Case Histories » Manufacturing

Gear Tooth Failure on a Triple Reduction Gearbox

Overall view showing the two fractured gear teeth
Overall view showing the two fractured gear teeth. For the areas of localized spalling, the spalling appears to start at the pitch circle and move upwards to the crest of the gear tooth
Magnaflux test on the gear
A Magnaflux test is performed on the gear in order to visualize the cracking by employing small magnetic particles and a fluorescent dye. Any cracks or fractures will become visible. The Magnaflux is considered non-destructive testing (NDT) as the specimen is not damaged after testing
Polished and etched transverse section of two gear teeth
Polished and etched transverse section of two gear teeth. Etching has revealed the case hardened layer (appears light grey in color). Two areas of the damage to the flank of the 2 adjacent gears (as indicated by the arrows). The subsurface cracking is visible at both locations

Specimen

Two fractured gear teeth with 185mm face width.

Background

A triple reduction gearbox had failed after two years of service. The gearbox has an input of 700 HP at 1780rpm on the expeller and is one of many other similar gear-boxes in use. An analysis was performed to determine the root cause for the gear failure and to provide information about whether a gear failure in the other gear-boxes may be imminent.

Service Life

Approximately two years.

Findings

Microscopic examinations of cross-sections of the gear teeth show the presence of subsurface cracking at the location of the pitch circle where the initial point contact occurs between meshing gears. Subsurface cracking is indicative of excessive loading. The presence of subsurface cracking is a characteristic of rolling contact fatigue. The gear teeth were not able to transmit a load of 700 HP. Consequently, other gear-boxes in service should be inspected for early stages of rolling contact failure of the gear teeth.