CONICAL FLUID BEARINGS

Field of the Invention

[0001] The present invention is related to conical fluid bearings for supporting a rotating shaft. In particular, the invention is related to conical foil fluid bearings .

State of the Art [0002] Foil fluid bearings can refer to tension foils, or bump foils. Foil fluid bearings, wherein the foil is tensioned around the shaft, are referred to in the art as "tension foils". Foil fluid bearings, wherein the foil is not tensioned, but is backed by a compliant support, are referred to as "bump foils", or compliant surface bearings. Foil fluid bearings, as used in the present invention, refer to tensioned foil fluid bearings.

[0003] Foil fluid (air) bearings have, like all fluid bearings, the advantage of running free of contact at high speeds. When the shaft is stationary, there is a small amount of preload between the shaft and the bearing. As the shaft turns, hydrodynamic pressure is generated, pushing the foil(s) away from the shaft and making the shaft completely airborne. This phenomenon occurs instantly during start-up at a very low speed. When the shaft is airborne, the friction loss due to shaft rotation is quite small. As the shaft grows, the foils get pushed further away, keeping the film clearance relatively constant. [0004] This air (fluid) bearing action, combined with the large tolerance to geometrical faults and thermal growth makes them very suited for use in turbo machines .

However the number of start-stops is limited because of friction and wear during start and stop cycles. [0005] Foil bearings for radial support are well developed and are capable of withstanding considerable loads. On the other hand performances of axial foil bearings are still very weak.

Aims of the Invention [0006] The present invention aims to provide a conical fluid (air) bearing which incorporates radial as well as axial bearing action in one single unit thereby extending the great radial performance of a classical fluid

(air) bearing to its axial bearing action. It is an aim of the invention to provide a conical fluid (air) bearing with improved characteristics compared to prior art fluid (air) bearings .

[0007] It is an aim of the invention to provide a conical fluid (air) bearing in the form of one or more tensioned foils. It is an aim of the invention to provide a conical foil fluid bearing having an improved tensioning support for the foil.

[0008] It is an aim of the invention to provide a conical fluid bearing in the form of one or more compliant surfaces, or compliant surface foils.

Summary of the Invention

[0009] Aims of the invention are achieved by providing a conical fluid (air) bearing, as set out in the appended claims. Conical fluid bearings of the invention comprise a flexible conical surface. There are several ways to generate a flexible conical surface that is able to work as a fluid bearing.

[0010] According to a first aspect of the invention, there is provided a conical foil fluid bearing comprising

one or more flexible foils adapted for enveloping a conically shaped portion of a shaft. The conical foil fluid bearing comprises a self-aligning mechanism which guarantees that the one or more foils envelope the shaft so as to minimise air lost and malfunctions.

[0011] Fluid bearings of the invention hence can comprise a frame and one or more foils. The one or more foils form a conical shape and are arranged for supporting a shaft (by providing bearing action to the shaft) . The air bearing further comprises means for self-alignment of the one or more foils to the shaft. The one or more foils are preferably tensioned around the shaft.

[0012] Preferably, the means for self-alignment comprise a kinematic support for the one or more foils. [0013] Preferably, the means for self-alignment comprise one or more cylinders, each cylinder being arranged for rotating around two axes normal to the geometrical axis (the rotation symmetry axis) of said cylinder. Alternatively, the means for self-alignment comprise one or more conical bodies, each conical body arranged for rotating around two axes normal to the geometrical axis (the rotation symmetry axis) of said conical body. In a more preferred embodiment, the means for self-alignment comprise both one or more cylinders and one or more conical bodies.

[0014] Conical bodies refer to bodies having as surface of revolution a cone segment.

[0015] Preferably, the means for self-alignment further comprise one or more pins. Each of said pins are coupled to one of said cylinders or conical bodies by a point contact.

[0016] Preferably, each of the one or more cylinders or conical bodies comprises a blind hole. The blind hole is more preferably provided on the surface of revolution of

the cylinder or conical body. Each of said one or more pins is coupled to one of said one or more cylinders or conical bodies by a point contact in the blind hole.

[0017] According to a preferred embodiment, the fluid bearing of the invention comprises a flat disc-shaped (i.e. annular shaped) foil and means for bending the flat disc-shaped foil to a conical shape. The fluid bearing in the latter case comprises one foil which envelopes the conically shaped portion of the shaft all around. [0018] According to a second aspect of the invention, there is provided a conical fluid bearing comprising a conical bearing surface made from a soft material such as a metal foam, plastic or any other suitable material. Hence, a fluid bearing is provided according to the invention, comprising a frame, the frame comprising a surface of conical shape arranged for use as a bearing surface. At least the surface of the frame is made of a plastic, a rubber or a metal foam. A metal foam is preferred as material for the bearing surface. A rubber is equally preferred. Preferably, said surface is coated. Alternatively, a foil can be provided on said surface. [0019] Preferably, the shaft for use with fluid bearings of the invention comprises means for pressurizing the fluid bearing. [0020] Preferably, the one or more foils, or the bearing surface of conical shape comprise means for pressurizing the fluid bearing.

[0021] More preferably, the fluid bearing or the shaft comprises one or multiple flexible fluid supplies for pressurization of the fluid bearing.

[0022] According to a third aspect of the invention there is provided a system comprising a shaft and one or more fluid bearings of the invention, wherein the shaft comprises one or more portions of conical shape.

Preferably, at least one of said one or more portions of conical shape is made of metal foam. Alternatively, said portion (s) can be made of rubber.

[0023] A conical fluid bearing refers to a fluid bearing providing axial as well as radial bearing action. Conical fluid bearings have a bearing surface in the form of (substantially) a cone segment. The angle between the bearing surface and the axis of the cone (segment) preferably falls in the range between and including about 5° and 85°, more preferably in the range between about 10° and 80° and most preferably in the range between about 20° and 70° .

[0024] Preferably, the fluid is air. Alternatively, working fluids other than air can be used, such as (lubricant) oil or water.

Brief Description of the Drawings

[0025] Figure 1 represents a segment of an air bearing comprising the self-aligning mechanism and a foil adapted to a conical shape.

[0026] Figure 2 represents the segment of air bearing of figure 1 from the back.

[0027] Figure 3 represents three segments as in figure 1 assembled to an air bearing, in which the foils are not represented.

[0028] Figure 4 represents an air bearing comprising one flat disc-shaped foil adapted to a conical shape.

[0029] Figure 5 represents the frame of a conical air bearing made of soft material. [0030] Figure 6 represents a shaft comprising two conical surfaces.

[0031] Figure 7 represents a test rig.

Detailed Description of the Invention

[0032] Embodiments of the present invention will now be described in detail with reference to the attached figures, the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not necessarily correspond to actual reductions to practice of the invention. Those skilled in the art can recognize numerous variations and modifications of this invention that are encompassed by its scope. Accordingly, the description of preferred embodiments should not be deemed to limit the scope of the present invention.

[0033] Furthermore, the terms first, second and the like in the description and in the claims are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein. [0034] Moreover, the terms top, bottom, left, right, over, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. The terms so used are interchangeable under appropriate circumstances and embodiments of the invention described herein can operate in other orientations than described or illustrated herein. For example, "left" and "right" of an element indicates being located at opposite sides of this element. [0035] It is to be noticed that the term "comprising", used in the claims, should not be interpreted

as being restricted to the means listed thereafter; it does not exclude other elements or steps. Thus, the scope of the expression "a device comprising means A and B" should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, A and B are relevant components of the device.

[0036] Where numerical values are given with regard to limitations of a quantity, or the outcome of a measurement, for the assessment of those values, account shall be taken of variations due to impurities, methods used to determine measurements, human error, statistical variance, etc.

[0037] Embodiments are described hereinafter referring to air bearings. However, same embodiments can be applicable to working fluids other than air.

[0038] According to a first aspect, a conical foil air bearing is provided. The air bearing comprises one or more thin foils, which are arranged in a flexible conical shape. The one or more foils are positioned and supported such that they can self-align with the corresponding conical (bearing) surface of the shaft. Therefore, the air bearing of the invention comprises means for self- alignment. The means for self-alignment are a mechanism providing a kinematic support for the one or more foils. [0039] According to a preferred embodiment, the self-alignment is obtained by one or more cylindrical or conical bodies which support the foil (s) . The foil is tensioned over the one or more cylindrical or conical bodies. The cylindrical or conical bodies are in turn supported by a pin through a point contact. Such point contact allows the cylindrical or conical bodies to move (rotate) around two axes perpendicular to the geometrical axis (rotation symmetry axis) of those bodies.

[0040] The location of the point contact of the pin along the surface of a cylinder or a conical body is preferably selected so as to ensure stable self correction of wrapping of the foil to the conical shape of the air bearing. Therefore the said location will generally depend on the cone angle of the shaft (or of the air bearing) . For a zero cone angle, i.e. in the cylindrical bearing case, this location will be at the centre of the aligning cylinders . [0041] The point contact between a supporting pin and the cylinder or conical body is preferably provided in a blind hole. The blind hole is provided on the surface of revolution of the cylinder or the conical body. Preferably, rotation of the cylinder or the conical body around the pin as indicated is obtained by providing some mechanical play between pin and hole.

[0042] According to an embodiment, the point contact between pin and cylinder or conical body is obtained by a spherical surface in the blind hole and on the pin. Preferably, the pin comprises a pin-head having a spherical surface. More preferably, the spherical surface of the blind hole has larger radius than the spherical surface of the pin head. Other ways of obtaining a rotation freedom of a cylinder or conical body around a point as known in the art can be used, e.g.: flexures or spherical joints.

[0043] The means for self-alignment may comprise a spring. Either multiple foils are used, each foil covering only a segment of the conical bearing surface, or one (annular) disc-shaped foil is used. Embodiment 1

[0044] According to a first embodiment, as illustrated in figure 1, an air bearing comprises multiple thin foils 11 arranged in a flexible conical shape. These foils 11 can be made of metal, plastic or any other

convenient material. Each foil 11 is extended between a first and a second attachment point 12 and 13 on a rigid frame 10. These attachment points 12, 13 can be outside or inside of the frame. In a more particular embodiment, at least one attachment point is on the outside of the frame. The foil can be attached or fixed to the frame by means of clamping members 14, by welding or by gluing. [0045] The perfect fitting of the foil is obtained by means of a mechanism 15 for self-alignment, more in particular by the provision of means 15 for supporting and/or positioning the foil, such means allowing self- alignment (i.e. because of a kinematic support). The self- aligning mechanism 15 allows the foil to adapt itself during the bearing action to envelope the shaft/axis in an optimal way.

[0046] The self-aligning mechanism 15 comprises two cylinders 151 as illustrated in figure 2. Each cylinder 151 is rotatably mounted, so as to be able to rotate around two axes normal to the geometrical axis of the cylinder. The cylinder 151 is provided with a blind hole. The blind hole is provided on the cylindrical surface. A small pin 152 is inserted in the blind hole of cylinder 151. The pin 152 is connected at one end to the cylinder by a point contact in the blind hole. The pin 152 is connected to the bearing frame at the other end.

[0047] Several bearing segments comprising foil 11 and self-aligning mechanism 15 can be combined to become a full radial bearing as is illustrated in figure 3. [0048] In an alternative embodiment, the cylinders 151 are substituted by conical bodies. Embodiment 2

[0049] According to a second embodiment, the air bearing comprises one foil only. The self-alignment method,

to guarantee a perfect fit described in the first embodiment, is also applicable in this configuration. [0050] A special structure 43 is needed to neutralize the nonconformity that results when bending a flat foil 41 (e.g. annular or disc-shaped) in a conical shape. In figure 4 a possible configuration is illustrated. Small cylinders 42 and 43, placed under an angle, overcome the difference in length between the flat and shaped foil. Furthermore these cylinders 42, 43 keep the foil 41 in place. Another possibility is the use of cones instead of cylindrical rods 42 in the self-aligning mechanism. [0051] The cylinders 43 can provide conical shaping of the foil. In addition, they can provide tensioning of the foil. The cylinders (or any other article) 43 can be spring loaded for tensioning the foil. Alternatively, they can be fixated (e.g. by bolts or screws) to the frame 40. Embodiment 3

[0052] In a third embodiment the bearing is made from a soft material like a metal foam, plastic or any other suitable material (see figure 5) . Due to the soft mechanical structure the geometry of the bearing surface will change under air pressure and/or contact, thereby giving it the same properties as traditional foil bearings. [0053] The conical surface can be coated to lower wear and friction and/or to make the surface impermeable in case of a porous material.

[0054] Alternatively, a (flexible) foil can be provided on the (soft) conical surface. In contrast to the previous embodiments, this foil is preferably not tensioned, as the self-aligning properties are primarily obtained by the soft understructure (support) . [0055] All the embodiments may comprise additional features. To obtain extra load capacity and reduce start- stop problems an additional air supply is possible. This

can be attached to the foil by means of a flexible hose. The air enters the air gap through a small hole in the foil or the conical bearing surface.

[0056] In the third embodiment the air can be introduced via a simple hole in the soft bearing material or coating.

[0057] The air bearing may equally be pressurized by supplying air via the shaft. [0058] Additional aerodynamic lift can be achieved by introducing spiral grooves, pockets,... on the foil, or on the shaft.

[0059] A corresponding shaft for use with an air bearing of the invention is illustrated in figure 6 and may comprise two conical surfaces around each of which an air bearing of the invention is provided. The position and orientation of the cones can be changed to any possible configuration. A bearing configuration with only one conical bearing combined with any other type of bearing is feasible as well. [0060] In order to achieve the right (peripheral) tension in the foils, a conical bearing according to the invention may be axially pre-loaded against the shaft. [0061] A test rig, as illustrated in figure 7 is developed to test the characteristics of the various embodiments.

[0062] While the foregoing description and drawings represent the preferred embodiments of the present invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the spirit and scope of the present invention .