Abstract:
An embodiment provides a drive train for a medium duty truck. The drive train includes an engine having an engine output member, and a centrifugal clutch. The clutch has a plurality of swing weights and a driving member. The driving member generally defines an axis and is rotatable with the engine output member. The clutch also includes a ramp portion having a generally planar ramped surface portion in selective engagement with at least a portion of the swing weights, and a clutch disk portion for transferring torque from the driving member to a driven member. The clutch is operable between an engaged condition and a disengaged condition. The swing weights are radially moveable with respect to the axis. Radial movement of the swing weights causes axial movement of at least a portion of the clutch between a disengaged position and an engaged position. The swing weights engage only the ramped surface portion when the at least a portion of the clutch is not in the disengaged position.

Description:
TECHNICAL FIELD  
       [0001]     The technical field is generally automotive drive trains, and particularly, clutch activation and configurations to reduce noise.  
       BACKGROUND  
       [0002]     Centrifugally operated friction clutches are well known in the art of vehicular drive train systems. They typically include an input member driven by a prime mover, usually an electric motor or internal combustion engine, and weights rotatable with the input member which, upon rotation of the driving member, will move radially outwardly under the effect of centrifugal force to cause the input member to frictionally engage a driven output member. Automatically actuated centrifugal clutches employed with heavy-duty electromechanical highway line-haul truck transmissions include so-called centrifugal actuation modules that house the centrifugally actuated weights. The centrifugal modules are drivingly connected to an engine flywheel, and each of a plurality of centrifugally actuated weights is adapted to swing in an arc about a pivot link fixed to the module housing structure. As such, the so-called swing weights contained within the modules are radially outwardly movable against resistive spring forces as a function of engine speed-the higher the speed, the greater the outward movement between limits. Rollers attached to the weights are adapted to roll atop ramp segments that are cammed for clutch engagement and disengagement.  
         [0003]     The swing weights are subjected to a number of forces, and thus give rise to competing concerns to achieve satisfactory operation of the modules over the useful life of a clutch. As an example, one feature of the above-described prior art centrifugal clutch is the use of two distinct frusto-conical ramp surfaces on the ramp segments. A first ramp surface exhibits a relatively steep slope and a second ramp surface exhibits a more gradual slope. These ramp surfaces are engaged by swing weight rollers and are used to create a clamp load as the centrifugal force acting on each swing weight increases. Particularly, as the centrifugal force increases, the swing weights will move from their original position on the relatively steep first ramp surface onto the more gradual sloping second ramp surface. Since a centrifugal clutch operates as a balance of forces, any tolerance in the centrifugal module components (e.g., swing weight springs, ramp segments, etc.) may cause a “staggered disengagement”, wherein one or more of the swing weights moves from the second ramp surface to the first ramp surface before the other swing weights. This condition is exacerbated in a swing weight style centrifugal clutch since operation of each individual swing weight is essentially independent of the other swing weights.  
         [0004]     Generally, the multi-ramp surface clutch described herein is used with a heavy duty truck (typically greater than about 30,000 lb GVW, more than about 10 liters engine displacement, and having an engine that develops maximum torque typically below about 1200 rpm). The multi ramp surface clutch, as disclosed in commonly owned U.S. Pat. No. 6,880,687, the disclosure of which is hereby incorporated by reference in its entirety, permits an engine that develops maximum torque at a lower rpm to generate a sufficient clutch clamping force to transmit the maximum torque.  
         [0005]     Another feature of the above-described prior art centrifugal clutch is the use of mechanical stops for the swing weights. As the swing weights move along the more gradual sloping second ramp surface during engine speed increase, it is desirable to provide a motion limiter for the swing weight to limit its radial travel relative the clutch. The stops prevent the full compression of the springs that are provided to return the swing weights to position during engine deceleration. The stops also provide for a maximum clutch engagement force, as the stops limit the axial displacement caused by the swing weight travel during engine speed increase.  
         [0006]     When these stops are used in a medium duty truck (typically about 16,000 to about 30,000 lb GVW, and having an engine that develops maximum torque typically above about 1200 rpm), in conjunction with the more gradual sloping second ramp surface, the swing weights tend to create an undesirable impact noise during engine speed increase that may be audible within the cabin of the vehicle. Additionally, second stops are typically provided to arrest the movement of the swing weights during engine deceleration. Since the conventional multi-ramp surface clutch will disengage as the swing weights traverse the he more gradual sloping second ramp surface then the relatively steep first ramp surface, the swing weights may accelerate on the relatively steep first ramp surface and impact the second stops if engine speed decreases dramatically. This undesirable impact of the swing weights and the second stops will typically create an undesirable impact noise.  
         [0007]     Accordingly, a need exists for an improved centrifugal clutch for a medium duty engine that avoids staggered disengagement of the centrifugally operated weights and reduces the undesirable impact noise associated with the stops.  
       SUMMERY  
       [0008]     An embodiment provides a drive train for a medium duty truck. The drive train includes an engine having an engine output member, and a centrifugal clutch. The clutch has a plurality of swing weights and a driving member. The driving member generally defines an axis and is rotatable with the engine output member. The clutch also includes a ramp portion having a generally planar ramped surface portion in selective engagement with at least a portion of the swing weights, and a clutch disk portion for transferring torque from the driving member to a driven member. The clutch is operable between an engaged condition and a disengaged condition. The swing weights are radially moveable with respect to the axis. Radial movement of the swing weights causes axial movement of at least a portion of the clutch between a disengaged position and an engaged position. The swing weights engage only the ramped surface portion when the at least a portion of the clutch is not in the disengaged position.  
         [0009]     Another embodiment provides a method of transferring torque from an engine to a transmission in a medium duty truck. The method includes increasing the speed of the engine and urging, at least partially through centrifugal force, a weight member away from a clutch axis. The method also includes urging a plate member toward a friction pad. The step of urging the plate member is at least partially in response to the step of urging the weight member. The method further includes engaging an output portion of the engine with a portion of the transmission for rotation therewith. The step of urging the plate member is initiated and completed as the weight member contacts a generally planar ramp surface. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a schematic illustration of a vehicular drive train system.  
         [0011]      FIG. 2  is a schematic partial sectional view of a prior art centrifugal clutch.  
         [0012]      FIG. 3  is a partial end view, in section with section graphics omitted for clarity, of a cover module of the prior art clutch of  FIG. 2 , showing the clutch in a disengaged state.  
         [0013]      FIG. 4  is a partial sectional side view of a centrifugal clutch according to an embodiment.  
         [0014]      FIG. 5  is a partial sectional end view of the centrifugal clutch of  FIG. 4 , with a portion of the cover removed for clarity, showing the clutch in an engaged state.  
         [0015]      FIG. 6  is a schematic partial sectional view of the centrifugal clutch of  FIG. 4 , with some section graphics omitted for clarity.  
         [0016]      FIG. 7A  is a partial end view, in section, of a cover module for the clutch of  FIGS. 4-6  with section graphics omitted for clarity, showing the clutch in a disengaged state.  
         [0017]      FIG. 7B  is a partial end view, in section, of a cover module for the clutch of  FIGS. 4-6  with section graphics omitted for clarity, showing the clutch in an engaged state. 
     
    
     DETAILED DESCRIPTION  
       [0018]     A vehicular drive train system  20  employing a centrifugally operated master friction clutch is schematically illustrated in  FIG. 1 . By way of example, system  20  may be fully automated, partially automated, or manual operated with controller assist.  
         [0019]     In system  20 , a change-gear transmission  22  comprising a main transmission section  24  connected in series with a splitter-type auxiliary transmission section  26  is drivingly connected to an internal combustion engine  28 , such as a well-known gasoline or diesel engine, by a centrifugal master friction clutch  30  of the present invention. Transmission  22 , by way of example, may be of the type well known in the prior art and sold by the assignee of this application, EATON CORPORATION, under the trademarks “Super-10” and “Lightning”, and may be seen in greater detail by reference to U.S. Pat. Nos.: 4,754,665; 6,015,366; 5,370,013; 5,1004,906; and 5,1004,354, the disclosures of which are incorporated herein by reference.  
         [0020]     Engine  28  includes a crankshaft  32 , which is attached to a driving member  34  of centrifugal master clutch  30  that frictionally engages with, and disengages from, a driven member  36  attached to an input shaft  38  of transmission  22 . A transmission output shaft  40  extends from the auxiliary transmission section  26  for driving connection to the vehicular drive wheels, as through a drive axle  41  or transfer case.  
         [0021]     The terms “engaged” and “disengaged” as used in connection with a master friction clutch refer to the capacity, or lack of capacity, respectively, of the clutch to transfer a significant amount of torque. Mere random contact of the friction surfaces, in the absence of at least a minimal clamping force, is not considered engagement.  
         [0022]     As may be seen from a  FIG. 1 , centrifugal clutch  30  requires no external clutch actuator and is operated as a function of the rotational speed (ES) of engine  28 . Centrifugal clutch  30  also requires no connections to operating linkages, command signal inputs, power electronics and/or compressed air and/or hydraulic conduits. The most economical application of the present invention is a dry clutch; however, the present invention is also applicable to wet clutch technology.  
         [0023]     As is known, rotation of input member  34  will cause clutch  30  to engage and drivingly connect the engine output, usually an engine flywheel or the like, to transmission input shaft  38 . The clamping force, and thus the torque transfer capacity of clutch  30  is a function of the rotational speed (ES) of engine  28  and clutch input member  34 . Clutch  30  should reach incipient engagement at an engine speed slightly greater than engine idle, and should fully engage at an engine speed lower than the engine speed at which a first upshift is required. Unlike typical spring applied master friction clutches, which are normally engaged, clutch  30  is disengaged at lower engine speeds.  
         [0024]     To allow proper vehicle launch and dynamic shifting with the master clutch engaged, clutch  30 , once fully engaged, should remain fully engaged at engine speeds greater than (i) the highest expected speed at which downshifts are initiated and (ii) the minimum expected engine speed after an upshift. Incipient engagement of clutch  30  is the initial torque transfer contact of clutch friction surfaces as may be seen by reference to U.S. Pat. Nos. 4,646,891 and 6,022,295, the disclosures of which are incorporated herein by reference.  
         [0025]     To fully appreciate the features of the present invention, reference is made to a prior art centrifugal clutch  30  shown in  FIGS. 2 and 3 .  FIG. 2  illustrates the operational components of clutch  30  shown in fragments as rotating about a rotational axis  52  of input shaft  38 . Clutch  30  includes a cover module  54  ( FIG. 3 ), a first friction disc assembly  56 , an intermediate pressure plate  58 , and a second friction disc assembly  60 . As is well known from conventional clutches, cover module  54  and intermediate pressure plate  58  mount to an engine flywheel  62  for rotation therewith and comprise the driving portion of clutch  30 . Friction disc assemblies  56  and  60  are typically splined to transmission input shaft  38  and comprise the driven portion of clutch  30 .  
         [0026]     As shown in  FIG. 3 , cover module  54  includes four swing weights  66 , which are movably attached to cover module  54  at pivot pins  68 . Return springs  70  bias swing weights  66  radially inwardly to rest on a first stop member  72 . A second stop member  74  limits the radially outward movement of swing weights  66 . As engine  28  and cover module  54  rotate, the effect of centrifugal force will cause swing weights  66  to move against the biasing force of springs  70  from a position abutting stops  72  toward stops  74 . Swing weights  66  each carry one or more rollers  76 , which act between a reaction surface and a ramp to provide an axial clamping force for engaging clutch  30 .  
         [0027]     As shown in  FIG. 2 , rollers  76  are received between a substantially flat surface  78  of a fixed reaction plate  80  and a ramped surface  82  of an axially movable ramp plate  84 . Ramp plate  84  acts on an axially movable main pressure plate  86  through a preloaded spring member  88 , which limits the axial force applied to the main pressure plate  86  by ramp plate  84 . Main pressure plate  86  applies a clamping force CF on friction pads  90  of the friction plates, which are trapped between surface  92  of main pressure plate  86  and intermediate pressure plate  58  and between intermediate pressure plate  58  and surface  94  of engine flywheel  62 . Hub portions  96  of friction plates  56  and  60  are adapted to be splined to input shaft  38  for rotation therewith while plates  80 ,  84 ,  86 , and  58  rotate with engine flywheel  62 . Clutch  30  also includes an adjustment mechanism  100  for modifying the axial position of reaction plate  80  to accommodate wear in friction pads  90  and, accordingly, maintain a more consistent engagement point.  
         [0028]     At rest, rollers  76  will engage a recessed portion  108  of ramp surface  82  and will not apply a leftward axial clamping force to friction pads  90 . As rollers  76  travel sufficiently radially outwardly, and onto a ramped portion  110  of ramp surface  82 , an increasing axial clamping force is applied. As rollers  76  move further radially outwardly onto a flat extended portion of  112  of ramp surface  82 , the clamping force will remain at a capped value as limited by preloaded spring member  88 . The swing weights  66  will hit stops  74  prior to full compression of springs  70 .  
         [0029]     As wear occurs in friction pads  90 , rollers  76  will be required to travel farther up ramped portion  110  to apply a given clamp load during clutch engagement. This wear, and the corresponding increased outward movement in swing weights  66 , causes the engagement point of clutch  30  to change due to the increased compression of biasing springs  70 .  
         [0030]     As the centrifugal force increases and overcomes the preload of spring member  88 , swing weights  66  will move from ramped portion  110  onto the relatively flat extended portion  112  of surface  82 . Once on flat extended portion  112 , clutch  30  can transmit a given torque at a lower engine speed without the swing weights  66  traveling back down ramped portion  110 . This feature is desired in commercial vehicles due to the high torque demand at relatively lower engine speeds. Because clutch  30  operates based on a balance of forces, any tolerance in the springs, compression of the springs or the dimensions of surfaces  110 ,  112 , for example, may cause one or more of swing weights  66  to prematurely move from flat extended portion  112  onto ramped surface  110 , resulting in a staggered disengagement of swing weights  66 . The following table illustrates the effects of a staggered disengagement on an exemplary implementation of the prior art centrifugal clutch that includes four (4) swing weights:  
                               TABLE 1                                       Number of Swing Weights   4   3           Engaged           Number of Swing Weights   0   1           Disengaged           Load On All Swing Weights   3820   3157           (Lbf)           Load On Each Disengaged Swing   0   292           Weight (Lbf)           Load On Each Engaged Swing   955   955           Weights (Lbf)           Additional Return Force Applied   0   0           To Engaged Swing Weight (Lbf)                      
 
         [0031]     As shown in the Table 1, when swing weights  66  are engaged, the load on all of the swing weights  66  collectively is about 3820 Lbf. In the above example, since there are four swing weights, the load on each engaged swing weight  66  is about 955 Lbf ( 3820 Lbf/4 engaged swing weights). If one of the swing weights  66  prematurely disengages from the generally flat surface  112  of ramp surface  82  and moves onto ramped portion  110  of ramp surface  82 , the disengaged swing weight  66  is subjected to a lesser load than the engaged swing weights (e.g., 292 Lbf) since there is still some centrifugal force acting on the swing weight positioned on ramped portion  110 . Because return springs  70  act on each swing weight  66  individually, there is generally no additional return force imposed on each of the remaining engaged swing weights. In other words, the load on each engaged swing weight remains at about 955 Lbf (3157 Lbf−292 Lbf/3 engaged swing weights). Thus, in clutch  30 , there is generally no additional return force applied to the remaining engaged swing weights after one or more of the swing weights prematurely disengage.  
         [0032]      FIGS. 4-6  illustrate a centrifugal clutch  130 , according to an embodiment.  FIG. 6  illustrates the operational components of clutch  130  shown in fragments as rotating about a rotational axis A-A of input shaft  38 . Clutch  130  includes a cover module  154  ( FIG. 5 ), a first friction disc assembly  156 , an intermediate pressure plate  158 , and a second friction disc assembly  160 . As is well known from conventional clutches, cover module  154  and intermediate pressure plate  158  mount to an engine flywheel  162  for rotation therewith and comprise the driving portion of clutch  130 . Friction disc assemblies  156  and  160  are typically splined to transmission input shaft  38  and comprise the driven portion of clutch  130 . Clutch  130  also includes a cover housing  164 .  
         [0033]     As shown in  FIGS. 5 and 6 , cover module  154  includes four swing weights  166 , which are movably attached to cover module  154  at pivot pins  168 . Return springs  170  bias swing weights  166  radially inwardly to rest on a first stop member  172 . A second stop member  174  is formed on the cover housing  164  and limits the radially outward movement of swing weights  166 . As engine  28  and cover module  154  rotate, the effect of centrifugal force will cause swing weights  166  to move against the biasing force of springs  170  from a position abutting stops  172  toward stop surfaces  174 . Swing weights  166  each carry one or more rollers  176 , which act between a reaction surface and a ramp to provide an axial clamping force for engaging clutch  130 . As best illustrated in  FIG. 6 , the return springs  170  exert a spring force, illustrated as Arrow F S , on swing weights  166  and rollers  176 .  
         [0034]     As shown in  FIG. 6 , rollers  176  are received between a substantially flat surface  178  of a fixed reaction plate  180  and a ramp plate surface  182  of an axially movable ramp plate  184 . While the ramp plate surface  182  is illustrated 2-dimensionally in  FIG. 6  as a ramped, or beveled surface on the ramp plate  184 , the ramp plate surface  182  may be a ramped surface, or a continuous or discontinuous generally frusto-conical surface. Ramp plate  184  acts on an axially movable main pressure plate  186  through a preloaded spring member  188 , which limits the axial force applied to the main pressure plate  186  by ramp plate  184 . That is, the ramp plate  184  applies an interim force on the preloaded spring member  188 , and at least a portion of this interim force is applied to the main pressure plate  186 .  
         [0035]     Main pressure plate  186  applies a clamping force CF on friction pads  190  of the friction plates, which are trapped between surface  192  of main pressure plate  186  and intermediate pressure plate  158  and between intermediate pressure plate  158  and surface  194  of engine flywheel  162 . Hub portions  196  of friction plates  156  and  160  are adapted to be splined to input shaft  38  for rotation therewith while plates  180 ,  184 ,  186 , and  158  rotate with engine flywheel  162 . Clutch  130  also includes an adjustment mechanism  200  for modifying the axial position of reaction plate  180  to accommodate wear in friction pads  190  and, accordingly, maintain a more consistent engagement point.  
         [0036]     The ramp surface  182  includes an annular surface  206 , an inner ramp portion  208 , a mid ramp surface  210 , and an outer ramp surface  212 . As best seen in  FIG. 7A , when the clutch  130  is not rotating, rollers  176  will engage the inner ramp portion  208  of ramp surface  182  and will not apply a leftward axial clamping force to friction pads  190 . Generally, the stops  172  will prevent the rollers  176  from contacting the annular surface  206 .  
         [0037]     As best illustrated in  FIG. 6 , rotation of clutch  130  will exert a centrifugal force, illustrated as Arrow F C , on swing weights  166  and rollers  176 . As the rotational speed of the clutch  130  increases, the centrifugal force F C  will increase. When the centrifugal force F C  exceeds the spring force F S , swing weights  166  will move away from the axis A-A generally in the direction of Arrow F C . as the swing weights lift from engagement with the stops  172 .  
         [0038]     As the speed of the clutch  130  increases, causing rollers  176  to travel sufficiently radially outwardly, along the mid ramp surface  210  of the ramp surface  182 , an increasing centrifugal force F C  will result in an increasing axial clamping force CF applied to the friction pads  190 . As rollers  176  move further radially outwardly onto the outer ramp surface  212  of ramp surface  182 , the clamping force CF will continue to increase until the swing weights contact the stop surfaces  174 . The clamping force CF will increase to a desired maximum value as limited by preloaded spring member  188 . The swing weights  166  will contact the stop surfaces  174  prior to full compression of springs  170 , as illustrated in  FIG. 7B .  
         [0039]     As wear occurs in friction pads  190 , rollers  176  will be required to travel farther up the mid ramp surface  210  to apply a given clamp load during clutch engagement. This wear, and the corresponding increased outward movement in swing weights  166 , causes the engagement point of clutch  130  to change due to the increased compression of biasing springs  170 .  
         [0040]     The surfaces  208 ,  210 ,  212 , when viewed normal the axis A-A as in  FIG. 6 , lie generally in a single plane. That is, the rollers  176  that contact the ramped surface  182  are in contact with a generally planar ramped surface portion  220  of surfaces  208 ,  210 ,  212  during axial movement of the ramp plate  184 . Generally, the rate of axial movement of the ramp plate  184  will be proportional to the rate of radial movement of rollers  176  (with both rates taken with respect to the axis A-A) when the surface  178  is generally flat. The rollers  176  that each contact a surface portion of the ramp surface  182  will travel in generally a rectilinear path (with respect to the clutch housing  164  ) during axial movement of the ramp plate  184 , since the rollers are guided on both surfaces  178  and  182 . The surfaces  208 ,  210 ,  212 , when viewed in orientations other than the view of  FIG. 6  may exhibit some curvature.  
         [0041]     With the rollers  176  in contact with generally a single plane during axial movement of the ramp plate  184 , the swing weights  166  will not impact the stop  172  and stop surface  174  with an undesirable amount of force, as may happen with a multi-plane ramp surface  82 . Preferably, the surfaces  208 ,  210 ,  212  are at an 11° angle with respect to the surface  206  when clutch  130  is mated to an engine that develops about 500 to about 550 ft·lb of torque at about 1450 rpm, although other ramp angles may be utilized. Generally, a higher ramp angle will result in a lower clamp rate and a higher wear capacity, and a lower ramp angle will result in a higher clamp rate and a lower wear capacity. Accordingly, the ramp angle may be tuned for the specific engine performance characteristics, operational characteristics of other components, and desired clutch performance.  
         [0042]     Generally, an operator of a heavy duty truck is not aware of the impact between the swing weights  66  and the stops  72 ,  74 . Primarily, this is due to greater insulation within the cab of a heavy duty truck and the production of a maximum engine torque at a low engine speed, such as 1100 rpm. Providing a single ramp plane  182  for the clutch  130  will lessen the impact of the swing weights  166  and the stop  172  and stop surface  174 , thereby reducing wear and increasing reliability of the clutch  130 .  
         [0043]     The inventor has discovered that the complexity of a multi-ramp, or multi-frusto-conical surface centrifugal clutch is unnecessary when an engine, such as the engine  28 , is a medium duty engine that develops maximum torque at a higher rpm than the typical heavy duty engine generally associated with a multi-ramp centrifugal clutch.  
         [0044]     The preceding description has been presented only to illustrate and describe exemplary embodiments of the methods and systems of the present invention. It is not intended to be exhaustive or to limit the invention to any precise form disclosed. It will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the claims. The invention may be practiced otherwise than is specifically explained and illustrated without departing from its spirit or scope. The scope of the invention is limited solely by the following claims.