Abstract:
A high load capacity journal foil bearing and more particularly undersprings therefor. The journal foil bearing preferably includes a number of individual pads or foils which are conventionally mounted to a journal and have a pad face proximate a rotating shaft. The undersprings each include a number of curvilinear support beams or corrugations which are varied in width and pitch to optimize the spring force supporting the overlying foil. The underspring is configured to have wider beams at their leading edge and progressively narrower beams for approximately the first one half of the circumferential length.

Description:
BACKGROUND OF THE INVENTION 
     Process fluid or gas bearings are utilized in a number of diverse applications. These fluid bearings generally comprise two relatively movable elements with a predetermined spacing therebetween filled with a fluid such as air, which, under dynamic conditions, form a supporting wedge sufficient to prevent contact between the two relatively movable elements. 
     Improved fluid bearings, particularly gas bearings of the hydrodynamic type, have been developed by providing foils in the space between the relatively movable bearing elements. Such foils, which are generally thin sheets of a compliant material, are deflected by the hydrodynamic film forces between adjacent bearing surfaces and the foils thus enhance the hydrodynamic characteristics of the fluid bearings and also provide improved operation under extreme load conditions when normal bearing failure might otherwise occur. Additionally, these foils provide the added advantage of accommodating eccentricity of the relatively movable elements and further provide a cushioning and dampening effect. 
     The ready availability of relatively clean process fluid or ambient atmosphere as the bearing fluid makes these hydrodynamic, fluid film lubricated, bearings particularly attractive for high speed rotating machinery. While in many cases the hydrodynamic or self-acting fluid bearings provide sufficient load bearing capacity solely from the pressure generated in the fluid film by the relative motion of the two converging surfaces, it is sometimes necessary to externally pressurize the fluid between the bearing surfaces to increase the load carrying capability. While these externally pressurized or hydrostatic fluid bearings do increase the load carrying capacity, they introduce the requirement for an external source of clean fluid under pressure. 
     In order to properly position the compliant foils between the relatively movable bearing elements, a number of mounting means have been devised. In journal bearings, it is conventional practice to mount the individual foils in a -slot or groove in one of the bearing elements as exemplified in U.S. Pat. No. 3,615,121. 
     To establish stability of the foils in most of these mounting means, a substantial pre-load is required on the foil. That is, the individual foils must be loaded against the relatively movable bearing element opposed to the bearing element upon which the foils are mounted. It has been conventional to provide separate compliant stiffener elements or underfoils beneath the foils to supply this required preload as exemplified in U.S. Pat. Nos. 3,893,733 and 4,153,315. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a high load capacity journal foil bearing and more particularly to undersprings therefore. The journal foil bearing preferably includes a number of individual pads or foils which are conventionally mounted to a journal and have a pad face proximate a rotating shaft. The undersprings each include a number of curvilinear support beams or corrugations which are varied in width and pitch to optimize the spring force supporting the overlying foil. The underspring is configured to have wider beams at their leading edge and progressively narrower beams for approximately the first one half of the circumferential length. This results in a weaker spring force supporting the overlying foil near the leading edge, and a gradually increasing underspring force which reaches and maintains a maximum at about the midpoint of the foil. 
     The undersprings also include means for reducing the exerted spring force at the axial edges of the foils, to thereby prevent rubbing contact where the dynamic fluid forces are diminished. Preferably, the means for reducing the spring force is accommodated by including cutouts or windows near the axial ends of the curvilinear support beams or corrugations. The cutouts are configured to approximate the spring force to the decrease in the overlying dynamic fluid pressure profile. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a foil journal bearing of the present invention. 
     FIG. 2 is a partial sectional view of the foil journal bearing taken along line 2--2 of FIG. 1. 
     FIG. 3 is a plan view of an underspring of the foil journal bearing of FIG. 1. 
     FIG. 4 is a cross sectional view along line 4--4 of FIG. 3 of an alternative configuration for the underspring. 
     FIG. 5 is a sectional view of an alternative configuration for the foil bearing taken along line 2--2 of FIG. 1. 
     FIG. 6 is an exploded view depicting an alternate mounting arrangement for the underspring of the foil journal bearing of FIG. 1. 
     FIG. 7 is a graphical illustration of the pressure exerted on the foil 20 along a circumferential length thereof for a compressible fluid and for an incompressible fluid. 
     FIGS. 8A, 8B, and 8C are exploded views of the axial edge of the foil journal bearing for two configurations of the underspring, and a graphical illustration of the pressure at the edge of the foil. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As generally illustrated in FIG. 1, The journal bearing 10 includes a shaft 12 rotatably supported within a bushing 14 by means of a foil bearing 16. The foil bearing 16, shown in FIG. 2, generally comprises a plurality of individual, preferably overlapping, compliant foils 20 and a number of individual foil stiffener elements or undersprings 22. Both the foils 20 and undersprings 22 are mounted in axial slots 24 in the bushing 14 in a conventional manner. While the individual curved foils 20, normally of a thin compliant metallic material, such as nickel alloy, stainless steel, or beryllium-copper, are illustrated as having a separate mounting bar 26 at the leading edge thereof, the mounting means may be formed integral with the individual foils or with the foils 20 having mounting means intermediate the ends thereof as shown in U.S. Pat. No. 4,178,046. 
     The undersprings 22, also normally of a thin compliant metallic material such as nickel alloy, stainless steel, or beryllium-copper, generally have a predetermined curvature. The undersprings 22 include a plurality of axially extending slots 28, defining therebetween a plurality of axially extending curvilinear beams 30, as illustrated in the plan view of FIG. 3. The slots 28 are preferably &#34;I&#34; shaped, and extend a majority of the axial length of the underspring 22. Thus, the curvilinear beams 30 between the slots 28 are connected only to the axial edges 32, 34 of the underspring 22 by short bars 36 defined by the top and bottom crossbeams of said &#34;I&#34; slots 28. The curvilinear beams 30 may also include windows 40, 42 near each axial end of the beams 30, slightly inset from the bars 36. The purpose of the windows 40, 42 at the ends of the curvilinear beams 30 is to reduce the spring force exerted upon the overlying foil 20 proximate the axially outer edges thereof. 
     As depicted in FIGS. 2 and 3, the underspring 22 has a leading edge 44 (with respect to the direction of rotation of shaft 12) at which it is mounted to the bushing 14, and a trailing edge 46 at the opposite end. In addition, the leading edge of the active surface area of the foils 20, i.e. the area exposed to the relatively rotating member, is configured to overlie the leading edge 44 of the undersprings 22, when assembled. 
     The curvilinear beams 30 are configured to have a varying width W, and pitch P, depending upon the location of the curvilinear beam 30 between the leading edge 44 and trailing edge 46. By way of example, for a given underspring 22 having curvilinear beams 30 a, b, c, d, e, f, g, and h, and corresponding widths Wa, Wb, Wc, Wd, We, Wg, Wh) the width Wa for the curvilinear beam 30a proximate the leading edge 44, is greater than the width Wb for the beam 30b. Additionally, Wb is preferably greater than Wc, Wc is greater than Wd, and Wd is greater than We. However, it is also desirable to have the widths of the beams 30 approximately equal from the midpoint of the undersprings 22 to near the trailing edge 46. Thus, the widths We, Wf, Wg are approximately equal, while the final width Wh may be greater than Wg. 
     It may be appreciated that the pitch of the successive curvilinear beams 30 is dependent upon the width of each beam 30, as well as, or in addition to the width of the slots 28 separating the beams 30. Preferably, the height of the beams 30 varies in order to provide a uniform support height or tapered support height for foils 20 while the width of slots 28 can be constant or variable. 
     The purpose of varying the width of beams 30 between leading edge 44 and trailing edge 46 is to maintain an optimum wedge shape spacing between the surface of the foil 20 and the rotating member 12 matched to the stiffness of the underspring 22 for the changing pressure force along the circumferential length of the foil 20 and underspring 22. By maintaining an approximately optimum wedge shape spacing between the foil 20 and the rotating member 12, the load capacity of the journal bearing 10 can be significantly increased over that of previous designs. 
     FIG. 4 depicts an alternate arrangement for the curvilinear beams 30, wherein the beams 30 have their edges curving radially inward, toward the bottom surface of the foil 20. The plan view of FIG. 3 is essentially the same for either of the end views of FIGS. 2 or 4. 
     FIG. 5 depicts an end view of another alternative arrangement of an underspring 52. In this view, the underspring 52 has corrugations 54 instead of the slot and beam configuration of the underspring 22. It should be recognized that while the corrugations 54 are defined by the underspring 52 having a serpentine-like shape in axial view, the underspring 22 also has &#34;corrugations&#34; in the sense that the curvilinear beams 30 cooperatively define alternating ridges and furrows in the axial view, recalling FIGS. 2 and 4. The corrugated underspring 52 is similar, however, in that the width and pitch of the corrugations 54 is longer proximate the leading edge and shorter in the middle of the underspring 52. This variation in the pitch of the corrugations 54 produces a weaker spring force at the leading edge of the foil 20 similar to the construction of FIGS. 2 and 4. 
     The undersprings 22, 52 of FIGS. 3, 4 and 5 provide varying spring force to support the foil 20 by virtue of the number and spacing of the points of contact between the undersprings beams 30 or corrugations 54 with the foil 20, and bushing 14. In addition, the length of travel or runout of the shaft against the spring force of the underspring before the underspring bottoms out is dependent upon the curvature or height of the curvilinear beams 30 or height of corrugations 54. 
     The present invention seeks to match the requirements of continuous support and easy fabrication by providing a high number of contact points spaced as close to one another as possible yet allowing significant runout or radial displacement for the shaft 12 in a configuration which can be readily manufactured. Accordingly it is preferable to have the width W for the beams 30 or corrugations 54 to be in the range between 0.025 cm to 1.0 cm. It is also preferable for the width of the first beam 22 or corrugation 54, to be between 2% to 50% greater than the width for any subsequent beam 22 or corrugation 54 for incompressible fluid applications, and up to 500% for compressible fluid applications. 
     It should also be noted that both the slot-beam configuration and the corrugated configuration are resilient in the event of a high load which bottoms out the underspring 22, 52. Thus, even if a high load is exerted, the beams 30 or corrugations 54 simply flatten out, and can recover their shape when the load is decreased. 
     An alternate mounting and arrangement for the underspring 22 is depicted in an exploded view in FIG. 6. In this alternative arrangement, the underspring 22 is mounted to the journal 14 by means of a mounting bar or tab 60 at the trailing edge of the underspring 22. This mounting arrangement can be used with any of the previously described configurations for undersprings 22 and 52. 
     The effect of incorporating an underspring having a variable spring force is graphically illustrated in FIG. 7 by the dashed line 60 for an incompressible fluid, and by the solid line 62 for a compressible fluid. The dashed and solid lines 60, 62 represent the pressure profile on the foil 20 supported by the variable pitch, variable spring rate underfoil 22 of the present invention. For comparison purposes, the pressure profile exerted on the surface of a foil is depicted by the dotted line 64, for a constant pitch, uniform spring rate underfoil of the prior art. As shown, the maximum pressure is increased, as is the total area under the curve which results in a greater load carrying capacity for the variable spring rate of underspring 22, 52 of the present invention. 
     By way of example only and for completeness of this disclosure, a preferred configuration of the incompressible fluid underspring 22 depicted in FIG. 3 having eight beams has the following relative widths for the active pad area. 
     
         ______________________________________Dimension   Range of Wn as % of Wa______________________________________Wa          --Wb          94%-98%Wc          85%-95%Wd          80%-90%We          80%-90%Wf          75%-85%Wg          75%-85%Wh          85%-90%______________________________________ 
    
     FIG. 8 depicts exploded cross sectional views of an edge portion of the foil bearing along an axial length of the rotating members 12 and 14, and the associated pressure profile on the foil 20. In these views, the effect of the windows 42, 44 on the loading close to the axial ends of the foil 20 is illustrated. An exploded view of a portion of an underspring 70 which does not include the windows is depicted by dashed lines superimposed on the cross-sectional view of a portion of the underspring 22 including the windows 42. The underspring 70 exerts a constant force across the entire axial length of the foil 20. However, the fluid pressure between the foil 20 and the shaft 12 drops to zero at the edge of the foil 20 as depicted by the graphical representation of the pressure. Thus, the underspring urges the edge of the foil 20 upward, causing rubbing contact with the shaft 12. By comparison, the edge of foil 20 for an underspring which includes the windows 40, 42, and which overhangs the underspring is shown by the solid lines. For this arrangement, the underspring exerts a decreasing spring force over the outer 1.5 cm to 0.1 cm of the foil. Thereby, the spring force at the axial edge of the foil 20 is substantially reduced in conjunction with the reduction of the fluid pressure between the foil 20 and shaft 12. By this arrangement, the rubbing contact at the axial edge of foil 20 is substantially reduced or even eliminated. 
     It should be evident from the foregoing description that the present invention provides advantages over foil journal bearings of the prior art. Although preferred embodiments are specifically illustrated and described herein, it will be appreciated that many modifications and variations of the present invention are possible in light of the above teaching to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.