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Edge Loading

Edge Loading – Gear failure

Edge loading, also called edge wear, is clearly identifiable from curved black markings at the end of active gear tooth flanks. It is caused by an uneven load distribution across mating tooth flanks. The markings are most commonly seen on driven gears. Many manufacturers consider light edge loading marks produced during run-in normal. As such, light edge loading marks are extremely common in wind turbine gearboxes. Light markings are usually non-progressive, arresting after sufficient run-in, and are not a cause for concern. In some cases, heavier edge loading may occur and progress into polishing or micropitting. This is likely an indication of poor design, or geometry changes or misalignment in operation.

Also referred to as: Edge Wear, Contact Marks

Causes

A contact patch is a representation of the load distribution when two gear tooth flanks mate. Gearbox design standards aim to centre the contact patch on the gear tooth (see Figure 1 a). However, these calculations use static load cases and other simplifications. More generally, the high infant mortality of wind turbine gearboxes illustrates a disparity between assumed loading conditions and reality. Therefore, there are many instances where the contact patch is shifted away from the centre, to one edge of the gear tooth (see Figure 1 b). This results in edge loading marks. Some examples include:

  • Poor design, with insufficient end relief or crowning
  • Transient loading from gusts, grid faults or emergency stops
  • Bearing or housing misalignment
  • Operational wear resulting in geometry changes that shift the contact patch.

Appearance

As the name implies, edge loading results in markings at the ends of active gear tooth flanks. These marks tend to have a distinctive curvature and are easily identified. They tend to form on the driven, rather than the driving gear. In the parallel stages of a wind turbine gearbox, this would mean the markings are most likely on pinion gears.

Acceptable mild edge loading tends to be superficial black marks with little or no depth. If it progresses, polishing and micropitting may develop on or around these marks.

Progression

Initial markings can develop very early in the service life of a gearbox, but will generally arrest after the run-in period. However, in some circumstances it may progress. Initially this will be into polishing, but later perhaps also micropitting. Nonetheless, although light markings are common, edge loading is rarely cause for immediate or serious concern. The main focus would be to increase the inspection frequency in case any micropitting developed into macropitting.

Detectability

MethodDetection EfficiencyNotes
Visual inspection✓✓✓The fixed location and characteristic markings mean edge loading is visually identifiable on accessible (parallel stage) gear teeth. Planetary gears will require a borescope.
Borescope inspection✓✓✓Edge loading is identifiable with a borescope.
Vibration analysisEdge loading will not be detectable from vibration data.
SCADA dataEdge loading will not be detectable from SCADA data.
Oil debris sensorEdge loading does not cause substantial enough material removal to be detectable.
Oil sample analysisEdge loading does not cause substantial enough material removal to be detectable.

Discussion

The formation of micropitting at the ends of multiple teeth would be indicative of underlaying design issues. These include a substantial lack of end relief, insufficient crowning, or misalignment. Ensuring suitable load distribution and microgeometry modification can help to eliminate this kind of wear.

Romax Technology have a large body of gearbox design, review and modification experience. We can accurately model gear tooth contact as our RomaxWIND software enables the construction of a fully integrated drivetrain model, in which misalignment from bearing and housing deflection can be accounted for. The image below compares a RomaxWIND contact patch with an operational gear that has a marking compound applied.

 

Severity Rating

RankDescriptionDetectionRecommended Action
1Small number of black lines at end of active gear tooth flank. Superficial level, with no depth.Visual, borescopeNone – run turbine as normal
2Larger number of edge loading marks with some appearance of depth. Possible evidence of polishing.Visual, borescopeNone – run turbine as normal
3Signs of micropitting developing around the edge wear and/or substantial polishing.Visual, borescopeRun turbine. Increase inspection frequency. Monitor for further progression to macropitting.
4N/AN/AN/A
1
Example of rank 1 edge loading (a gear failure)
2
Example of rank 2 edge loading (a gear failure)
3
Example of rank 3 edge loading (a gear failure)
4
Progresses to other failure modes
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