1.  Field of the Invention
The present invention relates to a lubricative sealing device which provides lubrication to rolling parts of ball screws, particularly between the screw groove of the ball screw shaft or the ball screw nut and the balls, and enables the ball screw to be used for a long time without replenishment of oil. 
2.  Description of the Related Art
Various sealing devices of this type have been proposed so far (see, for example, Japanese Unexamined Patent Application Publication No.  10-2395 and Japanese Patent No.  3288961). 
Japanese Unexamined Patent Application Publication No.  10-2395 discloses a structure in which a lubricant-impregnated rubber or synthetic resin ring is attached to the end of a ball nut with a pressing member which has projections to apply a preload to the ring in a circumferential direction. 
Japanese Patent No.  3288961 discloses a sealing device that is attached to sliding members such as nuts similarly to Japanese Unexamined Patent Application Publication No.  10-2395.  The lubricating oil applying member is made of polyester/wool blend felt, sintered resin, or the like, which has the merit of being able to absorb and hold more lubricant, as described in the specification. 
Another structure for the sealing device is known, as described in Japanese Examined Utility Model Application Publication No.  5-43310.  In this sealing device, thin seal plates having holes of shapes that correspond to the lateral cross-sectional shape of the screw groove of the ball screw shaft are circumferentially displaced and overlapped, and held in a frame-shaped ring to be attached to the end of the ball nut.  This structure is effective as a sealing device. 
A sealing device that can be used on any installed standard ball screw nuts by attaching to both ends of the ball screw nut is also known (see, for example, Japanese Unexamined Patent Application Publication No.  2002-147561). 
In the structure disclosed in Japanese Unexamined Patent Application Publication No.  10-2395, however, pressure is constantly applied to the lubricant supplying member making contact with the balls serving as rolling elements, to keep their contact for a long time.  This accordingly causes a problem of complexing lubrication mechanism. 
With the structure of Japanese Patent No.  3288961, while the member that makes contact with the rolling elements, or balls, to apply oil can absorb and hold a large amount of oil, it requires a lubricating oil reservoir member (tank function) that supply oil in order to achieve constant lubricating effect.  Another problem is that the lubricating oil applying and retaining member loses its resiliency in the areas contacting the balls as time passes, which leads to its plastic deformation, and as the surfaces contacting the rolling balls deteriorate, the lubricating effect may be reduced.  Also, since felt materials usually have large pores, absorbed oil can readily flow out.  Therefore, the structure needs to be modified such as provision of a flow rate control valve between the oil reservoir member and the oil application member, as described in the specification. 
Further, the device shown in Japanese Examined Utility Model Application Publication No.  5-43310 uses a common seal material, and therefore, while it has good sealing properties, it lacks a lubricating function and is unable to supply lubricating oil stably for a long time. 
Furthermore, with the device shown in Japanese Unexamined Patent Application Publication No.  2002-147561, the built-in lubricant holding member is made of a porous sintered resin material.  Therefore, similar to the other conventional devices described above, the lubricating oil applying and retaining member loses its resiliency as time passes in the areas contacting the balls, which leads to its plastic deformation, and as the surfaces contacting the rolling balls deteriorate, the lubricating effect can be reduced.  Accordingly, the device shown in this fourth patent document requires an oil replenishing member attached to one side, which is provided in addition to the lubricant applying member that contacts the screw groove, so that lubricant is replenished constantly from outside to the rolling parts of the screw.