Ferrofluidic seals, also known as magnetic liquid rotary seals, are implemented in rotating machinery to facilitate rotary movement while upholding a leak tight seal through the utilization of a physical barrier in the shape of a ferrofluid. This ferrofluid is held in position with the aid of a permanent magnet. Since their inception in the 1970s, these seals have been utilized in specific fields like computer disc drives, semiconductor, vacuum technology applications, and nuclear systems.
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 feedthroughs, which also contain a central shaft, ball bearings and an outer housing. The ball bearings 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, oil-based liquid, held magnetically between the rotor and stator. As the ferrofluid retains its liquid properties even when magnetized, drag torque is very low. With the use of permanent magnets, the operating life and equipment maintenance cycles are generally very long. Ferrofluid-sealed feedthroughs reach their greatest performance levels by optimizing features such as ferrofluid viscosity 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 (below 10−8 torr), 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 in order to try and solve this problem.
Each particular combination of construction materials and design features has practical limits with respect to 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.
