The first item to be realized is that radial or axial rubbing is not a machinery malfunction. A rub is secondary indicator that occurs when there is contact between rotating and non-rotating components. Some of the primary causes that can lead to a rub are:
tight or incorrect clearance
rotor bowing, etc.
distorted / twisted turbine casing or bearing housing
Richard Thomas has over 38 years of rotating equipment experience in industry and currently holds the position of Prinipal Engineer for Metrix Instrument Company. Prior to his current position, his work experience included 11 years as a rotating equipment engineer in the petrochemical industry; 17 years in the Engineering Services Department of Bently Nevada Corporation; and 6 years as the Global Services Manager for RoMaDyn Corporation.
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 syste
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.