Transient Lift Off Testing Results for a Radial Hybrid Bearing
Technical Report,01 May 2009,10 Feb 2015
AFRL/RQRC Edwards AFB United States
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An experimental test rig was designed and developed to test the performance of a hybrid bearing during its initial lift-off, referred to as start transient. The hybrid bearing studied was designed to replicate the geometry of the bearing used in a new Liquid Hydrogen Turbopump. The test rig uses a high-speed spindle motor capable of 20,000 RPM that drives a 782 Inconel rotor connected by a high speed flexible coupling. The rotor was supported by ceramic ball bearings on the coupling end, and the test hybrid bearing at the other end. A magnetic bearing is utilized to apply loads to the rotor at mid-span. During testing, several factors were varied including ramp rate rpmsec to vary drive torque, supply pressure, applied loading, and load orientation to simulate various start transient scenarios. This data was analyzed in an attempt to quantify hydrodynamic and hydrostatic lift-off speeds. Hydrodynamic lift-off occurs when the rotational speed of the rotor causes the wedge effect of the film to overcome forces acting on the rotor, while hydrostatic lift-off occurs when the pressure which is speed dependent developed in the bearing overcomes the forces acting on the rotor to cause lift-off. The lift-off speed is an important quantity to determine for a certain condition to prevent rotor and bearing damage due to a prolonged rotorbearing contact. With the rotor being loaded with a static force of 1X rotor weight in the vertical direction, it lifted-off hydrostatically at 4275 RPM. In the case of higher static vertical load, 8X rotor weight, it lifted-off hydrostatically at 7900 RPM. Testing showed that an increase in load and ramp rate increases the speed at which the rotor lifted-off. From this outcome, the hydrostatic lift-off is highly dependent on supply pressure and can be determined by graphical means. Although attempts were made to quantify hydrodynamic lift-off, the data showed no reliable indicators.