Patent Publication Number: US-RE38373-E

Title: Compliant foil fluid film thrust bearing with a tilting pad underspring

Description:
TECHNICAL FIELD 
     This invention relates to the general field of compliant foil fluid film bearings and more particularly to an improved thrust bearing employing fluid foils and multiple spring foils to support, position, damp and accommodate movements or excursions of the rotating portion of the bearing. 
     BACKGROUND OF THE INVENTION 
     Compliant foil fluid film thrust bearings are currently being utilized in a variety of high speed rotor applications. These bearings are generally comprised of a two sided thrust disk rotating element, non-rotating compliant fluid foil members that axially enclose the rotating element, non-rotating compliant spring foil members that axially enclose the fluid foil members and a non-rotating thrust plate element and a non-rotating housing element that axially enclose and provide attachments for the foil members. The space between the rotating element and the thrust plate element on one side of the bearing and the space between the rotating element and the thrust surface of the housing element on the other side of the bearing are filled with fluid (usually air) which envelops the foils. 
     The rotary motion of the rotating element applies viscous drag forces to the fluid and induces circumferential flow of the fluid between the smooth surface of the rotating element and the fluid foil. The space between the rotating element and the fluid foil is subdivided into a plurality of fluid-dynamic wedge channels. These wedge channels have typically been formed by resistance welding compliant, convex curved foil pads to an underlying support foil. The leading ramps of the foil pads relative to the fluid&#39;s circumferential flow and the smooth surface of the rotating element form the two primary surfaces of the converging wedge channels. The trailing ramps and the smooth surface of the rotating element form the primary surfaces of the diverging wedge channels. The fluid flowing circumferentially along a converging wedge channel experiences steadily decreasing flow area, increasing circumferential flow velocity and increasing static fluid pressure. 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. 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 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. 
     Conventional, compliant foil fluid film thrust bearings have fluid dynamic wedge channel ramps that converge or diverge circumferentially with no radial component to the ramp slopes. The converging wedge channel ramps have no side wall or other constraints to prevent fluid flow out of the channels at their inner and outer edges. At the trailing edge of the converging wedge channel, the high fluid pressure and lack of radial flow constraints induces radial flow leakage out of the channel, which in turn, results in a reduction in fluid pressure, a loss in bearing load capacity, and an increase in bearing drag. The radial flow leakage requires make-up flow at the beginning of the converging wedge channel. 
     Conventional, compliant foil fluid film thrust bearings have primary fluid flow patterns in the converging wedge channels that are single path recirculating loops. The fluid in the converging wedge channels adjacent to the rotating disk travels circumferentially in the same direction as the disk&#39;s motion (up the ramp) owing to viscous drag. The fluid in the converging wedge channels adjacent to the non-rotating fluid foil travels-circumferentially in the direction opposite to the disk&#39;s motion (down the ramp) owing to the circumferential pressure gradient along the channel. Much of the fluid that travels up the ramp near the disk while gaining static pressure turns back before reaching the end of the wedge channel and travels down the ramp near the fluid foil while losing pressure. Almost all of this fluid turns again before reaching the beginning of the wedge channels and travels up the ramp while again gaining pressure. The fluid traveling the single path recirculating loop flow patterns travels essentially the same path each loop and experiences the same pressure increases and pressure decreases each loop with no net pressure gain from one loop to the next. These bearings generate less fluid dynamic pressure and have less load capacity than bearings that utilize multi-path vortex flow patterns where the flow traveling each regenerative loop travels a different path and where there is a net increase in fluid pressure each loop 
     Conventional, compliant foil fluid film thrust 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 converging channel ramp trailing ends and the rotating thrust disk are typically less than 100 micro-inches when the bearing is heavily loaded at operating conditions. The bearing&#39;s drag coefficient is typically more than 0.005 at operating speed as defined by the ratio of the fluid dynamic drag induced shear forces applied to the disk by the bearing divided by the thrust load carried by the bearing. 
     Compliant foil fluid film thrust bearings tend to rely on backing springs to preload the fluid foils against the relatively moveable rotating element (thrust disk) so as to control foil position/nesting and to establish foil 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 disk 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 lift-off/touch-down speeds result in significant bearing wear each time the disk is started or stopped. 
     Many conventional, compliant foil fluid film thrust bearings have large sway spaces and loose compliance, i.e. they do not tightly restrict the axial or overturning motion of the bearing thrust disk, owing to poor control of spring deflection tolerances inherent in the spring designs. 
     It has been common for compliant foil fluid film thrust bearings to utilize a plurality of coated, convex curved, compliant fluid foil pads that are welded to a support foil to form the fluid foil member of the bearing. These two piece fluid foil members are typically thicker and have poorer thickness control than can single piece fluid foil members. Two piece fluid foil members also experience process fluid foil turbulence, increased drag at operating speeds and reduced load capacity owing to the flow discontinuities between the trailing edges of each foil pad and the weld attachment edge of the next circumferentially located pad. 
     Some conventional, compliant foil fluid film thrust bearings utilize spring foil elements that are formed by milling (chemically or otherwise) circumferentially offset recesses in opposing sides of flat foil stock so as to leave circumferentially offset unmilled ridges on opposing sides of the foil elements. Pressure applied to the offset ridges induces the spring foil element to deflect in a spring-like manner. Spring foil elements formed in this manner are prone to large variations in their spring rates due to small variations in milling depth. This milling process non-symetrically relieves any residual surface compressive stresses induced by previous foil rolling operations and thus induces foil warpage. 
     Other bearings utilize convolute shaped spring foil elements that are formed by pressing annealed Inconel 750X foil blanks between two contoured plates having matching wavy contours with constant plate to plate spacing and then heat treating the foil blanks at approximately 1300 degrees Fahrenheit for approximately 20 hours while they are still pressed between the contoured plates. Spring foils formed in this manner are prone to have large variations in undeflected thickness. 
     In some cases, the fluid foils may be attached to the spring foils by welding or brazing or various spring foil elements may be welded or brazed together to form a spring foil member. Those thrust bearings that utilize welding or brazing to attach one foil element to another are subject to foil distortions and foil fatigue failures, particularly at the bond sites. 
     The sides of the fluid foils that face the rotating element of the bearing can utilize low rubbing friction coatings to minimize bearing wear when disk speed is below the lift-off/touch-down speed. These coatings, however, may have large thickness tolerances that can adversely affect the foil pack thickness tolerance. 
     The latest development in compliant foil fluid film thrust bearings, described in U.S. Pat. No. 5,529,398 issued Jun. 25, 1996 to Robert W. Bosley entitled “Compliant Foil Hydrodynamic Fluid Film Thrust Bearing” includes a self shimming capability to compensate for variations in foil pack thickness and three (3) spring or support foils beneath the fluid foil. 
     SUMMARY OF THE INVENTION 
     In the present invention, the compliant foil fluid film thrust bearing generally comprises a single sided or two sided thrust disk rotor, fluid foils, spring /s, a thrust plate, a foil retaining housing and a spacer ring. The non-rotating but compliant fluid foils are located adjacent to the thrust face or faces of the rotatable disk. The fluid foils have open faced channels that induce regenerative vortex flow patterns in the process fluid. The forces applied by the thrust disk to the fluid foils vary inversely with fluid foil to disk gap and vary proportionally with disk deflection. 
     The spring foils provide a tilting pad support for the fluid foils but allow them to follow the axial and overturning motion of the disk. Each of the types of foils, namely fluid foils and spring foils are attached to the foil retaining housing by a compliant web structure and pins. The foils are formed as thin, flat, annular sheets with integral shim rings at their periphery and contoured cutout patterns that are unique to each type of foil. 
     As part of the forming process, the fluid foil blank is coated on one side with a compliant, wear resistant material then stamped with a forming tool to form the fluid flow channels. The thrust plate is preloaded towards the thrust surface of the foil retaining housing by a preload spring and is held away from the housing by the total thickness of the foil shim rings and the thickness of the spacer ring. This allows the bearing to essentially self shim itself to establish a small clearance between the fluid foils and the disk that is not affected by normal variations in foil or foil coating thicknesses. 
     The bearing has no preload force and has zero starting torque when the rotor&#39;s axis of rotation is oriented ninety degrees to the force of gravity. Owing to the vortex flow pattern of the process fluid, the bearing running clearances and load capacities are improved while lift-off speeds are reduced. In addition, good damping, low running torque and small sway space are achieved. This is all accomplished at a low manufacturing cost with a low parts count. 
     It is, therefore, a principal object of the present invention to provide an improved compliant foil fluid film thrust bearing. 
     It is another object of the present invention to provide such a bearing with enhanced axial and overturning load carrying capacity. 
     It is another object of the present invention to provide such a bearing with both squeeze film and coulomb damping. 
     It is another object of the present invention to provide such a bearing with small sway space clearances to tightly restrict bearing and thrust disk rotor deflections. 
     It is another object of the present invention to provide such a bearing with very low operating torque. 
     It is another object of the present invention to provide such a bearing with large running clearances between the fluid foil elements and the thrust disk. 
     It is another object of the present invention to provide such a bearing with fluid foil members that are not preloaded by spring forces against the thrust disk at zero speed. 
     It is another object of the present invention to provide such a bearing with zero starting torque when there is no gravity induced preload forces. 
     It is another object of the present invention to provide such a bearing with an extremely low lift-off/touch-down speed which is consistent with zero preload forces. 
     It is another object of the present invention to provide such a bearing with very low starting and stopping wear which is consistent with zero preload forces and a low lift-off/touch-down speed. 
     It is another object of the present invention to provide such a bearing with converging wedge channel features (formed on the surface of the fluid foil element) that limit fluid foil losses from the channel at the radial outer and radial inner edges of those channels. 
     It is another object of the present invention to provide such a bearing with converging wedge channel ramps formed on the surface of the fluid foil elements that have compound curve profiles with concave curvatures radially, flat slopes circumferentially at zero speed and convex curvatures at operating speed when fluid dynamic and spring forces are applied to the fluid foil elements. The profiles will form and function as scoops with radially wide fluid foil inlets, radially narrowing channel widths along the circumferential fluid foil paths, and rounded circumferentially trailing edges. 
     It is another object of the present invention to provide such a bearing with a fluid foil pattern that reduces fluid pressure losses when the process fluid travels “down the ramp”(in a nominally circumferential direction that is opposite to the rotation of the thrust disk) adjacent to the fluid foil element. 
     It is another object of the present invention to provide such a bearing with a fluid flow pattern that is regenerative with a different flow path for each regenerative flow loop. 
     It is another object of the present invention to provide such a bearing with a vortex flow pattern. 
     It is another object of the present invention to provide such a bearing with fluid flow element blanks and spring foil elements that are fabricated by optically masked chemical etch techniques. 
     It is another object of the present invention to provide such a bearing with foil elements that are extremely flat owing to the processes used to roll and heat treat the foil metal and the processes used to form (e.g. etch) the foil blanks and elements. 
     It is another object of the present invention to provide such a bearing with foil elements that have tightly held thickness tolerances. 
     It is another object of the present invention to provide such a bearing with fluid foil members that are single fluid foil elements, one for each side of the bearing. 
     It is another object of the present invention to provide such a bearing with fluid foil elements that are formed from blanks by pressing steeply sloped joggles to function as diverging wedge channels which allowing the gradually converging wedge channel ramps to result without plastic deformation as the straight line connection between the joggles. 
     It is another object of the present invention to provide such a bearing with fluid foil elements that are formed from annealed blanks of nickel steel, such as Inconel 750X, by pressing at room temperature. 
     It is another object of the present invention to provide such a bearing with a spring foil member that has local spring rates that vary with radial and circumferential location so as to accommodate variations in fluid pressure within the converging wedge channel adjacent to the local areas of the spring foil member. 
     It is another object of the present invention to provide such a bearing with a tilting pad spring support system that controls the relative support forces applied to the underside of the fluid foil at a multiplicity of locations circumferentially along the converging wedge channel. 
     It is another object of the present invention to provide such a bearing with a tilting pad spring support system having circumferentially closer spacing between support/pivot lines moving up the converging ramp from the leading edge of the fluid foil to the trailing edge of the fluid foil. 
     It is another object of the present invention to provide such a bearing with a tilting pad spring support system providing increasing support forces and support spring rates moving up the converging ramp from the leading edge of the fluid foil to the trailing edge of the fluid foil. 
     It is another object of the present invention to provide such a bearing with a tilting pad spring support system with radially curved support/pivot lines. 
     It is another object of the present invention to provide such a bearing with a tilting pad spring support system that assures retention of a reasonably hydrodynamically optimized scoop shape for the fluid foil converging ramps over a wide range of bearing operating speeds and bearing thrust loads. 
     It is another object of the present invention to provide such a bearing with foil elements that are not welded or brazed to form foil member assemblies. 
     It is another object of the present invention to provide such a bearing with pins (rigidly attached to the bearing housing) which position and resist rotation of the foil elements. 
     It is another object of the present invention to provide such a bearing with self shimming capability utilizing the resilient mounting and preload characteristics of the thrust disk, the spacer ring and the foil&#39;s self shimming rings to prevent variations in bearing axial play and sway space due to variations in foil thickness and foil coating thickness. 
     It is another object of the present invention to provide such a bearing with fluid foil elements, spacer ring element, thrust disk element and thrust plate element that can be installed in the thrust bearing quickly and easily. 
    
    
     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 thrust bearing of the present invention; 
     FIG. 2 is an enlarged partial view of oval  2  of FIG. 1 illustrating the thrust plate and spacer area of the compliant foil fluid film thrust bearing of the present invention; 
     FIG. 3 is a plan view of the fluid foil member of the compliant foil fluid film thrust bearing of the present invention; 
     FIG. 4 is an enlarged sectional view of the fluid foil member of FIG. 4 taken along lines  4 — 4 ; 
     FIG. 5 is another enlarged sectional view of the fluid foil member of FIG. 4 taken along lines  5 — 5 ; 
     FIG. 6 is a plan view of the outer spring foil element of the spring foil member of the compliant foil fluid film thrust bearing of the present invention; 
     FIG. 7 is a plan view of the intermediate outer spring foil element of the spring foil member of the compliant foil fluid film thrust bearing of the present invention; 
     FIG. 8 is a plan view of the middle spring foil element of the spring foil member of the compliant foil fluid film thrust bearing of the present invention; 
     FIG. 9 is a plan view of the intermediate inner spring foil element of the spring foil member of the compliant foil fluid film thrust bearing of the present invention; 
     FIG. 10 is a plan view of the inner spring foil element of the spring foil member of the compliant foil fluid film thrust bearing of the present invention; 
     FIG. 11 is a plan view, partially cut away foil-by-foil, of the complaint foil fluid film thrust bearing of the present invention; 
     FIG. 12 is a sectional view of the unloaded fluid foil member and spring foil member of FIG. 11 taken along line  12 — 12 ; 
     FIG. 13 is a sectional view of the loaded fluid foil member and spring foil member of FIG. 11 taken along line  12 — 12 ; 
     FIG. 14 is a plan view of an alternate compliant foil fluid film thrust bearing of the present invention; 
     FIG. 15 is a sectional view of the alternate compliant foil fluid foil thrust bearing of FIG. 14 taken along line  15 — 15 ; and 
     FIG. 16 is a partial plan view of the fluid foil member of an another alternate compliant foil fluid film thrust bearing of the present invention; 
     FIG. 17 is a partial plan view of the outer spring foil element of the spring foil member used with the fluid foil member of the alternate compliant foil fluid film thrust bearing of FIG. 16; 
     FIG. 18 is a partial plan view of the intermediate outer spring foil element of the spring foil member used with the fluid foil member of the alternate compliant foil fluid film thrust bearing of FIG. 16; 
     FIG. 19 is a partial plan view of the middle spring foil element of the spring foil member used with the fluid foil member of the alternate compliant foil fluid film thrust bearing of FIG. 16; 
     FIG. 20 is a partial plan view of the intermediate inner spring foil element of the spring foil member used with the fluid foil member of the alternate compliant foil fluid film thrust bearing of FIG. 16; and 
     FIG. 21 is a partial plan view of the inner spring foil element of the spring foil member used with the fluid foil member of the alternate compliant foil fluid film thrust bearing of FIG.  16 . 
    
    
     DETAILED DESCRIPTION OF THE 
     PREFERRED EMBODIMENTS A turbomachine utilizing the compliant foil fluid film thrust 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 bar  16 . The thrust and radial bearing rotor  18  is disposed around the tie bar  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 . 
     As best illustrated in FIG. 2, a thrust bearing spacer  32  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  38  in the recess  26  of the center bearing housing  22  and extend into holes  38  in the thrust bearing plate  28 . A thrust bearing fluid foil member  40  and thrust bearing spring foil member  42  are disposed on either side of the bearing rotor thrust disk  24  and thrust bearing spacer  32 . On one side, the fluid foil member  40  and spring foil member  42  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 fluid foil member  40  and spring foil member  42  are held in position radially and circumferentially by the pins  34  which extend from the center bearing housing  22 , through holes in one spring foil element  42 , through holes in one fluid foil element  40 , traverse the bore of the thrust bearing spacer  32 , through holes in the opposite side fluid foil element  40 , the holes in the opposite side spring foil member  42  and into holes  38  in the bearing thrust plate  28 . The bearing thrust plate  28  is biased towards the center bearing housing  22  by a Belleville washer  23  disposed between the lip  25  on the bearing thrust plate  28  and the compressor housing  30 . 
     The thickness of the thrust bearing spacer  32  is several thousandths of an inch greater than the thickness of the bearing rotor thrust disk  24 . Variations in the foil or foil coating thicknesses inherently cause compensating variations in the spacing between the thrust plate  28  and the housing  22 . Thus, variations in bearing sway space and bearing compliance due to foil thickness tolerances are prevented. 
     FIGS. 3-5 illustrate a fluid foil member  40  integrally formed from a single flat disk termed a foil blank. A plurality of individual fluid foils  41  are formed from a flat sheet of a nickel steel such as Inconel 750X by room temperature pressing steeply sloped joggles to function as diverging wedge channels while allowing the gradually converging wedge channel ramps to result without plastic deformation as the straight line connection between the joggles. The fluid foil members would normally be annealed both during forming and use and may be coated prior to forming the joggles with any number of a myriad of low friction or friction reducing coating materials which can protect the metal from abrasion during starting and stopping, and inadvertent and occasional high speed touch-downs. The coating would also provide for some embedment of contamination particles. 
     The individual fluid foils  41  (shown as twelve) are generally chevron shaped and connected to an outer self shimming ring  44  by support webs  45 . Fluid passages or openings  46  are formed between adjacent support webs  45 . Every fourth fluid passage  46  includes an indexing tab  47 . Each aerodynamic foil  41  has a trailing edge  48  with a rounded trailing point or nose  49  and a leading edge  50  a generally straight ramped contour from the leading edge  50  to the trailing edge  48 . The individual fluid foils  41  have a generally scoop shape as best illustrated in the two sectional views of FIGS. 4 and 5. One or two rows of openings  51  are provided at the leading edge  50  of each individual fluid foil  41  to allow fluid to enter the leading edge of the converging ramp adjacent to the thrust disk from under the fluid foil in the area of the spring foil members  42 . 
     As illustrated in FIGS. 6-10, the thrust bearing spring foil member  42  generally comprises an outer spring foil element  53  (FIG.  6 ), an intermediate outer spring foil element  54  (FIG.  7 ), a middle spring foil element  55  (FIG.  8 ), an intermediate inner spring foil element  56  (FIG.  9 ), and an inner spring foil element  57  (FIG.  10 ). 
     The outer support foil element  53 , shown in FIG. 6, generally includes an inner connector ring  60  and an outer self shimming ring  61  with a plurality (shown as twelve) of narrow width spring foils  62  extending therebetween. The narrow spring foils  62  consist of an inner curved foil section  63  extending outward from the inner connector ring  60  at a forward angle (in the direction of the thrust disk&#39;s rotary motion) and an outer curved foil section  64  extending inward from the self shimming ring  61  at a forward angle. The inner and outer foil sections  63 , 64  are nominally oriented approximately forty degrees from circumferential at all points along their length and join together to form a generally nose-shaped foil section  65 . The outer foil sections  64  are individually connected to the outer self shimming ring  61  by a radially extending connector or supporting web  66 . 
     The narrow spring foil  53  is narrowest at the connection to the inner connector ring  60  and gradually increases in width to the point or tip of the nose  65  and continues to gradually increase in width in the outer foil section  64  to the connector web  66 . The connector web  66  is of an even greater, generally radially increasing, width. A plurality of indexing tabs  67  (shown as four) extend inward from the outer self shimming ring  61  in order to enable precise alignment of the outer support foil element  53  with the other elements of the thrust bearing spring foil member  42  and with the fluid foil member  40 . 
     The intermediate outer support foil element  54 , shown in FIG. 7, generally includes an inner connector ring  73  and an outer self shimming ring  72  with a plurality (shown as twelve) of wide spring pads or foils  71  extending therebetween. The wide spring foils  71  consist of an inner curved foil section  75  extending outward from the inner connector ring  73  at a forward angle and an outer curved foil section  76  extending inward from the self shimming ring  72  at a forward angle. The inner and outer foil sections  75 ,  76  join together to form a generally nose-shaped foil section  77 . Each of the outer foil sections  76  are individually connected to the outer self shimming ring  72  by a pair of radially extending connector or supporting webs  74 . A plurality of concentric outer dam rings  78  (shown as three) extend between the adjacent wide spring foils  71  generally where the wide spring foils  71  and the pair of webs  74  connect. 
     The wide spring foil  71  is narrowest at the connection to the inner connector ring  73  and gradually increases in width to the point or tip of the nose  77  and continues to gradually increase in width in the outer foil section  76  to the connector webs  74 . A plurality of indexing tabs  79  (shown as four) extend inward from the outer self shimming ring  72  in order to enable precise alignment of the outer support foil element  54  with the other elements of the thrust bearing spring foil member  42  and with the fluid foil member  40 . 
     The middle spring foil element  55  is illustrated in FIG.  8  and includes an outer self shimming ring  81  and indexing tabs  82  identical to and aligned with the corresponding elements in the outer spring foil element  53 , intermediate outer spring foil element  54  and fluid foil member  40 . A plurality of pairs (shown as twelve) of generally nose-shaped narrow spring foils  83  extend between the inner connector ring  84  and the outer self shimming ring  81 . Each of the pair of narrow spring foils  83  generally straddle the narrow spring foils  62  of the outer spring foil element  53  and are aligned at the leading and trailing edges of the wide spring foil  71 . 
     A connector web  87  joins each of the narrow spring foils  83  to the outer self shimming ring  81 . A plurality of concentric outer dam rings  85  (shown as two) extend between adjacent pairs of narrow spring foils  83  where they connect to webs  87 . A plurality of concentric inner dam rings  86  (shown as three) extend between adjacent pairs of narrow spring foils  83  near the inner connector ring  84 . 
     FIG. 9 illustrates the intermediate inner spring foil element  56  which has an outer self shimming ring  89  and indexing tabs  91  which are common to the spring foil member  42 . The plurality of pairs of intermediate width spring foil elements  92  (shown as twelve) are disposed between the outer self shimming ring  89  and an inner connector ring  90 . Each of said pair of intermediate width spring foil elements are connected to the outer self shimming ring by a pair of webs  94 . By intermediate width is meant a width between the width of the narrow width outer and inner foils  62 ,  83  and the width of the wide intermediate outer foil  71 . 
     Each of the pairs of webs  94  for the intermediate width foils are joined by a circumferential stiffening ring  93 . In addition, a single outer dam ring  96  (which could alternately be a plurality of dam rings) and a plurality (shown as two) of concentric inner dam rings  95  extend between adjacent pairs of intermediate width foils  92 , at the connection to the webs  94  and adjacent to the inner connector ring  90 , respectively. Each of the pair of intermediate width foils  92  generally are positioned over one of said pair of narrow width spring foils  83  of the middle spring foil member  55 . 
     The inner spring foil element  57  is illustrated in FIG.  10  and includes an outer self shimming ring  100  and indexing tabs  101  identical to and aligned with the corresponding elements in the other spring foil elements  53 ,  54 ,  55 , and  56  and fluid foil member  40 . A plurality of trios (shown as twelve) of generally nose-shaped narrow spring foils  102  extend between the inner connector ring  103  and the outer self shimming ring  100 . Each of the trio of narrow spring foils  102  have widths that are generally the same as the narrow spring foils  62  of the outer spring foil element  53  and the pairs of narrow spring foils  83  of the middle spring foil element  55 . Of the trio of narrow spring foils  102 , two are aligned at the leading and trailing edges of one of the pair of intermediate width spring foils  92  and the third of the trio of narrow spring foils are aligned with the trailing edge of the other of the pair of intermediate width spring foils  92 . 
     A connector web  105  joins each of the trio of narrow spring foils  102  to the outer self shimming ring  100 . These webs  105  of each trio of narrow spring foils  102  are joined together by a circumferential stiffening ring  106 . 
     The precise relationship of the fluid foil member  40  and spring foil member  42 , including the five spring foil elements  53 ,  54 ,  55 ,  56  &amp;  57 , is best illustrated in FIGS. 11-13. FIG. 11 is a plan view of the assembled fluid foil member  40  and spring foil member  42  with the individual spring foil elements overlain and positioned by their respective indexing tabs. Proceeding in a counterclockwise rotation, the individual spring foil elements are individually cut away in a foil-by-foil manner to show their relationship with each other. 
     The outer spring foil  62  is shown in the arc identified as “A”. Arc “B” illustrates the outer intermediate foil  71  aligned over the outer spring foil  62 . A pair of middle spring foils  83  are laid over the outer spring foil  62  and intermediate spring foil  71  in arc “C”. Arc “D” then includes the pair of intermediate inner foils  92  and finally the trio of inner foils  102  are shown over the pair of intermediate inner foils  92  in arc “E”. The fluid foils  41  are then laid over the assembled spring foils in arc “F”. 
     An even more precise relationship of the various foils is illustrated in FIGS. 12 and 13. FIG. 12 illustrates an unloaded fluid foil member and spring foil member while a loaded fluid foil member and spring foil member are shown in FIG.  11 . It should be noted that the foil  62  is located nearer to the trailing edge of foil  71  than to the leading edge of foil  71 . Foils  83  are located nearer the trailing edge of foils  92  than to the leading edge of foils  92 . This assures that the tilting pad spring support will deliver more force to the underside of the fluid foil  40  near its trailing edge  48  than its leading edge  50 . 
     The fluid foil blank, as well as the individual elements of the thrust bearing spring foil member  42 , can be formed from flat metal sheets by optically masked chemical etch techniques. Nickel steels, such as Inconel 750X for the fluid foil and Inconel 718 for the spring foil elements can be used. Typically, foil thicknesses are between 0.004 inches and 0.007 inches. The nickel steel metal sheets from which the foil elements are formed normally are heat treated to full hardness in a vacuum oven (1300 degrees Fahrenheit for about twenty hours for Inconel 750X). The five individual elements of the thrust bearing spring foil member  42  can be assembled by stacking the spring foil elements without bonding. The relative micro movement of these foils in use provides coulomb damping. 
     The shape of the fluid foils  41 , namely an open-faced channel having a converging width and sloping “walls”, induces regenerative vortex flow patterns in the process fluid across the fluid foil. The process fluid generally enters the leading surface of the foil  41  from the trailing edge  48  of the preceding foil. Any make-up process fluid is provided from the inner and outer diameter of the leading edge or from openings  51 . There is some fluid leakage flow from the inner diameter and outer diameter, respectively, near the trailing edge  48 . 
     The compliant fluid foil members  40  are located adjacent to the two thrust faces of the thrust disk  24 . The spring foil members  42  provide support for the fluid foil members  40  but allow them to follow the axial and overturning motion of the disk  24 . The forces applied by the fluid foil members to the thrust disk through the process fluid vary inversely with foil to disk gap and vary proportionally with disk deflection. 
     The thrust plate  28  is held away from the center bearing housing  22  by the total thickness of the outer self shimming rings of the fluid foil members  40  and spring foil members  42  and the thickness of the thrust bearing spacer  32 . The thrust bearing spacer  32  is slightly thicker than the thrust disk  24  so that there is a small clearance between the fluid foil member  40  and the thrust disk  24  that is not affected by normal variations in foil or foil coating thickness. The bearing has no preload force and has zero starting torque when the disk&#39;s axis of rotation is oriented ninety degrees to the force of gravity. With the regenerative vortex flow pattern established by the contour of the fluid foil elements, the bearing running clearances are significantly improved (increased) and lift-off speeds are significantly less than previously possible. 
     The converging wedge channel ramps formed in the surface of the fluid foil members have compound curve profiles with concave curvatures radially, flat slopes circumferentially at zero speed and convex curvatures at operating speed when fluid dynamic and spring forces are applied to the foil elements. The tilting pad support for the fluid foil elements assures a near optimum convex curvature circumferentially over a wide range of bearing thrust loads. The profiles will form and function as scoops with radially wide fluid flow inlets, a radially narrowing channel width along the circumferential fluid foil paths, and rounded circumferentially trailing edges. This multi-path regenerative vortex fluid foil pattern reduces fluid pressure losses when the process fluid travels “down the ramp” in a nominally circumferential direction that is opposite to the rotation of the thrust disk adjacent to the fluid foil member. 
     The particular tilting pad bearing of the present invention has five spring foils and a fluid foil. The five spring foil elements have four support lines for each fluid foil ramp. The relative force exerted at each of these support lines is proportionally controlled by the circumferential spacing between several curved spring foil lines that generally match the curved shape of the formed fluid foil. 
     The outer spring foil element  53  has twelve individual narrow width spring foils  62  adjacent to either the thrust disk on one side of the thrust bearing or to the thrust surface of the housing on the other side of the thrust disk. Each of these narrow spring foils  62  provides a curved, generally nose shaped support line. 
     The wide intermediate outer spring foils  71  are supported by the individual narrow spring foils  62  and can articulate both by flexing and by tilting on the single support line of the narrow spring foils  62  to either a nose up or nose down position as required by the rest of the thrust bearing. 
     Above the wide spring foils  71  are the pair of narrow width spring foils  83  which provide two support lines per ramp and can give support at the nose and the leading edge of the wide spring foils  71 . The two foil support lines of narrow spring foils  83  push underneath and near the peripheral center of each of the two pairs of intermediate width foils  92  so that these intermediate width foils  92  can tilt nose up or nose down as required. Lastly, narrow width spring foils  102  have three support lines, one at the trailing edge of the leading spring foil  92 , one at the leading edge of the trailing spring foil  92  and one at the trailing edge of the trailing spring foil  92 . 
     Effectively, intermediate width foil  92  provide a line of support pushing on the underside of the fluid foil relatively close to the start of the converging ramp, and then the next three support lines moving up the ramp are provided to the fluid foil through spring foil  102 . This makes the entire spring structure behave like the articulated supports of tank treads, where as you go over uneven ground the tank treads themselves can articulate because you have a support structure that articulates. The circumferential spacing between these various support lines, now referring to the support lines of spring foil elements  53 ,  55 ,  56 , and  57 , control the percentage of the total force coming through the spring foil elements which is delivered to each circumferential location on the fluid foil above the spring foil element  57 . 
     The highest spring force will be at the nose of the formed fluid foil where the fluid compressed pressure is highest. The support forces and the spring rates produced are lower when moving away from the spring foil nose. While these foil elements are primarily tilting members to allow compliance to the fluid foil, they also have spring properties in their own right. 
     The concentric dam rings of spring foil elements  54 ,  55 , and  56  effectively serve as dams to limit leakage out of the thrust bearing particularly when the bearing is operated hydrostatically. The thrust bearing of the present invention is particularly useful in a hydrostatically augmented thrust bearing such as described in U.S. patent application Ser. No. 08/622,250 filed Jun. 14, 1996 by Robert W. Bosley and Ronald F. Miller entitled “Hydrostatic Augmentation of a Compliant Foil Hydrodynamic Fluid Film Thrust Bearing”, incorporated herein by reference. The openings  51  at the leading edge of the fluid foil  41  are specifically provided for this hydrostatic augmentation. The thrust bearing will, however, function as a hydrodynamic thrust bearing with or without openings  51 . 
     The tilting pad thrust bearing operates with its fluid foil flexed circumferentially in a convex fluid dynamically optimized shape on the surface adjacent to the thrust disk without relying upon air pressure in the spring area. Air pressure will, however, provide additional convex shaping and load capacity and provide good adaptability. 
     While the nose shape of the individual spring foils has been shown to be relatively identical for the five spring foils and the fluid foil, the shape of the spring foil nose can be varied to control the spring rate. As illustrated in FIGS. 14 and 15, the nose of the trailing edge of middle spring foil  83 ′ can be sharper, that is have a smaller radius than the noses of the trailing edge of inner spring foil  102  and intermediate inner spring foil  92 . This will stiffen up the spring rate at the end of the converging ramp so that there will be less fluid flow leakage and provide greater support for the fluid foil nose. 
     The generally scoop-shaped converging wedge channels formed on the surface of the fluid foil members induce vortex fluid foil patterns and limit process fluid foil losses from the channels at the radial inner and outer edges of the foils. This, together with the self shimming construction and other features of the present invention, provides a thrust bearing having a high load carrying capacity, good damping, small sway clearances, low running torque, high running foil to disk clearances, zero preload force, low starting torque, low lift-off/touch-down speeds, and low wear. In addition, all of this is achieved with a low parts count, low manufacturing cost, and ease of assembly. 
     Further, the fluid foil members and spring foil elements can have a straight radial shape as shown in FIGS. 16-21. Except for having a straight radial shape rather than the nose shape previously described, the fluid foil member  40 ′ having straight radial foils  141  and spring or support foil members  53 ′,  54 ′,  55 ′,  56 ′, and  57 ′ would function generally the same and have the same relative spacing and relative positioning with respect to each other. The outer support foil element  53 ′ of FIG. 17 includes narrow width radial spring foils  162 , intermediate outer support foil element  54 ′ of FIG. 18 includes wide radial pads or foils  171 , middle support foil element  55 ′ of FIG. 19 includes pairs of narrow width radial spring foils  183 , intermediate inner support foil element  56 ′ of FIG. 20 includes pairs of wide radial spring pads or foils  192 , while inner support foil element  57 ′ of FIG. 21 includes trios of narrow width radial spring foils  202 . The generally flat (radially) fluid foil surfaces, and underlying spring foils which maintain generally flat (radially) fluid foil surfaces of FIGS. 16-21, are not intended to establish a scoop shaped converging ramp and therefor do not generate vortex regenerative process fluid flows. 
     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.