Turbo-molecular pumps (turbo pumps)

Turbo-molecular pumps or ”turbo pumps” as they are called everyday are high vacuum pumps that are only used in combination with a vacuum pump. The pumps are generally used in applications that require a clean operating environment under high vacuum or ultra high vacuum. The principle of operation of a turbo pump is that a turbine with high speed spins in a pump casing where the majority of rotor stages in the turbine are provided with a rotor blade. Between each step of the rotor blade there are stationary stators which are angled in the opposite direction to the rotor blades and form a ”V” shape between them.

Using the kinetic energy through the turbine rotor blades, the rotor blades strike the gas molecules in an axial direction down through the pump body. For each rotor stage and stator stage, the molecules are further passed through the pump in the direction of the pre-vacuum flange.
Under molecular flow (<10-3 mbar), the medium-free path (the distance traveled by a molecule before colliding with another molecule) for gas molecules is greater than the distance between the rotor and stator blades (normally a few tenths of a millimeter). This means that the molecules instead collide with tightly seated rotor blades and result in a very efficient pump speed.

On some models, you also have an extra compression step that has slightly different names depending on the manufacturers and variants of the same type of solution. Sometimes it is called ”drag stage” sometimes also ”Siegbahn stage” we usually call it ”Holweck stage” after the inventor. This additional compression step adds another dimension to the pump type with respect to the pressure at which the pump can be started. With a Holweck stage, the pump can already start at such high pressures as 10-15 mbar (N2).

Another thing that is usually important for the users is what types of bearings the fast-rotating turbine uses. The older models had regular grease or oil-lubricated layers in order to last a long life. Over time, it became increasingly important for users of the pumps to limit the potential risk of contamination of the vacuum system due to the fact that hydrocarbons from grease and oil managed to get away from their encapsulation through the pump and into (towards the flow direction) into the vacuum system and potentially destroy products or samples that were inside.

To overcome this problem, alternative solutions to the problem were developed. Magnetic bearing turbo pumps. The new ”MAG” pumps had magnetic bearings that do not naturally have any contact with the rotating turbine, thus limiting potential leakage. The only time the bearings are subjected to mechanical wear in the classic sense is when the pump is stopped and when the lower part of the turbine ”lands” against the bearing surface and gets in contact there. However, this is a minor wear in the context and the technology with magnetic storage is classed as a major advance in the life of turbo pumps.

In recent years, it has begun to combine the two methods in order to be able to extend the service life and reduce downtime even more; historically, you always have (regardless of make) when the pump has broken down, it has to send it to a workshop with the right skills and the right tools to be able to repair the pump. However, minor measures could still be made in place, but for larger accidents this was the only solution.

The modern solution is called hybrid pumps, where a magnetic bearing has been taken in the upper part of the pump, supplemented by a permanent lubricant in the lower part of the pump. With experience from the users who thought it was difficult and expensive to send the pumps off to repair to the workshops, they accepted this and also solved the problem with the most common breakdowns so that these could be remedied on site by the users instead. Nowadays, the new standard on the market is just hybrid pumps for their versatility in pumping capacity and long service life.