Shaft Misalignment and Vibration - A Model

Misalignment of coupled rotating machinery shafts is a frequently occurring problem which can have a substantial influence on equipment reliability. Experience has shown that diagnosis of misalignment through vibration analysis can be extremely difficult due in large part to the observed substantial variability in the character of machinery vibration even when apparently identical alignment states exist. This paper presents the results of a theoretical study on a simple linear rotordynamic model, capable of simulating the effects of parallel and angular misalignment across a flexible-element coupling connecting drive and driven rotors. In contrast to other works the complex system forces and motions are derived by application of the Lagrange Method without the imposition of specific harmonic-excitation assumptions. The model results confirm that a system having purely linear properties when subjected to parallel misalignment can exhibit complex multi-harmonic vibration response. Support stiffness anisotropy is shown to be an important parameter in determining the presence and level of first (1X) and secondharmonic (2X) vibration response. Coupling of the lateral-torsional motions is demonstrated as being key to the production of multi-harmonic system response.The results provide significant insight into some of the major controlling elements of the vibration-misalignment relationship in a linear system.