TECHNICAL DOMAIN

The present invention relates to the domain of hydrodynamic foil bearings, more particularly to gas-lubricated foil bearings.

TECHNICAL BACKGROUND

Bearings are known in the state of the art for constraining relative motion of machine parts to a predefined motion and serve also to reduce friction between the moving parts. Known bearings include roller bearings or ball bearings, which have contacting parts and in which the load is transferred through a thin film of lubricant, usually oil. Other known bearings include gas-lubricated bearings, in which moving parts float in the lubricating fluid during operation, the lubricating fluid usually being a gas, often air.

Among the different known configurations of bearings are journal bearings, in which the load is predominantly perpendicular to the main axis, and thrust bearings, in which the load is predominantly in the same axis as the axis of rotation. An example of a journal bearing is a shaft rotating in a sleeve or housing. A clutch release bearing on an automobile would be an example of a thrust bearing.

Gas-lubricated bearings have found use in high-speed and oil-free turbomachinery, for example. Some of the advantages of this type of bearing are that they readily dissipate heat, they are robust and relatively simple to manufacture. They have a high load capacity and have a high tolerance towards misalignment errors.

Gas-lubricated foil bearings are hydrodynamic bearings. The bearing has a first member and a second member which can rotate relative to each other. Generally, one of the members is stationary and the other one rotates. In contact with the first member, is a compliant structure, generally a resilient corrugated foil, or bump foil, on top of which a flat, thin top foil is positioned. The top foil is also known as a compliant foil. The gas film between the top foil and the second member is developed through rotation of the rotor. The compliant underlying structure is the key feature for achieving both high misalignment tolerances and high load capacities. In addition, large rotor excitation activates structural damping on the compliant structure through Coulomb friction, thus stabilizing the rotordynamic system. Hydrodynamic bearings in which the lubricating fluid is a gas, especially when it is air, may be known as aerodynamic bearings.

Apart from bump foils, other configurations of compliant structure are also known, such as cantilever strips, foil strips, meshes etc. Such compliant structures are generally made from resilient metal and their inclusion in a bearing generally involves adding one or more external elements between the first element of the bearing and the top foil.

When foil strips are used as the compliant structure, the compliant structure, by definition, comprises a plurality of components, each component being an individual strip. Known bump foils may also comprise a plurality of components. For example, a thrust bearing may have a base plate and a thrust runner as the first and second members. The bump foil lies on the base plate, followed by the top foil, then the thrust runner. The bump foil may comprise a plurality of segments with a small space between each segment. This is a plurality of elements making up the bump foil. Similarly, the top foil may also comprise a plurality of segments. Likewise, in journal bearings, the compliant structure may be comprised of a plurality of elements. The top foil may comprise a plurality of elements too. Generally speaking, a compliant structure may comprise a plurality of elements and a top foil structure, or compliant foil structure, may comprise a plurality of elements.

BRIEF DESCRIPTION OF THE INVENTION

Although known types of foil bearing are relatively simple to manufacture, some drawbacks exist with respect to the bearing performance. Bump foils of the known bearings may potentially exhibit geometrical deviations due to manufacturing uncertainty, leading to significant effects on both the local stiffness and clearance at different points around the bearing. One known method for addressing such problems is selective shimming, which allows to alter the height and therefore the resilience of particular bumps of the bump foil depending on where the adjustment has to be made. Selective shimming effectively tailors the fluid film pressure to significantly delay the onset of whirl and to reduce sub-synchronous vibration.

According to certain embodiments, the present invention provides for a foil bearing, which may be deployed in an electric motor. The foil bearing supports two surfaces in relative motion. The two surfaces may be part of a stator and part of a rotor. The bearing has a compliant structure at the stator and a top foil between the compliant structure and the rotor. According to a particular embodiment of the invention, deployed within an electric motor, the stator of the electric motor is embedded within a polymer-based material and the compliant structure is made from protrusions of the polymer-based material provided within the cavity in which the rotor rotates, between the top foil and the inner wall of the cavity within the housing. Consequently, a simplified manufacturing process is provided where the operating characteristics of the bearing may be tailored by tailoring the compliant structure which provides the support for the compliant foil. According to all embodiments, the compliant structure forms an integral part of the housing.

Top foils, or compliant foils, disclosed herein may form a continuous sheet or may be made up of a plurality of elements. Consequently, in the case of a journal bearing, the top foil may form a single sheet substantially encircling at least a part of the rotor. In this case, the sheet encircles around 360 degrees or more of the rotor. Alternatively, the top foil may be made of a plurality of constituent foils, which when taken together substantially encircle at least part of the rotor. For example, the top foil may be formed of 3 constituent parts which each cover around 120 degrees of the stator such that the three parts together substantially encircle the stator. In a thrust bearing, the top foil may comprise either a single thin disc around the whole of the base plate, or it may comprise a plurality of segments, say six segments each covering around 60 degrees.

According to a first aspect, there is provided a foil bearing for supporting two surfaces in relative motion, the two supported surfaces being at least part of a first member of the foil bearing and at least part of a second member of the foil bearing, the foil bearing further comprising:

a top foil structure lying between the first member and the second member, the top foil structure substantially covering the supported surface of the second member, at least a part of the top foil structure being attached to a part of the first member; and

a compliant structure between the supported surface of the first member and the top foil. Advantageously, the compliant structure forms an integral part of the supported surface of the first member ft is shown that the compliant structure comprises a plurality of resilient protuberances protruding from the supported surface of the first member of the foil bearing, each protuberance therefore being an integral part of the supported surface. Due to the configuration of the compliant structure described herein, it can thus be said that each protuberance is an extension of the supported surface of the first member of the foil bearing.

More generally, there is disclosed a foil bearing for supporting two surfaces in relative motion, the two supported surfaces being at least part of a first member of the foil bearing and at least part of a second member of the foil bearing, the foil bearing further comprising:

a top foil structure lying between the first member and the second member, the top foil structure substantially overlapping the supported surface of the second member, at least a part of the top foil structure being attached to a part of the first member; and

a compliant structure between the supported surface of the first member and the top foil. Advantageously, the compliant structure comprises a plurality of resilient protuberances protruding from the supported surface of the first member, wherein the compliant structure forms a monolithic structure with the first member.

Preferably, the protuberances are distributed around substantially all of the supported surface of the first member.

According to a second aspect, provision is made for a static member for a foil bearing having a first member and second member mutually supported in relative motion, the first member being the static member, the static member having a supported surface. Advantageously, the supported surface of the static member comprises a plurality of resilient protuberances integral with the supported surface, the protuberances forming a compliant structure for the foil bearing. Due to the configuration of the compliant structure described herein, it can be said that all of the protuberances and the static member form a single monolithic unit.

According to a third aspect, provision is made for an electric motor comprising a foil bearing as described above, the stator comprising a stator core and motor windings encased within the stator by a polymer material, the supported surface of the stator comprising resilient polymer protuberances which form the compliant structure of the foil bearing.

According to a fourth aspect, a process for manufacturing a static member of a foil bearing having a first member and second member mutually supported in relative motion is disclosed, the first member being the static member, the static member having a supported surface, the process comprising: providing the supported surface of the static member with a plurality of protuberances, the protuberances forming a compliant structure for the foil bearing, wherein the supported surface of the static member and the compliant structure are formed as a monolithic unit of a polymeric material using a process selected from one of: an extrusion moulding process; a thermoforming process; a compression moulding process; a blow-moulding process; an injection moulding process; or an additive manufacturing process.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention and its advantages will be better understood with reference to the enclosed drawings and to the detailed description of various embodiments, wherein:

figures la, lb and lc show various sketches of parts of different compliant structures according to embodiments of the present invention, which may be deployed in a journal bearing;

figure 2 shows a sketch of a part of a compliant structure according to an embodiment of the present invention, which may be deployed in a thrust bearing;

figure 3 shows an axial view of a spindle and housing arrangement in a journal bearing according to an embodiment of the present invention, in which a compliant structure according to an embodiment can be seen;

figure 4 shows an axial view of a journal bearing according to an embodiment in which the stationary member is in the middle and the rotational member rotates around the stationary centre; figure 5 shows a cutaway view of a part of a stator core and windings of an electric motor according to an embodiment of the present invention; figure 6 illustrates a cutaway view of a part of a stator core and windings of an electric motor, where the stator can be seen encased in a housing which may be deployed in an electric motor according to an embodiment described herein;

figure 7 shows a cutaway view of a part of a stator core and windings of an electric motor, where the stator can be seen encased in a housing and the rotor can be seen with a foil journal bearing according to an embodiment described herein;

figure 8 shows a structure featuring both a journal bearing and a thrust bearing according to embodiments of the invention disclosed herein, in which the rotor rotates within the stator, the stator including the compliant structure; and

figure 9 showing a structure featuring both a journal bearing and a thrust bearing according to embodiments of the invention disclosed herein, in which the rotor rotates around the central stator, the stator including the compliant structure.

DETAILED DESCRPTION

Embodiments of the present invention provide for an improved foil bearing for supporting a stator and a rotor in relative motion, at least a part of the stator having a plurality of shaped resilient protuberances which form a compliant structure of the foil bearing. The foil bearing has a top foil structure between the compliant structure and the rotor. The part of the stator which has the protuberances is preferably a part of the surface of the stator which is supported when the bearing is in operation. Particular embodiments described herein support rotary motion of the bearing and are useable in either journal bearing applications or thrust bearing applications. Preferably, at least the stator surface, and therefore the protuberances, which form an integral part thereof, are made from a polymer. Resilient polymers are preferred in order to provide the compliant structure with a certain amount of resilience. Resilience around different parts of the stator can be tailored by tailoring the size, shape, thickness of the protuberances on a protuberance by protuberance basis if need be. In some embodiments a substantial part of the stator may be made from the polymer, such that the stator and compliant structure can be manufactured together as part of the same process step, for example by an injection moulding process step.

Figure la, lb and lc show parts of stators (10) and parts of their respective top foils (30). Each of the figures shows one protuberance of its respective stator. Top foils are preferably flexible, and the compliant structure serves to support the top foil on the stator while also providing resilience. Top foils are generally made from thin sheet metal but may be made from any flexible material with a low coefficient of friction. This resilience helps to ensure efficient operation of the bearing. Figures la, lb and lc illustrate how different shapes, different heights (h), different lengths, different cross-sectional widths and/or different levels of flexibility of the material can be used to tailor the resilience of the compliant structure (40). According to embodiments, various coatings may be applied to adjust the resilience of one or more of the protrusions. The general stiffness of the protrusions may be selected by selecting the type of polymer. Consequently, since the compliant structure is made from a plurality of these protuberances, the flexibility, or rigidity, can be tailored at different parts of the bearing as required and on a protuberance by protuberance basis, thus providing for very fine tuning of the resilience around the bearing and therefore improving the operating characteristics of the bearing.

According to a particular embodiment, the present invention provides for a low-cost foil bearing supported spindle with an integrated electric motor. The foil bearing is generally a hydrodynamic bearing configured to operate using a compressible fluid as a lubricant ln preferred embodiments the compressible fluid is in a gaseous phase.

Figure 2 shows the base plate of a foil bearing which can be classified as a thrust bearing since the load to be supported is predominantly in the same axis as the axis of rotation. The illustration shows the base plate, which can be called the stator (10). The base plate is the part to be supported, at the top of the spindle. The thrust runner is not shown in the drawing. The top surface of the base plate and the bottom surface of the thrust runner are supported in relative motion by the bearing. The top surface of the base plate has a plurality of protuberances which serve to form the compliant structure (40) of the bearing and the top foil structure (30) is placed between the compliant structure and the thrust plate (20). As shown in figure 2, the top foil structure may comprise a plurality of top foil segments to substantially cover the entire surface of the base plate to be supported. The protuberances are an integral part of the surface of the base plate ln embodiments where the top foil structure comprises a single top foil, one end of the top foil is attached to a part of the base plate while the other end is left free ln embodiments where the top file structure comprises a plurality of segments of top foil, each segment has one end attached to a part of the base plate and one end is left free. The top foil structure is therefore static, as is the stator to which a part of the top foil is attached. The protrusions form the compliant structure and are part of the main body of the spindle forming the base plate. The compliant structure allows to shape the aerodynamic wedge of the air film between the top foil and the thrust plate.

Figure 3 shows a view of a journal bearing, looking down its axis of rotation. The stator (10) surrounds the rotor (20), the rotor being free to rotate in the cavity within the stator. At least a layer of the stator is made from a polymer and forms the cavity within which the stator rotates. The cavity wall comprises a plurality of protrusions from the surface of the layer of polymer. The protrusions are made of the polymer and form an integral part of the cavity wall. The protrusions form the compliant structure (4) of the bearing, or flexible structure. Between the compliant structure and the rotor lies the top foil structure. The top foil structure (30) may comprise a plurality of top foil parts, with the combination of top foil parts substantially encircling the rotor. The top foil has a free end and an end which is attached to the surface of the inner wall of the cavity in the stator. Where the top foil structure comprises a plurality of parts, each part of the top foil structure has a free end an end which is attached to the inner wall of the cavity.

In figure 3, the rotor can be seen in the centre, with the top foil or compliant foil of the foil bearing located around the rotor and substantially surrounding the rotor, with a predetermined gap between the rotor and the top foil. In some embodiments, the top foil may be said to be substantially concentric around the rotor. As shown in figure 3, the top foil is supported by a compliant structure. The compliant structure in embodiments of the invention can be said to be equivalent to a bump foil of a state-of-the-art foil bearing. According to embodiments of the invention, the compliant structure forms an integral part of the sleeve or housing having a cavity within which the rotor rotates and is therefore also made from the same polymer material as the housing. According to an embodiment, the compliant structure has a number of protrusions of the polymer housing, protruding from the inner wall of the cavity of the housing.

According to a variation, the stator may be the spindle and the rotor rotates around the central spindle stator. An example of this is shown in figure 4. The stator (10) is the spindle in the middle and the rotor (20) is the part which surrounds the spindle. In this variation, the stator spindle has a plurality of polymer protrusions which form the compliant structure (40). The top foil (30) lies between the compliant structure and the rotor.

Figures 5, 6 and 7 illustrate how an electric motor according to an embodiment of the present invention may be realised. In figure 5, part of a stator of an electric motor is shown with its stator core and motor windings. The stator core may have any shape, as in known electric motors. An example shape could be a toroid or a generally toroidal shape. Figure 5, in this case, would show a section of a part of the stator core.

Figure 6 shows a cutaway view of a section of the motor stator of Figure 5 embedded within a housing. The housing may be of any shape as long as the housing can receive a rotor of the electric motor. In the example where the stator is of a generally toroidal shape, then the housing may generally follow the shape of the stator, resulting also in a toroidal shape having a cavity within which the rotor can rotate. As shown in Figure 6, at least part of the inner surface of the cavity within which the rotor is to rotate provides one or more locations for a journal foil bearing.

According to an embodiment of the invention, the housing may be made from a polymer-based material, which may be manufactured using any of the known processes for making polymer structures, such as an extrusion moulding process, a thermoforming process, a compression moulding process, a blow moulding process, an injection moulding process, an electronic printing process such as three- dimensional printing or any other process suitable for manufacturing monolithic polymer structures having resilient protuberances. Advantageously, by using a polymer housing to house the stator this provides an opportunity to create the compliant structure of the foil bearing of the motor using a simple manufacturing step which provides simple and accurate controllability of the characteristics of the compliant structure, as will be described below. The flexible structure, or compliant structure, becomes an integral part of the polymer housing. This embodiment is suitable for journal bearing designs.

Similarly, according to another embodiment of the invention, suitable for thrust bearing designs, the base plate may be made from a polymer-based material, which may be manufactured using any of the known processes for making polymer structures, such as an extrusion moulding process, a thermoforming process, a compression moulding process, a blow moulding process, an injection moulding process, an electronic printing process such as three-dimensional printing or any other process suitable for manufacturing monolithic polymer structures having resilient protrusions. Advantageously, by using a polymer base plate this provides an opportunity to create the compliant structure of the foil bearing using a simple manufacturing step which provides simple and accurate controllability of the characteristics of the compliant structure, as will be described below. The flexible structure, or compliant structure, becomes an integral part of the polymer base plate. In more general terms, where the thrust bearing has a first plate and a second plate, then according to the invention, at least one of the plates may be manufactured as a monolithic polymer structure having a plurality of resilient protuberances.

Figure 7 shows a cutaway view of a section of an electronic motor with its stator enveloped within its polymer housing and a part of the polymer housing serving as the compliant structure of a foil bearing. The top foil of the foil bearing is also shown. The top foil may also be referred to as a compliant foil. Figure 7 also shows the rotor of the motor with its magnet (70).

Going back to figure 7, the bearings can be seen located at either end of the stator. Consequently, the compliant foils and compliant structures of the bearings (one at each end of the rotor) are located at either ends of the rotor. This is just one example of a compliant foil, made from one or more constituent parts, encircling at least part of the rotatable member and being spaced at a predetermined distance from the at least part of the rotating member. It could even be said that in this example there are two bearings and so two compliant foils each encircling at least a corresponding part of the rotatable member. In another example, the at least part of the rotating member, or rotor, may be directly under the stator core of figure 7. In this case, the predetermined distance or gap for the bearing would be in the middle of the rotor. There could even be a single bearing at this position rather than two bearings, one at either end. The part under the ends of the windings of the stator may present a larger gap than the part in the middle where the bearing is. This is another example of a bearing according to an embodiment of the present invention, with the at least part of the rotating member being the part under the stator core. This allows for an even more compact version of a motor according to an embodiment of the present invention to be realised.

An electric motor according to an embodiment described herein may be made by making a stator of the motor as shown in figure 5, illustrating the core and windings of the motor. In a further step, a housing may be made for the stator. In a preferred embodiment the housing is made from a polymer-based material. Any known technique may be used for making the polymer body. According to a particular embodiment the housing may be made by moulding the polymer around the motor stator and arranging for the compliant foil, the flexible structure which may be tailored for the optimum operation of the foil bearing, to be fabricated during the moulding process. The compliant foil is thus directly integrated into the injected or otherwise moulded housing, forming an integral part thereof. The shape of this flexible structure, and more particularly the shape of each of the protrusions from the inner wall of the cavity of the housing, can be any generic geometry and can be tailored in order to tune the compliance level of the radial bearing both in axial and circumferential directions. Further, the compliant structure formed by the protrusions can be used to tune the clearance distribution by adjusting the height distribution of different parts of the flexible structure. According to an embodiment, the top foil, or compliant foil, may be made from a metallic material, which may be pre-shaped and then clipped onto the polymer structure. Materials other than metallic materials may also be used for the top foil, as is known in the state of the art.

The same principle applies to embodiments of the present invention which include a thrust bearing. The compliance of the flexible support will absorb any manufacturing errors which may result from the moulding process. A feature which is common to all embodiments is that the compliant structure, which is equivalent to the flexible support foil of state-of-the-art foil bearings, is obtained during the process of fabricating the polymer housing, resulting in the compliant structure forming an integral part of the housing.

Figure 8 illustrates an embodiment of the present invention in an application which can be said to be a combination of a journal bearing and a thrust bearing. Whereas in a journal bearing the force is applied in a direction perpendicular to the axis of rotation, in a thrust bearing a bearing surface is provided for forces acting axial to the shaft. In figure 8, the journal bearing discussed previously is shown to the left of the drawing. The rotor, or spindle of the journal bearing, is modified to feature a disc-shaped end to provide a thrust runner for thrust bearings for supporting forces which are predominantly axial to the rotation of the spindle and the thrust runner. The figure shows a cross- sectional view of the combined journal and thrust bearing, where part of the thrust bearing can be seen on the right of the drawing. The rotor (20) can be said to have a first, spindle-shaped part, on the left, forming a journal bearing, and second, disc-shaped parts, on the right, forming thrust bearings, the stator (10) having a first, toroidal shaped part and a second part having disc shapes. The parts of the stator (10) forming the journal bearing have protuberances on the inner wall of the cavity of the toroidal shaped part of the stator, forming the compliant structure (40) to support top foils (30) of the journal bearings. A second part of the stator (10) forms bearings with the disc-shaped second part of the rotor (20). Protuberances on the walls of the second part of the stator which face the disc-shaped rotor, form the compliant structures (40) of the thrust bearings.

Figure 9 shows an embodiment of the combined joumal/thrust bearings of figure 8 in“inverted” form, where the stator (10) forms the central spindle and the disc-shaped part, with the protuberances appearing at appropriate places to form the compliant structures (40) of the thrust and journal bearings. Top foils (30) are shown and the rotor (20) rotates around the stator spindle and over the base plate.

The invention allows for the number of parts of a gas-lubricated bearing to be minimised and for the series production cost to be decreased, while also increasing the robustness, reliability and tunability of such bearings. Foil bearings fabricated according to embodiments of the present invention are particularly suitable for high series and low power rotor applications.

Foil bearings according to embodiments of the present invention may be used in the aerospace, energy, automotive, bio-medical, and optical industries. Examples of applications in which embodiments of the present invention may find use include: space propulsion systems (Closed Brayton Cycles); Auxiliary Power Units; environmental control units; turbochargers; oil-free compressors for several thermodynamic cycles; and bio-medical oil-free high-speed applications.

It is clear from the information disclosed above and from figures 1, 2, 3, 4 and 9, that the compliant structure in all of the embodiments disclosed herein comprises a plurality of protuberances, each of which protrude from the supported surface of the first member of the foil bearing. Each protuberance is therefore an extension of the supported surface of the first member of the foil bearing. It follows therefore, that the compliant structure comprises a plurality of protrusions, each of which are extensions of the of the supported surface of the first member of the foil bearing. The protrusions of material extending from the supported surface of the first member are referred to herein as protuberances. The examples disclosed above and the figures 2, 3, 4 and 9 show that in preferred embodiments the protuberances are distributed over the supported surface of the first member.

Persons having ordinary skill in additive manufacturing techniques, including 3D printing methods, will recognise that these techniques cover a variety of processes in which material is joined or solidified under computer control to create a three-dimensional object, with material being added together, typically layer by layer. The material may be in the form of a filament of material, liquid molecules, gel, resin or powder grains. The layers of material may be fused together, melted, bonded, sprayed and cured, polymerised or otherwise solidified as part of the process. The layers meld to form a homogeneous three-dimensional object. One of the key advantages of additive manufacturing techniques is the ability to produce very complex shapes or geometries.

Embodiments of the present invention which may be used in the combined joumal/thrust bearings mentioned above, share the common feature of all embodiments where the compliant structure forms a monolithic structure with the stator.

It has been disclosed herein that the first member of the foil bearing may be manufactured economically using any from the various types of moulding processes discussed above. In embodiments where cost is less important and in which it is sought to achieve accurate tailoring of the resilience of the protrusions, on a protrusion by protrusion basis, then an additive manufacturing processes, such as a 3D printing process, may be preferred. The skilled person will recognise that a compliant structure of a foil bearing, manufactured using any of the above processes, may be formed as a single unit which integrates the compliant structure with the supported surface of the first member of the foil bearing. The single unit may be described as being monolithic because all of its parts are formed from a single piece of material, including the compliant structure and the supported surface of the first member. All of the protuberances of the compliant structure and the first member of the foil bearing form a monolithic unit. In other words, the compliant structure forms a monolithic structure with the first member. Indeed, it may further be said that each protuberance of the compliant structure forms a monolithic unit with the first member of the foil bearing.