Apparatus & method for forming a rotatable ferrofluidic seal between a shaft and a hub

Abstract

An apparatus and method are provided for sealing an outer surface 170 of a shaft 175 to an inner surface 165 of a hub 160 . The seal includes a magnet 155 , with top and bottom pole pieces 260, 265 , and a ferrofluid 270 magnetically held in a gap 275 between the pole pieces 260, 265 , and the hub 160 or the shaft 175 . The top pole piece 260 has a cross-section that is L-shaped, with a horizontal portion 260 a parallel to the magnet 155 and a shorter vertical portion 260 b facing the shaft 175 . In one version, the vertical portion 260 b or the portion of the shaft 175 facing it is contoured to provide a flux gradient that axially concentrates the ferrofluid 270 in the gap 275 . In another aspect, a catcher 335 is provided to reduce loss of ferrofluid 270 when the ferrofluidic seal 185 used to form an outside seal. In yet another aspect, a stationary ferrofluidic seal 345 is provided for sealing a stationary shaft 350 to a rotating hub 355.

1 - 20 . (cancelled)  
     
     
         21 : A seal for sealing an outer surface of a magnetically permeable shaft to an inner surface of a hub disposed about the shaft, the seal comprising: 
 (a) an annular magnet positioned between the shaft and the hub;    (b) a top pole piece and a bottom pole piece coupled to opposite poles of the magnet, both the top and the bottom pole pieces comprising a magnetically permeable material, and having annular shapes with interior radii that are larger than a radius of the outer surface of the shaft, the top pole piece having a cross-sectional area that is substantially L-shaped, and includes a long horizontal portion orientated substantially parallel to a surface of one of the opposite poles of the magnet, and a shorter vertical portion orientated in a facing, non-contact relationship to the outer surface of the shaft, and wherein a first gap separating the top pole piece from the outer surface of the shaft is smaller than a second gap separating the bottom pole piece from the outer surface of the shaft; and    (c) a ferrofluid magnetically held between the vertical portion of the top pole and the outer surface of the shaft to form a seal therebetween, wherein the shaft comprises a contoured portion, and wherein the seal is positioned between the shaft and the hub so that the vertical portion of the top pole piece faces the contoured portion.    
     
     
         22 : A seal for sealing an outer surface of a shaft to an inner surface of a magnetically permeable hub disposed about the shaft, the seal comprising: 
 (a) an annular magnet having a pair of annular pole pieces coupled to opposite poles thereof, the annular magnet positioned between the shaft and the hub, the pole pieces comprising a magnetically permeable material and having exterior radii that are smaller than a radius of the inner surface of the hub;    (b) a catcher affixed to the inner surface of the hub, the catcher made of a magnetically permeable material and comprising an annular ring having a curved surface on the interior radius thereof, the curved surface facing the exterior radii of the pole pieces; and    (c) ferrofluid magnetically held in a gap separating the pole pieces from the catcher,    whereby when the hub is rotated relative to the shaft, splashing or outward migration of the ferrofluid is substantially reduced.    
     
     
         23 : A seal according to  claim 22  wherein the curved surface 
 has a U-shaped cross section, and wherein open ends of the U-shape extend radially inward past the exterior radii of the pole pieces.    
     
     
         24 : A seal according to  claim 22  wherein the pole pieces, ferrofluid, catcher and hub are comprised of electrically conductive materials, and wherein the pole pieces are electrically coupled to the shaft, the ferrofluid is electrically coupled to the pole pieces and to the catcher, and the catcher is electrically coupled to the hub, and wherein the outer radii of the pole pieces and the inner radius of the hub are configured so that a surface area of ferrofluid electrically coupling the pole pieces to the catcher provides a resistance of less than about 1×10 9  ohms.  
     
     
         25 : A seal for sealing an outer surface of a stationary shaft to an inner surface of a hub that is supported for rotation about the shaft by at least one bearing having an inner race and an outer race affixed, respectively, to the shaft and to the hub, the seal comprising: 
 (a) an annular magnet having a pair of annular pole pieces coupled to opposite poles thereof, the annular magnet positioned between the shaft and the hub;    (b) a magnetic shield arm extending from said outer race to a position between the shaft and the magnet and pole pieces, wherein the magnetic shield arm extends over, but is not connected to the inner race; and    (c) a ferrofluid magnetically held between the pole pieces and the magnetic shield arm to form a seal between the shaft and the hub.    
     
     
         26 : A seal according to  claim 25  wherein the magnet has an interior radius that is larger than a radius of the outer surface of the shaft, and the pole pieces have interior radii that are larger than the radius of the outer surface of the shaft but smaller than the interior radius of the magnet, and wherein the magnetic shield arm extends between the inner radii of the pole pieces and the shaft.  
     
     
         27 : A seal according to  claim 25  wherein the magnetic shield arm comprises has a cross-sectional area that is substantially L-shaped and has a radial segment fastened to the outer race, the radial segment being of sufficient length to extend over an interior to the inner race, and an axial segment extending substantially parallel to the shaft and between the shaft and the poles of the magnetic seal.  
     
     
         28 : A seal according to  claim 27  further including a support arm extending axially from the stationary shaft towards the hub, a region of the support arm distal from the shaft supporting a radially outer end of the annular magnet and annular pole pieces, the annular magnet and annular pole pieces extending radially inward towards the shaft from the support arm.  
     
     
         29 : A seal according to  claim 25  wherein a Nyebar® coating is applied to the magnetic shield arm to reduce radial migration of the ferrofluid away from the seal.  
     
     
         30 : A seal according to  claim 25  wherein a nickel cladding is applied to the magnetic shield arm to provide a substantially smooth surface for contacting the ferrofluid,  
     
     
         31 : A seal according to  claim 25  wherein the top pole piece comprises a substantially L-shaped cross section having a long horizontal portion orientated substantially parallel to a surface of a pole of the magnet, and a shorter vertical portion facing and orientated substantially parallel to a shield portion of the magnetic shield arm.  
     
     
         32 : A seal according to  claim 31  wherein the vertical portion of the top pole piece comprises a contoured surface facing the shield portion of the magnetic shield arm to introduce a magnetic flux gradient between the top pole piece and the shield portion that axially concentrates the ferrofluid between a center of the contoured surface of the vertical portion and the shield portion of the magnetic shield arm.  
     
     
         33 : A seal according to  claim 25  wherein the support arm comprises a non-magnetic material.  
     
     
         34 : A seal according to  claim 25  wherein the magnetic shield arm comprises stainless steel