Principles of operation and benefits

A Ferrofluidic seal takes advantage of the response of a magnetic fluid to an applied magnetic field. The basic seal components include ferrofluid, a permanent magnet, two pole pieces and a magnetically permeable shaft.The magnetic circuit, completed by the stationary pole pieces and the rotating shaft, concentrates magnetic flux in the radial gap under each pole piece. When fluid is applied to this gap it assumes the shape of a liquid O-ring and produces a hermetic seal. Ferrofluidic vacuum rotary feedthroughs utilize multiple rings of ferrofluid contained in stages formed by grooves machined into either the shaft or pole pieces. Typically a single stage can sustain a pressure differential of 0.2 atmospheres (200 mbar). The pressure capacity of the entire feedthrough is approximately equal to the sum of the pressure capacities of the individual stages.

Benefits

Hermetic Sealing
A Ferrofluidic seal provides a hermetic seal under both static and dynamic conditions against gas, vapor and other contaminants. Leakage rates are in the region of 10-11 [He] mbar l / s.

Long Life
As the sealing medium is a fluid, there is virtually no friction between the rotating and stationary components so the seal does not wear. The fluids used in Ferrofluidic seals are inert, stable, low vapor pressure ferrofluids. The seal lifetime depends on the application, but many Ferrofluidic seals have been in operation over 10 years without maintenance.

High Reliability
A Ferrofluidic seal contains a simple permanent magnetic circuit and low volatility ferrofluid. The only parts subject to wear are self-contained ball bearings which are engineered to provide reliable operation.

Non-Contaminating
Since there is no mechanical friction within the ferrofluid or between the ferrofluid and the static seal components or shaft, no particles are produced which could contaminate the system. Low vapor pressure ferrofluids maintain seal integrity even in high vacuums better than 10-9 mbar.

High Speed Capability
Ferrofluidic seals provide hermetic sealing even at high rotational speeds. Current technology has produced configurations that perform at dN values of 500,000 (where d=shaft diameter in mm and N=rotational speed in rpm). For a 1" feedthrough this equates to a rotational speed of 20,000 rpm.

Optimum Torque Transmission
Through-shaft construction permits 100% of the engineered torque transmission and provides in-phase rotation without backlash or slip errors.

No Set-Leakage Failures
Ferrofluidic seals provide leak-free performance, even in intermittent and static conditions. Unlike elastomeric seals, they are not subject to plasticizing and relaxation effects during idle periods.

Smooth Operation
The low viscous drag of the ferrofluid is independent of the pressure applied across the seal. Operation therefore is extremely smooth.