SEALING MEMBER FOR SEALING MAGNETIC FLUID

Abstract: A sealing member for sealing magnetic fields according to the present invention is a sealing member (1) arranged so as to surround a periphery of a working member (5) such as shaft. The sealing member (1) comprises an annular magnet (2) and an annular pole piece (3), an electrically conductive magnetic fluid (6) being provided between the sealing member (1) and the working member (5) to construct an electrically conductive magnetic fluid-sealing member. And, an inner circumferential surface of the annular magnet (2) is coated with an electrically conductive non magnetic film (4) continuing to the annular pole piece (3) to down-size the sealing member (1) and reduce an electrical resistance.

Description: This invention relates to a sealing member for ferrofluid seal, more particularly to a sealing member comprising an annular magnet and annular pole pieces which can form an electrically conductive ferrofluid seal by interposing an electrically conductive ferrofluid between the sealing member and the above operation member. Sealing devices utilizing ferrofluid are known as described, for example, in Japanese Patent Publication No. 24585/1986. Such sealing device has an annular sealing member to be disposed so as to surround the circumference of a shaft. This sealing member comprises an annular magnet and a pair of pole pieces bonded onto both sides of the annular magnet, with a ferrofluid being interposed between the internal circumferential surfaces of the annular pole pieces and the external circumferential surface of the shaft, whereby to function as a sealer. In recent years, it is attempted in such sealing device to form an electric circuit utilizing an electrically conductive ferrofluid as the ferrofluid, in which electric current flows from the annular pole pieces through the ferrofluid to the shaft, which is, for example, utilized as a grounding circuit for preventing computer hard disks from being charged with static electricity. While it is preferred that when such ferrofluid is used as a part of the electric circuit, said part has a smallest possible resistance, the resistance value in said part depends on the conducting area and distance in the ferrofluid. The latter conducting distance, corresponds to the distance between the internal circumferential surface of the annular pole pieces and the external circumferential surface of the shaft, namely to the seal gap, and it is extremely difficult to further reduce the seal gap in view of the relationship with assembling accuracy and the like. On the other hand, the former conducting area is predominantly defined by the area of the internal circumferential surface of the annular pole pieces. Accordingly, in order to increase the conducting area, the thickness of the annular pole pieces must be increased, and thus more compact sealing devices in demand cannot be obtained. This invention has been accomplished for solving the above problems of the prior art and is directed to provide a sealing member for ferrofluid seal which can reduce the electrical resistance at the section of ferrofluid and form a compact sealing device. These and other objects of this invention will become more apparent from the description of the following embodiment. The sealing member for ferrofluid seal according to this invention is the one to be disposed to surround the circumference of a working member such as a shaft, consisting of an annular magnet and annular pole pieces, which forms an electrically conductive ferrofluid seal by interposing an electrically conductive ferrofluid between the sealing member and the working member, wherein the sealing member has an electrically conductive nonmagnetic coating on its internal circumferential surface. Preferably, the sealing member has the nonmagnetic coating over the entire surface. The sealing member for ferrofluid seal functions as follows: Namely, both the internal circumferential surface of the annular magnet or the electrically conductive coating and the internal circumferential surfaces of the annular pole pieces function as an electrode against the ferrofluid, whereby sufficient conducting area can be secured, and electrical resistance can be reduced without increasing the size of the sealing member as conventionally practiced. In this constitution, since the conductive coating is made of a nonmagnetic material, the affect which may be caused by the presence of this coating on the magneticity retaining mechanism of the ferrofluid will be negligible. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 illustrates an embodiment of the sealing member for ferrofluid seal according to this invention; and Fig. 2 illustrates a variation of such sealing member. Next, typical embodiments of the sealing member for ferrofluid seal of this invention will be described in detail referring to the attached drawings. In Fig. 1 illustrating the upper half of the sealing member in vertical cross section, the reference numeral 1 shows a sealing member, which is fabricated by bonding a pair of annular pole pieces 3,3 on both sides of an annular magnet 2. The annular magnet 2 and the annular pole pieces 3,3 are made of a rubbery magnet and the like (such as one obtained by incorporating a magnetic powder such as ferrite into a rubber) and a magnetic material such as stainless steel, respectively, and both members are bonded together using a synthetic rubber adhesive. The sealing member 1 has an electrically conductive nonmagnetic coating 4 over the entire surface. The coating 4 can be formed by applying a chrome plating, gold plating or electrolytic or nonelectrolytic nickel plating to the sealing member 1. Incidentally, in Fig. 1, the reference numerals 5 and 6 show a working member shaft and a ferrofluid, respectively. The above annular magnet and the annular pole pieces 3,3 each are of continuous shape and have uniform cross sections over the entire circumferences. The coating 4 is formed continuously to enclose the annular magnet and the annular pole pieces 3,3 to have uniform cross sections over the entire circumference thereof. Accordingly, the ferrofluid 6 also dwells between the sealing member 1 and the shaft 5 continuously and uniformly over the entire circumference. In the above sealing member 1, since the entire internal circumferential surface thereof functions as an electrode surface against the ferrofluid 6, electrical resistance can be reduced using the same thickness of sealing member 1 as used in conventional sealing members. In this embodiment, since the coating 4 is of a nonmagnetic material, the affect which may be caused by the presence of the coating 4 on the mechanism of retaining the ferrofluid will be negligible. In the prior art, for example as is disclosed in EP-A-0 208 391, an anti-dusting coating is applied on the annular magnet 2 made of, for example, a rubbery magnet to prevent dusting from the magnet in use. However, if the sealing member 1 is entirely coated as described above, such treatment of the magnet 2 with the anti-dusting coating can advantageously be omitted. When the sealing member has the coating over the entire surface, intrusion of ferrofluid 6 into the gaps between the annular magnet 2 and the annular pole pieces 3,3 which are liable to be formed due to the insufficient adhesion can be prevented, whereby the inconvenience of reduction in the level of the fluid to be retained can also be prevented. While Fig. 1 illustrates an embodiment in which the annular magnet 2 and the annular pole pieces 3,3 all have substantially equal inner diameters, this invention can also be practiced almost analogously in a constitution shown in Fig. 2 in which an annular magnet 2 has an inner diameter greater than that of the annular pole pieces 3,3 or in a constitution not shown, in which the annular magnet 2 has an annular pole piece 3 bonded only on one side. Fig. 2 also shows the upper half of the sealing member 1 in cross section. The coating 4 to be applied on the sealing 1 can also be obtained using a method other than plating, such as coating and vapor deposition. Further, such coating to be applied may not be present over the entire surface of the sealing member 1 but on the predetermined portion, and such localized coating may be a silver paste applied only to the internal circumferential surface of the sealing member 1. Since both the internal circumferential surface of the annular magnet or the electrically conductive coating and those of the annular pole pieces function as an electrode against the ferrofluid, sufficient conducting area can be secured and electrical resistance can be reduced without increasing the size of the sealing member as practiced in the prior art. As has been described above, the sealing member for ferrofluid seal according to this invention can be used as a member for sealing a working member such as a shaft, particularly suitably in an application for forming an electric connection between the sealing member and the shaft to be used as a grounding circuit for preventing computer hard discs from being charged with static electricity.

Claims: A sealing member for a ferrofluid seal to be disposed so as to surround the external circumference of an operation member such as a shaft (5), consisting of an annular magnet (2) and annular pole pieces (3), which form an electrically conductive ferrofluid seal by interposing an electrically conductive ferrofluid (6) between the sealing member (1) and the operation member (5), characterized in that said sealing member (1) has an electrically conductive non-magnetic coating (4) on its internal circumferential surface. The sealing member of claim 1, wherein the sealing member (1) has the non-magnetic coating (4) over the entire surface.

Inventors: ISHIKAWA RYUHEI NIPPON FERROFL, ISHIKAWA, RYUHEI NIPPON FERROFLUIDICS CORPORATION
Applicants: NIPPON FERROFLUIDICS KK, NIPPON FERROFLUIDICS CORPORATION
Application year: 1989
Main classification: F16J15
IPC codes: F16J15, H01F7