It is known that turbo-molecular pumps are to be operated at already low levels of pressure. So the turbo is always backed by another pump (usually a scroll, rotary pump) that ensures this. One should not operate a turbo pump at atmospheric pressure.
Why? Is that because of air resistance? The blades would get really hot and cook the bearings?
Something that is usually also specified for turbo pumps is the backing pressure. Is this the pressure below which the turbo can be operated? Does this affect the vacuum that the turbo can achieve, or does it only relate to the blades not getting too hot?