Centrifugal clutch with improved wear life and disengagement characteristics

A centrifugal clutch assembly is provided that includes a cover module having a pressure plate for applying a clamping force against a friction plate, a moveable plate adapted to rotate with the cover module, but axially displaceable with respect thereto to apply an axial force on the pressure plate through a preloaded plate spring, a fixed plate secured for rotation with the cover module, and a plurality of weights positioned between the moveable plate and the fixed plate that are adapted to move outward under the effects of centrifugal force to cause axial movement in the movable plate and a clamping force on the friction plate. The cover module also includes at least one return spring configured to apply a return force on the weights through the moveable plate, the return force being generally parallel to the axis of rotation of the cover module and independent of weight position.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a centrifugal master clutch for a vehicular drivetrain system and, more particularly, to a centrifugal master clutch having improved wear life and disengagement characteristics.

2. Description of the Related Art

Centrifugally operated friction clutches are well known in the art of vehicular drivetrain systems and 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.

The swing weights are subjected to a number of forces, and thus give rise to issues that work against satisfactory operation of the modules over the useful lives of the clutch. For example, in one known centrifugal clutch, each of the swing weights is biased by its own compression spring(s). In this design, the biasing force exerted on a swing weights by its compression spring(s) is dependent on the position of the swing weight-generally the farther outward the swing weight moves, the greater the spring force exerted on the swing weight. As the friction materials in the clutch wear, the swing weights move farther up the ramp segments to create a given clamp load and the engagement point of the clutch undesirably changes due to the additional compression of the swing weight springs.

Another feature of the above-described prior art centrifugal clutch is the use of two different 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.

Accordingly, a need exists for an improved centrifugal clutch that maintains the engagement point of the clutch and avoids staggered disengagement of the centrifugally operated weights.

SUMMARY OF THE INVENTION

A centrifugal clutch assembly is provided that includes an input portion fixed for rotation with an input member and an output portion fixed for rotation with an output member. The output portion includes at least one friction plate secured for rotation with the output member and the input portion includes a cover module secured for rotation with the input member. In an embodiment, the cover module includes a pressure plate for applying a clamping force against the at least one friction plate and a moveable plate adapted to rotate with the cover module, but is axially displaceable with respect thereto to apply an axial force on the pressure plate through a preloaded plate spring. A fixed plate is secured for rotation with the cover module and a plurality of weights are positioned between the moveable plate and the fixed plate. The weights are adapted to move outward under the effects of centrifugal force to cause axial movement in the movable plate and the pressure plate to exert a clamping force on the friction plate. The cover module also includes at least one return spring configured to apply a return force on the weights through the moveable plate, the return force being generally parallel to the axis of rotation of the cover module.

One or more limitations of the prior art are minimized in the clutch assembly of the present invention by operation of the return spring, which applies a return force on the centrifugally operated weights independent of the position of the weights.

DETAILED DESCRIPTION OF THE INVENTION

A vehicular drivetrain system20employing a centrifugally operated master friction clutch of the present invention is schematically illustrated inFIG. 1. By way of example, system20may be fully automated, partially automated, or manual operated with controller assist.

In system20, a change-gear transmission22comprising a main transmission section24connected in series with a splitter-type auxiliary transmission section26is drivingly connected to an internal combustion engine28, such as a well-known gasoline or diesel engine, by a centrifugal master friction clutch30of the present invention. Transmission22, 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; 5,974,906; and 5,974,354, the disclosures of which are incorporated herein by reference.

Engine28includes a crankshaft32, which is attached to a driving member34of centrifugal master clutch30that frictionally engages with, and disengages from, a driven member36attached to an input shaft38of transmission22. A transmission output shaft40extends from the auxiliary transmission section26for driving connection to the vehicular drive wheels, as through a drive axle41or transfer case.

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.

As may be seen from aFIG. 1, centrifugal clutch30requires no external clutch actuator and is operated as function of the rotational speed (ES) of engine28. Centrifugal clutch30also 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.

As is known, rotation of input member34will cause clutch30to engage and drivingly connect the engine output, usually an engine flywheel or the like, to transmission input shaft38. The clamping force, and thus the torque transfer capacity of clutch30is a function of the rotational speed (ES) of engine28and clutch input member34. Clutch30should 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, clutch30is disengaged at lower engine speeds.

To allow proper vehicle launch and dynamic shifting with the master clutch engaged, clutch30, 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 clutch30is 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.

To fully appreciate the features of the present invention, reference is made to a prior art centrifugal clutch50shown inFIGS. 2 and 3.FIG. 2is a schematic illustration of the operational components of clutch50shown in fragments as rotating about a rotational axis52of input shaft38. Clutch50includes a cover module54(FIG. 3), a first friction disc assembly56, an intermediate pressure plate58, and a second friction disc assembly60. As is well known from conventional clutches, cover module54and intermediate pressure plate58mount to an engine flywheel62for rotation therewith and comprise the driving portion of clutch50. Friction disc assemblies56and60are typically splined to transmission input shaft38and comprise the driven portion of clutch50.

As shown inFIG. 3, cover module54includes four swing weights66, which are movably attached to cover module54at pivot pins68. Return springs70bias swing weights66radially inwardly to rest on a first stop member72. A second stop member74limits the radially outward movement of swing weights66. As engine28and cover module54rotate, the effect of centrifugal force will cause swing weights66to move against the biasing force of springs70from a position abutting stops72toward stops74. Swing weights66each carry one or more rollers76, which act between a reaction surface and a ramp to provide an axial clamping force for engaging clutch50.

As shown inFIG. 2, rollers76are received between a substantially flat surface78of a fixed reaction plate80and a ramped surface82of an axially movable ramp plate84. Ramp plate84acts on an axially movable main pressure plate86through a preloaded spring member88, which limits the axial force applied to the main pressure plate86by ramp plate84. Main pressure plate86applies a clamping force CF on friction pads90of the friction plates, which are trapped between surface92of main pressure plate86and intermediate pressure plate58and between intermediate pressure plate58and surface94of engine flywheel62. Hub portions96of friction plates56and60are adapted to be splined to input shaft38for rotation therewith while plates80,84,86, and58rotate with engine flywheel62. Clutch50also includes an adjustment mechanism97for modifying the axial position of reaction plate80to accommodate wear in friction pads90and, accordingly, maintain a more consistent engagement point.

At rest, rollers76will engage a recessed portion98of ramp surface82and will not apply a leftward axial clamping force to friction pads90. As rollers76travel sufficiently radially outwardly, and onto a ramped portion100of ramp surface82, an increasing axial clamping force is applied. As rollers76move further radially outwardly onto a flat extended portion of102of ramp surface82, the clamping force will remain at a capped value as limited by preloaded spring member88. The swing weights66will hit stops74prior to full compression of springs70.

As wear occurs in friction pads90, rollers76will be required to travel farther up ramped portion100to apply a given clamp load during clutch engagement. This wear, and the corresponding increased outward movement in swing weights66, causes the engagement point of clutch50to change due to the increased compression of biasing springs70.

As the centrifugal force increases and overcomes the preload of spring member88, swing weights66will move from ramped portion100onto the relatively flat extended portion102of surface82. Once on flat extended portion102, clutch50can transmit a given torque at a lower engine speed without the swing weights66traveling back down ramped portion100. This feature is desired in commercial vehicles due to the high torque demand at relatively lower engine speeds. Because clutch50operates based on a balance of forces, any tolerance in the springs, compression of the springs or the dimensions of surfaces100,102, for example, may cause one or more of swing weights66to prematurely move from flat extended portion102onto ramped surface100, resulting in a staggered disengagement of swing weights66. 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 1Number of Swing Weights43EngagedNumber of Swing Weights01DisengagedLoad On All Swing Weights38203157(Lbf)Load On Each Disengaged Swing0292Weight (Lbf)Load On Each Engaged Swing955955Weights (Lbf)Additional Return Force Applied00To Engaged Swing Weight (Lbf)
As shown in the Table 1, when swing weights66are engaged, the load on all of the swing weights66collectively is about 3820 Lbf. In the above example, since there are four swing weights, the load on each engaged swing weight66is about 955 Lbf (3820 Lbf/4 engaged swing weights). If one of the swing weights66prematurely disengages from the generally flat surface102of ramp surface82and moves onto ramped portion100of ramp surface82, the disengaged swing weight66is 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 portion100. Because return springs70act on each swing weight66individually, 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 clutch50, there is generally no additional return force applied to the remaining engaged swing weights after one or more of the swing weights prematurely disengage.

An improved centrifugal clutch30according to an embodiment of the present invention is shown inFIGS. 4–12. In an embodiment, clutch30includes a cover module110, a first friction disc assembly112, an intermediate pressure plate114, and second friction disc assembly116. Cover module110and intermediate pressure plate114mount to an engine flywheel for rotation therewith and comprise the driving portion34of clutch30. Friction disc assemblies112and116are splined to transmission input shaft38and comprise the driven portion36of clutch30.

As shown inFIGS. 4–7, cover module110includes a plurality of movable roller weights118, which are positioned between a fixed reaction plate120and a ramp plate122. In an embodiment, cover module110includes seven (7) roller weights118arranged circumferentially about input shaft38; however, the number of roller weights118employed in clutch30is not limited thereto. Indeed, the number of roller weights118employed in clutch30may depend on a number of factors including, for example, the size of clutch30, the desired clamping force and the load bearing capacity of each roller weight118.

To minimize damage to reaction plate120due to engagement of roller weights118, an optional liner plate124of hardened steel or other durable material may be positioned between roller weights118and reaction plate120. In the embodiment shown inFIGS. 8 and 9, each roller weight118includes a shaft-like inner roller portion126, a cylindrical outer roller portion128and a bearing130positioned between inner and outer roller portions126,128. Bearing130is retained between inner and outer roller portions126,128by a pair of thrust washers132and134, such as a Teflon™ or steel thrust washer, each of which is secured in the illustrated position by a retaining member136, such as a snap ring. Once assembled, roller weights each exhibit a predetermined, yet substantially similar mass.

In the illustrated embodiment, bearing130includes a first needle bearing portion, such as a sealed heavy duty caged needle roller bearing manufactured by The Torrington Company having part number 101816, and a second needle bearing portion, such as a sealed full complement needle roller bearing manufactured by The Torrington Company having part number BH-1016. In a particular implementation of the invention, the first needle bearing portion is capable of a working load of about 2560 Lbf and a static load of about 4150 Lbf, and the second needle bearing portion is capable of a working load of about 5780 Lbf and a static load of about 10,300 Lbf. While a particular configuration of roller weight118has been shown and described in the illustrated embodiments, it will be appreciated that other configurations are within the scope of the present invention.

As shown inFIGS. 4–6, outer roller portion128of each roller weight118engages reaction plate120(or the optional liner plate124), while inner roller portion engages a ramp138on ramp plate122. Each ramp138may be integrally formed with ramp plate122or, alternatively, separately formed and attached thereto. In an embodiment, each inner roller portion126is engaged with a pair of adjacent ramps138(seeFIG. 7) that cooperatively support inner roller portion126such that outer roller portion128may freely roll therebetween. Each ramp138tapers radially outwardly and away from ramp plate122. In a particular configuration, ramps138include a single ramp surface140that tapers radially outwardly and away from ramp plate122at an increasing angle with respect to the ramp plate. For example, in the embodiment shown inFIG. 10, the surface140of each ramp138tapers from about 7 degrees adjacent its radially innermost point to about 13 degrees adjacent its radially outermost point. In another configuration, ramps138include a first ramp surface142and a second ramp surface144. In a representative embodiment shown inFIG. 11, first ramp surface142tapers at an increasing angle of around 10.5 degrees adjacent its radially innermost point to about 14 degrees adjacent its radially outermost point. Second ramp surface144tapers at an angle of about 5 degrees, which is relatively flatter than first ramp surface142.

Unlike the prior art clutch50shown inFIGS. 2 and 3, clutch30does not use coil return springs70to bias roller weights118toward their original or “disengaged” position shown inFIG. 4. Rather, clutch30includes a return spring member146, such as a diaphragm spring, which is mounted to the top of cover module110and engages an outer surface148of reaction plate120. In an embodiment, return spring member146has a height to thickness ratio (H/T ratio) of about 1.5. This ratio provides for a relatively consistent spring force over a long travel of spring member146. Return spring member146is held in position by a reaction member150, such as a generally cylindrical sleeve, which surrounds input shaft38. A first end of reaction member150includes a retaining member152, such as a retaining ring, for engaging a radially inner edge of return spring member146. A second end of reaction member150includes a lip154that engages ramp plate122. In operation, reaction member150is adapted to move axially with ramp plate122as clutch30is engaged and disengaged.

The spring force of return spring member146will react against reaction plate120and pull on ramp plate122through reaction member150to return the components to their disengaged positions. In the absence of centrifugal force, this “return force” will be applied through ramps138and will force roller weights118to remain at the bottom of ramps (seeFIG. 4). However, as the engine speed increases, centrifugal force will cause roller weights118to move radially outwardly over ramps138and overcome the preload of return spring member146. As the engine speed further increases, roller weights118will continue up ramps138and will force ramp plate122toward first friction disc assembly112(see upper section ofFIG. 6).

Clutch30may also include an installation device to facilitate installation of clutch30into drivetrain system20. In an embodiment, clutch installation device includes a generally cylindrical installation member156rotatably and/or axially disposed in cover module110radially inwardly of roller weights118. Installation member156includes a cam lobe158for engaging at least one of roller weights118during rotation and/or axial movement of installation member156relative to cover module110. Movement of cam lobe158forces the engaged roller weight118to roll outward and, accordingly, modify the position of the pressure plate, such that, when the cover module110is secured to the engine flywheel, the pressure plate provides a clamping force against friction disc assemblies112and116to inhibit movement thereof. Installation devices similar to that shown and described may be seen by reference to U.S. Pat. No. 6,609,602, which is owned by the assignee of the present invention and incorporated herein by reference in its entirety.

FIG. 12is a schematic illustration of the operational components of clutch30shown in fragments as rotating about the rotational axis160of input shaft38. As shown inFIG. 12, roller weights118are received between a substantially flat surface162of fixed reaction plate120and ramps138of the axially movable ramp plate122. Ramp plate122acts on an axially movable main pressure plate164through a preloaded plate spring member166, such as a diaphragm spring, which limits the axial force applied to main pressure plate164by ramp plate122. Main pressure plate164will apply a clamping force CF on friction pads168of friction disc assemblies112and116, which are trapped between main pressure plate164and intermediate pressure plate114and between intermediate pressure plate114and an engine flywheel170. Hub portions172and174of friction disc assemblies112and116, respectively, are adapted to be splined to input shaft38for rotation therewith while plates120,122,164, and114rotate with engine flywheel170.

In a disengaged state, roller weights118will engage the radially innermost portion of ramps138and will not apply a leftward axial clamping force to friction pads168. Spring members146,166bias roller weights118radially inwardly to rest on installation member156(seeFIG. 4). The biasing force of return spring member146is applied through reaction member150which pulls on ramp plate122in a direction toward the fixed reaction plate120. As engine flywheel170and cover module110rotate, the effect of centrifugal force will cause roller weights118to move against the biasing force of return spring member146from the disengaged position shown in the lower section ofFIG. 6toward the engaged position shown in the upper section ofFIG. 6. As the engine speed increases, roller weights118travel radially outwardly and an increasing axial clamping force is applied (see line175onFIG. 15). A radially inwardly facing surface176of reaction plate120limits the radially outward movement of roller weights118(seeFIG. 6).

The return spring force applied by spring members146,166is independent of roller weight position. Like the prior art clutch50, clutch30of the present invention functions based on a balance of forces. However, unlike the prior art clutch50, the return spring force imposed by spring members146,166in clutch30acts through all of roller weights118. If one or more roller weights118prematurely disengages before the other roller weights, the return force that was applied to the prematurely disengaged roller weight will be transferred to the remaining engaged rollers weights. This configuration provides additional return force to move the remaining roller weights118down ramps138, which disengages clutch30with less stagger of the individual roller weights118. In the prior art clutch50, the return spring force on a prematurely disengaged swing weight66is not applied to remaining engaged swing weights since each swing weight is biased by its own return spring(s).

Among other benefits, clutch30is inherently configured for extended wear life and exhibits a relatively consistent engagement point without the need for an adjustment system. These benefits are realized by properly configuring the ramp angle(s) and the spring load of return spring member146. Particularly, the return spring force provided by return spring member146acts through ramps138on ramp plate122. As roller weights118move progressively outward toward surface176of reaction plate120due to wear in friction pads168, roller weights118create additional clamping force. To compensate for this increased clamping force so that a consistent engagement point is achieved, the spring load of return spring member146may be progressively increased or the angle of ramps138may be progressively increased-either of which will increase the return force on roller weights118and resist radial outward movement of roller weights118. As will be appreciated, progressively increasing the angle at which ramps138taper, as shown inFIG. 10, will extend the wear life of the clutch assembly and provide for a relatively consistent engagement point without the need for the adjustment device required by the prior art.

FIGS. 13 and 14illustrate the effect of wear on the clamp force produced by clutch30configured with ramps138shown inFIG. 10.FIG. 13schematically illustrates the clamp force produced by a new clutch30as the engine speed increases andFIG. 14schematically illustrates the clamp force produced by clutch30with worn friction pads168(on the order of about 0.250 inches of wear). The clamp force produced by clutch30with worn friction pads is virtually identical to the clamp force produced by the new clutch30.

In addition to the benefits described above for the embodiment of ramps38shown inFIG. 10, additional benefits are realized with the embodiment of ramps138shown inFIG. 11. As engine speed is increased, roller weights118create a radial force that is transferred into a clamping force on ramps138as the roller weights move up first ramp surface142of ramps138. As the clamping force reaches the preload of plate spring member166, the axial force imposed on ramp plate122by outward movement of roller weights118will overcome the preload of plate spring member166and compress plate spring member166. When plate spring member166is compressed, roller weights188will continue to move radially outward onto second ramp surface144of ramps138, which is flatter that first ramp surface142. A greater centrifugal force is required to move roller weights118up first ramp surface142of ramps138and onto second ramp surface144than is required to retain roller weights118on second ramp surface142against the effect of the spring force imposed by spring members146,166. This accounts for the difference between the initial maximum clamping force engine speed value, point177inFIG. 15, and the release engine speed value, point179inFIG. 15.

Once clutch30is engaged, roller weights118are positioned on second surface144of ramps138and spring members146,166are compressed, the return spring force generated by spring members146,166is exerted on ramps138and roller weights118generally parallel to axis160. Both of these spring forces will act on roller weights118during disengagement to form a combined return spring force. During disengagement of clutch30, the engine speed is decreased and roller weights118will move from second ramp surface144of ramps138to first ramp surface142. If one or more of roller weights118disengages (i.e., moves from second ramp surface144to first ramp surface142) before the remaining roller weights118, the return spring force acting on all of the roller weights before disengagement is transferred to the remaining engaged roller weights118. Since there are fewer roller weights remaining on second ramp surface144, there is greater return force acting on each individual engaged roller weight118, which moves the remaining roller weights118from second ramp surface144to first ramp surface142. Once a roller weight118moves from second ramp surface144to first ramp surface142, there is a significantly reduced return spring force acting on that roller weight. This feature provides for a quieter disengagement of clutch30since there is no longer a significant return spring force acting on the disengaged roller weight to force the roller weight to impact installation member156. This feature is in contrast to the prior art swing weight66, which is continuously acted upon by the return spring force of coil springs70as the swing weight moves from surface102of ramp plate82to surface98and impacts stop72.

In a particular implementation of the present invention, first ramp surface142of ramps138exhibits a 10.5 degree ramp angle at a distance of approximately 3.3 inches from axis160and a 14 degree ramp angle at a distance of approximately 4.5 inches from axis160. Second ramp surface144of ramps138exhibits a 5 degree ramp angle at a distance of greater than about 4.5 inches from axis160. Return spring member146exhibits a spring load of approximately 2300 Lbf and plate spring member166exhibits a spring load of approximately 3500 Lbf. Additionally, various return straps (not shown) utilized to bias intermediate plate pressure plate114toward a disengaged position exhibit a combined spring load of about 175 Lbf. These angles and forces are approximate and it will be appreciated that slight variations are permissible given the features of the present invention. Thus, tolerances in the components are not as critical as they are in the prior art clutch50.

In the described implementation, the approximate load on roller weights118as clutch30transitions between various states of operation is as follows:

TABLE 2Load On All RollerWeights/Load onClutch StatePosition of Roller WeightsEach Roller WeightClutch10.0 degree ramp portion of first2300 Lbf/328 LbfDisengaged -ramp surface at a distance of aboutTouch Point2.9 inches from axis 160.Touch Point10.5 degree ramp portion of first2475 Lbf/353 Lbframp surface 142 at a distance ofabout 3.3 inches from axis 160.Clutch FullySecond ramp surface 144 at a5975 Lbf/854 LbfEngageddistance of about 4.8 inches fromaxis 160.

In the described implementation, system20is a heavy duty truck drivetrain, engine28is an electronically controlled diesel engine having an idle speed of about 600 RPM to 700 RPM, and a governed top speed of about 1800 RPM to 2000 RPM. Clutch30will move to incipient engagement at about 800 RPM, point181(ESIE), which is slightly above idle, and will have an increasing clamp load, line175, as engine speed increases. Clutch30will be fully engaged at or below a capped maximum clamp force, 5975 pounds, at about 1400 RPM, point177. Once at maximum clamp load, which is selected to lock-up clutch30under extreme conditions (i.e., substantially zero slip at considerably greater than expected torque loads), clutch30will remain locked-up, lines183and185, until the engine speed becomes less than about 970 RPM, point179. At the release point, clutch30will very rapidly disengage with decreasing engine speed, line187, to prevent engine stalling.

In the fully engaged state of the above described implementation of clutch30, the centrifugal force acting on each roller weight118is at least about 151 Lbf at an engine speed of about 967 RPM (which defines the line between engagement and disengagement of clutch30). When the engine speed drops below about 967 RPM, roller weights118will move from second ramp surface144of ramp138to first ramp surface142. Due to manufacturing tolerances in the clutch components, one or more roller weights118may prematurely move from second ramp surface144to first surface142before the remaining roller weights make the transition, resulting in a staggered disengagement of clutch30. As noted above, tolerances in the components are not as critical to the present invention given the discussed features of clutch30with respect to load redistribution. Table 3 below illustrates the various forces applied to roller weights118as one or more roller weights118prematurely transitions from second ramp surface144to first ramp surface142during disengagement of clutch30.

TABLE 3Number of Roller Weights76EngagedNumber of Roller Weights01DisengagedLoad On All Roller Weights59755975(Lbf)Load On Each Disengaged0607Roller Weight (Lbf)Load On Each Engaged854894Roller Weight (Lbf)Additional Return Force Per040Engaged Roller Weight (Lbf)
As shown in Table 3, the load on all of roller weights118collectively, due to the return roller force generated by roller members146,166and the return strap force, is the same regardless of whether one or more of roller weights118prematurely disengage. In an embodiment, since there are seven roller weights, the load on each engaged roller weight is about 854 Lbf (5975 Lbf/7 engaged roller weights) when all of the roller weights118are engaged. If one of roller weights118prematurely disengages from second ramp surface144and moves onto first ramp surface142, the disengaged roller weight is subjected to a lesser load than the engaged roller weights (e.g., 607 Lbf.) since there is still some centrifugal force acting on the roller weight on first ramp surface142. Because the return spring force generated by spring members146,166and the return strap load act on the engaged roller weights collectively, instead of individually as in the prior art clutch50, there is additional return force imposed on each of the remaining engaged roller weights118. In other words, the load on each remaining engaged roller weight118increases from about 854 Lbf to about 894 Lbf (5975 Lbf-607 Lbf/6 engaged roller weights). Thus, in the above embodiment, there is an additional 40 Lbf per roller weight118acting to force the remaining engaged roller weights118off the second ramp surface144.

In the prior art swing weight clutch50, the load on each engaged swing weight66remains the same, even as the one or more of its adjacent swing weights66prematurely disengages (see Table 1). However, in clutch30of the present invention, the load on each roller weight118increases as one or more of its adjacent roller weights118prematurely disengages (see Table 3). Therefore, unlike the prior art, the additional load acting on each engaged roller weight118will force the remaining roller weights118to move from second ramp surface144to first ramp surface142, resulting in a more consistent disengagement of clutch30.

As will also be appreciated, a greater number of roller weights118may be used in the clutch of the present invention, particularly when compared to the prior art clutch50. This feature is due in part to the more compact package exhibited by roller weights118as compared to swing weights66. In the embodiment of the invention shown inFIG. 7, a total of seven (7) roller weights were employed compared to four (4) swing weights used in the prior art clutch50, which is of comparable size to clutch30of the present invention. Because of the additional roller weights, a higher plate load is available for clutch30when compared to prior art clutch50.

Referring toFIG. 16, another embodiment of the invention is shown. In the illustrated embodiment, axial moveable plate122(ramp plate122in the embodiment ofFIG. 12) includes a plurality of relatively flat support members190that are positioned to support inner roller portions126of roller weights118in a manner similar to ramps138. Additionally, fixed plate120(reaction plate120in the embodiment ofFIG. 12) includes a plurality of ramps192positioned to engage outer roller portion128of roller weights118. Ramps192may be formed with fixed plate120or separately formed and attached thereto.

Ramps192may be configured with a single ramped surface or a pair of ramped surfaces, each of which is substantially similar in configuration to the ramped surfaces illustrated inFIGS. 10 and 11. Particularly, ramps192may include a single ramped surface tapering radially outwardly and away from fixed plate120at an increasing angle with respect to the fixed plate. Alternatively, ramps192may include first and second ramp surfaces that taper radially outwardly and away from fixed plate120at different angles with respect to the fixed plate. In contrast to the embodiment of clutch30shownFIG. 12, fixed plate120functions as a fixed ramp plate and the axially movable plate122functions as an axial movable reaction plate. Thus, as roller weights118move outwardly under the effects of centrifugal force, outer roller portion128of roller weights118rolls over ramps192, causing inner roller portion126to react against support members190and thereby move plate122. Further operation of the embodiment of clutch30shown inFIG. 16, and the function of its components beyond the differences described herein, is substantially similar to that described above in reference toFIG. 12and will not be described in further detail.

Referring toFIGS. 17 and 18, yet another embodiment of clutch30is shown. In the illustrated embodiment, the diaphragm return spring member146and reaction member150shown inFIGS. 4–6and12are replaced with a plurality of spring biased return members194positioned between roller weights118and configured to extend axially between moveable plate122and fixed plate120. In an embodiment, return members194each include a return spring196, such as a coil spring, a spring sleeve197and a reaction member198, such as a bolt. Spring sleeve197is secured to fixed plate120and reaction member198is secured to axially moveable plate122. Return spring196is positioned between an inner surface of spring sleeve197and a shoulder on reaction member198, and is preloaded to provide a biasing force in the disengaged position shown inFIG. 17. A stop member199may be positioned radially inwardly of roller weights118in place of reaction member150to support roller weights118in the disengaged position shown inFIGS. 17 and 18.

As roller weights118move outwardly under the effects of centrifugal force, movement of axial plate122and reaction member198will cause return springs196to compress and apply a return force to plate122through reaction member198that is generally parallel to axis160. Further operation of the embodiment of clutch30shown inFIGS. 17 and 18is substantially similar to that described above in reference toFIGS. 4–6and12and will not be described in detail.

The present invention has been particularly shown and described with reference to the foregoing embodiments, which are merely illustrative of the best modes for carrying out the invention. It should be understood by those skilled in the art that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention without departing from the spirit and scope of the invention as defined in the following claims. It is intended that the following claims define the scope of the invention and that the method and apparatus within the scope of these claims and their equivalents be covered thereby. This description of the invention should be understood to include all novel and non-obvious combinations of elements described herein, and claims may be presented in this or a later application to any novel and non-obvious combination of these elements. Moreover, the foregoing embodiments are illustrative, and no single feature or element is essential to all possible combinations that may be claimed in this or a later application.