  30 ,  32 ,  34  and  36  rotatably supported concentrically along the rotational axis  24  in a transmission casing  28 . Each planetary gear set  30 ,  32 ,  34  and  36  includes at least one sun gear, at least one planetary carrier, and at least one ring gear. 
     The transmission  10  may also include a number of control elements operatively coupled to the planetary gear sets  30 ,  32 ,  34  and  36 . As used herein, the term “control element” includes clutches (which are alternatively referred to in the industry as rotational clutches), brakes (which are alternatively referred to in the industry as stationary clutches), synchronizers (including dog and other types of synchronizing clutches) or other torque control components that may conventionally be used in a transmission. As shown in  FIG. 2 , the transmission  10  may include three rotational clutch assemblies  40 ,  42 , and  44  and three brake assemblies  50 ,  52 , and  54 . The rotational clutch assemblies  40 ,  42 , and  44  and brake assemblies  50 ,  52 , and  54  cooperate with and may selectively couple particular elements of the planetary gear sets to establish, for example, a set of ten forward gear ratios and one reverse gear ratio between the input member  14  and the output member  20 . 
       FIG. 3  illustrates a rotational clutch assembly  100  in accordance with aspects of the present disclosure. The rotational clutch assembly  100  may be used in the transmission  10 , for example, as one or more of the rotational clutch assemblies  40 ,  42 , and  44 . The clutch assembly  100  may include a drive member, generally indicated at  102 , and a driven member  104 , generally indicated at  104 , which rotate about a common axis. A clutch housing  106  is generally defined between the drive member  102  and the driven member  104  and is formed to house a clutch pack, generally indicated at  110 , that is engaged or disengaged through actuation of a piston  112 , such as through hydraulic actuation or spring force actuation. A balance piston assembly  114  may be included and housed in the clutch housing  106  along with the piston  112  in order to introduce reverse pressure on the low pressure side of the piston  112  to counteract the large thrust generated by the hydraulic pressure fluid on the high pressure side of the piston and prevent the piston from engaging the clutch at high rotational speeds. 
     The annular clutch pack  110  may be composed of annular clutch plates  116  that are splined to and extend inward from a drive hub portion  118  of the drive member  102  and annular friction discs  120  that are splined to and extend outward from a clutch hub  122  of the driven member  104 . The clutch plates  116  and friction discs  120  are interleaved as shown in  FIG. 3 . In accordance with aspects of the present disclosure, when the clutch assembly  100  is in a disengaged position, the drive member  102  maintains a certain rotational speed based on an input speed of the input member  14  of the transmission  10  and the driven member  104  is disengaged and not rotating or rotating at a different relative speed. When the clutch assembly  100  is in the disengaged position, the clutch plates  116  rotate freely past the friction discs  120  in a non-contacting manner. However, when the clutch pack  110  is to be placed into an engaged position during a particular gear change, for example, when moving from the fifth gear to the sixth gear in the transmission  10  described previously, pressurized hydraulic fluid is introduced into a pressure chamber  124  to produce axial movement of the piston  112 . In turn, actuation of the piston  112  forces a frictional engagement of the clutch plates  116  with the friction discs  120  to reduce or eliminate relative rotation between the clutch plates  116  and the friction discs  120 . 
     As discussed above, during engagement of the clutch pack  110 , the relative rotational speed of the drive member  102  and the driven member  104  may be synchronized. The frictional energy and kinetic energy generated by the engaged clutch pack  110  translates into a large amount of thermal energy that must be dissipated to reduce or eliminate wear or damage that may occur to the clutch plates  116  and the friction discs  120 . To facilitate cooling during this time, a continuous flow of coolant, such as automatic transmission fluid, may be provided to the clutch hub  122 . 
     In accordance with aspect of the present disclosure, the clutch hub  122  may be formed with an inner annulus  126  connected to an outer annulus  128  by a hub deck  130 . The inner annulus  126  and the outer annulus  128  extend from the hub deck  130  in a direction toward the piston  112  and cooperate with the shape of the hub deck  130  to form an interior space  132 . The outer annulus  128  may be provided with a series of radial bores  134 . The radial bores  134  provide fluid communication from the interior space  132  to the clutch pack  110  for a fluid to flow through the outer annulus  128  toward the clutch pack  110 . Thus, when the clutch pack  110  is engaged to cause rotation of the clutch hub  122  at the same speed as the drive member  102 , by way of centrifugal action, the transmission fluid is forced through the radial bores  134  and into the clutch pack  110  at an accelerated rate versus when the clutch pack is disengaged and motion of the transmission fluid into the clutch pack  110  is primarily by force of gravity. The radial bores  134  are sized to provide a maximum flow of coolant during a particular period of clutch engagement, for example, to provide sufficient cooling during engagement and disengagement between particular gears when thermal energy generation is greatest. Yet the radial bores  134  are also sized to restrict a majority of the coolant flow to the clutch pack  110  during a period of disengagement when centrifugal force is reduced or nonexistent. 
     As explained above, the issue of providing sufficient cooling during engagement of the clutch assembly  100  can also lead to inefficiencies due to transmission fluid sheer during disengagement.  FIG. 4  is a chart illustrating the relative speed difference between the constantly rotating drive member  102 , including the clutch plates  116 , and the driven member  104 , including the friction discs  120 , for a typical range of gears in a multi-speed transmission  10 . The relative speed line indicates that the relative rotational speed between the driven member and the drive member is greatest in forward gears one ( 1 F) and two ( 2 F). In forward gears three ( 3 F) through five ( 5 F), the driven member  104  is controlled to increase speed relative to the drive member  102  until the clutch assembly  100  is engaged during a shift from gear  5 F to forward gear six ( 6 F) where it remains engaged through forward gear ten ( 10 F). As illustrated by the chart, because the rotational speeds of the clutch plates  116  and the friction discs  120  are synchronized in gears  6 F- 10 F, the difference in relative speed is zero. When the transmission is controlled to provide the reverse gear (R), the relative speed difference between the clutch plates  116  and the friction discs is again at its highest. 
       FIG. 5  is provided to illustrate the estimated power loss experienced as a result of the shearing of lubrication fluid in the clutch pack  110  if lubrication fluid is not diverted in accordance with aspects of the present disclosure. As shown by the chart, if the input speed of the transmission is maintained at a steady speed of, for example, 1500 rpm, the power losses are clearly highest in gears  1 F,  2 F and Reverse when the relative speed difference between the drive member  102  and the driven member  104  are highest, as illustrated in  FIG. 4 . Conversely, the power loss decreases successively in gears  3 F- 5 F as the relative speed difference decreases until the power losses due to shearing are essentially zero in gears  6 F- 10 F because of the concurrent rotation of the clutch plates  116  and the friction discs  120  with the clutch assembly  100  engaged. It should be noted that the particular power numbers shown in  FIG. 5  may vary significantly for different transmissions depending on a number of factors, including the number and diameter of the clutch plates  116  and/or the friction discs  120 , for example. However, the relative amounts of power loss experienced across the gear range, e.g., power loss being highest in gears  1 F,  2 F, and  1 R, remains substantially as shown in  FIG. 5 . 
     As shown in  FIGS. 6 and 7 , a cooling system in accordance with aspects of the present invention may include a series of axial facing bores  136  provided in the hub deck  130  of the clutch hub  122 . The axial facing bores  136  may be spaced peripherally toward an outside diameter of the hub deck  130  and sized to permit a substantial quantity of the flowing coolant to escape the interior space  132 , primarily during a period of disengagement of the clutch assembly. Placement of the axial facing bores  136  near the outside diameter of the hub deck  130  also allows the clutch hub  122  to retain and direct a significant quantity of the flowing coolant toward and through the radial bores  134  during the period of engagement to lubricate and cool the clutch pack  110 . Lengthening the run of fluid flow allows development of a significant amount of centrifugal force to act on the fluid as it flows toward the radial bores  134 . 
     Referring back to  FIG. 3 , a lip flange  140  may be provided toward a distal end of the outer annulus  128  to extend a predetermined distance toward the inner annulus  126 . The axial bores  136  may be displaced radially inward from an inner diameter of the outer annulus  128  to create a step  138 . The lip flange  140  may be formed to extend toward the inner annulus  126  a distance greater than the radial dimension of the step  138 . Accordingly, a trough area  142  is formed in the outer annulus  128  between the step  138  and the lip flange  140  to maintain a small amount of coolant in the trough area  142  when the clutch hub  122  is not rotating or is rotating slowly. Thus, even during a period of clutch disengagement, although a majority of the coolant fluid flow is able to drain out of the interior space  132  through the axial bores  136 , the radial bores  134  may be appropriately dimensioned to allow an appropriate amount of coolant to drain by force of gravity from the trough area  142  into the clutch pack  110 . Thus, a minimal amount of fluid flow may be established during disengagement of the clutch assembly  100 , enough of a fluid flow from the interior space  132  to the clutch pack  110  to maintain viability and efficiency of the moving components without introducing the inefficiencies of shear caused by excessive fluid flow during the period of disengagement. As shown in  FIG. 3 , through-holes  139 , for example, may be provided in a housing component of the drive member for further routing of the fluid flow away from the clutch assembly. 
       FIGS. 6 and 7  illustrate that the axial bores  136  may be arcuately spaced at equal angles θ around the perimeter of the hub deck  130  such that at least a portion of one or more of the axial bores  136  will always be below the fluid level in the trough area  142  formed in the outer annulus  128  of the clutch hub  122  when the clutch hub  122  is not rotating, i.e., when the clutch pack  110  is disengaged. Accordingly, when the clutch hub  122  is stopped and the relative speed between the clutch plates  116  and the friction discs  120  is highest, such as in gears  1 F,  2 F and R, for example, the larger axial bores  136  are configured to drain away a majority of the transmission fluid and the smaller radial bores  134  are configured to allow only a select quantity of transmission fluid into the clutch pack  110 . Similarly, when the clutch hub  122  of the driven member  104  begins to rotate faster relative to the drive hub portion  118  of the drive member  102 , such as in gears  3 F- 5 F, or when the the clutch assembly  100  is engaged in gears  6 F- 10 F, centrifugal force will once again operate to pump the transmission fluid through the outer annulus  128  and into the clutch pack  110 . 
     In accordance with yet other aspects of the present disclosure, a seal (not shown) may be provided to close a gap  146  that may exist between the lip flange  140  and components of the driven member  104 , such as the balance piston  114  (see  FIG. 3 ), if it is determined that excess amounts of transmission fluid are spilling over the lip flange  140  into the clutch pack  110  during the disengagement period of the clutch assembly  100 . 
       FIG. 8  illustrates a rotational clutch assembly  200  in accordance with yet other aspects of the present disclosure. The rotational clutch assembly  200  may be used in the transmission  10 , for example, as one or more of the rotational clutch assemblies  40 ,  42 , and  44 . The clutch assembly  200  may include a drive member, generally indicated at  202 , and a driven member, generally indicated at  204 , which rotate about a common axis. A clutch housing  206  is generally defined between the drive member  202  and the driven member  204  and is formed to house a clutch pack, generally indicated at  210 , that is engaged or disengaged through actuation of a piston  212 , such as through hydraulic actuation or spring force actuation. A balance piston assembly  214  may be included and housed in the clutch housing  206  along with the piston  212  in order to introduce reverse pressure on the low pressure side of the piston  212  to counteract the large thrust generated by the hydraulic pressure fluid on the high pressure side of the piston and prevent the piston from engaging the clutch at high rotational speeds. 
     The annular clutch pack  210  may be composed of annular clutch plates  216  that are splined to and extend inward from a drive hub portion  218  of the drive member  202  and annular friction discs  220  that are splined to and extend outward from a clutch hub  222  of the driven member  204 . The clutch plates  216  and friction discs  220  are interleaved as shown in  FIG. 8 . In accordance with aspects of the present disclosure, when the clutch assembly  200  is in a disengaged position, the drive member  202  maintains a certain rotational speed based on an input speed of an input member of the transmission  10  and the driven member  204  is disengaged and not rotating or rotating at a slower relative speed. When the clutch assembly  200  is in the disengaged position, the clutch plates  216  rotate freely past the friction discs  220  in a non-contacting manner. However, when the clutch pack  210  is to be placed into an engaged position during a particular gear change, for example, when moving from the fifth gear to the sixth gear in the transmission  10  described previously, pressurized hydraulic fluid is introduced into a pressure chamber  224  to produce axial movement of the piston  212 . In turn, actuation of the piston  212  forces a frictional engagement of the clutch plates  216  with the friction discs  220  to reduce or eliminate relative rotation between the clutch plates  216  and the friction discs  220 . 
     As discussed above, during engagement of the clutch pack  210 , the relative rotational speed of the drive member  202  and the driven member  204  may be synchronized. To facilitate cooling during transition to and from an engaged state, and to reduce the transferred kinetic energy while the members are engaged, a continuous flow of coolant, such as automatic transmission fluid, may be provided to the clutch hub  222 . In accordance with aspects of the present disclosure, the clutch hub  222  may be formed with an inner annulus  226  connected to an outer annulus  228  by a hub deck  230 . The inner annulus  226  and the outer annulus  228  extend from the deck  230  in a direction toward the piston  212  and cooperate to form an interior space  232 . The outer annulus  228  may be provided with a series of radial bores  234  that provide fluid communication from the interior space  232  to the clutch pack  210  for a fluid to flow through the outer annulus  228  toward the clutch pack  210 . 
     A slinger plate  250  may be mounted to the hub deck  230  and configured to divide the interior space  232  into an upper space  233  and a lower space  235  while providing a trough  242  for collecting transmission fluid pumped into the interior space  232 . A series of axial facing bores  236  may be provided in the hub deck  230  of the clutch hub  222 . The axial facing bores  236  are spaced peripherally a radial distance from the center of the hub deck  230  and just above where the slinger plate  250  divides the interior space  232 . The axial bores  236  are sized to permit a substantial quantity of the flowing coolant to escape the upper space  233 , primarily during a period of disengagement of the clutch assembly  200 . 
     The slinger plate  250  is also provided with a series of radially situated slinger bores  244  at the bottom of the trough  242 . The slinger bores  244  provide fluid communication from the upper space  233  to the lower space  235  for the transmission fluid to flow therethrough. Thus, when the clutch pack  210  is engaged to cause rotation of the clutch hub  222  at the same speed as the drive member  202 , or during a period when rotation of the driven member  204  is increased relative to the rotation of the drive member  202 , such as in gears  3 F- 10 F, by way of centrifugal action transmission fluid is forced through the slinger bores  244  and into the lower space  235 . Once in the lower space  235 , the transmission fluid may continue under centrifugal force to flow through the radial bores  234  into the clutch pack  210 . The size of the slinger bores  244  and the radial bores  234  are determined in order to restrict a majority of the coolant flow to the lower space  235  and thus the clutch pack  210  during a period of disengagement, when most of the transmission fluid is intended to drain from the trough  242  by way of the axial bores  236 . 
     The axial bores  236  may be arcuately spaced at equal angles θ around the perimeter of the hub deck  230  such that at least a portion of one or more of the axial bores  236  will always be below the fluid level in the trough area  242  when the clutch hub  222  is not rotating, i.e., when the clutch pack  210  is disengaged. Accordingly, when the clutch hub  222  is stopped and the relative speed between the clutch plates  216  and the friction discs  220  is highest, such as in gears  1 F,  2 F, or R, the larger axial bores  236  may drain away a majority of the coolant before the coolant drains through the slinger bores  244  and the radial bores  234  into the clutch pack  210 . Once the clutch assembly  200  is engaged and/or the clutch hub  222  starts to rotate, the slinger bores  244  and the radial bores  234  will work again to pump the transmission fluid into the clutch pack  210  to provide adequate removal of the thermal heat generated therein. 
     Various aspects of systems and methods for cooling a clutch assembly may be illustrated by describing components that are connected, attached, and/or joined together. As used herein, the terms “connected”, “attached”, and/or “joined” are used to indicate either a direct connection between two components or, where appropriate, an indirect connection to one another through intervening or intermediate components. In contrast, if a component is referred to as being “directly coupled”, “directly attached”, and/or “directly joined” to another component, there are no intervening elements present. 
     Industrial Applicability 
     The disclosure includes a clutch cooling system and methods for cooling a clutch that include passively controlling the flow of coolant to a clutch pack in the clutch. The cooling system efficiently transfers thermal energy generated by the engagement and disengagement of the clutch pack while reducing drag losses of the clutch assembly when the clutch is disengaged. The clutch cooling system is disclosed for use in transmissions on vehicles, including heavy haul trucks or ground moving equipment, for example, but may be used in any machine that uses clutches for the engagement and disengagement of component members. 
     In a rotational clutch having a drive member and a driven member, the clutch cooling system employs a unitary clutch hub attached to the driven member that has axial bores and radial holes, the radial holes being in fluid communication with a clutch pack and the axial bores providing an outlet from the clutch assembly. Through placement and sizing of the axial bores and the radial holes, passive cooling control depends simply on the rotational speed of the clutch hub by increasing a flow of coolant to the clutch pack through the radial holes during engagement of the clutch pack, when the clutch hub is rotating, while diverting a larger portion of the flow away from the clutch pack via the axial bores when the clutch pack is disengaged, when the clutch hub is not rotating or rotating at a much slower speed. 
     The many features and advantages of the disclosure are apparent from the detailed specification, and, thus, it is intended by the appended claims to cover all such features and advantages of the disclosure which fall within the true spirit and scope of the disclosure. Further, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the disclosure to the exact construction and operation illustrated and described, and, accordingly, all suitable modifications and equivalents may be resorted to that fall within the scope of the disclosure.