WO1983003453A1

WO1983003453A1 - Ferrofin magnetic seal

Info

Publication number: WO1983003453A1
Application number: PCT/US1983/000365
Authority: WO
Inventors: Oscar Pinkus; Jed Walowit
Original assignee: Mechanical Technology, Incorporated
Priority date: 1982-03-25
Filing date: 1983-03-16
Publication date: 1983-10-13
Also published as: CA1208249A; JPS59500528A; IT1160834B; IT8320279A0; US4440402A; EP0104238A4; EP0104238A1

Abstract

A ferrofin magnetic-fluid seal (14) for hermetically sealing the space between a portion of a rotating member (11) of magnetically permeable material and a portion of a close-fitting, spaced-apart stationary member (12) also fabricated from magnetically permeable material and comprising a part of a housing for journaling the rotating member. The ferrofin magnetic-fluid seal (14) is comprised of a plurality of concentric, fin-like projections (16, 17) of magnetically permeable material formed on each of the rotating member (11) and the stationary member (12) in spaced-apart opposing relation to define a plurality of magnetic gap regions (18). The magnetic ferrofluid is disposed within the magnetic gap regions (18). A magnetic field producing means (21) is magnetically coupled in closed magnetic circuit relationship with at least the portions (11, 12) of the rotating and stationary members, the opposed fin-like projections (16, 17), and the magnetic ferro fluid trapped in the magnetic gaps (18), defined between the opposing ends of the fin-like projections. The new and improved ferrofin magnetic-fluid seal (14) can be used either alone or in conjuction with a centrifugal seal to form a combination magnetic/centrifugal fluid-seal.

Description

TITLE: FERROFIN MAGNETIC SEAL TECHNICAL FIELD

This invention relates to new and improved magnetic seals. More particularly, the invention relates to an improved ferrofin magnetic seal design wherein the geometry of the magnetic seal is optimized to provide maximum sealing capability in a given size magnetic seal structure. BACKGROUND PRIOR ART

U. S. Patent No. 3/620,584, issued November 16, 1971 for "Magnetic Fluid Seals", describes a modular magnetic- fluid seal wherein a magnetic fluid is captured in a gap between a rotating shaft and a stationary housing in which the shaft is journalled. The magnetic fluid thus captured, is designed to provide a leak-proof hermetic seal between the rotating shaft and the housing. There are a number of advantages provided by a magne¬ tic seal structure, namely, there is no leakage of the sealed substance across the magnetic-fluid seal, there is no rubbing or wear at the mating surfaces where the seal is formed, and structurally a magnetic seal is a very simple device. However, there are still a number of unanswered problems involved in the design and use of magnetic seals, the most significant of which is the capacity of a given magnetic seal construction to withstand differential pres¬ sure Δp across the seal. To resolve this problem, the present invention was devised. SUMMARY OF INVENTION

It is therefore a primary object of the invention to provide a new and improved ferrofin magnetic seal design wherein the seal geometry is optimized to provide maximum sealing capacity f-or a given physical size magnetic seal structure.

In practicing the invention a ferrofin magnetic-fluid seal is provided for hermetically sealing the space between a portion of a rotating member of magnetically permeable material and a portion of a close^* fitting spaced-apart stationary member - of magnetically permeable material. The. ferrofin magnetic- fluid seal comprises a plurality of concentric fin-like projections of magnetic-ally permeable material formed on eac of the rotating members and the stationary member portions i spaced-apart opposing relation to define'a plurality of magnetic gap regions. A magnetic fluid is disposed within the magnetic gap regions. A magnetic field producing means is magnetically coupled in closed magnetic circuit relation¬ ship with at least the said portions of the rotating and stationary members and further includes the opposed fin-like projections and the magnetic fluid trapped -in the magnetic gaps defined between the opposing ends of the in-like projections.

Another feature of the invention is the provision of a ferrofin magnetic fluid seal having the above-set forth characteristics wherein the fin-like projections each have a tooth width t and a tooth spacing s which are optimized so as to yield a maximum number of magnetic seal stages N over a given axial length L of the magnetic seal region in accordance with the expression N = . In preferred embodiments of the invention, the ends of the opposing fin¬ like projections are spaced-apart a distance h that defines the magnetic gaps in which the magnetic fluid is trapped to form the magnetic seal and optimum sealing capacity is obtained where t ranges in value from substantially h to 5h and s ranges in value from substantially 2h to 3h. In practical^" embodiments of the invention the gap spacing h is of the order of 10 millinches so that t ranges from 10 - 5 millinches and s ranges from 20 - 30 millinches.: The resultant ferrofin magnetic seal structure is somewhat comb¬ like in nature upon being thus dimensioned.

Still another feature of the invention is to provid a ferrofin magnetic seal having the above characteristics wherein maximum sealing capability is achieved when the desig is such that the dimension t is substantially equal to 2h and the dimension s is substantially equal to the dimension 3h where h is the gap spacing between the opposing ferrofin teeth on the ferrofin magnetic seal structure.

Still another feature of the invention is the pro- vision of a combination magnetic/centrifugal-fluid seal wherein the magnetic sealing regio is contructed . as described in the preceeding paragraphs and further in¬ cludes a circumferentially arranged centrifugal seal region radially disposed outwardly from the magnetic seal region and communicating therewith and physically located between the rotating and stationary members for receiving and pooling fluid centrifugally thrown there during high speed rotation of the rotating member to thereby form a centrifugal-hermetic seal between the stationary member and the rotating member during high speed rotation of the rotating member-

A still further feature of the invention is the pro¬ vision of a ferrofin magnetic fluid seal having the above- set forth characteristics wherein the source of magnetic flux in certain embodiments of the seal is provided by an electromagnet and in other working embodiments the source of magnetic flux may be provided by a permanent magnet. BRIEF DESCRIPTION OF DRAWINGS

These and other objects, features and many of the attendant advantages of the invention will become better understood upon a reading of the following detailed des¬ cription when considered in conjunction with the accom¬ panying drawings wherein like parts in each of the several figures are identified by the same reference character, and wherein:

Figure 1 is a partial, longitudinal sectional view of a new and improved ferrofin magnetic seal constructed according to the invention; Figure 2 is a partial fragmentary view of a portion of the ferrofin magnetic seal and illustrates details of the relative geometry and spacing of the fin-like projections formed on the shaft and housing in which the shaft is journalled and comprising a part of the ferrofin magnetic seal construction;

Figure 3 is a partial, fragmentary view of the ferrofin magnetic seal illustrating the length-wise dimension and lay-out geometry of the ferrofin construction; Figure 4 is a partial _f fragmentary view of an alternative embodiment of the ferrofin magnetic seal structur shown in Figure 1 wherein a permanent magnet is employed in place of the electromagent used in the Figure 1 embodiment of the invention; Figure 5 is a partial longitudinal sectional view of a combination magnetic/centrifugal-fluid seal constructed according to the invention wherein the magnetic seal region of the combined magnetic/centrifugal seal structure employs a ferrofin magnetic seal construction according to the invention and wherein Figure 5 illustrates the combined- magnetic/centrifugal seal while operating in the magnetic sealing mode;

Figure 6 is a similar longitudinal sectional view of a combination magnetic/centrifugal seal identical to that shown in Figure 5, but illustrates the seal in the centrifugal sealing mode;

Figure 7 is a longitudinal sectional view of still a different embodiment of a combined magne ic/cen r±fugal seal according to the invention wherein a rotatable -vane is employed in conjunction with the ferrofin magnetic seal structure and wherein Figure 7 illustrates the combined magnetic/centrifugal seal in the magnetic sealing mode; and

Figure 8 is a partial longitudinal sectional view similar to Figure 7 but illustrates the combined magnetic/ centrifugal seal in a centrifugal sealing mode. BEST MODE OF PRACTICING INVENTION"

Figure 1 is a partial, longitudinal sectional view of a new and improved ferrofin magnetic seal according to the invention. In Figure 1 a rotating shaft is shown at 11 which is. journalled by bearings (not shown) in a housing indicated generally at 12. The housing 12 may have cooling passageways shown at 13 formed therein for supply of a cool fluid for cooling the area surrounding a ferrofin magnetic seal region indicated generally at 14.- The ferrofin magnet sealing region 14 is disposed between the rotatable shaft 11 and a portion of the housing 12 indicated at 15 which is a section of housing 12 that forms the magnetic seal and is comprised of a magnetic permeable material such as stainless steel or other magnetically permeable alloys of iron. The magnetic seal forming housing section 15 has a plurality of ^' axially arrayed fin-like projections indicated at 16 which are concentrically arrayed around its inner periphery so as to surround the shaft 11. ^■ The concentric fin-like projectio 16 are secured to the inner periphery of magnetically per- meable housing section 15 by welding or other suitable means and are arrayed in opposing relationship to a corresponding set of concentric fin-like projections 17 axially secured around the shaft 11 which likewise is magnetically permeable The fin-like projections 16 and 17 are fabricated from stain less steel or other magnetically permeable -material and a ferromagnetic fluid, also referred to as a ferrofluid, shown at 18 is disposed^' between the gaps defined by the opposing opposite ends of the fin-like projections 16 and 17. The magnetically permeable housing section 15 which helps to define .the ferrofin magnetic seal has a portion 19 which is surrounded by an electromagnetic coil 21 that is-excited fro a source of electric current 22 through an on-off control switch 23. The electromagnetic coil 21 forms a source of magnetic lux, which threads through an extended part of the housing section portion 19 to the left of Figure 1 (not shown

gT} RE OMPI and spaced from the rotating shaft 11 to form a closed magnetic circuit through the shaft 11, the fin-like projecti 17, the ferrofluid^* 18 in the gaps between fin-like projectio 16 and 17, the fin-like projections16, magentically permeabl housing section 15 back through the magnetically permeable section 19 that is excited by electromagnetic coil 21. As known in the art, with the switch 23 closed, an electromagne field will be produced through the center of the coil 21 that induces the flow of magnetic flux through the closed magnetic circuit described above, thereby trapping droplets of the ferrofluid 18 between the opposed ends of the fin-like projections 16 and 17. In this manner, a hermetic ferrofin magnetic seal is formed.

Figure"2 is a partial fragmentary view of a portion of the ferrofin magnetic seal structure shown in Figure 1 and illustrates details of the relative geometry and spacing of the fin-like projections formed on shaft 11 and housing section 15. As best seen in Figure 2, each of the fin-like projections 16 and/or 17 has a tooth width t and tooth spacin between adjacent fin-like projection s which are optimized so as to yield a maximum number of magnetic seal stages N over a given axial length L of the shaft as shown in Figure 3 in accordance with the expression

The opposing spaced-apart ends of the sets of fin-like projections 16 and 17 are spaced-apart by a gap distance h which in effect determines the size of the droplet of ferro¬ fluid 18 that must be trapped within the space defined by this gap. In a seal such as illustrated in Figures 1 and 3 and which consists of a number of stages or tooth elements, there are basically two geometric variables whic can be considered, namely the minimal width of each stage t and the spacing between the stages s. In this regard, it should be kept in mind that the whole question of optimization rests on the postulate that the total extent of any given seal is

OM fixed, and an optimum design is required for a given allote axial space L. Within this constraint there are two oppos¬ ing effects; the larger the number of stages, the higher the differential pressure capacity (Δp) for the seal and it would thus be desireable to use as many as possible in the distance L provided by the stages. ^' On the other hand, too close a proximity of the teeth or ferrofin fin-like projec¬ tions can result in producing interference in each others magnetic field and prevents the attainment of the maximum possible Δp per stage. Within these opposing requirements, for a given ratio (t/h) there is an optimum corresponding ratio (s/h) which will yield the maximum total differential pressure that the multiple stage seal can withstand. The optimum values of the ratios (t/h and s/h) to yield a maximum differential pressure Δp per stage, is set forth in the below listing: t s h h

1. .00 1. .957

1. .50 2. .175

2. .00 2. ,363

2. .50 2. .529

3. 00 2. ,679

3. 50 - 2. 818

4. 00 2. 948

4. 50 3. 070

5. 00 3. 185

From the values set forth in the above listing, it will be appreciated that optimum values of Δp per stage, and hence maximum Δp for a given design multiple stage ferrofin magnetic seal according to the invention, are obtained in designs wherein the ends of the opposing fin-like projec¬ tions are spaced apart a distance h that defines the gaps in which the magnetic fluid is trapped to form the magnetic seal and wherein the value of t ranges in value from substantially h to 5h and the value s ranges-in value from .substantially 2h to 3h. In a preferred practical embodiment of a ferrofin magnetic seal according to the invention, the ends of the opposing fin-like projections are spaced apart a distance h that defines the gaps in which the magnetic ferrofluid is trapped to form the magnetic seal and wherein maximum sealin capacity is obtained by a design with t substantially equal to 2h and s substantially equal to 3h. In such a design the number of magnetic seal stages N preferably is at- least 4. As an example of the above-described preferred, magnetic seal according to- the invention providing maximum sealing capacity, a practical distance for the spacing h is 10 miili-inches in a design where the lenght L for accommo¬ dating the multiple stage seal is of the ord li- inches. According ³ to the.above formula, the ⁼

²⁰⁰ _ - 200 _ 200 _{= 4> τhe resultant}

^■ 2 h + 3h _. (20 + 30) 50 structure is somewhat comblike in nature and while it might be feared that tooth widths of only 20-30 mils would be structurally undesireable and spacings of somewhat the same order would raise the risk of flooding the interstage spaces, still the use of such fine comb-like sealing stages has been determined to offer optimum sealing conditions.

Figure 4 of the drawings is a partial, fragmentary view of an alternative embodiment of a ferrofin magnetic seal structure according to the invention wherein a permanent magnet 25 is employed in place of the electromagnet 21 used in the Figure 1 embodiment of the invention. In most appli¬ cations of ferrofin magnetic seals according to this inventio it is preferable to employ an electromagnet instead of a permanent magnet since the sealing capacity goes up with the strength of the applied magnetic field and it is easier to adjust the strength of the applied magnetic field while employing an electromagnet after a particular design seal has been fabricated. Besides allowing variations in field streng intensity, the use of an electromagnet also permits degaussin

OMP of the overall system. This latter feature may be required in order to facilitate fluid movement of the ferromagnetic fluid 18 employed with the seal. Finally, for a particular magnetic circuit, it is desirable to find the optimum magnetic field intensity, as above a certain point the pole pieces (ferro- fin-like projections) become saturated causin thereafter an actual drop in field .intensity across the magnetic fluid and consequently a drop in Δp capacity. For all of these reasons, use of an electromagnet rather than a permanent magnet is preferred. However, for some applicati the use of a permanent magnet may be dictated because of the unavailability of a source of electric power, etc. , for the particular application, in which case the permanent magnet embodiment illustrated in Figure 4 could be employed. Figures 5 and 6 of the drawings illustrate a combination magnetic/centrifugal fluid seal having a magneti sealing region defined by a plurality of axially arrayed, concentric ferro fin-like projections 16 which are arrayed oppositely from corresponding fin-like projections 17 extending from a tapered portion of the shaft 11 so that the ends oppose the ends of the fin-like projections 16 to define the gaps in which a ferrofluid 18 can be trapped in the manner described earlier with relation to Figure 1 of the drawings. The ferro fin-like projections 16 extend from a magnetically permeable portion 15 of a housing in which the shaft 11 is journalled (not shown) . In construction and operation, the magnetic sealing region thus comprised is entirely similar to that- described earlier with relation to Figures 1, 2 and 3 of the drawings. The magnetic sealing region thus comprised however, comprises a part of a combina tion agnetic/ceπtrifugal-fluid seal in that the gaps between opposing ends of the fin-like projections 16 and 17 communicate with a centrifugal seal region indicated generall at 26 and which is physically located between the housing section 15 and shaft 11 and radially disposed outwardly from shaft 11 so as to receive and pool the ferrofluid 18 that is thrown there through centrifugal action during high speed

_S^ K rotation of shaft 11 in the manner shown.in Figure 6 of the drawings. For this purpose, it is preferred that an elec¬ tromagnet be employed with the combined seal so that during high speed rotation while the combination magnetic/centrif- ugal fluid seal is in the centrifugal sealing mode shown in Figure 6, the electromagnet is de-energized to facilitate migration of the magnetically susceptible ferrofluid 18 from the magnetic sealing region into the centrifugal seal¬ ing region. For a more detailed description of the design considerations employed in fabricating the combination magnetic/centrifugal fluid seals as illustrated in Figures 5 and 6, reference is made to co-pending United States patent no. 4,304,411 for a Magnetic/Centrifugal-Fluid Seal, issued December 8, 1981 in the name of Donald F. Wilcock and Martin_^ W. Eusepi and assigned to Mechanical Technology, Incorpor¬ ated, the assignee of the present invention.

For suitable ferrofluids for use as the ferrofluid 18 described in this application, cooling features and other practical design considerations that would be employed in the fabrication of a practical ferrofin magnetic seal used alone or in conjunction with a centrifugal seal as shown in Figures 5 and 6, the disclosure of the above-noted patent no, 4,304,441 hereby is incorporated by reference in its entirety into the disclosure of this application. Briefly, however, it can be stated that at standstill and slow rota¬ ting speeds below say about 10,000 rp , the magnetic sealing region provides a hermetic seal between spaces on opposite ends of the shaft 11 between shaft 11 and housing section 15. At higher rotational speeds above say 10,000 rpm to perhaps 25,000-35,000 rpm, the centrifugal slinging action of vane 27 will cause migration of the magnetic ferrofluid 18 from between the opposing ends of the fin-like projections 16^" and 17, particularly if at the transition speed of 10,000 rpm the electromagnet used to produce the concentrated magnetic field between the opposing ends of 16 and 17, is discontinued. As a result of this centrifugal slinging action, the fluid will

OMPI pool in the space 26 thereby forming an hermetic centrifu¬ gal seal in the well-known manner of centrifugal seals as described more fully in the above-referenced patent no. 4,304,411. Figures 7 and 8 of the drawings illustrate an alter¬ native form of a combination magnetic/centrifugal-fluid seal construction according to the invention which operates in a similar manner to the combined magnetic/centrifugal seal shown and described with relation to Figures 5 and 6 but is constructed differently. In the embodiment of the invention shown in Figures 7 and 8, the fin-like projections 16 and 17 defining the magnetic sealing regions are con¬ structed so as to be arranged concentrically around the sides of an enlarged diameter vane 27 secured to and rotat- ing with the shaft 11. The fin-like projections 16 extend¬ ing from the housing section 15 oppose the fin-like projections 17 formed on the annular sides of the vane 27 by being concentrically arrayed around opposing side por¬ tions 15 of the magnetically permeable housing section to define the gaps in which the ferrofluid 18 is trapped while operating in the magnetic sealing mode. Thereafter, as the rotational speed of shaft 11 is increased above a critical speed, for example 10,000 rpm, the ferrofluid 18 migrates up into the centrifugal sealing region 26 in a similar fashion to that described with relation to Figures 5 and 6.

From the foregoing description it will be appreciated that the invention provides a new and improved ferrofin magnetic seal design wherein the magnetic seal geometry is optimized to provide maximum hermetic sealing capacity for a given size magnetic seal structure.

Having described several embodiments of a ndw and improved ferrofin magnetic seal used alone and in conjunc¬ tion with a magnetic/centrifugal-fluid seal constructed according to the invention, it is believed that other modi¬ fications,

TITUTE SHEET - variations and changes will be suggested, to those skilled in the_.art in the light of the above teachings. It is therefore to be^"understood that all such variations, modifications and changes which are within the capabilities of one having ordinary skill in the art are believed to come within the scope of the invention as defined by the appended claims.

Claims

_ .. . ^{^}^13- V7HAT IS CLAIMED IS:

.1. A ferrofin magnetic fluid seal comprising a shaft- having a magnetically permeable portion supporting a plurality of concentric fin-like projections of magnetic¬ ally permeable material circumferentially arrayed in parallel along a portion of the axial length of the shaft, a housing surrounding and rotatably supporting said shaft, said housing having a seal forming section having a plural of parallel, axially arrayed fin-like projections made fro magnetically permeable material circumferentially surroundi the interior of the housing and opposing said axially array parallel concentric fin-like projections on said shaft portion, a magnetic fluid disposed within the gaps defined between the ends of the opposing fin-like projections exten ing from said shaft portion and from said surrounding housi seal forming section, said housing seal forming section bei fabricated from magnetically permeable material and being physically disposed in a closed series magnetic circuit wit a source of magnetic flux, said closed series magnetic circuit further including the axially arrayed, concentric fin-like projections extending from said housing, the magnetically permeable shaft 'portion and the axially arraye concentric fin-like projections extending therefrom and the magnetic fluid droplets disposed in the gaps between the opposing ends of the sets of opposing fin-like projections.

2. A ferrofin magnetic fluid seal according to claim 1 wherein the fin-like projections on the magneticall permeable shaft portion and on the seal forming section eac have a tooth width t and a tooth spacing s which are optimiz so as to yield a maximum .number of magnetic seal stages N over a given axial lenght L of the shaft in accordance with the expression N max t + s

OMPI

3. A ferrofin magnetic fluid seal according to claim 2 wherein the ends of the opposing fin-like projecti are spaced apart-a distance h that defines the gaps in whi the magnetic fluid is trapped to form the magnetic sέal an wherein optimum sealing capacity is obtained where t range in value from substantially h to 5h and s ranges in value from substantially 2h to 3h.

4. A ferrofin magnetic fluid seal according to claim 3 wherein the optimum values of t and s are given by the ratios

.and wherei_n the ratios have the follow relations: t s_ h ^" h ^•

1.00 . 1.957

1.50 2.175

2.00 2.363

2.50 2.529 3.00 - 2.679

3-50 - 2.818

4.00 2.948

4.50 ^* 3.070

5.00 3.185

5. A ferrofin magnetic fluid seal according to claim 2 wherein the ends of the opposing fin-like projecti are spaced apart a distance h that defines the gaps in whi the magnetic fluid is trapped to form the magnetic seal an wherein maximum sealing capacity is obtained by a design where t is substantially equal to 2h and s is substantiall equal to 3h.

SUBSTITUTE SHEET