Thrust foil bearing, foil bearing unit, turbo machine, and foil

Each of foils (22) includes: a top foil portion (22a) including a bearing surface (S); and a back foil portion (22b), which is formed on an upstream side of the top foil portion (22a), and is arranged so as to overlap behind the top foil portion (22a) of the adjacent foil (22) (on a side opposite to the bearing surface (S)). An angle (E) covering a radially inner end of an overlapping portion (P) between the adjacent foils (22) is smaller than an angle (D) covering a radially outer end of the overlapping portion (P).

TECHNICAL FIELD

The present invention relates to a thrust foil bearing.

BACKGROUND ART

A bearing configured to support a main shaft of a turbo machine, such as a gas turbine or a turbocharger, is required to endure severe environments involving high temperature and high speed rotation. Focus has been given on a foil bearing being one kind of a fluid dynamic bearing as a bearing suitable for use under such conditions. The foil bearing includes bearing surfaces formed of flexible thin films (foils) having low flexural rigidity, and is configured to bear a load by allowing the bearing surfaces to be bent (for example, see Patent Literatures 1 and 2 described below).

CITATION LIST

Patent Literature 1: JP 2004-108485 A

Patent Literature 2: JP 2015-132309 A

SUMMARY OF INVENTION

Technical Problem

Lubricants to be used in the foil bearing is gas (air), and hence the foil bearing has an advantage of having lower torque than that of a fluid dynamic bearing that uses oil as the lubricants. However, a load capacity of the fluid dynamic bearing depends on a viscosity of the lubricants. Thus, the foil bearing that uses gas as the lubricants inevitably has a smaller load capacity than that of the fluid dynamic bearing that uses oil as the lubricants. Accordingly, in order to expand a field of application of the foil bearing, it is required to further increase the load capacity.

Here,FIG.16is an illustration of a related-art thrust foil bearing120of a leaf type. In the thrust foil bearing120, foils122each having a shape as illustrated inFIG.17are arranged so as to overlap each other with phases of the foils122being shifted as illustrated inFIG.18. Of each of the foils122, a portion arranged behind the adjacent foil122(on a side opposite to the bearing surface) functions as a back foil portion122b, and a portion overlaying on the adjacent foil122functions as a top foil portion122acomprising the bearing surface.

When a thrust collar106provided on a main shaft rotates in the direction indicated by the arrow ofFIG.19, a bearing gap C′ is defined between a bearing surface S′ formed on each of the foils122and an end surface of the thrust collar106. At this time, the top foil portion122aof each of the foils122overlays on the back foil portion122bof the adjacent foil122so that the bearing gap C′ between the top foil portion122aof each of the foils122and the thrust collar106forms a wedge shape having a gap width that becomes gradually narrower toward a downstream side. When the lubricants (air) are pushed into a small gap portion of the wedge-shaped bearing gap C′, the pressure of the lubricants are increased. With this pressure, the thrust collar106is supported in a non-contact manner.

A load capacity of the thrust foil bearing120as described above depends on a minimum width (hereinafter, the width is referred to as “floating gap”) h′ of the bearing gap C′. That is, in theory, as the floating gap h′ becomes smaller, the load capacity of the thrust foil bearing120is increased. Therefore, in order to increase the load capacity of the thrust foil bearing120, it is only required that the floating gap h′ be reduced as much as possible.

However, when it is intended to reduce the floating gap h′ of the thrust foil bearing120described above, the following problem arises. When the foils122are formed, as illustrated inFIG.20, in general, a foil material130having a hollow disc shape is divided at a plurality of positions in a circumferential direction, and an entire region of the foil material130in the circumferential direction is used as the foils122, thereby increasing a material yield. In this case, as illustrated inFIG.17, an angle “A′” covering an arc portion122cformed at a radially outer end of each of the foils122is equal to an angle “B” covering an arc portion122dformed at a radially inner end of each of the foils122. As a result, a circumferential length of the arc portion122cat the radially outer end is larger than a circumferential length of the arc portion122dat the radially inner end.

When the foils122are arranged so as to overlap each other while the phases of the foils122being shifted, at a near of the radially outer ends of the foils122, as illustrated inFIG.21, a circumferential pitch L1′ between the foils122is relatively large. At a near of the radially inner ends of the foils122, as illustrated inFIG.22, a circumferential pitch L2′ between the foils122is relatively small. As a result, rigidity near the radially inner end of each of the foils122is higher than rigidity near the radially outer end of each of the foils122. Accordingly, each of the foils122is less likely to be bent, and thus easily comes into contact with the thrust collar. In this case, it is required that the floating gap be set with respect to the radially inner end of each of the foils as a reference. Accordingly, there has been a problem in that the floating gap cannot be reduced satisfactorily.

Therefore, the present invention has an object to increase a load capacity of a thrust foil bearing of a leaf type by reducing a floating gap.

Solution to Problem

In order to solve the above-mentioned problem, according to the present invention, there is provided a thrust foil bearing, comprising a plurality of foils each comprising a bearing surface arranged to be opposed to a rotary member in an axial direction, and being arrayed in a rotation direction of the rotary member, each of the plurality of foils comprising: a top foil portion comprising the bearing surface; and a back foil portion, which is formed on an upstream side of the top foil portion, and is arranged so as to overlap a side of the top foil portion of the adjacent foil opposite to the bearing surface, wherein an angle covering a radially inner end of an overlapping portion between the adjacent foils is smaller than an angle covering a radially outer end of the overlapping portion.

In the present invention, as described above, the angle covering the radially inner end of the overlapping portion between the adjacent foils (that is, region of the top foil portion of each of the foils to be supported by the back foil portion from behind (from a side opposite to the bearing surface)) is smaller than the angle covering the radially outer end of the overlapping portion. With this configuration, rigidity near the radially inner end of each of the foils is relatively low so that a difference between rigidity near the radially outer end of each of the foils and rigidity near the radially inner end of each of the foils is reduced. Thus, an entire surface of each of the foils can be bent substantially equally, thereby being capable of setting the floating gap to a smaller gap.

In the thrust foil bearing described above, for example, in each of the foils, the angle covering the radially inner end of a main body portion comprising the top foil portion and the back foil portion is smaller than the angle covering the radially outer end of the main body portion. With this configuration, the angle covering the radially inner end of the overlapping portion between the adjacent foils can be smaller than the angle covering the radially outer end of the overlapping portion.

Further, in the thrust foil bearing described above, it is preferred that the different between the angle covering the radially inner end of the overlapping portion between the adjacent foils and the angle covering the radially outer end of the overlapping portion be equal to or larger than 10°.

Advantageous Effects of Invention

As described above, according to the present invention, it is possible to increase a load capacity of the thrust foil bearing of a leaf type by reducing the floating gap.

DESCRIPTION OF EMBODIMENTS

Now, description is made of embodiment of the present invention with reference to the drawings.

FIG.1is a schematic view for illustrating a configuration of a gas turbine as a type of a turbo machine. The gas turbine mainly comprises a turbine1and a compressor2, which comprise blade cascades, a power generator3, a combustor4, and a regenerator5. The turbine1, the compressor2, and the power generator3comprise a common main shaft6extending in a horizontal direction. The main shaft6, the turbine1, and the compressor2serve as an integrally rotatable rotor. Air sucked from an air-intake port7is compressed by the compressor2, heated by the regenerator5, and then fed into the combustor4. The compressed air is mixed with fuel and combusted so as to rotate the turbine1with a high-temperature and high-pressure gas. A rotational force of the turbine1is transmitted to the power generator3through the main shaft6so as to rotate the power generator3. Through the rotation of the power generator3, electric power is generated and output through intermediation of an inverter8. The gas having rotated the turbine1has a relatively high temperature. Thus, the gas is fed into the regenerator5so that heat exchange is performed with the compressed air prior to the combustion. Through the heat exchange, the heat of the gas after the combustion is reused. The gas that has been subjected to the heat exchange in the regenerator5passes through an exhaust heat recovery device9, and then is exhausted as an exhaust gas.

FIG.2is an illustration of an example of a support structure for the rotor in the gas turbine described above. In the support structure, radial bearings10are arranged at two positions in an axial direction, and thrust bearings20and20are arranged on both sides in the axial direction of a thrust collar6aprovided on the main shaft6. By the radial bearings10and the thrust bearings20, the main shaft6is supported so as to be freely rotatable in a radial direction and both thrust directions.

In the support structure, an area between the turbine1and the compressor2is adjacent to the turbine1to be rotated with high-temperature and high-pressure gas, and hence has a high-temperature atmosphere. In the high-temperature atmosphere, lubricants formed of, for example, oil or grease are changed in quality and evaporated. Thus, it is difficult to apply a normal bearing (such as a rolling bearing) that uses the lubricants described above. Accordingly, as the bearings10and20to be used in the support structure of this type, an air fluid dynamic pressure bearing, in particular, a foil bearing is suitable.

Now, with reference to the drawings, description is made of a configuration of a foil bearing (hereinafter, referred to as “thrust foil bearing20”) suitable for the thrust bearing20for the gas turbine described above.

As illustrated inFIG.3, the thrust foil bearing20comprises a foil holder21having a disc shape, and a plurality of foils22mounted on an end surface21aof the foil holder21. In this embodiment, the thrust foil bearings20and20are provided on both sides of the thrust collar6ain the axial direction. The thrust foil bearings20and20have structures symmetrical with respect to the thrust collar6ain the axial direction. In the following, a downstream side in a flowing direction of a fluid with respect to the foils22during rotation of the main shaft6is referred to as “downstream side”, and a side opposite thereto is referred to as “upstream side”.

The foil holder21is made of, for example, metal or a resin. The foil holder21has a hollow disc shape having an inner hole21binto which the main shaft6is to be inserted. The plurality of foils22are mounted on one end surface21aof the foil holder21. Another end surface21cof the foil holder21is fixed to a housing of an apparatus (gas turbine in this embodiment) into which the thrust foil bearings20are to be incorporated.

The foils22are each made of metal having a rich spring property and gool workability, for example, made of steel or copper alloy. The foils22are each formed of a metal thin sheet (foil) having a thickness of from about 20 μm to about 200 μm. In the air fluid dynamic pressure bearing that uses the air as a fluid film as in this embodiment, there is no oil in the atmosphere. Thus, it is preferred that the foils22be each made of stainless steel or bronze.

As illustrated inFIG.4, the foils22are arrayed in a circumferential direction while the phases of the foils22is shifted. As illustrated inFIG.5, the foils22each comprise a main body portion22ccomprising a top foil portion22aand a back foil portion22b. The top foil portion22acomprises a bearing surface S. The back foil portion22bis formed so as to be continuous with an upstream side of the top foil portion22a. In the illustrated example, a downstream-side end portion22f(that is, a downstream-side end portion of the top foil portion22a) and an upstream-side end portion22g(that is, an upstream-side end portion of the back foil portion22b) of the main body portion22ceach have such a herringbone shape that a middle portion thereof in a radial direction protrudes to the downstream side. In this embodiment, a shape of the downstream-side end portion22fof the top foil portion22aand a shape of the upstream-side end portion22gof the back foil portion22bare different from each other. Each of the foils22is fixed to the foil holder21by an appropriate method. For example, the upstream-side end portion22gof the main body portion22cis fixed to the end surface21aof the foil holder21by welding.

An arc portion22dis formed at a radially outer end of the main body portion22cof the foil22, and an arc portion22eis formed at a radially inner end of the main body portion22c. Both of a center of the arc portion22dand a center of the arc portion22eis the same with a rotation center O of the main shaft6. An angle “B” covering the arc portion22eat the radially inner end of the main body portion22cis smaller than an angle “A” covering the arc portion22dat the radially outer end of the main body portion22c. For example, the angle “B” is smaller than the angle “A” by 10° or more. In the illustrated example, the radially outer end and the radially inner end of the downstream-side end portion22fof the main body portion22care arranged with the same phase (circumferential positional phase), whereas the radially inner end of the upstream-side end portion22gof the main body portion22cis arranged more on the downstream side than the radially outer end of the upstream-side end portion22g.

The foils22are formed by performing punching or electric discharge machining on a foil material (metal thin sheet) having a flat sheet-like shape. In this embodiment, as illustrated inFIG.6, six foils22are formed from a foil material30having a hollow disc shape. In this case, the radially outer end of the downstream-side end portion22fand the radially outer end of the upstream-side end portion22gof the adjacent foils22are held in contact with each other, whereas the radially inner end of the downstream-side end portion22fand the radially inner end of the upstream-side end portion22gof the adjacent foils22are apart from each other. In the illustrated example, the downstream-side end portion22fand the upstream-side end portion22gof the adjacent foils22are held in contact with each other in a region on a radially more outer side than a top portion of the main body portion22cat the center thereof in the radial direction, but are apart from each other in a region on a radially more inner side than the top portion. InFIG.6, of the foil material30, each unnecessary portion31between the downstream-side end portion22fand the upstream-side end portion22gof the adjacent foils22is illustrated by the dotted pattern. In the illustrated example, the six foils22are formed from the foil material30, and hence the angle “A” (seeFIG.5) covering the arc portion22dat the radially outer end of each of the foils22is 60°. Meanwhile, the angle “B” covering the arc portion22eat the radially inner end of each of the foils22is smaller than 60°, for example, equal to or smaller than 50°.

Under a state in which the foils22described above are mounted on the foil holder21, as illustrated inFIG.7andFIG.8, the bearing surface S formed on the top foil portion22aof each of the foils22is opposed directly to the thrust collar6ain the axial direction. Behind the top foil portion22aof each of the foils22(on a side opposite to the bearing surface S), the back foil portion22bof the foil22adjacent on the downstream side is arranged. That is, the back foil portion22bof each of the foils22is arranged between the foil holder21and the top foil portion22aof the foil22adjacent on the upstream side. In this embodiment, the angle “B” covering the arc portion22eat the radially inner end of each of the foils22is smaller than the angle “A” covering the arc portion22dat the radially outer end of each of the foils22(seeFIG.5). With this configuration, an angle “E” covering a radially inner end of an overlapping portion P (illustrated by the dotted pattern inFIG.7) between the adjacent foils22is smaller than an angle “D” covering a radially outer end of the overlapping portion P. In the illustrated example, in each of the foils22, the radially inner end and the radially outer end of the top foil portion22acomprising the bearing surface S form the same angle, whereas an angle covering the radially inner end of the back foil portion22barranged behind the adjacent foil22is smaller than an angle covering the radially outer end of the back foil portion22b.

When the main shaft6rotates to one side in the circumferential direction (direction indicated by the arrow R ofFIG.8), a bearing gap C is defined between the bearing surface S of each of the foils22of the thrust foil bearing20and the end surface of the thrust collar6a. At this time, each of the foils22overlays on the adjacent foil22and curves so that the bearing gap C forms a wedge shape that becomes narrower toward the downstream side (inFIG.8, each of the foils22is simplified and illustrated as a flat sheet). When the air in a large gap portion C1of the wedge-shaped bearing gap C is pushed into a small gap portion C2, pressure of an air film in the bearing gap C is increased. With this pressure, the main shaft6is supported in the thrust direction in a non-contact manner. At this time, the foils22are elastically deformed in accordance with operating conditions such as a load, a rotation speed of the main shaft6, and an ambient temperature. Thus, the bearing gap C is automatically adjusted so as to have appropriate widths in accordance with the operating conditions. As a result, even under severe conditions involving high temperature and high speed rotation, the bearing gap C may be managed so as to have optimum widths, and hence the main shaft6may be stably supported.

At this time, a circumferential pitch L1(seeFIG.9) near the radially outer end of each of the foils22is smaller than a circumferential pitch L2(seeFIG.10) near the radially inner end of each of the foils22. Thus, at the near of the radially inner end of each of the foils22, rigidity of the bearing surface S is more likely to be higher than that near the radially outer end (that is, the bearing surface S is less liable to be displaced in the axial direction).

In the present invention, as described above, the angle “B” covering the arc portion22eat the radially inner end of each of the foils22is smaller than the angle “A” covering the arc portion22dat the radially outer end. With this configuration, the angle “E” covering the radially inner end of the overlapping portion P between the adjacent foils22is smaller than the angle “D” covering the radially outer end of the overlapping portion P. That is, near the radially outer ends of the foils22, the overlapping portions P between the adjacent foils22are arranged continuously with each other in the circumferential direction (seeFIG.9). In contrast, at the near of the radially inner ends of the foils22, the overlapping portions P between the adjacent foils22are arranged apart from each other in the circumferential direction (seeFIG.10). As described above, at the near of the radially inner end of each of the foils22, a ratio of a region of the top foil portion22ato be supported by the back foil portion22bis reduced so that the rigidity of the bearing surface S near the radially inner end of each of the foils22is reduced. As a result, a difference between the rigidity near the radially outer end of each of the foils22and the rigidity near the radially inner end thereof is reduced, thereby being capable of substantially equally bending an entire region of each of the foils22in the radial direction. Thus, a width (floating gap “h”) of the small gap portion C2of the bearing gap C can be further reduced, thereby being capable of increasing a load capacity of the thrust foil bearing20.

Further, in this embodiment, the radially outer end and the radially inner end of the top foil portion22aof each of the foils22form the same angle, whereas the angle covering the radially inner end of the back foil portion22bof each of the foils22is smaller than the angle covering the radially outer end of the back foil portion22b. In this case, as compared to the related-art foils122illustrated inFIG.18, there is no difference in area of the top foil portion22a, that is, an area of the bearing surface S. Accordingly, reduction in load capacity due to reduction in the area of the bearing surface S can be avoided.

During the low speed rotation immediately before the stop or immediately after the actuation of the main shaft6, the bearing surfaces S of the foils22and the end surface of the thrust collar6acome into sliding contact with each other. Thus, low-friction coating such as a DLC film, a titanium aluminum nitride film, a tungsten disulfide film, and a molybdenum disulfide film may be formed on one or both of the bearing surface S of each of the foils22and the end surface of the thrust collar6a. Further, during the rotation of the main shaft6, slight sliding is caused between the foils22and the foil holder21, and between the top foil portion22aand the back foil portion22bof the overlapping foils22. With frictional energy generated by the slight sliding, the vibration of the main shaft6can be damped. In order to adjust the frictional force generated by the slight sliding, the low-friction coating as described above may be formed on one or both of sliding surfaces.

The present invention is not limited to the embodiment described above. For example, in an embodiment illustrated inFIG.11, the foil22comprises a fixing portion22hextending from the main body portion22cto a radially outer side. The fixing portion22his fixed to the foil holder21by an appropriate method such as welding, to thereby fix the foil22to the foil holder21.

Further, in an embodiment illustrated inFIG.12, the downstream-side end portion22fand the upstream-side end portion22gof the main body portion22cof the foil22each have a straight shape. In the illustrated example, an entire region of the downstream-side end portion22fis arranged with the same phase (circumferential positional phase), and the radially inner end of the upstream-side end portion22gis arranged on the downstream side more than the radially outer end of the upstream-side end portion22g. With this configuration, the angle “B” covering the radially inner end of the main body portion22cis smaller than the angle “A” covering the radially outer end of the main body portion22c.

The thrust foil bearing20as described above is applicable not only to the gas turbine, but also to other turbo machines such as a turbocharger, and can be used for other machines configured to support a rotary shaft.

EXAMPLES

In order to confirm effects of the present invention, there were prepared a foil (Comparative Example) having a difference of 0° between the angle “B” covering the radially inner end of the main body portion and the angle “A” covering the radially outer end of the main body portion, a foil (Example 1) having the difference of 5°, and a foil (Example 2) having the difference of 10°. After a raceway stopping test was performed on thrust foil bearings comprising those foils, states of the foils were observed. As a result, in Comparative Example, as shown inFIG.13, at the near of the radially inner end of the top foil portion of each foil (in a region surrounded by the dotted line), there were marks made through sliding with the thrust collar. Meanwhile, in Example 1, as shown inFIG.14, sliding marks on each foil was reduced as compared to those in Comparative Example. Further, in Example 2, as shown inFIG.15, there were almost no sliding marks on each foil. Based on the results described above, it was confirmed that, when the angle “B” covering the radially inner end of each foil was smaller than the angle “A” covering the radially outer end of each foil, that is, when the angle covering the radially inner end of the overlapping portion between the adjacent foils was smaller than the angle covering the radially outer end of the overlapping portion, contact of each foil with the thrust collar at the radially inner end of the foil was reduced. In particular, in Example 2 having the difference of 10° between the angle “B” covering the radially inner end of each foil and the angle “A” covering the radially outer end of each foil, it is seen that an entirety of each of the foils is bent equally without involving contact of only the radially inner end of the foil with the thrust collar. When contact between the radially inner end of the foil and the thrust collar is thus suppressed, the bearing gap, in particular, the floating gap is further reduced, thereby being capable of increasing a load capacity.

REFERENCE SIGNS LIST

22cmain body portion

22fdownstream-side end portion

22gupstream-side end portion

C bearing gap

O rotation center

P overlapping portion between foils

S bearing surface