What types of bearings are used in Turbo pumps?

By Juliet D'cruz

Turbo molecular pumps were invented in 1958 by W. Baker  on the basis of the ancient molecular traction pump invented by Wolfgang Guide in 1913. Molecular pumps work on the principle of collision. By being hit repeatedly with a moving metal blade, the gas molecules can be guided in the direction of exhaust. In a turbine molecular pump, a rapidly rotating fan collides with gas molecules to create or maintain a vacuum, directing them from the pump inlet to the exhaust manifold. Most of these pumps have multiple stages, each consisting of a pair of rotors and stators. 

The system acts like a compressor providing  kinetic energy to  the gas to move it out from the vacuum chamber  . The gas is conveyed to the lower stage by the upper stage and is successively compressed to the vacuum level of the primary pump used to pump out the gas expelled by the turbopump. As the gas molecules enter the inlet, the rotor, with angular blades, hits the molecules. This transfers the mechanical energy of the spinning blades to the molecules. Due to this momentum, the gas molecules enter the gas transfer channels  of the stator pushing them down. This takes them to the next stage, where they collide with the surface of the propeller again, and the process continues, and eventually the gas molecules are ejected from the exhaust. To find out more about how these pumps work, visit Agilent.

Due to the motions of the rotors and the stators at specific angles with respect to each other, the molecules hit the undersides of the rotors. Blades for high pressure operation should be strong and stable and as thin as possible so that they can be bent slightly for maximum compression. The compression capacity of each step is approximately 10 times and the  efficiency of the molecular pump is closely related to the frequency of the rotor. As the speed increases, the rotor blades deflect more material. To increase speed and reduce warping, sturdier materials and different designs are being considered for the blades. Thus, multiple types of bearings are used in these turbo pumps for smooth operations.

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  1. Mechanical bearings:

Some models of turbine molecular vacuum pumps use  ceramic ball mechanical bearings. They are coupled to the rotor shaft and allow a completely hydrocarbon free operation as thanks to the rotor “cantilever” design, bearings and lubricant can be located out of the high vacuum area. Molecular turbine pumps   operate at very high speeds. The high rotational speed of the turbo (some small diameter units run at 80’000  rpm) requires great attention and advanced technologies to guarantee the rotor positioning and vibration balancing (see Agilent Technologies web site for more detailed info) Grease and lubricants used ceramic ball bearings are specifically designed to operate in high vacuum environment and can guarantee very low vapor pressure zeroing the outgassing of hydrocarbon molecules.  Mechanical bearings are highly resistant to external impact or pump’s sudden ventilation and also have a small footprint.  

  1. Magnetic Levitated Bearing:

Magnetic bearings typically used on large turbomolecular pumps running for very long time with a very limited number of start-stop cycles.  These pumps have 5 magnetic bearings  placed at the ends of the turbomolecular pump shaft. Because of the magnetic nature of operation, there is no contact of the bearings with any surface. This means that these bearings operate without friction. Pumps using magnetic bearings are more susceptible to external influences than turbine molecular pumps with mechanical bearings. In addition, they are larger and more expensive than pumps using other types of bearings.  

  1. Hybrid Bearings:

Another type of bearing used in turbine molecular pumps is the hybrid bearing. Hybrid bearings have mechanical bearings located at the lower end of the shaft rotor and a   passive magnetic bearings located at the upper end of the shaft. Like the turbine molecular pumps equipped by magnetic bearings, the hybrid bearing pumps have less resistance to external shocks  than the two mechanical bearing systems. The design of the hybrid bearing requires a magnet to stay on top of the pump rotor, in the high vacuum chamber environment, in some cases this layout can be a problem in terms of compatibility to hydrocarbon free environments and applications sensitive to scattered magnetic field.

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