Abstract:
A clutch for a lift fan comprises at least one output shaft lug key positioned in a keyway of at least two output clutch plates and extending axially from one to the other of the two output clutch plates. The output shaft lug key couples the rotation of the output clutch plates to the rotation an output shaft and allows axial movement of the two output clutch plates relative the output shaft lug key. The width of an inner surface of the output shaft lug key varies along the axial length as a function of the proximity to a home axial position of each of the output clutch plates when in the disengaged position.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority to U.S. Provisional Patent Application No. 62/304,811, filed Mar. 7, 2016, the entirety of which is hereby incorporated by reference. 
     
    
     GOVERNMENT RIGHTS 
       [0002]    The present application was made with the United States government support under Contract No. N00019-02-C-3003, awarded by the Joint Program Office. The United States government has certain rights in the present application. 
     
    
     BACKGROUND 
       [0003]    With reference to  FIG. 1 , there is illustrated a schematic representation of one form of an aircraft engine  50  used as a power plant for an aircraft  52 . In one form the aircraft engine  50  can be a gas turbine engine having a variety of forms including turbo jet, turbo fan, and turbo shaft, to set forth just a few non-limiting examples. The aircraft  52  includes a lift fan  54  capable of producing a vertical lift force useful for providing hovering or short takeoff and vertical landing operations, among other uses. As used herein, the term “aircraft” includes, but is not limited to, helicopters, airplanes, unmanned space vehicles, fixed wing vehicles, variable wing vehicles, rotary wing vehicles, unmanned combat aerial vehicles, tailless aircraft, hover crafts, and other airborne and/or extraterrestrial (spacecraft) vehicles. Further, the present inventions are contemplated for utilization in other applications that may not be coupled with an aircraft such as, for example, industrial applications, power generation, pumping sets, naval propulsion and other applications known to one of ordinary skill in the art. 
         [0004]    A clutch  56  is depicted in  FIG. 1  and is used to selectively couple the aircraft engine  50  to lift fan  54 . In the illustrative form the clutch  56  is depicted as coupling an input shaft  58  from the aircraft engine  50  to an output shaft  60  of the lift fan  54 . In some embodiments the input shaft  58  can be directly coupled to a spool shaft (not depicted) of the aircraft engine  50 , but in other embodiments the input shaft  58  can be coupled through any combination of lay shafts, clutches, and/or gearboxes, to set forth just a few non-limiting examples. Furthermore, the output shaft  60  can be coupled to a fan rotor and/or shaft (not shown) of the lift fan  54  through any combination of lay shafts, clutches, and/or gearboxes, to set forth just a few non-limiting examples. The clutch  56  of the illustrative embodiment is a multi-plate clutch having friction surfaces coupled at an inner diameter to one shaft, and friction surfaces coupled at an outer diameter to the other shaft. Further details are provided below. 
         [0005]    Turning now to  FIGS. 2 and 3 , perspective views are shown of an input shaft  58  and an output shaft  60  coupled with respective input clutch plates  62  and  64  and output clutch plates  77 ,  78  and  80 .  FIG. 4  is a composite illustration of features depicted in  FIGS. 2 and 3 . The input shaft  58  is arranged coaxially with the output shaft  60 . In the illustrative form the input shaft  58  is smaller in diameter than the output shaft  60  and is thus located radially inward of the output shaft  60 . The terms “input” and “output” are not intended to be limiting regarding the use of the shafts. For example, the input shaft  58  can be used to drive the lift fan  54  while the output shaft  60  can be driven by the aircraft engine  50 . In these forms the shaft  60  can be used to drive the lift fan  54  and the shaft  58  can be driven by the aircraft engine  50 . 
         [0006]    In one form the input shaft  58  includes input clutch plates  62  and  64 . The input clutch plates  62  and  64  are slidably engaged with the input shaft  58  and are operable to be engaged with corresponding plates associated with the output shaft  60 . The input clutch plates  62  and  64  include friction surfaces on one or more of its sides  74  and  76 , respectively. The friction surfaces of the sides  74  and/or  76  can be any surface having attributes associated with abradable surfaces or wear surfaces in the brake, clutch, and/or transmission arts such as, but not limited to, toughness, strength, heat resistance, adequate frictional properties, and/or relatively long life. In some forms the friction surfaces can be textured, roughened, and/or grooved. The friction surfaces can be made from a variety of materials including, but not limited to, steel, bronze, iron, iron-bronze, ceramic, metallic ceramic, graphitic carbon and metallic graphite. Though only two input clutch plates  62  and  64  are depicted in the illustrative embodiment, any number of input clutch plates can be used in other embodiments. 
         [0007]    The output clutch plates  77 ,  78 , and  80  are operable to be engaged with the input clutch plates  62  and  64 . The output clutch plates  77 ,  78 , and  80  include friction surfaces on one or more of its sides  90 ,  92 , and  94 , respectively. The outer periphery of the output clutch plates is axially recessed from the friction surfaces on the sides. The friction surfaces of the sides  90 ,  92 , and/or  94  can be any surface having attributes associated with abradable surfaces or wear surfaces in the brake, clutch, and/or transmission arts such as, but not limited to, toughness, strength, heat resistance, adequate frictional properties, and/or relatively long life. In some forms the friction surfaces can be textured, roughened, and/or grooved. The friction surfaces can be made from a variety of materials including, but not limited to, steel, bronze, iron, iron-bronze, ceramic, metallic ceramic, carbon fiber, graphitic carbon and metallic graphite. Though only three output clutch plates  77 ,  78 , and  80  are depicted in the illustrative embodiment, any number of output clutch plates can be used in other embodiments. 
         [0008]    The output shaft lug key  88  is coupled to the output shaft  60  through attachment members  96  and  98 . The attachment members  96  and  98  can be bolts threadingly received in apertures formed in the output shaft lug key  88 , but other forms of attachment members are contemplated herein. To set forth just one non-limiting example, the attachment members  96  and  98  can be fasteners such as rivets. 
         [0009]    The output shaft lug key  88  maintains the relative clocked orientation of the output clutch plates  77 ,  78 , and  80  and is received in the illustrative embodiment by a keyway  100  formed by cutouts  101  in the outer periphery of the output clutch plates  77 ,  78 , and  80 . In other embodiments the output shaft lug key  88  can be received by apertures formed radially inward of the outer periphery of the output clutch plates  77 ,  78 , and  80 . Although one output shaft lug key  88  is depicted, multiple lug keys  88  can be used. The cutouts  101  need not be identical in each of the output clutch plates  77 ,  78 , and  80 . In addition, the output shaft lug key  88  need not have the same shape along its length. The output shaft lug key  88  permits the output clutch plates  77  and  78  to slide relative to the output shaft lug key  88 . 
         [0010]    In a first, uncoupled state (disengaged), the input shaft  58  and the output shaft  60  are not coupled by the clutch  56 . The output clutch plates  77 ,  78 , and  80  are in an uncompressed state and do not contact, or partially contact, the input clutch plates  62  and  64 . The actuator  104  is withdrawn and the output clutch plates  77 ,  78  and  80  axially separate naturally due to non-synchronous rotational speeds when left in the uncompressed state. Output clutch plates  77 ,  78  and  80  separate away from the other plates and out of contact with the input clutch plate  62 . 
         [0011]    In a second, coupled state (engaged), the actuator  104  provides a force that moves a pressure plate (not shown) axially in the direction of plate  77  and continues moving plate  77  such that it and all clutch plates  77 ,  78 ,  80 ,  62  and  64  are compressed and synchronized. Compression of the output clutch plate  77  with the input clutch plate  62 , the output clutch plate  78  with the input clutch plates  62  and  64 , and the input clutch plate  64  with the output clutch plate  80  engages the friction surfaces of the sides  74 ,  76 ,  90 ,  92 , and  94  to couple rotation of the input shaft  58  with the output shaft  60 . 
         [0012]    A more detailed discussion of a clutch, clutch plates and their operation may be found in U.S. Pat. No. 8,567,713, the entirety of which is incorporated by reference. 
         [0013]    It has been observed that when the lift fan clutch system with interleaved sets of plates is in the disengaged or uncoupled position, the output clutch plates  77 ,  78 , and  80  can, under certain circumstances, tend to flutter, wobble, or have other gyroscopic instability. Such instability may suddenly cause a relatively large increase in drag torque, heat and wear at the keyway  100  which exacerbates the vibration, flutter, wobble or gyroscopic instability. This excessive wear is believed to stem from axial movement, off center plates, tilting plates, and/or interaction between keys and keyways associated with free floating output clutch plates  77 ,  78  and  80  as described above. 
         [0014]    In order to obviate the deleterious effects described above, the disclosed subject matter provides identifies several types of mitigating systems and methods. A set of solutions are directed to preventing the output clutch plates from falling off center, another set of solutions are directed to maintaining the proper axial position of the output clutch plates during disengagement, others to reducing wear. All the solutions are ultimately directed to preventing an unbalanced condition developing in the output clutch plates and the deleterious resultant vibration, flutter, and wobble. 
         [0015]    These and many other objects and advantages of the present subject matter will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of preferred embodiments. 
       SUMMARY 
       [0016]    According to an aspect of the present disclosure, a clutch for a lift fan comprises a rotatable input shaft and a rotatable output shaft having a common axis of rotation; two substantially disc shaped output clutch plates oriented normal to the axis, each of the two output clutch plates having a keyway extending radially inward from a mouth at an outer diameter of the output clutch plate to a seat at an inner diameter of the output clutch plate; at least one output shaft lug key positioned in the keyway of the two output clutch plates and extending axially from one of the two output clutch plates to the other of the two output clutch plates, the output shaft lug key coupling the rotation of the output clutch plates to the rotation the output shaft and allowing axial movement of the two output clutch plates relative the output shaft lug key, the output shaft having an outer radial surface and an oppositely disposed inner radial surface; at least one input clutch plate positioned axially between the two output clutch plates, wherein the input clutch plate is rotationally fixed to the input shaft. In an engaged position, the two output clutch plates are in frictional communication with the input clutch plates and a load is transferred from the input shaft to the output shaft via the two output clutch plates, the input clutch plates and the output shaft lug key. In a disengaged position, each of the two output clutch plates are axially displaced from input clutch plate. The width of the inner surface of the output shaft lug key varies along the axial length as a function of the proximity to a home axial position of each of the output clutch plates when in the disengaged position. 
         [0017]    In some embodiments the width of the inner surface is at a maximum between the home axial position of adjacent output clutch plates. In some embodiments the width of the inner surface is at a minimum at each of the respective home positions of the output clutch plates. In some embodiments each of the output clutch plates have an engaged axial position different from their respective home position. In some embodiments the lug key fully fills the keyway. 
         [0018]    In some embodiments the inner surface of the output shaft lug key is semi-circular and corresponds to a semi-circular keyway seat. In some embodiments the inner surface of the output shaft lug key is planar and corresponds to a planer keyway seat. In some embodiments the cross section of the inner surface of the shaft lug key and the cross section of the seat of the keyway is selected from the group consisting of a trapezoidal, parabolic, rectangular, triangular, chinned and polygonal shape. 
         [0019]    According to another aspect of the present disclosure, a clutch for a lift fan comprises a rotatable input shaft and a rotatable output shaft having a common axis of rotation; two substantially disc shaped output clutch plates oriented normal to the axis, each of the two output clutch plates having a keyway extending radially inward from a mouth at an outer diameter of the output clutch plate to a seat at an inner diameter of the output clutch plate; at least one output shaft lug key positioned in the keyway of the two output clutch plates and extending axially from one of the two output clutch plates to the other of the two output clutch plates, the output shaft lug key coupling the rotation of the output clutch plates to the rotation the output shaft and allowing axial movement of the two output clutch plates relative the output shaft lug key, the output shaft having an outer radial surface and an oppositely disposed inner radial surface; at least one input clutch plate positioned axially between the two output clutch plates, wherein the input clutch plate is rotationally fixed to the input shaft. In an engaged position, the two output clutch plates are in frictional communication with the input clutch plates and a load is transferred from the input shaft to the output shaft via the two output clutch plates, the input clutch plates and the output shaft lug key. In a disengaged position, each of the two output clutch plates are axially displaced from input clutch plate. The radial depth of the output shaft lug key varies along the axial length as a function of the proximity to the home axial position of each of the output clutch plates when in the disengaged position. 
         [0020]    In some embodiments the depth of the inner surface is at a maximum between the home axial position of adjacent output clutch plates. In some embodiments the depth of the inner surface is at a minimum at each of the respective home positions of the output clutch plates. In some embodiments each of the output clutch plates have an engaged axial position different from their respective home position. In some embodiments the lug key fully fills the keyway. 
         [0021]    According to another aspect of the present disclosure, a clutch for a lift fan comprises a rotatable input shaft and a rotatable output shaft having a common axis of rotation; two substantially disc shaped output clutch plates oriented normal to the axis, each of the two output clutch plates having a keyway extending radially inward from a mouth at an outer diameter of the output clutch plate to a seat at an inner diameter of the output clutch plate; at least one output shaft lug key positioned in the keyway of the two output clutch plates and extending axially from one of the two output clutch plates to the other of the two output clutch plates, the output shaft lug key coupling the rotation of the output clutch plates to the rotation the output shaft and allowing axial movement of the two output clutch plates relative the output shaft lug key, the output shaft having an outer radial surface and an oppositely disposed inner radial surface; at least one input clutch plate positioned axially between the two output clutch plates, wherein the input clutch plate is rotationally fixed to the input shaft. In an engaged position, the two output clutch plates are in frictional communication with the input clutch plates and a load is transferred from the input shaft to the output shaft via the two output clutch plates, the input clutch plates and the output shaft lug key. In a disengaged position, each of the two output clutch plates are axially displaced from the input clutch plate. A plurality of tabs extends radially from the outer diameter of the output clutch plates, the plurality of tabs circumferentially distributed around the outer diameter of the output clutch plates. 
         [0022]    In some embodiments the clutch further comprises a second key way, wherein at least one of the plurality of tabs is located circumferentially midway between the key way and the second keyway. In some embodiments the tab extends from the outer periphery of the output clutch plate to an inter surface of the output shaft. In some embodiments the tab is integral to the output clutch plate. 
         [0023]    In some embodiments the tab is metallic. In some embodiments the tab is configured to serve as a stand-off to the output clutch plate. In some embodiments the interaction of the tab with the clutch plate restricts motion of the clutch plate to an off center position. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0024]      FIG. 1  is a schematic of an aircraft with an illustrative lift fan clutch. 
           [0025]      FIG. 2  depicts an input shaft and input clutch plates of an illustrative lift fan clutch. 
           [0026]      FIG. 3  depicts a partial perspective view of illustrative input clutch plates and output clutch plates. 
           [0027]      FIG. 4  depicts an illustration of the illustrative lift fan clutch depicted in  FIGS. 2 and 3 . 
           [0028]      FIGS. 5 a  and 5 b    illustrate an output shaft lug key according to an embodiment of the present subject matter. 
           [0029]      FIG. 6  is an illustration of retaining fingers according to an embodiment of the present subject matter. 
           [0030]      FIG. 7  is an output shaft lug key and output clutch plate keyway according to an embodiment of the present subject matter. 
           [0031]      FIGS. 8 a  and 8 b    are illustrations of a lug key spline according to an embodiment of the present subject matter. 
           [0032]      FIG. 9  is a detent system for output clutch plates according to an embodiment of the present subject matter. 
           [0033]      FIG. 10  is an output clutch plate keyway with a coating of wear resistance material according to an embodiment of the present subject matter. 
           [0034]      FIGS. 11 a  and 11 b    are illustrations of a shaft lug key spacer plate according to an embodiment of the present subject matter. 
           [0035]      FIGS. 12 a  and 12 b    are illustrations of an output clutch plate keyway clip according to an embodiment of the present subject matter. 
           [0036]      FIG. 13  is an output clutch plate with a tab according to an embodiment of the present subject matter. 
           [0037]      FIG. 14  is an output clutch plate keyway channel flange according to an embodiment of the present subject matter. 
       
    
    
     DETAILED DESCRIPTION 
       [0038]    For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Any alterations and further modifications in the described embodiments, and any further applications of the principles of the invention as described herein are contemplated as would normally occur to one skilled in the art to which the invention relates. 
         [0039]    One solution to prevent vibration, wobble, and instability of the output clutch plates  77 ,  78 , and  80  is to dampen the response of the output clutch plates  77 ,  78 , and  80  such that wear of the keyway  100  is prevented. There are several approaches to restrict the movement of output clutch plates  77 ,  78 , and  80 . One approach is to restrain the output clutch plates  77 ,  78 , and  80  from axial movement in the disengaged clutch position. 
         [0040]      FIG. 5 a    illustrates a shaft lug key  88  with an outer radial surface  501  and an inner radial surface  503 . The shaft lug key  88  is received by the keyway  100  as shown in  FIG. 5 b   . The keyway  100  in the illustrative embodiment has a circular seat  505  shape to receive the inner radial surface  503 . With the shaft lug key  88  fully filling the keyway  100 , axial and radial movement, as well as wobble, are damped. The shaft lug key  88  of  FIGS. 5 a  and 5 b   , as a result of extending to the seat  505  of the keyway  100  has an increased mass which in turn may be minimized by removing material on the outer surface  501  as shown by recess  507 . 
         [0041]    Engagement protrusions  509  restrict the output clutch plates  77 ,  78 , and  80  from being able to move off center. 
         [0042]    The cross section of the inner surface  503  of the shaft lug key  88  and its receiving keyway  100  may also be of a trapezoidal, parabolic, rectangular, triangular or polygonal shape. The relevant characteristic with respect to this embodiment is that the lug key  88  substantially fills the keyway  100  to limit axial, radial, and tilting movement when the clutch is not engaged. 
         [0043]    Another embodiment of the present subject matter is the implementation of stainless steel fingers  601 ,  602  and  603  or springs on the inner surface  503  of the shaft lug key  88 . The fingers  601 ,  602  and  603  would dampen any axial movement as well as push the output clutch plates  77 ,  78  and  80  to a “neutral” or home position when the clutch is disengaged. The home position is the preferred location of the output clutch plates  77 ,  78 , and  80  when the clutch is disengaged. 
         [0044]    The existing bolts that mount the lug key  88  to the output shaft  60  may be used to mount the fingers  601 ,  602  and  603 . The lug key inner surface  503  may be modified to allow the fingers to be imbedded within the key  88  and may offset any additional weight added by the fingers  601 ,  602  and  603  by removing stock from the lug key  88 . 
         [0045]    The fingers  601 ,  602  and  603  would extend between the inner surface of the lug key through the radial gap  611  between the seat  505  of the keyway  100  and the lug key  88  as shown in  FIG. 6 . The shape of the fingers  601 ,  602  and  603  may be dependent upon the position of the output clutch plates  77 ,  78  and  80 . The fingers  601 ,  602  and  603  impede axial and radial movement when the clutch is in the disengaged position. Each finger  601 ,  602  and  603  has a connection point  610 , an elbow  620  and an arm  630  angled with respect to the clutch plate to urge the plate forward when in the disengaged position as shown in  FIG. 6 . Similarly, the clutch plate  80  is urged backwards to its home position and the middle clutch plate  78  is urged to its middle home position by the respective fingers  601 ,  602  and  603 . The fingers  601 ,  602  and  603  are configured to allow the axial movement required to engage the clutch, but otherwise dampen unwanted axial movement. 
         [0046]    Another approach is shown in  FIG. 7 , where the keyway  100  is designed to have a slightly angled sides such that the keyway  100  has a trapezoidal cross-section. For example, a 7 degree taper may be used on the keyway  100  and the inner surface  503  of the key  88 . In some embodiments the mouth  511  of the keyway  100  is wider than the seat  505  of the keyway  100  and the outer surface  501  of the lug key  88  is greater than the base of the inner surface  503  of the keyway  100 . Any torque transmitted to the shaft lug key  88  applies a centering force to the clutch plate  77 . Any wear in the keyway  100  allows the clutch plate  77  to move radially with respect to the lug key  88  but a centering force is still applied to the plate  77 . 
         [0047]      FIG. 8 a    shows another embodiment of the subject matter. The inner surface  503  of the lug key  88  is configured axially as a spline to encourage the plates  77 ,  78 ,  80  to stay in their home region when the clutch is not engaged. The lug key  88  increases its width between the home regions of the plates  77 ,  78 ,  80  and as shown in  FIG. 8 a    the width of the inner surface  503  of the lug key  88  is narrowest at the clutch plate&#39;s home location. Alternatively as shown in  FIG. 8 b   , the depth as the inner surface  503  extends radially may also be varied axially, such that the inner surface  503  extends radially further between the home location and is shallowest in the home locations of the clutch plates  77 ,  78 ,  80 . The interaction of the clutch plates  77 ,  78 ,  80  and the lug key  88  given its shape will urge the clutch plates to the home position when the clutch is not engaged and retain the plates  77 ,  78 ,  80  in their respective homes, while allowing axial movement required for engaging the clutch. 
         [0048]      FIG. 9  illustrates a detent system that may be employed to retain the clutch plates  77 ,  78 ,  80  at their home position while the clutch is disengaged. A detent is a device (as a catch, dog, or spring-operated ball) for positioning and holding one mechanical part in relation to another in a manner such that the device can be released by force applied to one of the parts. A spring  903  is retained within the shaft lug key  88  and provides a force on ball  901 . The ball  901  in the embodiment shown acts as an urger. The spring  903  may also take the form of an elastic material. The keyway  100  in output clutch plates  77 ,  78  and  80  is shaped to provide a recess  905  or indentation that cooperates with the ball  901  to position and retain the respective clutch plates  77 ,  78 ,  80  in their home positions. When in the engaged position the ball  901  is forced up and out of the way to allow the output clutch plates  77 ,  78 ,  80  to engage with the input clutch plates  62 ,  64 . It is also envisioned that the detent system may be reversed with the ball  901  and spring  903  in the clutch plate and the recess  905  in the inner surface  503  of the shaft lug key  88 . 
         [0049]    Another approach to prevent wear and subsequent imbalance of the output clutch plates  77 ,  78 ,  80  is the addition of a coating to the shaft lug key interface surface (i.e. the surface of keyway  100 ) of the output clutch plates  77 ,  78 ,  80 . A ceramic or similar style barrier coating could add wear resistance to the keyway surface of the output clutch plates that are typically made of relatively brittle carbon. A silicon carbine coating or other oxidant coating would protect the underlying material and prevent imbalance as a result of removal of the underlying material. In  FIG. 10 , the coating is shown as being applied to the seat surfaces  1100  of the output clutch plate keyway  100 . The coatings are preferably selected from carbides, ceramics, cerametatillic and other high temperature and wear resistant material. Additionally, the coating may also be applied to the face of the clutch plates proximate the keyway  100 . The area proximate the keyway  100  face does not frictionally engage with the other plates and thus a coating, while protecting from wear induced from interfaces with the shaft lug key  88 , will not interfere with the engagement of the output plates  77 ,  78  and  80  with the input plates  62  and  64 . 
         [0050]      FIGS. 11 a  and 11 b    illustrate an output shaft lug key  88  with a spacer  1501  positioned between the output shaft  60  and the lug key  88 . The spacer  1501  takes up additional radial space between the output shaft  60  and the periphery of the output clutch plates  77 ,  78  and  80 . The spacers  1501  serve to limit the radial movement of the output clutch plates  77 ,  78  and  80  and thus limit damage that may occur if the plates  77 ,  78  and  80  were to contact the output shaft  60 .  FIG. 11 b    shows the spacer  1501  with a curved shape to fit securely against the inner periphery  1111  of the output shaft  60  and holes  1510  which correspond to the bolts used to secure the output shaft lug key  88 . As shown in  FIG. 11 a   , the spacers  1501  are distributed around the inner periphery  1111  of the output shaft  60  wherever an output shaft lug key  88  is positioned. The spacers  1501  may be constructed of a non-abrasive material to avoid damaging the periphery of the output clutch plates  77 ,  78  and  80  when in contact, or alternatively may be made of a sacrificial material to also avoid damaging the output clutch plates  77 ,  78  and  80 . 
         [0051]    Another approach to reduce wear is illustrated in  FIGS. 12 a  and 12 b   . A U-shaped metallic clip  1205  is attached to the output clutch plate  77  and the keyway  100  is defined by the clip  1205 . Because the clip  1205  is formed of a metallic material, such as steel, titanium, etc. it wears slower than the material from which the output clutch plates  77 ,  78  and  80  are constructed. The metallic clip  1205  is attached by rivets  1207  or other appropriate fastening means, with the rivets  1207  passing through slots  1209  in the metallic clip  1305  and corresponding holes  1210  in the output clutch plate  77 ,  78  and  80  as shown in  FIGS. 12 a  and 12 b   , or the rivets  1207  may pass through holes in the clips and slots in the output clutch plates  77 . This latter configuration is not shown in  FIGS. 12 a  and 12 b   . The slots  1209  either in the metallic clip  1205  or in the output clutch plates  78  allow the clip  1205  to slide radially inward/outward as needed to occupy any available free-space. This would provide a metal contact to prevent the keyway  100  from wearing. Importantly the slots  1209  allow radial movement so that the radial load would be reacted by pressing against the corresponding lug key  88  rather than pulling on the brittle clutch plates which may have limited strength margins depending on the material, which is necessary in high speed clutches that have plates with large diameters. As shown in  FIG. 12 b   , the metallic clip  1205  encompasses the recessed outer periphery of the output clutch plate  77 . Alternatively, the metallic clip  1205  may be attached via adhesive, crimping or bolting to the output clutch plate  77 ; however this alternative may unduly radially load the output clutch plate  77 . 
         [0052]      FIG. 13  shows a plurality of tabs  1310  extending from the outer periphery of the output clutch plate  77 . The tabs  1310  serve as a stand-off to the clutch plates. The tabs  1310  occupy the free space outboard of the clutch plates  77 ,  78  and  80 , preventing tilting of the clutch plates  77 ,  78  and  80 . The tabs  1310  interact with the output shaft  60  restrict the clutch plate  77 ,  78  and  80  from being able to move off center and thus keep the clutch plates  77 ,  78  and  80  balanced within the desired operating limits. These tabs  1310  may also serve as sacrificial tabs to prevent damage to the frictional surfaces of the clutch plate  77 . The tabs  1310  may be distributed around the outer periphery of the output clutch plate  77  and may be positioned with respect to the keyways  100 . For example, the tabs  1310  may be placed circumferentially between adjacent keyways  100 , or as shown on either side of the keyway  100 . The tabs  1310  may be integral to the clutch plates  77 ,  78  and  80  or made of another material such as metal that exhibits better wear characteristics. The tabs  1310  may also be arranged in pairs and attached to opposite axial faces of the outer periphery of the clutch plate  77 . The use of tab pairs will further limit pitch and yaw of the clutch plate  77 . 
         [0053]    A final approach to preventing wear and imbalance is shown in  FIG. 14 . Metal clips  1401  received in the outer periphery of the clutch plate  77  define the keyway  100 . The metal clips  1401  extend axially forward and aft of the clutch plate  77 . These axially longer metal clips  1401  prevent the clutch plate  77  from tilting. By limiting the axial movement due to tilting, the metallic clip can help reduce the amount of wear on the output clutch plate  77 . In addition, by restricting tilting of the output clutch plate  77 , wobbling and imbalance of the clutch plates  77 ,  78  and  80  are also dampened. As shown in  FIG. 14 , the metallic clip  1401  forms an elongated channel that is adapted to receive the shaft lug key  88  and extends axially along a shaft lug key  88  which increases the contact area of the interface between the keyway  100  and the lug key  88  and thus reduces stress concentrations and more effectively resists axial movement than key/keyway interfaces with less contact area. The extended metallic clips  1401  reduce pitching, yawing and axial translation of the output clutch plates  77 ,  78  and  80 . Because of the possibility of increased torque transmitted from the metal clips  1401  to the periphery of the clutch plates  77 ,  78  and  80 , addition flanges may extend circumferentially from the metal clip  1401  in order to attach the clip  1401  to the clutch plates  77 ,  78  and  80  in a manner similar to that shown in  FIGS. 12 a  and 12 b   . The flanges may reduce localized loads and distribute the loads over a greater area. The axial length of the metallic clips  1401  are a function of the axial thickness of the adjacent input clutch plates and the proximity of the output clutch plates  77 ,  78  and  80  to the axial bounds of the clutch. The metallic clips  1410  may take the form of a C-channel as shown in  FIG. 14 . The clip  1410  may also be tapered radially as it extends axially, the reducing the weight load of the clip  1410 . 
         [0054]    While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiments have been shown and described and that all changes and modifications that come within the spirit of the inventions are desired to be protected. It should be understood that while the use of words such as preferable, preferably, preferred or more preferred utilized in the description above indicate that the feature so described may be more desirable, it nonetheless may not be necessary and embodiments lacking the same may be contemplated as within the scope of the invention, the scope being defined by the claims that follow. In reading the claims, it is intended that when words such as “a,” “an,” “at least one,” or “at least one portion” are used there is no intention to limit the claim to only one item unless specifically stated to the contrary in the claim. When the language “at least a portion” and/or “a portion” is used the item can include a portion and/or the entire item unless specifically stated to the contrary.