CA1247180A

CA1247180A - Ferrofluid bearing and seal apparatus

Info

Publication number: CA1247180A
Application number: CA000466362A
Authority: CA
Inventors: Philip Stahl; Dudley D. Fuller; Donald F. Wilcock
Original assignee: Ferrofluidics Corp
Current assignee: Ferrofluidics Corp
Priority date: 1983-10-27
Filing date: 1984-10-26
Publication date: 1988-12-20
Also published as: GB2149027B; GB2149027A; FR2554189A1; FR2554189B3; GB8427113D0; DE3439081A1; JPS60192116A

Abstract

ABSTRACT OF THE DISCLOSURE
A composite ferrofluid bearing and seal apparatus which comprises a ferrofluid seal apparatus which contains disposed therein a bearing element, to form a ferrofluid-film bearing with the surface of a shaft, the ferrofluid seal apparatus retaining the bearing ferrofluid film within the bearing cavity, by forming a ferrofluid seal O-ring, held by magnetic flux, at each end of the bearing element, the composite ferrofluid bearing and seal apparatus acting to contain the bearing ferrofluid film within the fluid-film bearing cavity and to exclude gases from being entrained in the ferrofluid.

Description

:~t~ ~7~8C) Single-, dual- or multiple-stage ferrofluid seals are usefu:Lly em-ployed for Eorming one or more ferroEluid 0-rings about a shaft element, to provide for sealing about the shaft element. Ferrofluid seals are employed with shafts having moderate speed rotation and are useful for sealing in bearing lubricants, in order to prevent the bearing lubricants from reaching areas that must be free of contamination. While such ferrofluid seals can with-stand typically a gas-pressure difference of about 3 to 5 psi per ferrofluid stage, such ferrofluid seals are generally incapable of withstanding liquid pressure; for example, a magnetic fluid pressure, and even low pressure, during dynamic operations; that is, when the shaft is being rotated. Such liquid-pressure situations may arise, for example, when a bearing lubricant reservoir is attached for fluid replenishment of the bearing fluid, or when a reservoir which is combined with circulation from the bearing through a cooler is desired, in order to maintain the desired bearing temperature during dynamic operation of the shaft.
Under dynamic operating conditions, the ferrofluid, which includes ferrolubricants which perform the function of providing a fluid film, as well as a lubricating film for a ferrofluid bearing, tends to move longitudinally outwardly, due to the rise in temperature under -the operating conditions, and expansion of the ferrofluid, by virtue of the heat generated by the shearing forces on the ferrofluid used as the fluid film in the bearing and the dif-ferential expansion of tlle component parts of the bearing.
Bifluid, hydrodynamic bearing systems have been described, for ex-ample, in United States Patent 3,439,961 in which scavenger grooves are employed, to impel the bearing fluid toward the a~ial center of the bearing cavity, so as to reduce any lea~age of the bearing fluid under load or dynamic conditions.

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This patent also provides a ferromagnetic Eluid as a bearing lubricant, wherein the bearing lubricant is magneti~ed and retained in position not only through the magnetic field, but through the use oE nonwetting TEFLON ~Dupont trade mark) material at each end of the bearing cavity. In addition, United States Patent 3,891,2~2 discloses an assembly, wherein use is made of spiralling channels which urge a lubricant into the bearing gap of a lubricating bearing assembly, where the shaft of the bearing assembly is sealed by a magnetic seal.
It is desirable to provide a composite ferrofluid-seal, fluid-film bearing, either radial, thrust or a combination of radial and thrust, apparatus of simple design and construction and which provides an effective means -to retain the ferrolubricant employed in the bearing cavity, particularly under dynamic operating conditions.
The invention relates to a ferrofluid seal-bearillg apparatus and method of retaining a ferrofluid within a fluid-film bearing assembly. In par-ticular, the invention relates to a composite ferrofluid seal-bearing assembly, wherein a ferrolubricant is maintained within a fluid-film bearing radial or thrust cavity by a magnetic ferrofluid seal, which prevents the escape of the ferrolubricant under dynamic conditions.
In one aspect, the invention comprises a ferrofluid seal-bearing apparatus Eor use with a magnetically permeable shaft element, which apparatus comprises:
a) a bearing apparatus containing a bearing element to provide a thin-film, radial or thrust bearing cavity, the bearing assembly surrounding the shaft element;
b) a first ferrofluid seal apparatus comprising an annular permanent magnet adapted to surround the shaft element and at least one pole piece in ~ 7~ 3 27737_7 a magnetic flux relationship with the permanent magnet and one end of which .is adapted to extend into a close noncontacting relationship with the surface of the shaft element to form a radial gap;
c) a ferrofluid in the bearing cavity to act as a ferro-lubricant and ferrofluid in the radial gap adapted to form a ferrofluid O-ring seal about the surface of the shaft element;
d) the ferrofluid seal positioned about the shaft element and at least at one end of the radial or thrust bearing cavity to prevent in operation the escape of ferroEluid from the radial or thrust bearing cavity of the bearing apparatus.
In a further aspect, the invention relates to a radial/
thrust bearing assembly for a housing with a cylindrical bore which surrounds a magnetically-permeable shaft, the bearing assembly comprising: a cylindrical bearing collar mounted on the shaft, the outer surface of the collar forming a fluid-film radial bearing with the inner surface of the bore, a pair of thrust rings attached to the housing on either side of the collar, the rings bearing against the ends of the collar to form two fluid-film thrust bearings, a ferrolubricant lubricating both the radial and thrust bearings, and a pair of ferrofluid seals for retaining the ferrolubricant within the bearing, one of said seals being located on the outside of each of the thrust rings and each of the seals being comprised of a magnet and pole piece which magnet and pole pieces are constructed to confine the magnetic field to the vicinity of the seal and avoid subjecting the ferrolubricant film in the bearings to the magnetic field.
The invention generally comprises a composite, compact, ~,"~ 7~

simple, ferrofluid seal-bearing assembly or apparatus, wherein the use of a magnetic field, to form a ferrofluid seal a-t either end of a bearing cavity or of a fluid-film bearing cavity or gap, is employed, to prevent the longitudinal movement or escape of the ferrofluid from the thin, fluid-film bearing cavity under dynamic operations, and which seal, at either end of the bearing cavity, prevents the entrainment of gases or air into the bearing ferrofluid. Under dynamic operating conditions, the thin film of the ferrofluid or ferrolubricant, since the ferrofluid acts both as a ferrofluid and also has lubricant properties, moves longitudinally outwardly along the shaft, due to the pressure generated in the thin ferrofluid bearing film; and due to the heat generated by the shearing force resulting from the relative motion of the bearing surface and the rotary shaft surface.
In the invention, the ferrofluid is retained within the fluid-film bearing cavity, by placing one or more ferrofluid magnetic seals at each end of the bearing cavity, to entrap the ferrofluid within the bearing cavity and to ~',f'~

inhibit the longitudinal, outward movement of the Eerrofluid from the bearillg cavity. One or more ferroEluid O-ring seals are provided at each end of the bearing cavity, the O-ring seals forming an exclusion or pressure seal and held in position by virtue of a concentrated magnetic-Elux field about the shaft.
The compact, integral, ferrofluid seal-bearing apparatus of the invention avoids the difficulties associated with prior-art practices, such as in United States Patent 3,~39,961, wherein the ferrofluid within the entire bearing cavity is subjected to a magnetic force. Generally, a ferrofluid within a concentrated magnetic field tends to increase in viscosity and to change significantly its properties, which may affect the proper operation of the ferrofluid as a fluid-film bearing fluid, at least to an increase in shearing force and an increase in the expansion of the ferrofluid employed. By employing one or more ferro-fluid sealing O-rings at each end of the cavity, and in the absence of a magnetic field throughout the fluid-film bearing cavity, the ferrofluid is retained in position, without affecting the viscosity or properties of the bearing ferrofluid.
Thus, the invention comprises in its simplest form employing a dual-stage, ferrofluid O-ring seal and encompassing the bearing element within the two pole pieces, ~o provide for a bearing cavity directly adjacent and contigu-ous with the ferrofluid O-ring seals at each end of the bearing cavity.
The composite ferrofluid seal-bearing apparatus of the invention comprises a movable, magnetically permeable shaft element, typically a rotat-able shaft element, to be placed within the radial or radial-thrust seal bearing assembly, and an annular permanent magnet which surrounds the shaft element and which has a one and another end of opposite polarity. The apparatus includes first and second pole pieces, one end of each pole piece being in a -magnetic-flux relationship, typically adjacent, with tho one and tho other ollds, respectively, of the annular permanent magnet. 'I'he other ends of each of the pole pieces extend into a close, noncontacting relationsilip with tho surfaco of the shaft element, to form one or more radial gaps under the other end of each pole piece. In the simplest form, the radial gap will be a single- or a dual-stage ferrofluid, with one radial gap at each end of the bearing cavity, while, in other embodiments, particularly where a higher-pressure seal is desired, one or the other, or both, ends of the pole pieces may be grooved or have knife edges, or the shaft under the pole pieces grooved or having knife edges, to form a plurality of radial gaps underneath each end of the pole pieces, with interstage air cavities between the respective gaps.
A ferrofluid is employed within the radial gaps, to form one or more ferrofluid 0-ring seals extending about the surface of the magnetically per-meable shaft element. A nonmagnetic bearing element, typically a cylindrical element, is disposed between the first and second pole pieces and extends into a close, noncontacting relationship with the surface of the shaft element, and typically in a closer relationship than the radial gap at the ends of the pole pieces, such as, for example, 0.1 to 1 mils, while the pole pieces typi-cally range from about 2 to 8 mils in radial g~p. The internal surface of the bearing element forms a generally thin, tubular fluid-film bearing cavity or gap about the surface of the magnetically permeable shaft element. A ferro-fluid is placed within the bearing cavity, which ferrofluid may be the same or a different ferrofluid than that employed in the radial gaps under the respec-tive pole pieces. However, the ferrofluid typically should extend continuously through the radial bearing cavity and into the radial gaps of the seal at each end of the bearing cavity, without any air or gas spaces therein, so that air lt,~J~

or gas is not entrapped in the ferrofluid during rotary movement of the shaft.
The composite ferrofluid seal-bearing assembly described is compact, simpLc :in design, is low-cost and effective in preventing the longitudinal movement or expansion outwardly of the ferrofluid from the -film-bearing cavity, and without affecting the film-bearing properties of the ferrofluid so employed in the bearing cavity.
The ferrofluid employed, for the purposes of this invention, may be any ferrofluid, but usually comprises a very low-volatility ferrofluid, which also has fluid-film bearing and/or lubricant-type properties. Generally, the type of ferrofluid employed is the same ferrofluid underneath the radial gaps and the fluid-film bearing cavity, and generally comprises a very low-volatility, synthetic hydrocarbon or ester carrier fluid with colloidal magnetic particles dispersed therein, and generally would comprise, for example, a synthetic hydrocarbon having a very narrow boiling-point distribution, to avoid the pre-sence of high-volatility materials. The viscosity of the ferrofluid may vary, - depending uponthe fluid-film bearing properties desired, but typically ranges from about 50 cp to 500 cp at 25C, and, for use as an 0-ring seal, has a magnetic saturation generally ranging from about 100 to 500 gauss.
In a further embodiment of the ferrofluid seal-bearing apparatus of the invention, it has been found desirable to alter the geometry of the exter-nal outer edges of the other ends of each of the pole pieces, in order to pro-vide a variable magnetic gradient extending toward the outer edges of each of the pole pieces, such as by chamfering, contouring or tapering the edges of the pole pieces, to provide a variable gradient. The altering of the outer edge provides additional space for the expansion longitudinally outwardly of the ferrofluid from the 0-ring seal, without loss of the ferrofluid. The alteration to)~B~

of the outer edges of the pole pieces provides an axial variation of the mag-netic gradient beneath tile ends of each oE the pole pieces, so that, as the ferrofluid expands axially, there is additional volume created under the pole pieces, to retain the ferrofluid. Thus, the ferrofluid seal-bearing apparatus provides for very low or no magnetic flux within the bearing cavity, and a generally uniform flux under the interior portion of the pole pieces and, optionally, a variable declining magnetic flux under the outer-edge portions of the pole pieces.
In another embodiment of the ferrofluid seal-bearing apparatus of the invention, a reservoir of ferrofluid is positioned on one or typically on both sides of the bearing element and adjacent the pole-piece elements, so that the excess ferrofluid within the ferro:Eluid reservoir acts as a reserve ferro-lubricant and mixes with the ferrofluid which is in the fluid-film bearing cavity, so as to aid in cooling the ferrolubricant and to limit the change in bearing clearance which might be caused by differential thermal distortion of the component parts of the apparatus. Thus, in one preferred embodiment, a ferrofluid reservoir space is created on either side of the bearing element, to contain excess ferrofluid. This ferrofluid reservoir is completely filled with ferrofluid, so as to exclude the entrainment of gases, such as air, in ; the ferrofluid within the bearing assembly. Generally, ferrofluid reservoir spaces comprise an annular disc-like space of defined height extending about the shaft and generally adiacent each end of the bearing element and between the bearing element and the ferrofluid O-ring seal created by the pole pieces~
Another embodiment of the invention comprises placing a thin layer or coating of a nonmagnetic bearing-type material at the end of the pole pieces.
The thickness of the nonmagnetic bearing material is insufficient to effect ~Jl-~7,,l ~

substantially the magnetic flux through the ends oE the pole pieces; that is, the pole pieces will still act to retain a ferrofluid 0-ring in position;
however, the bearing material; for example, 0.1 to 1 mil in thickness or more, extends within the same or slightly greater bearing thickness than the bearing element. This embodiment is particularly useful where there is a difference between the ends of the pole pieces and the bearing elements, the bearing ele-ments being closest to the shaft. Where there is a possibility of the shaft or the bearing moving, so as to touch the pole pieces, it is desirable to provide a thin bearing material at the ends of the pole pieces and to extend the pole pieces at the same or slightly greater distance than the fluid-film gap in the fluid-film bearing cavity.
In another embodiment of the invention, the ferrofluid reservoir em-ployed within the ferrofluid seal-bearing apparatus may be used alone or be supplemented by an external ferrofluid reservoir in which the ferrofluid is then fed from a feed groove into the bearing cavity and withdrawn from the bearing cavity during dynamic operation of the shaft, which arrangement is par-ticularly adaptable to a large bearing apparatus, where SUCil an external cir-culation of bearing lubricants is known and is employed -to provide a fresh bearing film to either bearing cavity, and also to control the temperature of the ferrolubricant, by external cooling of the ferrolubricant withdrawn from the bearing cavity and supplying cool lubricant to the bearing cavity during opera-tion. Thus, the use of an external heat-exchange-type ferrofluid reservoir, alone or in conjunction with the ferrofluid reservoir of the composite ferro-fluid seal-bearing assembly, also may be employed to prevent excess expansion of the ferrofluid through heating up of the ferrofluid in conjunction with the ferrofluid 0-rings at each end of the bearing cavity.

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The ferrofluid seal-bearing appara-tus of the invention also may in-clude the employment of grooves, either on the bearing surface or on the adjacent surface of the shaft adjacent the bearing surface, or on both surEaces, in order to force inwardly the ferrofluid away Erom the ferrofluid 0-ring seals at each end of the bearing assembly and inwarclly towarcl the bearing cavity; thus, helping to retain the ferrofluid in the bearing cavity in a manner like a screw, to force gradually the fluid inwardly on rotation of the shaft.
In such embodiment, there are generally at least two grooves which extencl to-gether at least 180 around the perimeter of the shaft, or there may be a plura-lity of grooves spaced apart, overlapped and canted. The grooves need not be connected and may extend peripherally generally at an angle to the axis of the shaft in the bearing apparatus, typically a few degrees for up to about 10 or more, in order to provide a pumping pressure inwardly. If the grooves are placed at 0 or 90, then, of course, there would be no pumping action. Gener-ally, the grooves should be shallow in depth and greater in width, typically, for example, from 100 to 1,000 microinch in depth or less than 1 mil, and from

2 to 10 mils in width. For example, the grooves may be placed in the surface of the bearing element, and there may be, for example, ten grooves, each groove extending some~hat more than 36, to form a peripheral circle about the shaft, the grooves spaced apart and canted 1 to 10, so that ferrofluid is forced toward the center of the bearing cavity.
The grooves should be positioned adjacent each end of the bearing element ancl adjacent the ferrofluid 0-ring seal. The employment of grooves to pump the ferrofluicl in the bearing cavity may be used in conj~mction with exter-nal fluid circulation, such as by pumping the ferrofluid, by providing for the step of a pressure-dam bearing configuration using an exit port or a hole in oJ ~

front of the dam. In addition, a multiple-daM or multiple-step desigll may be employed, if desired, as well as a herring-bone design, to provide a pumping action. Typically, ferrofluid is fed to the bearing via a return passage connected to the circumferential grooves in the bearing. The bearing feed grooves are connected to the circumferential grooves, and the circumferential grooves serve to maintain the low-pressure ambient ferrofluid supply to the annular film under the ferrofluid seals at each end. The bearing designs may be of varying types, including cylindrical, multiple-groove, multiple-lobe, pressure-dam, multiple-step, multiple-shrouded-step, herring-bone, porous or the like. Thus, the ferrofluid film under the ferrofluid seal section also may contain spiral grooves over part of the axial sections, to pump inwardly to the bearing section and assist in the sealing action of the ferrofluid seal.
The circulation of the ferrofluid in the bearing cavity permits the isolation of the high-pressure region under the bearing, and avoids high pressure under the ferrofluid 0-ring seal at each end of the bearing cavity; thus, preventing the excess expansion of ferrofluid from each end of the bearing. The ferrofluid seal-bearing apparatus of the invention permits the employment of any type and geometry of fluid-film bearing assembly within the confines of the ferrofluid 0-ring seal apparatus. However, for the purpose of illustra-tion only, a cylindrical, fluid-film bearing assembly will be illustrated.
For the purpose of illustration only, the ferrofluid seal-bearing apparatus of the invention will be described in connection with particular embodiments; however~ it is recogni3ed that various changes, additions, modi-fications and improvements to the illustrated embodiments may be made by those persons s~illed in the art, all falling within the spirit and scope of the invention.
In drawings which illustrate embodiments of the invention, Figure 1 is an illustrative sectional view of a portion of the ferro-fluid seal radial bearing apparatus of the invention;
Figure 2 is an illustrative sectional view of another embocliment of the ferrofluid seal radial bearing apparatus of the invention; and Figure 3 is an illustrative sectional view of a ferrofluid seal radial-thrust bearing apparatus of the invention.
Figure 1 illustrates a sectional view of the top half of a ferrofluid seal-bearing apparatus 10 of the invention, comprising an annular permanent magnet 12 and pole pieces 14 and 16, the pole pieces extending toward a shaft element 20, to form a radial gap at each end thereof of about 2 to 3 mils, and extending between each of the pole pieces. Occupying the space below the annular permanent magnet 12 is a nonmagnetic bearing-type material, such as bronze bearing material, in which the interior surface thereof extends into a close, fluid-film bearing relationship with the surface of the shaft 20, to form a fluid-film bearing cavity, and which surface contains peripheral shallow-type grooves 30 disposed a few degrees offset from the axis of the shaft 20.
The fluid-film bearing cavity is typically about 0.1 to 0.8 mils from the surface of the shaft. The ends of the pole pieces contain a thin coating of a bearing material 2S positioned to make the ends of the pole pieces generally flush with the surface of the bearing element lS, so as to present a bearing surface which extends over the entire ferrofluid seal-bearing apparatus 10. A
ferrofluid, which also acts as a ferrolubricant 26, is placed in the bearing cavity and underneath the radial gap of each pole piece, to form a continuous flow of ferrofluid 26, and wherein, at each end of the apparatus 10, there is ~ 7J ~ 8 formed, as illustrated by the dotted lines across the surface of the shaft, ferrofluicl 0-ring seals 22 and 24, with the ferrofluid underneath the radial gaps of the pole pieces 14 and 16 retained in position, by virtue of the mag-netic-flux. The magnetic-flux surface passes through the permanent magnet 12, the pole pieces 14 and 16, the ferrofluid 26 beneath each of the ends of the pole pieces 14 and 16 and the magnetically permeable shaft 20.
In operation, the magnetic-flux from the pole pieces 14 and 16, which form the 0-ring seals 22 ancl 24, retains the ferrofluid between the 0-ring seals 22 and 24 as a fluid-film ferrolubricant or ferrofluid across the length of the bearing element 18. The shallow grooves 30 on the surface of the bear-ing element, which may or may not be connected via grooves as an inlet and outlet into the bearing apparatus to an externally cool ferrofluid reservoir, are offset slightly from the access of the shaft 20 and are placed adjacent the pole pieces, so as to force the ferrofluid, on the inward side of the 0-rings 22 and 24, inwardly toward the center of the bearing l~. In this embodiment, the ends of the pole pieces are capped with bronze, nonmagnetic bearing material which extends the ends of the pole pieces in line with the surface of the bear-ing element 18, to extend the bearing surface. In this arrangement, then any canting of the bearing apparatus of the shaft does not cause any scoring damage, while the thinness of the nonmagnetic material does not affect substantially the magnetic-flux ~hich retains in position the ferrofluid 0-ring seals 22 and 24 and prevents the outward, longitudinal, axial expansion of the ferrofluid under dynamic conditions, and, further, prevents the entrapment of air or gases in the ferrofluid underneath the bearing element 18.
~igure 2 is a half-sectional view of a ferrofluid seal-bearing apparatus 50 of the invention, wherein the apparatus includes a ferrolubricant ~ 7 ~ 27737-7 reservoir. The apparatus 50 includes a nonmagnetic housing 52 containing an axially polarized, annular permanent magnet 54 surrounding a shaft 70, the magnetically permeable shaEt 70 having opposed magnetically permeable pole pieces 56 and 58, the ends of the pole pieces 56 and 58 being generally parallel with the surface of the shaft and extending into a close, noncontacting relationship with the surface of the shaft, to form a radial gap 7~ and 76 therewith, for example, 2 mils. Positioned within the housing and between the pole pieces 56 and 58 is generally cylindrical, non-magnetic, for example, bronze, bearing element 60 having a surface which is spaced apart, to form a fluid-film bearing cavity 72 adjacent the surface of the shaft 70. On either side of the bearing 60, there is formed, by the pole pieces 56 and 58, ferro-fluid reservoirs 82 and 84 in which excess ferrofluid 86 is employed, which ferrofluid acts to form a ferrofluid O-ring at the end of pole pieces 56 and 58, the O-rings illustrated by dotted lines 88 and 90 extending across the surface of the shaft 70.
The bearing 60 generally extends into a closer relationship with the surface of the shaft 70 than do the ends of the pole pieces 56 and 58. Further, the exterior edges of the pole pieces 56 and 58 are chamfered, to provide surfaces 78 and 80, so as to provide additional volume under the ends of the pole pieces and a variable magnetic-flux gradient at the end of each of the pole pieces.
In operation, the magnetic-flux passes through the permanent magnet 54, the pole pieces 56 and 58 and the ends of the pole pieces, both the flat ends and the chamfered ends 78 and 80, to retain the ferrofluid 86 as O-ring seals 88 and 90 at each end 1~?J ~7.~8~3 of the appara-tus 50. The ferrofluid 86 is shown in an expanded condition, but still retained within the external edges 78 and 80 of the pole pieces 56 and 58 by the variable magnetic-flux.
The chamfered edges 78 and 80 permit the expansion outwardly of the ferrofluid, -13a-~,~f~

without loss of ferrofluid as the ferrofluid expands, due ~o heating up of the ferrofluid by shearing forces md by the expansion of the components of the seal 50, due to differential thermal expansion of different materials.
In operation, by rotation of shaft 70, the ferrofluid in the narrow fluid-film cavity 72 circulates and moves into cavities 82 and 84 at each end thereof, providing new ferrolubricant from the cavity into the bearing cavity 72; thus aiding in reducing the rise in temperature of the ferrofluid. The ferrofluid completely occupies the apparatus 50 and both cavities 82 ancl 84, in order not to provide the source of air for entrapment of the ferrofluid, which would alter the ferrofluid properties and would affect the bearing proper-ties in the bearing cavity 72. As illustrated, the ferrofluid is retained through the employment of adjacent ferrofluid reservoirs 82 and 84 and by the chamfered edges 78 and 80 within the ferrofluid seal-bearing apparatus 50, with the excess ferrofluid S6 in the reservoirs 82 and 84 acting as a reserve ferro-lubricant, and which excess ferrofluid mixes with the ferrofluid in the high-pressure portion of the bearing within the film bearing cavity 72, which aids to cool the ferrofluid and limit the change in bearing clearance. Optionally, of course, the bearing 60 may contain grooves and inlets and outlets, in order to remove ferrofluid from the ferrofluid reservoir and to cool externally the ferrofluid and to recirculate the ferrofluid bac~ into the reservoirs, and/or to contain scavenger grooves on the surface of the bearing in the bearing cavity, to force inwardly the ferrolubricant.
Figure 3 illustrates a sectional view of a ferrofluid seal radial-thrust bearing assembly 100, which comprises a steel, magnetically permeable rotatable shaft 102 having secured thereto for rotation therewith a hardened steel collar 106 positioned within a bron~e outer housing 104. The exterior ~.~J~ 7~13V

radial surface of the collar 106 and the interior radial surface o the housing 104 are spaced apart, for example, 0.1 to 1 mil, and form opposing radial bear-ing surfaces 128. At each end of the radial-thrust bearing collar 106 are positioned lower and upper collars 108 and 110 which form thrust-bearings. The upper surface of the lower collar 108 and the lower surface of the upper collar 110 are spaced apart from the respective lower and upper ends of the collar 106 to form thrust-bearing sur:Eaces 124 and 126. The radial bearing 106 is further characterized by a radial reservoir cavity 112 for the storage of a ferrolubri-cant 114.
At the lower and upper ends of the radial-thrust bearing assembly, as illustrated~ are single-stage, single-pole-piece ferrofluid seals, such seals are described in Canadian patent application Serial No. 443,394, filed December 15, 1983. The ferrofluid seals each comprise an annular permanent magnet 116 and 118 and a magnetically permeable, for exampleJ steel, pole piece 120 and 122. One end of each pole piece extends toward and into a close non-contacting relationship with the exterior surface of the rotatable shaft 102 to form upper and lower radial gaps of, for example, 1-3 mils. Ferrolubricant 114 in the gaps forms respectively upper and lower seals which contain the ferrofluid between such upper and lvwer seals. The exterior side of the upper and lower pole pieces 120 and 122 are chamfered at an oblique angle illustrated at 45 degrees as in Figure 2 to permit some outward radial movement of the ferro-lubricant 114 in operation and to form a radiant density magnetic-flux field under the pole piece.
As illustrated, the ferrolubricant 114 extends between the thrust and radial bearing surfaces and into the radial cavity be~ween the pole piece magnet and upper and lower collars 108 and 110 to form a continuous layer of fcrro-fluid 11~ on both the radial and thrust-bearing surfaces. 'I'ho bearing assombly 100 illustrated is short in length, and, therefore, the single-pole-piece seal may be used at its lower level, since the amount of hydrostatic pressure on the pole piece lower seal is not high, for example, 2-~ inches of water; how-ever, if desired, a multiple-stage ferrofluid seal may be employed as a lowor ferrofluid seal with or without pressurization in order to obtain the ferro-fluid where the hydrostatic head becomes excessive. As illustrated, lower and upper collars 108 and 110 are spaced apart a greater distance from the exterior surface of the shaft 102 to permit the passage of ferrolubricant but does not form, but may, a radial-thrust bearing surface. The single-stage ferrofluid seal which extends about the periphery of the shaft 102 is illustrated by the dotted lines extending from the radial gap and the ferrolubricant retained therein by the magnetic-flux.
In operation, the ferrolubricant extends between the upper and lower ferrofluid seal while the ferrolubricant in the cavity 112 permits the ferro-lubricant therein to mix with the ferrolubricant on the thrust and radial bear-ing film surfaces to aid in cooling the ferrolubricant while the chamfered edge at each end of pole pieces 120 and 122 aids in retaining the ferrolubricant within the ferrofluid seal bearing assembly. The apparatus of Figure 3 illus-trates a ferrofluid seal with a combined radial and thrust bearing apparatus.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A radial/thrust bearing assembly for a housing with a cylindrical bore which surrounds a magnetically-permeable shaft, the bearing assembly comprising:
a cylindrical bearing collar mounted on the shaft, the outer surface of the collar forming a fluid-film radial bearing with the inner surface of the bore, a pair of thrust rings attached to the housing on either side of the collar, the rings bearing against the ends of the collar to form two fluid-film thrust bearings, a ferrolubricant lubricating both the radial and thrust bearings, and a pair of ferrofluid seals for retaining the ferrolubricant within the bearing, one of said seals being located on the out-side of each of the thrust rings and each of the seals being comprised of a magnet and pole piece which magnet and pole pieces are constructed to confine the magnetic field to the vicinity of the seal and avoid subjecting the ferrolubricant film in the bearings to the magnetic field.

2. A bearing assembly according to claim 1 which includes a fluid reservoir for retaining a predetermined quantity of ferrolubricant within the bearing assembly.

3. A bearing assembly according to claim 2 in which the reservoir is formed by a groove cut into the collar.

4. A bearing assembly according to claim 1 in which the ferrofluid seal comprises an annular, axially-polarized magnet sandwiched between a pair of the pole pieces which extend close to the shaft to form two gaps and which completes a magnetic circuit with the shaft, the ferrolubricant being trapped in both gaps by the magnetic field to retain the ferrolubricant in the bearing assembly.

5. A bearing assembly according to claim 4 in which the inner surface of the magnet has a diameter substantially greater than the diameter of the shaft and the reservoir consists of an annular space formed by the magnet, the pole piece and the shaft.

6. A bearing assembly according to claim 4 in which the pole piece located towards the exterior of the magnet assembly has an exterior face which is chamfered at the edge next to said shaft to accommodate expansion of the ferrolubricant.

7. A bearing assembly according to claim 4 in which the ferrofluid seal gaps are about 2-8 mils wide.

8. A bearing assembly according to claims 1 to 3 in which the fluid-film bearings have fluid gaps of about 0.1-1 mils.

9. A bearing assembly according to claims 1 to 3 in which the bearing collar is formed of non-magnetic material.

10. A bearing assembly according to claim 9, wherein the bearing collar includes shallow scavenger grooves on the surface of the collar facing the bore walls, the grooves being angularly offset from the axis of rotation of the shaft in order to force the ferrolubricant toward the center of the bearing assembly.

11. A bearing assembly according to claim 10, in which the scavenger grooves are positioned at each end of the collar and in which the grooves peripherally encircle the shaft.

12. A bearing assembly according to claims 4 to 6, in which the ferrofluid seal pole pieces each include a thin layer of non-magnetic bearing material on the annular surfaces facing the seal gaps, the gearing material being sufficiently thin to avoid affecting the magnetic field distribution in the seal gaps and sufficiently thick to provide a bearing surface in case of inadvertent contact between the shaft and the pole piece.

13. A ferrofluid seal-bearing apparatus for use with a cylindrical, magnetically-permeable shaft, which apparatus comprises:
an annular permanent magnet polarized axially with respect to said shaft, said magnet having a first end and a second end and an inside diameter greater than the diameter of said shaft, a first annular pole piece having a first end substantially abutting said first end of said magnet and a second end extending close to, but not contacting, the surface of said shaft to form a first radial gap, a second annular pole piece having a first end substantially abutting said second end of said magnet and a second end extending close to, but not contacting, the surface of said shaft to form a second radial gap, a bearing element located in the annular cavity formed by said magnet and said first and second pole pieces, said bearing element being formed of non-magnetic material which extends from the outer diameter of said cavity close to, but not contacting, said shaft and which material also extends over at least the second end of said first pole piece, and a ferrofluid filling said annular cavity and first and second radial gaps whereby said ferrofluid acts as a sealing agent and a lubricant.

14. A seal-bearing apparatus according to claim 13 wherein said bearing material extends over the second ends of said first and said second pole pieces.

15. A seal-bearing apparatus according to claim 13 wherein said first and second pole pieces have a first face which substantially abuts said magnet and a second face which faces the environment external to said bearing and wherein the second face of at least one of said first and second pole pieces is chamfered at the second end to produce a tapered gap between said pole piece and said shaft.

16. A seal-bearing apparatus according to claim 15 wherein the second face of both of said first and second pole pieces is chamfered at the second end to produce a tapered gap between said pole piece and said shaft.

17. A seal-bearing apparatus according to claim 13 wherein said bearing material does not fill said cavity so that at least one annular space is formed by said magnet, said bearing element and one of said pole pieces, which annular space is filled with ferrofluid and acts as a ferrofluid reservoir.

18. A ferrofluid seal-bearing apparatus for use with a cylindrical, magnetically-permeable shaft, which apparatus comprises:
an annular permanent magnet polarized axially with respect to saidshaft, said magnet having a first end and a second end and an inside diameter greater than the diameter of said shaft, a first annular pole piece having a first face which substantially abuts said magnet and a second face which faces the environment external to said bearing, a first end substan-tially abutting one end of said magnet and a second end extending close to, but not contacting, the surface of said shaft to form a first radial gap, said second face of said first pole piece being chamfered at the second end to produce a tapered gap between said pole piece and said shaft, a second annular pole piece having a first end substan-tially abutting one end of said magnet and a second end extending close to, but not contacting, the surface of said shaft to form a second radial gap, a bearing element located in the annular cavity formed by said magnet and said first and second pole pieces, said bearing element being formed of non-magnetic material which extends from the outer diameter of said cavity close to, but not contacting, said shaft, and a ferrofluid filling said annular cavity and first and second radial gaps whereby said ferrofluid acts as a sealing agent and a lubricant.

19. A seal-bearing apparatus according to claim 18 wherein said second pole piece has a first face which substan-tially abuts said magnet and a second face which faces the environment external to said bearing and wherein the second face of said second pole piece is chamfered at the second end to produce a tapered gap between said pole piece and said shaft.

20. A seal-bearing apparatus according to claim 18 wherein said bearing material does not fill said cavity so that at least one annular space is formed by said magnet, said bearing element and one of said pole pieces, which annular space is filled with ferrofluid and acts as a ferrofluid reservoir.

21. A seal-bearing apparatus according to claim 18 wherein said bearing material substantially fills said cavity and extends over the second ends of said first and second pole pieces.

22. The apparatus of claim 13 wherein the first and second radial gaps are 2 to 8 mils wide, and wherein the gap between the surface of the non-magnetic bearing element and the surface of the shaft ranges from about 0.1 to 1 mil.

23. The apparatus of claim 13 wherein the bearing element includes shallow scavenger grooves on the surface of the bearing element, the grooves angularly offset from the axis of the shaft, to force ferrofluid inwardly toward the center of the bearing element.

24. The apparatus of claim 23 wherein the scavenger grooves are positioned toward each end of the bearing, which grooves peripherally encircle the shaft.

25. The apparatus of claim 22 wherein the bearing element is a cylindrical element which forms a generally tubular, thin-film, bearing cavity with the surface of the shaft.