Spindle motor and method for manufacturing the same

Abstract

To provide a spindle motor which can be highly accurately attached with a shaft without impairing the fixing rigidity of the shaft.SOLUTION: In a spindle motor 10, a rotor hub 30 rotates with respect to a sleeve 14 via a fluid dynamic pressure radial bearing 14c in which a minute clearance between the shaft 20 and the sleeve 14 is filled with a lubrication fluid. The shaft 20 is constituted of a material harder than the rotor hub 30, and pressure-inserted into a shaft insertion hole 30a formed at the rotor hub 30. An internal peripheral face of the shaft insertion hole 30a includes a first internal peripheral face being a portion to be in contact with the shaft 20, an external peripheral face of the shaft 20 includes a first external peripheral face being a portion to be in contact with the first internal peripheral face, and the arithmetic average roughness of the first internal peripheral face is higher than an arithmetic average roughness of the first external peripheral face.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a spindle motor used in, for example, a hard disk drive, and particularly relates to a technique for attaching a shaft to a base with high precision.

Background Art

[0002] Recently, spindle motors of recording disk devices are required to have high rotational accuracy, so the rotating part rotates with respect to the fixed part via a hydrodynamic bearing part. The hydrodynamic bearing part has a structure in which a shaft inserted into a bearing member faces the inner peripheral surface of the bearing member with a minute gap therebetween, and the minute gap is filled with lubricating oil. Further, a dynamic pressure generating groove is formed on at least one of the inner peripheral surface of the bearing member and the shaft. When the bearing member and the shaft rotate relatively, dynamic pressure is generated and the bearing member and the shaft are in a non-contact state, and the rotating part of the spindle motor rotates with high precision.

[0003] Here, the hydrodynamic bearing part may be such that the shaft is fixed to the rotating part and rotates, or the shaft is fixed to the fixed part and the bearing member rotates. Taking a hard disk drive as an example of a recording disk device, a shaft rotation type motor in which a shaft is fixed to a rotor hub (rotating part) on which a magnetic disk is mounted, and a shaft fixed type motor in which a shaft is fixed to a base (fixed part) that holds a stator can be considered. Further, it is known to use press fitting and adhesion in combination as a method of fixing the shaft.

[0004] For example, Patent Document 1 discloses a spindle motor including a turntable made of a deformable iron-based material, a turntable inner diameter part provided at the center of the turntable, having a through hole, and having a coupling part with a groove or an embossed shape formed on the inner peripheral surface, and a rotating shaft inserted into the through hole of the turntable inner diameter part and contacting the coupling part to perform shaft rotation. And the inner diameter of the coupling part is characterized by being the same as or smaller than the outer diameter of the rotating shaft.

[0005] According to the embodiment described in Patent Document 1, a rotating shaft 130 is inserted into the central through-hole of the turntable inner diameter portion 120, and the rotating shaft 130 comes into contact with a coupling portion 140 formed in the turntable inner diameter portion 120 when inserted. The contact between the rotating shaft 130 and the coupling portion 140 during insertion causes local deformation of the coupling portion 140 and fixes the rotating shaft 130 to the center of the turntable inner diameter portion 120. The coupling portion 140 of the turntable inner diameter portion 120 may have a groove or embossed shape. In this case, after inserting the rotating shaft 130 into the turntable inner diameter portion 120, the gap can also be fixed with a bonding agent. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2013-25856 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] In the shaft fixing method described above, the space between the shaft and the shaft insertion hole is an interference fit that causes local deformation, requiring a large pressing force, and making galling likely when pressing in the shaft. In other words, if the central axis of the shaft is even slightly tilted or eccentric with respect to the central axis of the shaft insertion hole when inserting the shaft, the tip of the shaft will bite into the groove or embossed shape of the insertion hole, damaging the inner surface. While shaft galling creates resistance to press-fitting, the large pressing force can overcome this resistance, and the shaft may be inserted. As a result, the shaft is installed with mounting errors due to the tilt, which worsens the rotational accuracy of the rotating part (turntable or rotor hub) on which the recording disc is placed. For example, repeated runout synchronized with the rotation of the rotating part, so-called synchronous runout (RRO), is worsened due to shaft mounting errors. On the other hand, in the case of a fixed-shaft motor, the shaft is press-fitted and fixed to the base, and the rotating part is fixed to the bearing member side, but the parallelism of the disc mounting surface of the rotating part with respect to the base surface is worsened due to shaft mounting errors.

[0008] However, reducing the press-fit allowance to avoid galling results in insufficient fixing strength of the shaft. In order to compensate for the insufficient fixing strength, increasing the gap between the shaft and the inner surface of the insertion hole and increasing the amount of adhesive, as in Patent Document 1, is undesirable in the case of hard disk drives because it increases the amount of outgassing generated from the adhesive that contaminates the hard disk.

[0009] This invention has been made in view of the above circumstances, and aims to provide a spindle motor that can mount a shaft with high precision without impairing the fixing strength of the shaft. [Means for solving the problem]

[0010] The present invention relates to a spindle motor in which a rotating part rotates relative to a stationary part via a fluid dynamic bearing mechanism in which a lubricating fluid is filled into a minute gap between a shaft and a bearing member, wherein the shaft is made of a material harder than the rotating part or the stationary part and is press-fitted into a shaft insertion hole provided in either the rotating part or the stationary part, the inner circumferential surface of the shaft insertion hole includes a first inner circumferential surface which is the part that contacts the shaft, the outer circumferential surface of the shaft includes a first outer circumferential surface which is the part that contacts the first inner circumferential surface, and the arithmetic mean roughness of the first inner circumferential surface is greater than the arithmetic mean roughness of the first outer circumferential surface.

[0011] Furthermore, the present invention relates to a method for manufacturing a spindle motor in which a rotating part rotates relative to a stationary part via a fluid dynamic bearing mechanism in which a lubricating fluid is filled into a minute gap between the outer circumferential surface of a shaft and the inner circumferential surface of a bearing member into which the shaft is inserted, the method comprising the steps of: preparing the rotating part, the stationary part, and a shaft made of a material harder than the rotating part and the stationary part; providing a shaft insertion hole in the rotating part or the stationary part in which the arithmetic mean roughness of the inner circumferential surface is greater than the arithmetic mean roughness of the outer circumferential surface of the shaft; applying a lubricating liquid to at least one of the inner circumferential surface of the shaft insertion hole and the outer circumferential surface of the shaft; and pressing the shaft into the shaft insertion hole after applying the lubricating liquid. [Effects of the Invention]

[0012] According to the present invention, it is possible to provide a spindle motor that can mount a shaft with high precision without compromising the fixing strength of the shaft. [Brief explanation of the drawing]

[0013] [Figure 1] This is a cross-sectional view showing a spindle motor according to a first embodiment of the present invention. [Figure 2] This is a cross-sectional view showing a spindle motor according to a second embodiment of the present invention. [Figure 3] This is a cross-sectional view showing a spindle motor according to a third embodiment of the present invention. [Figure 4] This is a plan view of a hard disk drive device showing the location for measuring parallelism in an embodiment of the present invention. [Figure 5] This is a cross-sectional view of Figure 4. [Figure 6] This is a rear view of a hard disk drive device showing the reference surface used when measuring parallelism in an embodiment of the present invention. [Modes for carrying out the invention]

[0014] 1. First Embodiment Figure 1 shows a first embodiment of the present invention, which is an example of applying the present invention to an axial rotation type spindle motor used in a hard disk drive. The spindle motor 10 includes a base 11, the base 11 having a cylindrical portion 11a extending upward. A stator core 12 is fixed to the outer circumference of the cylindrical portion 11a. The stator core 12 is made of multiple thin, ring-shaped soft magnetic material (for example, electromagnetic steel sheet) stacked in the axial direction, and has multiple pole teeth protruding radially outward. The multiple pole teeth are arranged at equal intervals along the circumferential direction, and a coil 13 is wound around each of them.

[0015] A sleeve (fixing part, bearing member) 14 is fixed to the inner circumference of the cylindrical part 11a by means of press-fitting or adhesive, and a shaft 20 is inserted into the bearing hole 14a of the sleeve 14 with a small gap between them. A larger diameter part 14b is formed at the lower end of the bearing hole 14a, and a ring-shaped flange part 21, fixed to the outer circumference of the lower end of the shaft 20 by press-fitting, is positioned in the larger diameter part 14b. The lower side of the larger diameter part 14b is closed by a counter plate 22 fixed to the lower end opening of the sleeve 14. As a result, a small gap is formed between the shaft 20 and the sleeve 14, with only the upper end open. Lubricating oil O is continuously filled into this small gap.

[0016] A minute gap is formed between the outer peripheral surface of the shaft 20 and the inner peripheral surface of the sleeve 14, and dynamic pressure grooves are formed at a plurality of positions (two positions in this example) spaced apart in the axial direction of either one of their opposing surfaces. And by filling the minute gap with lubricating oil, a fluid dynamic pressure radial bearing is constituted. Also, minute gaps are formed between the lower and upper surfaces of the flange portion 21, the end surface of the sleeve 14 facing the flange portion 21, and the counter plate 22, and dynamic pressure grooves are formed on either one of their opposing surfaces. And by filling the minute gap with lubricating oil, a fluid dynamic pressure thrust bearing is constituted. When the shaft 20 rotates, a dynamic pressure force that supports the shaft 20 in the radial direction is generated by the fluid dynamic pressure radial bearing 14c, and a dynamic pressure force that causes the shaft 20 to float in the axial direction is generated by the fluid dynamic pressure thrust bearing, and the shaft 20 and the flange portion 21 are in a non-contact state with respect to the sleeve 14.

[0017] The shaft 20 is made of martensitic stainless steel (for example, SUS420J2) whose hardness is increased by heat treatment. The outer peripheral surface of the shaft 20 is finished to have an arithmetic mean roughness of Ra0.03 by polishing.

[0018] A rotor hub (rotating part) 30 is fixed to the upper end of the shaft 20 by press fitting. The rotor hub 30 is made of aluminum or an aluminum alloy. Therefore, the hardness of the rotor hub 30 is lower than that of the shaft 20. Incidentally, the rotor hub 30 may be made of a ferritic stainless steel such as SUS430. The rotor hub 30 is provided with a shaft insertion hole 30a for press fitting the shaft 20.

[0019] The inner peripheral surface of the shaft insertion hole 30a is finished into a smooth cylindrical surface having an arithmetic mean roughness larger than the outer peripheral surface of the shaft 20 by cutting. In this embodiment, the arithmetic mean roughness of the inner peripheral surface of the shaft insertion hole 30a is finished to Ra0.2 while the arithmetic mean roughness of the outer peripheral surface of the shaft 20 is Ra0.03. The arithmetic mean roughness of the inner peripheral surface of the shaft insertion hole 30a is not limited to this, and other combinations of arithmetic mean roughness may be used as long as the surface is rougher than the outer peripheral surface of the shaft 20 and mounting accuracy and fixing strength can be obtained.

[0020] For example, a combination may be used where the arithmetic mean roughness of the outer peripheral surface of the shaft 20 is Ra0.05 and the arithmetic mean roughness of the inner peripheral surface of the shaft insertion hole 30a is Ra0.1. However, considering the processing cost, it is desirable that the arithmetic mean roughness of the outer peripheral surface of the shaft 20 is in the range of Ra0.025 to Ra0.2, and the arithmetic mean roughness of the inner peripheral surface of the shaft insertion hole 30a is in the range of Ra0.1 to Ra0.8. However, the arithmetic mean roughness of the inner peripheral surface of the shaft insertion hole 30a must be larger than the arithmetic mean roughness of the outer peripheral surface of the shaft.

[0021] Here, for the inner peripheral surface of the shaft insertion hole 30a, after press-fitting, it is sufficient that the arithmetic mean roughness of at least the portion (first inner peripheral surface) that contacts the outer peripheral surface of the shaft 20 is larger than the arithmetic mean roughness of the portion (first outer peripheral surface) of the outer peripheral surface of the shaft 20 that contacts the first inner peripheral surface. That is, even when the shaft 20 is press-fitted, the arithmetic mean roughness of the portion of the inner peripheral surface of the shaft insertion hole 30a that does not contact the outer peripheral surface of the shaft 20 (for example, the chamfer portion or the counterbore portion) is not particularly limited.

[0022] When press-fitting the shaft 20, a lubricating liquid may be applied to the outer surface of the shaft 20 and / or the inner surface of the shaft insertion hole 30a to facilitate press-fitting. Liquid adhesives, lubricating oils, and isopropyl alcohol (IPA) can be used as the lubricating liquid. Since press-fitting is an interference fit, the applied lubricating liquid will be pushed out by the shaft. If the arithmetic mean roughness of the inner surface of the shaft insertion hole 30a is the same as (e.g., 0.03) or less than that of the outer surface of the shaft 20, the surface of the shaft insertion hole 30a will be too smooth, and almost no lubricating liquid will remain in the press-fit area, potentially causing galling and degrading the mounting accuracy of the shaft 20. However, according to the present invention, since the inner surface of the shaft insertion hole 30a is rougher than the outer surface of the shaft 20, lubricating liquid remains in the press-fit area, providing a lubricating effect between the inner surface of the shaft insertion hole 30a and the outer surface of the shaft 20. Therefore, galling is prevented, and high mounting accuracy is achieved.

[0023] However, to achieve the same effect, the surface roughness of the outer surface of the shaft 20 cannot be made greater than the surface roughness of the inner surface of the shaft insertion hole 30a. As mentioned above, the shaft 20 is harder than the rotor hub 30, so if the outer surface of the shaft 20 is rougher, the shaft insertion hole 30a of the lower-hardness rotor hub 30 may be worn down by contact. Furthermore, it is undesirable to form macroscopic irregularities on the shaft insertion hole 30a of the rotor hub 30 that make it uneven, thereby forming lubricating fluid reservoirs. Macroscopic irregularities here refer to, for example, helical grooves, circumferential grooves, or pattern shapes of irregularities formed in the shaft insertion hole 30a. Such macroscopic irregularities make it difficult to achieve the inner diameter dimension through polishing, impairing the dimensional accuracy of the inner diameter of the shaft insertion hole 30a, and consequently potentially impairing the mounting accuracy and fixing strength of the shaft 20. Moreover, macroscopic irregularities are undesirable because they result in too much residual adhesive or lubricating fluid, causing outgassing to increase beyond the acceptable range.

[0024] Therefore, it is preferable that the inner circumferential surface of the shaft insertion hole 30a be a smooth cylindrical surface without macroscopic irregularities in the press-fit portion. This allows for a suitable lubrication effect to prevent galling. In this embodiment, an acrylic anaerobic adhesive is used as the lubricating liquid. Using an anaerobic adhesive is preferable because the adhesive remaining between the shaft 20 and the inner circumferential surface of the shaft insertion hole 30a is completely cured by being isolated from the air, thereby suppressing outgassing and increasing the fixing strength of the shaft 20 due to the adhesive effect. Alternatively, a thermosetting epoxy adhesive can be used instead of an anaerobic adhesive.

[0025] A flat disk mounting section 31 is formed on the outer circumference of the rotor hub 30. Multiple hard disks are stacked on the disk mounting section 31 via spacers. A cylindrical section 32 extending downward is formed on the outer edge of the disk mounting section 31, and an annular rotor magnet 34 is fixed to the inner circumference of the cylindrical section 32 via a yoke 33. The rotor magnet 34 is magnetized such that adjacent sections of SNSN·· are alternately polarized along the circumferential direction. The inner circumference of the rotor magnet 34 faces the outer circumference of the pole teeth of the stator core 12 with a gap between them. When a drive current is supplied to the coil 13, a driving force is generated that attempts to rotate the rotor magnet 34, causing the rotor hub 30 to rotate relative to the base 11 around the shaft 20.

[0026] In the spindle motor 10 with the above configuration, the arithmetic mean roughness of the first inner surface of the shaft insertion hole 30a that contacts the shaft 20 is greater than the arithmetic mean roughness of the first outer surface of the shaft 20 that contacts the first inner surface. Therefore, when the shaft 20 is press-fitted into the shaft insertion hole 30a, the tips of the protruding parts of the minute irregularities on the inner surface of the shaft insertion hole 30a are pressed against the shaft 20 and undergo plastic deformation, causing the shaft insertion hole 30a to expand in diameter and reducing the press-fitting resistance. As a result, galling is less likely to occur when press-fitting the shaft 20, improving the mounting accuracy of the rotor hub 30, and increasing the press-fitting allowance, thereby increasing the fixing strength of the rotor hub 30.

[0027] On the other hand, if the mounting accuracy is poor when inserting the shaft 20 into the shaft insertion hole 30a of the rotor hub 30, the perpendicularity of the shaft 20 to the rotor hub 30 will be poor. As a result, the repeated runout of the rotor hub 30, which rotates around the shaft 20, so-called synchronous runout (RRO), will worsen. In this respect, the spindle motor 10 with the above configuration improves the mounting accuracy of the shaft 20, so the RRO of the rotor hub 30 is improved.

[0028] 2. Second Embodiment Figure 2 is a cross-sectional view showing a spindle motor 100 according to a second embodiment of the present invention. The spindle motor 100 includes a base (fixed part) 110. The base 110 is made of aluminum or an aluminum alloy and has a shaft insertion hole 112.

[0029] A roughly cylindrical shaft 220 is fixed to the shaft insertion hole 112 by press-fitting. The shaft 220 is made of martensitic stainless steel (e.g., SUS420J2) whose hardness has been increased by heat treatment. Therefore, the shaft 220 is harder than the base 110.

[0030] The outer circumferential surface of the shaft 220 is finished to an arithmetic mean roughness of Ra 0.2 by polishing. On the other hand, the inner circumferential surface of the shaft insertion hole 112 is finished to a smooth cylindrical surface with a larger arithmetic mean roughness than the outer circumferential surface of the shaft 220 by machining. In this second embodiment, the outer circumferential surface of the shaft 220 has an arithmetic mean roughness of Ra 0.2, while the inner circumferential surface of the shaft insertion hole 112 has an arithmetic mean roughness of Ra 0.8.

[0031] A conical bearing portion 214 is formed at the upper end of the shaft 220. Furthermore, a conical bearing member 220a is fixed to the axial middle portion of the shaft 220 by means of press-fitting, adhesive bonding, or other means. The conical bearing member 220a is made of SUS303, an austenitic stainless steel.

[0032] A cylindrical portion 114 extending upward is formed on the base 110. A stator core 120, similar to that in the first embodiment, is fixed to the outer circumference of the cylindrical portion 114, and a coil 130 is wound around each of the multiple pole teeth of the stator core 120.

[0033] A sleeve (bearing member) 370 is positioned on the outside of the shaft 220. A minute gap is formed between the conical bearing portion 214 of the shaft 220, the axial intermediate portion of the shaft 220, and the outer circumferential surface of the conical bearing member 220a and the inner circumferential surface of the sleeve 370. A dynamic pressure groove is formed on one of the opposing surfaces of these components, and lubricating oil is continuously supplied to the minute gap, thereby forming a fluid dynamic pressure bearing.

[0034] A rotor hub (rotating part) 360 is fixed to the outer circumference of the sleeve 370 by means of press-fitting or adhesive. A flat disk mounting portion 361 is formed on the outer circumference of the rotor hub 360. A cylindrical portion 362 extending downward is formed on the outer edge of the disk mounting portion 361, and an annular rotor magnet 330 is fixed to the inner circumference of the cylindrical portion 362 via a yoke 320. The inner circumference of the rotor magnet 330 faces the outer circumference of the pole teeth of the stator core 120 with a gap between them.

[0035] In the spindle motor 100 with the above configuration, the arithmetic mean roughness of the outer surface of the shaft 220 is Ra0.2, and the arithmetic mean roughness of the inner surface of the shaft insertion hole 112 of the base 110 is Ra0.8. Therefore, when the shaft 220 is press-fitted into the shaft insertion hole 112, the tips of the protruding parts of the minute irregularities on the inner surface of the shaft insertion hole 112 are pressed against the shaft 220 and undergo plastic deformation, causing the shaft insertion hole 112 to expand in diameter and reducing the press-fitting resistance. As a result, galling is less likely to occur when press-fitting the shaft 220, the mounting accuracy of the rotor hub 360 can be improved, and the press-fitting allowance can be increased, thereby increasing the fixing strength of the rotor hub 360.

[0036] 3. Third Embodiment Figure 3 is a cross-sectional view showing a spindle motor 500 of a third embodiment. The spindle motor 500 includes a base 510. The base 510 is made of aluminum or an aluminum alloy and includes a through hole 512 and a cylindrical portion 513 that extends upward in a manner continuous with the through hole 512. A stator core 540, similar to that of the first embodiment, is fixed to the outer circumference of the cylindrical portion 513, and a coil 550 is wound around each of the multiple pole teeth of the stator core 540. A cup member (fixing portion) 520 is fixed to the through hole 512 by press-fitting and adhesive. The cup member 520 is made of ferritic stainless steel such as SUS430, has a shaft insertion hole 521 formed in its center, and has a cylindrical portion 522 that extends upward at its upper edge.

[0037] A cylindrical shaft 530 is fixed to the shaft insertion hole 521 by press-fitting. The shaft 530 is made of martensitic stainless steel (e.g., SUS420J2) whose hardness has been increased by heat treatment. Therefore, the shaft 530 is harder than the cup member 520. A large-diameter portion 531 is formed at the upper end of the shaft 530, which is larger in diameter than the rest of the shaft.

[0038] The outer circumferential surfaces of the shaft 530 and the large-diameter portion 531 are finished to an arithmetic mean roughness of Ra 0.03 by polishing. On the other hand, the inner circumferential surface of the shaft insertion hole 521 is finished to a smooth cylindrical surface with a larger arithmetic mean roughness than the outer circumferential surfaces of the shaft 530 and the large-diameter portion 531 by cutting. In this third embodiment, the arithmetic mean roughness of the outer circumferential surfaces of 530 and the large-diameter portion 531 is Ra 0.03, while the inner circumferential surface of the shaft insertion hole 521 is finished to an arithmetic mean roughness of Ra 0.1.

[0039] A rotor hub 560 is positioned on the outside of the shaft 530. A through hole 561 is formed in the center of the rotor hub 560, and a small gap is formed between the inner surface of the through hole 561 and the outer surface of the shaft 530. Dynamic grooves are formed in the upper and lower parts of the inner surface of the through hole 561, which is divided into approximately three equal parts vertically, or on the outer surface of the shaft 530 opposite these parts, and a fluid dynamic radial bearing 532 is constructed by filling the small gap with lubricating oil.

[0040] Furthermore, a cylindrical portion 562 extending upward is formed on the upper end surface of the rotor hub 560, and the large-diameter portion 531 of the shaft 530 is housed in the cylindrical portion 562. A minute gap is formed between the inner circumferential surface of the cylindrical portion 562 and the outer circumferential surface of the large-diameter portion 531. A pumping seal portion 533, consisting of a helical groove, is formed on the inner circumferential surface of the cylindrical portion 562 or the outer circumferential surface of the large-diameter portion 531, and as the rotor hub 560 rotates, lubricating oil is supplied to the fluid dynamic radial bearing 532. side It is designed to revert back to its original state.

[0041] A tapered portion 563 is formed at the lower end of the rotor hub 560, which widens in diameter as it extends downwards. A wedge-shaped gap 523 is formed between the outer surface of the tapered portion 563 and the cylindrical portion 522 of the cup member 520, serving as an oil reservoir. A small gap is also formed between the lower end surface of the rotor hub 560 and the bottom surface of the cup member 520. A hydrodynamic groove is formed on the lower end surface of the rotor hub 560 or the bottom surface of the cup member, and lubricating oil is filled into the small gap, thereby forming a fluid hydrodynamic thrust bearing 524.

[0042] The rotor hub 560 has a circulation passage 564 that communicates between the lower end surface of its tapered portion 563 and the upper end surface facing the large-diameter portion 531 of the shaft 530. The circulation passage 564 is filled with lubricating oil, and when the rotor hub 560 rotates, centrifugal force acts on the lubricating oil, causing it to move downward and supply it to the fluid dynamic thrust bearing 524. The lubricating oil is also supplied to the fluid dynamic radial bearing 532 and then returned to the circulation passage 564. In this way, the lubricating oil circulates due to the pumping action generated by the rotation of the rotor hub 560.

[0043] A cylindrical portion 565 extending downward is formed on the outer circumference of the rotor hub 560, and an annular rotor magnet 570 is fixed to the inner circumference of the cylindrical portion 565. The inner circumference of the rotor magnet 570 faces the outer circumference of the pole teeth of the stator core 540 with a gap between them. A flange-shaped disc mounting portion 566 is formed on the lower end edge of the cylindrical portion 565.

[0044] In the spindle motor 500 with the above configuration, the arithmetic mean roughness of the outer surface of the shaft 530 is Ra0.03, and the arithmetic mean roughness of the inner surface of the shaft insertion hole 521 of the cup member 520 is Ra0.1. Therefore, when the shaft 530 is press-fitted into the shaft insertion hole 521, the tips of the protruding parts of the minute irregularities on the inner surface of the shaft insertion hole 521 are pressed against the shaft 530 and undergo plastic deformation, causing the shaft insertion hole 521 to expand in diameter and reducing the press-fitting resistance. As a result, galling is less likely to occur when press-fitting the shaft 530, the mounting accuracy of the rotor hub 560 can be improved, and the press-fitting allowance can be increased, thereby increasing the fixing strength of the rotor hub 560.

[0045] 4. Example of changes The present invention is not limited to the embodiments described above, and various modifications are possible as follows. i) Although the above embodiment applies the present invention to a spindle motor equipped with a hard disk, it can also be applied to spindle motors equipped with other magnetic disks or optical disks.

[0046] ii) In the second embodiment shown in Figure 2, the rotor hub 360 and the sleeve 370 are constructed as separate components, but they may be formed integrally. iii) Fluid dynamic thrust bearings are not necessarily required. [Examples]

[0047] 1. Examples and Comparative Examples The effects of the present invention will be explained in detail with reference to specific examples. The spindle motor 10 shown in Figure 1 (arithmetic mean roughness of the outer surface of the shaft: Ra0.03, arithmetic mean roughness of the inner surface of the shaft insertion hole: Ra0.2) is designated as Example 1, and Example 2 is a version in which the arithmetic mean roughness of the outer surface of the shaft is changed to Ra0.05.

[0048] The spindle motor 100 shown in Figure 2 (arithmetic mean roughness of the outer shaft surface: Ra0.2, arithmetic mean roughness of the inner shaft insertion hole surface: Ra0.8) was designated as Example 3. The spindle motor 500 shown in Figure 3 (arithmetic mean roughness of the outer shaft surface: Ra0.03, arithmetic mean roughness of the inner shaft insertion hole surface: Ra0.1) was designated as Example 4, and Example 5 was created by changing the arithmetic mean roughness of the outer shaft surface to Ra0.1 and the arithmetic mean roughness of the inner shaft insertion hole surface to Ra0.5.

[0049] In the spindle motor 100 shown in Figure 1, Comparative Example 1 was defined as having an arithmetic mean roughness of Ra0.05 on the outer surface of the shaft and an arithmetic mean roughness of Ra0.04 on the inner surface of the shaft insertion hole, while Comparative Example 2 was defined as having an arithmetic mean roughness of Ra0.04 on the outer surface of the shaft and an arithmetic mean roughness of Ra0.03 on the inner surface of the shaft insertion hole. In addition, in the spindle motor 10 shown in Figure 2, Comparative Example 3 was defined as having an arithmetic mean roughness of Ra0.2 on the outer surface of the shaft and an arithmetic mean roughness of Ra0.1 on the inner surface of the shaft insertion hole.

[0050] 2. RRO trial The RRO runout during rotation was measured for Examples 1, 2, and 5, which are axial rotation type spindle motors, and Comparative Examples 1 and 2, and the RRO failure rate was compared. This test was conducted to confirm how the failure rate is affected when a shaft with a large surface roughness and a shaft with a small surface roughness are combined, in a spindle motor with the same structure, with a rotor hub having a large surface roughness and a rotor hub having a small surface roughness.

[0051] The RRO test was performed as follows: 100 spindle motors with different surface roughness combinations were rotated at 7200 rpm, and the RRO runout of the disk mounting surface 31 of the rotor hub 30 was checked to determine the defect rate. Specifically, the axial displacement was measured using a non-contact capacitance probe P, and its amplitude was defined as the RRO runout. Next, spindle motors with a measured RRO runout of 8 μm or more were considered defective, and the defect rate (%) was calculated as 100 × (number of defects / number of tests). The results of the defect rate (%) obtained in this way are shown in Table 1. As shown in Examples 1, 2, and 5 and Comparative Examples 1 and 2, it was confirmed that in axial rotation type spindle motors, the RRO defect rate was significantly reduced when the inner surface roughness of the hub was greater than the surface roughness of the shaft.

[0052] [Table 1]

[0053] 3. Parallelism Test Parallelism was measured by using a reference plane A, defined by three measurement points A1 to A3 on the underside of the base plate shown in Figure 6, as the measurement reference plane. The height was measured by bringing a height measuring probe into contact with four points H1 to H4 on the disc mounting surface 31 (361) shown in Figure 4 while the device was stationary, and the parallelism was calculated from these values. The parallelism was calculated using the formula shown in Equation 1 below.

[0054]

number

[0055] Next, spindle motors with a parallelism measurement of 30 μm or more were considered defective, and the defect rate (%) was calculated as 100 × (number of defects / number of tests). Note that if the parallelism is 30 μm or more, the hard disk placed on the disk mounting surface 361 will come into contact with the base 510. There is a possibility The defect rate (%) obtained in this manner is shown in Table 1. As shown in Examples 3 and 4 and Comparative Example 3, it was confirmed that in the fixed-shaft spindle motor 100, the parallelism defect rate was significantly reduced when the roughness of the inner circumferential surface of the shaft insertion hole 112 was greater than the surface roughness of the shaft 220. [Industrial applicability]

[0056] This invention can be used in electronic devices such as spindle motors and hard disk drives that incorporate magnetic disks or optical disks. [Explanation of Symbols]

[0057] 10, 100, 500… Spindle motor, 11, 110, 510… Base (fixed part), 11a, 114, 513… Cylindrical part, 12, 120, 540… Stator core, 13, 130, 550… Coil, 14, 370… Sleeve (fixed part, bearing member), 14a… Bearing hole, 14b… Large diameter part, 14c, 532,… Fluid dynamic radial bearing, 20, 220, 530… Shaft, 21… Flange part, 22… Counter plate, 30, 360, 560… Rotor hub (rotating part), 30a, 112, 521… Shaft Insertion hole, 31, 361, 566…Disk mounting section, 32, 362, 565…Cylindrical section, 33, 320…Yoke, 34, 330, 570…Rotor magnet, 214…Conical bearing section, 220a…Conical bearing member, 512…Through hole, 520…Cup member (fixing section), 522…Cylindrical section, 523…Gap, 524…Fluid dynamic thrust bearing, 531…Large diameter section, 533…Pumping seal section, 561…Through hole, 562…Cylindrical section, 563…Tapered section, 564…Circulation passage, A…Measurement reference surface, O…Lubricating oil, P…Probe.

Claims

1. In a spindle motor, a rotating part rotates relative to a fixed part via a fluid dynamic bearing mechanism in which a lubricating fluid is filled in a minute gap between a shaft and a bearing member, the shaft is made of a material harder than the rotating part or the fixed part, and is press-fitted into a shaft insertion hole provided in either the rotating part or the fixed part; an inner circumferential surface of the shaft insertion hole includes a first inner circumferential surface that is a portion that contacts the shaft, the outer circumferential surface of the shaft includes a first outer circumferential surface that is a portion that contacts the first inner circumferential surface, A spindle motor, wherein the arithmetic mean roughness of the first inner peripheral surface is greater than the arithmetic mean roughness of the first outer peripheral surface.

2. 2. The spindle motor according to claim 1, wherein the first inner peripheral surface is a smooth cylindrical surface.

3. 3. The spindle motor according to claim 1, wherein at least one of a lubricating oil component, an isopropyl alcohol component, and a hardened adhesive remains between the first inner peripheral surface of the shaft insertion hole and the first outer peripheral surface of the shaft.

4. 4. The spindle motor according to claim 1, wherein the shaft is press-fitted into the rotating portion, the bearing member is fixed to the fixed portion, and the shaft rotates together with the rotating portion.

5. 4. The spindle motor according to claim 1, wherein the shaft is press-fitted into the fixed portion, the bearing member is fixed to the rotating portion, and the bearing member rotates together with the rotating portion.

6. a spindle motor according to any one of claims 1 to 5 for rotating a rotating part; the rotating portion includes a disk mounting portion, a recording disk device provided with a recording disk that is attached to the disk mounting portion and rotates;

7. 7. A recording disk device according to claim 6, wherein said recording disk is a magnetic disk.

8. 7. The recording disk device according to claim 6, wherein the recording disk is an optical disk.

9. A method for manufacturing a spindle motor in which a rotating part rotates relative to a fixed part via a fluid dynamic bearing mechanism in which a lubricating fluid is filled in a minute gap between an outer peripheral surface of a shaft and an inner peripheral surface of a bearing member into which the shaft is inserted, the method comprising: providing the rotating portion, the fixed portion, and a shaft made of a material harder than the rotating portion and the fixed portion; providing a shaft insertion hole in the rotating portion or the fixed portion, the inner circumferential surface of which has an arithmetic mean roughness greater than the arithmetic mean roughness of the outer circumferential surface of the shaft; applying a lubricating liquid to at least one of an inner circumferential surface of the shaft insertion hole and an outer circumferential surface of the shaft; a step of press-fitting the shaft into the shaft insertion hole after applying the lubricating liquid; A method for manufacturing a spindle motor comprising:

10. 10. The method for manufacturing a spindle motor according to claim 9, wherein the lubricating liquid applied to at least one of the inner peripheral surface of the shaft insertion hole and the outer peripheral surface of the shaft is an adhesive, lubricating oil, or isopropyl alcohol.

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