One of the first commercial products with magnetic bearings offering big advantage and added value to the customer was the turbo molecular vacuum pump. Only magnetic bearing could achieve hydrocarbon free pumps. This was a great achievement at this time and would change the whole market.
The first mass produced pump with magnetic bearing had a PMB that resemble the first figure in the first row and first column seen in Fig. 2.1. It was a vertical rotor with one ball bearing at the bottom on the exhaust side and a permanent magnetic bearing on the high vacuum side at the top with the motor between the bearings.
By doubling the amount of magnets seen in Fig. 2.2, one achieved approximately 3 times the stiffness according to Lembke [6]. An optimization of PMB geometry was recently done by Lang & Fremerey[7], who showed that many thin magnets should be
used instead of a few thick ones.
Fig. 2.2 Radial passive bearing with doubled amount of magnets
Passive bearings of these kinds are unstable in one direction according to the Earnshaw theorem.
This can be seen in Fig. 2.3. If the inner magnet on the rotor is in the centre of the
outer magnet the axial force is zero, but when it’s not kept in its centre it is shot out in that direction in which it was unaligned in.
The axial force F increases as the rotor is displaceduntil it reaches it maximum and than decreases.
In the case of the earlier mentioned pump theaxial force is taken up by the ball bearing, were some force is good to keep contact in the ball bearing.