Dynamic pressure bearing and fluid dynamic pressure bearing device equipped with same

Abstract

To provide a hydrodynamic bearing excellent in bearing rigidity at low cost. [Solution] A dynamic pressure generating portion P is provided on each of two radial bearing surfaces A1, A2 spaced apart in the axial direction, and a large-diameter cylindrical surface 8b larger in diameter than the radial bearing surfaces A1, A2 is provided between the two radial bearing surfaces A1, A2, and the dynamic pressure generating portion P is a rough surface with a plurality of dynamic pressure grooves 10 extending in the axial direction and convex ridges 11 arranged alternately in the circumferential direction in a dynamic pressure bearing (bearing member 8), one radial bearing surface A1 and the large-diameter cylindrical surface 8b are connected via a tapered surface 8c that is inclined with respect to the axial direction, and the other radial bearing surface A2 and the large-diameter cylindrical surface 8b are connected via a tapered surface 8d that has a smaller inclination angle with respect to the axial direction than the tapered surface 8b, and the dynamic pressure grooves 10 provided in the other radial bearing surface A2 open to the tapered surface 8d.

Description

[Technical Field]

[0001] The present invention relates to a dynamic pressure bearing and a fluid dynamic pressure bearing including the same. [Background technology]

[0002] For example, a fluid dynamic bearing device for a fan motor equipped with fan blades includes a cylindrical bearing member that supports the motor's main shaft in a non-contact manner so that it can rotate freely in the radial direction. This bearing member can be a true circular bearing, in which the radial bearing surface that forms a radial bearing gap between itself and the outer peripheral surface of the shaft is formed as a smooth cylindrical surface without any irregularities, or a dynamic pressure bearing, in which a dynamic pressure generator is provided on the radial bearing surface. When a dynamic pressure bearing is used as the bearing member, as the shaft rotates relative to the dynamic pressure bearing, the dynamic pressure action of the dynamic pressure generator increases the pressure (rigidity) of the fluid film that forms in the radial bearing gap (strictly speaking, the narrowest gap width within the radial bearing gap). For this reason, dynamic pressure bearings are considered to have better bearing rigidity than true circular bearings.

[0003] As hydrodynamic bearings, (1) A dynamic pressure generating portion that is composed of a plurality of dynamic pressure generating grooves that are inclined with respect to the axial direction and spaced apart in the circumferential direction, and convex hill portions that define the dynamic pressure generating grooves, the hill portions having a herringbone shape. (2) A bearing having a dynamic pressure generating portion having an uneven surface formed by alternately arranging a plurality of linear dynamic pressure grooves extending in the axial direction and convex ridges in the circumferential direction. are well known. When a radial bearing gap is formed between the radial bearing surface and the outer peripheral surface of the shaft as the shaft rotates relative to these hydrodynamic bearings (1) and (2), the opposing area of ​​the convex hills becomes the "minimum width portion of the radial bearing gap." Note that when describing the hydrodynamic bearings (1) and (2) separately below, the hydrodynamic bearing (1) above will also be called a "herringbone type hydrodynamic bearing," and the hydrodynamic bearing (2) above will also be called a "step type hydrodynamic bearing."

[0004] In order to ensure the required bearing rigidity while reducing torque during relative rotation with the shaft, a hydrodynamic bearing often has two radial bearing surfaces spaced apart in the axial direction. In this case, a large-diameter cylindrical surface with a larger diameter than the radial bearing surfaces is provided on the inner circumferential surface of the hydrodynamic bearing in the region axially between the two radial bearing surfaces. This large-diameter cylindrical surface (the portion where it is provided) is also referred to as a "middle relief portion" (see, for example, Patent Document 1).

[0005] Of the hydrodynamic bearings having the above configuration, herringbone hydrodynamic bearings have the advantage of being superior to step-type hydrodynamic bearings in terms of their ability to form a fluid film in the radial bearing gap, and therefore in terms of bearing rigidity, but they are less costly than step-type hydrodynamic bearings for reasons such as the need to change the inclination direction of the hydrodynamic grooves depending on the direction of rotation of the shaft, which necessitates measures to prevent incorrect assembly into the bearing device, and the time and effort required to finish the hydrodynamic pressure generating section to the required precision. For this reason, step-type hydrodynamic bearings are preferably used in applications where cost is a particular concern. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-275306 Summary of the Invention [Problem to be solved by the invention]

[0007] As mentioned above, step-type hydrodynamic bearings are superior to herringbone-type hydrodynamic bearings in terms of cost, but are said to be somewhat inferior in terms of performance (especially bearing rigidity). Therefore, if the bearing rigidity of step-type hydrodynamic bearings could be increased without compromising their cost advantage, it would be possible to adopt step-type hydrodynamic bearings in applications requiring high bearing rigidity, which would contribute to reducing the cost of motors.

[0008] In view of the above circumstances, the main object of the present invention is to provide a hydrodynamic bearing having excellent bearing rigidity, in which the hydrodynamic pressure generating portion provided on the radial bearing surface is formed of an uneven surface in which a plurality of hydrodynamic grooves extending in the axial direction and protrusions are alternately arranged in the circumferential direction, and which can be inexpensively molded. [Means for solving the problem]

[0009] The bearing rigidity of a step-type hydrodynamic bearing can be increased, for example, by increasing the area of ​​the ridges of the hydrodynamic pressure generating portion, which form the narrowest portion of the radial bearing gap during relative rotation with the shaft. The area of ​​the ridges can be increased by elongating the ridges in the axial and / or circumferential directions. However, elongating the ridges in the axial direction counteracts this by shortening the central relief portion formed between the two radial bearing surfaces in the axial direction, which may reduce the torque reduction effect. Furthermore, elongating the ridges in the circumferential direction counteracts this by shortening the hydrodynamic grooves in the circumferential direction. In this case, the distance over which the fluid can flow within the radial bearing gap is shortened, weakening the fluid's pushing force toward the ridges, which may reduce the ability to form a fluid film in the area facing the ridges and, ultimately, the bearing rigidity. Therefore, in the present invention, we have adopted a configuration that allows the hydrodynamic grooves to be elongated in the axial direction without changing the area (axial dimension and / or circumferential dimension) of the ridges that make up the hydrodynamic pressure generating portion, thereby improving the ability to form a fluid film in the area facing the ridges (bearing rigidity).

[0010] That is, the present invention, which has been devised to achieve the above object, is a hydrodynamic bearing having two radial bearing surfaces spaced apart in the axial direction, which form a radial bearing gap between themselves and the outer peripheral surface of a shaft to be supported, and an inner peripheral surface having a large-diameter cylindrical surface larger in diameter than the radial bearing surfaces, and each radial bearing surface is provided with a hydrodynamic pressure generating portion for generating a hydrodynamic pressure action on the fluid in the radial bearing gap, and this hydrodynamic pressure generating portion is made of a concave-convex surface having a plurality of hydrodynamic pressure grooves extending in the axial direction and convex ridge portions arranged alternately in the circumferential direction, one radial bearing surface and the large-diameter cylindrical surface are connected via a steeply inclined surface inclined with respect to the axial direction, and the other radial bearing surface and the large-diameter cylindrical surface are connected via a gently inclined surface whose inclination angle with respect to the axial direction is smaller than that of the steeply inclined surface, The dynamic pressure groove provided on the other radial bearing surface is characterized in that one end on one axial side opens onto the gently inclined surface.

[0011] As described above, by making one axial end of the hydrodynamic groove provided on the other radial bearing surface open to a gentle slope, the hydrodynamic groove can be elongated in the axial direction without elongating the ridge portion that constitutes the hydrodynamic pressure generating portion in the axial and / or circumferential directions. If the hydrodynamic groove is elongated in the axial direction, the amount of fluid flowing in the radial bearing gap during relative rotation between the shaft and the hydrodynamic bearing, and ultimately the amount of fluid pushed toward the ridge portion, can be increased accordingly, thereby improving the ability to form a fluid film in (the narrowest portion of) the radial bearing gap and increasing bearing rigidity.

[0012] In the above configuration, when the dynamic pressure generating portion is developed on a plane, the ratio (= Z2 / Z1) of the circumferential dimension Z2 of the ridge portion to the circumferential dimension (groove width) Z1 of the dynamic pressure groove is set to 1 or less. In other words, the circumferential dimension of the dynamic pressure groove is set to be equal to or greater than the circumferential dimension of the ridge portion. In this way, when the shaft and dynamic pressure bearing rotate relative to each other, the fluid in the radial bearing gap can be caused to move significantly in the circumferential direction, thereby improving the ability to form a fluid film in the narrowest part of the radial bearing gap.

[0013] In the above configuration, the difference in height between the inner diameter end face of the ridge portion and the bottom face of the dynamic pressure groove is preferably 2 μm or more.

[0014] The hydrodynamic bearing according to the present invention may be made of a porous sintered metal. A hydrodynamic bearing made of sintered metal allows the hydrodynamic pressure generating portion of the radial bearing surface to be formed easily and accurately. In this case, the other radial bearing surface may be a formed surface formed by subjecting the sintered metal to plastic processing.

[0015] A fluid dynamic bearing device comprising a bearing member comprising the dynamic pressure bearing according to the present invention described above, a bottomed cylindrical housing that houses this bearing member on its inner periphery, lubricating oil as a fluid present in the radial bearing gap, and a radial bearing portion that supports the shaft in a non-contact manner in the radial direction by an oil film of lubricating oil formed in the radial bearing gap, can exhibit high bearing performance at low cost because the dynamic pressure bearing according to the present invention has the above-mentioned characteristics. [Effects of the Invention]

[0016] As described above, according to the present invention, it is possible to provide a hydrodynamic bearing in which the hydrodynamic pressure generating portion provided on the radial bearing surface can be molded inexpensively, yet which has excellent fluid film formation properties in the radial bearing gap and excellent bearing rigidity. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a cross-sectional view conceptually illustrating an example of a fan motor. [Figure 2] 1 is a longitudinal cross-sectional view conceptually showing a fluid dynamic bearing device into which a dynamic bearing according to an embodiment of the present invention can be incorporated. [Figure 3] 1 is a vertical cross-sectional view of a hydrodynamic bearing according to an embodiment of the present invention. [Figure 4] The drawing on the left side of the page is a developed plan view of the inner peripheral surface of the bearing member, and the drawing on the right side of the page is a cross-sectional view taken along line XX of the drawing on the left side. [Figure 5] FIG. 2 is a schematic vertical cross-sectional view showing a state in which a cavity of a compression molding die is filled with raw material powder. [Figure 6] FIG. 2 is a schematic vertical cross-sectional view showing a state in which raw material powder filled in a cavity of a compression molding die is being compressed. [Figure 7] FIG. 10 is a schematic vertical cross-sectional view showing a state in which a powder compact has been discharged outside the compression molding die. [Figure 8] FIG. 1(a) is a schematic longitudinal cross-sectional view showing a state in which a sintered body has been placed in a sizing die, and FIG. 1(b) is a schematic longitudinal cross-sectional view showing a state in which the sintered body placed in the sizing die is being subjected to sizing processing. [Figure 9]1A and 1B are schematic longitudinal cross-sectional views showing a sizing die used to obtain a hydrodynamic bearing according to another embodiment of the present invention, in which FIG. 1A is a schematic longitudinal cross-sectional view showing a state in which a sintered body has been placed in the sizing die, and FIG. 1B is a schematic longitudinal cross-sectional view showing a state in which the sintered body placed in the sizing die is being subjected to sizing processing. DETAILED DESCRIPTION OF THE INVENTION

[0018] An embodiment of the present invention will now be described with reference to the drawings. First, an example of a motor (here, a fan motor) that uses a fluid dynamic bearing device as a component will be described, followed by a description of a fluid dynamic bearing device that incorporates a fluid dynamic bearing according to an embodiment of the present invention as a bearing member.

[0019] FIG. 1 is a longitudinal cross-sectional view conceptually illustrating an example of a small fan motor that sends cooling air (airflow) toward an object to be cooled. This fan motor includes a fluid dynamic bearing device 1, a motor base 5 that forms the stationary side of the motor, a rotor 3 fixed to a shaft member 2 of the fluid dynamic bearing device 1 that also functions as the motor's main shaft, impellers 4 attached to the rotor 3, and a stator coil 6a and rotor magnet 6b arranged opposite each other with a radial gap between them. The stator coil 6a is attached to a housing 7 of the fluid dynamic bearing device 1, and the rotor magnet 6b is attached to the rotor 3. In a fan motor configured as described above, when current is applied to the stator coil 6a, the rotor magnet 6b rotates due to electromagnetic force between the stator coil 6a and the rotor magnet 6b, causing the shaft member 2 and the rotor 3 to rotate together. As the rotor 3 rotates, the impellers 4 attached to the rotor 3 generate an axial airflow.

[0020] FIG. 2 is a longitudinal cross-sectional view conceptually illustrating a fluid dynamic bearing device 1 that can be incorporated into the fan motor shown in FIG. 1 , with components such as the shaft member 2 and bearing member 8 depicted in simplified form. The fluid dynamic bearing device 1 shown in FIG. 2 is a so-called rotating shaft type bearing device, primarily comprising the shaft member 2 constituting the rotating side, a housing 7, bearing member 8, and seal member 9 constituting the stationary side, and lubricating oil (not shown) as a lubricating fluid filled in the interior space of the housing 7. The rotating shaft member 2 is supported in a non-contact manner for radial rotation by two radial bearing portions R1 and R2 spaced apart in the axial direction. A fluid dynamic bearing according to an embodiment of the present invention can be used for the bearing member 8. For ease of explanation, the upper side of FIG. 2, where the seal member 9 is located, will be referred to as the "upper side," and the lower side of FIG. 2 will be referred to as the "lower side." However, this is not intended to limit the orientation of the fluid dynamic bearing device 1 during use.

[0021] The shaft member 2 is made of a highly rigid metal material such as stainless steel, and its outer peripheral surface 2a is formed into a smooth cylindrical surface without any irregularities, while its lower end surface 2b is formed into a convex spherical surface. The rotor 3, to which the blades 4 and the rotor magnet 6b (see Figure 1) are attached, is fixed to the upper end of the shaft member 2.

[0022] The housing 7 is cylindrical and has a cylindrical portion 7a and a bottom portion 7b that closes the opening at the lower end of the cylindrical portion 7a. In the illustrated example, the cylindrical portion 7a and the bottom portion 7b are integrally formed from a resin or metal material. The inner peripheral surface 7a1 of the cylindrical portion 7a is formed into a cylindrical surface of a constant diameter, and the outer diameter end of the lower end is connected to the outer peripheral surface 7a2 of the cylindrical portion 7a. The stator coil 6a and motor base 5, which are components of the fan motor, are fixed to the outer peripheral surface 7a2 of the cylindrical portion 7a.

[0023] In this embodiment, a thrust plate 7c made of a material with better sliding properties than the material of the housing 7 is placed on the inner bottom surface 7b1 of the housing 7, and the upper end surface of the thrust plate 7c contacts and supports the lower end surface 2b of the shaft member 2 (contact supports the shaft member 2 in the thrust direction). However, the thrust plate 7c is not necessarily provided and may be omitted. If the thrust plate 7c is omitted, the lower end surface 2b of the shaft member 2 is contact-supported by the inner bottom surface 7b1 of the housing 7.

[0024] The seal member 9 is formed in an annular shape from a resin or metal material, and its inner peripheral surface 9a forms an annular seal space S with the opposing outer peripheral surface 2a of the shaft member 2. This seal space S prevents as much as possible the leakage of lubricating oil filled in the housing 7 to the outside of the device.

[0025] The fluid dynamic bearing device 1 may be used with the entire interior space of the housing 7 filled with lubricating oil, or with lubricating oil present in a portion of the interior space of the housing 7 (lubricating oil and air mixed in the interior space of the housing 7). In the former case, the volume of the seal space S is determined so that the lubricating oil level will always be within the axial range of the seal space S, even if the lubricating oil level position fluctuates in the axial direction due to temperature changes.

[0026] The bearing member 8 is fixed to the inner periphery of the cylindrical portion 7a of the housing 7 with its lower end surface abutting against the shoulder surface 7b2 of the housing 7. The bearing member 8 can be fixed to the inner periphery 7a1 of the cylindrical portion 7a by press-fitting, bonding, or press-fit bonding (a combination of press-fitting and bonding), or it can also be fixed to the inner periphery of the cylindrical portion 7a by a clearance fit (see JIS B 0401-1) to the inner periphery of the cylindrical portion 7a and then sandwiching the bearing member 8 from both axial sides between the seal member 9 and the shoulder surface 7b2 of the housing 7. In particular, the latter fixing method allows the bearing member 8 to be fixed to the housing 7 at the same time as the seal member 9 is fixed to the housing 7, thereby reducing the effort required to assemble the components. Furthermore, for example, if the bearing member 8 is press-fitted into the inner periphery of the cylindrical portion 7a with a large interference, deformation of the bearing member 8 caused by the press-fitting will spread to the inner periphery 8a of the bearing member 8, which may have an adverse effect on the accuracy of the radial bearing gap and, ultimately, the bearing performance of the radial bearing portions R1, R2. In contrast, the method of sandwiching the bearing member 8 between the seal member 9 and the housing 7 from both axial sides can prevent such problems from occurring.

[0027] The bearing member 8 is formed into a cylindrical shape from a porous sintered metal body whose main components are, for example, copper and iron, and is fixed to the inner periphery of the cylindrical portion 7a of the housing 7 in an oil-impregnated state with its internal pores impregnated with lubricating oil. The dimensions of this bearing member 8 are, for example, an inner diameter of φ4 mm or less, an outer diameter of φ10 mm or less, and an axial dimension of 15 mm or less.

[0028] Fig. 3 is a longitudinal cross-sectional view of a hydrodynamic bearing according to one embodiment of the present invention, which is used as the bearing member 8 in the fluid dynamic bearing device 1 shown in Fig. 2. The drawing on the left side of Fig. 4 is an expanded plan view of the inner circumferential surface of the bearing member 8 shown in Fig. 3, and the drawing on the right side of Fig. 4 is a cross-sectional view taken along line XX in the drawing on the left side. As shown in Figs. 3 and 4, the inner circumferential surface 8a of the bearing member 8 is provided with two radial bearing surfaces A1 and A2 spaced apart in the axial direction, which form a radial bearing gap between the inner circumferential surface 2a of the shaft member 2 and the bearing member 8. In the illustrated example, the radial bearing surface A1 is provided at the upper end of the inner circumferential surface 8a, and the radial bearing surface A2 is provided at the lower end of the inner circumferential surface 8a.

[0029] Between the two radial bearing surfaces A1, A2 on the inner peripheral surface 8a of the bearing member 8, there is provided a large-diameter cylindrical surface 8b that is larger in diameter than the radial bearing surfaces A1, A2 and has a cylindrical shape with a constant diameter and no irregularities. By providing such a large-diameter cylindrical surface 8b between the two radial bearing surfaces A1, A2, it is possible to reduce the torque when the shaft member 2 rotates while ensuring the bearing rigidity required for the entire bearing member 8. In short, the large-diameter cylindrical surface 8b forms a so-called "middle relief portion."

[0030] The upper (one) radial bearing surface A1 and the large-diameter cylindrical surface 8b provided on the inner peripheral surface 8a of the bearing member 8 are connected via a tapered surface 8c inclined at an angle θ1 with respect to the axial direction, and the lower (other) radial bearing surface A2 and the large-diameter cylindrical surface 8b are connected via a tapered surface 8d inclined at an angle θ2 with respect to the axial direction. There is a magnitude relationship between the above angles θ1 and θ2, θ1 > θ2. In other words, a steep slope is a slope with a larger inclination angle than a gentle slope, and the tapered surfaces 8c and 8d respectively constitute the "steep slope" and "gentle slope" as defined in the present invention.

[0031] Each of the radial bearing surfaces A1, A2 is formed with a dynamic pressure generating portion P for generating a dynamic pressure action on the lubricating oil in the radial bearing gap. Each of the dynamic pressure generating portions P has an uneven surface with a plurality of axially extending dynamic pressure grooves 10 and convex ridge portions (regions indicated by cross-hatching in FIGS. 3 and 4) 11 alternately arranged in the circumferential direction. Therefore, the bearing member 8 of this embodiment is a type of the aforementioned "step-type dynamic pressure bearing." The height difference h between the inner diameter surface (inner diameter end surface) 11a of the ridge portion 11 and the groove bottom surface 10a of the dynamic pressure groove 10 (groove depth of the dynamic pressure groove 10) is set to 2 μm or more.

[0032] In the dynamic pressure generating portion P provided on the upper radial bearing surface A1, each dynamic pressure groove 10 is formed so that its lower end (the other axial side) opens onto the tapered surface 8c, which serves as a "steep slope," while each hill portion 11 is formed so that its lower end terminates at the upper end of the tapered surface 8c (the boundary between the radial bearing surface A1 and the tapered surface 8c). Also, in the dynamic pressure generating portion P provided on the lower radial bearing surface A2, each dynamic pressure groove 10 has its upper end (one axial side) opening onto the tapered surface 8d, which serves as a "gentle slope," while each hill portion 11 has its upper end terminated at the lower end of the tapered surface 8d (the boundary between the radial bearing surface A2 and the tapered surface 8d). Therefore, in both the upper and lower dynamic pressure generating portions P, the dynamic pressure grooves 10 are longer in the axial direction than the hill portions 11.

[0033] In the fluid dynamic bearing device 1 having the above configuration, when the shaft member 2 rotates about its central axis, a radial bearing gap, with lubricating oil present, is formed between the opposing outer peripheral surface 2a of the shaft member 2 and the radial bearing surfaces A1, A2 of the bearing member 8. Furthermore, due to the generation of pressure accompanying the rotation of the shaft member 2 and the thermal expansion of the lubricating oil, the lubricating oil impregnated in the internal pores of the bearing member 8 gradually seeps out through the surface perforations of the bearing member 8 to the outside of the bearing member 8 and is drawn into the radial bearing gap.

[0034] In this embodiment, the dynamic pressure generating portions P provided on the radial bearing surfaces A1, A2 are formed as uneven surfaces with a plurality of axially extending dynamic pressure grooves 10 and convex ridges 11 arranged alternately in the circumferential direction. As the shaft member 2 rotates, the lubricating oil flowing circumferentially within the radial bearing gap (strictly speaking, the arc-shaped gap within the radial bearing gap defined by each dynamic pressure groove 10) collides with each ridge 11 and is forced into the opposing region of the inner diameter end face 11a of each ridge 11 (the narrowest portion of the radial bearing gap), thereby increasing the pressure (rigidity) of the oil film within the radial bearing gap. This dynamic pressure action of the dynamic pressure generating portions P forms two radial bearing portions R1, R2 spaced apart in the axial direction that support the shaft member 2 in a non-contact manner so that it can rotate freely in the radial direction.

[0035] Furthermore, when the shaft member 2 rotates, the lower end surface 2b of the shaft member 2 is contact-supported by the upper end surface of the thrust plate 7c placed on the bottom 7b of the housing 7. This forms a thrust bearing portion T that supports (contact-supports) the shaft member 2 in the thrust direction.

[0036] In the bearing member 8 of the present embodiment described above, one (upper) radial bearing surface A1 and the large-diameter cylindrical surface 8b are connected via a tapered surface 8c, which is a steeply inclined surface inclined with respect to the axial direction, and the other (lower) radial bearing surface A2 and the large-diameter cylindrical surface 8b are connected via a tapered surface 8d, which is a gently inclined surface whose inclination angle with respect to the axial direction is smaller than that of the steeply inclined surface. The upper ends of the dynamic pressure grooves 10 of the dynamic pressure generating portion P provided in the lower radial bearing surface A2 open to the tapered surface 8d, which is a gently inclined surface. With this configuration, in the radial bearing surface A2, the dynamic pressure grooves 10 can be made axially longer than the convex ridge portions 11 constituting the dynamic pressure generating portion P provided thereon, without increasing the axial and / or circumferential lengths of the convex ridge portions 11. In this case, the axial lengthening of the dynamic pressure grooves 10 increases the amount of lubricating oil flowing in the radial bearing gap during rotation of the shaft member 2, and therefore the amount of lubricating oil pushed toward the ridge portions 11. Therefore, the oil film formation in the radial bearing gap of the radial bearing portion R2 can be improved, and the bearing rigidity of the radial bearing portion R2 can be increased.

[0037] In the bearing member 8 of this embodiment, the lower ends of the dynamic pressure grooves 10 provided in the upper radial bearing surface A1 open to the tapered surface 8c as a steeply inclined surface, so that in the radial bearing surface A1, the dynamic pressure grooves 10 are (slightly) longer in the axial direction than the lands 11. In this case, even in the radial bearing gap of radial bearing portion R1, although not as much as in the radial bearing gap of radial bearing portion R2, the flow rate of lubricating oil, and therefore the amount of lubricating oil pushed toward the lands 11, can be increased, and therefore the oil film formation in the narrowest part of the radial bearing gap of radial bearing portion R1 can be improved, and the bearing rigidity of radial bearing portion R1 can also be increased.

[0038] Incidentally, the deeper the groove depth h of the hydrodynamic grooves 10 that constitute the hydrodynamic pressure generating portion P, the greater the amount of lubricating oil that can be present in the radial bearing gap, which is advantageous in preventing oil film breakdown in the radial bearing gap, but the greater the amount of lubricating oil present in the radial bearing gap, the greater the viscous resistance of the lubricating oil, which reduces the flow rate of the lubricating oil when it flows circumferentially in the radial bearing gap as the shaft member 2 rotates, and this may make it difficult to improve oil film formation in the narrowest part of the radial bearing gap. Therefore, from the perspective of increasing the bearing rigidity of the radial bearing portions R1, R2, it is preferable to set the groove depth h of the hydrodynamic grooves 10 as small as possible within the range that prevents oil film breakdown in the radial bearing gap, and specifically to set it to 20 μm or less, more preferably 10 μm or less, and even more preferably 5 μm or less.

[0039] As shown in FIG. 4, when the groove width (circumferential dimension) of the hydrodynamic grooves 10 is Z1 and the circumferential dimension of the ridge portions 11 is Z2, the ratio of Z2 to Z1 (= Z2 / Z1) is 1 or less, preferably in the range of 0.3 to 0.7. In other words, the circumferential dimension Z2 of the ridge portions 11 is preferably set to be smaller than the circumferential dimension Z1 of the hydrodynamic grooves 10 by a predetermined amount. The hydrodynamic pressure generating portion P of this embodiment has five hydrodynamic grooves 10 and five ridge portions 11, and the ratio (= Z2 / Z1) is 0.5. This allows the lubricating oil in the radial bearing gap (within the arc-shaped gap of the radial bearing gap defined by the hydrodynamic grooves 10) to move significantly in the circumferential direction as the shaft member 2 rotates. This is advantageous for improving oil film formation in the radial bearing gap and increasing the bearing rigidity of the radial bearing portions R1 and R2. The number of hydrodynamic grooves 10 and the ratio can be arbitrarily changed depending on the required characteristics, etc.

[0040] The bearing member 8 made of the metal sintered body having the above-mentioned structure is obtained by undergoing a compression molding step, a sintering step, a sizing step, and an oil impregnation step in this order. Each of these steps will be described below.

[0041] [Compression molding process] In this step, raw material powder filled into the cavity of a compression molding die (details will be described later) is compression molded to obtain a green compact 18 as shown in Figures 6 and 7, which is the base material of a bearing member 8 as a dynamic pressure bearing.

[0042] 5 and 6 show schematic vertical cross-sectional views of the compression molding die 20. The compression molding die 20 has a cylindrical die 21 that forms the outer peripheral surface of the powder compact 18, a core pin 22 that forms the inner peripheral surface of the powder compact 18, a lower punch 23 that forms the lower end surface of the powder compact 18, including the lower outer peripheral chamfer and lower inner peripheral chamfer of the powder compact 18, and an upper punch 24 that forms the upper end surface of the powder compact 18, including the upper outer peripheral chamfer and upper inner peripheral chamfer of the powder compact 18, and the core pin 22, lower punch 23, and upper punch 24 are provided so as to be able to move up and down relative to the die 21.

[0043] The die 21 has a small-diameter inner circumferential surface 21a, a large-diameter inner circumferential surface 21b provided above the small-diameter inner circumferential surface 21a, and a tapered inner circumferential surface 21c connecting the inner circumferential surfaces 21a and 21b. The core pin 22 has a stepped shaft shape, with a large-diameter outer circumferential surface 22a, a small-diameter outer circumferential surface 22b provided above the large-diameter outer circumferential surface 22a, and a tapered outer circumferential surface 22c connecting the outer circumferential surfaces 22a and 22b. The small-diameter outer circumferential surface 22b is provided with a molding die portion 22d corresponding to the shape of the dynamic pressure generating portion P having an uneven surface to be provided on the radial bearing surface A1 of the bearing member 8. The molding die portion 22d has a convex portion 22e that forms the dynamic pressure grooves 10.

[0044] In the compression molding die 20 having the above configuration, first, as shown in Fig. 5, a core pin 22 and a lower punch 23 are arranged on the inner periphery of a die 21 to form a cavity 25 to be filled with raw material powder, and raw material powder M is then filled into this cavity 25. The raw material powder M is, for example, a metal powder mainly composed of copper or iron, to which a solid lubricant powder such as graphite or molybdenum disulfide, or a low-melting-point metal powder such as tin powder or zinc powder is appropriately added and mixed. When forming the cavity 25, the core pin 22 is arranged so that the small-diameter outer peripheral surface 22b and the tapered outer peripheral surface 22c are located radially inside the large-diameter inner periphery 21b of the die 21.

[0045] As shown in Fig. 6, after the raw material powder M is filled into the cavity 25, the upper punch 24 is lowered to compress the raw material powder M in the axial direction and form a cylindrical green compact 18. Thereafter, as shown in Fig. 7, the core pin 22, the lower punch 23, and the upper punch 24 are raised together to eject the green compact 18 to the outside (upper side) of the die 21, and then the upper punch 24 is raised (releasing the axial restraining force exerted by the lower punch 23 and the upper punch 24), and the core pin 22 is removed from the inner periphery of the green compact 18. The inner and outer peripheral shapes of the green compact 18 obtained in this manner follow the outer peripheral shape of the core pin 22 and the inner peripheral shape of the die 21, respectively.

[0046] That is, the inner peripheral surface of the powder compact 18 is provided with a small-diameter inner peripheral surface 18a, a large-diameter inner peripheral surface 18b, and a tapered inner peripheral surface 18c that respectively follow the shapes of the small-diameter outer peripheral surface 22b, the large-diameter outer peripheral surface 22b, and the tapered outer peripheral surface 22c of the core pin 22, and the outer peripheral surface of the powder compact 18 is provided with a large-diameter outer peripheral surface 18e1, a small-diameter outer peripheral surface 18e2, and a tapered surface connecting both outer peripheral surfaces 18e1, 18e2 that respectively follow the shapes of the large-diameter inner peripheral surface 21b, the small-diameter inner peripheral surface 21a, and the tapered inner peripheral surface 21c of the die 21. The small-diameter inner peripheral surface 18a of the powder compact 18 is provided with a dynamic pressure generating portion P (concaves and recesses corresponding to the shape of) formed by the molding die portion 21d of the core pin 21.

[0047] [Sintering process] In this sintering process, the green compact 18 obtained in the compression molding process is heated for a predetermined time, for example, at a temperature equal to or higher than the sintering temperature of the main component metal, to obtain a sintered compact 18' (see Figure 8) in which the metal powders that make up the green compact 18 are neck-bonded together.

[0048] [Sizing process] In this sizing step, a sizing die described below is used to plastically deform a portion of the sintered body 18', thereby finishing the sintered body 18' into its final shape (the shape of the bearing member 8 shown in FIG. 3).

[0049] 8(a) and 8(b) show schematic vertical cross-sectional views of the sizing die 30. The sizing die 30 includes a cylindrical die 31, a core pin 32 that is provided so as to be able to move up and down relative to the die 31, a lower punch 33, and an upper punch 34.

[0050] A forming die 32a having an uneven surface corresponding to the shape of the dynamic pressure generating portion P to be provided on the radial bearing surface A2 of the bearing member 8 is provided at a predetermined axial position on the outer peripheral surface of the core pin 32. That is, the forming die 32a is configured by circumferentially arranging convex portions 32b extending in the axial direction and spaced apart to form the dynamic pressure grooves 10 of the dynamic pressure generating portion P. The area of ​​the outer peripheral surface of the core pin 32 excluding the forming die 32a is a smooth cylindrical surface without any unevenness, and is formed with a diameter smaller than the small-diameter inner peripheral surface 18a of the sintered compact 18' by a predetermined amount so that when the sintered compact 18' is subjected to a sizing process, the area of ​​the inner peripheral surface of the sintered compact 18' excluding the area to be formed of the radial bearing surface A2 (a part of the large-diameter inner peripheral surface 18b) is in non-contact or light contact to the extent that it does not deform.

[0051] The inner surface of the die 31 is provided with a large-diameter inner surface 31a formed into a cylindrical surface of a constant diameter, a cylindrical constriction surface 31b formed with a smaller diameter than the large-diameter inner surface 31a and constricting the lower end of the small-diameter outer surface 18e2 of the sintered body 18', and a tapered inner surface 31c connecting the large-diameter inner surface 31a and the constriction surface 31b.

[0052] In the sizing die 30 having the above configuration, first, as shown in Fig. 8(a), the sintered body 18' is placed between the die 31 and the core pin 32. At this time, the sintered body 18' is placed so that the lower region of the large-diameter inner circumferential surface 18b, which is the region to be molded for the radial bearing surface A2, faces the molding die 32a of the core pin 32.

[0053] Then, while maintaining the relative positional relationship between the sintered compact 18' and the core pin 32 in the axial direction, the core pin 32, the lower punch 33, and the upper punch 34 are moved downward together, and (the lower region of) the small-diameter outer peripheral surface 18e2 of the sintered compact 18' is guided by the tapered inner peripheral surface 31c of the die 31 and gradually pressed into the drawing surface 31b of the die 31. As a result, the lower region of the small-diameter outer peripheral surface 18e2 of the sintered compact 18' is drawn by the drawing surface 31b of the die 31 (receives a radially inward compressive force), so that the vicinity of the lower end of the sintered compact 18' is entirely reduced in diameter and deformed, and the lower region of the large-diameter inner peripheral surface 18b of the sintered compact 18' (the region intended for molding the radial bearing surface A2) is pressed against the forming die 32a of the core pin 32 [see FIG. 8(b)]. As a result, a radial bearing surface A2 having substantially the same diameter as the radial bearing surface A1 and including a dynamic pressure generating portion P is formed in the lower region of the inner peripheral surface of the sintered body 18'.

[0054] The dynamic pressure generating portion P provided on the radial bearing surfaces A1, A2 has a simple shape consisting of an uneven surface with multiple axially extending dynamic pressure grooves 10 and convex ridges 11 alternately arranged in the circumferential direction, so the required precision can be ensured simply by molding using the sizing die 30 (and the aforementioned compression molding die 20). In contrast, with a "herringbone-type dynamic pressure bearing" in which the ridges that make up the dynamic pressure generating portion have a herringbone shape, the required precision cannot be ensured simply by molding, and separate finishing processing (such as rotational sizing) must be performed after molding (after the sizing process). For this reason, the bearing member 8 of this embodiment, which is a "step-type dynamic pressure bearing," requires few man-hours and processes until completion, and can be manufactured at low cost.

[0055] At the same time that the radial bearing surface A2 is formed on the inner peripheral surface of the sintered body 18', a tapered surface 8d (see Figure 4) as a gently sloping surface is formed on the portion of the inner peripheral surface of the sintered body 18' adjacent to the upper side of the radial bearing surface A2, and the lower region of the small diameter outer peripheral surface 18e2 of the sintered body 18' that is narrowed by the narrowing surface 31b of the die 31 is formed into the small diameter outer peripheral surface 8e3 of the bearing member 8.

[0056] By performing the sizing process on (a part of) the sintered compact 18' as described above, the radial bearing surface A2 having the dynamic pressure generating portion P and the tapered surface 8d are formed on the large-diameter inner peripheral surface 18b of the sintered compact 18'. During this process, the axial region of the outer peripheral surface of the sintered compact 18', excluding the lower region of the small-diameter outer peripheral surface 18e2, is not subjected to a compressive force in the diameter-reducing direction from the die 31 and is therefore not deformed. Therefore, the axial central region of the large-diameter inner peripheral surface 18b of the sintered compact 18' becomes the large-diameter cylindrical surface 8b of the bearing member 8. Furthermore, the upper region of the small-diameter outer peripheral surface 18e2 of the sintered compact 18' that is not drawn by the drawing surface 31b of the die 31 becomes the medium-diameter outer peripheral surface 8e2 provided on the outer peripheral surface 8e of the bearing member 8.

[0057] [Oil impregnation process] In this step, lubricating oil is impregnated into the internal pores of the sintered body 18' (bearing member 8) that has been finished to its final shape in the sizing step by a known method such as vacuum impregnation. This results in an oil-impregnated bearing member 8 (dynamic bearing) that can be incorporated into a fluid dynamic bearing device 1.

[0058] As in the embodiment described above, the surface open area ratio of the radial bearing surface A1 (radial bearing surface A1 having the dynamic pressure generating portion P) formed by compression molding rather than sizing is likely to be greater than the surface open area ratio of the radial bearing surface A2 formed by sizing, and as a result, of the two radial bearing portions R1, R2, the bearing rigidity of radial bearing portion R1 may be lower than the bearing rigidity of radial bearing portion R2. Therefore, although the bearing performance of the bearing member 8 of this embodiment may be somewhat inferior to that in a case where both radial bearing surfaces A1, A2 are formed by sizing, when supporting the main shaft of a fan motor (shaft member 2), it does not matter if the required bearing performance is somewhat lower than when supporting the main shaft of a spindle motor for an HDD, for example. If there is a difference in bearing rigidity between the radial bearing portions R1 and R2, it is preferable to position the radial bearing portion with superior bearing rigidity (here, radial bearing portion R2) near the center of gravity of the motor's rotating body (in this embodiment, the entire component that rotates integrally with the rotating shaft member 2, such as the shaft member 2 and rotor 3).

[0059] The dynamic pressure bearing and its manufacturing method according to an embodiment of the present invention, as well as the fluid dynamic pressure bearing device 1 that uses this dynamic pressure bearing as the bearing member 8, have been described above, but the embodiments of the present invention are not limited to these.

[0060] For example, the bearing member 8 as a hydrodynamic bearing shown in FIG. 3 has two radial bearing surfaces A1 and A2 each having a hydrodynamic pressure generating portion P. Of these, the radial bearing surface A1 is formed by compression molding, and the other radial bearing surface A2 is formed by sizing. However, both radial bearing surfaces A1 and A2 may be formed by sizing. In this case, the compression molding step uses a compression molding die (not shown) having a core pin 22 on its outer circumferential surface that does not have a molding die portion 22d corresponding to the shape of the hydrodynamic pressure generating portion P. Meanwhile, the sizing step uses a sizing die 30′ having a core pin 32 on its outer circumferential surface that is provided with a molding die 32a that can mold the hydrodynamic pressure generating portion P on both the radial bearing surfaces A1 and A2, as shown schematically in FIGS. 9(a) and 9(b). In the sizing die 30′ shown in FIG. 9, the protrusions 32b for forming the hydrodynamic pressure grooves 10 are longer in the axial direction than the protrusions 32b provided in the sizing die 30 shown in FIG. 8.

[0061] 2, the thrust bearing portion T that supports the shaft member 2 in the thrust direction is configured as a pivot bearing, but the thrust bearing portion T can also be configured as a so-called dynamic pressure bearing. Although not shown in the figures, such a configuration can be realized, for example, by forming the lower end surface of the shaft member 2 into a flat surface in a direction perpendicular to the axial direction, and providing a dynamic pressure generating portion such as a dynamic pressure groove on the lower end surface of the shaft member 2 or on the inner bottom surface 7b1 of the housing 7 that faces this.

[0062] Furthermore, in the fluid dynamic bearing device 1 described above, the shaft member 2 constitutes the rotating side and the bearing member 8 constitutes the stationary side, but the opposite may also be true, with the shaft member 2 constituting the stationary side and the bearing member 8 constituting the rotating side. In other words, the bearing member 8 made up of a dynamic bearing according to an embodiment of the present invention can be incorporated into and used not only in so-called rotating-shaft type fluid dynamic bearing devices 1, but also in so-called fixed-shaft type fluid dynamic bearing devices 1.

[0063] Furthermore, the fluid dynamic bearing device 1 can be used not only for fan motors such as the one shown in Figure 1, but also as a bearing device for other small motors such as spindle motors for HDDs and polygon scanner motors for LBPs. When the fluid dynamic bearing device 1 is used in a spindle motor, a rotor holding one or more recording disks is mounted on the rotating member of the fluid dynamic bearing device 1 (for example, shaft member 2) so that it can rotate integrally with the fluid dynamic bearing device 1, and when the fluid dynamic bearing device 1 is used in a polygon scanner motor, a rotor holding a polygon mirror is mounted on the rotating member of the fluid dynamic bearing device 1 so that it can rotate integrally with the fluid dynamic bearing device 1. [Explanation of symbols]

[0064] 1. Fluid dynamic bearing device 2 Shaft member 8 Bearing components (hydrodynamic bearings) 8a Inner surface 8b Large diameter cylindrical surface 8c Tapered surface (steep slope) 8d Tapered surface (gentle slope) 8e Outer surface 10 Hydrodynamic groove 11 Hill 18 Powder Compacts 18' sintered body 20 Compression molding mold 30 Sizing mold A1, A2 radial bearing surfaces P Dynamic pressure generating part R1, R2 radial bearing section T Thrust bearing Circumferential dimension of Z1 hydrodynamic groove Circumferential dimension of Z2 hill

Claims

1. Two radial bearing surfaces are provided spaced apart in the axial direction, forming a radial bearing gap between them and the outer peripheral surface of the shaft to be supported, and the bearing has an inner peripheral surface provided with a large-diameter cylindrical surface having a diameter larger than that of the radial bearing surfaces between the two radial bearing surfaces, A dynamic pressure bearing is provided on each radial bearing surface, and the dynamic pressure generating portion is configured as a concave-convex surface having a plurality of axially extending dynamic pressure grooves and convex ridges alternately arranged in the circumferential direction, one of the radial bearing surfaces and the large-diameter cylindrical surface is connected via a steeply inclined surface inclined with respect to the axial direction, and the other of the radial bearing surfaces and the large-diameter cylindrical surface is connected via a gently inclined surface whose inclination angle with respect to the axial direction is smaller than that of the steeply inclined surface, The hydrodynamic bearing is characterized in that the hydrodynamic groove provided on the other radial bearing surface has one axial end opening onto the gently inclined surface.

2. When the dynamic pressure generating portion is developed on a plane, the circumferential dimension Z of the dynamic pressure groove １ The circumferential dimension Z of the hill portion relative to ２ The ratio (= Z ２ / Z １ 2. A hydrodynamic bearing according to claim 1, wherein ≈1 is 1 or less.

3. 3. A hydrodynamic bearing according to claim 1, wherein the difference in height between the inner diameter end face of the ridge portion and the bottom face of the hydrodynamic groove is 2 [mu]m or more.

4. 2. A hydrodynamic bearing according to claim 1, which is made of a porous sintered metal body.

5. 5. A hydrodynamic bearing according to claim 4, wherein the other radial bearing surface is a formed surface formed by subjecting the metal sintered body to plastic working.

6. a bearing member comprising the hydrodynamic bearing according to claim 1; a housing that accommodates the bearing member on its inner periphery; lubricating oil as the fluid present in the radial bearing gap; a radial bearing portion that supports the shaft in a non-contact manner in the radial direction by an oil film of the lubricating oil formed in the radial bearing gap.

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