By: T.J. Garten, IR/MCA Specialist
Bean Conditioner – Horizontal Rotary Vessel
Motor: TECO-Westinghouse 75 HP 1775 RPM
Gearbox: Falk 2120Y3-1 Ratio: 70.09
Pinion: 19 tooth
Synopsis
The Bean Conditioner is a rotary vessel used to condition soy beans prior to transportation to flaking equipment. This particular vessel rotates on a slight uphill axis and is driven by a 75 HP 1775 RPM motor coupled to a 70.09 reduction gearbox and a 19 tooth pinion. The vessel has a bull gear assembly that the 19 tooth pinion drives. During the months leading up to the repair, a visible shaking of the feed screw platform was noted during every revolution of the vessel. In addition, the pedestals for the pillow block bearings for the pinion gear shaft could be seen bending inward during the revolution. This bending motion coincided with the vibration of the feeder screw platform. Due to guarding of the pinion assembly, vibration data collection was not practical and remote provisions had not been installed.
Fault
During the quarterly Motor Circuit Analysis (MCA) online data collection, the July 6, 2009 data showed a marked increase in the dB for the peak associated with the 19 tooth pinion. Identification of the anomaly peak in the demodulation spectrum was accomplished by identifying the motor turning speed (TS), calculating the gearbox output shaft TS, and then multiplying that TS by the number of teeth on the pinion gear. This calculation gives the RPM of the pinion gear mesh, which then needs to be converted into hertz (divide the RPM of the pinion gear mesh by 60) to locate the peak in the demodulation spectrum. (See Figure 1.)
TSMotor / Gearbox Ratio = TSGearboxOut
(TSGearboxOut * Number of Teeth on Pinion) / 60 = Pinion Frequency
Using the July 6, 2009 data:
1785 / 70.09 = 25.47 RPM
25.47 * 19 = 483.93 RPM / 60 = 8.07 Hz
Note: Slight deviations (less than 1%) may occur due to variations in the turning speed of the motor.
Historically, the 8.09 Hz baseline peak operated in a 1.1 to 1.5 dB range, depending on loading. From the initial alarm trip until the next scheduled shutdown during September 2010, the dB peak increased to 4.81 dB, with one data capture of over 5.1 dB in April 2010. (See Figure 2.) During this time, the visible shaking of the feed screw platform increased from minor to severe.
Visual inspection of the pinion shaft assembly noted broken welds on the pedestals for the pillow blocks (Figure 4), as well as inward bending during every rotation of the vessel. The flexible coupling
that joined the gearbox output shaft with the pinion gear shaft also exhibited wrap flex heating, as determined through thermography scans, and appeared to have visible angular misalignment.
Summary of Action
During discussions with the site, it was determined that before the 2009 shutdown, a trunnion pillow block bearing had failed. During the repair of this bearing, the vessel height had to be adjusted due to a different roller height and consequential opening of the pinion gear-to-bull gear mating. This correction was made at the pinion shaft utilizing dial indicators. No testing had been performed after this repair due to plant outages and scheduling concerns. The first testing time was a period of four months after the repair.
During the 2010 shutdown, the contracted mechanical crew noticed the following:
• The pinion shaft was lower than the gearbox output shaft.
• There was a high spot on the vessel bull gear.
• The clearance between the pinion gear and the bull gear was too small.
• There were increased levels of mating rub on the pinion gear teeth.
The shaft alignment was corrected, the pinion gear was flipped to change the wear pattern on the teeth, the pinion-to-bull gear clearance was changed to account for the high spot on the vessel, and the pedestals were welded.
During the subsequent site visit, the vibration of the feed screw platform was no longer present. In addition, the
MCA online demodulation data showed a return to normal dB ranges (Figure 5).
However, over the next year, the same pattern began to develop again. The welds had broken again and required additional welding during the 2011 shutdown.
Supporting Data
Figure 1: Pre-repair Demodulation Spectrum
Figure 2: Pre-repair Historical Data
Figure 3: Pinion Wear
Figure 4: Broken Welds on Pinion Gear Pillow Block Pedestal
Figure 5: Post-repair Demodulation Data
Conclusion
The corrective action taken during the repair of the failed trunnion bearing resulted in an altered elevation of the rotary vessel. The attempts to adjust for this were inadequate and resulted in a changed mating pattern between the pinion gear and bull gear assemblies. Over time, the increased impacting caused the pinion shaft to deflect, breaking the welds on the pedestals. This deflection also caused an angular misalignment between the pinion shaft and the gearbox output shaft. The level of misalignment may also have been affected by utilizing dial indicators for precision shaft alignment rather than the on-site laser alignment equipment. As the trunnion rollers wear, the clearance between the pinion gear and bull gear’s high spot decreases, resulting in increased shaft deflection. The increased shaft deflection results in transmitted vibration to the feed screw platform and torsional stress on the gearbox output coupling.Future corrections need to take into account all clearances and their effect on correlating components. Monitoring of adjusted components should be within days of adjustments and load changes. Installation of remote accelerometers on the pinion gear shaft pillow blocks would allow for quicker analysis of shaft misalignment, bearing condition, and pinion gear mesh frequencies.
Tagged as:
machine parts,
motor circut analysis,
pinion,
planning and scheduling,
shaft misalignment,
thermography,
ultrasonic,
vibration,
vibration analysis
