3.2 Design of a Hydrodynamic damper support
Hydrodynamic damper (HD) principles can be investigated in consideration of main forces acting (Fig. 9).
Fig 9. Forces in a Hydrodynamic damper.
mew2 - rotor unbalance force; Fg - hydrodynamic force; Fg - bearing radial force; Ft- damping tangential force.
A HD starts operating only from some speed of rotation. At smaller speed the shaft journal lies on the inner bearing surface and the rotor is not centred.
At rotation the rotor starts precession within the clearance due to the unbalance loads. Large precession speed could produce negative pressures in the oil film, but actual liquids cannot exist at negative pressures, so the oil film loses its solidity, there develop cavities filled with oil vapour. An annular pressure distribution becomes non-uniform, which creates a hydrodynamic force. The force radial component determines the HD bearing ability, the circular component - its damping performance.
The greater is the cavitating region the larger is the hydrodymamic force and the bearing ability. The cavitating region is influenced by the rotor eccentricity and oil supply pressure. The pressure increase reduces the cavitating region which is mostly observed at small speeds of precession and small eccentricities.
It is worth mentioning that if the oil film completely fills the clearance with no cavitation the HD loses its bearing ability but has remarkable damping.