Patent Publication Number: US-9404529-B2

Title: Foil journal bearing applicable to high speed machining center

Description:
This application is a divisional of application Ser. No. 13/950,107, filed Jul. 24, 2014 (issuing Oct. 6, 2015, as U.S. Pat. No. 9,151,322), which is a divisional of application Ser. No. 13/441,807, filed Apr. 6, 2012 (now U.S. Pat. No. 8,801,290), which is a continuation of international application PCT/US2010/051451, with an international filing date of Oct. 5, 2010, and which claims and it is hereby claimed the benefit of U.S. Provisional Application 61/278,385, filed Oct. 6, 2009. Each of these prior applications is hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The technical field relates to the development and application of high speed machining and grinding machines, particularly those suitable for fabrication of microscopic features, and of high speed foil journal and thrust bearings suited for use in such a micro-machine. 
     BACKGROUND 
     It is well known in the machining arts that cutting tools perform best when urged into contact with a workpiece at a specific speed or within a specific range of speeds. Although the particulars of the speed range may vary with workpiece composition or workpiece attributes such as hardness or ductility, this behavior is generally observed. In particular, it is observed in both metal and ceramic work-pieces, for tool steel, carbide, coated carbide, and ceramic tools, and for cutting tools of specified geometry such as milling cutters as well as for tools comprising a bonded assemblage of a more or less randomly-oriented cutting edges such as diamond or ceramic grinding tools. 
     Many rotating cutting tools, such as mills, burrs, and drills, mount the cutting edges at their periphery. Thus as the tool diameter is reduced to enable the creation of smaller features in the workpiece, commonly termed micro-machining, the tool is required to rotate faster to maintain the preferred peripheral cutting speed range since the linear velocity is given by the product of the angular velocity and the tool radius. 
     For purpose of illustration only, a reasonable value for the preferred cutting speed of aluminum is about 75 meters (about 246 feet) per minute. Thus, a rotary tool with a radius of about 500 micrometers (0.5 millimeter or about 0.02 inch) should be operated at a rotational speed of about 25000 revolutions per minute (rpm). Reducing the tool diameter to about 50 micrometers (about 0.002 inch) leads to a tool rotational speed of 250,000 rpm. With a still further reduction to about 25 micrometers (about 0.001 inch), it would necessitate a tool rotational speed of about 500,000 rpm for the cutting tool to operate in the preferred range. 
     Thus, micro-machines capable of micro-machining must, for robust cutting performance, operate at significantly higher rotational speeds than conventional machine tools. More specifically, for machined features 100 micrometers (about 0.004 inch) in width or less, the micro-machine should be capable of operation at several hundreds of thousands of rpm, which poses significant challenges in the manufacture and operation of such devices. 
     As the size of the machined feature shrinks, the need for high precision in the micro-machine increases. For example, control of tool run-out to micrometer levels is required, placing stringent requirements on tool-micro-machine attachment systems and on machine spindle alignment and run-out, among other issues. Since the machine spindle will be supported on bearings, many of the required micro-machine features promote a need for innovative bearing designs. 
     In turn, the machine spindle and bearings must be assembled into a support structure or housing. It may therefore be important that the housing, bearing, and spindle design be consistent with assembly practices which assure high precision in the assembled micro-machine. The assembly practices should be robust, that is, accepting of normal part or component tolerances and assembler skill level, without significant prejudice to performance. The assembly practices should also enable disassembly and reassembly without significant prejudice to performance. 
     SUMMARY 
     One embodiment may include a high speed micro-machine with a drive system for rotation of a spindle about a rotation axis and supported by at least one gas-cooled foil journal bearing and at least one gas cooled foil thrust bearing. The bearings in turn may be supported by a housing, with a pressurized gas reservoir. The housing may be split along at least one joint line into at least two parts. The housing may contain a pressurized gas reservoir. The joint line may lie in a plane substantially containing the axis of rotation or may lie in a plane substantially perpendicular to the axis of rotation. Both types of joint lines may be present simultaneously. 
     The micro-machine may be powered by a gas turbine or an electric motor or both, and, if powered by an electric motor, may incorporate a compressor for generation of pressurized gas for cooling the bearings. The drive system may be partitioned with a portion located on the spindle and a portion located on the housing. 
     The spindle may have a hollow portion bounded by an end-cap with inclined through holes for capture and retention of machining debris. Extending from the end-cap there may be a solid cylinder with, on its end, a tool holder for acceptance and retention of a tool shank. 
     The foil bearings, both thrust and journal, may be constructed of a number of top foil segments, supported by a like number of bump foil segments and mounted to and supported by a housing or support. The journal bearings may have the form of a hollow cylinder with the top foil and bump foil segments arranged around the interior surface; the thrust bearings may have the form of a disc-like thrust plate with bump foils and top foils mounted on one of the planar disc surfaces. The foil bearings, both thrust and journal, may be split and reassembled to facilitate assembly of the micro-machine. The foil bearings may be cooled by provision of pressurized gas flow directed along the channels in the housing. The gas flow may be directed along the cylindrical axis of the journal bearings, and radially inward, that is, from the edge of the generally disc-like thrust plate toward its center, for the thrust bearings. 
     In one embodiment, a foil journal bearing may include a plurality of top foils overlying a like number of bump foils, each supported by the interior circumference of a hollow, generally cylindrical, housing having a length. The widths of the top foils and bump foils may be substantially equal to the length of the housing. The bump foils may have a plurality of ridges and flats oriented generally parallel to the cylinder axis. Each of the top and bump foils may be attached to the housing. Attachment may be by engagement of mounting features on the foils which extend across substantially their width, with complementary features in the housing. The complementary features in the housing may be uniformly distributed around the interior circumference of the housing. 
     The mounting feature of a top foil may be located between the ends of the top foil to thereby divide the top foil into a leading segment and a trailing segment. The top foils may be arranged so that the leading segment of a first top foil overlies the trailing segment of an adjacent top foil. Each bump foil may have a length generally equal to the length of the leading segment and secured at a single location to underlie the leading segment of one of the top foils. 
     In a second embodiment, a foil journal bearing may include a top foil overlying a bump foil, each supported by the interior circumference of a hollow, generally cylindrical, housing having a length. The widths of the top foil and the bump foil may be substantially equal to the length of the housing. The bump foil may have a plurality of ridges and flats oriented generally parallel to the cylinder axis. The top foil and the bump foil may each have a length substantially equal to the interior circumference of the housing, and a width substantially equal to the length of the housing. Each of the top foil and bump foil may have a mounting feature extending substantially across its width for engagement with a feature of complementary shape in the housing. The bump foil may have a plurality of regions, each including groups of generally uniformly-spaced ridges and flats, the regions being separated by extended flat regions. 
     In a third embodiment, a foil journal bearing may include a single foil secured to and supported by a hollow, generally cylindrical, housing with a cylinder axis, an interior circumference, and a length. The width of the foil may be substantially equal to the length of the housing. The length of the foil may be substantially equal to twice the interior circumference of the housing, and the foil may have a mounting feature extending across its width. The engagement feature may engage a feature of complementary shape in the housing. The foil may have two portions of approximately equal length where one portion of the foil may be a bump foil having a plurality of regions each having groups of generally uniformly-spaced ridges and flats, the regions being separated by extended flat regions, and where the other portion of the foil may be a generally flat top foil which may overlie the bump foil which may overlie the interior of the housing. The mounting feature may be positioned at about the mid-length of the foil, or, alternatively, at the end of the bump foil portion of the foil. 
     One embodiment may include a foil thrust bearing comprising a plurality of generally planar top foils overlying a like number of coextensive bump foils, which may be supported by a generally disc-like thrust plate with a center and a circumference. 
     The top foils and their associated bump foils may be positioned in the annular region formed between two circles, an inner circle and an outer circle, where each circle may be centered on the thrust plate center. The foils may be bounded by four edges; on two opposing edges, the edges may have the form of circular arcs whose radii correspond to the radii of the inner and outer circle. The two other opposing edges are linear and may be portions of radial lines lying between the inner and outer circle. One of the linear ends of both the bump foil and thrust foil may be free and not secured to the thrust plate. One of the linear edges of the top foil and one of the linear edges of the bump foil may be secured to the thrust plate. The foils may be welded to the thrust plate or mechanically secured, for example, by means of a structure on the edge of the foil engaging a slot or other structure of complementary shape in the thrust plate. The foils may be generally equally spaced around the annular region and separated by gaps between adjacent foils. 
     Each of the bump foils may be divided into a series of circumferential tabs by a number of circumferentially-oriented slots extending from the free end of the foil part-way toward the secured end of the foil. Each of the tabs may be corrugated to form a series of substantially parallel ridges separated by flats, each of the ridges and flats being uniformly and substantially equally spaced apart and each of the ridges and flats being oriented generally parallel to the secured edge. Each of the ridges may be characterized by a peak with a height, with each of the flats having a centerline oriented generally parallel to the ridges. 
     The thrust plate may have a plurality of openings which permit the radial inflow of cooling gas to the bearing. The openings may be positioned on a circle with a radius greater than the radius of the outer circle. The openings may be positioned in the gaps between adjacent foils. 
     The peaks of the ridges in each of the bump foil tabs may be generally aligned and collinear. Alternatively, the peaks of the ridges in one tab may be aligned with the center-lines of the flats in adjacent tabs, or, equivalently, the center-lines of the flats in one tab may be aligned with the peaks of the ridges in adjacent tabs. The peaks of all of the plurality of ridges of the bump foil tabs may be of the same height or may be of differing heights. 
     The top foils may be coated with a hard lubricious layer to minimize wear during startup and shut down of a machine when the bearing runner will contact and rub against the top foil. 
     A second embodiment may incorporate all of the bump foils and all of the top foils in individual planar sheets, a bump foil sheet and a top foil sheet. 
     The bump foil sheet may be stamped and/or pierced to create a number of spaced-apart circumferentially-arranged bump foils, each with four edges and each of the bump foils being unsecured on three edges and continuous with the sheet on its fourth edge. The bump foils, as in the first embodiment, may have the general configuration of annular arcs with the foils disposed about a bump foil center. 
     The top foil sheet may be stamped and/or pierced to create a number of spaced-apart circumferentially-arranged top foils, coextensive with the bump foils, each with four edges and each of the top foils being unsecured on three edges and continuous with the sheet on its fourth edge. The top foils, as in the first embodiment, may have the general configuration of annular arcs, with the foils disposed about a top foil center. 
     The foil thrust bearing may then be assembled by assembling the bump foil sheet to the thrust plate and overlying the bump foil sheet with the top foil sheet, ensuring that the centers of the thrust plate, bump foil sheet, and top foil sheet coincide and that the top foils overlie the bump foils. The top foil and bump foil sheets may be attached to the thrust plate in any convenient fashion, but welding is preferred. 
     The details of the top foils and bump foils of the second embodiment may parallel those of the first embodiment. Also, a plurality of openings for ingress of cooling gas, similarly positioned as in those of the first embodiment, may be formed in at least the thrust plate and, if required, in the top foil and bump foil sheets. 
     The thrust bearings of both the first and second embodiments may be split along a line passing through the center of the thrust plate and passing through the gaps between the foils for bearing disassembly and reassembly. Guidance features may be incorporated into the thrust plate for ease of alignment during reassembly. 
     Other illustrative embodiments of the invention will become apparent form the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while disclosing embodiments of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows in quarter cut-away perspective view a first embodiment of a micro-machine capable of high rotational speeds comprising a single piece hollow rotor with integral turbine wheel suitable for imparting rotation. 
         FIG. 2  shows the embodiment of  FIG. 1  in cross-section to better illustrate the placement of the thrust and journal bearings and the manner in which air is bled from the turbine exhaust for bearing cooling. 
         FIG. 3  shows in partial cut-away perspective view the embodiment of  FIG. 1  to better show some additional features, particularly flow paths for cooling air. 
         FIG. 4  shows a foil journal bearing adapted for receiving and distributing cooling air flow. 
         FIG. 5  shows further detail of attachment means for the bump foil and  FIG. 6  shows further detail of attachment means for the top foil in the bearing of  FIG. 4 . 
         FIG. 7  shows the complementary top foil bearing shell attachment feature of the bearing of  FIG. 4 . 
         FIG. 8  shows, in partial cut-way, a plan view of foil thrust bearing with a slotted bump foil for improved bearing compliancy and cooling. 
         FIG. 9  shows a cross-sectional view of a portion of the foil thrust bearing of  FIG. 8 . 
         FIG. 10  shows a fragmentary detail of flange  15  of  FIG. 1  to better illustrate features for capture and retention of machining debris. 
         FIG. 11  shows, in quarter cutaway perspective, a second embodiment of the invention. 
         FIG. 12  shows the second embodiment of the invention in cross-section. 
         FIG. 13  shows a fragmentary view of the abutting faces of two elements of an assembled rotor suitable for application in the second embodiment of the invention. 
         FIG. 14  shows a means by which the magnets may be assembled to and retained in a one-piece rotor to enable its use in the second embodiment of the invention. 
         FIG. 15  shows a first embodiment of a foil journal bearing adapted to generate increased hydrodynamic pressure. 
         FIG. 16  shows the top foil configuration and the generated hydrodynamic pressure profile of the foil journal bearing of  FIG. 11  during operation, and compares its top foil configuration and hydrodynamic pressure profile during operation with that of a more conventional foil journal bearing. 
         FIG. 17  shows a second embodiment of a foil journal bearing adapted to generate increased hydrodynamic pressure. 
         FIG. 18  shows a third embodiment of a foil journal bearing adapted to generate increased hydrodynamic pressure and a second embodiment of a cooperative foil bearing shell retainer groove geometry. 
         FIG. 19  shows a fourth embodiment of a foil journal bearing adapted to generate increased hydrodynamic pressure and a second embodiment of a cooperative foil bearing shell retainer groove geometry. 
         FIG. 20  shows a fragmentary view of an alternate embodiment of a bump foil. 
         FIGS. 21, 22 and 23  show exemplary composite bump foils fabricated from two individual bump foils. 
         FIG. 24  shows, in exploded perspective view, an embodiment of a foil thrust bearing. 
         FIG. 25  shows in fragmentary perspective view an embodiment of a bump pad similar to that shown in  FIG. 24 . 
         FIG. 26  shows a first preferred embodiment of a bump foil pad. 
         FIG. 27  shows the bump foil profile taken along line  27 - 27  shown in  FIG. 26 . 
         FIG. 28  shows the streamlines of fluid flow over an operating foil bearing pad. 
         FIG. 29  shows the streamlines of fluid flow over an operating foil bearing pad when radially-inflowing pressurized air is introduced at the outer diameter of the bearing pad. 
         FIG. 30  shows a second embodiment of a foil thrust bearing. 
         FIG. 31  shows an embodiment of a bump foil pad adapted for mechanical attachment to a thrust plate. 
         FIG. 32  shows an embodiment of a bump foil pad for promoting development of a turbulent boundary layer on the top foil during operation. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows, in quarter cut-away perspective view, a first embodiment of a micro-machine  10 , capable of high rotational speed and comprising a single piece hollow rotor with integral turbine wheel suitable for imparting rotation. In one embodiment, the components of the micro-machine may be constructed and arranged so that the micro-machining center is capable of high operating speeds of greater than 700,000 rpm, or greater than 1,000,000 rpm, or up to 1,500,000 rpm. 
     The micro-machine employs both journal foil bearings  28 ,  30  and thrust foil bearings  60 ,  62  (best seen at  FIG. 2 ). Foil bearings support the shaft on a self-generated film of air, so that, at operating speeds, in either a journal or thrust bearing, there is no contact between the shaft and the bearing. Also, because of the low viscosity of the operating fluid (air or any operating process gas or liquid), frictional losses are lowered and temperature rise, though not insignificant, is inherently lower than most liquid lubricated bearings. Thus, issues of thermal expansion and its influence on shaft-bearing tolerances are reduced. 
     Foil bearings, for example, foil journal bearing  100  shown in  FIG. 4 , typically include three major elements: a smooth, thin top foil  108  which provides a smooth bearing surface; a corrugated compliant support foil or bump foil  106  which underlies the top foil and provides resilient support to the top foil; and a supporting shell  102  which positions and secures the foils. 
     As described in greater detail later, interaction between the top foil  108  and the rotating shaft  112  generates the air film which supports the shaft, while the corrugated compliant bump foil  106  contributes both stiffness and damping to the bearing. The pressure supporting the shaft load is conveyed by the air film to the smooth top foil  108 , which deflects and elastically deforms the corrugations of the bump foil  106  and thereby imparts stiffness to the bearing. Also, the peaks  106 ′ of the ridges of corrugated bump foil  106  are in contact with the underside of the top foil  108  while the valleys  106 ″ of the bump foil are supported by the inner surface of bearing shell  102 . The geometry of the corrugations assures that, as the corrugations are displaced vertically, they will simultaneously spread laterally. Hence, the peaks of the corrugations will rub against the underside of the top foil, and valleys of the corrugations will rub against the inner surface of the supporting shell. The friction associated with the rubbing of the foil will dissipate energy and impart damping to the bearing. 
     Bearing stiffness and damping is important in micro-machine applications because rotating machine tools such as mills, burrs, or drills generate complex, time-varying, three-dimensional loads even under invariant or steady-state cutting conditions. Cutting loads may also change abruptly, for example, when the tool enters or exits a cut. Thus, micro-machine bearings must be selected to provide sufficient stiffness and damping to accommodate both steady state and transient loads without generating an instability or excessive deflection. 
     Even with air as an operating fluid, operating speeds of up to 1,500,000 rpm may result in some increase in bearing temperature. Since foil bearings employ compliant elements, they may be made more tolerant of thermal expansion or of shaft-bearing misalignment than rolling contact bearings, but only at the expense of reduced bearing stiffness and damping. It may therefore be preferred to locate and position bearings to minimize bearing misalignment and to apply enhanced temperature management strategies to the bearings to minimize thermal effects. 
     The supporting air film is self-generated, resulting from the relative motion of the shaft and the bearing. The ability of the bearing to support the load imparted by the shaft depends on the relative motion of shaft and bearing and only after the shaft is rotating rapidly is the air film capable of fully supporting it. Hence, during periods of low shaft rotational speeds, for example, during start-up and shut-down, the shaft may contact, rub on, and wear the bearing surfaces, potentially limiting useful bearing life. The bearing surfaces, particularly the top foil surfaces which may contact the moving shaft, may therefore be coated with a wear-reducing surface coating. The coating may be both hard and lubricious. A suitable coating may be KOROLON (trademark) 1350 coating, a proprietary, spray-gun-applied, nickel-chrome coating with solid lubricants developed by MiTi, Albany, N.Y. 
     Referring to  FIGS. 1 and 2 , the micro-machine  10  may include a hollow rotor  12  connected to an extended overhung shaft  14  by means of flange  15 , which has an outer surface  16  and an inner surface  16 ′. Flange  15  may be an integral part of rotor  12  with each machined from a common stock or may be attached to the front end of rotor  12  by suitable means, including welding, brazing and mechanical fasteners such as screws  18  ( FIG. 2 ) which pass through holes  17  to engage threaded holes on rotor  12  (not shown). Shaft  14  is adapted to incorporate tool holder mechanism  20  for support and releasable retention of tool  22 . Tool holder mechanism  20  may be received in a cylindrical cavity of precise dimension intended for shrink-fit retention of shank  23  of cutting tool  22 . 
     The rotor  12  may include integral turbine wheel and thrust disc  42  (best seen at  FIGS. 2 and 3 ) driven by a pressurized gas or gas mixture, including air, as a source of power. The pressurized gas may be introduced at nozzle ring  24 , where, by means of guide-vanes  82  (indicated in  FIG. 2 ), the flow is converted to supersonic jet-streams and then directed at a series of reaction turbine blades  40  ( FIG. 3 ) mounted on turbine wheel/thrust disc  42  ( FIG. 2 ). 
     Turbine wheel/thrust disc  42  has an axis of rotation  36  which is generally coaxial with the centerline of rotor  12 . The thrust disc portion of turbine wheel/thrust disc  42  may preferably be coated with a wear resistant coating including, for example, thin, dense, tribological chromium alloys, titanium nitride, and others. The coating may be both hard and lubricious. As with the journal bearings, the proprietary nickel-chrome coating with solid lubricants, KOROLON (trademark) 1350 coating developed by MiTi, Albany, N.Y. may be suitable. The direction of rotation for the turbine blade configuration shown is indicated by arrow  37 . 
     Rotor  12  may be supported by two foil journal bearings  28 ,  30 , mounted inside machine  10  split housing  50 , comprising first portion  52  and second portion  54 , as shown best at  FIG. 2 . The foil journal bearings, which incorporate several novel features and will be described more fully later, may have very thin foil retaining shells ( 102 ,  FIG. 4 ) comparable to the thickness of the smooth top foil ( 108 ,  FIG. 4 ). Thin bearing shell  102  need not be a cylinder as shown in  FIG. 4 , but may instead be a wrapped foil in a quasi-cylinderical shape. After insertion of compliant elements, bump foil  106  and top foil  108 , the complete journal bearing may be positioned, as bearings  28 ,  30  in split housing  50 . An analogous construction, described in detail later, may be employed for the thrust bearings  60  and  62 , arranged in opposition to accommodate axial loads applied to spindle  12 . 
     The relative thickness and stack-up heights of the foil bearing compliant elements are extremely small relative to the thickness of split housing  50 . The stack-up height may range from a few micrometers up to 1.25 mm (about 0.05 inch), while the wall thickness of the machines&#39; bearing housing may range from a few mm up to 10 cm (3.9 inches) or more, depending on the size of the machine. The bearings shown have a thin (bearing) shell relative to the wall thickness of split shell  50 . But it will be appreciated that with suitable adjustment to the geometry of split shell  50 , in ways well know to those skilled in the art, thick shell bearings like those shown in  FIG. 4  may also be employed. 
     The journal bearings may be positioned on either side of turbine wheel/thrust disc  42 , that is, one bearing is positioned within the interior diameter of each of first and second portions  52  and  54  respectively of split housing  50 . The bearings are suitably dimensioned to accommodate the outer diameter of hollow rotor  12 . 
     Rotor  12  may be supported by a split housing  50  comprising a first portion  52  and a second portion  54 . Portions  52  and  54  may be releasably attached, for example, with mechanical fasteners, along a common attachment plane generally positioned on the mid-plane of the turbine wheel/thrust disc  42 . 
     Opposed thrust bearings  60  and  62  (for clarity shown only in  FIG. 2 ) may be mounted in recessed openings  61  and  63  ( FIG. 2 ) of housings  52  and  54 , and, more particularly, within rings  24  and  32  respectively. Opposed thrust bearings  60  and  62  will accommodate axial loads, that is, loads applied along the rotation axis  36  and directed either toward or away from tool  22 . With this configuration, the micro-machine may be capable of operation in all orientations and attitudes. 
     The micro-machine may be assembled with the following procedure. Rotor  12  (after assembly to flange  16  and overhang  14 , if required) may be balanced to 0.1 microgram-meter (about 1.4 microinch-ounce avdp.) for a 6 to 8 mm (about 0.24 to 0.32 inch) shaft, 0.3 to 0.6 microgram-meter (about 4.2 to 8.4 microinch-ounce avdp.) for a shaft with a 4 mm (about 0.16 inch) diameter with a micro-balancing machine. Thrust bearing  60  may be positioned on ring  32 , possibly in a mounting recess, not shown, of housing  52 . The rotor may then be advanced into housing  52  containing foil journal bearing  30  in a direction corresponding to ‘A’ shown on  FIG. 2  until turbine wheel/thrust disc  42  just contacts thrust bearing  60 . Thrust bearing  62  may be inserted in recess  63  of ring  24  of housing  54 . Housing  54  containing foil journal bearing  28  may then be inserted over rotor  12  in a direction shown as ‘A’ in  FIG. 2  until turbine wheel/thrust disc  42  just contacts thrust bearing  62 . 
     Housings  54  and  52  may then be releasably attached, for example, through the use of bolts (not shown) inserted into hole  70  and which engage the thread in aligned threaded hole  72 . A V-clamp, sized to engage the cylindrical and end surfaces of flanges  24  and  32 , may also be used. Generally, separate and independent alignment and attachment features may be employed for housings  52  and  54 . Suitable alignment features may include mating features such as dowel pins on one housing engaging mating holes on the second housing (not shown). 
     Housing surfaces  28 ′ and  30 ′ support journal foil bearings  28  and  30  respectively. The relative alignment of all the bearings, but particularly of the journal bearings  28  and  30 , will depend on the alignment, both angular and positional, achieved between housings  52  and  54 . Because of the inherent compliance afforded by the foil bearings, both thrust and journal, some misalignment of the housings may be tolerated. However, it will be appreciated that compliant element foil bearings&#39; internal components may be extremely thin with a total stack height of only 0.01 to 0.02 inches (0.0254 to 0.05 cm) so that any misalignment of housings  52  and  54  is likely to be minor. 
     More generally, it will be appreciated that the housings must be assembled and arranged to at least not exceed the maximum allowable bearing tolerance. Inasmuch as some of the maximum allowable bearing tolerance will be required to accommodate the dimensional changes undergone by the bearing as it expands due to temperature rise in use, in one embodiment the housing misalignment should preferably be maintained at no more than half of the total allowable tolerance. 
     The temperature rise of the bearing and thus the dimensional changes undergone by the bearing in use may be minimized by provision of features to promote enhanced cooling. A representative foil journal bearing adapted for such enhanced cooling is shown in  FIGS. 4, 5, 6, and 7 .  FIG. 4  shows a foil journal bearing  100  with bearing center  110  in cross-section. The bearing may include a bearing shell  102  with features  104  such as one or more notches or grooves formed in an inner surface and adapted for retention of the bump foil  106  and top foil  108 . Top foil  108  may incorporate features  105 , for example, a tongue constructed and arranged to be generally complementary to shell feature  104 , intended to compliantly engage shell features  104  and thereby retain the top foil under rotation of shaft  112  about center  114  in the direction indicated by arrow  111 . 
     Bump foil  106  may have a similar retaining feature  105 ′ to feature  105  of top foil  108 , as shown in fragmentary view  FIG. 5 , compliantly positioned and restrained between features  105  and  104 . Alternatively, bump foil  106  may be secured by spot welding, as at  109 , to shell  202  ( FIG. 15 ). It will be appreciated that these attachment methods are alternate methods and are not to be employed in conjunction. As shown in  FIG. 6 , shell retaining feature  105  may incorporate slots  107  or similar features. Such features may be incorporated to maintain a more constant compliance of the top foil by at least partially offsetting the geometric stiffening resulting from introducing shaped retaining feature  105  and impart axial compliance to the bearing. These features are effective in imparting a self-alignment capability to the bearing to accommodate minor axial shaft misalignment. 
     The inclusion of grooved features  104  in bearing shell  102  may also be effective in promoting improved ingress of cooling air, and facilitates its distribution within the foil bearing to promote improved cooling. The process may be made even more effective by ‘damming’ or obstructing one end of groove  104  to induce circumferential airflow, as illustrated in  FIGS. 6 and 7 , which illustrates airflow in the direction of arrow  118  entering groove  104  at a first end  115  and exiting at second end  116 . By blocking end  116 , by a bent-up feature on top foil  108 , or by forming groove  104  only partway into the bearing shell, to create, by means of the remaining shell portion, an endwall at  116 , or by any other means known to those skilled in the art, incoming airflow  118  may be redirected circumferentially by means of slots  107  ( FIG. 6 ) in directions shown by arrows  119  and  119 ′ to more fully participate in cooling the journal bearing. 
     The benefits of such cooling may be appreciated by consideration of the relative locations of bearing center  110  and shaft  112  center  114 . Because the bump foil is compliant, it may flex and displace when loaded by the film of air on which the shaft is supported. In operation, bearing heating may result in an increase in the shaft or journal temperature, causing it to expand and more closely approximate the diameter of the bearing shell. Thus, the greater the temperature rise of the journal, the greater the initial clearance and the greater the initial compliance which must be designed into the bearing, thereby compromising bearing stiffness and degrading the micro-machine accuracy. Hence, as will be discussed in greater detail in subsequent sections, it may be preferred that all bearings be gas-cooled and that the bearings, as indicated in the exemplary design of  FIG. 4 , be adapted to more efficiently receive and distribute the cooling gas. 
     Similar considerations apply to foil thrust bearings, a representative example of which  120  is shown in partial cut-away plan view in  FIG. 8  and in selected cross-section  9 - 9  in  FIG. 9 . Here, foil thrust bearing  120  may include bearing plate  122 , suitably positioned with respect to shaft  130  rotating about its center  131  in a direction indicated by arrow  137 . Bearing plate  122  may be overlaid by a plurality of bump foils, an example of which is shown as  126  and by a foil sheet  138  which has been slit and formed to develop a plurality of top foil segments  128  separated from foil disc  138  along a three-sided path shown as  136  and deformed to create an elevated lip  125  along its remaining line of attachment to foil disc  138 . Bump foil  126 , as best shown in  FIG. 9 , comprises a series of ridges  129  of varying heights separated by flats  127 . The designators ‘a-g’ associated with the ridges of cross-section  9  correspond to the ‘a-g’ designators associated with bump foil  126  in  FIG. 8 . Foil  126  may include a plurality of circumferentially-oriented slots  135  to promote improved gas flow within the bearing and improved bearing cooling. Other features of foil thrust bearings adapted for use in micro-machining centers will be addressed in a subsequent section. 
     Returning to  FIG. 2 , ring  24  may receive pressurized air flow (or other process gas) through inlet port  80 , which discharges at a higher, even supersonic speed, after passing through a series of guide-vanes  82 . The developed jet flow may be directed toward reaction turbine blades  40  where it may transfer its kinetic energy to the blades to induce rotation of the rotor  12 . After interacting with the turbine blades the process gas may expand and cool and primarily discharges into annular exit port  84 . 
     As will be appreciated from the prior discussion of the characteristics of foil thrust bearings, clearance exists between the backing plate and bump foil/top foil combination of a thrust bearing. Thus, the pressurized gas of annular exit port  84  can bleed from the outer radius of thrust bearing  60  to its inner radius where it may then be constrained to flow through foil journal bearing  30  and between rotor  12  and housing  52 . The overall flow path is indicated at  90  ( FIG. 3 ). After traversing the length of housing  52 , the gas flow may exit at end  53  of housing  52 . A slinger washer (not shown) may be located at the point of exit to redirect the gas flow away from the cutting tool to minimize airborne debris. By this scheme of flow passages, the bearings may be both cooled and kept free of external contamination, such as the debris generated during cutting. 
     A parallel flow scheme, providing equivalent benefits, and shown as  92  in  FIG. 3 , may be followed for foil thrust bearing  62  and foil journal bearing  28 . Here, however, there is no corresponding plenum to  84 , and flow through the bearings simply results from the small but necessary clearances between the rotating turbine wheel/thrust disc  42  and the stationary flow shaping features of ring  24 . Provision may also be made for bleed-off of excess gas if flow paths  90  or  92  become obstructed and create excessive back pressure. Openings  74  and  74 ′, located on rings  32  and  24  respectively, may create pathways for release of vent flows  76  ( FIG. 3 ) and  76 ′ ( FIG. 2 ). 
     Flow passages  90  and  92  assure that the thrust bearings  60  and  62  will not be deprived of cooling air as long as the rotor is rotating. Thus, there will always be cooling gas, which may also serve as a lubricant, available for the bearings. An additional benefit is that the gas flow may serve to exclude debris from the bearings. 
     Another means of managing machining debris is indicated in  FIG. 1 , which shows a series of channels  21  extending through flange  16  into the interior of rotor  12 . These channels may extend radially outward from surface  16  to surface  16 ′ of flange  15  and are inclined to rotation axis  36 , as may best be seen in  FIG. 10 . Thus, the openings  21 ′ of channels  21  on surface  16  may be located on the circumference of a first circle  37  on face  16 , and the channels  21  may extend radially outward and at some inclination to rotation axis  36  to form openings  21 ″ on surface  16 ′, where openings  21 ″ may be located on the circumference of a circle whose diameter is greater than that of circle  37 . 
     Under rotation, this channel  21  configuration may act as a scoop and ensures that any debris entering openings  21 ′ may be transported through channels  21  and deposited within hollow rotor  12 , thereby minimizing airborne debris. 
     A second embodiment of the invention is shown in partial cutaway perspective in  FIG. 11  and in section in  FIG. 12 . In this embodiment, the housing may be split longitudinally, that is, along the axis of rotation of the rotor. 
     Micro-machine  140  may include a rotor assembly  142  adapted to accommodate a cutting tool (not shown) in tool-holder portion  151 , comprising tool-holder cavity  150 . Rotor assembly  142  may be an assembled multi-piece rotor comprising permanent magnet motor rotor  144 , tool-holder portion  151 , and impeller attachment portion  153 , with all three pieces secured and attached to one another through shrink-fitted sleeve  158 . As shown, radial flow compressor impeller  146  may be a separate element attached to rotor assembly  142 , specifically to impeller attachment portion  153 , for example, by mechanical fastener  148 . However, radial flow compressor impeller  146  and impeller attachment portion  153  may also be fabricated as a single piece. Stator  160  may be incorporated in split machine housing  165  and positioned to cooperatively interact with permanent magnet motor rotor  144  to induce rotation of rotor assembly  142 . 
     Rotor assembly  142  may be supported on split journal foil air bearings  154  and restrained from motion along the direction of rotation axis  152  by housing-mounted, opposed thrust bearings  156 ,  156 ′ acting against rotor disc  157 . Cooling gas inlets  164 ,  166  may be provided to direct pressurized cooling gas to journal bearings  154  (inlet  164 ) and to thrust bearings  156  (inlet  166 ). After passing over the bearings, the cooling gas may be discharged at outlet  162 . 
     Pressurized cooling gas may be derived from any convenient source. The micro-machine shown may be capable of providing pressurized air without recourse to an external source. Here, incoming air flow  170 , induced by rotation of impeller  146 , passes through air passage  171  and may be compressed by cooperative interaction of impeller  146  and the shaped inner surfaces of air passage  171  and discharged through ducts  172  into storage tank  174 , where it may be accessed at outlet  176  and fed through cooling ports  164  and  166  in controlled fashion. 
     Rotor assembly  142  has been described as a multi-piece rotor comprising tool-holder portion  151 , impeller attachment portion  153 , and permanent magnet rotor  144 , which may be permanently attached using a shrink-fitted sleeve. The shank or solid portion (i.e., not containing the tool-holder cavity  150 ) of the tool-holder portion is illustrated at  149 . As depicted, the various elements are shown in butt-joint configuration so that only the frictional interaction between the sleeve and the individual elements enables torque transmission from one element to another. Another approach may be to incorporate complementary features on the abutting members to improve the mechanical interlock. An example is shown in  FIG. 13 , which, without limitation, shows permanent magnet rotor  144  with slot  180  and tool-holder portion  151  with complementary key  181 . When properly aligned, key  181  will tightly engage slot  180  when face  182  abuts face  183 . Thus, the mechanical engagement of key  181  and slot  180  will be effective in transmitting torque while a shrink-fitted sleeve may overlie shaft surfaces  184  and  185  and hold them in longitudinal alignment. Of course, more complex mechanically-engaging features than the slot and key configuration shown may also be adopted. These may include configurations which also tend to axially align the shaft surfaces  183 ,  184 . 
     It may also be possible to fabricate rotor assembly  142  as a one piece non-magnetic shaft, not incorporating compressor impeller  146 , comprising slots or pockets for incorporation of magnets for the rotor and a short shrink-fitted sleeve to aid in magnet retention under rotation. Such a configuration is shown in  FIG. 14 , which may show a segment of rotor assembly  142  which may have been suitably pocketed or slit at locations  190  to accept appropriately oriented magnets which may be retained at least by shrink-fitted sleeve  158 , and may be supplemented by adhesive or other retention means. 
     Fabrication of the micro-machine may include; finishing the assembly of the rotating group first; conducting final machining/polishing and balancing to achieve acceptable rotor dynamic behavior; positioning the rotor in bearings; positioning the rotor and bearings in one of the parts of the split housing; and finalizing assembly by positioning and releasably attaching the remaining parts of the split housing. The assembly may be performed in this sequence to ensure acceptable rotor dynamic behavior, which is not achievable if the rotor is not balanced as a complete assembly. It will be appreciated by reference to  FIGS. 11 and 12  that this assembly sequence may be facilitated by the journal and thrust bearings being split bearings. Details of the split bearing designs which may be employed will be discussed in a later section. 
     The assembled complete rotor, as shown in  FIGS. 11 and 12 , may be driven by the electric motor comprised of permanent magnet rotor  144  and stator  160 , which together form a brushless DC electric motor. As is well known to those skilled in the art, the stator of such a motor comprises electrically conducting coils energized by an electronic commutation controller system (not shown). Stator  160 , like the bearings, may be split along the split axis or axes of the housing, requiring that electrical connection (not shown) be made between the windings associated with the split portions of the stator wiring for motor operation. Upon reaching its maximum operating speed, compressor impeller  153  in cooperation with the shaped interior surfaces of air passage  171  produces pressurized air to greater than ambient pressure which may be temporarily stored in tank  174 . When required, the pressurized air may be directed through exit port  176  to the bearings and the motor rotor and stator in order to minimize any temperature rise. After cooling the machine elements, the air may be vented at port  162 , though it may be preferred that some air bleed past the clearance between tool holder portion  149  and housing  165  and thereby be directed toward the work piece, in order to remove the debris and cool the cutting edges of tool  151 . 
     Thrust bearings  156  and  156 ′ may be cooled from their outer diameters toward their inner diameters by directing cooling flow from port  166 . This is effective in enhancing the cooling because it is in opposition to the flow of frictionally-heated air impelled by centrifugal force imparted by rotor disk  157  toward the outer diameter. By directing cooling air flow, in the radially inward direction, the two opposing fluid flows are in “counter-flow” configuration which maximizes heat exchange and more effectively cools the thrust bearings. 
     The micro-machine design shown in  FIGS. 11 and 12  may, with minor modification, be adapted to operate on pressurized gas and thereby eliminate the electric motor. In a first design variant, the motor may be operated on pressurized gas introduced at air passage  171 . By modification of compressor impeller  146 , it may be adapted to function as a radial turbine inducing rotation of rotor assembly  142  and discharging air, still at greater than atmospheric pressure, into tank  174 . Alternatively, the machine may be constructed with the drive systems, electric and gas shown and used either in either mode. 
     In a second design variant, an independent turbine may include an impeller suitably surrounded by gas flow shaping surfaces formed in housing  165  and may be located on and coupled to rotor assembly  142  and operated by pressurized gas. No other modifications need be made to the micro-machine. In this design variant, the gas discharged from the independent turbine may simply be vented, and the cooling air stored and discharged from tank  174  may be generated by impeller  146  as previously described. 
     Foil journal and thrust bearings adapted for improved cooling and thereby suited for use in micro-machining centers have been previously described. These bearings may incorporate other novel features as described below. 
     Another embodiment of a foil journal bearing  200  may be shown in  FIG. 15 . In this design, each of a plurality of top foils  206  may be secured to a split housing comprising (housing) shell elements  202  and  203  by spot welds  109 . In common with the bearing  100  of  FIG. 4 , the top foils of bearing  200  may overlie a number of suitably-secured corrugated bump foils  206 . Bump foils  206  may be secured to the housing  202  by spot welds  109  and compliantly distance top foils  208  from the inner diameter of housing  202 . It should be noted that bearing housing  202  may be split along line x-x and readily and consistently re-assembled. Shell elements  202 ,  203  may include complementary alignment or engagement features such as guide pins  220  fitting into reamed holes  221 , or shoulder screws engaging complementary partially-threaded holes (not shown) to ensure alignment. Alternatively, alignment may be achieved through dimensional control and alignment of the bearing support features in a split housing. 
     Top foils  208  may circumferentially extend on either side of their mounting locations and have both a trailing edge segment  208 ′ and a leading edge segment  208 ″ relative to their attachment location. As shown, each top foil  208  necessarily comprises a leading edge and a trailing edge. With appropriate modification to the top foil  208  mounting and retention procedure, top foil lengths corresponding to the leading edge length and to the trailing edge length may be independently mounted adjacent to one another without prejudice to their performance. However, there may be a definite relationship between the bump foil strips and the top foil and in the way they are anchored to the bearing shell. As shown in  FIG. 15 , attachment points  109  may always be located in the vicinity of the trailing end of the uppermost of the overlying top foil segments. This may be done to systematically vary the structural stiffness of each foil segment from leading end to trailing end of top foil  108 . 
     As shown in  FIG. 15 , the lengths of trailing  208 ′ and leading  208 ″ edge segments may be unequal but, in a design with N foils, their lengths are so chosen as to enable the trailing edge of a first foil to overlie the leading edge of a second foil and the trailing edge of a second foil to overlie the leading edge of a third foil. This relative foil placement continues around the inner circumference of the bearing housing until the trailing edge of the Nth foil overlies the leading edge of the first foil, thereby extending the composite top foil surface around the entire inner circumference of the housing, as shown by the four-top-foil configuration shown in  FIG. 15 . 
     In operation, the shaft&#39;s surface is initially in rubbing contact with the top foil surfaces until, with increasing shaft rotation speed, a thin hydrodynamic film develops between the shaft and top foil surface. The shaft is then levitated from the bearing&#39;s surface and separated from it by an air film. 
     The top foil  208  may be free, subject to any restoring forces exerted by compliant bump foil  206 , to bend and pivot about the center of the bearing mounting groove  204 , responsive to the influence of the dynamic, static, or thermal movements of the shaft with respect to the bearing. Top foil  208  may also deform elastically, and such elastic deformation may be local. For example, the trailing edge  208 ″ of a first top foil, partially supported by the leading edge  208 ′ of a second top foil, as shown in their unloaded configuration in  FIG. 15 , may locally deform, as shown in the fragmentary view of bearing  200  shown in  FIG. 16 , to create a stepped bearing surface at  209 . This stepped bearing surface  209  creates an “elasto-pressure dam” which is effective in enhancing the hydrodynamic pressure profile resulting from the shaft-bearing interaction, enabling higher bearing loads and more rapid development of the hydrodynamic film. 
     In one embodiment, curve  240  ( FIG. 16 ) shows the pressure versus distance, measured along the bearing, developed by shaft  212  rotating in a direction indicated by arrow  224  for a top foil deformed to develop feature  209 . The top foil configuration shown may be typical, for example, of the configuration which would be adopted by the bearing of  FIG. 11  under the same conditions.  FIG. 16  also shows, as curve  230 , the pressure versus distance profile which may be developed by a more conventional top foil configuration  219 , shown in ghost. The increase in hydrodynamic pressure generated by the elasto-pressure dam according to one embodiment may be identified by the cross-hatched area  235  representing the difference between the hydrodynamic pressure profiles. The elasto-pressure dam may also increase damping and improve bearing stability and provide pressure variation that is effective in pumping additional air through the bearing for more efficient bearing cooling. 
     The novel stepped bearing surface may also accommodate two-phase (gas-liquid) flow, for example, compressed air with entrained water droplets. In conventional foil journal bearings, any liquid mixed with the gas vaporizes. Because the mixture is heated by passage through the bearing clearance, it results in a significant volume expansion and precipitates a rapid pressure rise, which if severe may interfere with proper bearing operation. But a compliant elasto-pressure dam may respond to the localized rise in pressure by deforming further, thereby relieving the pressure increase and promoting stable bearing operation. 
     Other embodiments are also within the scope of the invention. The structural compliance of foil bearings, as described here, may be established through the interaction of the top foil with the underlying bump foil, which is deformed or corrugated to a form comprising an alternating series of ridges and flats. These bump foils  106 ,  206 , as shown in  FIGS. 4 and 15 , are anchored at one end and free at the other end and thus may slide circumferentially, and provide damping, when pressure is applied radially, for example, if the shaft is displaced from its original central position toward the bearing housing wall. 
     Generally, the bump foil  206  height or the change in height between adjacent ridge tops and flat bottoms in the bump foil is constant and independent of position in the foil, leading to a uniform elastic response at any location along their length. However, as illustrated in  FIG. 16 , the elasto-pressure dam may arise in response to a local geometric irregularity  209 . In the case shown in  FIG. 16 , geometric irregularity  209  is attendant on the geometric discontinuity resulting from the overlapping top foil geometry, where portion  208 ″ overlies portion  208 ′ of the adjacent top foil. But similar results may be obtained through the use of a bump foil whose compliance varies with position in a predictable manner. 
     A suitable configuration is shown in  FIG. 17 , which shows a foil journal bearing  250  with a single foil  260  with a retaining feature  255  engaging complementary groove  254  in housing  252 . Foil  260  may comprise a first, partially corrugated, portion of foil  261  in contact with the inner diameter of shell  252  and approximately equal in length to the circumference of the inner shell diameter; and a second, planar, portion  262 , again of length approximately equal to the circumference of the inner shell diameter, overlying portion  261 . Foil  260  is shown in its operating configuration. 
     At locations  264 , top foil  262  is unsupported by any corrugations like those at locations  266 . At locations  264 , therefore, top foil  262  may adopt, as shown, a configuration similar to that shown at  209  in  FIG. 16 , that is, it may exhibit a configuration like that of the elasto-pressure dam described previously ( 209  at  FIG. 16 ). 
     Similar performance may be obtained with other top foil and bump foil configurations. For example, in  FIG. 18 , bearing  270  may comprise a separate bump foil  276  secured at retainer groove  274 ′ in housing  272 , again exhibiting corrugated and non-corrugated portions, and a separate top foil  278  secured at retainer groove  274  in housing  272 . In another exemplary foil journal bearing  280 , shown in  FIG. 19 , a single foil  281  may comprise a bump foil  286  exhibiting corrugated and non-corrugated sections and a smooth top foil  288  which overlies it. The individual sections are disposed on either side of retainer groove  284  located in housing  282  and complementary foil retaining feature  285 . Bump foil segment  286  extends in counterclockwise orientation and underlies top foil section  288  which extends clockwise. It will be appreciated that housings  252 ,  272 , and  282  for the journal bearings shown in  FIGS. 17, 18, and 19  may also be split for ease of assembly and disassembly. In this case, the bump foils and top foils may be unwrapped from the shaft for bearing disassembly or wrapped around the shaft for assembly. 
     In  FIG. 19 , the retaining groove  284  and complementary retaining feature  285  are not trapezoidal as previously shown, but generally resemble the upper case Greek letter omega. It will be appreciated that the specific design approaches described and depicted are illustrative and not limiting. A variety of tongue and groove or mating or nested retaining features are within the scope of the invention. 
     In other select embodiments, the longitudinally-varying bump foil geometry of  FIGS. 17 to 19  need not symmetrically increase and decrease but may slowly ramp up and rapidly decrease, as shown in the partial bump foil segment  296  shown in  FIG. 20 . The ability to systematically vary bump foil stiffness is not limited to only the designs shown but may be employed in any foil journal bearing including the designs of  FIGS. 4 and 15 . Finally, the bearing may employ more than a single bump foil so that the bump foils will act cooperatively. Exemplary configurations are shown in  FIGS. 21, 22, and 23 . 
     In  FIG. 21 , portions of two identical bump foil segments  306  and  306 ′ are shown arranged in opposition to cooperatively deform under all loadings and, in combination, form composite bump foil  307 . 
     In  FIG. 22 , two bump foils  316  and  316 ′ of varying height are nested to form composite bump foil  317 . Bump foil  317  enables abruptly varying bump foil stiffness with displacement since bump foil  316 ′ will contribute to the over bump foil stiffness only after bump foil  316  has been deflected by the amount shown in  FIG. 22 . It will be appreciated that the spacing between individual ridges, shown as constant in  FIG. 22 , may be varied as in  FIG. 23  to enable more progressive stiffness variation with displacement. In fragmentary view in  FIG. 23 , nested individual bump foils  326  and  326 ′ comprise composite bump foil  327 . In this configuration, however, the separation between the bump foils may vary. Thus, using in this paragraph for convenience the letter S instead of the symbol shown in  FIGS. 22 and 23  along with the associated numbers not as sub-scripts, at location  324 , the maximum separation is S1, at location  323  the maximum separation is S2, and at location  322  the maximum separation is S3. Because S1&lt;S2&lt;S3, the stiffness of composite bump foil  327  will progressively increase with displacement. 
     All the composite bump foil configurations shown were fabricated from two individual bump foils. This is not intended as a limitation and it is recognized that the concepts may be readily extended to comprehend more than two individual foils. 
     One embodiment of a foil thrust bearing design  400  is shown in exploded perspective view in  FIG. 24 . Thrust foil bearing  400  may include an underlying thrust plate  402 , bump foil sheet  404 , and an overlying top foil sheet  406  positioned and located axially about centerline  408 , which is the axis of rotation of runner  410  (shown in ghost) rotating in a direction indicated by arrow  412 . Bump foil sheet  404  may include a plurality of equally-spaced corrugated annular segments  414  each of which may be overlaid by a top foil pad  416  bounded by three slits  417 ,  418 , and  419  and may remain secured to top foil sheet  406  only along line  420 . Pad  416  may therefore be free to flex and pivot about line  420  in response to any applied load with a component directed along centerline  408 . In similar manner to the foil journal bearing, because pad  416  may elastically deform under load, the shape which it adopts will be moderated by the corrugated foil segment  414  which underlies it. Again corrugated bump foil pad  414  should be suitably profiled to induce in the top foil a preferred configuration when under load. 
     The corrugated bump foil pads  414  may be fabricated as individual corrugated segments and attached directly to thrust plate  402 ; examples of this configuration will be shown later. The configuration shown in  FIG. 18  may however be formed in similar fashion to that employed to form the top foil pads  416 . Thus, as shown in  FIG. 25 , bump foil sheet  404 ′ may be shaped and formed by stamping then slit along slit lines  422 ,  424  and  426  to form bump foil tab  414 ′ secured to bump foil sheet  404 ′ along line  428 . The tab may then be shaped and formed by stamping to form bump foil tab  414 ′. The embodiment shown in  FIG. 24  in which a plurality of bump foil pads  414  and pads  416  may be formed from, and remain attached to, a larger foil sheet ( 404  and  406  respectively) may be preferred for fabrication of small bearings, for example, those bearings with outer diameters of between 0.1 inch and 2 inches (0.254 cm and 5 cm). It will be appreciated that as the bearing size decreases the dimensions of pads  414  and  416  will likewise decrease and may pose increasing challenges in reliably positioning and attaching these pads to a thrust plate  402 . Larger bearings, say up to 100 inches (2.54 meters) in diameter, may optionally employ individual bump foils and pads or may continue to employ the foil sheet construction elements of the small bearings. 
     It may be noted that because the bearing may include a number of equally spaced bearing elements  414 ,  416  the bearing may be readily split, for example, along C-C ( FIG. 24 ) to facilitate installation. As with the split journal bearing, it may be preferred to introduce features such as guide pins fitting into reamed holes, or shoulder screws engaging complementary partially-threaded holes (not shown) to assure alignment on re-attaching the split bearing segments. To facilitate reassembly, it may be preferred to split thrust plate  402  but to only partially split bump foil sheet  404  and top foil sheet  406 , for example, from their outer circumference to centerline  408 . For such a thrust bearing, the sheets  404 ,  406  may be elastically flexed to enable sufficient clearance around a shaft, for example, tool-holder portion  149  in  FIG. 11 , for bearing installation and removal. 
     Another embodiment of a bump foil configuration  414  is shown in plan view in  FIG. 26 . As with the journal bearing, the bump foil may include a series of alternating flats  430  and ridges  432  here separated into individual sectors by circumferential slits  434 . The heights of the ridges  430  may vary with their location in the foil. In this figure, the ridge to ridge or flat to flat spacing ‘S’ is constant, but other configurations in which the dimension ‘S’ will vary with position on the foil are also comprehended in this description. The relative ridge height may be indicated by the numbers 1 to 5 associated with the ridges respectively, where  5  may represent a large ridge height and 1 a small ridge height Like the bump foil shown in  FIG. 8 , the foil of  FIG. 26  may be circumferentially slit. The slits  434  may minimize ‘cross-talk’ between adjacent ridge-flat tabs and thereby enable each ridge-flat tab to respond to an applied load more independently of its neighbors. Slits  434  may also facilitate cooling gas flow through the bearing. 
     Thus, each ridge  432  may comprise only a portion of the overall radial distance spanned by the overall bump foil  414 . The ridge  432  heights may vary systematically with position and the ridge height variation generally conforms to a compound wedge. The compound wedge may taper upward, both circumferentially from the leading edge of the foil to the trailing edge and also from the leading edge of the bump foil outer circumference to the trailing edge of the inner circumference.  FIG. 27  is a section taken along line  27 - 27  in  FIGS. 18 and 20  and shows the relative ridge  432  height variation from the leading edge of the bump foil outer circumference to the trailing edge of the inner circumference. As will be appreciated by consideration of items 1 to 5, the height of the bumps may gradually increase from item 1 to item 5. 
     As a consequence of this configuration, a cross flow may be induced in the fluid in the composite diverging wedge region. In this region the fluid may be subject to a circumferential pressure gradient which may encourage the fluid film to move along circumferential stream lines. However, there may also be centrifugal forces promoting radial flow. 
     These two components of the flow velocities may be orthogonal to each other in the compound tapered region. This flow behavior, depicted as a series of streamlines overlaid on the outline of a bearing pad segment  414 , may be shown in  FIG. 28 . The runner velocity may be in the direction of arrow  440 . The circumferential flow (stream lines  450 ) entering the bearing gap space may interact with radial, centrifugally-induced flow (streamlines  460 , shown as dotted) and may be very effective in transferring momentum to the radial flow and imparting a circumferential component to the stream lines. Thus, little side leakage (streamlines  462 ) may occur, and most of the flow may exit the pad at the trailing edge, flowing in a generally circumferential direction (streamlines  470 ). It will be obvious by reference to  FIG. 24  that this circumferential flow shown exiting pad  414  may become an incoming circumferential flow for the next downstream pad. 
     This flow behavior may have consequences for bearing cooling. The radially-flowing air, drawn from the bearing inner circumference, will be cool. Some portion of the circumferentially-directed air may be cool air drawn from beyond the outer bearing circumference, as indicated at  452 , but a significant proportion will be previously-heated air drawn from the upstream pad. Since little side leakage  460  occurs, the small volume of heated air which may be lost to side leakage may make little contribution to bearing cooling. 
     In  FIG. 29 , the effect of providing a pressurized airflow (streamlines  480 ) at the outer circumference of the pad according to one embodiment is illustrated. Obviously, the cooling airflow, acting in opposition to the centrifugally-directed fluid (streamlines  460 ), may promote ingress of a much larger volume of cooling air (streamlines  450 ) and dramatically expand the region where an incoming flow of cool air is dominant, providing enhanced cooling. This may result from the fact that the inwardly directed flow of cool air, in continuing to act on the exiting air flow, may direct a significant portion of the flow exiting at pad trailing edge inward, that is, toward the axis of rotation. The inward flow may remove the heated air and permit expanded access of cooling air, as described above. Thus, incoming streamlines  470 ′ from the upstream pad may be redirected toward the axis of rotation ( 408  in  FIG. 24 ), as shown figuratively by streamlines  472 . 
     If the bearing is mounted on the outer diameter of the thrust plate, it may be fully accessible to an inwardly-directed radial airflow. However, if the bearing is mounted interior to the outer diameter, it may be beneficial to introduce openings or channels into the thrust plate to enable air access. The openings may be aligned with the gaps between pads. Such a configuration is illustrated in  FIG. 30 , which shows an alternate bearing design  500  in which the top foil pads  512  and bump foil pads  514  are individually attached to the thrust plate  506 . The top foils shown in  FIG. 30  have tabs  520  which may be secured, for example, by welding to thrust plate  506 ; edges  521 ,  522 , and  523  are unsecured. Note that this bearing may also be split, for example, along line C′-C′, and that if split (not shown), the individual elements of the bearing may incorporate features constructed and arranged for reassembly and alignment as described previously (not shown). 
     It is well known that a turbulent boundary layer is better able to maintain its attachment to a surface than a laminar boundary layer when the fluid film flow velocities are transonic or supersonic so that a turbulent boundary layer may result in less pressure drag and less heat generation. For at least this reason, it may be preferred to develop a turbulent boundary layer on the top foil. 
     Smooth or flat surfaces may promote a laminar boundary layer while uneven surfaces or those of irregular height may be more likely to promote development of the more desirable turbulent boundary layer. Surfaces with generally uniformly-spaced height irregularities of similar scale in regular spaced-apart configuration, for example, dimpled surfaces, may be especially effective in promoting turbulent boundary layer formation across the entire surface. 
     Textured surface patterns with concavity and convexity, analogous to dimples, may be fabricated on the top foil and/or runner or on coatings applied to them, using various techniques (laser beam, EDM, chemical etching, etc.). The depths of such recesses may be about a fraction of fluid film thickness (0.00002 to 0.0004 inch or 0.00005 to 0.0001 cm). However, such fine features may be worn away by any rubbing of the runner on the top foil which may occur during start-up and shut-down. 
     The wear process which may occur on start-up and shut-down may be used to advantage, since the wear may promote the development of the desired spaced-apart surface irregularities. Consider the configuration shown in  FIG. 26 . Before a supporting air film is fully developed, runner  410  may rub against top foil pad  416  ( FIG. 24 ). However, top foil pad  416  may be supported by the corrugated annular segments  414  of bump foil sheet  404 . Hence, the stiffness, and thus the local pressure during wear of the top foil, may be greatest in those locations where the bump foil ridges  432  ( FIG. 27 ) directly support the top foil. This may produce non-uniform and local wear of the top foil creating an array of worn regions distributed across the top foil. These worn regions may form a series of depressions in the top foil during normal bearing operation and may promote formation of the desirable turbulent boundary layer. 
     Wear of the top foil may be minimized by the addition of lubricious, wear-resistant, coatings such as KOROLON (trademark) 1350 coating, a proprietary, spray-gun-applied, nickel-chrome coating with solid lubricants. However, the more effective the wear-resistant coating, the greater the likelihood that the top foil may not acquire the wear-induced surface features, resulting in a greater tendency for the boundary air flow to remain laminar. 
     In another embodiment, an alternative bump foil geometry  416 ′ may be used, as shown in  FIG. 32 , in which each of the series of the flats  430 ′ and ridges  432 ′ in each of the tabs  421 ′,  423 ′,  425 ′,  427 ′,  429 ′ is offset from the flats and ridges of its neighbor. Under the applied pressure generated during use, such as shown in  FIG. 16 , those portions of the top foil overlying the flats may be displaced downward more than the surrounding regions overlying the ridges to form a dimpled surface on the top foil. Since this dimpled surface may develop due to the positional variation in stiffness of the top foil-bump foil combination, it may develop in the absence of top foil wear and similarly promote boundary layer transition from laminar to turbulent. 
     In  FIG. 32 , the ridges and flats on adjacent tabs are shown as 180° out of phase. However, lesser degrees of phase mis-match may be employed. Obviously, the dimple pattern will be modified in the event that other than 180° phase mismatch is selected, but, as described above, even at 0 degrees phase mis-match, the desired dimpling may develop due to wear. 
     A similar concept may be employed for journal bearings. In one embodiment, a journal bearing may have fewer than 9 ridges per tab. If, for a specific bearing and ridge spacing, the number of ridges would exceed nine, then multiple bump foils, each with fewer than 9 ridges, in sufficient number to fully cover the bearing surface, may be employed. Preferably, the ridges may be oriented perpendicular to the axis of shaft rotation, but ridges inclined at up to ±45° to the shaft rotation axis will yield acceptable results. 
     In  FIG. 30 , foil thrust bearing  500  comprising thrust plate  506  with through holes  502  and  504  for attachment or alignment may have through holes  508  for ingress of cooling air to a plurality of bearing pads  510  comprising top foil pads  512  and bump foil pads  514 . In this embodiment, top foil pads  512  and bump foil pads  514  may be attached to thrust plate  506  only along line  520 , and edges  521 ,  522 , and  523  of top foil pad  512  may be unattached. A similar procedure may be adopted for bump foil pad  514 . The attachment of the bump foil is not shown in  FIG. 30 , but bump foil pad  414  shown in  FIG. 26  or bump foil pad  416 ′ shown in  FIG. 32  may be adapted for such procedure where flats  436  ( FIG. 26 ) or  436 ′ ( FIG. 32 ) may be employed to weld the bump foil pad to thrust plate  506  ( FIG. 30 ). 
     In addition to the configurations and procedures identified for attachment of the bump foil pad to the thrust plate, the trapezoidal-shaped (or shaped like the Greek capital letter omega) retaining groove employed to retain the foil in the journal bearing may be adapted to attach the bump foil pad to the thrust plate. An example of such a bump foil pad is shown as  514 ′ in  FIG. 31 . The Greek capital letter omega-shaped retaining feature  535  is evident, and only minor changes to the slit configuration, here shown as two sets of circumferential slits  534  and  534 ′, are necessary to accommodate this attachment means. 
     It has been noted that foil bearings, either journal or thrust, may only generate their own supporting air film only after the shaft surface has attained some suitable and subsequent rotation speed. Hence, prior to exceeding that suitable speed on start up or subsequent to falling below that speed on shut-down, the shaft and top foil may be in loaded contact. Thus, wear of both the shaft and top foil may occur. The occurrence of wear may be reduced by appropriately coating the surfaces of the shaft and at least the shaft-contacting surface of the top foil. It has been found that an effective combination may be a hard, wear-resistant, coating applied to the shaft and a soft, lubricious coating applied to the top foil. It has also been found beneficial to retain the wear debris, mainly contributed by the softer lubricious coating, within the foil bearing, since, even though detached from the surface, they continue to contribute lubricity to the foil surface. Debris retention may be well promoted by the top foil geometry which leads to the ‘elasto-pressure’ dam shown at  209  in  FIG. 12 , which may be promoted by the top foil-bump foil configurations herein described. 
     The above description of select examples of embodiments of the invention is merely illustrative in nature, and, thus, variations or variants thereof are not to be regarded as a departure from the spirit and scope of the invention.