Off-shore vessel investigation identifies damaged teeth/crack in a gear

PCMS was asked to provide an engineer to confirm the location and possible cause of an impact noise coming from a Port Azipull Pod Gearbox area.

The Solution

Data collection was carried out using the CSI 2140 Machinery Analyser with the AMS suite software, which is used for data storage and analysis.

The Challenge

Technical data on the gearbox was made available but contained no gear teeth or bearing type information. Accessibility for data collection was limited to the motor, coupling and upper gearbox area. Based on the technical data available, a collection data template was built to allow the following points to be collected:

  • Non-Drive and Drive End of the Motor.
  • Either end of the input shaft of the upper gearbox.
  • Additional readings were collected from the gearbox with reduced Fmax to ensure longer capture of data so that any events from the slower moving components in the lower gearbox are captured.

The Results

A Review of the data from all points collected showed no cause for concern with visible gear mesh and bearing frequency activity is acceptable. The data collected with the low Fmax for capturing the lower gearbox events showed an impacting taking place in the time waveform data.

The data Graph 1 below shows the time waveform with the impacting. The speed reference for the graph has been set at 79 rpm to match the output speed of the lower gearbox.

Highland Prestige Gearbox Impacting

Note: the clear impacting taking place once per revolution of the lower gearbox
output speed.

The impacting once per revolution of the output shaft is characteristic of damaged teeth or a crack in a gear. This is further confirmed by the lack of problematic frequencies i.e. imbalance, alignment, bearing or gear mesh, etc, in the rest of the data collected. The impacting taking place at a rate equal to the lower gearbox output speed gives the location of the impacting to the gear on the output shaft.