Magnetic fluid seals are commonly utilized to provide a seal that will protect devices against the introduction of gas or other contaminants. These seals may be installed to provide a barrier between various components present in a device that are either stationary with respect to each other or in a rotational relationship. For example, magnetic fluid seals have been utilized in computer magnetic disc storage units as a barrier against contaminants being transmitted between the motor area and the disc area. Magnetic fluid seals also have been designed to seal robotic actuators and to seal around rotatable x-ray tubes that are used in high vacuum environments, as well as to seal rotary components incorporated into pumps used in refineries and chemical plants.
Magnetic fluid seals generally operate through the placement of a magnetic fluid (e.g., a ferrofluid) in the gap established between the surface of a rotating shaft and stationary surface for use as a dynamic seal. The stationary surface normally includes an annular magnetic structure whose peripheral edge forms a close, non-contacting gap with the surface of the rotating shaft. The magnetic flux path generated by the magnet retains and concentrates the magnetic fluid in the gap forming a tight seal that resembles a liquid o-ring. Since the rotating and stationary surfaces do not directly contact each other, they are subject to very little wear. Thus the serviceable life of the magnetic fluid seal is remarkably extended in comparison to the life expectancy of a mechanical seal.
In order for a magnetic fluid seal to operate properly, it is important that the annular magnetic structure, including any magnets and corresponding pole pieces, is mounted concentrically about the rotatable shaft. Inaccurate centering about the shaft will result in a non-uniform width in the annular gap established between the magnetic structure and the shaft. Generally, it is the pole pieces which are in closest proximity with a shaft and for which centering is most critical.
When a magnetic seal is mounted non-concentrically about the shaft, the resulting magnetic field will not be symmetrically distributed about this shaft, but rather the magnetic flux will be elevated near the narrowest portion of the gap and reduced near the gap's widest portion. An uneven distribution of magnetic flux within the annular gap may cause the magnetic fluid to concentrate towards the narrowest portion of the gap, thereby, leaving the widest portion of the gap with an insufficient volume of fluid to maintain the desired sealing strength. A reduction in sealing strength can lead to seal “bursting” at a lower threshold pressure differential across the seal than one skilled-in-the-art would normally expect or predict. Accordingly, there exists a continual desire and need to provide magnetic fluid sealing devices, and a method of centering the magnetic seals about rotatable shafts.