Self-Activating Ferrofluid Seal Apparatus and Method

Magnetic fluid or ferrofluid magnetic seals have been used as exclusion seals or as multiple-stage seal apparatuses, where a difference in pressure environments is involved. Ferrofluid exclusion seal apparatuses are particularly useful in providing effective seals in computer-disc-drive systems and in other high-performance precision spindle applications. The difficulties associated with the application of the particular ferrofluid or magnetic fluid into the seal by the user, either exclusion seals or multiple-stage seals, have led to the development of self-activating fluid seal apparatuses, such as described in U.S. Pat. Nos. 4,252,328 and 4,252,353, both issued Feb. 24, 1981 (both hereby incorporated by reference).

In U.S. Pat. No. 4,252,328, a self-activating ferrofluid seal apparatus is described which is particularly useful with a memory disc-drive shaft, wherein the seal apparatus contains a cavity between the pole pieces, and a closed loop of a tubular container with a sealing amount of ferrofluid is positioned within the cavity. Just prior to insertion of the magnetically permeable shaft element to be sealed, a penetration clamp is employed to penetrate the plastic tubular container, to permit the ferrofluid in the container to move from the container into a sealing relationship in the radial gap about the ends of the pole pieces.

In U.S. Pat. No. 4,252,353, a self-activating seal apparatus is disclosed, wherein the pole pieces form a cavity for storing a sealing amount of a ferrofluid, until the magnetically permeable shaft is inserted within the seal apparatus. The ferrofluid is retained within the cavity generally adjacent the intersection of the magnetic ring element and the first and second pole pieces, while a nonmagnetic sealing ring is also employed to retain the sealing volume of ferrofluid within the cavity in the stored position. On removal of the nonmagnetic ring element and the insertion of the magnetically permeable shaft, the magnetic field is so altered and the flux path diverted from the intersection of the permanent-magnet ring element and the pole pieces to the radial gap separating the ends of the pole pieces and the surface of the shaft element, so that the ferrofluid then forms a self-activating, ferrofluid, O-ring seal with the surface of the shaft element. While such self-activating ferrofluid seal apparatuses are effective, it is desirable to provide for a self-activating seal apparatus and method of improved design, cost and simplicity.

The invention relates to an improved, self-activating, ferrofluid seal apparatus and to a method of self-activating sealing employing such seal apparatus. In particular, the invention concerns a self-activating, ferrofluid exclusion seal apparatus and its use particularly in providing an effective exclusion seal for a computer-disc-drive shaft or for other high-performance precision spindle-type applications.

The self-activating seal of the invention comprises a permanent-magnet ring adapted to surround a shaft to be sealed and pole pieces in a magnetic flux relationship with the poles of the permanent-magnet ring, and a sealing amount of a ferrofluid. The pole pieces form an air cavity therebetween, the air cavity containing a ferrofluid-storage-retaining gap of defined gap width formed by magnetically permeable extensions of one or both of the pole pieces. The retaining gap formed must be significantly larger than the radial gap formed by or at the ends of the pole pieces. The elements, forming the ferrofluid-retaining gap within the air cavity between the pole pieces, may be formed by altering or preforming the pole pieces, themselves, or by magnetically permeable inserts or extensions in a magnetic flux relationship with the pole pieces or permanent magnet. The ferrofluid-retaining gap is typically placed within the air cavity between the pole pieces in such a manner as to avoid contact of the stored ferrofluid in the retaining gap during handling of the seal apparatus, and typically is placed generally intermediate the air cavity.

It is essential, in the manufacturing and operating of the self-activating seal of the invention, that the distance in the ferrofluid-retaining gap be significantly magnetically greater than the distance of the radial gap which forms the ultimate ferrofluid O-ring seal. The distance across the retaining gap within the air cavity determines the relative magnetic-field intensity, which field intensity must be sufficient to retain and hold magnetically the sealing amount of ferrofluid during storage and prior to the insertion of the magnetically permeable shaft element in the seal apparatus, so that, even though subject to handling and movement, ferrofluid will not be dislodged from the retaining gap. The radial gap between the surface of the shaft and the ends of the pole pieces, which form the O-ring ferrofluid seal, must be much shorter in distance and have a significantly higher magnetic flux intensity, so that, when the magnetically permeable shaft is inserted within the seal apparatus, ferrofluid stored and retained within the retaining gap will move substantially or completely away from the retaining gap to the radial gap and into a sealing relationship with the surface of the shaft.

Therefore, in operation, on the insertion of the magnetically permeable shaft in the seal apparatus, the magnetic flux across the retaining gap essentially should go to zero, substantially zero, or to a very low magnetic intensity, so that there is a complete shift in movement of the magnetic flux intensity to the ends of the pole pieces which form the radial gap with the surface of the shaft element. In such operation, then the ferrofluid is retained within the retaining gap in a secure manner, but, on insertion of the shaft element and the alteration of the magnetic field flux by the presence of the magnetically permeable shaft element, the ferrofluid moves into the sealing relationship within the radial gap.

The retaining gap formed in the cavity should be of a defined, accurate dimension and may vary in distance, but typically may range, for the purpose of illustration, from about 12 to 30 mils, such as, for example, from 15 to 25 mils, while the distance of the sealing radial gap may vary and may range, for example, from about 1 to 15 mils, and typically 2 to 10 mils. The ratio in distance between the ferrofluid-retaining gap to the radial sealing gap should range from about 1.5 to 5.0. The radial sealing gap, being significantly smaller than the retaining gap, creates, on the insertion of the magnetically permeable shaft into the seal apparatus, a new magnetic flux path which is significantly lower in reluctance than the prior reluctance path across the retaining gap, and which radial gap, therefore, preferentially attracts all of the ferrofluid from the retaining gap to the radial gap.

The self-activating, magnetic ferrofluid seal of the invention provides for the secure retention of the ferrofluid within the cavity in the retaining gap during shipment and storage, without any risk of ferrofluid leakage due to shock and vibration or due to the presence of other magnetic seals or other magnetic fields, and, further, by being placed internally within the cavity avoids dislodgement or contamination by the hands of the user. At the point of usage of the seal apparatus, the seal apparatus, normally placed within a nonmagnetic seal housing, is installed and the shaft inserted, to form the new magnetic flux path of lower reluctance, and then rotated to form an O-ring seal or multiple-stage seals, if a multiple-stage seal apparatus is employed. The seal apparatus of the invention is simple, inexpensive and of an effective design. Since the ferrofluid moves directly from the retaining gap to the radial gap, any difficulty concerning the employment and use of high-tolerance passageways in a nonmagnetic ring element, which provide for controlled leakage rate of the ferrofluid, as in the prior self-activating seal patent, is avoided. In U.S. Pat. No. 4,252,353, the ferrofluid is retained at the intersections of the pole pieces and the magnetic ring element, where the magnetic gradient is not great. The employment of a retaining gap, as described in this invention, of carefully controlled dimensions, but larger than the sealing radial gap, provides for a stable and more securely stored ferrofluid. Thus, the seal apparatus and method of the invention provide for a magnetically permeable focusing structure within an air cavity which provides a high magnetic gradient to retain the ferrofluid, but which also permits the insertion of the shaft and the movement of the ferrofluid to the sealing gap. The selection of the retaining-gap and the radial-gap distance is based on the fact that the magnetic field intensity is inversely proportional to the square of the gap distance.

The self-activating ferrofluid seal apparatus and method of the invention will be described for the purpose of illustration only in connection with certain embodiments; however, it is recognized that various changes and modifications may be made in those embodiments by those persons skilled in the art, all falling within the spirit and scope of the invention. For example, this invention will be described in connection with the use of permanent magnets and magnetically permeable pole pieces and inserts of various shapes and structures, which pole pieces and inserts may be formed by machining, molding, bending or other operations. It is also recognized that the seal apparatus may be formed wholly or partially from permanently magnetic or magnetizable material, such as material containing magnetic material therein, such as elastomeric or polymeric material, which may be molded, such as injection-molded, into the desired shape and form for the seal apparatus. For example, the seal apparatus, the permanent magnet or pole pieces, alone or in combination, may be formed of a polymer-based material, such as nylon, or other polymeric material containing a ferrite, such as barium ferrite or other dipole magnetic-type material, to provide a permanent magnet element. The injection-molding of the pole pieces and permanent magnet of magnetic-containing material provides for significant flexibility in various shapes and sizes of the seal apparatus. Such molded design would have an enhanced ability to be assembled easily and may be employed with single ple pieces, double pole pieces or as a multiple-stage seal with a retaining gap within all or some of the multiple-stage seal cavities, in accordance with the present invention.

FIGS. 1-7 are illustrative, partial, sectional views of various self-activating ferrofluidic seal apparatuses of the invention.

FIG. 1 is a partial, sectional, schematic view of a self-activating ferrofluid seal apparatus of the invention which comprises an annular permanent-magnet ring element 12 having north and south poles, and which includes pole pieces 14 and 16 composed of a magnetically permeable material or formed of a permanent magnet. The pole pieces 14 and 16 are preformed, so as to provide for angular elbow sections or edges 18 and 20, with the flat ends 24 and 26 of the pole pieces forming a ferrofluid storage and retaining gap therebetween, to retain a sealing amount of ferrofluid 22, by virtue of the magnetic flux lines across the retaining gap from the ends of the respective pole pieces 14 and 16. As illustrated, the pole pieces 14 and 16 are preformed, such as by bending, with the inside diameter of the respective pole pieces formed in a symmetrical fashion, so that their ends 24 and 26 are in a general opposing relationship to form the retaining gap. Of course, if desired, the ends 24 and 26 of the pole pieces may be parallel to each other rather than at an angle. In any event, the ferrofluid 22 captured within the retaining gap comprises an amount of ferrofluid sufficient to effect a seal. As illustrated, the nonuniform retaining gap may range from about 10 mils at the low portion of the pole piece ends 24 and 26 to 15 mils at the top. The ferrofluid 22 in the retaining gap is disposed within the air cavity formed between the two pole pieces 14 and 16 and is disposed in such a manner that the seal apparatus may be encompassed within a nonmagnetic housing (not shown) and may be shipped and handled prior to use, without dislodgement of the ferrofluid 22 held in the retaining gap.

FIG. 2 is an illustrative, partial, sectional, schematic view of the self-activating seal 10 of the invention in a sealing mode, wherein a magnetically permeable, rotary shaft 28 has been inserted within the ring seal apparatus 10 containing the permanent-magnet ring 12 and preformed ring pole pieces 14 and 16. The ferrofluid 22 is shown moved from the retaining gap to the respective radial gap; for example, of 2 to 10 mils, formed between the ends of the elbow sections 18 and 20 and the opposite surface of the shaft 28. The magnetic flux path now passes, rather than through the retaining gap which has diminished substantially to zero magnetic flux, through the permanent magnet 12, the pole pieces 14 and 16, the sealing O-ring amounts of the ferrofluid and the shaft 28. The pair of O-rings formed by the ferrofluid in the radial gap across the surface of the shaft element 28 is illustrated by dotted lines. This self-activating seal apparatus has been illustrated by the formation of an exclusion seal of two O-rings; however, it is recognized that a single O-ring may be formed as an exclusion seal.

FIGS. 3 and 4 show another embodiment of the self-activating seal apparatus 30 of the invention comprising an annular permanent-magnet ring 32 with annular, preformed pole pieces 34 and 36 having ends 44 and 46, and having extension elements intermediate the pole pieces extending toward each other to form flat ends 38 and 40, to provide a uniform, ferrofluid-storage-retaining gap therebetween, to retain the ferrofluid 42. FIG. 4 is an illustration, wherein the magnetically permeable shaft 48 has been inserted in the seal apparatus 30, illustrating that the ferrofluid 42 has moved from the retaining gap to the radial gap under each end 44 and 46 of the pole pieces 34 and 36, to form an O-ring-type seal.

FIGS. 5-7 are illustrative, partial, sectional views of other various embodiments of the self-activating seal of the invention, with the ferrofluid in each seal apparatus shown in the retaining-gap position, and with the ferrofluid in the radial-gap sealing position illustrated by dotted lines under the respective pole pieces, with the dotted line under each of the seal apparatuses representing the surface of a magnetically permeable shaft element.

FIG. 5 is a partial, sectional view of a seal apparatus 50 comprising an annular permanent-magnet ring element 52 having preformed annular pole pieces 54 and 56, with flat ends 58 and 60 which form, with the surface of the shaft element, a defined radial gap for a sealing amount of a ferrofluid 66. The pole pieces 54 and 56 are preformed to have facing, rounded end extensions 64 and 62 extending toward each other in the air cavity formed between the pole pieces 54 and 56, to form a retaining gap therebetween, with a sealing amount of ferrofluid 66 disposed and retained in the sealing gap by the magnetic flux lines of the permanent magnet.

FIG. 6 is a schematic, partial, sectional illustration of a self-activating seal apparatus 70 comprising a permanent-magnet ring 72 and pole pieces 74 and 76, with pole piece 74 having an end 82 extending into the air cavity formed between the pole pieces, and having an elbow end section 78 which forms a radial gap with the surface of the shaft element. The pole piece 76 is a straight pole piece having an end 80 which forms a radial gap with the surface of the magnetically permeable shaft element. A ferrofluid-storage-retaining gap is formed between the end 82 and the opposite interior surface of the pole piece 84 of pole piece 76. If desired, the end of pole piece 82 may be flat and parallel with the surface 84 of pole piece 76.

FIG. 7 shows an illustrative, partial, sectional view of a self-activating seal apparatus 90 having an annular permanent-magnet ring element 92, and straight pole pieces 94 and 96, with ends 98 and 100 forming radial gaps thereunder with the surface of the magnetically permeable shaft element. In the air cavity formed between the pole pieces 94 and 96, there is disposed and secured an L-shaped, preformed, magnetically permeable insert element 104 having a flat end 106. The end 106 is parallel to and opposite the surface 102 of the pole piece 96 and forms therebetween a ferrofluid-storage-retaining gap, to retain the sealing amount of ferrofluid 108 therebetween. The insert 104 may also be extended at one end and placed in direct contact with and across the north pole of magnet 32.

In the embodiments illustrated in the drawings, a cavity is formed within pole pieces and extensions either of the pole pieces or by magnetically permeable inserts used to form a retaining gap within the air cavity, the gap being greater in distance than the radial gap formed by the pole pieces with the surface of the shaft to be sealed, thereby providing a simple, effective and economical self-activating seal, so that, on insertion of a magnetically permeable element in the seal apparatus, the magnetic flux path changes, to concentrate the magnetic-field strength at the radial gap, thereby providing a single-/multiple-stage sealing of the shaft element.