Abstract:
An improved foil thrust bearing includes a pair of members arranged for relative rotation with respect to one another. One of the pair of members is adapted to rotatably support the other. A thrust bearing disk is operatively disposed between the pair of relatively rotatable members. The thrust bearing disk has a plurality of diverging surface ramps. The surface ramps can comprise an inner ramp and an outer ramp. The thrust bearing disk can also comprise a plurality of radial slots. A plurality of separately formed, converging surface compliant foils are disposed on the thrust bearing disk and between the ramps. An underspring element is operatively disposed between the thrust bearing disk and one of the rotatable members, with the underspring element having a plurality of alternating upper and lower ridges.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of an application filed on Jan. 22, 1999, Ser. No. 09/235,849. 
     This application also claims the benefit of U.S. Provisional Application filed Feb. 10, 1999, Ser. No. 60/119,581. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to bearings and, more particularly, to foil thrust bearings. 
     The ready availability of ambient atmosphere as a bearing fluid makes fluid bearings particularly attractive for high speed rotating machinery. Some applications might include, for example, a turboalternator-generator and turbocompressor. 
     Fluid bearings generally comprise two relatively rotatable members (i.e., a bearing and a runner). A predetermined spacing between the bearing and runner is filled with a fluid such as air. Foils (or thin sheets of a compliant material) disposed in the spacing are deflected by the hydrodynamic film forces between the adjacent bearing surfaces. The foils thus enhance the hydrodynamic characteristics of the fluid bearing 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 members and further provide a cushioning and dampening effect. 
     To properly position the foils between the movable bearing members, it has been common to mount a plurality of individually spaced foils on a foil or thrust bearing disk and position the disk on one of the bearing members. Another common practice has been to provide separate compliant stiffener elements or undersprings beneath the foils to supply the required compliance. Examples of typical foil thrust bearings are shown in U.S. Pat. Nos. 5,547,286; 4,871,267; 4,682,900; 4,668,106; 4,624,583; 4,621,930; 4,597,677; 4,459,047; 4,331,365; 4,315,359; 4,300,806; 4,277,113; 4,277,111; and 4,247,155. 
     Notwithstanding the inclusion of the above design characteristics, the load capacity of a foil thrust bearing still depends on the compliance of the bearing with pressure exerted by a fluid film developed between the bearing and the runner. The pressure profile for a thrust bearing varies, and in order to accommodate the optimal pressure profile and attendant fluid film thickness associated with maximum load capacity, the thrust bearing should be designed to provide a fluid film that correlates to the pressure profile. 
     To correlate the fluid film with the varying pressure profile, the shape of the fluid film can be altered. Such alteration can be primarily achieved by varying the design of three components—namely, the thrust bearing disk, the foils supported by the thrust bearing disk, and the underspring element or thrust bearing stiffener that supports the thrust bearing disk. However, a design variation in one of the three components can have a performance impact on one or both of the other two components—either advantageously or disadvantageously. Accordingly, if two (and even three) of the components are altered in design, the ability to predict the performance impact (either positively or negatively) on the thrust bearing decreases more than linearly. 
     As can be seen, there is a need for an improved foil thrust bearing. In particular, there is a need for a foil thrust bearing that provides improved performance, including increased load capacity. A further need is for a thrust bearing that has increased fluid film pressure to increase the load capacity. Also needed is an improved thrust bearing that includes a fluid film shape that better correlates to the pressure profile. Another need is for a thrust bearing that has increased damping for increased vibration load capability. 
     SUMMARY OF THE INVENTION 
     In an improved foil thrust bearing, the present invention provides a pair of members arranged for relative rotation with respect to one another, one of the members being adapted to rotatably support the other; a thrust bearing disk operably disposed between the relatively rotatable members, with the thrust bearing disk having at least one surface ramp and at least one separately formed foil disposed on the thrust bearing disk. 
     Also, in another improved foil thrust bearing, the present invention provides a pair of members arranged for relative rotation with respect to one another, one of the members being adapted to rotatably support the other; a thrust bearing disk operatively disposed between the relatively rotatable members, with the thrust bearing disk having a plurality of radial slots, a plurality of surface ramps, and a plurality of separately formed foils disposed on the thrust bearing disk and between the ramps; and an underspring element operatively disposed between the thrust bearing disk and one of the rotatable members, with the underspring element having a plurality of alternating upper and lower ridges. 
    
    
     These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded view of a foil thrust bearing according to one embodiment of the present invention; 
     FIG. 2 is a plan view of a thrust bearing disk which can be used in the foil thrust bearing shown in FIG. 1; 
     FIG. 3 is a cross sectional view of the thrust bearing disk taken across line  3 — 3  of FIG. 2; 
     FIG. 4 is a plan view of a thrust bearing disk according to another embodiment of the present invention which can be used in the foil thrust bearing shown in FIG. 1; 
     FIG. 5 is a cross sectional view of the thrust bearing disk taken across line  5 — 5  of FIG. 4; 
     FIG. 6 is a cross sectional view of a foil thrust bearing according to another embodiment of the present invention; and 
     FIG. 7 is a plot of fluid film pressure versus circumferential distance, and fluid film shape versus circumferential distance, for the foil thrust bearing shown in FIG. 1 and a prior art design. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a foil thrust bearing  10  according to one embodiment of the present invention. The bearing  10  generally comprises the components of a thrust runner  11 , a thrust bearing disk  14 , an underspring element or thrust bearing stiffener  22 , and a thrust plate  28 . The above components are typically constructed of nickel-based alloys. While various applications for the bearing  10  are within the scope of the present invention, the bearing  10  is considered to have particular benefit in high speed rotating machinery, such as turbo-generators and turbocompressors. 
     In more particularly describing a first embodiment of the present invention, it can be seen in FIG. 1 that the thrust runner  11  is engaged to a rotating shaft  12 , thereby causing the runner  11  to rotate in the direction of the arrow shown in FIG.  1 . The runner  11  includes a runner surface  13  that faces opposite a bearing surface  15  of the thrust plate  28 . Intermediate the runner  11  and thrust plate  28  is the thrust bearing disk  14  and an underspring  22 . 
     In this embodiment of the present invention (FIG.  2 ), the bearing disk  14  has an overall stepped configuration and is of the type shown in U.S. Pat. No. 4,624,583. However, in contrast to U.S. Pat. No. 4,624,583, a plurality of bearing pads or foils  16  in the present invention are not integrally formed with the bearing disk  14 . Instead, the pads  16  are separately formed and affixed along one side (e.g., a leading edge  17  as discussed below) to the bearing disk  14 , such as by welding. A similar pad construction is shown in U.S. Pat. No. 4,668,106. But the use of separately formed pads  16  for the bearing disk shown in U.S. Pat. No. 4,624,583 is, in fact, opposite to the teaching therein. Specifically, U.S. Pat. No. 4,624,583 teaches that it is disadvantageous to use individual foils or pads (col. 1, lines 43 to col. 2, line 17), at least from a cost perspective. 
     As depicted in FIG. 2, each of the separately formed pads  16  are substantially annular sector in shape, although other shapes may be employed such as trapezoidal. The surface of the foils  16  can have a slight crown (FIG. 5) or can be relatively flat (FIG. 6) depending upon the desired operating characteristics. The pads  16  are circumferentially positioned about the entire surface of the bearing disk  14  that faces the runner surface  13 . Thereby, each pad  16  is described by a leading edge  17  and a trailing edge  19 , as the runner  11  rotates in the direction shown in FIG.  1 . While the present embodiment shows the pads  16  as being substantially equidistant from one another in a circumferential direction, the present invention envisions that unequal spacing may be used. Further, even though FIG. 2 depicts ten (10) pads  16  being employed, the present invention envisions that more or less than ten pads  16  may be useful. 
     In contrast to the embodiment of FIG. 2, the pads or foils  16  can be alternately positioned with a plurality of slots  18  in the bearing disk  14 , as seen in the alternative embodiment shown in FIG.  4 . Accordingly, and for such alternative embodiment, one pad  16  is alternately positioned with one slot  18 . The function of the slots  18  is to allow a substantially unrestricted flow of fluid (i.e., air) to pass through the bearing disk  14  and form a fluid film between the runner surface  13  and the bearing surface  15 . In this alternative embodiment, all of the slots  18  are of an L-shaped configuration. Yet, it can be appreciated that all of the slots  18  can be of other configurations, such as U-shaped. Further, the slots  18  need not be of the same configuration, and can be varied from one another. Additionally, a plurality of radially aligned holes or slots may be used to form the slots  18 . 
     In referring again to the embodiment of the thrust bearing disk  14  shown in FIGS. 1 and 2, surface ramps or transition areas  30  extend between adjacent foils  16 . Overall, the ramps  30  provide the bearing disk with a stepped configuration. The individual ramps  30  have a diverging configuration when viewed from an outer diameter of the bearing disk  14  and to an inner diameter. The diverging configuration is due to the converging configuration of the foils  16  when viewed from the outer diameter to the inner diameter of the bearing disk  14 . Thus, the bearing disk  14  provides alternately converging foils  16  and diverging surface ramps  30 . It can be appreciated that the configuration of the ramps  30  can change depending upon the shape of the foils  16 . Further, the ramps  30  need not all be of the same configuration. 
     Similarly, in the embodiment of the bearing disk  14  having slots  18 , ramps  30  extend between the inner diameter of the disk  14  and one of the distal ends of the slots  18 . Ramps  30  also extend between the outer diameter of the disk  14  and the other distal end of the slot  18 . Generally, the ramps  30  are radially aligned with the slots  18 . Again, the ramps  30  have a diverging configuration when the foils  16  have a converging configuration. 
     With respect to various embodiments of the bearing disk  14  above, it should be recognized that the ramps  30  can be provided at the outer diameter, inner diameter or both. Further, there may be applications where the extent or degree of divergence and/or the length of the inner ramps  30  adjacent the inner diameter (and thus the ramp height) may vary from the degree of divergence and/or the length of the outer ramps  30  adjacent the outer diameter. Likewise, the degree of divergence and/or the length of the ramps  30  can be varied along the radial direction. The actual angle or degree of divergence and height of the ramps  30  can be varied to provide for particular operating conditions. The height of the individual ramps  30  would typically be between 0.0005 to 0.010 inches with a preferred range of 0.001 to 0.002 inches. 
     The bearing disk  14  further includes a plurality of notches  21  positioned about the outer or circumferential edge of the bearing disk  14  (FIG.  2 ). The notches  21  can be aligned with a plurality of notches  23  of the underspring element  22  to fix the rotational position of the disk  14  to the underspring  22 , as further described below. 
     In the embodiment shown in FIG. 1, the underspring element  22  comprises a plurality of upper ridges  24  and lower ridges  26 . All of the upper ridges  24  of the underspring  22  have substantially the same configuration and dimensions, as do the lower ridges  26 . Nevertheless, it is contemplated by the present invention that all of the upper ridges  24  and lower ridges  26  need not respectively be of the same configuration and dimensions. Further, although different spacing can be employed, the present embodiment has the upper ridges  24  and lower ridges  26  substantially equidistant from one another in their circumferential positions. In making the underspring or stiffener  22  of the present invention, conventional methods can be utilized. For example, most of the underspring  22 , including the ridges  24 , 26 , can be stamped. 
     The underspring  22  is shaped to substantially match the configuration and dimensions of the bearing disk  14 . FIG. 4 depicts the relative position of the upper ridges  24  of the underspring  22  with respect to the foils  16  of the thrust bearing disk  14 . The angle θ 1  is defined between the radial line extending from the base of the ramps  30  (i.e., the leading edge  17  of the foil  16 ) and the radial centerline of the upper ridge  24 . The angle θ 2  is defined between the leading edge  17  of the foil  16  and the trailing edge  19  of the foil  16 . In order to provide the proper pre-load and support for the individual foils  16 , the relationship between θ 1  and θ 2  should be approximately 2:3 to provide optimum results in most operating conditions. It should be understood, however, that the relationship between θ 1  and θ 2  range can be from approximately 1:2 to almost 1:1. 
     Notwithstanding the foregoing, the present invention contemplates that other designs of an underspring element  22  can be employed. As in U.S. Pat. No. 5,110,220, which is incorporated herein by reference, the underspring element  22  can have a plurality of spring sections. Each spring section includes a plurality of corrugated spring elements arranged radially adjacent to one another and traversing radially increasing arc lengths. The pitch of the corrugations in the spring elements increases from the outermost to the innermost spring element. Like in U.S. Pat. No. 5,248,205, which is incorporated herein by reference, the underspring element  22  can include a plurality of trapezoidal areas. From the leading edge of each area and towards but not to the trailing edge extend a plurality of corrugated arcuate springs. Each spring may contain a plurality of slots extending circumferentially and radially over the spring. Alternatively, the underspring element  22  can be formed, as shown in U.S. Pat. No. 5,318,366 and incorporated herein by reference, whereby trapezoidal areas are provided. From the trailing edge of each area and towards but not to the leading edge extend a plurality of corrugated arcuate springs. The springs are defined by widths that increase from the innermost spring to the outermost spring. Also, the width of each individual spring decreases from the trailing edge and towards the leading edge. 
     When the foil thrust bearing  10  is operative, the shaft  12  rotates and the runner  11  likewise rotates. As the runner  11  rotates, a fluid film is built up between the runner surface  13  and the bearing surface  15 . For each of the pads or foils  16 , the fluid film pressure increases from the leading edge  17  and to the trailing edge  19 . At the same time, each of the upper ridges  24  provides load support to their respective pads  16 . 
     FIG. 7 depicts fluid film shape versus circumferential distance about the bearing disk  14 , as curve  38 , for a preferred embodiment of the present invention. In conjunction with curve  38 , curve  40  depicts fluid film pressure versus circumferential distance. In contrast, curve  42  depicts a fluid film shape and curve  44  depicts a fluid film pressure, both for a prior art design that does not have a stepped configuration to the thrust bearing disk of a foil thrust bearing. As can be seen, a comparison between film shape curves  38  and  42  indicate excessive gap near the trailing edge and insufficient gap near the leading edge in the prior art design. A comparison of pressure curves  40  and  44  indicates that the present invention provides greater film pressure. 
     To those skilled in the art, it can be appreciated that the present invention provides an improved foil thrust bearing and, specifically, increased performance, including increased load capacity. The present invention provides increased fluid film pressure to increase the load capacity. Another advantage provided by the present invention is increased damping for improved vibration load capability. The increased damping is realized through coulomb friction and squeeze film forces from the relative motion between the pad and disk interface. 
     It should be understood, of course, that the foregoing relates to preferred embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.