Abstract:
A composite clutch shaft assembly has an annular hub with a shaft extending therefrom. The shaft has a generally tubular outer portion and a core portion. The core portion has higher internal damping than the outer portion, thereby improving the vibration and noise damping characteristics of the clutch shaft while having little or no effect on the external dimensions of the clutch shaft.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to torque transferring devices used within vehicle transmissions, and, more particularly, to a composite clutch shaft assembly.  
       BACKGROUND OF THE INVENTION  
       [0002]     Clutch shafts for use in vehicle transmissions are well known in the art. Generally, a shaft extends from a clutch hub and supports a gear. A clutch selectively engages teeth formed within the clutch hub to selectively transfer torque between the clutch and the gear. The vehicle transmission may experience an audible noise or “squawk” when the clutch is applied or released at elevated temperatures. This “squawk” may be a result of instability of the clutch hub and shaft system. The spring rate and inertia of the system may be such that the shaft behaves as a one degree of freedom system, with the clutch hub and clutch plates acting as the inertia and the output end of the shaft acting as the ground.  
         [0003]     The hub is subject to the friction force of the slipping clutch, which can exhibit a negative coefficient of friction versus slip speed characteristic when the clutch becomes hot, aged, or subject to high unit loading. This negative friction slope emulates negative damping, which may cause the one degree of freedom system to become unstable if the negative slope and the positive internal damping of the shaft sum to a negative value. In such situations, the oscillation of the hub (inertia) across the shaft (spring) will increase exponentially until a non-linearity is encountered. Such non-linearities may be that the clutch plate splines no longer contact the hub or that the rotational velocity of the clutch moves the friction characteristics out of the negative slip zone. Engineers have improved “squawk” characteristics in the past by increasing the diameter of the shaft, increasing the inertia of the hub, increasing heat extraction from the clutch pack, increasing clutch surface area, and/or the addition of a damper. The damper may be either a coulomb type or a tuned mass damper.  
       SUMMARY OF THE INVENTION  
       [0004]     Provided is a composite clutch shaft having a hub and a shaft extending from the hub. The shaft has a core portion disposed within a generally tubular outer portion. The core portion has a higher internal damping characteristic than the outer portion. The core portion may be either solid or hollow. Additionally the core portion may be formed from grey iron and press fit into the outer portion which may be formed from steel. The core portion may extend substantially the entire length of the shaft.  
         [0005]     Also provided is a composite clutch shaft including a hub having a shaft extending therefrom. The shaft is formed from a heat treatable material and the shaft has a heat treated outer portion and a non-heat treated core portion. The non-heat treated core portion has a higher internal damping characteristic than the heat treated outer portion.  
         [0006]     The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  is a sectional perspective view of a composite clutch shaft assembly broken away to show a core portion within an outer portion of a shaft portion according to the present invention;  
         [0008]      FIG. 2  is a side view of the composite clutch shaft assembly of  FIG. 1 , showing the core portion in phantom;  
         [0009]      FIG. 3   a  is a schematic cross sectional view of one embodiment of the shaft portion of the composite clutch shaft assembly taken along line A-A of  FIG. 2 ;  
         [0010]      FIG. 3   b  is a schematic cross sectional view of a second embodiment of the shaft portion of the composite clutch shaft assembly also taken along line A-A of  FIG. 2 ;  
         [0011]      FIG. 4   a  is a schematic cross sectional view of a third embodiment of the shaft portion of the composite clutch shaft assembly also taken along line A-A of  FIG. 2 ; and  
         [0012]      FIG. 4   b  is a schematic cross sectional view of a fourth embodiment of the shaft portion of the composite clutch shaft assembly also taken along line A-A of  FIG. 2 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0013]     Referring to  FIGS. 1 and 2 , a composite clutch shaft or composite clutch shaft assembly according to the present invention is shown at  10 . The composite clutch shaft assembly  10  comprises an annular hub or hub portion  12  with a shaft or shaft portion  14  extending therefrom. The hub  12  and the shaft  14  may be formed integrally as a single piece or may be two pieces joined together. In the preferred embodiments, the shaft assembly  10  is a fourth clutch shaft for a vehicle transmission. As such, the hub  12  is configured to be matable with a clutch (not shown), while the shaft  14  is configured to support a gear (not shown). However, it should be appreciated that the present invention may be used to transfer torque in a variety of applications within the inventive concept.  
         [0014]     The hub  12  includes a circumferential wall  16  having a plurality of teeth  18  protruding radially therefrom. The plurality of teeth  18  preferably extend about the entire perimeter of the circumferential wall  16 . Lubricant openings  20  extend through at least some of the plurality of teeth  18  to allow lubricant to flow into and out of the composite clutch shaft  10 . The hub  12  is preferably configured to engage the clutch within the vehicle transmission. When the clutch is applied, splined clutch plates transfer torque from the clutch to the hub  12  for substantially unitary rotation therewith.  
         [0015]     The shaft  14  extends from the hub  12  to a splined end portion  22 . Splines  24  are formed on an outer surface  26  of the shaft  14  at the splined end portion  22 . In the preferred embodiment, the splines  24  extend around the entire outer surface  26  of the shaft  14 . The splines  24  are preferably configured to support a gear. Preferably, the splines  24  are induction hardened following formation, thereby reducing spline degradation caused by the gear.  
         [0016]     With reference to  FIG. 3   a,  the shaft  14  of the present invention includes a generally tubular outer portion  28  having an inner core portion  30  disposed therein. In the preferred embodiment, the core portion  30  will extend substantially the length of the shaft  14  from end  31 , as shown in phantom in  FIG. 2 . Those skilled in the art will recognize that the core portion  30  may extend for less than the length of the shaft  14  as design constraints of the composite clutch shaft  10  dictate. Preferably, the core portion  30  is press fit into the tubular outer portion  28 ; however, those skilled in the art will recognize other methods operable to retain the core portion  30  relative to the outer portion  28 , such as bonding or staking.  FIG. 3   a  illustrates one embodiment of the present invention in which a cross sectional view of the shaft  14  illustrates the outer portion  28  and the inner core portion  30 . The inner core portion  30  may be solid or hollow.  FIG. 3b  illustrates a second embodiment of the present invention in which a cross sectional view of the shaft  14  illustrates a core portion  30 ′ defining a hollow center  33 .  
         [0017]     An exemplary embodiment of the shaft  14  in  FIGS. 3   a  and  3   b  would be to form the core portions  30  and  30 ′ from a stiff material with a high internal damping characteristic, such as grey iron. Grey iron core portions, such as  30  and  30 ′, would be press fit into a steel outer portion, such as  28 , having a known or predetermined internal damping characteristic. The grey iron has approximately half the stiffness of steel, but has nearly eighty times the internal damping of quenched and tempered high carbon steel having a martensite microstructure. The damping characteristic of the shaft  14  is thus improved compared to hollow or solid shafts formed from a single material. This increased damping characteristic is achieved without changing the exterior dimensions of the shaft  14  or significantly increasing stresses within the steel constituting the outer portion  28 .  
         [0018]     A third and fourth embodiment of the present invention is shown respectively in  FIG. 4   a  and  4   b.  In  FIG. 4   a  there is shown a cross section of a shaft  14 ′. In this embodiment, the shaft  14 ′ is formed from a heat treatable material, such as steel. In this embodiment, the outer portion  28 ′ is formed by heat treating the shaft  14 ′ sufficiently to a predetermined depth while leaving core portion  30 ″ unaffected. By doing so, the core portion  30 ″ will maintain high internal damping compared to the outer portion  28 ′.  FIG. 4   b  is a cross sectional view of the shaft  14 ′ illustrating a core portion  30 ′″ defining a hollow center  33 , similar to that shown in  FIG. 3   b.    
         [0019]     Exemplary of the embodiments shown in  FIGS. 4   a  and  4   b,  the shaft  14 ′ may be made of steel. The shaft  14 ′ may be sufficiently heat treated to form a martensite microstructure in the outer portion  28 ′ while the core portions  30 ″ and  30 ′″ will maintain a ferrite microstructure, which has approximately eight times the internal damping of steel with a martensite microstructure. The shaft  14 ′ will have a greater damping ability than a shaft having a martensite microstructure for both the outer portion  28 ′ and the core portions  30 ″ and  30 ′″.  
         [0020]     Those skilled in the art will recognize that the relative radial thicknesses of the outer portions  28 ,  28 ′ and the core portions  30 ,  30 ′,  30 ″, and  30 ′″ will be dictated by engineering constraints such as torsional loading on the shaft  14 ,  14 ′. The composite clutch shaft  10  of the present invention may reduce unwanted noise and vibration within the power transmission by increasing the damping effectiveness of the shaft  14  and  14 ′.  
         [0021]     While the best modes for carrying out the invention have been described in detail, it is to be understood that the terminology used is intended to be in the nature of words and description rather than of limitation. Those familiar with the art to which this invention relates will recognize that many modifications of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced in a substantially equivalent way other than as specifically described herein.