Ferrofluid-Pole-Piece-Magnet Apparatus

This application is an improvement in the single-pole-piece ferrofluid seal apparatus and the method of manufacturing the single-pole-piece-magnet apparatus used in the seal apparatus as described in U.S. patent application Ser. No. 450,339, filed Dec. 16, 1982, entitled "Single-Pole-Piece Ferrofluid Seal Apparatus and Exclusion Seal System" by Kuldip Raj and Raoul Casciari, the disclosure of which is incorporated by reference herein.

Single, dual, and multiple pole piece ferrofluid seal-type apparatus are known and are designed to provide a ferrofluid seal about a moving element, such as a rotatable shaft element, either as an exclusion-type single or dual pole piece seal or as a pressure-type multiple-stage seal.

In single pole piece-permanent magnet ferrofluid seal apparatus of the type disclosed in U.S. Ser. No. 450,339, as well as in other seal apparatus, the washer-like pole piece element is secured to the annular permanent magnet by staking the elements together or more commonly by bonding the flat opposing surface of the pole piece element and the permanent magnet together employing a high strength adhesive material.

It is however desireable to provide a simple, rapid, low cost and yet effective method for the manufacture of a single or dual pole piece-permanent magnet apparatus and to provide an improved single and dual pole piece ferrofluid seal apparatus, which method and apparatus overcome certain disadvantages of prior art methods and apparatus and provide new and improved advantages.

The invention relates to an improved single pole piece ferrofluid seal-magnet apparatus and to the pole piece-permanent magnet used in the seal, and to a method of manufacturing the single-pole-piece magnet combination used in ferrofluid-type seal apparatus. In particular, the invention concerns the method of attaching one surface of a single- or dual-pole piece to a molded polymeric permanent magnet.

A method of manufacturing a pole-piece element-permanent magnet combination has been discovered which method avoids the use of high strength adhesives and yet provides a simple, rapid, low cost, direct bonding of one face surface of a washer-type pole-piece element directly into intimate bonding contact with a permanent magnet. The pole piece-permanent magnet combination apparatus so prepared is useful in a single-pole-piece-magnet ferrofluid seal, i.e., single-stage seals, or in the preparation of dual or multiple-stage ferrofluid seals.

The method comprises striking the one surface of a selected magnetically permeable typically metal, thin, pressure-deformable sheet material with a die element such as in a coining operation to form a plurality of spaced-apart, generally peripherally formed indentations in the sheet material and to form raised projections or corresponding raised dimple elements on the opposite side of the sheet material by the coining or die striking operation.

The sheet material used may comprise any magnetically-permeable-type deformable material suitable for use as a pole piece element in a ferrofluid seal apparatus. Generally the sheet material is a metal sheet material such as a stainless sheet. The sheet material may vary in thickness, but may, for example, in small single-stage-ferrofluid seals range from about 1/64" to 1/16" in thickness. It is preferred that the sheet material be selected to have sufficient hardness to be deformable in the die operation and in the subsequent die cutting operation. It has been found for example that sheet material of about 1/4 hard stainless steel or slightly softer metal sheet material is suitable for the purpose of the invention. The softness-hardness of the sheet material is selected to provide acceptable raised dimples and indentations and also to form the desired amount of overhang or swaging during the subsequent spanking operation to form an anchoring means for the permanent magnet material.

The die striking of the solid sheet material may be accomplished with a single or multiple pin-like die element to form generally annular or other shaped indentations of defined depth in the sheet material, e.g., about 10 to 50% of the sheet material thickness. The plurality of indentations may be formed in one or multiple striking operations. It is preferred that the forming of indentations and the raised dimples be carried out prior to cutting or removing any washer-like pole-piece elements from the sheet material to avoid deformation of the washer-shaped sheet material.

After or with formation of the indentations and dimples in the sheet material, the surface of raised dimple elements is then struck to force outwardly the deformable material from the outer periphery of the raised dimple element in a spanking operation to provide an anchoring means for the subsequent molding and bonding of the permanent magnet material to this side of the pole piece material. For example, the raised surface of the dimple element is struck with a bar of knife-like die element across the center of the surface to splay outwardly the material on each side forming a elliptical V-like shape with the splayed dimple element material extending at a low angle to the surface of the sheet material and importantly extending beyond, i.e., overhanging the underlying sheet material of the raised dimple element. In the spanking operation it is desireable to prevent the downward movement of the raised dimple elements into the indentations and thus the pin die element after the die striking of the indentation may be held in place, while the spanking operation carried out on the opposite side of the sheet material. The in-place die element prevents the further inward movement of the raised dimple element and provides solid suport for the spanking operation. A variety of spanking or other pressure-type operations may be used to provide an anchoring means on the raised dimple element surface. The amount, form, nature and extent of the spanking operation and the splaying of the material may vary as desired to provide the necessary material overhang.

After die striking and spanking, the desired pole-piece element is removed from the sheet material, typically in a die-cutting operation to provide a washer- or other type pole-piece element of defined inner and outer diameter and dimensions as required for the particular shaft and ferrofluid seal appartus. Optionally, the washer pole-piece element may be plated, such as nickel plated, to reduce corrosion or other operation carried out, for example, degreasing, corrosion treatments, deburring and the like. The washer pole-piece element is cut from the sheet material generally to provide for a plurality of uniformly spaced-apart indentations on one flat washer surface and splayed raised dimple elements on the other flat washer surface, such as, at least two and generally three to four or more, anchoring means to provide proper and secure bonding of the permanent magnet.

The washer pole-piece element is then directly bonded to a polymeric moldable magnetic material by molding or otherwise forming the polymeric material in direct contact with the flat surface of the pole piece containing the anchoring means thereon. In one method, the pole-piece element is placed in an injection mold cavity and the polymeric magnetic material is injection molded under heat and high pressure into the mold cavity and against the surface of the pole piece with the anchoring means to form an annular washer-type permanent magnet element having generally an outer diameter the same or similar to the outer diameter of the pole piece element and an inner diameter slightly greater than the inner diameter of the pole piece element. The injected molded polymer material is molded about and under the anchoring mean surface and provides a direct, close direct bond with the flat surface of the pole-piece element. This technique provides for a secure bond and avoids the problems associated withthe use of too much or too little adhesive, is a cleaner operation than the use of glue or adhesives, provides a close and more effective bond without any gap through which ferrofluid might escape and reduces the amount of and errors associated with manual-type adhesive operations.

Where a dual stage or dual pole piece-magnet combination is desired, then two pole pieces may be placed in the mold and the polymeric material injected between the pole pieces to secure each pole piece to the molded magnet material.

Any moldable or formable polymeric magnetic material may be employed to form the hard molded permanent washer magnet. Generally, the polymeric material comprises an engineering-type moldable resin particularly a thermoplastic material capable of being injection molded in single or multiple cavity molds or an elastomeric magnetic cureable material. Typically the polymer material contains high amounts of finely divided magnetic particles like iron or iron oxides dispersed therein to provide for the desired magnetic properties. The polymeric material is often employed in pellet, powder or granular form and is used alone or in combination with additives or reinforcing material such as glass fibers and spheres, silica, metal oxides, carbon fibers and particles or other fiber and particulate materials. In use the polymeric material is heated, e.g., to over 200.degree. C., and then in a melt or semi-melt condition forced, i.e., injected under high pressure, e.g., over 500 psig, into the mold cavity and then allowed to cool or react in situ and the molded hard injection molded piece then removed.

In one embodiment it has been found that a commercially available polyamide magnetic material, i.e., a magnetic nylon resin sold by 3M Company under the designations PLASTIFORM B-1055 and B-1060 having a gauss of 265 and a B-H curve maximum of 1.7.times.10.sup.-6 gauss-oersted and containing magnetic particles is a suitable material for injection molding and forming the bonded pole-piece-nylon permanent magnet combination. Nylon resin, thermoplastic engineering-type resins, thermoset resins and elastomers can also be used. Nylon resins based on condensation or addition reactions generally have a melting point of 200.degree.-300.degree. C. or higher, and well known for use in injection molding techniques; however, other injection molding type resins containing magnetic properties may be used, such as having over 250 gauss.

The pole piece-molded-magnet combination apparatus of the invention is employed typically within a nonmagnetic housing as a single-stage ferrofluid seal apparatus with the use of a high gauss ferrofluid and the insertion of a magnetically permeable shaft element. The flux flow path is through the ferrofluid in a radial gap and through an air gap to the molded magnet.

For the purposes of illustration only, the invention will be described in connection with certain embodiments; however, it is recognized that certain changes, additions and modifications may be made of the illustrated embodiments all falling within the spirit and scope of the invention.

FIG. 1 is a plan view of one side of a pole-piece element;

FIG. 2 is a plan view of the opposite side of the pole-piece element of FIG. 1;

FIG. 3 is a sectional view of a ferrofluid single-pole-piece molded magnet seal apparatus;

FIG. 4 is an enlarged, sectional, fragmentary view of a portion of FIG. 3; and

FIG. 5 is a sectional view of the upper half of a ferrofluid dual-pole-piece molded magnet seal apparatus.

FIG. 1 is a top plan view of a metal stainless steel nickel plated washer pole piece 12 having an outer diameter 14 and an inner diameter 16, the inner diameter designed to provide a radial gap of 1-6 mils about the surface of a magnetically permeable rotatable shaft element, used in the ferrofluid seal apparatus. The pole piece element 12 has four equal spaced-apart small circular indentations 18 therein caused by a multiple pin die striking the one surface of the element. The opposite surface of the sheet material contains before spanking similar cylindrical but raised dimple elements formed by the deformation of the sheet material.

FIG. 2 is bottom plan view of the opposite side of the pole piece element 12 of FIG. 1 showing a raised splayed dimple element 20 having an overhanging splayed surface as an anchoring means, the splayed surface extending at a low outward angle from the plane of the pole piece element 12. The splayed surface is formed by die striking the top surface of raised dimple element across the entire diameter 22 with sufficient pressure to deform the raised cylindrical stainless steel dimple sheet material, while the pin die element is held in position against the surface of the indentations.

FIG. 3 is a sectional view of a single pole piece single stage ferrofluid seal apparatus 10 with a rotatable shaft 26 extending through the inner diameter 16 of the single pole piece element 12 with a high gauss ferrofluid 28 in a radial gap and with the magnetic flux path shown in dotted lines for the upper position of the seal apparatus 10. The seal apparatus shows a hard molded magnetic nylon polymer 24 as the washer or annular permanent magnet of the seal apparatus, with the nylon about the splayed anchoring extensions 20 to retain and bond the nylon directly to the face of the pole piece 12.

FIG. 4 is a fragmentary enlarged schematic view of a portion of FIG. 3 showing the anchoring surface and splayed extension 20 with the center line 22 when struck by the spanking bar.

FIG. 5 is an illustration of a top portion of a dual pole piece dual stage ferrofluid seal wherein two washer metal pole pieces 32 are directly bonded to the washer nylon permanent magnet 40 therebetween by the splayed extensions 34 on the opposite side of indentations 36 of each pole piece. The magnetic flux is illustrated by the rectangular dotted line showing the flux passing through both ferrofluid O-ring seals 28 and with the extension of the ferrofluid seals on the surface schematically illustrated by parallel dotted lines extending from the ends of each pole piece.

Thus, the invention comprises employing mechanical anchoring means on at least one surface of a pole piece element and molding or otherwise forming a polymeric pressure magnetic material over and about the anchoring means typically in a molding operation to bond the polymeric material to the surface of the pole piece element and to the pole piece-magnet combination so produced.