Hydrodynamic bearing device

This is a divisional of U.S. application Ser. No. 11/195,813, filed Aug. 3, 2005.

This application claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2004-229825. The entire disclosure of Japanese Patent Application No. 2004-229825 is hereby incorporated herein by reference.

1. Field of the Invention

The present invention generally relates to a hydrodynamic bearing device utilizing a dynamic pressure and a method for manufacturing the device.

2. Background Information

In recent years, a storage capacity of a recording apparatus or the like using a disk or the like has been increasing, and a data transmission rate thereof has been increasing as well. A high speed and a high precision of rotation is necessary for a spindle motor that is used for such a recording apparatus. Therefore, a hydrodynamic bearing device is used for a rotation shaft portion of the spindle motor. A conventional hydrodynamic bearing device will be described below with reference to FIGS. 9 and 10.

FIG. 9 is a cross sectional view of a spindle motor including a conventional hydrodynamic bearing device. As shown in FIG. 9, a sleeve 101 having a bearing bore 101a is made of a sintered metal that is made of sintering powdered metal, such as a copper alloy. The sleeve 101 is made of a sintered metal mainly to reduce manufacturing costs. If the sleeve 101 is produced by machining a metal bar or the like, a lot of chips will be generated as waste material. In contrast, a sintered metal does not generate such chips. In addition, the time necessary for producing a sleeve using a sintered metal is a fraction of the time necessary for producing the same by machining. Accordingly, the production using a sintered metal is suitable for low-cost mass production.

On the outer surface of the sleeve 101, a sleeve cover 114 is provided. The sleeve cover 114 is made of a metal that is not a sintered metal. A shaft 102 is inserted in the bearing bore 101a of the sleeve 101 in a rotatable manner. A thrust flange 103 is fixed to a lower end portion of the shaft 102. The thrust flange 103 is housed in a space enclosed by the sleeve 101, the sleeve cover 114 and a thrust plate 104. A lower face of the thrust flange 103 in FIG. 9 is opposed to the thrust plate 104. An upper face of the thrust flange 103 is opposed to a lower end face of the sleeve 101.

A rotor hub 105 is fixed to an upper end portion of the shaft 102. A rotor magnet 106 is fixed to an inner surface of the rotor hub 105. The rotor magnet 106 is opposed to a motor stator 107 that is fixed to a base 108. An inner surface of the bearing bore 101a of the sleeve 101 is provided with dynamic pressure generating grooves 109a and 109b in the radial direction that are well known in the art. In addition, a face of the thrust plate 104 that is opposed to the thrust flange 103 is provided with dynamic pressure generating grooves 110a in the thrust direction that are also well known. Dynamic pressure generating grooves 110b may be formed on at least one of the opposed faces of the thrust flange 103 and the sleeve 101, if necessary. Oil 111, as working fluid, is filled in the space between the shaft 102 and the sleeve 101, including the dynamic pressure generating grooves 109a, 109b, 110a and 110b, as well as in the space between the thrust flange 103 and the sleeve 101 and the space between the thrust flange 103 and the thrust plate 104.

An operation of the conventional hydrodynamic bearing device will be described with reference to FIG. 9. When the motor stator 107 is supplied with power, a torque is generated by the rotor magnet 106, so that the rotor hub 105, the shaft 102 and the thrust flange 103 rotate as one body. As a result of this rotation, the dynamic pressure generating grooves 109a, 109b, 110a and 110b respectively give a pumping pressure to the oil 111 in the corresponding portions. Accordingly, radial bearings are formed at the area of the dynamic pressure generating grooves 109a and 109b for supporting the shaft 102 in the radial direction, while thrust bearings are formed at the area of the dynamic pressure generating grooves 110a and 110b for supporting the flange 103 in the thrust direction. Thus, the shaft 102 and the flange 103 rotate without contacting the bearing bore 101a and the thrust plate 104.

Since the sleeve 101 is made of a sintered metal, it has pores at 2-15% of volume (small spaces contained in the sintered metal). The pores include those called “tissue pores” existing inside the sintered metal and those called “surface pores” opening on the surface of the sintered metal. In a usual sintered metal, the surface pores and the tissue pores are communicated with each other. Although the sleeve 101 made of the sintered metal is impregnated with oil at a pressure lower than an atmospheric pressure in advance, the oil can pass through the sleeve 101 by moving in the pores. In the conventional example, the sleeve 101 is surrounded by the sleeve cover 114 so that the oil does not leak externally by moving through the pores.

According to the structure of the conventional hydrodynamic bearing device shown in FIG. 9, it is necessary to insert the sleeve 101 inside the sleeve cover 114 in the manufacturing process, which increases the number of man-hours in production. Since the sleeve 101 and the sleeve cover 111 are individual parts, the number of parts increases and the cost is also increased. In addition, if the sleeve 101 is inserted in the sleeve cover 114 with an inclined position, as shown in FIG. 10 in the insertion process, the axis of the bearing bore 101a is not kept perpendicular to the surface of the thrust plate 104. In this state, the gap of the thrust bearing or the radial bearing shown in FIG. 9 becomes uneven so that the shaft 102 cannot be supported in a stable manner. If the unevenness of the gap increases, the shaft 102 contacts the bearing bore 101a of the sleeve 101 and the bearing is seized up. The same problem can occur if the axis of the bearing bore 101a of the sleeve 101 is not precisely perpendicular to the opposed face of the thrust flange 103 fixed to the shaft 102.

When the shaft 102 rotates in the conventional hydrodynamic bearing described above, a hydraulic pressure within the range of 2-5 atmospheric pressures is generated by the radial dynamic pressure generating grooves 109a and 109b. If the oil is driven by this hydraulic pressure to flow in the pores of the sleeve 101, the hydraulic pressure is reduced to 70% of the above-mentioned pressure. As a result, stiffness of the radial bearing is also reduced to approximately 70%. Japanese Unexamined Patent Publication No. 2003-322145 discloses a method of covering the entire surface of the sleeve 101 with a coating layer that is not permeable to oil in order to prevent the oil from entering the pores of the sleeve 101. This method includes a step of forming the coating layer. Consequently, the method has many steps and a high cost.

An object of the present invention is to provide a hydrodynamic bearing device including a sleeve made of sintered metal that can prevent working fluid, such as oil, from leaking externally. A further object of the present invention is to prevent stiffness of the radial bearing from decreasing. Still yet another object of the present invention is to maintain an appropriate bearing gap of a thrust bearing or a radial bearing to ensure stable non-contact rotation.

A hydrodynamic bearing device according to the present invention includes a sleeve having a bearing bore, a shaft that is inserted in the bearing bore of the sleeve in relatively rotatable manner, a thrust flange that is provided to an end of the shaft, a thrust member that is opposed to the thrust flange, a dynamic pressure generating groove formed on the inner surface of bearing bore and is, a dynamic pressure generating groove formed on at least one of the opposed surfaces of the thrust flange and a thrust member, and working fluid filled in a gap between the shaft and the bearing bore, and between the thrust flange the thrust member. The sleeve is a sintered body that is obtained by sintering a sintering material containing at least one selected from a group containing iron, an iron alloy, copper and a copper alloy. Pores of the sintered body are independent pores in which neighboring pores are independent from each other, and a diameter of the independent pores is smaller than each of a width and a height of a crest portion of the radial dynamic pressure generating groove.

According to the present invention, since pores in the sleeve made of a sintered body are independent pores, working fluid does not enter the sleeve, and therefore the working fluid does not leak through the sleeve. In addition, since a diameter of the independent pores is smaller than each of a width and a height of a crest portion of the radial dynamic pressure generating groove, functions of the radial dynamic pressure generating groove is hardly deteriorated even if an independent pore exist in the crest portion.

According to another aspect of the present invention, the hydrodynamic bearing device includes a sleeve having a bearing bore, a shaft that is inserted in the bearing bore of the sleeve in relatively rotatable manner, a thrust flange that is provided to an end of the shaft, a thrust member that is opposed to the thrust flange, a radial dynamic pressure generating groove formed on the inner surface of bearing bore so as to work as a radial bearing and is, a thrust dynamic pressure generating groove formed on one of the opposed surfaces of the thrust flange and a thrust member so as to work as a thrust bearing, and working fluid filled in a gap between the shaft and the bearing bore, and between the thrust flange the thrust member. The sleeve is a sintered body that is obtained by sintering a sintering material containing at least one selected from a group containing iron, an iron alloy, copper and a copper alloy. At least one of sintered body forming conditions including a molding pressure the sintered body, sintering temperature and sintering period that are sintering conditions, and an average grain size of metal grains of the sintering material is selected so that a diameter of the independent pores becomes smaller than each of a width and a height of a crest portion of the radial dynamic pressure generating groove.

According to the present invention, at least one of sintered body forming conditions of the sleeve made of a sintered body including, the molding pressure of the sintered body, the sintering temperature, the sintering period and the average grain size of metal grains of the sintering material is selected so that a diameter of the independent pores becomes smaller than each of a width and a height of a crest portion of the radial dynamic pressure generating groove. Thus, pores of the sleeve become independent pores. Accordingly, working fluid does not enter the sleeve, and therefore the working fluid does not leak through the sleeve. In addition, since a diameter of the independent pores is smaller than each of a width and a height of a crest portion of the radial dynamic pressure generating groove, functions of the radial dynamic pressure generating groove is hardly deteriorated even if an independent pore exist in the crest portion.