The present invention relates to an improvement in a fluid injection apparatus for a fluid dynamic pressure bearing, which injects a lubricating fluid to the fluid dynamic pressure bearing by using a vacuum injection method.
As shown in FIG. 4 for instance, the fluid dynamic pressure bearing comprises a flanged shaft 1 formed by pressure-inserting a ring member 3 that is a thrust member to a cylinder member 2, a stepped cylindrical sleeve 4 in which the flanged shaft 1 is rotatably fitted, and an annular lid member 5 functioning also as a thrust presser member. Fine gaps R1, R2, R3, R4 and R5 formed between these members constituting the bearing are filled with a lubricating oil F. A taperlike fine gap S formed between an upper outer peripheral face of the cylinder member 2 and an inner peripheral face of the annular lid member 5 is a capillary seal functioning so as to prevent the lubricating fluid F from leaking outside by utilizing a capillary tube phenomenon and surface tension. A radial dynamic pressure groove G1 such as a herringbone groove is formed in a lower outer peripheral face of the cylinder member 2, and a spiral thrust dynamic pressure generating groove G2 such as a herringbone groove is respectively formed in an upper face and a lower face of the ring member 3. The taper-like fine gap S is a solitary opening of a single bag-like fluid filling portion comprised of the fine gaps R1, R2, R3, R4 and R5 which are in communication with each other and becomes an injection port for injecting the lubricating oil F. Incidentally, an example of the thrust dynamic pressure generating groove G2 is shown in FIG. 5.
In short, the fluid dynamic pressure bearing is a dynamic pressure bearing which has bearing members including a shaft and a sleeve, and in which lubricating fluid is filled in the single bag-like fluid filling portion including a radial gap and a thrust gap that are formed between the bearing members, and the radial dynamic pressure generating groove is formed in the radial gap and a thrust dynamic pressure generating groove is formed in the thrust gap.
Since the fine gap of a small and thin fluid dynamic pressure bearing is several μm—several hundred μm, the lubricating fluid is injected into the fluid dynamic pressure bearing by a vacuum injection method. As a conventional apparatus using the vacuum injection method, there is a lubricating fluid injection apparatus disclosed in U.S. Pat. No. 5,862,841.
As shown in FIG. 3, this conventional apparatus is a fluid injection apparatus for a fluid dynamic pressure bearing, which comprises a bearing holding case 70 having an O-ring through which a predetermined face of a fluid dynamic pressure bearing 27 is seated on a bearing holding face, bearing fixing means 44 for mounting and fixing the fluid dynamic pressure bearing 27 to the bearing holding base 70, a vacuum pump 52 for decompressing an inside of the fluid dynamic pressure bearing 27 through a seal space formed by the predetermined face of the fluid dynamic pressure bearing 27, the bearing holding face and the O-ring, a reservoir 74 in which a lubricating fluid is stored, a fluid injection tube 90 for injecting the lubricating fluid into the inside of the fluid dynamic pressure bearing 27 through the seal space, and liquid level control means for controlling a liquid level of the lubricating fluid to a 1st level separating from a tip of the fluid injection tube 90 when decompressed and to a 2nd level at which the tip of the fluid injection tube 90 is dipped when injected.
The liquid level control means is one in which the reservoir 74 is attached to the bearing fixing means 44 through a bellows 80, an actuator 78 driven by an oil pressure or air pressure cylinder is attached to the bearing fixing means 44, and liquid level control is performed by means of moving the reservoir 74 up and down by the actuator 78 as shown with an arrow mark. That is, since the liquid level control means is one for controlling upper and lower positions of the reservoir in which the lubricating fluid is stored, it is a device having a comparatively complex structure. Moreover, as to the bellows 80, it is necessary to use a bellows which has a sufficient strength against deforming even if an internal pressure becomes 50 mTorr or lower, and which has a high durability against frequent expansion and contraction over an extended period. However, there has been a problem in that a bellows having such high durability is very expensive. Moreover, even a high durability bellows must be exchanged at a suitable frequency. Accordingly, in the conventional fluid injection apparatus, there is also a problem in that the cost of periodic maintenance and part replacement is high.