Gear Failure

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.

Also referred to as:
Edge Wear, Contact Marks

An overview of edge loading in wind turbine gears

Edge loading, also referred to as edge wear or contact marks, is a type of gear failure characterised by curved black markings at the ends of active gear tooth flanks. These markings are particularly visible on driven gears and are typically a result of uneven load distribution. While light edge wear on many types of gear (such as helical gears or spiral bevel gears) is common during the run-in phase, more severe edge loading can progress to micropitting or even macropitting, which may signal issues like poor design, gear misalignment, or changes in geometry over time.

Causes of edge loading

In gearboxes, a contact patch reveals the load distribution between mating gear teeth. Proper gearbox design ideally centers this patch on the gear tooth; however, due to operational conditions, it often shifts, creating edge wear.

Some specific causes include:

Insufficient end relief or crowning in the gear design
External factors like transient loading from wind gusts, grid faults, or emergency stops
Misalignment of bearings or housings, which leads to uneven gear tooth contact
Operational wear that causes geometry changes over time

Appearance of edge wear in gearboxes

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

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Detectability

Method

Detection Efficiency

Notes

Visual inspection

Easy

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

Easy

Edge loading is identifiable with a borescope.

Vibration analysis

Not applicable

Edge loading will not be detectable from vibration data.

SCADA data

Not applicable

Edge loading will not be detectable from SCADA data.

Oil debris sensor

Not applicable

Edge loading does not cause substantial enough material removal to be detectable.

Oil sample analysis

Not applicable

Edge loading does not cause substantial enough material removal to be detectable.

Severity rating

Rank 1 Small number of black lines at end of active gear tooth flank. Superficial level, with no depth.

Detection:

Visual, borescope

Recommended action:

None – run turbine as normal

Rank 2 Larger number of edge loading marks with some appearance of depth. Possible evidence of polishing.

Edge loading – gear failure severity rating 2

Detection:

Visual, borescope

Recommended action:

None – run turbine as normal

Rank 3 Signs of micropitting developing around the edge wear and/or substantial polishing.

Edge loading – gear failure severity rating 3

Detection:

Visual, borescope

Recommended action:

Run turbine. Increase inspection frequency. Monitor for further progression to macropitting.

Rank 4 Progresses to other failure modes.

Detection:

N/A

Recommended action:

N/A