Identification of Journal Bearing Force Coefficients under High Dynamic Loading Centered Static Operation

Abstract

Lightly damped rotor bearing systems experience large amplitudes of vibration when traversing critical speeds. Bearing linearized force coefficients, strictly valid for minute motions about an equilibrium position, may not be reliable for design or troubleshooting in rotordynamics predictive analyses. Experiments assessing the dynamic forced response of a plain journal bearing undergoing large orbital motions due to single-frequency excitation forces were conducted in a test rig. The short test bearing of slenderness ratio L/D = 0.25 has a nominal radial clearance of 0.127 mm (5 mils). Tests were conducted at three rotor speeds (900, 1800, and 2700 rpm), three feed pressures (1, 3, and 6 psig), and three excitation frequencies (15, 30, and 45 Hz). Baseline bearing motions due to shaft runout are recorded and subtracted in the parameter identification procedure. The forces exerted on the bearing induce large orbital motions with peak amplitudes exceeding 50% of the nominal bearing clearance. Identified cross-coupled stiffness and direct damping coefficients fall within value bands predicted by the π and 2π models of the fluid film, even for the largest amplitudes of motion. The bearing whirl frequency ratio approaches the typical 50% value at the highest speed tested. Excitation frequency has a marked influence of the test direct dynamic stiffness coefficients with added mass coefficients at least twice as large as predicted values.

KEY WORDS:

• Oil Film Bearings
• Force Coefficients
• Experimental Identification

Notes

Presented at the STLE/ASME Tribology Conference in Long Beach, California