Journal Bearing Rub Detection Using Voyager FDS

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

Summary

A rub between a shaft and the Babbitt of a fluid film bearing can occur due to operation near a rotor critical speed, a thermal bow, or a cocked bearing. This case study discusses data that was collected on a laboratory rotor at the Voyager Dynamics office as it passed through its first critical speed using the Voyager FDS dynamic strain sensor. Time waveform data from two FDS sensors mounted to the rotor’s inboard bearing were used for examples of how a rub presents in dynamic strain data.These two sensors were mounted at bottom dead center (BDC) and 45° from BDC in the direction of rotation (45R) as shown in Figure 1.

Figure 1: Voyager FDS Sensor Mounting Locations

As the rotor passes through its first critical speed, it deflects following its first mode shape. This significant rotor deflection can take up the total available bearing clearance leading to a rub between the shaft and the bearing surface. The Voyager FDS sensor is sensitive to bearing rubs as the contact forces between the shaft and the Babbitt creates high compressive stress in the bearing housing.

Bearing Rub During Rotor Start-up

Data was collected from the FDS sensors mounted at the inboard bearing of the laboratory rotor during the rotor start-up. The rotor’s first critical speed is approximately 1,332 CPM. Figure 2 shows the response of the measured dynamic strain when the rotor passed through its first critical speed.

Figure 2: Overall Strain Amplitudes During Startup

Figures 3 and 4 show the time waveform data collected with the BDC FDS sensor mounted on the inboard rotor bearing at rotor speeds of 983 RPM and 1,332 RPM, respectively.

Figure 4: IB BDC FDS Time Waveform at 1,332 RPM

By plotting the BDC FDS time waveforms at these speeds with the same y-axis scaling, the dramatic increase in dynamic strain amplitudes at this location as the rotor passed through its first critical speed is clear. This significant increase in dynamic strain amplitudes is caused by a rub between the shaft and the bearing surface due to the significant dynamic deflection of the rotor caused by the shaft bending mode. This rub causes a significant negative bias of the time waveform as seen in Figure 4 due to the compressive loading on the bearing housing as the rubbing rotor passes by the bottom dead center FDS sensor mounting location. The same event occurring in the negative half of the time waveform is seen in the time waveform measured from the 45R FDS sensor.Figure 5 shows the time waveforms measured from these two FDS sensors mounted on the IB bearing while the rotor passes through its first critical speed.

Figure 5: Comparison of IB BDC and 45R FDS Time Waveform at 1,332 CPM

One important item of note is the 45° phase difference between the negative events seen in the dynamic strain data measured by the 45R FDS sensor compared to the BDC FDS sensor. This phase difference between the two sensors is caused by the physical angle the rotor rotates between their mounting locations. The significant amplitude difference between the two sensors is caused by the pressure distribution of the oil in the bearing.Since the oil pressure in the bearing is higher at the BDC location, the BDC FDS sensor shows a more dramatic increase in strain due to the rub. When diagnosing a potential bearing rub using dynamic strain data the time waveform will show the most obvious change due to the rub. Apart from the significant increase in overall dynamic strain amplitudes, the presence of the large negative event in the time waveform is a telltale sign of the high compressive loads typical of a rub.