CENTRIFUGAL FAN

Abstract

A centrifugal fan includes an impeller that rotates inside the elongated cylindrical housing and a motor for driving the impeller. The impeller and the motor are arranged tandem in the axial direction. The impeller includes a blade portion having plural blades elongated in the axial direction, and the blades are arranged at a predetermined pitch in the circumferential direction. When the impeller rotates, air is taken in through a suction opening provided to the tip portion in the axial direction of the housing and is blown out through an outlet opening that is provided to a part in the circumferential direction of the housing. A bearing portion that constitutes the motor is a sleeve bearing including a shaft member and a sleeve having a cylindrical shape that engages the shaft member with a clearance. One of the shaft member and the sleeve is fixed to a rotational member, while the other is fixed to a base member. One of the shaft member and the sleeve is made of a ceramic, while the other is made of a ceramic or a metal.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a centrifugal fan for cooling embedded in electronic equipment.

2. Description of the Prior Art

Recently, cooling fans embedded in electronic equipment such as a personal computer have become smaller and thinner. Conventionally, most electronic equipment incorporates a cooling fan of an axial flow type. It is because the axial flow type cooling fan typically has a structure suitable for a thin shape or a low profile. In contrast, although centrifugal fans have an advantage of having higher static pressure than axial fans, they also have a disadvantage of difficulty in reducing a dimension in the axial direction for a low profile compared with axial fans. In order to realize a centrifugal fan that can be embedded in low profile electronic equipment such as a note type personal computer, it is possible to adopt a structure in which a dimension in the radial direction of an impeller is reduced while a length in the axial direction of the same is increased, and the impeller is rotated at a high speed.

FIG. 6 is a cross sectional view in the axial direction showing a structure of a long thin centrifugal fan. In addition, FIG. 7 is a cross sectional view in the direction perpendicular to the rotational axis of an impeller in a typical centrifugal fan. Although shapes and scales of each element are not identical between FIG. 6 and FIG. 7, elements having the same functions are denoted by the same reference numeral. The long thin centrifugal fan shown in FIG. 6 includes a housing 101 having a substantially cylindrical shape elongated in the axial direction, which houses an impeller 102 and a motor 103 for rotating the impeller 102. The impeller 102 is disposed at the top side (the right side in FIG. 6), in the axial direction within the housing 101, while the motor 103 is disposed at the bottom side (the left side in FIG. 6) in the axial direction within the housing 101.

The impeller 102 includes a blade portion 104 (at the top side) having plural blades 104a of a shape elongated in the axial direction that are arranged at a predetermined pitch in the circumferential direction and a bottom end portion 105 having a substantially cylindrical shape for supporting the blade portion 104. The tip of the blade portion 104 is provided with a ring-like linking portion 106 for linking and supporting tip portions of the plural blades 104a. When the impeller 102 rotates, external air is taken in through a suction opening 107 provided to the tip portion in the axial direction of the housing 101 as illustrated by an arrow IN. Then, the air is blown out through an outlet opening (corresponding to 108 in FIG. 7) externally that is provided to a part in the circumferential direction of the housing 101 (as illustrated by an arrow OUT in FIG. 7).

The motor 103 includes a rotor yoke 111 and a rotor magnet 112 that corresponds a rotational member rotating with the impeller 102, a stator armature 113 and a fixed shaft 114 on a stational side, a pair of ball bearings 115, and a base member 116 for fixing the fixed shaft 114 to the housing 101. The bottom end portion 105 of the impeller 102 is engaged with and fixed to the outer surface of the rotor yoke 111 that is a cylindrical magnetic member. A plurality of rotor magnets 112 is arranged in the circumferential direction at a predetermined pitch on the inner surface of the rotor yoke 111 at the middle portion in the axial direction, and a pair of outer ring portions of the ball bearings 115 is disposed separately in the axial direction at both sides of the rotor magnets 112.

The stator armature 113 is fixed substantially at the middle portion in the axial direction of the fixed shaft 114 so as to be opposed to the rotor magnets 112 with a constant gap. In addition, a pair of inner ring portions of the ball bearings 115 is fixed to the fixed shaft 114 separately in the axial direction at both sides of the stator armature 113. The bottom side of the fixed shaft 114 is engaged with and fixed to a center hole of the base member 116 so that an axis of the fixed shaft 114 agrees the rotation axis of the rotor yoke 111.

When the stator armature 113 is driven (excited) to generate a revolving magnetic field, the rotor magnet 112, the rotor yoke 111 and the impeller 102 are rotated as one unit body responding to the revolving magnetic field. Then, as described above, the rotation of the impeller 102 causes air flow that is taken in through the suction opening 107 at the tip portion in the axial direction of the housing 101 and is blown out through an outlet opening formed at a part in the circumferential direction of the housing 101.

The long thin centrifugal fan having the above-mentioned structure is required to have a large length of the impeller in the axial direction and to be rotated at a high rotation speed in order to compensate low capacity of air blowing due to a small diameter of the impeller. For example, dimensions of the impeller is designed so that its radius r and its length in the axial direction h satisfies the relationship 2r<=h<=20r, and the rotation speed of the impeller is set to a value of 15,000 revolutions per minute or more. In this case, since the diameter of the impeller is small, noise (wind noise) due to the rotation of the impeller is not so large despite of the high rotation speed. Indeed, its noise is smaller than an ordinary centrifugal fan.

However, the desire to realize lower noise level has grown recently particularly in portable electronic equipment such as a note type personal computer. Therefore, there is a tendency that an acceptable level of noise generated by a cooling fan becomes lowered. Concerning the long thin centrifugal fan having the above-mentioned structure, noise generated by ball bearings has become larger than the wind noise generated by the impeller.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a centrifugal fan that has a novel structure of bearing so as to suppress generation of noise. Another object of the present invention is to reduce further a dimension in the radial direction of the long thin centrifugal fan.

According to a first aspect of the present invention, a centrifugal fan is provided that includes an impeller including a blade portion having plural elongated blades arranged at a predetermined pitch in the circumferential direction and a motor for rotating the impeller. The impeller and the motor are arranged tandem in the axial direction. A bearing portion that constitutes the motor is a slide bearing including a shaft member and a sleeve having a cylindrical shape that engages the shaft with a clearance. One of the shaft member and the sleeve is made of a ceramic, and the other is made of a ceramic or a metal.

According to this structure, since a slide bearing (also referred to as a sleeve bearing) is used for the bearing portion, noise due to rotation can be reduced compared with the conventional ball bearing. Furthermore, since at least one of the shaft member and the sleeve that constitute the slide bearing is made of a ceramic, a good durability can be obtained. Note that it is preferable that both of them are made of a ceramic for a long life because seizing up occurs hardly between them. If one of them is made of a ceramic and the other is made of a metal, the sliding surface of the metal is preferably hardened by a surface treatment or coating so that a long life is obtained.

According to a second aspect of the present invention, a reduced diameter portion is formed on the shaft member at a middle portion in the axial direction of the sliding portion with the sleeve, the reduced diameter portion having a diameter a little smaller than other portions. In this way, an area of the sliding surface is reduced so that a bearing loss due to sliding friction can be reduced. Note that although it is possible to cut a part of the inner surface of the sleeve instead of the shaft member for reducing an area of the sliding surface, cutting of the shaft member is easier than cutting of the inner surface of the sleeve.

According to a third aspect of the present invention, means for keeping lubricating oil that is supplied to the sliding surfaces of the shaft member and the sleeve are attached to the shaft member or the sleeve. For example, a felt member having a ring-like shape impregnated with lubricating oil can be attached to the shaft member or the sleeve as the means for keeping lubricating oil. When the lubricating oil is supplied to and is kept on the sliding surfaces of the shaft member and the sleeve, a bearing loss due to a friction on the sliding surface can be reduced.

According to a fourth aspect of the present invention, the shaft member is fixed to a rotational member, and the sleeve that retains the shaft member in a rotatable manner is fixed to the housing directly or via a base member. This structure is a so-called rotational shaft (or fixed sleeve) structure, and the number of components can be reduced by fixing the sleeve to a base member (that is fixed to the housing) directly. This means a structure in which a sleeve and a sleeve holder are made as one unit member. It is particularly preferable to make such a member using a ceramic. In addition, as the sleeve holder is attached to the housing directly, the number of components can be reduced further. This means a structure in which the base member, the sleeve holder and the sleeve are made as one unit member.

According to a fifth aspect of the present invention, a sleeve made of a ceramic is fixed to the rotational member, and a shaft member is fixed to the housing directly or via a base member. This structure is a so-called rotational sleeve (or fixed shaft) structure, and the number of components can be reduced by fixing the shaft member to a base member (that is fixed to the housing) directly. In addition, the number of components can be further reduced by attaching the shaft member to the housing directly. This means a structure in which the base member and the shaft member are made as one unit member. Note that although a sleeve bearing that uses a sleeve made of a sintered metal impregnated with lubricating oil is used conventionally, it is difficult to use such a sleeve in this case of rotating sleeve. It is because that when the sleeve is rotated, the lubricating oil is moved to the outer side by centrifugal force so that the sliding surface of the inner side will be lack of the lubricating oil. In contrast, there is not such a problem if a sleeve made of a ceramic is used as a ceramic bearing so that the structure of the rotating sleeve can be realized easily.

According to a sixth aspect of the present invention, the top side of the shaft member is disposed within the inner space of the blade portion of the impeller. When the top side of the shaft member is positioned within the inner space of the blade portion of the impeller, a dimension in the axial direction of the entire centrifugal fan can be reduced.

According to a seventh aspect of the present invention, the bottom side of the shaft member is a free end without a thrust bearing. According to this structure, a member (thrust plate, for example) for a thrust bearing can be eliminated, and generation of noise due to friction at the thrust bearing portion as well as wear of the tip portion of the shaft member on the bottom side can be avoided. Note that it is possible to support the bottom side of the shaft member by using air dumping or oil dumping.

According to an eighth aspect of the present invention, among elements that constitute the motor, a torque generating portion including an armature and a field magnet, and a bearing portion are arranged tandem in the axial direction. According to this structure, a dimension in the radial direction of the centrifugal fan can be reduced. Namely, if the torque generating portion and the bearing portion are arranged substantially at the same position in the axial direction in a coaxial manner, an outer diameter of the motor depends on a total sum of the outer diameter of the bearing portion and a size (thickness) of the torque generating portion in the radial direction. In contrast, if the torque generating portion and the bearing portion are arranged tandem in the axial direction, the outer diameter of the motor depends on either the outer diameter of the bearing portion or the size (outer diameter) of the torque generating portion that is larger one than the other. Therefore, it is possible to reduce the outer diameter of the motor, thereby reducing further the dimension in the radial direction of the centrifugal fan. A combination of this structure and the sleeve bearing utilizing a ceramic member as described above can reduce the number of components so that the dimension in the radial direction can be further reduced.

According to a ninth aspect of the present invention, the rotational member rotates integrally as one element, the bearing portion is disposed at a barycenter of the element or at vicinity of the barycenter. This structure facilitates stabilization of rotation of the impeller. Namely, vibration of the impeller accompanying rotation thereof can be reduced, and a load to the bearing portion can be suppressed so that a life of the bearing portion can be increased. This structure is useful particularly in the case where the impeller is rotated at a high rotation speed. Note that the structure that the bearing portion is disposed at a barycenter of the element or at vicinity of the barycenter means a structure that an intermediate point in the axial direction of the slide bearing (sleeve bearing) is disposed at a barycenter or at vicinity of the barycenter.

According to a tenth aspect of the present invention, a centrifugal fan is provided that includes an impeller including a blade portion having plural blades arranged at a predetermined pitch in the circumferential direction and a motor for rotating the impeller. The impeller is rotated at a rotation speed more than or equal to 15,000 revolutions per minute, a bearing portion that constitutes the motor is a slide bearing including a shaft member and a sleeve having a cylindrical shape that engages the shaft member with a clearance, one of the shaft member and the sleeve is made of a ceramic, and the other is made of a ceramic or a metal.

According to this structure, since a slide bearing (also referred to as a sleeve bearing) is used for the bearing portion, noise due to rotation can be reduced compared with the conventional ball bearing even at high speed rotation. Furthermore, since at least one of the shaft member and the sleeve that constitute the slide bearing is made of a ceramic, a good durability can be obtained even the high speed rotation is maintained. When the present invention is embodied, a small centrifugal fan can be rotated silently and at high speed. Therefore, a centrifugal fan can be downsized while increasing quantity of airflow and static pressure thereof. Note that it is preferable that both of them are made of a ceramic for a long life because seizing up occurs hardly between them. If one of them is made of a ceramic and the other is made of a metal, the sliding surface of the metal is preferably hardened by a surface treatment or coating so that a long life is obtained. In addition, when using a ceramic, liquid lubricant can be eliminated, thereby a seal mechanism for the liquid lubricant can be eliminated so that the structure can be simplified.

According to a eleventh aspect of the present invention, the sleeve and the shaft member are both made of the same type of ceramic. Although a ceramic is an insulator, generation of static electricity due to electrification of the surface can be prevented by using the same type of ceramic for the sliding surfaces. If static electricity is generated, the bearing may adsorb dust or micro particles, which may deteriorate performance of the bearing. The present invention can be preferably used particularly in an environment where a lot of dust or the like exists.

According to a twelfth aspect of the present invention, a plurality of shallow grooves is formed on at least one of the inner surface of the sleeve and the outer surface of the shaft member. The shallow grooves can be linear grooves extending in the vertical direction (axial direction) or grooves having a herringbone shape, a screw-like shape (a spiral shape) or a curved shape. As a plurality of grooves having these shapes is arranged, pressure varies along the axial direction so that stiffness of the bearing is enhanced at a part of high pressure when relative rotation between the sleeve and the shaft member is generated. Therefore, contact between the sleeve and the shaft member can be prevented, and stability when an external force is applied can be increased. Note that a set of grooves may be provided, or plural sets of grooves may be provided separately in the axial direction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view showing a structure of a long thin centrifugal fan according to a first embodiment of the present invention.

FIGS. 2(a) and 2(b) show partial cross sections of the bearing portion according to variations of the embodiment shown in FIG. 1.

FIGS. 3(a) and 3(b) show partial cross sections of the bearing portion according to other variations of the embodiment shown in FIG. 1.

FIG. 4 is a cross sectional view showing a structure of a long thin centrifugal fan according to a second embodiment of the present invention.

FIG. 5 is a cross sectional view showing a structure of a long thin centrifugal fan according to a third embodiment of the present invention.

FIG. 6 is a cross sectional view showing a structure of a conventional long thin centrifugal fan.

FIG. 7 is a cross sectional view in the direction perpendicular to the axis of a housing and an impeller in a typical centrifugal fan.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described in detail with reference to attached drawings. Note that when referring to a relationship of positions or directions of structural elements in the following description, they mean a relationship of positions or directions in a drawing and do not mean a relationship of positions or directions in a state being incorporated in actual equipment.

Embodiment 1

FIG. 1 is a cross sectional view showing a structure of a long thin centrifugal fan according to a first embodiment of the present invention. This long thin centrifugal fan includes a housing 11 having a substantially cylindrical shape elongated in the axial direction, which houses an impeller 12 and a motor 13 for rotating the impeller 12. The impeller 12 is disposed mainly at the top side (the right side in FIG. 1) in the axial direction within the housing 11, while the motor 13 is disposed at the bottom side (the left side in FIG. 1) in the axial direction within the housing 11.

The impeller 12 includes a blade portion 14 (at the top side) having plural blades of a shape elongated in the axial direction that are arranged at a predetermined pitch in the circumferential direction, and a bottom end portion 15 having a substantially cylindrical shape for supporting the blade portion 14. The tip of the blade portion is provided with a ring-like linking portion 16 for linking and supporting tip portions of the plural blades. When the impeller 12 rotates, external air is taken in through a suction opening 17 provided to the tip portion in the axial direction of the housing 11 as illustrated by an arrow IN. Then, the air is blown out through an outlet opening that is provided to a part in the circumferential direction of the housing 11.

The impeller 12 having a shape elongated in the axial direction has a diameter less than or equal to 25 millimeters. In addition, a radius r and a length h in the axial direction of the impeller satisfies the relationship of 2r<=h<=20r. Since the impeller 12 has such an elongated shape, a long thin centrifugal fan is realized that can be incorporated in low profile equipment. In addition, since it is a centrifugal fan, it has higher static pressure than an axial fan and is suitable for being embedded in compact electronic equipment of a high density of mounting components. In addition, the impeller 12 is rotated at a rotation speed more than or equal to 15,000 revolutions per minute, or at a relative speed more than or equal to 0.8 meters per minute between the shaft and the bearing surface. Such a high rotation speed of the impeller 12 secures sufficient quantity of airflow even by the long thin centrifugal fan with a reduced dimension (particularly in the radial direction).

The motor 13 includes a stator armature 21, a rotor magnet 22, a rotor yoke 25, a sleeve 31 and a sleeve holder 32 that constitute a sleeve bearing as a slide bearing, a rotational shaft 33 and other members. The rotor yoke 25 that is a rotational member is provided with step portions so that its diameter decreases from the bottom side to the top side by three steps. In other words, the rotor yoke 25 has a large diameter portion 25a, a medium diameter portion 25b and a small diameter portion 25c. For example, a plurality of the rotor magnets 22 is arranged on the inner surface of the large diameter portion 25a of the rotor yoke 25 at a predetermined pitch in the circumferential direction, and each of the rotor magnets 22 is opposed to the stator armature 21 with a constant gap.

The bottom end portion 15 of the impeller 12 is engaged with and fixed to the outer surface of the medium diameter portion 25b and the small diameter portion 25c of the rotor yoke 25. The rotational shaft 33 that is a rotatable shaft portion is fit in and fixed to the small diameter portion 25c of the rotor yoke 25. The rotational shaft 33 is retained by the cylindrical sleeve 31 in a rotatable manner, so that the rotational shaft 33, the rotor yoke 25 and the impeller 12 are rotated as one unit body. The sleeve 31 is engaged with the inner surface of the sleeve holder 32 and is fixed to the same.

In this embodiment, both the rotational shaft 33 and the sleeve 31 are made of a ceramic. For example, alumina, silicon nitride, artic, zirconia or the like can be used as the ceramic material. It is possible to use a ceramic for making one of the rotational shaft 33 and the sleeve 31 and use a metal for making the other. In this case, it is preferable to use a martensitic stainless steel as the metal material for example, and to perform a surface hardening process (nitriding treatment, for example) or DLC (diamond-like carbon) coating so that hardness of the surface (sliding surface) is increased.

The sleeve holder 32 has a cylindrical shape with an opening at the top side, and its bottom end portion is fixed to a center through-hole of a base member 24 that is a stational member by press fitting. The base member 24 is fixed to the inner wall at the bottom side of the housing 11 made of a resin (or a metal). The inner surface of the sleeve holder 32 at the bottom side is provided with a thrust plate 34 made of a metal or a ceramic to which the bottom end portion of the rotational shaft 33 can abut. Thus, a thrust bearing is constituted. If the rotational shaft 33 is made of a ceramic, it is preferable that the thrust plate 34 is also made of a ceramic or a metal having a hardened surface by the above-mentioned surface hardening process or the like. Alternatively, it is possible to keep the bottom end portion of the rotational shaft 33 as a free end so that the thrust plate 34 (the thrust bearing) can be eliminated. In this case, air dumping or oil dumping may be used for retaining the bottom end portion of the rotational shaft 33. In addition, a seal member is provided for sealing a ring-like gap between the opening of the sleeve holder 32 at the top side and the rotational shaft 33. Thus, it is possible to prevent dust from entering the inside of the sleeve holder 32.

FIGS. 2(a)-2(b) and 3(a)-3(b) show partial cross sections of the bearing portion according to some variations of the embodiment shown in FIG. 1. In a structure shown in FIG. 2(a), the sleeve 31 and the sleeve holder 32 in the structure shown in FIG. 1 are made as one unit member (hereinafter, it is simply referred to as a sleeve 31). In other words, the sleeve holder 32 is eliminated, and the bottom end portion of the cylindrical sleeve 31 is directly fixed to the base member 24. Furthermore, it is possible to make the base member 24 and the sleeve 31 as one unit member, which is directly fixed to the housing 11. It is preferable to use a ceramic as a material of these unit members. In addition, the seal that is necessary in the structure shown in FIG. 1 is not necessary in the structure shown in FIG. 2(a). Furthermore, the thrust plate 34 is also eliminated in the structure shown in FIG. 2(a). Thus, the number of components is reduced in the structure shown in FIG. 2(a), and the man-hour for assembling is also reduced. In addition, a dimension in the radial direction of the long thin centrifugal fan can be reduced compared with the structure shown in FIG. 1.

FIG. 2(b) is an enlarged cross section of the sleeve 31 and the rotational shaft 33 in the structure shown in FIG. 2(a). As understood from this diagram, the rotational shaft 33 has a reduced diameter portion 33a at a middle portion in the axial direction of the part sliding with the sleeve 31, and the reduced diameter portion 33a has its diameter that is slightly smaller than other portion. Thus, an area of the sliding surface is reduced. Namely, there is no sliding action at the reduced diameter portion 33a between the rotational shaft 33 and the sleeve 31, and the sliding action occurs only at the parts at both sides of the reduced diameter portion 33a (i.e., a bottom side part 33b and a top side part 33c). As a result, a bearing loss due to the sliding friction is reduced.

Note that although it is possible to cut a part of the inner surface of the sleeve 31 instead of cutting the rotational shaft 33 (forming the reduced diameter portion 33a) for reducing an area of the sliding surface, cutting of the rotational shaft 33 is easier than cutting of the inner surface of the sleeve 31. Particularly, if the rotational shaft 33 is made of a metal, cutting of the rotational shaft 33 is not difficult. After forming the reduced diameter portion 33a by cutting the rotational shaft 33, the above-mentioned surface hardening process or coating of the sliding surface may be performed so as to increase its hardness.

In this way, the method of forming the reduced diameter portion 33a on the rotational shaft 33 at the part sliding with the sleeve 31 so as to reduce an area of the sliding surface for reducing a bearing loss can be applied to the structure of the first embodiment shown in FIG. 1. In addition, the method can also be applied to a structure with rotational sleeve (fixed shaft) that will be described later, namely a structure in which the shaft member is not the rotational shaft but a fixed shaft.

In the structure shown in FIG. 3(a), the sleeve 31 is divided into two members in the axial direction, and between them there is a felt tube 36 that is means for keeping lubricating oil and is attached to the rotational shaft 33. The felt tube 36 is impregnated with a lubricating oil such as an ester lubricating oil having a coefficient of viscosity less than 0.02 Pa-s. Thus, the lubricating oil is supplied to and kept on the sliding surfaces of the rotational shaft 33 and the sleeve 31, so that a bearing loss due to friction between the sliding surfaces can be reduced.

In the structure shown in FIG. 3(b), the step level (cut portion) of the reduced diameter portion 33a that is formed on the rotational shaft 33 in the structure shown in FIG. 2(b) is a little larger, and the felt tube 36 that is means for keeping lubricating oil is attached to that portion. Also in this case, the lubricating oil that is impregnated in the felt tube 36 is supplied to and kept on the sliding surfaces of the rotational shaft 33 and the sleeve 31 so that a bearing loss due to friction between the sliding surfaces can be reduced similarly to the structure shown in FIG. 3(a). Note that the means for keeping lubricating oil is not limited to the felt tube 36 as shown in FIGS. 3(a) and 3(b). In addition, even if means for keeping lubricating oil is not provided separately, it is possible to keep lubricating oil between the sliding surfaces of the rotational shaft 33 and the sleeve 31 so that the above-mentioned lubricating effect can be obtained during a certain period.

Embodiment 2

FIG. 4 is a cross sectional view showing a structure of a long thin centrifugal fan according to a second embodiment of the present invention. This long thin centrifugal fan includes a housing 11 having a substantially cylindrical shape elongated in the axial direction, which houses an impeller 12 and a motor 13 for rotating the impeller 12. The impeller 12 is disposed mainly at the top side (the right side in FIG. 4) in the axial direction within the housing 11, while the motor 13 is disposed mainly at the bottom side (the left side in FIG. 4) in the axial direction within the housing 11.

The impeller 12 includes a blade portion 14 (at the top side) having plural blades of a shape elongated in the axial direction that are arranged at a predetermined pitch in the circumferential direction, and a bottom end portion 15 having a substantially cylindrical shape for supporting the blade portion 14. The tip of the blade portion is provided with a ring-like linking portion 16 for linking and supporting tip portions of the plural blades. When the impeller 12 rotates, external air is taken in through a suction opening 17 provided to the tip portion in the axial direction of the housing 111 as illustrated by an arrow IN. Then, the air is blown out through an outlet opening that is provided to a part in the circumferential direction of the housing 11.

The motor 13 has a structure in which a torque generating portion 13a including a stator armature 21 and a rotor magnet 22, and a bearing portion 13b that retains a rotational member in a rotatable manner are arranged tandem in the axial direction. In addition, the bearing portion 13b is disposed between the impeller 12 and the torque generating portion 13a. According to this structure, it is possible to reduce a dimension in the radial direction can be reduced compared with the structure of the first embodiment shown in FIG. 1. Namely, although an outer diameter of the motor 13 depends on a total sum of the outer diameter of the bearing portion 13b and a size (thickness) of the torque generating portion 13a in the radial direction in the structure shown in FIG. 1, the outer diameter of the motor 13 depends on either the outer diameter of the bearing portion 13b or the size (outer diameter) of the torque generating portion 13a that is larger one than the other (the outer diameter of the torque generating portion 13a in the embodiment shown in FIG. 4) in the structure shown in FIG. 4. As a result, a dimension in the radial direction of the long thin centrifugal fan can be reduced.

More specifically, when plural members rotating as one unit body including the impeller 12 and the rotational member (the rotor yoke 25) are regarded as one element, the bearing portion 13b (i.e., the intermediate point thereof in the axial direction) is disposed at a barycenter of the element or at vicinity of the barycenter. According to this structure, rotation of the impeller 12 can be stabilized easily. Namely, vibration of the impeller 12 accompanying its rotation can be reduced, and a load to the bearing portion can be suppressed so that a long life can be realized. This structure is useful particularly in the case where the impeller 12 is rotated at a high rotation speed.

In the case of this embodiment too, the bearing portion 13b includes the rotational shaft 33 and the sleeve 31 having a cylindrical shape that engages the outer surface of the rotational shaft 33 with a clearance, and both the rotational shaft 33 and the sleeve 31 are made of a ceramic. However, it is possible that one of the rotational shaft 33 and the sleeve 31 is made of a metal, on which a surface hardening process or coating of the sliding surface is performed so as to increase its hardness as mentioned in the first embodiment. The rotational shaft 33 is fixed to the rotor yoke 25 that is the rotational member, and the sleeve 31 is engaged with and fixed to the inner surface of the sleeve holder 32.

The rotor yoke 25 having a substantially cylindrical shape is elongated in the axial direction and extends from the torque generating portion 13a to the bearing portion 13b. As understood from the cross section shown in FIG. 4, the rotor yoke 25 is provided with step portions so that its diameter decreases from the bottom side to the top side by three steps. In other words, the rotor yoke 25 has a large diameter portion 25a, a medium diameter portion 25b and a small diameter portion 25c. For example, a plurality of the rotor magnets 22 is arranged on the inner surface of the large diameter portion 25a of the rotor yoke 25 at a predetermined pitch in the circumferential direction. Each of the rotor magnets 22 is opposed to the stator armature 21 with a constant gap.

The bottom end portion 15 of the impeller 12 is engaged with and fixed to the outer surface of the medium diameter portion 25b and the small diameter portion 25c of the rotor yoke 25. The rotational shaft 33 is fixed to the small diameter portion 25c of the rotor yoke 25 by press fitting. Therefore, the rotational shaft 33, the rotor yoke 25 and the impeller 12 rotate as one unit body. In addition, as understood from FIG. 4, the top end portion 33d of the rotational shaft 33 is disposed within the inner space of the blade portion 14 of the impeller 12. Namely, the rotational shaft 33 extends from the inside of the bottom end portion 15 of the impeller 12 to the inside of the blade portion 14 in the axial direction. Accompanying with this, the small diameter portion 25c of the rotor yoke 25 also extends from the inside of the bottom end portion 15 of the impeller 12 to the inside of the blade portion 14. Thus, sufficient length in the axial direction for secure fixing and aligning between the rotational shaft 33 and the rotor yoke 25 is ensured, while a dimension in the axial direction of the long thin centrifugal fan can be reduced.

In addition, the sleeve holder 32 that is made of a metal includes a cylindrical portion 32a for engaging with the outer surface of the sleeve 31 so as to retain the same and a shaft portion 32b that extends from the center of the bottom end surface of the cylindrical portion 32a toward the bottom side. The axis of the cylindrical portion 32a agrees the axis of the shaft portion 32b. The shaft portion 32b fits in the stator armature 21, and further the bottom end portion thereof fits in the center through-hole of the base member 24 made of a resin (or a metal) and fixed to the same. The base member 24 is fixed to the inner wall of the bottom side of the housing 11 made of a resin (or a metal). Thus, the sleeve holder 32 is fixed so that the axis of the sleeve holder 32 agrees the center axis of the substantially cylindrical housing 11.

In the cylindrical portion 32a of the sleeve holder 32, the inner surface at the bottom side is provided with a thrust plate 34 made of a metal to which the bottom end portion of the rotational shaft 33 can abut. Thus, a thrust bearing is constituted. If the rotational shaft 33 is made of a ceramic, it is preferable that the thrust plate 34 is made of a ceramic too or a metal having enhanced surface hardness by the surface hardening process or the like as described above. Alternatively, it is possible to keep the bottom end portion of the rotational shaft 33 as a free end so that the thrust plate 34 can be eliminated. In this case, air dumping or oil dumping may be used for retaining the bottom end portion of the rotational shaft 33. In addition, a seal member is provided for sealing a ring-like gap between the opening of the sleeve holder 32 at the top side and the rotational shaft 33, so as to prevent dust from entering the inside of the cylindrical portion 32a of the sleeve holder 32.

Note that the structure of the bearing portion that is described as a variation of the first embodiment can be applied to this second embodiment, too. Namely, as shown in FIGS. 2 and 3, the sleeve 31 and the sleeve holder 32 can be made as one unit member, and/or the reduced diameter portion 33a can be formed on a part of the rotational shaft 33 so that an area of the sliding surface (i.e., the bearing loss) can be reduced, and/or means for keeping lubricating oil that is supplied to the sliding surface can be provided.

Embodiment 3

FIG. 5 is a cross sectional view showing a structure of a long thin centrifugal fan according to a third embodiment of the present invention. This long thin centrifugal fan includes a housing 11 having a substantially cylindrical shape elongated in the axial direction, which houses an impeller 12 and a motor 13 for rotating the impeller 12. A structure of the impeller 12 is the same as the embodiments described above, so overlapping description is omitted here.

A motor 13 of the long thin centrifugal fan in this embodiment is similar to that of the second embodiment. Namely, the motor 13 has a structure in which a torque generating portion 13a including a stator armature 21 and a rotor magnet 22, and a bearing portion 13b that retains a rotational member in a rotatable manner are arranged tandem in the axial direction, and a bearing portion 13b is disposed between the impeller 12 and the torque generating portion 13a. An effect of this structure is also the same as described in the second embodiment. In addition, when plural members rotating as one unit body including the impeller 12 and the rotational member (the rotor yoke 25) are regarded as one element, the bearing portion 13b (i.e., the intermediate point thereof) is disposed at a barycenter of the element or at vicinity of the barycenter. This structure and its effect are also the same as described in the second embodiment.

The long thin centrifugal fan of this embodiment has a bearing portion 13b of the motor 13 whose structure is substantially different from the embodiments described above. Namely, unlike the embodiments described above in which the bearing portion 13b has a rotational shaft (fixed sleeve) structure, the bearing portion 13b in the third embodiment has a rotating sleeve (fixed shaft) structure. In other words, although the bearing portion 13b in this embodiment is also a sleeve bearing including a shaft member and a sleeve having a cylindrical shape that engages the outer surface of the shaft with a clearance, the shaft member is a fixed shaft 33 that is fixed to a base 24, and a sleeve 31 is fixed to a rotor yoke 25 that is a rotational member via a sleeve holder 32. Note that although there is a difference between the fixed side and the rotating side, the fixed shaft that is the shaft member is denoted by the reference numeral 33 that is the same as the rotational shaft in the embodiments described above. Other members including the sleeve 31 and the sleeve holder 32 are also denoted by the same reference numerals as the embodiments described above.

In FIG. 5, the rotor yoke 25 having a substantially cylindrical shape extends from the torque generating portion 13a to the bearing portion 13b in the axial direction. The rotor yoke 25 has step portions so that its diameter decreases from the bottom side to the top side by three steps. Namely, the rotor yoke 25 has a large diameter portion 25a, a medium diameter portion 25b and a small diameter portion 25c. A plurality of the rotor magnets 22 is arranged on the inner surface of the large diameter portion 25a of the rotor yoke 25 at a predetermined pitch in the circumferential direction. Each of the rotor magnets 22 is opposed to the stator armature 21 with a constant gap.

The bottom end portion 15 of the impeller 12 is engaged with and fixed to the outer surface of the medium diameter portion 25b and the small diameter portion 25c of the rotor yoke 25. The small diameter portion 25c of the rotor yoke 25 has a closed end surface at the top side, which seals the opening of the impeller 12 at the boundary between the blade portion 14 and the bottom end portion 15. A cylindrical sleeve holder 32 having a closed end surface at the top side is engaged with and fixed to the inner surface of the medium diameter portion 25b and the inner end surface of the small diameter portion 25c of the rotor yoke 25, while a cylindrical sleeve 31 is fixed to the inner surface of the sleeve holder 32.

The top side of the fixed shaft 33 that is the shaft member is engaged with the inner surface of the sleeve 31 with a clearance, and the bottom side of the fixed shaft 33 fits in the stator armature 21 and further fits in and fixed to the center through-hole of the base member 24 made of a resin (or a metal). The base member 24 is fixed to the inner wall of at the bottom side of the housing 11 made of a resin (or a metal). Thus, the fixed shaft 33 is fixed so that the axis of the fixed shaft 33 agrees the center axis of the substantially cylindrical housing 11, and the sleeve 31, the sleeve holder 32, the rotor yoke 25 and the impeller 12 rotates as one unit body around the top side of the fixed shaft.

The inner surface of the sleeve holder 32 at the top side is provided with a thrust plate 34 made of a metal to which the top end portion of the fixed shaft 33 can abut. Thus, a thrust bearing is constituted. If the fixed shaft 33 is made of a ceramic, it is preferable that the thrust plate 34 is made of a ceramic too or a metal having enhanced surface hardness by the surface hardening process or the like as described above. Alternatively, it is possible to keep the bottom end portion of the rotational shaft 33 as a free end so that the thrust plate 34 can be eliminated. In this case, air dumping or oil dumping may be used for retaining the bottom end portion of the rotational shaft 33. In addition, a seal member is provided for sealing a ring-like gap between the opening of the sleeve holder 32 at the bottom side and the fixed shaft 33, so as to prevent dust from entering the inside of the sleeve holder 32.

The rotating sleeve (fixed shaft) bearing structure of this embodiment can be realized easily by structure in which the torque generating portion 13a and the bearing portion 13b for retaining the rotational member in a rotatable manner are arranged tandem in the axial direction, and the bearing portion 13b is disposed between the impeller 12 and the torque generating portion 13a. In addition, use of the sleeve made of a ceramic also contributes largely to realizing the structure. Although a sleeve bearing using a sleeve made of a sintered metal impregnated with lubricating oil has been used conventionally, it is difficult in this case to realize the rotating sleeve structure because lubricating oil moves to the outer side by the centrifugal force when the sleeve rotates and lack of lubricating oil occurs on the inner sliding surface. In contrast, if a sleeve made of a ceramic is used as a ceramic bearing, such a problem does not happen so that the rotating sleeve structure can be realized easily.

Note that variations of the bearing portion described in the first embodiment can be applied to this third embodiment, too. Namely, as shown in FIGS. 2 and 3, the sleeve 31 and the sleeve holder 32 can be made as one unit member, and/or the reduced diameter portion 33a can be formed on a part of the rotational shaft 33 so that an area of the sliding surface (i.e., the bearing loss) can be reduced, and/or means for keeping lubricating oil that is supplied to the sliding surface can be provided.

Note that if both the sleeve and the shaft are made of a ceramic, it is preferable to use the same ceramic material for making them. Although a ceramic is an insulator, generation of static electricity due to electrification of the surface can be prevented by using the same type of ceramic for the sliding surfaces. If static electricity is generated, the bearing may adsorb dust or micro particles, which may deteriorate performance of the bearing. According to the present invention, the ill effect of dust or micro particles to the bearing can be prevented, so that a long life of the centrifugal fan can be secured. The present invention is useful especially in an environment with much dust.

In addition, a plurality of shallow grooves may be formed on at least one of the inner surface of the sleeve and the outer surface of the shaft member that are sliding surfaces of the bearing. The shallow groove is preferably a linear groove extending in the vertical direction (axial direction) or a groove having a herringbone shape, a screw-like shape (a spiral shape) or a curved shape. As a plurality of grooves having these shapes is arranged, pressure varies along the axial direction so that stiffness of the bearing is enhanced at a part of high pressure when relative rotation between the sleeve and the shaft member is generated. Therefore, contact between the sleeve and the shaft member can be prevented, and stability when an external force is applied can be increased. Note that a set of grooves may be provided, or plural sets of grooves may be provided separately in the axial direction. The grooves can be formed by coining, pressing, electrochemical machining, cutting or other processes.

When the grooves are formed, bearing stiffness is enhanced, but a shaft loss increases. If the centrifugal fan is very small and if the fluid that generates dynamic pressure is a gas, increase of the shaft loss due to forming of the grooves is not so large. If the shaft loss is outstandingly large, it is preferable to decrease an outer diameter of the shaft in parts where the dynamic pressure is not increased (as the structure described in claim 2) so that the shaft loss can be reduced without lowering the stiffness.

Although embodiments and variations of the present invention are described above, the present invention is not limited to these embodiments and variations but can be embodied variously. In addition, materials and shapes of the members shown in the above description are merely embodiments, and it should not be interpreted that the structure of the present invention is limited to the materials and the shapes.

While example embodiments of the present invention have been shown and described, it will be understood that the present invention is not limited thereto, and that various changes and modifications may be made by those skilled in the art without departing from the scope of the invention as set forth in the appended claims and their equivalents.

Claims

1. A centrifugal fan comprising:

an impeller including a blade portion having plural elongated blades arranged at a predetermined pitch in the circumferential direction; and

a motor for rotating the impeller, wherein the impeller and the motor are arranged tandem in the axial direction,

a shaft having a cylindrical outer peripheral surface,

a sleeve having a cylindrical inner peripheral surface opposed to the outer peripheral surface of the shaft in the radial direction with a minute gap,

a bearing portion that constitutes the motor including said shaft and said sleeve,

wherein one of the shaft and the sleeve is made of a ceramic, and the other is made of a ceramic or a metal.

2. The centrifugal fan according to claim 1, wherein the impeller is rotated at a rotation speed more than or equal to 15,000 revolutions per minute.

3. The centrifugal fan according to claim 1, wherein the sleeve and the shaft are both made of the same type of ceramic.

4. The centrifugal fan according to claim 2, wherein the sleeve and the shaft are both made of the same type of ceramic.

5. The centrifugal fan according to claim 1, wherein a reduced diameter portion is formed on the shaft member at a middle portion in the axial direction of the sliding portion with the sleeve, the reduced diameter portion having a diameter a little smaller than other portions.

6. The centrifugal fan according to claim 2, wherein a plurality of shallow grooves is formed on at least one of the inner surface of the sleeve and the outer surface of the shaft member.

7. The centrifugal fan according to claim 3, wherein a plurality of shallow grooves is formed on at least one of the inner surface of the sleeve and the outer surface of the shaft member.

8. The centrifugal fan according to claim 4, wherein a plurality of shallow grooves is formed on at least one of the inner surface of the sleeve and the outer surface of the shaft member.

9. The centrifugal fan according to claim 1, wherein among elements that constitute the motor, a torque generating portion including an armature and a field magnet, and a bearing portion are arranged tandem in the axial direction.

10. The centrifugal fan according to claim 1, wherein the shaft member is fixed to a rotational member, and the sleeve that retains the shaft member in a rotatable manner is fixed to the housing directly or via a base member.

11. The centrifugal fan according to claim 9, wherein the shaft member is fixed to a rotational member, and the sleeve that retains the shaft member in a rotatable manner is fixed to the housing directly or via a base member.

12. The centrifugal fan according to claim 1, wherein a sleeve made of a ceramic is fixed to a rotational member, and a shaft member is fixed to the housing directly or via a base member.

13. The centrifugal fan according to claim 9, wherein a sleeve made of a ceramic is fixed to a rotational member, and a shaft member is fixed to the housing directly or via a base member.

14. The centrifugal fan according to claim 13, wherein the top side of the shaft member is disposed within the inner space of the blade portion of the impeller.

15. The centrifugal fan according claim 10, wherein the bottom side of the shaft member is a free end without a thrust bearing.

16. The centrifugal fan according to claim 1, wherein when plural members rotating as one unit body including the impeller and the rotational member are regarded as one element, the bearing portion is disposed at a barycenter of the element or at vicinity of the barycenter.

17. The centrifugal fan according to claim 2, wherein when plural members rotating as one unit body including the impeller and the rotational member are regarded as one element, the bearing portion is disposed at a barycenter of the element or at vicinity of the barycenter.

18. The centrifugal fan according to claim 9, wherein when plural members rotating as one unit body including the impeller and the rotational member are regarded as one element, the bearing portion is disposed at a barycenter of the element or at vicinity of the barycenter.

19. The centrifugal fan according to claim 1, wherein means for keeping lubricating oil that is supplied to the sliding surfaces of the shaft member and the sleeve are attached to the shaft member or the sleeve.