3.7 Magnetic Fluid Seals

Magnetic fluid seals have been available since the early 1970s. The major application is as a rotary seal in vacuum applications and other specialist high integrity gas sealing.

A magnetic, or ferrofluid, 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, Figure 3.173. When fluid is applied to this gap it assumes the shape of a liquid O-ring and produces a hermetic seal. A rotary feed-through will comprise multiple rings of ferrofluid contained in stages formed by grooves machined into either the shaft or pole pieces, Figure 3.174. Typically a single stage can sustain a pressure differential of 200 mbar. The pressure capacity of the entire feed-through is approximately equal to the sum of the pressure capacities of the individual stages; a typical industrial seal is shown in Figure 3.175.

Figure 3.173: Basic concept of a ferrofluid magnetic seal. (Source: Sealing Technology)

Figure 3.174: A typical magnetic seal arrangement with a series of magnetic ferrofluid rings and poles between a pair of bearings. (Source: Sealing Technology)

Figure 3.175: A multi-axial feed-through can provide a number of feeds through one shaft. (Source: Sealing Technology)