1.  Field of the Invention
The present invention relates to a bearing unit which incorporates a countermeasure for preventing leakage of lubricating oil and a rotating apparatus which incorporates a bearing unit of the type described. 
2.  Description of the Related Art
First, a bearing unit of a related art is described with reference to FIGS.  6 to 9. 
In particular, FIG.  6 is a side elevational sectional view showing the bearing unit of the related art mentioned.  FIGS.  7A to 7C show a path forming member which is a component of the bearing unit shown in FIG.  6, and wherein FIG.  7A is a perspective view of the entire path forming member; FIG.  7B is a top plan view of the path forming member; and FIG.  7C is a sectional view taken along line 7C-7C of FIG.  7B. Further, FIG.  8 is a perspective view of a path forming member lid which is a component of the bearing unit shown in FIG.  6 and cooperates with the path forming member shown in FIG.  6.  FIG.  9 is a perspective view of a radial bearing which is another component of the bearing unit shown in FIG.  6. 
Referring first to FIG.  6, the bearing unit of the related art is denoted by a numeral 30.  The bearing unit 30 is the same as a bearing unit disclosed in Japanese Patent Laid-Open No.  2005-69382 (pages 9 to 18, FIG.  3).  The bearing unit 30 includes a radial bearing 33, a path forming member 34, a path forming member lid 40, and a housing 60.  The radial bearing 33 supports a shaft 31 in a circumferential direction at a portion 31a thereof which is supported for rotation by dynamic pressure and has dynamic pressure generating grooves 32 formed on an outer periphery thereof.  The radial bearing 33 has a shaft opening side end face 33a and a thrust bearing side end face 33b formed thereon.  The path forming member 34 covers a circumferential face of part of the radial bearing 33 and has a thrust bearing 46 formed at a bottom portion thereof.  The path forming member lid 40 has a shaft fitting hole 40h formed at a central portion thereof, as particularly seen in FIG.  8, and has a diameter sufficient to allow the shaft 31 to pass through the shaft fitting hole 40h.  The path forming member lid 40 covers the shaft opening side end face 33a of the radial bearing 33. 
Referring to FIG.  9, the radial bearing 33 has a cylindrical structure and a through-hole 33h formed axially at a central portion thereof.  The upper end face of the radial bearing 33 is formed as a shaft opening side end face 33a while the lower end face of the radial bearing 33 is formed as a thrust bearing side end face 33b.  The radial bearing 33 further has a plurality of grooves, three in the arrangement shown, with first grooves 51 formed on an outer circumferential face thereof.  The radial bearing 33 further has an equal number of second grooves 52 and an equal number of third grooves 53 formed on the thrust bearing side end face 33b and the shaft opening side end face 33a thereof, respectively, and both communicating with the first grooves 51 to form communicating paths 50. 
Referring now to FIGS.  7A to 7C, the path forming member 34 has a cup-shaped configuration and a thrust bearing 46 formed on a bottom face thereof.  An upper cylindrical portion of the path forming member 34 has three arcuate plates 36 separated by three cutaway portions 35 for covering the communicating paths 50 to assure voids for the grooves of the communicating paths 50.  The path forming member 34 has a space 37 and another space 38 formed at a central portion of the bottom portion thereof.  The space 37 can be expanded outwardly to accommodate a flexible coming off preventing washer 49 when the shaft 31 is pushed into the same, while the space 38 can accommodate a lower end portion 31b of the shaft 31 therein.  The spaces 37 and 38 form a thrust space which is communicated also with the communicating paths 50 such that lubricating oil 42 can be circulated.  Further, engaging pawls 39 are formed to extend inwardly from a circumferential edge of the path forming member 34 at the upper opening. 
Referring now to FIG.  8, the path forming member lid 40 has a donut-shaped flat disk shape and a shaft fitting hole 40h formed at a central portion thereof.  The path forming member lid 40 has a size a little greater than the diameter of the engaging pawls 39 formed at the opening of the path forming member 34 and is fitted in through the engaging pawls 39 while pushing away the engaging pawls 39.  The path forming member lid 40 is locked by the engaging pawls 39 so that it does not move upwardly. 
Upon assembly of the bearing unit 30, the coming off preventing washer 49 is disposed in the space 37 of the path forming member 34 in advance, and the radial bearing 33 is fitted into the coming off preventing washer 49 with the thrust bearing side end face 33b directed downwardly.  Then, the path forming member lid 40 is fitted in from above the path forming member 34 and secured by the engaging pawls 39 such that it closely contacts with the shaft opening side end face 33a of the radial bearing 33. 
Where the bearing unit 30 is assembled in this manner, the three third grooves 53 of the radial bearing 33 are closed up with the path forming member lid 40 to form pipe-shaped space paths, and the three first grooves 51 of the radial bearing 33 are closed up with the arcuate plates 36 of the path forming member 34 to form pipe-shaped space paths.  Further, the three second grooves 52 of the radial bearing 33 are open to the space 37 to form space paths.  The space paths formed in this manner form communicating paths 50 through which lubricating oil 42 can be supplied and circulated to the dynamic pressure generating grooves 32 of the shaft 31, and which have a function of short-circuiting pressure, which is generated when the shaft 31 and the radial bearing 33 rotate relative to each other. 
If an outer surface of the path forming member 34 and an upper face of the path forming member lid 40 are sealed with a resin material using a metal mold while the bearing unit 30 is in the assembled state described above, then the bearing unit 30 can be covered with the housing 60 of the resin material.  In particular, the housing 60 made of a resin material can cover the bearing unit 30 over part of the outer circumferential face of the radial bearing 33 which is exposed by the cutaway portions 35 of the path forming member 34 and the outer surface of the arcuate plates 36 and over the upper surface of the path forming member lid 40. 
Thereafter, if the shaft 31 is inserted and pushed through the shaft fitting hole 40h of the path forming member lid 40 and the through-hole 33h of the radial bearing 33 into a central portion of the coming off preventing washer 49, then the bearing unit 30 having the structure shown in FIG.  6 is obtained.  After the bearing unit 30 is completed, the lubricating oil 42 is poured in from the shaft opening end 30a side of the bearing unit 30 so as to fill between the shaft fitting hole 40h of the radial bearing 33 and the shaft 31 and the communicating paths 50. 
The bearing unit 30 is assembled in such a manner as described above.