Abstract:
A bi-directional, conical carbon-carbon clutch bearing engagement mechanism for the inner race of a conventional rolling element bearing comprising a plurality of wave springs for axially urging the clutch into a non-engaged position, and a guide consisting of contiguous pairs of non-parallel ball raceways and a ball positioned in both pairs of the ball raceways whereby relative rotation of the ball raceway pairs in opposite directions results in axial movement of the clutch ring housings in opposing axial directions to an engaged position. The mechanism includes a pair of conical carbon-carbon rings mounted in the movable clutch ring housings conformed to engage opposing conical shaft runners.

Description:
RIGHTS OF THE GOVERNMENT 
     The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to clutch mechanisms for turbine engines, turbopumps and the like, and more particularly to a bearing engagement mechanism in a bidirectional conical carbon-carbon clutch structure. 
     Standard lubrication systems are being pushed to the limit in current turbine engines. Conventional rolling element bearings and liquid lubricants become unusable as the thrust-to-weight ratio for engines increase. One way to increase the temperature and speed capability of a bearing sump is by using magnetic bearings. To meet thrust-to-weight goals of advanced turbine engines, magnetic bearings are generally excessively large and heavy for handling short duration, high load maneuver conditions and auxiliary conventional rolling element bearings must be used to support the shaft. 
     Conventional rolling element bearings used as auxiliaries must have a clearance between the shaft and the inner race (0.005-0.010-0.020 inch diametral clearance) which is less than the magnetic bearing clearance. However, in the event of touchdown on the auxiliary bearings, this gap usually produces dynamic instability for the rotating shaft. An example of shaft instability is a condition known as backward whirl where the shaft bounces inside the tolerance of the inner race in a direction opposite to rolling. Additionally, when the auxiliary bearing is engaged the inner race tries to attain shaft speed instantaneously, resulting in skidding damage to the bearing. Simple sleeve bearings can be used to bring the shaft to a stop after a magnetic failure, but sleeve bearings are not adequate for a shared load condition because they experience high wear unless they are lubricated under a fully flooded condition of up to two gallons of lubricant per minute. This would necessitate a lubricant reservoir, pump, and cooler. 
     Continuously engaged rolling element bearings with softly mounted races have the problem of limited life since high temperature experimental lubrication schemes with marginal lubrication capability must be used. These high temperature experimental lubrication schemes produce high wear and bearing life is typically less than 30 hours. 
     Magnetically levitated rotors for aerospace turbine engines require auxiliary bearings for shaft support in case of magnetic bearing failure or overload. For engine applications a magnetic bearing system cannot be sized to handle full maneuver loads because it becomes unrealistically large and heavy. Thus, auxiliary bearings are required to handle loads above the capacity of the magnetic bearings. To use conventional rolling element bearings on an as-needed basis, means must be used to close the tolerance from a disengaged to an engaged status while centering the shaft. Closing the clearance around the shaft prevents backward whirl. Centering the shaft minimizes rotating unbalance. A relatively gradual acceleration of the bearing is required to avoid skidding damage and inertial welding. Ideally the rolling element bearing is brought up to speed quickly, in the order of a few seconds rather than almost instantaneously. The present invention allows a gradual startup of the auxiliary bearing through the use of a slip surface or clutch consisting of one or a pair of carbon-carbon clutch plates or rings. The clutching action of the present invention allows the bearing elements to come up to shaft speed gradually during engagement, thereby minimizing skidding damage. The closed clearance avoids backward whirl. The centering minimizes shaft unbalance. 
     The invention is a solution that uses a rolling element bearing and provides a gradual engagement to minimize skidding damage of the bearing, provides closed clearance to avoid backward whirl, and provides shaft centering to minimize rotating unbalanced loads. 
     An auxiliary bearing engagement mechanism using a carbon-carbon clutch enables the use of conventional rolling element bearings as auxiliary bearings for as-needed use in magnetically supported rotors. Rolling element bearings must have a gradual engagement and provide shaft centering. Conventional rolling element bearings with 0.005-0.010-0.020 inch diametral clearance between the shaft and bearings have been shown to be dynamically unstable with skidding damage, inertial welding, and a catastrophic backward whirl condition when used for auxiliary support. 
     It is therefore a principal object of the invention to provide an improved clutch mechanism. 
     It is a further object of the invention to provide an improved carbon-carbon clutch mechanism having particular utility within turbine engines, turbo pumps and the like. 
     It is another object of the invention to provide a novel bearing engagement mechanism in a bi-directional conical carbon-carbon clutch structure. 
     It is another object of the invention to allow a gradual startup of a conventional rolling element auxiliary bearing through the use of a slip surface or clutch consisting of one or a pair of carbon-carbon clutch plates or rings. 
     It is another object of the invention to allow conventional rolling bearing elements to come up to shaft speed while preventing skidding damage and rotor backward whirl. 
     These and other objects of the invention will become apparent in the detailed description of representative embodiments. 
     SUMMARY OF THE INVENTION 
     In accordance with the foregoing objects of the invention, a bearing engagement mechanism for a bi-directional conical carbon-carbon clutch for turbine engines, turbo pumps and the like is provided which includes a carbon-carbon clutch bearing engagement mechanism for the inner race of a rolling element bearing. The invention comprises a set of wave springs for axially urging the clutch into a non-engaged position, and guide means for guiding the clutch into an engaged position. The clutch includes a pair of conical carbon-carbon rings mounted in movable clutch ring housings. The guide means comprises opposing, contiguous pairs of non-parallel ball raceways and a ball positioned in both pairs of the ball raceways whereby relative rotation of the ball raceway pairs in opposite directions results in axial movement of the clutch ring housings in opposing axial directions. The carbon-carbon clutch provides a gradual engagement to minimize skidding damage of the bearing and provides shaft centering to eliminate the initial tolerance required to allow beneficial operation of the magnetic bearing while avoiding the backward whirl phenomenon. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross sectional view of a first embodiment of the invention in a disengaged state; 
     FIG. 2 is a flattened view of the arched portion of the housing of a first embodiment of the invention in a disengaged state; 
     FIG. 3 is a cross sectional view of a first embodiment of the invention in an engaged state; 
     FIG. 4 is a flattened view of the arched portion of the housing of a first embodiment of the invention in an engaged state; 
     FIG. 5 is a cross sectional view of a second embodiment of the invention in a disengaged state; 
     FIG. 6 is a cross sectional view of a second embodiment of the invention in an engaged state; 
     FIG. 7 is a cross sectional view of a third embodiment of the invention in a disengaged state; 
     FIG. 8 is a cross sectional view of a third embodiment of the invention in an engaged state; 
     FIG. 9 is a cross sectional view of a fourth embodiment of the invention in a disengaged state; 
     FIG. 10 is a cross sectional view of a fourth embodiment of the invention in an engaged state; 
     FIG. 11 is a cross sectional view of a fifth embodiment of the invention in a disengaged state; 
     FIG. 12 is a cross sectional view of a fifth embodiment of the invention in an engaged state; 
     FIG. 13 is a cross sectional view of a sixth embodiment of the invention in a disengaged state; and 
     FIG. 14 is a cross sectional view of a sixth embodiment of the invention in an engaged state. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 1, an axial cross sectional view of the essential components of a representative embodiment of the auxiliary carbon-carbon clutch bearing structure  101  of the present invention is shown. In this invention, carbon-carbon rings are used as high temperature clutch surfaces. Carbon-carbon clutch mechanism  102  is press fitted on bearing inner race  103  with mechanism housing pairs  104  in direct contact with bearing inner race  103 . Outer bearing race  116  is press fit or otherwise hard mounted in an outer housing member not shown. Mechanism housing pairs  104  have U-shaped ball raceways  105  machined around the circumference. Clutch ring housing  106  holds carbon-carbon rings  107  and tangentially mates to mechanism housing  104 . Referring to FIGS. 2 and 4, clutch ring housing  106  has ball raceways  108  machined around the circumference at approximately a 30° angle to the tangential portion of mechanism housing ball raceways  105 . The axial positions of clutch ring housings  106  are controlled by axial movement balls  109 , which set in ball raceways  105  and  108  of mechanism housings  104  and clutch ring housings  106  respectively. Clutch ring housings  106  are held in a non-engaged position shown in FIG. 2 by wave springs  110 . 
     The above-described carbon-carbon clutch is located coaxial with shaft  111  and a pair of oppositely and contiguously situated shaft runners  112 . As depicted in FIGS. 1 and 3, the pair of carbon-carbon rings  107  are tapered at approximate 45° with decreasing radii in a direction radially and axially inward of clutch ring housing  106 . The pair of shaft runners  112  are accordingly tapered at a like angle as the corresponding carbon-carbon rings in order to mate with the carbon-carbon rings and to center shaft  111 . 
     In operation shaft  111  is magnetically levitated with magnetic bearings located at opposite sides of auxiliary carbon-carbon clutch bearing  101 . An aerospace turbine engines will, for example, require auxiliary carbon-carbon clutch bearing  101  for shaft support in case of magnetic bearing failure or overload. An overload could occur, for example, during a high torque turning maneuver. The clearance between carbon-carbon rings  107  and shaft runners  112  is exaggerated in the figures. Under normal operating conditions the shaft would be supported entirely by the magnetic bearing system with a small clearance between carbon-carbon rings  107  and shaft runners  112  on the order of 0.005-0.010 inches. If the shaft were to become unstable or exceed predetermined positional limits as indicated by two-way arrows  113  and  115 , the axial shaft position controlled by a thrust magnetic bearing can axially force the carbon-carbon clutch to contact tapered mating surfaces  114  of shaft runners  112 . 
     Initially shaft runners  112  slip on carbon-carbon clutch rings  107 . The slip is controlled by the amount of axial or radial shaft load that requires support. Upon first contact inner race  103  begins to rotate in the same direction as shaft  111 . During the slippage between the carbon-carbon clutch rings and shaft runners, fine powder portions of the carbon-carbon clutch rings are created by the frictional contact, thereby providing a lubricant. This lubricant acts as a prophylaxis against frictional welding and reduces heat generation. As the load on the auxiliary bearing increases the slip rate diminishes to zero and the auxiliary bearing achieves full shaft speed. The carbon-carbon clutch also centers the shaft geometrically through the active force of the shaft. 
     Referring again to FIGS. 2 and 4, as the load increases and the slip rate diminishes, clutch rings housings  106  move tangentially with respect to mechanism housing  104  as indicated by arrows  115 . Axial movement balls  109  are co-located in both ball raceways  105  of mechanism housings  104  and ball raceways  108  of clutch ring housing  106 . Because balls  109  travel in ball raceways  105  which are axially fixed and because ball raceways  108  are at an angle to the tangential portion of the mechanism housing ball raceways  105 , clutch ring housings  106  are forced apart from one-another in opposite axial directions as indicated by arrows  116 . Prior to contact of carbon-carbon rings  107  with shaft runner surface  114 , the clutch ring housings are maintained in mutual contact by wave springs  110  as depicted in FIG.  2 . During employment of the auxiliary bearing, wave springs  110  remain compressed as depicted in FIG.  4 . 
     As stated in the previous paragraph, as clutch ring housings  106  rotate they move axially as balls  109  roll through ball raceways  105  and  108  machined at an angle in the clutch ring housing. Both rings rotate together through the action of the anti-rotation device  120  as they move axially outward toward the shaft runners. The outward movement continues until both carbon-carbon rings are contacting the runners and the shaft is centered due to the tapered geometry of the carbon-carbon rings and the shaft runners. Additional force from the shaft now translates directly through the auxiliary rolling bearing of the present invention. When the overload force diminishes, the mechanism will reverse its previous action due to the force of wave springs  110  and the clutch will disengage the runner as the springs push clutch ring housings  106  and carbon-carbon rings  107  mounted therein back to the initial non-engaged position. 
     Referring to FIGS. 5 and 6, a second embodiment of carbon-carbon clutch bearing  201  may be effected by reversing the location of carbon-carbon rings  207  by mounting carbon-carbon rings  207  in shaft runners  212 . The invention represented by the second embodiment would function in all other respects as the first embodiment functions. 
     Referring to FIGS. 7 and 8, a third embodiment of clutch bearing  301  is depicted. In this third embodiment of the invention, outer auxiliary bearing race  316  is soft mounted in outer housing  320 . The soft mounting allows the axial position of outer race  320  to be changed by an engagement force F which could be provided by an electrical, hydraulic or pneumatic actuator in a well-known manner. Magnetic bearing are controlled by feeding back the shaft position as an input to the magnetic bearing controller which converts the shaft displacement from centerline to a force requirement that is output to the magnetic bearing via changes in voltage and current. Therefore, since shaft position is know and mechanical clearance is know, limits of the shaft position may be established that would trigger auxiliary bearing actuation. The engagement actuator could be part of the magnetic bearing system and be controlled by the magnetic bearing controlling or it could have its own controlling with shaft position sensors. When the shaft position limits were exceeded the engagement actuator would be signaled to move the outer race from an non-engaged position shown in FIG. 7 to an engaged position shown in FIG.  8 . In this third embodiment a single carbon-carbon ring  307  is mounted in clutch ring housing  306  which is in turn mounted on shaft runner  312 . As engagement force F moves outer race  316  in an axial direction towards carbon-carbon ring  307 , inner race  303  is forced into contact with carbon-carbon ring  307 . Auxiliary rolling bearing  301  assumes a support role for a shaft (not shown) which supports shaft runner  312  as in the first embodiment. As in the first embodiment, when the overload force diminishes, the mechanism will reverse its previous action due to the force of wave springs  310  and disengage the runner as the springs push outer race  316  to the initial non-engaged position. 
     Referring to FIGS. 9 and 10, a fourth embodiment of carbon-carbon clutch bearing  401  may be effected by reversing the location of the carbon-carbon rings  407  by mounting the carbon-carbon rings  407  in the rolling bearing outer race  403 . The invention represented by the fourth embodiment would function in all other respects as the third embodiment functions. 
     Referring to FIGS. 11 and 12, a fifth embodiment of clutch bearing  501  is depicted. In this fifth embodiment of the invention outer auxiliary bearing race  516  is hard mounted in outer housing  520 . Carbon-carbon clutch ring  507  is mounted through clutch ring housing  506  and disk or shaft runner  512  to shaft  522 . Under normal operating conditions shaft  522  would be supported entirely by magnetic bearings with a small clearance, on the order of 0.005-0.010 inches, between carbon-carbon clutch ring  507  and tapered inner race  503 . The magnetic bearing supporting thrust loads would control the engagement. If the rotor were to become unstable or exceeded predetermined positional limits, the axial shaft position controlled by thrust magnetic bearing  524  could allow the force F of the turbine engine to cause axial displacement of shaft  522  and subsequent contact of the carbon-carbon clutch  507  and tapered inner race  503 . This approach allows a considerably simpler mechanical design. 
     Referring to FIGS. 13 and 14, a sixth embodiment of carbon-carbon clutch bearing  601  may be effected by reversing the location of carbon-carbon rings  607  by mounting carbon-carbon rings  607  in rolling bearing outer race  603 . The invention represented by this sixth embodiment would function in all other respects as the fifth embodiment functions. 
     Our invention provides a novel bearing engagement mechanism in a bidirectional conical carbon-carbon clutch structure. While the above description contains many specificities, these should not be construed as limitations of the scope of the invention, but rather as an exemplification of preferred embodiments thereof. Modifications to the invention may be made as might occur to one with skill in the field of the invention within the scope of the appended claims. All embodiments contemplated hereunder that achieve the objects of the invention have therefore not been shown in complete detail. Other embodiments may be developed without departing from the spirit of the invention or from the scope of the appended claims. 
     Many other variations are possible. For example, the mechanism as described uses a 45° angle for the carbon-carbon ring and shaft runner interface which provides equal displacement before engagement in both the radial and axial directions. This angle can be changed to tune the mechanism for either axial or radial direction operation which is within the spirit of this invention. Also, by altering the engagement area of the carbon-carbon rings, the engagement speed and required force to attain equal bearing and shaft speed can be modified. Furthermore, the angle of ball raceways  105  and/or  108  could be altered. Accordingly, the scope of the invention should be determined not by the embodiments illustrated, but by the appended claims and their legal equivalents.