1. Context

Often, operation in contaminated environments or conditions that prohibit the use of oil are an important factor in choice (cryogenic machines). Books published over four decades ago cover these aspects ( [2] , [3] , [4] ). The use of bearings and aerodynamic thrust bearings does not stop here. It is now recognized that the most effective way of reducing the weight of small and medium-sized turbomachines (< 400 kW) is to reduce their size. In order to maintain their performance, the reduction in diameter requires an increase in rotational speed. Targeted rotational speeds are of the order of several tens of thousands of revolutions per minute, possibly reaching, and even exceeding, 10...