Fluid-Induced Instabilities Measured with Dynamic Strain

Alexander M. Tomsick - P.E. Director of Engineering

Case Study - Summary

Oil whirl and shaft whip are fluid-induced instabilities that affect fluid film bearings, with plain bore bearings being more susceptible than other bearing types. The laboratory rotor used by Red Wolf Reliability for internal research and development, as well as education, is prone to these instabilities due to the low load, small bearing length-to-diameter ratio, and the plain bore bearings that are used. This case study discusses data collected on the rotor using the mounted Voyager FDS sensors, which shows content related to both oil whirl and shaft whip.

Oil Whirl

Oil whirl is a sub-synchronous fluid-induced instability that causes vibratory response at the mean oil velocity in the bearing. The frequency of oil whirl depends greatly upon several factors, but is most strongly controlled by a bearing's eccentricity ratio. The frequency of the sub-synchronous oil whirl can vary from 20% to 60% of the rotor’s operating speed. The lower the eccentricity ratio of the bearing is, the closer the whirl frequency will be to 50% of the rotor’s speed. Figure 1 shows a visual representation of the expected operating position of a rotor in a fixed-geometry hydrodynamic bearing.

Figure 1: Hydrodynamic Bearing Eccentricity

Oil Whip

Oil whip is a severe fluid-induced instability that causes the rotor to vibrate at a rotor system natural frequency regardless of the rotor’s operating speed. This typically occurs at the rotor’s first natural frequency. The occurrence of oil whip depends on the threshold of instability of the rotor-bearing system, which depends on the mean fluid velocity in the bearing, the bearing’s stiffness, and the rotor’s mass.

The mean oil velocity in a fixed geometry bearing increases as the bearing eccentricity ratio decreases. Given this relationship, turbomachinery is more prone to experience oil whip when the rotor is centered in the bearing. Additionally, while oil whip is typically excited by existing oil whirl, oil whip can occur without oil whirl being present.

Data was collected on the laboratory rotor at Red Wolf Reliability. The rotor was balanced to a G1.7 ISO balance quality and was precision aligned to the motor using a Prüftechnik RotAlign Touch. Given the high balance quality and precision alignment, the rotor was operated under very low load. Voyager FDS sensors were installed on the bearing housings at the bottom dead center (BDC) location as well as 45° with and against rotation from the BDC position, as shown in Figure 2. For this case study, the Voyager FDS sensors mounted at the BDC location will be discussed.

Figure 2: Voyager FDS Sensor Mounting Locations

Data was collected at the rotor’s full operating speed of 1,800 rpm under this low load condition. Figure 3 shows the spectrum collected from the BDC FDS sensor under this condition. In the aligned condition, the BDC FDS sensor contained sub-synchronous content at both 0.5 and 0.75 orders. The content at 0.5 orders is consistent with oil whirl in the bearing. While present, this amplitude of the oil whirl frequency is relatively low compared to those seen at the rotor’s running speed. More interestingly, the amplitude of 0.75 orders is much higher. This order is consistent with the first critical speed of the rotor system and is caused by oil whip in the bearing. The presence of oil whip supports that the low load on the bearing has caused the bearing to operate with a low eccentricity.

Figure 3: Inboard BDC FDS Order Spectrum while aligned

Following the data collected in the aligned condition, the train was intentionally misaligned by offsetting the motor to the left of the rotor by 8 thousandths of an inch. The rotor was then operated at 1,800 rpm, and data was collected. Figure 4 shows the spectrum computed from the BDC FDS sensor under this misaligned condition. In the misaligned condition, the oil whirl frequency and the oil whip frequencies both increased compared to the aligned condition, with a very significant increase seen in the amplitude of Oil Whip from 0.37 µε rms to 1.53 µε rms. This increase in oil whip vibration amplitudes is consistent with the effect the offset misalignment has on the bearing’s stability. By offsetting the motor to the left, the misalignment resists the natural attitude angle of the rotor in the bearing and increases the minimum oil film thickness. This, in turn, reduces the bearing stability, increasing the severity of oil whip.

Figure 4: Inboard BDC FDS Order Spectrum with the Motor Offset to the Left

Conclusion

The operating position of the rotor, which is supported by fixed-geometry hydrodynamic bearings, plays a critical role in the stability of the rotor system. Fluid-induced instabilities, such as oil whirl, can indicate the rotor is not being operated under its design conditions and can cause the rotor to respond poorly to transient loading conditions due to the lack of bearing stiffness. Oil whip is a more severe fluid-induced instability that causes significant vibration amplitudes due to the excitation of a rotor system's natural frequency. With both oil whirl and oil whip, it is important to be able to identify the presence of either fluid-induced instability and respond quickly to correct it. From the data in this case study, it was shown that the dynamic strain data collected with the Voyager FDS sensor can identify both oil whirl and oil whip when present in turbomachinery.