Ferrofluidic is the brand name of a staged magnetic liquid rotary sealing mechanism made by the Ferrotec Corporation. Ferrofluidic seals, also known as magnetic liquid rotary seals, are employed in various rotating equipment to facilitate rotary motion while ensuring a

hermetic seal A hermetic seal is any type of sealing that makes a given object airtight (preventing the passage of air, oxygen, or other gases). The term originally applied to airtight glass containers but, as technology advanced, it applied to a larger ca ...

. This is achieved through a physical barrier constituted by a

ferrofluid Ferrofluid is a dark liquid that is attracted to the poles of a magnet. It is a colloidal liquid made of nanoscale ferromagnetic or ferrimagnetic particles suspended inside a carrier fluid (usually an organic solvent or water). Each magnetic ...

, which is held in position by a

permanent magnet A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nickel, c ...

. Developed in the 1970s, ferrofluidic seals have been utilized in various specialized applications, including computer

disk drives Data storage is the recording (storing) of information (data) in a storage medium. Handwriting, phonographic recording, magnetic tape, and optical discs are all examples of storage media. Biological molecules such as RNA and DNA are cons ...

vacuum systems A vacuum (: vacuums or vacua) is space devoid of matter. The word is derived from the Latin adjective (neuter ) meaning "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressur ...

, and nuclear technologies.''''

Origins

Ferrofluidic seals rely on the general principle of

s - fluids that display magnetic attraction. Following research on ferrofluids during the 1960s, the ferrofluidic seal was first patented in 1971 by R.E. Rosensweig ( USP 3,620,584), who subsequently founded Ferrofluidics Corporation with R. Moskowitz.

Benefits and limitations

Magnetic liquid rotary seals operate with little maintenance and minimal leakage in a range of applications. Ferrofluid-based seals used in industrial and scientific applications are most often packaged in mechanical seal assemblies called rotary feed-throughs, which also contain a central shaft, ball bearings, and outer housing. The

ball bearings A ball bearing is a type of rolling-element bearing that uses balls to maintain the separation between the bearing races. The purpose of a ball bearing is to reduce rotational friction and support radial and axial loads. It achieves this ...

provide two functions: maintaining the shaft's centering within the seal gap and supporting external loads. The bearings are the only mechanical wear items, as the dynamic seal is formed with a series of rings of ultra-low

vapor pressure Vapor pressure or equilibrium vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. The equilibrium vapor pressure is an indicat ...

, oil-based liquid, held magnetically between the rotor and

stator The stator is the stationary part of a rotary system, found in electric generators, electric motors, sirens, mud motors, or biological rotors (such as bacterial flagella or ATP synthase). Energy flows through a stator to or from the rotat ...

. As the ferrofluid retains its liquid properties even when magnetized,

drag Drag or The Drag may refer to: Places * Drag, Norway, a village in Tysfjord municipality, Nordland, Norway * ''Drág'', the Hungarian name for Dragu Commune in Sălaj County, Romania * Drag (Austin, Texas), the portion of Guadalupe Street a ...

torque In physics and mechanics, torque is the rotational analogue of linear force. It is also referred to as the moment of force (also abbreviated to moment). The symbol for torque is typically \boldsymbol\tau, the lowercase Greek letter ''tau''. Wh ...

is very low. With the use of permanent magnets, the operating life and equipment maintenance cycles are generally very long. Ferrofluid-sealed feed-throughs reach their greatest performance levels by optimizing features such as ferrofluid

Viscosity Viscosity is a measure of a fluid's rate-dependent drag (physics), resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for e ...

and magnetic strength, magnet and steel materials, bearing arrangements, and using water cooling for applications with extremely high speeds or temperatures. Ferrofluid-sealed feedthroughs can operate in environments including

ultra-high vacuum Ultra-high vacuum (often spelled ultrahigh in American English, UHV) is the vacuum regime characterised by pressures lower than about . UHV conditions are created by pumping the gas out of a UHV chamber. At these low pressures the mean free path of ...

(below 10⁻⁸ mbar), temperatures over 1,000 °C, tens of thousands of RPM, and multiple-atmosphere pressures. Magnetic liquid seals can be engineered for a range of applications and exposure, but are generally limited to sealing gases and vapors, not direct pressurized liquid. This is due to premature failure of the ferrofluid seal when it seals a liquid. In 2020, research was underway to try and solve this problem. Each particular combination of construction materials and design features has practical limits concerning temperature, differential pressure, speed, applied loads, and operating environment, and as such devices must be designed to meet the criteria for their applications. Necessary features may include multiple ferrofluid stages, water cooling, customized materials, permanent magnets, and exotic bearings. Ferrofluid-based seals have extremely low leak rates however they cannot reach the levels of welded connections or other all-metal, static (non-rotating) seals.

References

External links

Video demonstration of a magnetic liquid rotary seal
Seals (mechanical) Magnetic devices