Hermetic seals

In the case of agitators with magnetic drives, static hermetic sealing against the atmosphere is achieved by employing a can. A permanent magnet coupling, as shown in Fig. 12, provides contactless transmission of the motive energy right through the can. If properly laid out and used, these couplings operate without any mechanical wear and thus achieve a very long service life.

The hermetic seal makes it necessary to support the agitator shaft, which is usually long and supported at one end only, on the product side. A fundamental distinction is made between product-lubricated bearings (variant "a" in Fig. 12) and oil- or grease-lubricated bearings (variant "b" in Fig. 12).

• Bearings lubricated by the product (a gas or liquid) employ plain or antifriction bearings of ceramics. Ceramic antifriction bearings have excellent resistance to attack by products and are usable even at very high temperatures with hardly any loss of performance. On the other hand, they are vulnerable to shocks and impacts. Media contaminated with abrasive or magnetizable substances are not suitable for lubrication.

• Ordinary, rugged steel antifriction bearings are used in bearings lubricated with oil or grease. They are capable of absorbing high axial and radial forces, but require an additional seal for separation from the product space.

 
Table 4 compares the two alternative designs with regard to important application-related criteria.