ELECTRIC COMPRESSOR

Abstract

An electric compressor includes a housing; a rotary shaft; an impeller connected to at least a first end portion of the rotary shaft in an axial direction of the rotary shaft, of the first end portion and a second end portion of the rotary shaft in the axial direction; and a pair of air bearings supporting the rotary shaft such that the rotary shaft is rotatable relative to the housing. A load on the first end portion is larger than a load on the second end portion. The pair of air bearings includes a first air bearing, and a second air bearing supporting the rotary shaft at a position closer to the second end portion of the rotary shaft than the first air bearing is. A load carrying capacity of the first air bearing is larger than a load carrying capacity of the second bearing.

Description

TECHNICAL FIELD

The present invention relates to an electric compressor.

BACKGROUND ART

Patent Literature 1 mentions an electric compressor that includes a housing having therein a space, a rotary shaft accommodated in the housing, an impeller connected to one end of the rotary shaft in an axial direction of the rotary shaft, and a pair of air bearings supporting the rotary shaft such that the rotary shaft is rotatable relative to the housing. The rotation of the rotary shaft forms an air film between the outer peripheral surface of the rotary shaft and the air bearings, thereby causing the rotary shaft to float off the air bearings. This allows the air bearings to support the rotary shaft without coming into contact with the rotary shaft.

CITATION LIST

Patent Literature

Patent Literature 1: Japanese Patent Application Publication No. 2011-188612

SUMMARY OF INVENTION

Technical Problem

In the electric compressor mentioned in Patent Literature 1, although one end of the rotary shaft in the axial direction of the rotary shaft is connected to the impeller, the other end of the rotary shaft is not connected to an impeller.

Accordingly, compression is not performed near the other end of the rotary shaft although compression is performed by the impeller near the one end of the rotary shaft, so that a load applied by the rotation of the rotary shaft on the one end of the rotary shaft is different from a load applied by the rotation of the rotary shaft on the other end of the rotary shaft. Some of electric compressor may include impellers respectively connected to opposite ends of the rotary shaft. However, the impellers of such electric compressors may provide different compression capacities between the one end and the other end of the rotary shaft due to a difference in size between the impellers, so that the load applied by the rotation of the rotary shaft on the one end of the rotary shaft may be different from the load applied by the rotation of the rotary shaft on the other end of the rotary shaft.

If different loads, i.e., a large load and a small load, are applied by the rotation of the rotary shaft on the opposite ends of the rotary shaft, a necessary load carrying capacity is different between the air bearings respectively disposed on the opposite ends of the rotary shaft. If the air bearings have the same load carrying capacity, this load carrying capacity is excessive for one of the air bearings but deficient for the other of the air bearings. Deficient load carrying capacity of the air bearing may deteriorate the air bearing early. Excessive load carrying capacity of the air bearing may increase the manufacturing cost of the air bearing.

The present invention, which has been made in light of the above-mentioned problem, is directed to providing an electric compressor that is capable of preventing an excess or a deficiency of load carrying capacity of an air bearing relative to a necessary load carrying capacity of the air bearing.

Solution to Problem

An electric compressor to improve the above-mentioned problem comprising: a housing having therein a space; a rotary shaft accommodated in the housing; an impeller connected to at least a first end portion of the rotary shaft in an axial direction of the rotary shaft, of the first end portion and a second end portion of the rotary shaft in the axial direction; and a pair of air bearings supporting the rotary shaft such that the rotary shaft is rotatable relative to the housing, wherein a load on the first end portion is larger than a load on the second end portion, the pair of air bearings includes a first air bearing, and a second air bearing supporting the rotary shaft at a position closer to the second end portion of the rotary shaft than the first air bearing is, and a load carrying capacity of the first air bearing is larger than a load carrying capacity of the second bearing.

When the rotation of the rotary shaft applies a larger load on the first end portion than on the second end portion because the impeller is connected only to the first end portion, the rotary shaft applies a larger load on the first air bearing than on the second air bearing. Also, when the rotation of the rotary shaft applies a larger load on the first end portion than on the second end portion although the first end portion and the second end portion are respectively connected to impellers, the rotary shaft applies a larger load on the first air bearing than on the second air bearing.

Accordingly, the first air bearing needs a relatively large load carrying capacity, and the second air bearing needs a relatively small load carrying capacity. This load carrying capacity is a maximum load that each air bearing can receive without a deformation and a performance deterioration.

According to this configuration, the load carrying capacity of the first air bearing is larger than the load carrying capacity of the second bearing. This prevents a deficiency of the load carrying capacity of the first air bearing and an excess of the load carrying capacity of the second air bearing. This therefore prevents an excess or a deficiency of the load carrying capacity of each air bearing relative to a necessary load carrying capacity of the air bearing.

In the electric compressor, a length of the first air bearing may be preferably greater than a length of the second air bearing in the axial direction, so that the load carrying capacity of the first air bearing may be preferably larger than the load carrying capacity of the second air bearing.

According to this configuration, the length of the first air bearing is greater than the length of the second air bearing in the axial direction, so that a supporting surface of the first air bearing for supporting the rotary shaft is larger than a supporting surface of the second air bearing for supporting the rotary shaft. This allows the load carrying capacity of the first air bearing to be larger than the load carrying capacity of the second air bearing just by making a difference in length in the axial direction between the first air bearing and the second air bearing without changing the shapes of the first air bearing and the second air bearing. This therefore easily prevents an excess or a deficiency of the load carrying capacity of each air bearing relative to a necessary load capacity of the air bearings.

In the electric compressor, the first air bearing and the second air bearing may have different shapes so that the load carrying capacity of the first air bearing is larger than the load carrying capacity of the second air bearing.

ADVANTAGEOUS EFFECTS OF INVENTION

This disclosure prevents an excess or a deficiency of a load carrying capacity of each air bearing relative to a necessary load carrying capacity of each air bearing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of an electric compressor.

FIG. 2 is an exploded perspective view of a rotary shaft and a first air bearing.

FIG. 3 is a sectional view of an air bearing mounted on the rotary shaft.

FIG. 4 is an enlarged sectional view of the air bearing mounted on the rotary shaft.

FIG. 5 is a schematic view explaining the length of the first air bearing and the length of a second air bearing.

FIG. 6 is a sectional view of an air bearing mounted on a rotary shaft according to an example.

FIG. 7 is a sectional view of an air bearing mounted on a rotary shaft according to another example.

FIG. 8 is a sectional view of an air bearing mounted on a rotary shaft according to another example.

FIG. 9 is an exploded perspective view of a rotary shaft and a first air bearing according to another example.

DESCRIPTION OF EMBODIMENTS

The following will describe an embodiment of an electric compressor with reference to accompanying FIGS. 1 to 5.

As illustrated in FIG. 1, an electric compressor 10 includes a housing 11 having a cylindrical shape and therein a space, and an electric motor 20 accommodated in the housing 11. The housing 11 includes a first housing member 12 having a plate-like shape and a second housing member 13 having a bottomed-cylindrical shape and connected to the first housing member 12. The first housing member 12 and the second housing member 13 are each made of a metallic material, such as aluminum. The second housing member 13 has a bottom wall 13a having a plate-like shape and a peripheral wall 13b having a cylindrical shape and extending from an outer peripheral portion of the bottom wall 13a. The first housing member 12 is connected to the second housing member 13 with an opening of the peripheral wall 13b distant from the bottom wall 13a closed by the second housing member 13.

The first housing member 12 has a housing hole 12c that is formed through the first housing member 12 in the thickness direction of the first housing member 12. The housing hole 12c is a circular hole. The second housing member 13 has a cylindrical boss 13c protruding from the inner surface of the bottom wall 13a. The axis of the housing hole 12c is coaxial with the axis of the boss 13c.

The electric motor 20 includes a stator 21 and a rotor 22. The stator 21 includes a cylindrical stator core 21a that is fixed to the inner peripheral surface of the peripheral wall 13b of the second housing member 13, and a coil 21b that is wound around the stator core 21a. The rotor 22 is rotatably disposed radially inside the stator 21 in the housing 11.

The rotor 22 includes a cylindrical member 23, a permanent magnet 24 as a magnetic body, and a rotary shaft 25. The cylindrical member 23 has a circular cylindrical shape. The axis of the cylindrical member 23 corresponds to the axes of the housing hole 12c and the boss 13c. In this embodiment, a direction along the axis of the cylindrical member 23 is called an axial direction. A direction along the radius of the cylindrical member 23 is called a radial direction. The cylindrical member 23 has a first opening 23a and a second opening 23b respectively at opposite ends of the cylindrical member 23 in the axial direction. The cylindrical member 23 is made of a metallic material, such as titanium.

The permanent magnet 24 has a solid column shape and is magnetized in the radial direction. The permanent magnet 24 is press-fitted in the inner peripheral surface of the cylindrical member 23 so as to be fixed into the cylindrical member 23. The axis of the permanent magnet 24 corresponds to the axis of the cylindrical member 23. The length of the permanent magnet 24 is shorter than that of the cylindrical member 23 in the axial direction.

The rotary shaft 25 has a column-shaped first shaft portion 26 and a column-shaped second shaft portion 27 respectively located opposite sides in the axial direction with respect to the permanent magnet 24. The first shaft portion 26 and the column-shaped second shaft portion 27 are made of metal, for example. The first shaft portion 26 has a first small-diameter shaft portion 26a, and a first large-diameter shaft portion 26b having a diameter larger than that of the first small-diameter shaft portion 26a and aligned with the first small-diameter shaft portion 26a in the axial direction. The axis of the first small-diameter shaft portion 26a and the axis of the first large-diameter shaft portion 26b extend along the axial direction. The second shaft portion 27 has a second small-diameter shaft portion 27a, and a second large-diameter shaft portion 27b having a diameter larger than that of the second small-diameter shaft portion 27a and aligned with the second small-diameter shaft portion 27a in the axial direction. The axis of the second small-diameter shaft portion 27a and the axis of the second large-diameter shaft portion 27b extend along the axial direction. The first small-diameter shaft portion 26a and the second small-diameter shaft portion 27a have the same diameter. The first large-diameter shaft portion 26b and the second large-diameter shaft portion 27b have the same diameter.

The first large-diameter shaft portion 26b is located in the housing hole 12c of the first housing member 12. The second large-diameter shaft portion 27b is located in the boss 13c. The first small-diameter shaft portion 26a is inserted through the first opening 23a of the cylindrical member 23 and fixed to the cylindrical member 23 so as to close the first opening 23a. The second small-diameter shaft portion 27a is inserted through the second opening 23b of the cylindrical member 23 and fixed to the cylindrical member 23 so as to close the second opening 23b. This configuration allows the first shaft portion 26 and the second shaft portion 27 to be rotatable together with the cylindrical member 23 and the permanent magnet 24.

The axis of each of the first shaft portion 26 and the second shaft portion 27, i.e., the axis of the rotary shaft 25, corresponds to the cylindrical member 23. The axis of the rotary shaft 25 is illustrated by the axis L.

One end of the opposite ends of the first large-diameter shaft portion 26b in the axial direction is connected to the first small-diameter shaft portion 26a, and the other end of the opposite ends serves as a first end portion 25a of the rotary shaft 25. One end of the opposite ends of the second large-diameter shaft portion 27b in the axial direction is connected to the second small-diameter shaft portion 27a, and the other end of the opposite ends serves as a second end portion 25b of the rotary shaft 25. In this embodiment, the first end portion 25a of the rotary shaft 25 is connected to an impeller 32.

The impeller 32 includes an impeller shaft 32a extending in the axial direction, a hub 32b fixed to an outer peripheral surface of the impeller shaft 32a and configured to rotate together with the impeller shaft 32a, and a plurality of vanes 32c arranged in the circumferential direction of the hub 32. The impeller shaft 32a extends from the first end portion 25a of the rotary shaft 25 in the axial direction so that the impeller shaft 32a protrudes outside the housing 11. The hub 32b has an approximately conical shape and an outer diameter of the hub 32b expands as the hub 32b extends from one side to the other side in the axial direction. The vanes 32c are disposed on the outer surface of the hub 32b and equally spaced from each other in the circumferential direction of the hub 32b.

The first housing member 12 is connected to a compressor housing 31 that has a cylindrical shape and has an inlet 31a. The compressor housing 31 has the inlet 31a at one end thereof in the axial direction. The inlet 31a extends in the axial direction. The other end of the compressor housing 31 has an opening that is closed by the first housing member 12. The compressor housing 31 has therein an impeller chamber 33 in which the impeller 32 is accommodated. The impeller chamber 33 is communicated with the inlet 31a. The impeller shaft 32a extends in the axial direction in the impeller chamber 33.

The compressor housing 31 has a discharge chamber 34 in which air compressed by the impeller 32 is discharged, and a diffuser passage 35 through which the impeller chamber 33 is communicated with the discharged chamber 34. The diffuser passage 35 is located outward of the impeller chamber 33 in the radial direction of the impeller shaft 32a and formed into a ring shape surrounding the impeller chamber 33. The discharge chamber 34 is located outward of the diffuser passage 35 in the radial direction of the impeller shaft 32a and formed into a ring shape.

In the electric compressor 10, the rotor 22 including the rotary shaft 25 is rotated by energization of the coil 21b. The rotation of the rotary shaft 25 rotates the impeller 32 so as to compress the air drawn from the inlet 31a into the impeller chamber 33. The air compressed by the impeller 32 is further compressed via the diffuser passage 35 and is discharged to the discharge chamber 34. The air in the discharge chamber 34 is discharged outside the compressor housing 31 from an outlet (not illustrated) of the compressor housing 31.

In the electric compressor 10 according to the embodiment, although the first end portion 25a of the rotary shaft 25 is connected to the impeller 32, the second end portion 25b of the rotary shaft 25 is not connected to an impeller. That is, in the electric compressor 10 according to the embodiment, although compression is performed by the impeller 32 near the first end portion 25a of the rotary shaft 25, compression is not performed near the second end portion 25b of the rotary shaft 25. Accordingly, in the electric compressor 10 according to the embodiment, a load applied by the rotation of the rotary shaft 25 on the first end portion 25a is larger than that on the second end portion 25b.

The rotary shaft 25 is rotatably supported by a pair of air bearings 40 relative to the housing 11. The pair of air bearings 40 includes a first air bearing 41 supporting the first shaft portion 26 and a second air bearing 42 supporting the second shaft portion 27. That is, the second air bearing 42 supports the rotary shaft 25 at a position closer to the second end portion 25b of the rotary shaft 25 than the first air bearing 41 is.

The first air bearing 41 and the second air bearing 42 have a cylindrical shape. The axis of the first air bearing 41 and the axis of the second air bearing 42 correspond to the axis L of the rotary shaft 25. The first air bearing 41 is disposed between the inner peripheral surface of the housing hole 12c of the first housing member 12 and the outer peripheral surface of the first large-diameter shaft portion 26b. The second air bearing 42 is disposed between the inner peripheral surface of the boss 13c of the second housing member 13 and the outer peripheral surface of the second large-diameter shaft portion 27b. The rotary shaft 25 is supported by the housing 11 via the first air bearing 41 and the second air bearing 42 such that the rotary shaft 25 is rotatable relative to the housing 11.

The rotary shaft 25 is supported by the first air bearing 41 and the second air bearing 42 with the rotary shaft 25 in contact with the first air bearing 41 and the second air bearing 42 until the rotational speed of the rotary shaft 25 reaches a floating rotational speed at which the rotary shaft 25 floats off the first air bearing 41 and the second air bearing 42. When the rotational speed of the rotary shaft 25 reaches the floating rotational speed, a dynamic pressure is generated between the first air bearing 41 and the first shaft portion 26 and between the second air bearing 42 and the second shaft portion 27. The dynamic pressure allows the rotary shaft 25 to float off the first air bearing 41 and the second air bearing 42, so that the rotary shaft 25 is supported by the first air bearing 41 and the second air bearing 42 without coming into contact with the first air bearing 41 and the second air bearing 42. The first air bearing 41 and the second air bearing 42 are air dynamic bearings that support the rotary shaft 25 in the radial direction.

Next, the following will describe the air bearings 40 in more detail. The first air bearing 41 and the second air bearing 42 have the same base configuration. Accordingly, the following description will focus on the configuration of the first air bearing 41, and will not elaborate the same components of the second air bearing 42 as that of the first air bearing 41.

As illustrated in FIGS. 2 and 3, the first air bearing 41 includes a top foil 45 that has an approximately cylindrical shape and surrounds the rotary shaft 25 so as to support the rotary shaft 25, and a bump foil 50 that has an approximately cylindrical shape and surrounds the top foil 45. The outer peripheral surface of the bump foil 50 is supported by a bearing housing 55 that has a cylindrical shape and surrounds the bump foil 50. The axis of each of the top foil 45, the bump foil 50, and the bearing housing 55 corresponds to the axis L of the rotary shaft 25.

The first air bearing 41 has a configuration in which the top foil 45, the bump foil 50, and the bearing housing 55 are disposed between the outer peripheral surface of the first large-diameter shaft portion 26b of the first shaft portion 26 and the inner peripheral surface of the housing hole 12c of the first housing member 12. The second air bearing 42 has a configuration in which the top foil 45, the bump foil 50, and the bearing housing 55 are disposed between the outer peripheral surface of the second large-diameter shaft portion 27b of the second shaft portion 27 and the inner peripheral surface of the boss 13c of the second housing member 13. The rotary shaft 25 rotates in a clockwise direction indicated by an arrow X in FIG. 3.

The top foil 45 is formed of a flexible metallic plate, such as a nickel alloy plate, curved into a cylindrical shape. One of the opposite ends of the top foil 45 in a circumferential direction of the top foil 45 is a first fixed end 45a that is fixed to the bump foil 50. The first fixed end 45a extends outwardly in the radial direction of the top foil 45. The other of the opposite ends of the top foil 45 is a first free end 45b that is not fixed to the bump foil 50. The first free end 45b is spaced from the first fixed end 45a in the circumferential direction of the top foil 45. Since the top foil 45 has an approximately cylindrical shape, the distance between the first fixed end 45a and the first free end 45b is small.

The bump foil 50 is formed of a flexible metallic plate, such as a nickel alloy plate, and extends along the outer peripheral surface of the top foil 45. One of the opposite ends of the bump foil 50 in the circumferential direction of the bump foil 50 is a second fixed end 50a that is fixed to the inner peripheral surface of the bearing housing 55. The first fixed end 45a of the top foil 45 is placed on and fixed to the second fixed end 50a. That is, the first fixed end 45a is fixed to the inner peripheral surface of the bearing housing 55 via the second fixed end 50a. The other of the opposite ends of the bump foil 50 is a second free end 50b that is not fixed to the bearing housing 55. The second free end 50b is spaced from the second fixed end 50a in the circumferential direction of the bump foil 50. Since the bump foil 50 has an approximately cylindrical shape, the distance between the second fixed end 50a and the second free end 50b is small.

As illustrated in FIG. 4, the bump foil 50 has a plurality of projections 51 that project in the radial direction of the bump foil 50. The projections 51 are spaced from each other in the circumferential direction of the bump foil 50. Each of the projections 51 is semi-circular in cross-section in a direction perpendicular to the axial direction. In the bump foil 50, the adjacent projections 51 are connected to each other by an extending portion 52 that extends in the circumferential direction of the bump foil 50. The extending portion 52 extends along the inner peripheral surface of the bearing housing 55, and each of the projections 51 projects so as to be radially and inwardly spaced from the inner peripheral surface of the bearing housing 55. The bump foil 50 is formed into a corrugated shape as a whole.

The extending portion 52 of the bump foil 50 and the top of the projection 51 are respectively in contact with the inner peripheral surface of the bearing housing 55 and the outer peripheral surface of the top foil 45 when the rotary shaft 25 is not rotated. The top foil 45 is elastically and radially outwardly deformed when the rotary shaft 25 is rotated, so that air enters a gap between the outer peripheral surface of the rotary shaft 25 and an inner peripheral surface 45c of the top foil 45 to form an air film. That is, the rotary shaft 25 is supported by the inner peripheral surface 45c of the top foil 45 via the air film. The inner peripheral surface 45c of the top foil 45 serves as a supporting surface that supports the rotary shaft 25. The elastic and radially outward deformation of the top foil 45 along with the formation of the air film causes the bump foil 50 to be elastically and radially outwardly deformed via the projections 51 in contact with the outer peripheral surface of the top foil 45.

The bump foil 50 has a first thickness T1 in both of the first air bearing 41 and the second air bearing 42. The thickness of the bump foil 50 corresponds to the thickness of the metallic plate that forms the bump foil 50. The bump foil 50 of the first air bearing 41 and the bump foil 50 of the second air bearing 42 have the same number of the projections 51 in a predetermined length L3 in the circumferential direction of the bump foil 50. In other words, the first air bearing 41 and the second air bearing 42 have the same area density of the projections 51 in their bump foils 50. Each of the projections 51 forms a first angle A1 with the corresponding extending portion 52 in a boundary between the projection 51 and the extending portion 52 in the circumferential direction of the bump foil 50 in both of the first air bearing 41 and the second air bearing 42. The first angle A1 is greater than 0 degrees and less than 90 degrees. In this embodiment, the first air bearing 41 and the second air bearing 42 have the same thickness of the bump foil 50, the same area density of the projections 51, and the same angle formed by each projection 51 and the corresponding extending portion 52, so that the first air bearing 41 and the second air bearing 42 have the same shape.

The circumferential length of the top foil 45 of each of the first air bearing 41 and the second air bearing 42 is determined so that the whole of the inner peripheral surface 45c of the top foil 45 is in contact with the outer peripheral surface of the rotary shaft 25 when the rotary shaft 25 is not rotated. The first air bearing 41 and the second air bearing 42 have the same length of the inner peripheral surface 45c in the circumferential direction of the top foil 45. Similarly, the first air bearing 41 and the second air bearing 42 have the same length of the bump foil 50 and the same length of the bearing housing 55 in the circumferential direction.

As illustrated in FIG. 2, the top foil 45, the bump foil 50, and the bearing housing 55 of the first air bearing 41 have the same length in the axial direction. The top foil 45, the bump foil 50, and the bearing housing 55 of the second air bearing 42 have the same length in the axial direction. The length of the bearing housing 55 may be slightly greater than the length of the top foil 45 and the length of the bump foil 50 in the axial direction.

As illustrated in FIG. 5, the top foil 45 of the first air bearing 41 has a first length L1 in the axial direction, and the top foil 45 of the second air bearing 42 has a second length L2 that is shorter than the first length L1. That is, the area of the inner peripheral surface 45c of the top foil 45 of the first air bearing 41 is larger than that of the second air bearing 42. The larger area of the inner peripheral surface 45c of the top foil 45 extends the supporting surface, which supports the rotary shaft 25 via the air film when the rotary shaft 25 is rotated, thereby increasing the load carrying capacity of the air bearings 40. Accordingly, in this embodiment, the load carrying capacity of the first air bearing 41 is larger than the load carrying capacity of the second air bearing 42.

Next, the following will explain the operation of the electric compressor according to the embodiment.

When the rotary shaft 25 is rotated, air enters a gap between the outer peripheral surface of the rotary shaft 25 and the inner peripheral surface 45c of the top foil 45 and forms an air film. This causes the top foil 45 to be elastically and radially outwardly deformed and therefore the bump foil 50 to be elastically and radially outwardly deformed via the projections 51 in contact with the outer peripheral surface of the top foil 45.

The rotation of the rotary shaft 25 applies a larger load on the first air bearing 41, which supports the rotary shaft 25 at a position adjacent to the first end portion 25a to which the impeller 32 is connected, than that on the second air bearing 42. Accordingly, the first air bearing 41 needs a relatively large load carrying capacity, and the second air bearing 42 needs a relatively small load carrying capacity.

In this embodiment, since the area of the inner peripheral surface 45c of the top foil 45 of the first air bearing 41 is larger than that of the second air bearing 42, the load carrying capacity of the first air bearing 41 is larger than that of the second air bearing 42. This allows the respective first air bearing 41 and the second air bearing 42 to have a load carrying capacity satisfying a necessary load carrying capacity.

This embodiment provides following advantageous effects.

(1) The load carrying capacity of the first air bearing 41 is larger than the load carrying capacity of the second air bearing 42. This prevents a deficiency of the load carrying capacity of the first air bearing 41 and an excess of the load carrying capacity of the second air bearing 42. This therefore prevents an excess or a deficiency of the load carrying capacity of each of the air bearings 40 relative to a necessary load carrying capacity of each of the air bearings 40.

(2) The length of the first air bearing 41 is greater than that of the second air bearing 42 in the axial direction, so that the area of the inner peripheral surface 45c of the top foil 45 of the first air bearing 41 is larger than that of the second air bearing 42. This allows the load carrying capacity of the first air bearing 41 to be larger than that of the second air bearing 42 just by a difference in length in the axial direction between the first air bearing 41 and the second air bearing 42 without changing the shapes of the first air bearing 41 and the second air bearing 42. This therefore easily prevents an excess or a deficiency of the load carrying capacity of each of the air bearings 40 relative to the necessary load carrying capacity of each of the air bearings 40.

This embodiment may be modified as below. The embodiment may be combined with the following modification examples within technically consistent range.

• • As illustrated in FIG. 6, the bump foil 50 of the first air bearing 41 may have a second thickness T2 that is thicker than the first thickness T1 of the second air bearing 42. The difference in thickness between the bump foil 50 of the first air bearing 41 and the bump foil 50 of the second air bearing 42 makes a difference in shape between the first air bearing 41 and the second air bearing 42. The thicker bump foil 50 increases the stiffness of the bump foil 50, thereby increasing the load carrying capacity in the air bearings 40. In this configuration, the bump foil 50 of the first air bearing 41 is thicker than that of the second air bearing 42, so that the load carrying capacity of the first air bearing 41 is larger than that of the second air bearing 42.
  • As illustrated in FIG. 7, the number of the projections 51 of the bump foil 50 of the first air bearing 41 may be larger than that of the second air bearing 42 in the predetermined length L3 in the circumferential direction of the bump foil 50. In other words, the area density of the projections 51 of the bump foil 50 of the first air bearing 41 may be greater than that of the second air bearing 42. The difference in area density of the projections 51 between the bump foil 50 of the first air bearing 41 and the bump foil 50 of the second air bearing 42 makes a difference in shape between the first air bearing 41 and the second air bearing 42. The greater area density of the projections 51 of the bump foil 50 increases the stiffness of the bump foil 50, thereby increasing the load carrying capacity in the air bearings 40. In this configuration, the area density of the projections 51 of the bump foil 50 of the first air bearing 41 is greater than that of the second air bearing 42, so that the load carrying capacity of the first air bearing 41 is larger than that of the second air bearing 42.
  • As illustrated in FIG. 8, each projection 51 of the bump foil 50 may be divided with respect to the circumferential direction in both of the first air bearing 41 and the second air bearing 42. In this configuration, each projection 51 is formed of a first projection 51a and a second projection 51b adjacent to each other in the circumferential direction of the bump foil 50. The first projection 51a is curved in the rotational direction of the rotary shaft 25 so as to approach the outer peripheral surface of the top foil 45 from one end of the extending portion 52. The second projection 51b is curved in the rotational direction of the rotary shaft 25 so as to approach one end of another extending portion 52 from the outer peripheral surface of the top foil 45. The top of the first projection 51a is spaced from the top of the second projection 51b in the circumferential direction. Each of the projections 51 is formed by the first projection 51a and the second projection 51b into approximately semi-circular in cross-section in a direction perpendicular to the axial direction.
  • In the second air bearing 42 of this modification example, each of the first projection 51a and the second projection 51b forms the first angle A1 with the corresponding extending portion 52 in a boundary between the projection 51 and the extending portion 52 in the circumferential direction of the bump foil 50. Alternatively, in the first air bearing 41, this angle may be a second angle A2 that is greater than the first angle A1 and less than 90 degrees. The first angle A1 and the second angle A2 are angles when the rotary shaft 25 is not rotated. The difference in angle formed by each projection 51 and the corresponding extending portion 52 between the first air bearing 41 and the second air bearing 42 makes a difference in shape between the first air bearing 41 and the second air bearing 42. The greater angle formed by the projection 51 and the extending portion 52 and not exceeding 90 degrees increases the stiffness of the bump foil 50, thereby increasing the load carrying capacity in the air bearings 40. In this configuration, the angle formed by the projection 51 and the extending portion 52 of the first air bearing 41 is greater than that of the second air bearing 42, so that the load carrying capacity of the first air bearing 41 is larger than that of the second air bearing 42. In this configuration, similarly to the embodiment, each projection 51 may not be divided in both of the first air bearing 41 and the second air bearing 42. This configuration allows the angle formed by the projection 51 and the extending portion 52 of the first air bearing 41 to be greater than that of the second air bearing 42, so that the load carrying capacity of the first air bearing 41 is larger than that of the second air bearing 42.
  • As illustrated in FIG. 9, the bump foil 50 of the first air bearing 41 may be divided with respect to the axial direction. In this configuration, the bump foil 50 is formed of a first bump foil member 150a and a second bump foil member 150b adjacent to each other in the axial direction. The length of the first bump foil member 150a and the length of the second bump foil member 150b are half the length of the top foil 45 in the axial direction. The first bump foil member 150a and the second bump foil member 150b are fixed to the bearing housing 55 with the first bump foil member 150a and the second bump foil member 150b in contact with each other in the axial direction. Accordingly, in the first air bearing 41, the length of the whole bump foil 50 is equal to that of the top foil 45 in the axial direction. In this configuration, similarly to the embodiment, the bump foil 50 of the second air bearing 42 is not divided in the axial direction. Accordingly, the top foil 45 of the first air bearing 41 has the second length L2 that is equal to the length of the top foil 45 of the second air bearing 42 in the axial direction. The length of each of the first bump foil member 150a and the second bump foil member 150b is a third length L4, which is half the length of the second length L2 in the axial direction.

In this configuration, the bump foil 50 of the first air bearing 41 is divided into more members than the bump foil 50 of the second air bearing 42 in such a manner, so that the first air bearing 41 and the second air bearing 42 have different shapes. More divisions of the bump foil 50 in the axial direction allow distribution of the load applied by the rotary shaft 25 on the bump foil 50, thereby increasing the stiffness of the bump foil 50 and the load carrying capacity in the air bearings 40. In this configuration, the bump foil 50 of the first air bearing 41 is divided into more members than the bump foil 50 of the second air bearing 42 in the axial direction, so that the load carrying capacity of the first air bearing 41 is larger than that of the second air bearing 42.

• • In the modification example as illustrated in FIG. 9, the bump foil 50 of the first air bearing 41 may be divided into three or more members. The bump foil 50 of the second air bearing 42 may be divided with respect to the axial direction. That is, the bump foil 50 of the first air bearing 41 and the bump foil 50 of the second air bearing 42 may be divided into any number of members in the axial direction as long as the bump foil 50 of the first air bearing 41 is divided into more members than the bump foil 50 of the second air bearing 42.
  • The bump foil 50 of the first air bearing 41 and the bump foil 50 of the second air bearing 42 may be made of different materials. For example, if the bump foil 50 of the first air bearing 41 may be made of a material with higher Young's modulus than that of the material of the bump foil 50 of the second air bearing 42, the load carrying capacity of the first air bearing 41 is larger than that of the second air bearing 42.
  • The top foil 45 and the bump foil 50 may be made of a flexible metal other than nickel alloy, such as stainless steel.
  • Both of the first end portion 25a and the second end portion 25b of the rotary shaft 25 may be respectively connected to impellers 32. In this configuration, the impeller 32 connected to the first end portion 25a may be larger than the impeller 32 connected to the second end portion 25b, so that the compression capacity provided by the impeller 32 on the first end portion 25a may be larger than that provided by the other impeller 32 on the second end portion 25b. That is, a load applied by the rotation of the rotary shaft 25 on the first end portion 25a may be larger than that on the second end portion 25b. If the load carrying capacity of the first air bearing 41 is set larger than the load carrying capacity of the second air bearing 42 as explained in the embodiment and the modification examples for the electric compressor 10 with such a difference in load, the same advantageous effects as in the above embodiment can be obtained.

REFERENCE SIGNS LIST

10 electric compressor

11 housing

25 rotary shaft

25a first end portion

25b second end portion

32 impeller

40 air bearing

41 first air bearing

42 second air bearing

Claims

1. An electric compressor comprising:

a housing having therein a space;

a rotary shaft accommodated in the housing;

an impeller connected to at least a first end portion of the rotary shaft in an axial direction of the rotary shaft, of the first end portion and a second end portion of the rotary shaft in the axial direction; and

a pair of air bearings supporting the rotary shaft such that the rotary shaft is rotatable relative to the housing, wherein

a load on the first end portion is larger than a load on the second end portion,

the pair of air bearings includes a first air bearing, and a second air bearing supporting the rotary shaft at a position closer to the second end portion of the rotary shaft than the first air bearing is, and

a load carrying capacity of the first air bearing is larger than a load carrying capacity of the second bearing.

2. The electric compressor according to claim 1, wherein

a length of the first air bearing is greater than a length of the second air bearing in the axial direction so that the load carrying capacity of the first air bearing is larger than the load carrying capacity of the second air bearing.

3. The electric compressor according to claim 1, wherein

the first air bearing and the second air bearing have different shapes so that the load carrying capacity of the first air bearing is larger than the load carrying capacity of the second air bearing.