WO1984004372A1

WO1984004372A1 - An improved magnetic seal assembly

Info

Publication number: WO1984004372A1
Application number: PCT/US1984/000472
Authority: WO
Inventors: Donald J Macleod
Original assignee: Applied Motion Products Inc
Priority date: 1983-04-20
Filing date: 1984-03-29
Publication date: 1984-11-08
Also published as: JPS60501122A; EP0143800A1; EP0143800A4

Abstract

An improved magnetic liquid seal assembly (10) including an inner magnetic ring (18) concentrically bonded to the shaft (12), an outer magnetic ring (20) concentrically surrounding the inner magnetic ring (18) with a uniform air gap (22) therebetween and a quantity of magnetic fluid (24) disposed about the air gap (22) such that there is no passage of air or debris through the air gap (22). The inner and outer magnetic rings (18 and 20) are permanently magnetized units while the magnetic fluid (24) is a suspension of permanently magnetized particles in a carrier fluid. The direction of polarization in both the inner and outer magnetic rings (18 and 20) is parallel to the axis of the shaft (12), but is opposite in the two rings, at least in the vicinity of the air gap (22). In the preferred embodiment, the magnetic rings (18 and 20) are provided with iron pole pieces (26) on the planar surfaces to intensify the field strength. Furthermore, both the inner and outer magnetic rings (18 and 20) are provided with surface magnetizations in the nature of guard rings (28 and 30) which prevent the magnetic fluid (24) from migrating along the surface of the pole piece (26) to either the shaft (12) or the housing (16). The primary use of the improved seal (10) is in disk drive assemblies in the data processing and computer peripheral industries.

Description

AN IMPROVED MAGNETIC SEAL ASSEMBLY

TECHNICAL FIELD

The present invention relates generally to methods of sealing apertures and more particularly to magnetic fluid seals. The predominant current usage of the improved magnetic seal of the present invention is to provide a complete seal about a rotating shaft entering the housing of a Winchester disk drive apparatus.

BACKGROUND ART

Problems frequently exist regarding providing an effective seal at a point where a rapidly rotating shaft passes through an aperture in a housing. These problems are particularly noticeable in modern data processing applications, and especially in regard to Winchester disk drive apparatus. The Winchester data storage disk is susceptible to contamination from any debris which may enter the disk chamber. This contamination can result in erroneous recording or reading of data and can have far reaching) consequences. Accordingly, it is important to eliminate, in as much as is possible, any entry of debris of any kind into the disk chamber.

One possible source of contamination occurs in the vicinity where the rotating spindle shaft enters the disk chamber through the chamber housing. It is necessary that there be an aperture for shaft entrance but it is difficult to seal that aperture against entry of debris. Contamination may enter the chamber along the shaft from a variety of sources, including the shaft bearings. Various methods have beeh attempted to properly seal this aperture.

Some methods which have been utilized in the prior art are rubbing surfaces such as rubber washers, controlled clearances, pressurized chambers and hydrodynamic pumping. Rubbing surfaces are not applicable to high precision data processing applications since the irregular friction could cause positioning errors. Furthermore, the seal material may generate debris upon wearing. Controlled clearances are insufficient since milling techniques are hot available to provide clearances which are sufficiently small to prevent entry of any debris whatsoever. Even minute debris can cause contamination. Pressurized chamber methods require continuous power and are subject to equipment failures, Hydrodynamic fluid pumping seals have proved ineffective because of fluid drift and contamination.

One important prior art method for sealing the apertures about rotating shafts Utilize a magnetic fluid, ferrofluid or magnetic liquid. In this sort of device, the material utilized to block the aperture from debris entrance is a magnetic fluid or ferrofluid is a permanent dispersion of exceedingly fine microscopic sized particles of permanently magnetized material in a liquid carrier such as a mineral oil.

The technology of seals utilizing; magnetic fluids was described in the article "Magnetic-Fluid Seals" by R. E. Rosenweig, G. Miskolczy and F. D. Ezekiel which appeared in the March 28, 1968 issue of Machine Design.

The method of uitiϋzihg magnetic fluids as part of liquid seals for shafts is described in U.S. Patent no. 4,171,818, issued to R. Moskowitz et al and in U.S. Patent No. 4, 293, 137, issued to F. D. Ezekiel. These patents described various ways in which a liquid magnetic seal may be achieved to prevent debris passage about a rotating shaft.

Some disadvantages which have been encountered with prior art magnetic fluid seals are that , since the magnetic liquid is ordinarily in direct contact with the shaft it has been known to creep along the shaft and directly cause contamination. This creepage may also eventually result in the loss of enough magnetic fluid to destroy the integrity of the seal. Another disadvantage is that with most of the prior art methods, the shaft itself must be a magnetic material in order to provide a return flux path for the magnetic field. This prevents the use of lighter more efficient materials sϋch as aluminum. Another disadvantage is that the field strength in the prior art magnetic seals is inherently limited by spatial consideration. This results in lower integrity of the magnetic fluid and shorter lifetime for the seals. None of the prior art methods efficiently provide a high integrity, long life seal about rotating shafts of any material. There still exists a heed for magnetic seal assemblies with higher field strength in the area of the Magnetic fluid and which totally prevent the phenomenon of magnetic fluid creep along the shaft.

DISCLOSURE OF INVENTION Accordingly, it is an obj ect of the present invention to provide an improved magnetic seal assembly which prevents the magnetic fluid from creeping along the shaft. It is another object of the present invention to provide a magnetic seal assembly wherein the effective field strength about the ferrofluid is increased without modifying the magnet material.

It is a further object of the present invention to provide a magnetic seal assembly which reduces the possibility of ferrofluid escape due to mechanical shock or other physical factors.

The present invention is an improved magnetic fluid seal assembly particularly adapted for use about a rotating shaft. The invention is especially intended for use in situations where a spindle shaft enters the housing of a Winchester disk drive apparatus.

Briefly, a preferred embodiment of the present invention is an improved magnetic liquid seal assembly including an inner magnetic ring concentrically bonded to the shaft, an outer magnetic ring concentrically surrounding the inner magnetic ring with a uniform air gap therebetween and a quantity of magnetic fluid disposed about the air gap such that there is no passage of air or debris through the air gap. The inner and outer magnetic rings are permanently magnetized units while the magnetic fluid is a suspension of permanently magnetized particles in a carrier fluid. The direction of polarization in both the inner and outer magnetic rings is parallel to the axis of the shaft, but is opposite in the two rings, at least in the vicinity of the air gap. In the preferred embodiment, the magnetic rings are provided with iron pole pieces on the planar surfaces to intensify the field strength. Furthermore, both the inner and outer magnetic rings are provided with surface magnetizations in the nature of guard rings which prevent the magnetic fluid from migrating along) the surface of the pole piece to either the shaft or the housing.

It is an advantage of the present invention that the magnetic fluid does not come into direct contact with the shaft and is thus prevented from creeping alohg the shaft into the sealed chamber.

It is another advantage of the present invention that the opposing polarizations of inner and outer rings intensifies the magnetic field strength at the edges of the air gap and thus maintains the integrity of the magnetic fluid. It is a further advantage of the present invention that the magnetic guard rings on the surface of the inner and outer ring magnets prevent any magnetic fluid from migrating to the shaft or the housing.

It is yet another advantage of the present invention that the magnetic fluid seal assembly may be utilized with shafts and housings constructed of any material desired.

It is another advantage of the present invention that the magnetic liquid in the seal may be maintained at a lower temperature due to the increased size of the air gap, and thus the seal has an increased life span.

These and other objects and advantages of the present invention will become clear to those skilled in the art in view of the description of the best presently known mode of carrying out the invention and the industrial applicability of the preferred embodiment as described herein and as illustrated in several figures of the drawing.

BRIEF DESCRIPTION OF THE DRAWING

Fig. 1 is a perspective view of an improved magnetic seal assembly according to the present invention, shown installed upon a rotary shaft passing through a housing aperture; Fig. 2 is a cross sectional view taken along lines 2-2 of Fig. 1; and

Fig. 3 is a schematic top plan view of the magnetic seal assembly showing the polarization of the guard rings and other magnetic elements. BEST MODE OF CARRYING OUT INVENTION

The best presently known mode of practicing the present invention is an improved magnetic fluid seal assembly particularly characterized by concentric opposingly polarized magnets in a magnetic-fluidic seal therebetween, the improved magnetic seal assembly is particularly adapted for providing a seal of an aperture between a rotary shaft and a housing.

The primary present use of the improved seal is in regard to spindle shafts entering the housing of Winchester disk drive apparatus. The presently preferred embodiment of the improved magnetic seal assembly is illustrated in Fig. 1 and designated by the general reference character 10. The seal assembly 10 is shown as installed upon a rotary shaft 12 which passes through an aperture 14 in a chamber housing 16. This relationship is typical to that found as a rotary shaft enters the disk chamber of a disk drive apparatus.

The improved magnetic fluid s£al assembly 10 includes an inner magnetic ring 18 and a concentric outer magnetic ring 20. The outer magnetic ring 20 is concentrically disposed about the inner magnetic ring 18 with a uniform air gap 22 disposed laterally therebetween. The air gap 22 is sealed against the passage of matter by a magnetic fluid 24 inserted therein.

The structure of the improved magnetic fluid seal assembly 10 is shown in a cross sectional view in Fig. 2. In this illustration it may be seen that the inner magnetic ring 18 is rigidly bonded to the shaft 12. Similarly, the outer magnetic ring 20 is rigidly bonded to the housing 16. In both cases, the bonding must be sufficiently strong to hold the rings 18 and 20 in place. The bonding must furthermore form a complete seal such that no matter or debris may pass into the chamber at the bonding interface.

Since the inner magnetic ring 18 is securely bonded to the rotary shaft 12, it will rotate with the shaft. For this reason, the inner magnetic ring 18 must be carefully circumferentially balanced such that it does not in any way degrade the purity of rotation of the shaft 12. The illustration of Fig. 2 shows that the inner magnetic ring 18 and the outer magnetic ring 20 have a definite thickness such that the air gap 22 extends through the total thickness of the rings. Because of magnetic properties and fields the magnetic fluid 24. will be disposed only at the edges of the air gap 22 and will ordinarily not be found in the center of the air gap 22.

The illustration of Fig. 2 shows the direction of magnetization of the magnetic rings 18 and 20. As is shown, both the magnetic rings are polarized in a direction parallel to that of the axis of rotation of the rotary shaft 12. However, the inner magnetic ring 18 is polarized in a direction opposite to that of the outer magnetic ring 20. In Fig. 2 the illustration shows the inner magnetic ring 18 to be polarized in the South direction while the outer magnetic ring 20 is polarized in a North direction. However, these directions could be reversed with no alteration of effect. The opposing polarities in the magnetization of the inner and outer magnetic rings 18 and 20 results in an additive effect on the magnetic field in the vicinity of the edges of the air gap 22. This concentration in the magnetic field acts to hold the magnetic fluid 24 within such vicinity. The stronger the magnetic field the greater the integrity of the magnetic fluid 24 and the less chance of any leakage through the air gap 22. The opposing polarity arrangement of the magnetic rings results in an increase in the magnetic field strength about the ferrofluid 24 without requiring any strengthening of the isolated magnetic field of the individual ring magnets.

As is also shown in Fig. 2 the magnetic field strength is increased and concentrated by the addition of a pair of ferromagnetic pole pieces 26 on the upper and lower planar edges of the inner ring magnet 18 and the outer ring magnet 20. These pole pieces 26, ordinarily constructed of soft iron,or magnetic steel act as magnetic conductors and concentrators to increase the integrity of the magnetic particle-fluiddispersion 24. The pole pieces 26 are ridigly adhered or otherwise bonded to the inner ring magnet 18 and the outer magnetic ring 20. Fig. 3 illustrates, in schematic fashion, a further improvement to the preferred embodiment. In this illustration, the magnetic seal assembly 10 is shown with the polarity of the various portions of the inner magnetic ring 18 and the outer magnetic ring 20 schematically illustrated. In this view, the inner magnetic ring 18 is shown to include an inner guard ring 28 while the outer magnetic ring 20 is shown to include an outer guard ring 30. The inner guard ring 28 and the outer guard ring 30 are in the nature of magnetically polarized rings on the surface of the rings 18 and 20 which are polarized in a direction opposite to the remainder of the ring. Ordinarily, this is a surface magnetization only. The net result of the guard rings 28 and 30 is that any magnetic fluid 24 which in any manner escapes the vicinity of the air gap 22 will be prevented by the guard rings 28 and 30 from migrating or creeping either to the shaft 12 or the housing 16. Ordinarily, the guard rings 28 and 30 will be required only on the secured chamber side of the assembly 10. However, it may be desirable to place similar guard rings on the exterior surface in order to prevent loss of magnetic fluid 24.

The inner magnetic ring 18 and the outer magnetic ring 20 of the preferred embodiment may be constructed of any suitable magnetic material. Due to the field concentration caused by this spatial array materials generating weaker fields but having superior milling capabilities may be utilized. The preferred material is ordinarily strontium ferrite embedded in a nylon matrix.

The pole pieces 26, which act to conduct and concentrate the field, are typically constructed of soft iron or magnetic steel. The pole pieces 26 are constructed to coincide with the axial surfaces of the inner magnetic ring 18 and the outer magnetic ring 20. They do not extend significantly into the air gap 22. The magnetic liquid or ferrofluid 24 is selected to be a very fine dispersion of solid particles of magnetic material in a carrier fiuid. For best results, the fluid is selected to have extremely fine permanently magnetized particles disbursed throughout the fluid. A carrier liquid such as mineral oil is selected to prevent the particles from coagulating or settling within the fluid. in this manner, even though the liquid portion of the magnetic fluid 24 has no magnetic properties the entire magnetic fluid 24 acts as if it were a flowable magnetic material. Appropriate ferrofluids 24 are commercially available. The dimensions of the various elements of the improved magnetic seal assembly 10 are predominantly a matter of choice. Since the magnetic fields of the inner magnetic ring 18 and the outer magnetic ring 20 are to some extent affected by the ring radius it is desirable to select the rings to have sufficient size to generate a reasonable field. Although the air gap 22 is shown in the drawing as having a significant width, this is merely for illustration purposes. In practice it is desirable to make the air gap as. narrow as possible to maintain higher field strengths. A typical air gap should have a width of approximately 5.0 x 10-5 meters to 7.5 x 10-5 meters (0.002-

0.003"). The extremely narrow air gap is desirable because it prevents the possibility of gross debris sufficient to disrupt the magnetic field or the magnetic fluid from entering into the air gap 22. The larger chunks of debris are physically restrained and may only disrupt the magnetic fluid 24 at one edge of the air gap 22 and would not reach the still intact magnetic fluid 24 seal at the other edge.

One advantage of the magnetic arrangement of the present invention is that an equivalent field strength to prior art seals may be accomplished with a wider air gap 22. This may be desirable in some applications since the wider the air gap 22 the lesser the frictional heat generated in the magnetic liquid 24 by the shaft rotation. A reduction of frictional heat may have the effect of reducing fluid loss by evaporation and thus increasin the effective lifespan of the seal 10. The reduced shear stress may become a relevant factor over the fifty thousand hour lifespan of the typical motor on which the seal is Utilized With the exception of the surface magnetization of he inner guard ring 28 and the outer ring 30, the inner magnetic ring 18 and the outer magnetic ring 20 are uniformly polarized. The balanced symetrical field resulting from uniform polarization is useful in maintaining the continuity and integrity of the seal.

It is to be noted that while the inner magnetic ring 18 must be in the form of a balanced symetrical ring for proper rotation there is no.such requirement on the exterior dimensions of the outer ring 20. As long as the inner edge of the outer magnetic ring 20 informed to concentrically enclose the inner ring 18 with appropriate air gap 22 therebetween, there is no restriction preventing the outer ring 18 from having any exterior shape required to fit into the opening in the housing 16.

The shaft 12 and the housing 16 may be of any material desired. Since the magnetic fluid 24 is displaced from contact with any elements other than the inner ring 18 and the outer ring 20 the selection of materials of other elements is irrelevant to the operation of the invention.

Although the invention has been described above exclusively in terms of an application utilizing a rotary shaft, there is no requirement limiting the invention to this sort of circular seal. The magnetic liquid seal of the present invention will operate in any application wherein there is an air gap between one element and another. It is especially useful in the instances wherein the elements are in motion with respect to one another. Another typical usage of the magnetic seal of the present invention would be in an application wherein two co-planar elements move with respect to one another along an air gap. An alternating forward and back motion would be similar to that of the rotary motion described above. The magnetic seal of the present invention would operate equally well in a linear air gap operation such as this as it will in the radial air gap described above.

INDUSTRIAL APPLICABILITY The improved magnetic liquid seal assembly of the present invention is adaptable to any Use wherein an air gap must be sealed against matter passage. The present invention is especially adapted for use with rotating shafts passing through apertures in housings, the particular application for which the invention was developed is the entry of a spindle shaft into the disk chamber of a Winchester disk drive. the present invention is easily manufactured and may be readily installed upon Winchester disk drive systems. it may also be readily installed on other applications in which a rotary shaft must pass into a secure sealed chamber. The incorporation of a seal according to the present invention will not in any way modify the required construction of other elements of the data processing devices.

The improved magnetic liquid seal of the present invention is constrcuted of readily available materials using readily available tooling. It is therefore economical, to manufacture as well as advantageous and economical to use. it is therefore expected that the present invention wiil have wide industrial applicaiblity, especially in the data processing industry.

Claims

IN THE CLAIMS

1. An improved magnetic seal assembly for sealing a passage aperture in a housing about a rotary shaft, comprising: an inner magnetic ring bonded to and concentric with the shaft so as to rotate therewith, the outer portion of the inner ring being magnetically polarized in a direction parallel to the axis of rotation of the shaft; an outer magnetic ring bonded to the housing providing the passage aperture and stationary therewith, the outer ring being formed to include a cylindrical aperture therethrough, said cylindrical aperture being concentric with the inner ring and having a radius greater than that of the inner ring such that a gap is formed therebetween, the inner portion of the outer ring being magnetically polarized in a direction opposite to that of the inner ring; and a quantity of ferromagnetic fluid placed in and about the vicinity of the gap between the inner magnetic ring and the outer magnetic ring and restrained in such vicinity by magnetic forces.

2. The assembly of claim 1 wherein: said gap is selected to be as narrow as tolerances allow.

3. The assembly of claim 1 wherein: the ferromagnetic fluid selected includes a plurality of microscopic particles of ferromagnetic material in a stable dispersion within a carrier liquid.

4. The assembly of claim 1 and further including: magnetically conducting pole pieces bonded to at least one planar surface of the inner magnetic ring and the outer magnetic ring.

5. The assembly of claim 1 wherein: a magnetic guard ring portion is disposed on the surface of the inner magnetic ring, the polarity of said guard ring portion being opposite to that of the remainder of the inner magnetic ring such that the migration of ferromagnetic fluid across said guard ring portion is precluded.

6. The assembly of claim 5 wherein: a magnetic guard ring portion is disposed on the surface of the outer magnetic ring, the polarity of said guard ring portion being opposite to that of the remainder of the outer magnetic ring such that the migration of ferromagnetic fluid across said guard ring portion is precluded.

7. The assembly of claim 6 and further including: magnetically conducting pole pieces bonded to at least one planar surface of the inner magnetic ring and the outer magnetic ring.

8. The assembly of claim 7 wherein: said gap is selected to be as narrow as tolerances allow; the ferromagnetic fluid selected includes a plurality of microscopic particles of ferromagnetic material in a stable dispersion within a carrier liquid.

9. A magnetic seal assembly for sealing a narrow gap against matter passage, comprising: a first magnetic element extending along one side of the gap and being magnetically polarized in a direction perpendicular to the plane containing the edges of the gap; a second magnetic element extending along the opposite side of the gap from the first magnetic element and being opposingly polarized thereto; and a magnetic fluid disposed within and about the entire extent of the gap so as to prevent the passage of matter therethrough.

10. The assembly of claim 10 and further including: magnetically conducting pole pieces disposed along the surface of the first and second magnetic elements so as to conduct and concentrate the magnetic fields generated thereby.