WO1992011469A1

WO1992011469A1 - Stepped foil journal foil bearing

Info

Publication number: WO1992011469A1
Application number: PCT/US1991/009393
Authority: WO
Inventors: Marshall P. Saville; Alston Lee-Van Gu
Original assignee: Allied-Signal Inc.
Priority date: 1990-12-20
Filing date: 1991-12-11
Publication date: 1992-07-09

Abstract

A compliant foil bearing disposed between shaft (12) and bushing member, and including a plurality of circumferentially arranged and overlapping foil elements (20) mounted to said bushing member and having a thickness, each foil element having a radially inwardly disposed surface portion confronting said shaft member and defining therewith in operation of said bearing a variable radial spacing dimension, each foil element (20) with a next adjacent foil element overlappingly defining a circumferentially extending gap of radial spacing dimension discontinuity according to said thickness; a like plurality of circumferentially arranged underspring elements (22) also mounted to said bushing member; and means for decreasing the circumferential extend of said gap.

Description

STEPPED FOIL JOURNAL FOIL BEARING 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.

The present invention is directed to a high load capacity journal foil bearing and more particularly to a stepped foil arrangement. The journal foil bearing preferably includes a plurality of individual foils mounted to one of a pair of relatively rotatable members, and a plurality of undersprings which provide resilient support for the foils. The foils are formed from a relatively thin sheet of metallic foil, and extend circumferentially. In certain unidirectional rotation arrangements, it is desirable to have the trailing edge of the foil overlap the leading edge of the next successive foil. In order to minimize the air gap between the surface of the foil and the rotating element at the leading edge of the foil, the foils include a transition arm defined for example by a step, adjacent the trailing edge of the preceding foil. The use of the stepped foil optimizes the geometry of the air gap profile, and thereby increases the load capacity of the journal foil bearing. Alternative configurations for the transition zone for approximating the preferred air gap geometry are also detailed.

Fig. 1 is a perspective view of a foil journal bearing of the present invention. Fig. 2 is a partially cross sectional, partially perspective, view of the journal foil bearing in the assembly of Fig. 1. Fig. 3 is an exploded view of a portion of a prior art design.

Fig. 4 is an exploded view showing the area enclosed by circle 4 in Fig. 2. Fig. 5 is an exploded view showing an alternate configuration for the foil of Fig. 4.

Fig. 6 is an exploded view showing a second alternate configuration for the foil of Fig. 4.

Fig. 7 is depicts the pressure on two different foil bearings as a function of circumferential length.

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, overlapping compliant foils 20(Fig. 2A) and a number of individual foil stiffener elements or undersprings 22 (Fig. 2B) . Both the foils 20 and undersprings 22 are mounted in axial slots 24 in the bushing 14 in a conventional manner. The individual curved foils 20, normally of a thin compliant metallic material, are illustrated as having a mounting means 26 at the leading edge thereof. The mounting means 26 may be formed integral with the individual foils 20 alternatively the foils 20 having may have a means 28 intermediate the ends as shown for example in Fig. 6. The underspring 22, also normally of a thin compliant metallic material, generally has a predetermined circumferential curvature greater than the curvature of the individual foils 20.

The foil journal bearing 16 is designed to provide non-contacting bearing support for the rotating shaft 12 for shaft speeds in excess of a nominal, "lift off" speed. Essentially, air or another gaseous or liquified gas, is delivered to the journal bearing and acts as the working "fluid". Due to the relative rotation of the shaft 12 and bushing 14 boundary layers of the working fluid exits at the surfaces of the shaft 12 and foils 20, and the space therebetween is subjected to a pressure gradient. It is desirable to minimize the spacing between the surface of the foils 20 and the shaft 12 to maintain a high pressure gradient, while simultaneously maximizing the distribution of a uniform pressure on the surface of the foils 20. Previous foil journal bearing designs which incorporated a plurality of overlapping pads would have a transition area with a proportionally larger air gap, at least equal to the thickness of the preceding foil, in the area of the foil immediately downstream of the trailing edge of the preceding foil. This condition is illustrated in Fig. 3, wherein the air gap is depicted having a thickness of _*_■_ before the trailing edge of a foil and T₂ downstream of the trailing edge of the preceding foil.

In order to minimize the air gap between the surface of the foil 20 and the rotating element at the leading edge of the foil 20,, the foils 20 of the present invention are is provided with a transition or step 30 adjacent the trailing edge of the preceding foil 20, as shown in Fig. 4. The use of the stepped foil 20 optimizes the geometry of the air gap profile, and thereby increases the load capacity of the journal foil bearing. The step 30 is preferably formed during the manufacture of the foils 20 by a stamping or forging step.

Fig. 5 illustrates an alternative method of providing the transition in the foils 20, wherein a groove 32 is formed along the underside of the foil 20. The groove 32 provides a structurally weak area which will deform under operating pressure. The groove 30 is preferably formed by either a mechanical or chemical machining process during fabrication of the foil 20.

A second alternative construction for a foil bearing 36 including foils 40 and undersprings 42 is depicted in Fig. 6. For this embodiment, the foils 40 do not overlap one another. Instead, the leading edge of the foils 40 extended forward of the mounting means 28. Thus, the trailing edge of a preceding foil 40 is closely spaced with respect to the leading edge of a subsequent foil 40. By providing adequate spring force to the underside of the foils 40, a very small air gap can be maintained at the foils leading edge. The spring force is preferably applied by overlapping the leading edge of the foils 40 atop the trailing edge of the preceding underspring 42.

While it is desirable to have thin, compliant foils 20, 40 proximate the rotating shaft 12, it may also be desirable to have relatively stiffer spacer foils 44 inserted between the bottom of the foils 20, 40 and the undersprings 22, 42, as is shown in Figs. 4 and 6. The spacer foils 44 distribute the forces exerted by the undersprings 22, 42 to prevent localized deformation of the overlying foils 20, 40, and increase the resilient spring force of the journal bearing 16 due to the addition of frictional forces between the foils 20, 40 and spacer foils

44.

Fig. 7 illustrates the increase in load capacity for a foil bearing 16 including the step 30 as depicted in Fig. 6. The pressure on the surface of a foil is depicted overlying the foil for a circumferential length for a journal bearing which includes the step 30 (solid line) , and the pressure profile on a bearing without the step (dashed lined) . As may be appreciated, the pressure profile on the foils increases more rapidly for the stepped foil 20. In addition, the total area under the curve is greater for the stepped foil 20, thereby the load capacity of the journal bearing 10 is increased proportionately.

For each of the designs according to Figs. 4-6, the foils 20, 40 are preferably a thin metallic foil such as nickel alloy having a thickness in the range of between .0025 cm to .05 cm,, preferably about .01 to .02 cm for a 4.5 cm diameter shaft 12. In the case of the steps 30 of Fig. 4, the step height will be equal to or less than the thickness of the foils 20, thus the steps 30 will have a height in the range of between .0025 cm and .05 cm. For the groove 32 of Fig. 5, the dimensions of the groove will be defined by the thickness of the foil 20, and the aterial characteristics. In general, the grooves 32 will have a depth approximately one-half the thickness of the foils 20, and a width of between one-half and three times the thickness of the foils 20. The undersprings 22, 42 as well as the spacer foils 44 are also formed from thin metallic sheets such as nickel alloy or stainless steel having thickness in the range of between .0025 cm and .075 cm. It should be noted that the thicknesses of the undersprings and spacer foils are generally greater than the thickness of the foils 20, 40, preferably from twenty to one hundred percent greater.

Claims

CLAI S :

1. A journal foil bearing comprising: a bushing member; a shaft member arranged for relative coaxial rotation within said bushing member; a compliant foil bearing disposed between said shaft and bushing member, and including a plurality of circumferentially arrayed and overlapping foil elements mounted to said bushing member and having a thickness, each foil element having a radially inwardly disposed surface portion confronting said shaft member and defining therewith in operation of said bearing a variable radial spacing dimension, each foil element with a next adjacent foil element overlappingly defining a circumferentially extending gap of radial spacing dimension discontinuity according to said thickness; a like plurality of circumferentially arrayed underspring elements also mounted to said bushing member; and means for decreasing the circumferential extent of said gap.

2. The invention of claim 12 wherein said means for decreasing includes each foil element defining a transition step located adjacent said surface portion and an edge of a next adjacent overlapping foil element.

3. The journal foil bearing of claim 2 wherein said foil elements are formed from a thin compliant metallic material having a thickness of between about .0025 cm. and .05 cm.

4. The journal foil bearing of claim 3 wherein said transition step in said foil elements has a height approximately equal to said thickness of said foil elements.

5. The journal foil bearing of claim 1 wherein said foil elements overlap one another and said means for decreasing the gap comprises: an axially aligned groove formed along the underside of said foil elements, said groove providing a structurally weak area which will deform under operating pressure, said groove located proximate the trailing edge of the preceding foil element.

6. The journal foil bearing of claim 5 wherein said foil elements are formed from a thin compliant metallic material having a thickness of between about .0025 cm. and .05 cm.

7. The journal foil bearing of claim 6 wherein said groove in said foil elements has a depth of approximately one-half said thickness of said foil elements, and a width between about one-half and three times said thickness of said foil elements.

8. The journal foil bearing of claim 1 wherein said means for decreasing the gap comprises: said foil elements arranged to have a trailing edge of a preceding foil element closely spaced with respect to a leading edge of a subsequent foil element, and underspring elements providing adequate spring force to the underside of said foil elements for maintaining an air gap thickness at a leading edge of said foil elements between about one to fifty times the air gap thickness at the trailing edge of the proceeding of said foil elements.

9. The journal foil bearing of claim 8 wherein said foil elements are mounted to said bushing by a mounting bar attached to said foil elements, said mounting bar inserted into a cutout in said bushing and said leading edge of said foil elements extends forward of said mounting bar, said leading edge of said foil elements supported by a trailing end of said underspring supporting said preceding foil element.

10. The journal foil bearing of claim 1 wherein said compliant foil bearing further comprises: a plurality of spacer foils inserted between the underside of said foil elements and said underspring elements, said spacer foils distributing the forces exerted by said underspring elements to prevent excessive localized deformation of said foil elements.

11. The journal foil bearing of claim 1 wherein said underspring further comprises: means for mounting said underspring to said journal, said means defined by an axially aligned mounting bar or tab located at a circumferential edge of said underspring.