DYNAMIC-PRESSURE GAS BEARING STRUCTURE, METHOD OF PRODUCING THE SAME AND METHOD OF USING IT

Abstract: The transverse cross section of a shaft (1) perpendicular to the axis O has a shape defined by closed curves having a plurality of maximum points (M1, M2, M3) at which distances from the axis O which is located at the center become maximum and a plurality of minimum points (N1, N2, N3) at which the distances from the axis become minimum. The transverse cross section has a groove-forming region in which at least one groove (1b1) is formed between maximum points M1 and M2. When a bearing (2) rotates counterclockwise relative to the shaft (1), the groove (1b1) is so arranged that the outer peripheral length (a1) in the forward direction of rotation is longer than the outer peripheral length (b1) in the reverse direction of rotation. There is thus obtained a dynamic-pressure gas bearing structure which prevents scuffing between the shaft and the bearing by shifting the shaft and the bearing into a noncontact state at a low rotational speed.

Description: BACKGROUND OF THE INVENTION The present invention relates to a bearing device using a dynamic-pressure bearing where a lubricant is sealed to the bearing surface and also relates to a spindle motor provided with the bearing device. In most of the spindle motors used in information equipment, ball. bearings have been employed for supporting a rotating body. However, the ball bearing has limitations on high-accurate rotation and high-speed rotation and has become an obstacle to the high-accurate and high-speed operation of information equipment. When, on the other hand, a rotating body is supported by a sliding bearing, very high accurate rotation is possible because the rotating body can be supported by a film of dynamic-pressure oil with non-contact, and the sliding bearing is suitable for high-speed rotation. A bearing device adopting the sliding bearing is disclosed, for example, in Japanese Patent Unexamined Publication No. 61-201916 on which the preamble of claims 1 and 4 is based. In this publication, axial and radial gaps are provided between a rotational shaft and a housing as a means of preventing axial and radial vibrations, and magnetic fluid is sealed in the gaps so that the rotational shaft can be rotatably supported. In addition, seal devices are provided on the opposite ends of the housing, and step-shaped bearing-force generating maintenance of the rotational accuracy is expected also during high-speed rotation, due to the employment of the dynamic pressure gas bearing. In the aforementioned radial gas bearing, a wedge-shaped clearance is formed by eccentricity of a shaft body in the bearing body, as described in "Gas Bearing" by Shinichi Togo, Kyoritsu Shuppan (1989). Pressure is generated since air is compressed when the air passes through this wedge-shaped clearance. Thus, it is possible to support the shaft body and the bearing body in a noncontact manner. A concrete structure of such a dynamic pressure gas bearing is described in Japanese Patent Publication No. 4-21844, for example. The structure described in this gazette is now described as a conventional dynamic pressure gas bearing structure. Fig. 18 is a sectional view of a principal part of the conventional dynamic pressure gas bearing structure. Referring to Fig. 18, a shaft body 31 is arranged in a hollow cylinder of a bearing body 32. A sectional shape of this shaft body 31 in the radial direction has such a shape that a plurality of grooves 31b in the form of substantially L-shaped notches are equally distributed and provided on the outer periphery of a circular cylinder 31a having a section of a substantially complete circle. In this conventional dynamic pressure gas bearing structure, the shaft body 31 is so structured as to rotate with respect to the bearing body 32. When the shaft body 31 rotates, air is caught in the grooves 31b provided on the outer periphery of the shaft body 31, and dynamic pressure is generated by combination of vortex motion of the caught air and wedge action with respect to the inner peripheral surface of the bearing body 32. Due to this dynamic pressure, it comes to that the shaft body 31 and the bearing body 32 are supported in the radial direction in a noncontact manner in the rotational operation thereof. In the conventional dynamic pressure gas bearing shown in Fig. 18, however, dynamic pressure cannot be efficiently generated between the shaft body 31 and the bearing body 32 in a low rotational speed area after rotating/starting the shaft body 31. Therefore, the shaft body 31 and the bearing body 32 cannot be shifted from a contact state to a noncontact state at a low rotational frequency. Thus, there has been such a problem that abrasion powder results from continuous contact between the shaft body 31 and the bearing body 32 up to a high rotational frequency shifting the same to the noncontact state, and galling is caused between the shaft body 31 and the bearing body 32 by the abrasion powder. Disclosure of the Invention The present invention is to provide a dynamic pressure gas bearing structure which can prevent abrading of a shaft body and a bearing body by shifting the shaft body and the bearing body to a non-contact state at a low rotational frequency and a method of manufacturing the same as well as a method of using the same. The present invention provides a dynamic pressure gas bearing structure according to claim 1. In the dynamic pressure gas bearing structure according to one aspect of the present invention, the cross-sectional shape of the shaft body perpendicular to its axis has plural maximal points and minimal points and the groove is formed on its outer periphery, whereby the space between the shaft body and the bearing body changes along the circumferential direction. Namely, the space between the shaft body and the bearing body becomes relatively narrow in the regions of the maximal points and relatively wide in the regions of the minimal points, and becomes wider in the region provided with the groove. The region provided with the groove has no dynamic pressure effect, but becomes a pressure state close to the atmospheric pressure. The dynamic pressure gas bearing starts rotating from a speed 0, and the centers of the shaft body and the bearing body are remarkably decentered until the shaft body and the bearing body shift from a contact state to a noncontact state. A taper consisting of the circumference between a maximal point present on a cross section of the shaft body and an edge of the groove forms such a region that its clearance is gradually reduced between the same and the inner surface of the bearing body with respect to the eccentric direction. If the length A of the taper is sufficient with respect to an air flow direction in the bearing clearance, the pressure in the clearance is increased by an effect of the wedge-shaped clearance, and the bearing body can be supported in a noncontact state with respect to the shaft body even at a relatively low speed. On the other hand, there has been such a problem that the effect of the wedge-shaped clearance cannot be sufficiently attained at a low speed and the bodies do not shift to the noncontact state unless a high speed is attained if the shaft body is an incomplete circle and the taper with respect to the rotational direction is shortened due to provision of the groove. According to the present invention, a dynamic pressure effect by the wedge-shaped clearance can be efficiently caused by relatively increasing the length A of the taper with respect to the rotational direction. Therefore, it is possible to float the bearing body with respect to the shaft body at a low rotational frequency, for bringing the same into the noncontact state. Thus, it is possible to suppress generation of abrasion powder caused by direct contact of the shaft body and the bearing body at a high speed in a rotation start time and a stop time. Due to this, a failure caused by abrasion of the shaft body and the bearing body can be suppressed, and it is possible to use the structure in clean environment not contaminating the surroundings since occurrence of dust is small. The dynamic pressure gas bearing structure according to the aforementioned one aspect can attain the intended object by being so used as to feed the air in the clearance which is defined when the shaft body and the hollow cylindrical bearing body are oppositely arranged from the first maximal point to the second maximal point through the groove forming region in relation to relative rotational motion of the shaft body and the bearing body. It is possible to suppress generation of abrasion powder caused by direct contact between the shaft body and the bearing body at a high speed in a rotation start time and a stop time. Due to this, a failure caused by abrasion of the shaft body and the bearing body can be suppressed, and it is possible to use the structure in clean environment not contaminating the surroundings since occurrence of dust is small. The dynamic pressure gas bearing structure according to the aforementioned another object can attain the intended object by being so used as to feed air in a clearance which is defined when the shaft body and the hollow cylindrical bearing body are oppositely arranged from a first minimal point to a second minimal point through the groove forming region in relation to relative rotational motion of the shaft body and the bearing body. The present invention further provides a dynamic pressure gas bearing structure according to claim 4. The clearance between the shaft body and the bearing body is changed along the groove, whereby a dynamic pressure effect can be effectively caused, and it is possible to float the bearing body with respect to the shaft body at a low rotational frequency for bringing the same into a noncontact state. Thus, it is possible to suppress generation of abrasion powder caused by direct contact between the shaft body and the bearing body at a high speed in a rotation start time and a stop time. Due to this, a failure caused by abrading of the shaft body and the bearing body can be suppressed, and it is possible to use the structure in clean environment not contaminating the surroundings since occurrence of dust is small. The dynamic pressure gas bearing structure according to the aforementioned still another aspect can attain the intended object by being so used as to feed air into the clearance which is defined when the shaft body and the hollow cylindrical bearing body are oppositely arranged from the boundary point E 2 side to the boundary point E 1 side through the groove(s) in relation to relative rotational motion of the shaft body and the bearing body. It is also possible to form a magnetic circuit and a control circuit of a motor into which the dynamic pressure gas bearing structures according to the aforementioned three aspects used in this manner are assembled. The dynamic pressure effect in the portion of the groove can be attained by changing angles of first and second side walls with respect to a bottom wall, i.e., by rendering the angle of the taper forming the groove asymmetric, whereby dynamic pressure opposable to the dynamic pressure effect by the aforementioned taper on the outer periphery, can be obtained. Even if the aforementioned relation does not hold in the taper on the outer periphery, therefore, it is possible to float the bearing body with respect to the shaft body at a low rotational frequency, for bringing the same into the non-contact state. The present invention further provides a method of manufacturing a dynamic pressure gas bearing structure according to claim 6. According to the methods of manufacturing dynamic pressure gas bearing structures according to the aforementioned two aspects, it is possible to obtain dynamic pressure gas bearing structures which can generate dynamic pressure by effects of wedge-shaped clearances, and can suppress occurrence of falling of the shaft bodies and the bearing bodies. Brief Description of the Drawings FIG. 1 is a sectional view of a principal part showing a dynamic pressure gas bearing structure of the present invention including a radial section of a shaft body; Fig. 2 is a diagram showing displacement of outer peripheral positions with respect to rotation angles at a time of rotating the shaft body about an axis in the dynamic pressure gas bearing structure shown in Fig. 1; Fig. 3 is a diagram for illustrating characteristics associated with formation of a shaft body of a structure according to the invention; Fig. 4 is a sectional view showing a shaft body in which at least two grooves are provided between adjacent maximal points, in a dynamic pressure gas bearing structure of the present invention; Fig. 5 is a diagram showing displacement of outer peripheral positions with respect to rotation angles at a time of rotating a shaft body about an axis in the dynamic pressure gas bearing structure shown in Fig. 4; Fig. 6 is a sectional view showing a shaft body in which grooves are provided at minimal points, in a dynamic pressure gas bearing structure of the present invention; Fig. 7 is a diagram showing displacement of outer peripheral positions with respect to rotation angles at a time of rotating a shaft body about an axis in a dynamic pressure gas bearing structure shown in Fig. 6. Fig. 8 is a sectional view of a shaft body in which grooves are provided, at maximal points in a dynamic pressure gas bearing structure of the present invention. Fig. 9 is a diagram showing displacement of outer peripheral positions with respect to rotation angles at a time of rotating a shaft body about an axis in the dynamic pressure gas bearing structure shown in Fig. 8. Fig. 10 is a sectional view of a shaft body having groove shapes defined by prescribed position vectors in a dynamic pressure gas bearing structure of the present invention. Fig. 11 is a diagram for a shaft body for defining shapes of grooves having taper shapes in a dynamic pressure gas bearing structure of the present invention. Fig. 12 is a diagram showing displacement of outer peripheral positions with respect to rotation angles at a time of rotating a shaft body about an axis in such case that maximal points and minimal points on the outer peripheral shape of the shaft body are present in short cycles and grooves include a plurality of positions to become the maximal points and the minimal points. Fig. 13 is a model diagram showing a method of measurement of a floating rotational frequency. Fig. 14 is a graph showing the relation between a vibration signal and a rotational frequency for illustrating the measurement of the floating rotational frequency. Fig. 15 is a schematic diagram showing one example of a dynamic pressure bearing assembly to which the dynamic pressure gas bearing structure of the present invention is applied. Fig. 16 is a schematic diagram showing another example of a dynamic pressure bearing assembly to which the dynamic pressure gas bearing structure of the present invention is applied. Fig. 17 is a schematic diagram showing a spindle motor whose rotor hub rotates, to which the dynamic pressure gas bearing structure of the present invention is applied. Fig. 18 is a sectional view of a principal part of a dynamic pressure gas bearing structure disclosed in Japanese Patent Publication No. 4-21844. Best Mode for Carrying Out the Invention Fig. 1 is a sectional view of a principal part of a dynamic pressure gas bearing structure in one embodiment of the present invention. Fig. 2 is a diagram showing displacement of outer peripheral positions with respect to rotation angles at a time of rotating a shaft body about an axis in the bearing structure of Fig. 1. Referring to Fig. 1 and Fig. 2, the dynamic pressure gas bearing structure of this embodiment comprises a shaft body 1, and a hollow cylindrical bearing body 2 which is opposed to the shaft body 1 with a clearance in the radial direction. A radial sectional shape (i.e., a cross-sectional shape perpendicular to an axis O) of the shaft body 1 has a shape 1a defined by a closed curve having a plurality of maximal points M 1 , M 2 and M 3 closest to the bearing body 2 so that their distances from this axis O are maximised about the axis O and a plurality of minimal points N 1 , N 2 and N 3 whose distances from the axis O are minimized. At least one groove 1b 1 is formed at least between adjacent maximal points M 1 and M 2 . Only one groove 1b 1 is formed between the adjacent maximal points M 1 and M 2 here, and hence only a region provided with this groove 1b 1 becomes a groove region. The bearing body 2 is so supported as to relatively rotate about the axis O in the anticlockwise direction (CCW direction) side in the Figure with respect to the shaft body 1. Assuming that a 1 represents the outer peripheral length from groove 1b 1 , and strictly speaking from an edge of the groove 1b 1 on the maximal point M 2 side to the maximal point M 2 and b 1 represents the outer peripheral length from an edge of the groove 1b 1 on the maximal point M 1 side to the maximal point M 1 side to the maximal point M 1 at this time, the relation of a 1 ≥ b 1 is satisfied. Namely, the outer peripheral length a 1 in the forward rotational direction has a dimension exceeding the outer peripheral length b 1 in the reverse rotational direction. Due to such positioning of the maximal points, the minimal points and the groove, the structure has a taper which is relatively long in the rotational direction with respect to the minimum clearance when the bearing body 2 is eccentric with respect to the shaft body 1. When air flows in a clearance defined between the shaft body 1 and the bearing body 2, therefore, dynamic pressure is efficiently generated by the taper which is relatively long with respect to the rotational direction. Thus, the shaft body 1 can be floated with respect to the bearing body 2 at a low rotational frequency. Therefore, galling of the shaft body 1 and the bearing body 2 resulting from abrasion powder caused by contact between the shaft body 1 and the bearing body 2 in a rotation start time and a stop time, for example, is suppressed. Respective grooves 1b 1 , 1b 2 and 1b 3 may be so arranged that the sum Σa (= a 1 + a 2 + a 3 ) of the outer peripheral lengths in the forward rotational direction exceeds the sum Σb (= b 1 + b 2 + b 3 ) of the outer peripheral lengths in the reverse rotational direction. So far as this relation is satisfied, the outer peripheral length a 2 of the groove 1b 2 in the forward rotational direction may be smaller than the outer peripheral length b 2 in the reverse rotational direction. Similarly, the outer peripheral length a 3 in the forward rotational direction may be smaller than the outer peripheral length b 3 in the reverse rotational direction. A cross section of the shaft body 1 perpendicular to the axis O before formation of the grooves 1b 1 , 1b 2 and 1b 3 is shown in Fig. 3. Referring to Fig. 3, the cross-sectional shape of this shaft body 1 is defined by δ and C. δ represents the radial distance between two virtual concentric circles 4a and 4b in such case that all points on the closed curve 1a defining the outline of the shaft body 1 before groove formation are between the two virtual concentric circles 4a and 4b and the difference of the radial space between the two virtual concentric circles 4a and 4b is minimized. C represents the mean value of the diameters of the two virtual concentric circles 4a and 4b. When a plurality of grooves 1b 1 and 1b 4 are provided between adjacent maximal points M 1 and M 2 as shown in Fig. 4 and Fig. 5, the outer peripheral lengths in the forward rotational direction and the reverse rotational direction are defined as follows: Referring to Fig. 4 and Fig. 5, the outer peripheral length in the forward rotational direction becomes the distance a 1 from an end portion of the groove 1b 4 closest to the maximal point M 2 to the maximal point M 2 along the outer periphery. The outer peripheral length in the reverse rotational direction becomes the distance b 1 from an end portion of the groove 1b 1 closest to the maximal point M 1 to the maximal point M 1 along the outer periphery. When the groove 1b 3 is provided in a portion forming the maximal point M 3 and the groove 1b 2 is provided between this groove 1b 3 and the maximal point M 2 , the outer peripheral length becomes the distance a 2 from an end portion of the groove 1b 2 on the groove 1b 3 side to an end portion of the groove 1b 3 on the groove 1b 2 side along the outer periphery, and the outer peripheral length in the reverse rotational direction becomes the distance b 2 from an end portion of the groove 1b 2 on the maximal point M 2 side to the maximal point M 2 along the outer periphery. When the grooves 1b 1 , 1b 2 and 1b 3 are present in portions forming the minimal points N 1 , N 2 and N 3 as shown in Figs. 6 and 7, the outer peripheral length in the forward rotational direction becomes the distance a 1 from an end portion of the groove 1b 1 on the maximal point M 2 side to the maximal point M 2 , for example, and the outer peripheral length in the reverse rotational direction becomes the distance b 1 from an end portion of the groove 1b 1 on the maximal point M 1 side to the maximal point M 1 along the outer periphery, for example. When the grooves 1b 1 , 1b 2 and 1b 3 are present in portions forming the maximal points M 1 , M 2 and M 3 as shown in Figs. 8 and 9, the outer peripheral length in the forward rotational direction becomes the distance a 1 from an end portion of the groove 1b 1 on the minimal point N 3 side to the minimal point N 3 along the outer periphery, for example, and the outer peripheral length in the reverse rotational direction becomes the distance b 1 from an end portion of the groove 1b 1 on the minimal point N 1 side to the minimal point N 1 along the outer periphery, for example. The aforementioned maximal points and minimal points may be maximal points and minimal points derived from the outer peripheral shape (Figs. 4, 6 and 8) of the shaft body 1 after groove part formation by a curve interpolation method such as a least squares method or a spline method, for example. When the grooves 1b 1 , 1b 2 and 1b 3 are provided at the portions providing the minimal points N 1 , N 2 and N 3 respectively as shown in Fig. 6 and Fig. 7, it is preferable that the relation between the sum of the outer peripheral lengths in the forward rotational direction and the sum of the outer peripheral lengths in the reverse rotational direction is: a 1 + a 2 + a 3 ≧ b 1 + b 2 + b 3 It is possible to float the bearing body 2 with respect to the shaft body 1 at a low rotational frequency by providing the grooves 1b 1 , 1b 2 and 1b 3 in the portions providing the maximal points M 1 , M 2 and M 3 as shown in Fig. 8 and Fig. 9. In this case, it is preferable that the relation between the sum of the outer peripheral lengths in the forward rotational direction and the sum of the outer peripheral lengths in the reverse rotational direction is: a 1 + a 2 + a 3 ≥ b 1 + b 2 + b 3 Fig. 10 is a sectional view of a shaft body and enclosing bearing body of a dynamic pressure gas bearing structure in accordance with another embodiment of the present invention. Referring to Fig. 10, the dynamic pressure gas bearing structure of this embodiment comprises a shaft body 1 and a hollow cylindrical bearing body 2 opposed to the shaft body 1 with a clearance in the radial direction, similarly to the aforementioned embodiment. Grooves 1b 1 , 1b 2 and 1b 3 are formed at least in one portion, e.g. in three portions, on the outer periphery of the shaft body 1 in a radial section of the shaft body 1. Particularly with reference to Figure 10, the number of grooves and the geometry of a majority of the grooves are inter-related. For a groove such as groove 1b 1 , a position vector r 1 from a central point O to a boundary point E 1 (boundary point between one end of the groove 1b 1 and the outer periphery of the shaft body 1) and a position vector r 2 from the central point O to a boundary point E 2 (boundary point between another end of the groove 1b 1 and the outer periphery of the shaft body 1) satisfy the following relation in at least the majority of the total number of the grooves where the central point O represents the axis in the cross-section: | r 2 | = k | r 1 | where k is a value greater than 1 such that air pressure can be exerted against the side wall of the or each such groove to cause air to be directed towards the bearing body to maintain the bearing body in spaced relationship to the shaft body when relative rotation occurs therebetween. Namely, when having three grooves as shown in the Figure, for example, the number of such grooves for which the position vector r 1 is larger than the position vector r 2 is at least two. The shape of each groove shown in Fig. 10 may be in a shape shown in Fig. 11. With reference to Figs. 10 and 11, description is now made with reference to the groove 1b 1 . This groove 1b 1 is formed between the maximal points M 1 and M 2 , and the bearing body 2 is so supported as to rotate about the axis O in the anticlockwise direction (CCW direction: i.e. the direction of (position vector r 1 ) - (position vector r 2 )) relatively with respect to the shaft body 1. Assuming that a 1 represents the outer peripheral length from an end portion (bound point E 1 ) of the groove 1b 1 on the maximal point M 1 side to the maximal point M 1 and b 1 represents the outer peripheral length from an end portion (boundary point E 2 ) of the groove 1b 1 on the maximal point M 2 side to the maximal point M 2 , a 1 < b 1 is satisfied at this time. Namely, the outer peripheral length a 1 of the groove 1b 1 in the forward rotational direction has a dimension less than the outer peripheral length b 2 in the reverse rotational direction. When the shapes of the respective grooves 1b 1 , 1b 2 and 1b 3 satisfy the aforementioned relation, the relation between the sum Σa of the outer peripheral lengths in the forward rotational direction and the sum Σb of the outer peripheral lengths in the reverse rotational direction may be arbitrary relation. The shape of the groove 1b 1 is defined by a bottom wall 1b A and first and second side walls 1b B and 1b C . The first side wall 1b B reaches the boundary point E 2 from one end of the bottom wall 1b A , and the second side wall 1b C reaches the boundary point E 1 from another end of the bottom wall 1b A . This groove 1b 1 has such a taper shape that the distance (i.e., opening dimension) between the first and second side walls 1b B and 1b C reduces toward the bottom wall 1b A from the outer peripheral side of the shaft body 1. The relation between an angle (taper angle)  1 formed by a virtual plane 1b D extending from one end of the bottom wall 1b A along the bottom wall 1b A and the first side wall 1b B and an angle (taper angle)  2 formed by a virtual plane 1b E extending from another end of the bottom wall 1b A along the bottom wall 1b A and the second side wall 1b C is:  1 >  2 where  1 =tan -1 (d/w 1 )  2 = tan -1 (d/w 2 ) d = (d 1 + d 2 )/2 In case of relatively short tapers with respect to the rotational direction as hereinabove described in relation to the structure having the relation between the maximal points, the minimal points and the grooves in this manner, dynamic pressure cannot be efficiently generated from a low rotational frequency even if air flows in the clearance formed between the shaft body 1 and the bearing body 2, and there is the possibility of causing abrasion powder due to contact between the shaft body 1 and the bearing body 2 in a rotation start time and a stop time, for example. Due to formation of the groove 1b 1 having the taper shape in the aforementioned mode, a portion of the groove 1b 1 having a large taper angle effectively supplies air into the bearing clearance due to communication with the exterior of the bearing, and a portion having a small taper angle generates dynamic pressure by an effect of a wedge-shaped clearance with air supplied from the exterior by the taper. It is conceivable that, when the tapers on both sides of the groove 1b 1 are equal to each other, differentiation of the function of the aforementioned groove is not sufficiently performed, and does not effectively act with occurrence of turbulence in flow of air communicating with the outside air and flow of air caused by dynamic pressure generation in the groove. When the relation between the taper angles is reversed, no dynamic pressure is generated in the groove part since the direction where the wedge-shaped clearance in the groove narrows and the direction where the air flows become reversed, and the aforementioned problem of not floating at a low rotational frequency arises since generation of dynamic pressure is more dominant in the taper portion on the outer periphery than the groove part. If the shapes of the respective grooves 1b 1 , 1b 2 and 1b 3 satisfy the aforementioned relation, the relation between the sum Σa of the outer peripheral lengths in the forward rotational direction and the sum Σb of the outer peripheral lengths in the reverse rotational direction may be arbitrary direction. When maximal points M and minimal points N on the outer peripheral shape of the shaft body 1 are present in short cycles and a groove 1b is therefore formed over a plurality of portions to become the maximal points M and the minimal points N as shown in Fig. 12, the outer peripheral length in the forward rotational direction becomes the distance between an edge E and the maximal point M in case of including no minimal point N between adjacent maximal points M with respect to the edge E of the groove, and becomes the distance between the minimal point N and the maximal point M if any minimal point N is present. Examples of the present invention are now described on the basis of the drawings. Example 1 In a dynamic pressure bearing comprising a circular cylindrical shaft body and a hollow cylindrical bearing body, a cylinder substantially circular in cross-section of a section having m maximal points as shown in Fig. 1 and Fig. 2 was prepared by employing a centerless grinder. It was assumed here that m = 3, and the ratio of δ/C defining the shape of the circular cylinder section described with reference to Fig. 3 was set at 1 x 10 -4 . Silicon nitride-based ceramics was employed for the shaft body 1 and the bearing body 2. The silicon nitride-based ceramics was prepared as follows: Material powder of Si 3 N 4 having a mean particle diameter of 0.3 µm, particle size distribution of 3σ = 0.20 µm, an α crystallization ratio of 96.5 % and an oxygen content of 1.4 weight % was prepared. Wet mixing was performed in ratios of 90 weight % of this material powder of Si 3 N 4 , 4 weight % of Y 2 O 3 powder having a mean particle diameter of 0.8 µm, 3 weight % of Al 2 O 3 powder having a mean particle diameter of 0.5 µm, 1 weight % of AlN powder having a mean particle diameter of 1.0 µm, and 2 weight % of MgO powder having a mean particle diameter of 0.5 µm in ethanol for 100 hours through a ball milling by using nylon ball. Thereafter mixed powder obtained by drying was CIP (cold isostatic pressing)-formed with pressure of 3000 kgf/cm 2 . The obtained compact was held in a nitrogen gas atmosphere of 1 atmospheric pressure at a temperature of 1450°C for six hours. Further, the compact was primarily sintered at a temperature of 1550°C for three hours. The obtained sintered body was secondarily sintered at a temperature of 1600°C in a nitrogen gas atmosphere of 1000 atmospheric pressure for one hour. The silicon nitride-based sintered body obtained in this manner contained crystal grains whose linear density per length of 30 µm was at least 35, and the volume ratio of its grain boundary phase was not more than 15 volume %. Further, the silicon nitride-based sintered body contained pores whose maximum diameter was not more than 20 µm, and the content of the pores was not more than 3 %. With respect to the obtained substantially circular shape grooves 1b 1 , 1b 2 and 1b 3 were prepared in prescribed positions by machining, for preparing the shaft body 1. The groove working on this surface was executed as follows: While the case of vertically using the bearing body 2, rendering the shaft body 1 and the bearing body 2 a fixed shaft and a rotator, and rendering the rotational direction on the CCW direction side here, this also applies to the case of transversely using the bearing body, and the positions of groove working may be reversed from this Example in case of using the bearing body while rotating the same in the clockwise direction (CW direction) in the figure. Marking indicating a measurement start position was performed on a position of an upper end surface of the circular cylinder forming the shaft body 1 close to a side surface, the circular cylinder was rotated for measuring a profile of a section, and the profile shown in Fig. 2 was obtained, for example. However, the grooves 1b 1 , 1b 2 and 1b 3 were not yet formed at this point of time, and the sectional profile becomes a profile with no displacement of positions by the grooves in Fig. 2. In the profile shown in Fig. 2, the axis of abscissas shows rotation angles at times of rotating the circular cylinder from the marking position, and the axis of ordinates shows displacement of the positions with respect to the same. The positions of the grooves were so decided that the direction where the angle increased was the rotational direction of the shaft body 1 and the grooves were on sides of larger angles than the maximal points with respect to the rotational direction. Namely, the positions of the respective grooves were so decided that the groove 1b 1 , the groove 1b 2 and the groove 1b 3 were on the sides of larger angles than the maximal point M 1 , the maximal point M 2 and the maximal point M 3 respectively. Groove working was performed with reference to the marking position made on the circular cylinder for preparing the shaft body 1. While this groove formation was executed by cutting work, the actual working method is not particularly important. These grooves 1b 1 , 1b 2 and 1b 3 were so arranged that the sum (a 1 + a 2 + a 3 ) of the outer peripheral lengths in the forward rotational direction was in excess of the sum (b 1 + b 2 + b 3 ) of the outer peripheral lengths in the reverse rotational direction when the bearing 2 rotated relatively in the CCW direction side with respect to the shaft body 1, as hereinabove described. The current ratio of the depth to the diameter of the grooves was set at 0.01. In this bearing body structure shown in Fig. 1, rotating the bearing body 2 in the CCW direction side with respect to the shaft body 1 was regarded as an inventive sample, and rotating the same in the CW direction side was regarded as a comparative sample 1, for measuring floating rotational frequencies. The bearing body structure shown in Fig. 18 was also similarly prepared, and rotating the bearing body 32 relatively in the CCW direction side with respect to the shaft body 31 was regarded as a comparative example 2 while rotating the same in the CW direction side was regarded as a comparative sample 3 for measuring floating rotational frequencies. As to this floating rotational frequency, measurement was made in the following manner with a floating rotational frequency measuring apparatus shown in Fig. 13. Referring to Fig. 13, the bearing body 2 was first rotated at several 1000 rpm with respect to the shaft body 1. Thereafter vibration and a rotational frequency up to a time when the bearing body 2 naturally stopped were detected by a vibration sensor 9 and a rotational speed sensor 5 respectively, and information thereof was monitored by an oscilloscope 7 through amplifiers 6 and 8. A rotational speed (rotational frequency) R at a time when a floating support effect by dynamic pressure disappeared and the level of a vibration signal rapidly increased due to contact between the bearing body 2 and the shaft body 1 was regarded as the floating rotational frequency as shown in Fig. 14. Results thereof are shown in the following Table 1. Inventive Sample Comparative Example 1 Comparative Sample 2 Comparative Sample 3 Floating Rotational Frequency [rpm] 350 6000 2600 3500 First it was proved that the floating rotational frequency remarkably reduced in the inventive sample rotating the bearing body 2 in the CCW direction with respect to the shaft body 1 as compared with the comparative sample 1 rotating the bearing body 2 in the CW direction. It was further proved that the floating rotational frequency remarkably reduced in the inventive sample also as compared with the comparative samples 2 and 3 shown in Fig. 18. This is described as follows: In the bearing structure of the comparative samples 2 and 3 shown in Fig. 18, the shaft body 31 is in a structure merely arranging the grooves 31b with respect to the outer peripheral shape of the substantially complete circle 31a. Dynamic pressure caused between the shaft body 31 and the bearing body 32 in low-speed rotation is not sufficiently generated since flow of air is disturbed due to irregularity by the grooves 31b on the surface of the shaft body 31. In the inventive sample, on the other hand, not only the outer peripheral shape has the grooves 1b 1 , 1b 2 and 1b 3 butalso the maximal points M 1 , M 2 and M 3 and the minimal points N 1 , N 2 and N 3 as shown in Fig. 1. The tapers formed by the maximal points and the grooves are relatively long on the rotational direction sides along the maximal points. It has been conceivably possible to thereby float the shaft body with respect to the bearing body at a low rotational frequency for bringing the same into a noncontact state. Further, the positions for working the grooves were changed in Fig. 1, the ratio of the outer peripheral length a 1 in the forward rotational direction to the outer peripheral length b 1 in the reverse rotational direction was changed, and floating rotational frequencies as to the case of a 1 /b 1 ≧ 1 and the case of a 1 /b 1 < 1 were measured. Results thereof are shown in the following Table 2. a/b ≧ 1 a/b < 1 Floating Rotational Frequency [rpm] 420 2800 From the results of Table 2, the floating rotational frequency was low, and a result of start/stop endurance was also excellent in the inventive sample of a 1 /b 1 ≧ 1 as compared with the comparative sample of a 1 /b 1 < 1. In the comparative sample of a 1 /b 1 < 1, on the other hand, sufficient dynamic pressure necessary for floating was not obtained in a low rotational speed area, and it is conceivable that the shaft body 1 and the bearing body 2 directly slide up to a high speed to cause friction damage as a result. Example 2 Then in the bearing body structure shown in Fig. 1, floating rotational frequencies in case of changing the ratio of the sum Σa (= a 1 + a 2 + a 3 ) of the outer peripheral lengths in the forward rotational direction to the sum Σb (= b 1 + b 2 + b 3 ) of the outer peripheral lengths in the reverse rotational direction were measured. Results thereof are shown in the following Table 3. Σa/Σb ≧ 1 Σa/Σb < 1 Floating Rotational Frequency [rpm] 450 3200 From the results of Table 3, it was proved that the floating rotational frequency remarkably reduced and a start/stop endurance test was also excellent in the inventive sample of Σa/Σb ≧ 1, as compared with the comparative sample of Σa/Σb < 1. Example 3 In a shaft body 1 having a substantially circular cross-section with at least four maximal points, grooves were formed in three portions in accordance with the aforementioned Examples 1 and 2, and floating rotational frequencies were measured. That prepared by deciding the positions of the grooves so that the sum Σa of the outer peripheral lengths in the forward rotational direction was larger than the sum Σb of the outer peripheral lengths in the reverse rotational direction and performing working in relation to the rotation component of the circumference defined in the above was regarded as an inventive sample. On the other hand, that prepared by deciding the positions of the grooves so that the sum Σa of the outer peripheral lengths in the forward rotational direction was smaller than the sum Σb of the outer peripheral lengths in the reverse rotational direction and performing working was regarded as a comparative sample. Respective floating rotational frequencies in case of setting the numbers of maximal points at 5, 9 and 23 as to these inventive sample and comparative sample respectively were measured. Results thereof are shown in the following Table 4. m = 5 m = 9 m = 23 Inventive Sample [rpm] 400 420 380 Comparative Sample [rpm] 4500 5000 5200 From the results of Table 4 it was proved possible to reduce the floating rotational frequency according to the present invention even if the number of the maximal points increases. Example 4 Study was made on floating rotational frequencies as to the case of setting δ/C defined in relation to the outer periphery of the shaft body 1 with reference to Fig. 3 at 1 x 10 -5 and 4 x 10 -5 . Bearing structures having shaft bodies 1 provided with these respective δ/C and subjected to groove working in accordance with the inventive samples of the aforementioned Examples 1 and 2 were regarded as inventive samples. Further, bearing structures having shaft bodies 1 provided with the respective δ/C and subjected to working of grooves at random were regarded as comparative samples. As to measurement of the floating rotational frequencies, 100 samples were prepared for each bearing structure, the shaft bodies 2 of each bearing structure were rotated in the CCW direction with respect to the shaft bodies 1, and the ratio of those having floating rotational frequencies of not more than 2000 rpm was obtained. A floating rotational frequency of not more than 2000 rpm is a rotational frequency causing no abrasion by contact until the bearing body 2 floats with respect to the shaft body 1. Results thereof are shown in the following Table 5. Inventive Sample Comparative Sample Roundness 1 × 10 -5 100% 80% Roundness 4 × 10 -5 100% 50% From the results of Table 5, 80 % exhibits floating rotational frequencies of low values when the roundness of the circular cylinder is excellent (1 x 10 -5 ), while the ratio becomes 50 % when the roundness is inferior (4 x 10 -5 ). In the inventive samples, on the other hand, it is possible to control the floating rotational frequencies to low values of not more than 2000 rpm by 100 %. From these results, it is understood that when groove working is performed in accordance with the inventive samples of Examples 1 and 2, floating rotational frequencies with respect to specific directions can be reduced even if working accuracy of the shaft bodies 1 is low, and hence it is industrially useful too. Example 5 As shown in Fig. 8 and Fig. 9, a cylinder with a substantially circular cross-section having m maximal points was prepared. Each combination of a shaft body 1 and a bearing body 2 in this case was made of silicon nitride ceramics, and the silicon nitride ceramics was assumed to be as follows: This silicon nitride ceramics was obtained by forming mixed powder prepared by adding 0.02 weight % of Fe having a mean particle diameter of 1.0 µm to mixed powder of 91 weight % of commercially available Si 3 N 4 powder (α crystallization ratio: 95 %, mean particle diameter: 0.5 µm), 4.5 weight % of Y 2 O 3 powder having a mean particle diameter of 0.6 µm and 4.5 weight % of Al 2 O 3 powder having a mean particle diameter of 0.7 µm (blending volume ratio of a sintering assistant in this case becomes 7.0 volume % assuming that theoretic densities of Si 3 N 4 , Y 2 O 3 and Al 2 O 3 are 3.4 g/cm 3 , 5.0 g/cm 3 and 4.0 g/cm 3 respectively), performing heat treatment for two hours under such conditions that pressure of N 2 was 380 torr and a temperature was 1200°C, and thereafter performing sintering under nitrogen normal pressure at 1700°C for three hours. In the silicon nitride-based sintered body thus obtained, the porosity was 2.1 %, while the maximum diameter of the pores was 30 µm, and the grain boundary phase was 7 volume %. When areas of all silicon nitride crystal grains present in an arbitrary two-dimensional section were measured, the total area occupied by silicon nitride crystal grains in an area of 0.1 to 10 µm 2 was 77 % of the silicon nitride crystal grain total area, and the number ratio occupied by silicon nitride crystal grains having aspect ratios of 2 to 10 in all silicon nitride crystal grains was 35 %. With respect to the obtained circular cylinder, grooves were formed by machining. The ratio of the depth of each groove to the diameter (C defined in Fig. 3) at this time was set at 0.01, and the number of the grooves was set at 3. Respective grooves 1b 1 , 1b 2 and 1b 3 are formed in positions of the shaft body 1 to become maximal points M 1 , M 2 and M 3 . These maximal points M 1 , M 2 and M 3 may be maximal points M 1 , M 2 and M 3 which can determine the outer peripheral shape of the shaft body 1 other than the groove forming region by the least squares method. Those rotating the bearing body 2 in the CCW direction with respect to the shaft body 1 in the bearing structure shown in Fig. 8 were regarded as inventive samples and that rotating the bearing body 2 in the CW direction with respect to the shaft body 1 in the bearing structure shown in Fig. 1 was regarded as a comparative sample, for measuring respective floating rotational frequencies. Results thereof are shown in the following Table 6. No. 1 is that of a taper length Σa = Σb, and No. 2 is that of Σa > Σb. Inventive Sample Comparative Sample No. 1 No. 2 Floating Rotational Frequency [rpm] 400 230 6000 From the results of Table 6, it was proved that remarkably low floating rotational frequencies can be obtained in the inventive samples as compared with the comparative sample. As to this, it is conceivable that the tapers from the minimal points to the groove edges are sufficiently long and dynamic pressure is efficiently generated in the inventive samples, while no sufficient dynamic pressure was generated and the contact rotational frequency increased since the taper contributing to generation of dynamic pressure was short in the comparative sample. On the other hand, that rotating the bearing body 2 in the CW direction with respect to the shaft body 1 in the bearing structure shown in Fig. 8 were regarded as an inventive sample, and its floating rotational frequency was measured. Comparative samples were prepared from the comparative sample of Table 6 and No. 2. Results thereof are shown in the following Table 7. No. 1 No. 2 Comparative Sample Floating Rotational Frequency [rpm] 400 2400 6000 From the results of Table 7, it was proved that a low floating rotational frequency is attained in whichever one of the CCW direction and the CW direction the bearing body 2 is rotated with respect to the shaft body 1 when the structure is the bearing structure shown in Fig. 8 and the taper lengths Σa and Σb on the circumference satisfy Σa = Σb through the grooves. This is conceivably because dynamic pressure generation by an effect of a wedge-shaped clearance is effectively performed since the taper lengths are sufficient with respect to any rotational direction of CW and CCW in the inventive sample. Example 6 Study was made on floating rotational frequencies in case of setting δ/C defined in relation to the shaft body 1 with reference to Fig. 3 at 1 x 10 -5 and 4 x 10 -5 . Those having the respective δ/C and rotating the bearing body 2 with respect to the shaft body 1 in the CCW direction in the bearing structure shown in Fig. 8 were regarded as inventive samples. Further, those having the respective δ/C and having shaft bodies 1 working grooves at random were regarded as comparative samples. As to measurement of the floating rotational frequencies, 100 samples were prepared for each bearing structure, the bearing bodies 2 of each bearing structure were rotated, and the ratio of those having floating rotational frequencies of not more than 2000 rpm was obtained. Results thereof are shown in the following Table 8. Inventive Sample Comparative Sample Roundness 1 × 10 -5 100% 80% Roundness 4 × 10 -5 100% 50% From the results of Table 8, it was possible to obtain floating rotational frequencies of not more than 2000 rpm in all bearing structures in the inventive samples in relation to any δ/C. Thus, it was proved that the floating rotational frequency can be reduced in the bearing structure shown in Fig. 8 even if working accuracy of the shaft body 1 is low, and it is industrially useful too. Example 7 In a dynamic pressure bearing of combination of a circular cylindrical shaft body 1 and a hollow cylindrical bearing body 2, a circular cylinder of substantially a circular cross-section having m maximal points was prepared as shown in Fig. 6. With respect to the obtained circular cylinder, grooves were formed by machining. The ratio of the depth of the grooves to the diameter (C defined in Fig. 3) at this time was set at 0.01, and the number of the grooves was set at three. In inventive samples, respective grooves 1b 1 , 1b 2 and 1b 3 are formed in positions to define minimal points N 1 , N 2 and N 3 . The minimal points N 1 , N 2 and N 3 coincide with minimal points obtained as a result of curve-interpolating an outer peripheral shape other than the grooves with the least squares method. In this shape, the grooves are so formed that the sum Σa of outer peripheral lengths a 1 , a 2 and a 3 in the forward rotational direction and the sum Σb of outer peripheral lengths b 1 , b 2 and b 3 in the reverse rotational direction are Σa = Σb (No. 1), or Σa > Σb (No. 2). Those rotating the bearing body 2 in the CCW direction with respect to the shaft body 1 in the bearing structure shown in Fig. 6 were regarded as inventive samples and that rotating the bearing body 2 in the CW direction with respect to the shaft body 1 in the bearing structure shown in Fig. 1 was regarded as a comparative sample for measuring floating rotational frequencies. Results thereof are shown in the following Table 9. Inventive Sample Comparative Sample No. 1 No. 2 Floating Rotational Frequency [rpm] 320 190 6000 From the results of Table 9, it was proved that the inventive samples exhibit low floating rotational frequencies as compared with the comparative sample. This is conceivably because dynamic pressure is effectively generated since tapers from the groove edges to the maximal points are sufficiently long in the inventive samples, while sufficient dynamic pressure was not generated but the contact rotational frequency increased since tapers contributing to generation of dynamic pressure were short in the comparative sample. On the other hand, that rotating the bearing body 2 in the CW direction with respect to the shaft body 1 in the bearing structure shown in Fig. 6 was regarded as an inventive sample, and its floating rotational frequency was measured. Comparative samples were prepared from the comparative sample of Table 6 and No. 2. Results thereof are shown in the following Table 10. No. 1 No. 2 Comparative Sample Floating Rotational Frequency [rpm] 320 2350 6000 From the results of Table 10, it was proved that the bearing structure shown in Fig. 6 in which the taper lengths on the outer periphery are Σa = Σb can obtain a low floating rotational frequency in whichever one of the CCW direction and the CW direction the bearing body 2 is rotated with respect to the shaft body 1. This is conceivably because dynamic pressure generation by an effect of a wedge-shaped clearance is effectively performed since the lengths of tapers are sufficient with respect to any rotational direction of CW and CCW in the inventive sample. Example 8 Study was made on floating rotational frequencies in case of setting δ/C defined in relation to the shaft body 1 with reference to Fig. 3 at 1 x 10 -5 and 4 x 10 -5 . Those having the respective δ/C and having shaft bodies 1 subjected to groove working as shown in Fig. 6 were regarded as inventive samples. Further, those having the respective δ/C and having shaft bodies 1 working grooves at random were regarded as comparative samples. As to measurement of the floating rotational frequencies, 100 samples were prepared for each bearing structure, the bearing bodies 2 of each bearing structure were rotated with respect to the shaft bodies 1, and the ratio of those having floating rotational frequencies of not more than 2000 rpm was obtained. Results thereof are shown in the following Table 11. Inventive Sample Comparative Sample Roundness 1 × 10 -5 100% 80% Roundness 4 × 10 -5 100% 50% From the results of Table 11, it was possible to obtain floating rotational frequencies of not more than 2000 rpm in all bearing structures in relation to either δ/C in the inventive samples. Thus, it was proved that floating rotational frequencies with respect to specific directions can be reduced even if the working accuracy of th shaft bodies 1 is low in the inventive samples, and these are industrially useful too. Example 9 In relation to the silicon nitride shaft body and the bearing body prepared in Example 5, working of grooves was executed so that distances (OE 1 and OE 2 ) from an axis O of the shaft body 1 to respective edges E 1 and E 2 of the grooves were OE 1 < OE 2 in such case that air in a bearing clearance formed when the shaft body 1 and the bearing body 2 were oppositely arranged as shown in Fig. 10 flew in the direction from the edge E 2 to the edge E 1 . The grooves were worked to have the shapes shown in Fig. 10, by forming the grooves in the longitudinal direction of the axis by cutting and grinding a circumferential surface including edge portions on single sides of the grooves with free abrasive grains or the like, for example. Inventive samples were prepared from those satisfying the aforementioned conditions and forming grooves in positions corresponding to the comparative sample of Example 2, and roundness of the shaft bodies 1 was set at 1 x 10 -5 and 4 x 10 -5 . The comparative samples 1 of Example 1 were employed as domparative samples. Roundness Inventive Sample Comparative Sample Floating Rotational Frequency 1 × 10 -5 250 rpm 2800 rpm 4 × 10 -5 620 rpm 4500 rpm From results of Table 12, floating rotational frequencies are remarkably low in the inventive samples rendering the shapes of the grooves OE 1 > OE 2 as compared with the comparative samples 1 in which taper angles of the grooves are symmetrical. Thus, it was proved that the floating rotational frequency can be reduced by making the groove shapes as described above even if the groove positions are the worst in Example 2, regardless of the forming positions of the grooves. Example 10 In relation to the silicon nitride shaft body and the bearing body prepared in Example 5, grooves having the taper shape shown in Fig. 11 were formed on the shaft body 1. The grooves were so formed that taper angles  1 and  2 were  1 >  2 in such case that air in a bearing clearance formed when the shaft body 1 and the bearing body 2 were oppositely arranged flew from the maximal point M 2 to the maximal point M 1 side through the groove 1b 1 , and this was employed to prepare inventive samples. Further, comparative samples were prepared from those obtained by forming grooves so that taper angles of the grooves were  1 ≦  2 . In both of the inventive samples and the comparative samples, the grooves were formed in positions corresponding to the comparative samples of Example 2, roundness of the shaft bodies was set at 1 x 10 -5 and 4 x 10 -5 , and the taper angles  1 and  2 were changed as in the following Table 13 for measuring floating rotational frequencies. Results thereof are shown in the following Table 14. No. 1 No. 2 No. 3 No. 4 No. 5  1 90° 60° 45° 30° 45°  2 30° 45° 15° 90° 45° Roundness No. 1 No. 2 No. 3 No. 4 No. 5 Floating Rotational Frequency [rpm] 1 × 10 -5 330 270 240 2600 2800 4 × 10 -5 510 560 610 3700 4100 From the results of Table 14, the floating rotational frequencies were remarkably low in the inventive samples rendering the taper shapes of the grooves  1 >  2 as compared with No. 5 of the comparative sample in which the taper angles of the grooves were symmetrical, and it was proved that the floating rotational frequency can be reduced by making the groove shapes as described above even if the groove positions are the worst in Example 2, regardless of the forming positions of the grooves. Example 11 Methods of using dynamic pressure gas bearing structures according to inventive examples are now described with reference to Fig. 15 and Fig. 16. In each of dynamic pressure bearing structures shown in Fig. 15 and Fig. 16, radial supporting consists of a shaft body 1 and a bearing body 2 forming the inventive dynamic pressure bearing structure, an end is so fixed that the shaft body 1 is perpendicular to a base 106, and the bearing body 2 is rotatably set on the shaft body 1. In the dynamic pressure bearing structure shown in Fig. 15, thrust supporting is formed by an end surface of the bearing body 2 and a dynamic pressure thrust bearing 103 having a spiral groove 103a opposed thereto. In the dynamic pressure bearing structure shown in Fig. 16, thrust supporting is formed by a ring-shaped permanent magnet 104 which is fixed to an end surface of the bearing body 2 and a ring-shaped permanent magnet 105 which is fixed to a base 106 to be coaxial with the shaft body 1 oppositely to this ring-shaped permanent magnet 104. Both of these ring-shaped permanent magnets 104 and 105 are magnetized in the axial direction, and so arranged that magnetic poles thereof are in repulsive directions to each other. If the aforementioned dynamic pressure gas bearing structure of the present invention is used only in the CCW direction, the shaft body 1 is so set that the relations between the outer peripheral tapers of the grooves and the shaft body and the shapes of the grooves are those in Figs. 1, 6, 8, 10 and 11 as viewed from an end surface opposite to the fixed end. At this setting, the structure may be fixed in a regular direction with the shaft body 1 subjected to groove working while previously defining the rotational direction by the method heretofore described in detail, or the grooves may be formed while providing reversibility to the shaft body 1, i.e., so that the grooves are present in sliding parts with the bearing body 2 whichever end portion of the shaft body 1 is fixed, for confirming the shapes before setting the shaft body 1 and deciding the fixing direction. It is possible to hold the shaft body 1 and the bearing body 2 in a noncontact manner from low-speed rotation by deciding the fixing direction of the shaft body 1 in this manner and rotating/using the shaft body 1 and the bearing body 2 in a prescribed rotational direction, and the time and the speed for direct contact sliding of the shaft body 1 and the bearing body 2 following starting/stopping of the dynamic pressure gas bearing structure can be reduced. Consequently, it is possible to prevent damage of the bearing structure caused by abrasion. Example 12 Fig. 17 is a diagram schematically showing a spindle motor whose rotor hub rotates. Referring to Fig. 17, a motor is formed by a motor rotor magnet 108 provided on an inner peripheral side of a hub 107 which is on a rotating side and a stator coil 109 provided on a fixed side, so that the hub 107 can be rotated at a high speed. The stator coil 109 is fixed to a base 110, and a shaft body 1 is similarly fixed as a fixed shaft coaxially with the stator coil 109. A bearing body 2 is fixed to the hub 107 by either method of shrinkage fitting or adhesion. The shaft body 1 and the bearing body 2 form a radial bearing. Grooves 1b are formed on the shaft body 1 in a longitudinal direction of its axis, whereby a dynamic pressure effect is attained as a result of interaction of outer peripheral tapers of the shaft body 1 and the groove 1b portions, and the bearing body 2 is rotated/supported in a noncontact manner in the radial direction at a rotational frequency in excess of a floating rotational frequency. While a thrust bearing supporting a load in a thrust direction is assembled into an end of the hub 107 under the bearing body 2, ring-shaped permanent magnets 104 and 105 are provided on a lower end of the bearing body 2 and the base to repulsively support each other with a proper space in this Example. In the motor of this structure, no 1/2 whirl takes place even in high-speed rotation of at least 30000 rpm due to an aerodynamic effect of the grooves 1b. When a control circuit or the like is so set that this motor rotates only in the CCW direction, the shaft body 1 is formed as shown in Figs. 1, 6, 8, 10 and 11 as viewed from an end surface opposite to its fixed side. Therefore, the floating rotational frequency can be controlled low even if starting/stopping of rotation of this motor is repeated, whereby it is possible to obtain a motor which is capable of high-speed rotation in a simple structure, has high rotational accuracy, and causes no abrasion damage. As to the grooves in all embodiments and Examples of the present invention described above, it is preferable that at least single ends communicate with the outside air. This is because at least single ends of the grooves must communicate with the outside air for causing axial air flows. The outer peripheral tapers of the shaft body, which are defined by the edges of the grooves and the lengths up to the maximal points adjacent thereto, are defined by the lengths of the minimal points and the maximal points when the minimal points are present therebetween. This is a unified concept in this specification, regardless of the number of the grooves and the number of the maximal values of the closed curve forming the shaft. The Examples and embodiments disclosed this time must be regarded as illustrative in all points and not restrictive. The scope of the present invention is shown not by the scope described in the above but by claims, and it is intended that all changes within the meaning and scope equal to claims are included. As hereinabove described, a low floating rotational frequency can be implemented in the dynamic pressure gas bearing structure according to the present invention, and hence occurrence of galling of the shaft body and the bearing body is suppressed also when the same is employed for an HDD or the like, for example. Industrial Availability The present invention relates to a dynamic pressure gas bearing structure and a method of manufacturing the same as well as a method of using the same, and is advantageously applicable particularly to a dynamic pressure gas bearing structure comprising a circular cylindrical shaft body having a groove on its outer periphery and a hollow cylindrical bearing body opposed to the shaft body with a space in the radial direction.

Claims: A dynamic pressure gas bearing structure comprising a grooved shaft body (1) having an axis (0) and having at least one axially-extending groove formed therein, a hollow cylindrical bearing body (2) opposed to said shaft body (1) with an annular clearance therebetween, the structure being characterized by the shaft body (1) having where no groove is formed, a substantially-cylindrical external surface with a substantially-circular cross-section of non-uniform gradually changing, radius perpendicular to the axis (0) , the cross-section of the shaft body (1) having maximal points (M) at which distances of the shaft body surface from the shaft axis (0) are maximized and minimal points (N) at which distances of the shaft body surface from the shaft axis (0) are minimised, the minimal points (N) being interposed with the maximal points (M), the at least one groove (a) being formed between adjacent, first and second, maximal points at a surface distance A measured from one adjacent maximal point to an adjacent edge of the groove and at a distance B measured from the other adjacent maximal point to the other edge of the groove, where A ≥ B, or (b) being formed at a respective maximal point, or (c) being formed at a respective minimal point, wherein two maximal points (M) are formed on the surface where no groove is formed such that the groove lies between two maximal points. A dynamic pressure gas bearing structure in accordance with Claim 1, wherein said surface has four maximal points having third and fourth maximal points (M) adjacent to each other, and has a second groove (1b) between said third and fourth maximal points (M), and A + A 1 ≥ B + B 1 is satisfied where A 1 represents the distance from said second groove (1b) to said fourth maximal point (M) along said closed curve and B 1 represents the distance from said second groove (1b) to said third maximal point (M) along said closed curve. The dynamic pressure gas bearing structure in accordance with Claim 4, having said at least two minimal points (N) arranged with said at least one groove (1b) therebetween such as to satisfy the relationship a ≥ b where a represents the distance from said groove (1b) to said first minimal point (N), along said closed curve thereby to present a part of said surface to air introduced into said annular clearance to effect separation of the shaft body (1) from the bearing body (2) when relative rotation occurs therebetween when the structure is in use. A dynamic pressure gas bearing structure comprising a grooved shaft body (1) having an axis (0), and having a plurality of axially-extending grooves formed therein, the grooves being of a cross section having a bottom wall and first and second opposed walls defining edges E 1 and E 2 where each of said first and second opposed walls intersects the external surface of the shaft body a hollow cylindrical bearing body (2) opposed to said shaft body (1) with an annular clearance therebetween, the structure being characterized by the shaft body (1) having a substantially-cylindrical external surface with a substantially-circular cross-section of non-uniform gradually changing radins perpendicular to the axis (0) where no groove is formed and for a majority of said plurality of grooves the radial dimension of the each wall being such that each of the edges E 1 and E 2 has a position vector r1, r2 respectively relative to the axis of the shaft body which satisfy the equation |r2| = k x |r1| where k>1. The dynamic pressure gas bearing structure in accordance with Claim 4, wherein the or each said groove (1b) has a taper shape in cross-section such that the distance between said first and second side walls reduces toward said bottom wall from the outer surface of said shaft body, and an angle formed by a first virtual plane extending from said one end of said bottom wall along said bottom wall and said first wall is larger than an angle formed by a second virtual plane extending from said another end of said bottom wall along said bottom wall and said second side wall. A method of manufacturing a dynamic pressure gas bearing structure comprising a grooved shaft body (1) having an axis (0) and a closed exterior surface and a hollow cylindrical bearing body (2) opposed to said shaft body (1) with a substantially annular clearance therebetween, the method being characterized by : forming the shaft body (1), to shape said external surface to have a substantially-circular cross-section, perpendicular to said axis, the cross-section being of non-uniform gradually changing radius and thereby providing a plurality of maximal points (M) whose distances from said axis are maximised and a plurality of minimal points (N) whose distances from said axis are minimised; and forming at least one groove (1b) in the outer periphery of said shaft body (1) (a) between adjacent ones of said plurality of maximal points (M), said shaft body (1) being so formed that the distance A from one edge of the or each groove adjacent one maximal point (M) to said one maximal point (M) is at least equal to the distance B from an opposed edge of the at least one groove to another adjacent maximal point (M), such that A≥B, or (b) the or each groove being located at a respective maximal point, or (c) being formed at a respective minimal point, whereby two maximal points (M) are formed on the surface where no groove is formed such that the groove lies between two maximal points. A method of using the dynamic pressure gas bearing structure of any one of Claims 1 to 5 which method comprises introducing air under pressure into the annular clearance between said shaft body (1) and said hollow cylindrical bearing body (2) in the desired direction of relative rotation to effect separation of said shaft body (1) and said bearing body (2) during relative rotation thereof.

Inventors: KOMURA OSAMU, MURABE KAORU, TAKEUCHI HISAO, KOMURA, OSAMU, MURABE, KAORU, TAKEUCHI, HISAO, KOMURA, Osamu, Itami Works, MURABE, Kaoru, Itami Works, TAKEUCHI, Hisao, Itami Works
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD
Application year: 1997
Main classification: F16C17
IPC codes: F16C33, G11B19, F16C3