Closed type thrust dynamic pressure bearing with through hole

Abstract: There is provided a closed type thrust dynamic pressure bearing which is capable of generating enough dynamic pressure for axially bearing a rotating member at the time of start-up and which never damages an end face of a flange of the rotating member. The flange 1 of the rotating member 3 is housed in a housing 2 in a closed state. V-shaped dynamic pressure generating grooves 8 are provided at upper and lower end faces 5, 6 of the flange 1, while a plurality of holes 11 axially passing through the flange 1 are provided radially inside an annular region where the dynamic pressure generating grooves 8 are provided. The total sum of cross-sectional areas of the holes 11 is at least 1/30 of the area of an annular region 7. Further, oil sumps 13 are provided at a receiving surface 12 of the housing 2 opposite to the lower end face 6 of the flange 1.

Description: BACKGROUND OF THE INVENTION The present invention relates to a closed type thrust dynamic pressure bearing. There has been available hitherto a closed type thrust dynamic pressure bearing which comprises a rotating member consisting of a shaft body and a flange provided at one end of the shaft body, and a housing for housing the flange of the rotating member in a closed state, where dynamic pressure generating grooves are formed at both axial end faces of the flange. In this conventional closed type thrust dynamic pressure bearing, when the rotating member is installed vertically, the rotating member lowers due to gravity before activation so that the lower end face of the flange and the receiving surface of the housing opposite to the lower end face of the flange are brought into a contact state or into a state that only an extremely thin membrane of working fluid is present therebetween. In this state, with the rotating member activated, for example, V-shaped dynamic pressure generating grooves try to pull in the working fluid toward a radial center of these grooves simultaneously in both axial end faces of the flange. However, because the housing has the flange housed therein in a closed state, a negative pressure is generated between the upper end face of the flange and the housing, making it impossible to introduce the working fluid sufficiently to a place, particularly a radially inner portion, between the receiving surface of the housing and the lower end face of the flange where only a small amount of working fluid is present originally, as a problem. On this account, at the time of activation of the rotating member, there would arise a state that the working fluid is insufficient between the lower end face of the flange and the receiving surface. As a result, enough dynamic pressure for bearing the flange could not be generated, disabling the thrust dynamic pressure bearing from fulfilling the function as a thrust dynamic pressure bearing, and damaging the end face of the flange, as another problem. U.S. 5,433,529 describes a dynamic thrust pressure bearing of the open type, wherein the shaft extends beyond the flange. The flange has holes passing axially through the flange. The holes are provided radially inside an annular region, where dynamic pressure generating grooves are provided. SUMMARY OF THE INVENTION It is an object of the present invention to provide a closed type thrust dynamic pressure bearing which is capable of generating enough dynamic pressure for axially bearing the flange at a start-up of activation, which never damages the end face or the like. The closed type thrust dynamic pressure bearing of the present invention is defined by claim 1. The present invention provides a closed type thrust dynamic pressure bearing comprising: a rotating member which comprises a shaft body and a flange provided at an end of the shaft body; and a housing for housing the flange of the rotating member in a closed state, wherein dynamic pressure generating grooves are formed at an end face of the flange or at a receiving surface of the housing opposite to the end face, characterized in that the flange has at least one hole which passes axially through the flange and which is provided radially inside an annular region where the dynamic pressure generating grooves are provided. According to the closed type thrust dynamic pressure bearing of the present invention, when the rotating member rotates in the state where the shaft body of the rotating member is vertical, the working fluid placed between the upper end face of the flange and the receiving surface of the housing passes through the holes passing through the flange so as to be supplied to between the lower end face of the flange and the receiving surface of the housing, so that enough working fluid can be supplied to the dynamic pressure generating grooves. Therefore, enough dynamic pressure to effect the axial bearing of the flange is generated, allowing the thrust dynamic pressure bearing to fulfill the function as a thrust dynamic pressure bearing, and eliminating the possibility of damage at the lower end face of the flange and the receiving surface of the housing opposite thereto. According to the invention a total sum of cross-sectional areas of the hole or holes is at least 1/30 of an area of the annular region. In the closed type thrust dynamic pressure bearing of this embodiment, the total sum of cross-sectional area of the holes is at least 1/30 of the annular region, so that enough working fluid is supplied to the lower end face of the flange at a specified rotating speed, allowing enough dynamic pressure to be generated. This fact was verified by experiment. In an embodiment, an oil sump is provided at the receiving surface of the housing or at the end face of the flange in such a way that the oil sump is at least partly opposed to or overlapped with the annular region. According to the closed type thrust dynamic pressure bearing of this embodiment, because an oil sump is provided at the receiving surface of the housing or at the end face of the flange so that the oil sump is at least partly opposed to or overlapped with the annular region, the supply of the working fluid to the lower end face of the flange at the activation of the rotating member is done also from this oil sump. This allows the generation of dynamic pressure by the dynamic pressure generating grooves to be achieved more reliably. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, wherein: Fig. 1A is a sectional view of a dynamic pressure bearing according to an embodiment of the present invention; Fig. 1B is a plan view of the upper end face of a flange; and Fig. 2 shows a floating amount of a rotating member versus the ratio of a total area of holes to an annular region. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinbelow, the present invention is described in detail by an embodiment thereof illustrated in the accompanying drawings. Fig. 1A is a sectional view of an embodiment of the present invention. As shown in Fig. 1A, a rotating member 3 is comprised of a shaft body 4 and a flange 1 fixed to its one end. The shaft body 4 and the flange 1 are supported radially and axially by the housing 2. The flange 1 is housed in the housing 2 in a closed state. At both upper and lower end faces 5, 6 of the flange 1, are provided V-shaped dynamic pressure generating grooves 8 in an annular region 7 as shown in Figs. 1A, 1B. This annular region 7 is a region between a circle 9 and a circle 10 in Fig. 1B. Radially inside the annular region 7 of the flange 1, eight through holes 11 are provided equidistantly on a circumference. The total sum of the cross-sectional areas of these through holes 11 is not less than 1/30 of the area of the annular region. On the other hand, at a receiving surface 12 opposite to the lower end face 6 of the flange 1, are provided a plurality of oil sumps 13 partly opposed to the annular region 7. These oil sumps 13 are curved smoothly at their radial inside so that a working fluid 14 easily goes out toward the dynamic pressure generating grooves 8. Meanwhile, dynamic pressure generating grooves 15 are provided on the shaft body 4 so that the shaft body 4 is radially supported by dynamic pressure. When the closed type thrust dynamic pressure bearing of the above construction has the rotating member 3 installed vertically, the flange 1 lowers due to its dead weight before start-up, so that the lower end face 6 of the flange 1 and the receiving surface of the housing 2 opposite to the lower end face 6 of the flange 1 are brought into a contact state or into a state that only an extremely thin membrane of the working fluid 14 is present therebetween. In this state, with the rotating member 3 activated, the V-shaped dynamic pressure generating grooves 8 try to pull in the working fluid 14 toward a radial center of these grooves 8. When this occurs, a space formed between the upper end face 5 of the flange 1 and the receiving surface 12 of the housing 2 and a space formed between the lower end face 6 of the flange 1 and the receiving surface 12 of the housing 2 are communicated with each other by the holes 11 drilled through the flange 1 radially inside the annular region 7. Therefore, the working fluid 14 flows through the holes 11 as shown by arrows E, F in Fig. 1A so that the working fluid 14 is supplied to between the lower end face 6 of the flange 1 and the receiving surface 12 of the housing 2. As a result, even if the housing 2 has housed the flange 1 in a closed state, negative pressure is not generated between the upper end face 5 of the flange 1 and the housing 2. Thus, between the receiving surface 12 of the housing 2 and the lower end face 6 of the flange 1, the working fluid 14 can be sufficiently supplied particularly to radially inside part as shown by arrows D, E, F so that dynamic pressure can be generated from the beginning of activation. As shown above, when the rotating member 3 is started up, the working fluid 14 is sufficiently supplied to the annular region 7 of the lower end face 6 of the flange 1 so that enough dynamic pressure to axially support the flange 1 can be generated to the annular region 7 where the dynamic pressure generating grooves 8 are provided. Thus, the closed type thrust dynamic pressure bearing is enabled to fulfill the function as a thrust dynamic pressure bearing. Also, the dynamic pressure generating grooves 8 of the flange 1 pull in the working fluid 14 from the oil sumps 13. That is, the dynamic pressure generating grooves 8 pull in the working fluid 14 from the oil sumps 13 located immediately under the dynamic pressure generating grooves 8 during the rotation of the flange 1. Therefore, the working fluid 14 is even more easily supplied to between the lower end face 6 of the flange 1 and the receiving surface 12 of the housing 2, as compared with the case where no oil sumps 13 are provided. Fig. 2 shows the relationship of the floating amount of the rotating member 3 versus the ratio of the total area of all the holes 11 that allow the working fluid 14 to be circulated to the area of the annular region 7. More specifically, Fig. 2 is a view showing the relationship of the floating amount of the rotating member 3 versus the ratio of the total area of all the holes 11 for the working fluid 14 to the area of the annular region 7, in a case where the rotating speed of the rotating member 3 is varied to 1000 r.p.m., 3000 r.p.m. and 5000 r.p.m. Given a 1/30 or more ratio of the total area of the holes 11 to the area of the annular region 7, when the rotating speed of the rotating member 3 had changed to 1000 r.p.m., 3000 r.p.m. and 5000 r.p.m., the floating amount of the rotating member 3 becomes about 5 µm, 5.5 µm and 6 µm, being not less than a set floating amount of 5 µm. Accordingly, if the ratio of the total area of the holes 11 for the working fluid 14 drilled through the flange 1 to the area of the annular region 7 is not less than 1/30 and if the rotating speed of the rotating member 3 is not less than 1000 r.p.m., then the rotating member 3 is enabled to ensure a floating amount not less than the set floating amount of 5 µm. Although the dynamic pressure generating grooves 8 are provided at the upper and lower end faces 5, 6 of the flange 1 in the above embodiment, the dynamic pressure generating grooves may alternatively be provided at the receiving surface of the housing opposite to the end faces. Also, the oil sumps, although not shown, may also be provided at end faces of this flange. These oil sumps may be opposed to the annular region where dynamic pressure generating grooves are provided, or partly overlapped with this annular region. Further, although the oil sumps 13 are provided in a plural number at outer peripheral portion of the receiving surface 12 of the housing 2 in the embodiment, they may also be provided at inner peripheral portion of the receiving surface of the housing. The shape of the oil sumps may also be round-hole, circular-arc or ring shaped. As apparent from the foregoing description, the dynamic pressure bearing comprises: a rotating member which comprises a shaft body and a flange provided at an end of the shaft body; and a housing for housing the flange of the rotating member in a closed state, wherein dynamic pressure generating grooves are formed at an end face of the flange or at a receiving surface of the housing opposite to the end face, characterized in that the flange has at least one hole which passes axially through the flange and which is provided radially inside an annular region where the dynamic pressure generating grooves are provided. Therefore, the working fluid can be sufficiently supplied to between the lower end face of the flange and the surface of the housing opposite to the lower end face of the flange, through the hole. Consequently, enough dynamic pressure to effect the axial bearing of the flange can be generated, allowing the thrust dynamic pressure bearing to fulfill the function as a thrust dynamic pressure bearing at start-up, and preventing the end face of the flange and the receiving surface of the housing from damage. Also, in the dynamic pressure bearing of the embodiment of the invention, the total sum of cross-sectional areas of the hole or holes is at least 1/30 of the annular region. Therefore, the working fluid can be sufficiently supplied to between the lower end face of the flange and the receiving surface of the housing opposite thereto so that the set floating amount can be ensured. Also, in the dynamic pressure bearing of an embodiment, an oil sump is provided at the receiving surface of the housing or at the end face of the flange in such a way that the oil sump is at least partly opposed to or overlapped with the annular region. Therefore, the working fluid can be supplied to between the lower end face of the flange and the receiving surface of the housing opposite to thereto, more reliably.

Claims: A closed type thrust dynamic pressure bearing comprising: a rotating member (3) which comprises a shaft body (4) and a flange (1) provided at an end of the shaft body (4), and a housing (2) for housing the flange(1) of the rotating member (3) in a closed state, wherein dynamic pressure generating grooves (8) are formed at an end face (6) of the flange (1) or at a receiving surface (12) of the housing (2) opposite to the end face (6), characterized in that the flange(1) has at least one hole(11) which passes axially through the flange (1) and which is provided radially inside an annular region (7) where the dynamic pressure generating grooves (8) are provided, wherein a total sum of cross-sectional areas of the hole or holes (11) is at least 1/30 of the area of the annular region (7). The closed type thrust dynamic pressure bearing according to claim 1, wherein an oil sump (13) is provided at the receiving surface (12) of the housing (2) or at the end face (6) of the flange(1) in such a way that the oil sump (13) is at least partly opposed to or overlapped with the annular region (7).

Inventors: TAKAHASHI TAKESHI, TAKAHASHI, TAKESHI
Applicants: KOYO SEIKO CO, KOYO SEIKO CO., LTD.
Application year: 1998
Main classification: F16C33
IPC codes: F16C17, F16C33