How to choose a turbomolecular pump
A turbomolecular pump is a stack of angled rotor and stator blades spinning tens of thousands of revolutions per minute. Above the rotor tip speed the blades give gas molecules a preferred direction, so the pump works only in molecular flow — it always needs a backing pump to bring the foreline down first. Compression ratio is strongly gas-dependent: enormous for nitrogen, far lower for hydrogen and helium, which is why residual spectra in a turbo-pumped chamber are hydrogen-rich.
Match four things: pumping speed for the gas that matters to you, the inlet flange (CF for bakeable UHV, ISO-K for high vacuum), the bearing type, and the backing pump. Grease-lubricated ceramic hybrid bearings are inexpensive and orientation-limited; magnetic levitation bearings are contact-free, vibration-free, need no maintenance and mount at any angle — worth the premium on analytical instruments and long-uptime tools. If the process carries particles or corrosives, ask about purge ports and coated rotors.
Typical duties: mass spectrometry and electron microscopy, thin-film deposition and sputtering, semiconductor process tools, leak detection, space and accelerator simulation chambers. Damage is nearly always mechanical — sudden venting to atmosphere at full speed, a dropped particle, or a bearing run past its interval — so the vent valve and service schedule matter as much as the pump.
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FAQ
What backing pump does a turbo molecular pump need?
A turbo pump cannot start against atmosphere. A backing pump must bring the foreline into the turbo's start-up range, usually below 1 mbar. Oil-sealed rotary vane pumps are the cheapest choice; dry scroll or diaphragm pumps are used where hydrocarbon backstreaming would contaminate the process.
Grease-lubricated or oil-lubricated bearings — which should I choose?
Grease-lubricated pumps can be mounted in any orientation and need no oil level checks, which suits mobile and multi-position installations. Oil-lubricated pumps generally carry higher gas throughput and longer bearing life but must be mounted within the specified orientation. Magnetic-levitation models remove the bearing question altogether at higher cost.
Why is my turbo pump not reaching its rated ultimate pressure?
In order of likelihood: the foreline pressure is too high (backing pump worn or undersized), the chamber has a real leak, surfaces are outgassing because the system was never baked, or the seal at the inlet flange is damaged. Check the foreline pressure first — it is the fastest measurement and the most common cause.
Can a turbo molecular pump handle corrosive process gases?
Only in a corrosion-resistant version with purge gas. Standard pumps have aluminium rotors that halogen chemistries attack, and the bearing lubricant degrades. Etching and CVD lines use pumps with protective coatings, purge ports and, upstream, an isolation valve that closes on failure.
How long do turbo pump bearings last?
Mechanical-bearing pumps are usually specified for 25,000 to 40,000 running hours before bearing service, provided the pump runs clean, at rated foreline pressure and without repeated hard vents. Frequent venting to atmosphere at full speed and particulate ingress are what actually shorten bearing life.