(Richard Thomas dealt with shaft rubs in two blog posts: Back to Basics - Rubbing or not and Differentiating between fluid induced instability (oil whirl and oil whip) and a rub. In today's post, he answers a reader's question on how rubs can sometimes lead to reduced vibration amplitude)
I have seen examples where a rub results in reduced vibration amplitude. The two most common mechanisms for this are when the rub occurs it adds heat to the rotor which can bow the rotor. If the rotor bows in a direction opposite of the 1X vibration, the combined effect would be a lower 1X vibration amplitude.
Additionally, a strong rub can result in lower vibration levels at one location due to the force of the rub restricting shaft movement at that location.
Figure 1 is a graphical representation showing that in reality the overall system Dynamic Stiffness is comprised of a series of springs from the rotor to ground with the weakest (smallest) spring being the fluid film stiffness of the bearing itself. This fact is logical in that it is the weakest spring that will control vibration. Furthermore, the resulting Dynamic Stiffness resulting from these springs in series will always be less than the stiffest spring in the series.
(A strong rub can result in lower vibration levels at one location)
In the case of the bearing fluid film, vibration energy is absorbed and dissipated via the direct damping term, which is the rotor pushing on the fluid – the shock absorber effect. However, for adequate damping to occur within the bearing, the bearing must be rigidly mounted to the foundation spring and the foundation spring rigidly connected to ground. Any looseness between the bearing and the housing, the housing to 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.
What we must remember is that that the vibratory energy will be / must be dissipated to ground via the dynamic stiffness of the rotor / bearing / support structure. If the vibratory energy is not dissipated at the bearing fluid film (due to a rub restricting shaft motion), then some other element of the system's dynamic stiffness will experience increased vibration.