A test rig for air bearings rotordynamic coefficients measurement

This paper describes the design and manufacturing of an experimental facility for measurement of equivalent stiffness and damping of air bearings. For these preliminary tests, the shaft moves only in two perpendicular directions, laying in the rotation plane, thus producing 2×2 characteristic matrices. However, the rig can be easily modified to measure rotordynamic characteristics related to angular motion of the journal and measuring 4×4 matrices. The testing facility uses an experimental magnetic bearing suspension system that allows imposing any given orbit to the shaft, during the testing experiments. All individual parts, as well as the assembly, were dynamically studied to determine their modal response and optimize it according to the test rig's operating frequency range. The principle of operation is to produce a shaft orbit using the magnetic suspension system and measuring the forces generated on the test bearing housing. Then, the stiffness and damping coefficients are calculated using an iterative parameter identification algorithm (a modification of the IVF method). The force measurement is performed via three load cells placed in a triangle configuration around the test bearing housing. All data is gathered and processed using PC based data acquisition boards and software. The present design allows testing air bearings up to 44 mm in external diameter and a bandwidth of 0 Hz to 1.000 Hz. Preliminary testing was performed on this research that demonstrates the capability of the apparatus to measure the dynamic properties with ease and accuracy.