Abstract:
A compliant foil fluid film radial bearing having a contoured profile bearing rotor to provide for the greater thermal expansion of the hot end of the compliant foil fluid film radial bearing at operating temperature. The rotor may have a uniform taper over its entire length, may be tapered for a portion of its length, or the taper may vary over the entire length of the bearing rotor.

Description:
TECHNICAL FIELD 
     This invention relates to the general field of compliant foil fluid film radial bearings and more particularly to an improved radial bearing having a contoured or tapered bearing rotor. 
     BACKGROUND OF THE INVENTION 
     Compliant foil fluid film radial bearings are currently being utilized in a variety of high speed rotor applications. These bearings are generally comprised of a bushing, a rotating element such as a rotor or shaft adapted to rotate within the bushing, non-rotating compliant fluid foil members mounted within the bushing and enclosing the rotating element, and non-rotating compliant spring foil members mounted within the bushing underneath the non-rotating compliant fluid foil members. The space between the rotating element and the bushing is filled with fluid (usually air) which envelops the foils. 
     The motion of the rotating element applies viscous drag forces to the fluid in the converging wedge channels. This results in increases in fluid pressure, especially near the trailing end of the wedge channels. If the rotating element moves toward the non-rotating element, the convergence angle of the wedge channel increases, causing the fluid pressure rise along the channel to increase. Conversely, if the rotating element moves away, the pressure rise along the wedge channel decreases. Thus, the fluid in the wedge channels exerts restoring forces on the rotating element that vary with and stabilize running clearances and prevent contact between the rotating and non-rotating elements of the bearing. Flexing and sliding of the foils causes coulomb damping of any axial or overturning motion of the rotating element of the bearing. 
     Owing to preload spring forces or gravity forces, the rotating element of the bearing is typically in physical contact with the fluid foil members of the bearing at zero or low rotational speeds. This physical contact results in bearing wear. It is only when the rotor speed is above what is termed the lift-off/touch-down speed that the fluid dynamic forces generated in the wedge channels assure a running gap between the rotating and non-rotating elements. 
     Compliant foil fluid film radial bearings typically rely on backing springs for preload, stiffness, and damping. The fluid foils are preloaded against the relatively movable rotating element so as to control foil position/nesting and to establish dynamic stability. The bearing starting torque (which should ideally be zero) is directly proportional to these preload forces. These preload forces also significantly increase the rotor speed at which the hydrodynamic effects in the wedge channels are strong enough to lift the rotating element of the bearing out of physical contact with the non-rotating members of the bearing. These preload forces and the high liftoff/touch-down speeds result in significant bearing wear each time the rotor is started or stopped. 
     Conventional compliant foil fluid film radial bearings operate with extremely small running clearances and moderate, as opposed to low, drag and power consumption. The clearances between the non-rotating fluid foil&#39;s minimum film region and the rotating element are typically less than 100 micro-inches at operating conditions. 
     In some instances, the compliant fluid foil elements may comprise a plurality of individual compliant foils to form a plurality of wedge shaped channels which converge in thickness in the direction of the rotation of the rotor, while in other instances the compliant foils may be formed on a single sheet enveloping the rotor within a cylindrical or lobed bushing. 
     One of the high speed applications in which these compliant foil fluid film radial bearings are utilized is a gas turbine engine. Such an engine could include a gas turbine and a gas compressor at opposite ends of a shaft with the compliant foil fluid film radial bearing in between. Since hot combustion gases, as high as 1,750 degrees Fahrenheit are expanded in the gas turbine and atmospheric air is compressed in the compressor, there is a high degree of temperature variation between the turbine or hot end of the compliant foil fluid film radial bearing and the compressor or cooler end of the compliant foil fluid film radial bearing. In some instances the hot end of the compliant foil fluid film radial bearing will be as much as 500 degrees Fahrenheit hotter than the cooler end of the compliant foil fluid film radial bearing. 
     In most cases, the compliant foil fluid film radial bearing will be generally uniform from the turbine end to the compressor end and the bushing and rotor will also be a constant diameter. As a result of the temperature gradient from the hot end to the cooler end of the compliant foil fluid film radial bearing, the bearing&#39;s radial play (sway space or running clearance) may be severely restricted at the turbine end of the bearing. The restriction in radial play, the result of greater thermal expansion at the turbine end of the bearing, can cause the bearing to lose load capacity, have an excessively high touch-down speed, have excessive drag, have excessive power consumption, or be forced to operate at an unacceptable temperature due to self heating. All of the above will adversely impact bearing life and reliability. 
     U.S. Pat. Nos. 4,502,795 issued Mar. 5, 1985, and 4,555,187 issued Nov. 26, 1985, both entitled Foil Bearing Alignment, propose the use of shims between the compliant foil fluid film radial bearing and the axially extending bore to radially outwardly diverge the surface of the bearing along its axial length. Alternately, an axially varying underspring rib thickness is proposed as is a conical bearing bushing to correct or minimize bearing rotor misalignment and accommodate bearing rotor deflection or tilt. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a compliant foil fluid film radial bearing having a contoured or tapered bearing rotor at ambient temperature. The rotor may have a uniform taper over its entire length or may only be tapered for a portion of its length. Alternately, the taper may vary over the length of the bearing rotor. 
     In this manner the greater thermal expansion of the hot end of the compliant foil fluid film radial bearing is accommodated such that at operating speed and temperature the bearing rotor will have a relatively constant diameter and the sway space between the bearing and the rotor will be relatively uniform. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Having thus described the present invention in general terms, reference will now be made to the accompanying drawings in which: 
     FIG. 1 is a sectional view of a turbomachine having the compliant foil fluid film radial bearing of the present invention; 
     FIG. 2 is an enlarged sectional view of a portion of FIG. 1 illustrating the compliant foil fluid film radial bearing of the present invention; 
     FIG. 3 is an enlarged sectional view of a portion of FIG. 1 illustrating an alternate compliant foil fluid film radial bearing of the present invention; and 
     FIG. 4 is an enlarged sectional view of a portion of FIG. 1 illustrating another alternate compliant foil fluid film radial bearing of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A turbomachine utilizing the compliant foil fluid film radial bearing of the present invention is illustrated in FIG.  1 . The turbomachine  10  generally includes turbine wheel  12  and compressor wheel  14  at opposite ends of a common shaft or tie bolt  16 . The thrust and radial bearing rotor  18  is disposed around the tie bolt  16  between the turbine wheel  12  and the compressor wheel  14 . A journal bearing cartridge  20  in center bearing housing  22  rotatably supports the bearing rotor  18 . 
     The compressor end of the bearing rotor  18  includes a radially extending thrust disk  24  which extends into a recess  26  in the compressor end of the center bearing housing  22 . A bearing thrust plate  28  is disposed on the opposite side of the bearing rotor thrust disk  24 . The outer periphery of the compressor end of the center bearing housing  22  engages the compressor housing  30 . 
     A thrust bearing spacer  33  is positioned radially outward from the thrust disk  24  of the bearing rotor  18  and is positioned radially by a plurality of circumferentially spaced pins  34  which are fixed in holes in the recess  26  of the center bearing housing  22  and extend into holes in the thrust bearing plate  28 . A thrust bearing fluid foil member and thrust bearing spring foil member are disposed on either side of the bearing rotor thrust disk  24  and thrust bearing spacer  32 . On one side, the fluid foil member and spring foil member are positioned in the recess  26  of the center bearing housing  22  and on the other side they are adjacent to the bearing thrust plate  28 . 
     The compliant foil fluid film radial bearing of the present invention is illustrated in FIG.  2 . The bearing retainer cartridge  20  mounts a bearing underspring  30  and a bearing foil  32  in conventional fashion around the bearing rotor  18 . The bearing rotor  18  is tapered from the compressor or cooler end  40  down to the turbine or hot end  42 . 
     In the alternate compliant foil fluid film radial bearing illustrated in FIG. 3, the bearing rotor  18 ′ includes a generally cylindrical section  50  at the cooler end  40  and a tapered section  52  which extends from the cylindrical section  50  to the hot end  42 . The rotor  18 ″ of the alternate compliant foil fluid film radial bearing of FIG. 4 includes a cylindrical cooler end section  60 , a central tapered section  62  and a more highly tapered hot end section  64 . 
     Each of FIGS. 2-4 illustrate the bearing rotor  18 ,  18 ′, or  18 ″ in an at rest, nonoperating, condition at ambient temperature. When the bearing rotor is operated at temperature, the hot end of the rotor will thermally expand such that the bearing rotor will become generally cylindrical at operating temperature since the hot end  42  will expand more than the cooler end  40 . Depending upon the temperature profile of the bearing rotor at operating temperature, either of the tapered bearing rotors of FIGS. 2-4 can be utilized. 
     In other words, instead of starting with a generally cylindrical bearing rotor and ending up with the hot end of the bearing rotor pinching the running clearance at operating conditions, the bearing rotor starts out tapered (smaller) at the hot end and the bearing rotor achieves a generally cylindrical surface at operating conditions which insures a more uniform bearing clearance along the entire bearing rotor. The differential thermal growth, greater at the hot end than at the cooler end, is used to provide a more uniform bearing clearance rather than producing a pinching of the clearance at the hot end. 
     The interaction of the axially tapered or contoured diameter of the bearing rotor with the change in the rotor&#39;s axial temperature profile as it comes up to its operating temperature and with the bearing rotor&#39;s thermal coefficient of expansion can assure that the bearing&#39;s radial clearance is optimized at operating temperature at all axial locations along the bearing. This can result in a bearing which, under operating conditions, achieves high load carrying capacity, good damping, low running torque, low starting torque, low lift-off/touch-down speeds, low power consumption, and reduced bearing cooling air flow requirements. All of the above are important to the successful manufacture and operation of high speed turbomachinery. 
     The present invention is equally applicable to hydrodynamic or hydrostatic fluid film radial bearings. Also, while the rotating group has been illustrated as having a gas turbine as the hot rotating component and a gas compressor as the cooler rotating component, there are applications, such as an air cycle turbine system, where the gas compressor would be the hot rotating component and the cooling turbine would be the cooler rotating component. It is only significant that there be a temperature difference between the rotating components for there to be a need for the present invention. 
     While specific embodiments of the invention have been illustrated and described, it is to be understood that these are provided by way of example only and that the invention is not to be construed as being limited thereto but only by the proper scope of the following claims.