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As the journal shaft (gray) starts to rotate it drags a film of air between it and the top foil (violet). As a the hydrodynamic pressure increases a force is exerted on the circumferential top foil. This pressure pushes the top foil away from the journal in accordance with the compliance of the backing bump foil. At the liftoff speed, the journal 'floats' on this hydrodynamic film of air without touching the top foil.
Torque and Speed Graphs
These graphs show the velocity and torque curves during a typical start/stop cycle. The torque is measured as the journal is driven by a motor. These are typical torque and speed curves for foil bearings. The journal (shaft) is initially in direct contact with the top foil. The starting torque is the result of this sliding friction. As the journal speed increases, a hydrodynamic film of air is produced that forces the journal to 'lift off' and thus become airborne without any contact between the journal and top foil. On shutting off the motor, the journal coasts to a stop with the torque spike occuring just after the journal reaches 'touchdown' and sliding friction occurs.