The following is a continuation of our previous discussion on rubs. We will begin with a brief review of some basic concepts and then give an example of a rub with supporting data plots. In upcoming blog articles which will continue this discussion, additional rub examples will be depicted.
Dissipating Rotor Vibration Energy via a Fluid Film Bearing
Any looseness between the bearing and the housing, the housing and the foundation, the foundation to ground will result in unwanted seismic relative motion which will adversely affect damping within the bearing. Any looseness between the bearing and its housing, the housing and the foundation, or the foundation and ground will reduce overall damping which can lead to elevated vibration amplitudes at one or more frequencies.
Most vibration data plots, including: Timebase, Orbit, Polar, and Bode, present dynamic vibration data. For seismic transducers, this dynamic data is inertially referenced to ground. For the case of the shaft relative proximity probe, the dynamic vibration occurs about a –dc gap voltage, with the –dc gap voltage being proportional to the average distance from the probe tip to the target (shaft). These data plots display dynamic vibration data, but do not show changes in the average shaft radial position, an important response characteristic of the rotor system.
The root cause analysis of vibration problems related to critical turbomachinery is an engineering science based on what is often referred to as the "First Law of Machinery Diagnostics". The 1st Law states that displacement (vibration) is equal to the summation of the dynamic forces acting on the rotor divided by the complex dynamic support stiffness of the rotor bearing system.