Gearbox failure due to metal spray fatigue

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Hi All, this is the last post for 2017 – Enjoy

We were called to inspect a gearbox as the client had reported an abnormal sound. This was a very large old extruder high torque gearbox with a single input and dual output shafts.

Executive Summary:
Through onsite vibration analysis we were able to pinpoint the shaft that was generating the abnormal noises, this enabled the bearings for the shaft to be pre-ordered so they arrived at the repair shop the same time as the gearbox. This ensured a quick turnaround was completed with minimal production loss.

On Site Initial Assessment:
The gearbox vibrational levels as measured under full load conditions were >20mm/s RMS. This is considered “Vibration Causing Damage” as per ISO 10816-3. The Acceleration Peak to Peak impactions at Gearmesh #1 was excessive at 162G’s. There was also indications of misalignment on the 1st intermediate shaft and considerable looseness present. The 1st intermediate shaft ‘binds’ for 1/4 to 1/3 of a revolution when turned by hand.

Vibration Data:
The Input shaft high frequency Acceleration spectra clearly shows a high 2x gearmesh frequency for the gearmesh 1. This indicates there is misalignment within the gearing setup. The sidebanding at 19.20Hz indicates that it is relative to the 1st intermediate shaft.

The plot above is the Acceleration Spectrum from the Gearbox NDE Horizontal.

The Peak to Peak measurements on the Acceleration Time Waveform below indicates the Acceleration forces are within the 1st Intermediate shafting. The total reading of 162G’s is highly destructive and is impacting at 19.2Hz, the 1st intermediate shaft speed.

The Velocity spectrum taken from the NDE of the 1st intermediate shaft shows a considerable amount of run speed harmonics attributed to the shaft speed. This is an indication of looseness.

Cause of Failure:

On inspection the tab washer on the first intermediate shaft outer bearing had failed. In addition the suspected gear on the 1st intermediate shaft was extremely loose. It was found that this shaft had been previously repaired with metal spray and this had failed. On closer inspection the stress raiser appears to be around the keyway, as there was no strengthening welds around the keyway to support the metal spray.

Strip Down Images:

This is an image of the gearbox internal layout.

Images of the failed tab washer found in the bearing cap from the 1st Intermediate shaft.

Image of the key that supported the 1st intermediate gear that was loose.

Metals spray coating that was under the 1st Intermediate shaft gear. This failed initially at the metal spray coating at the keyway.



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