Adhesive wear - gear failures

Contributing factors of adhesive wear

Purely rolling motion only occurs along the operating pitchline of mating gear teeth. Elsewhere, at tooth tips and roots, motion is a combination of rolling and sliding. Provided the lubricant separates the mating teeth, such sliding motion is not an issue. However, when the lubricant fails to prevent metal-to-metal contact, sliding motion under load will generate substantial frictional heat. If temperatures are sufficiently high, asperities on the mating gear teeth will momentarily weld as they contact. As the gears separate, these asperity-scale micro-welds will be torn apart and plastically deformed material transferred from one surface to the other. The markings this produces are known as adhesive wear.

There are many possible conditions which could cause the lubrication film to be insufficient. Some examples include:

• Gear geometry errors, from poor design or operational wear.
• Insufficient lubricant selection, with inadequate anti-scuffing additives.
• Poor lubrication system design.
• Overheating – Even if there is a sufficient lubrication film, it may be vaporised by frictional heating.
• Gear misalignment, possibly resulting from bearing or housing deflections.
• Transient events, such as gusts or grid faults.

The severity of adhesive wear is determined by the scale of welding and tearing which takes place. This in turn is generally proportional to the load and distance over which sliding contact occurs.

Adhesive wear is a direct result of the welding and tearing instigated by metal-to-metal contact of loaded components at high temperatures.

Appearance of adhesive wear

As adhesive wear results from sliding contact, it forms at tooth tips and roots where such motion occurs. Furthermore, as material is transplanted from one gear flank to another, adhesive wear will be evident in corresponding areas on both.

While broadly similar, the appearance of adhesive wear varies with severity:

Mild adhesive wear occurs on the micro-scale when discrete asperities weld and tear. In many cases the damage will not be visible to the naked eye, however close-up inspection will reveal small, dark furrows which represent weld-spots. Overall the appearance of the gear teeth will be largely unaffected.

Moderate adhesive wear results from the welding and tearing of multiple but discrete asperities. It is visible to the naked eye, with scuffed, silver markings. It may occur in localised patches and could be mistaken for abrasive wear.

Severe adhesive wear is frequently called scuffing, but is also known as smearing or galling. Material transfer happens on a macroscopic scale, as multiple adjacent asperities are welded and torn. This results in a raised patch of material being built-up and growing as it moves along the surface. Transferred material will be rough, torn and will mask machining marks. It may also appear “smeared” up and possibly over the top of a gear tooth. Torn areas are dark, jagged furrows.

Progression

Adhesive wear is not a fatigue-related phenomenon and can occur instantly. Unlike contact fatigue failures such as micropitting and macropitting, which develop over time, adhesive wear can result from a single brief event and may or may not be progressive.

Mild adhesive wear is common during the run-in phase and usually stops as surface roughness is smoothed out. Since the importance of following proper run-in procedures and replacing the lubricant afterward is well understood, mild adhesive wear is not a major concern.

Moderate adhesive wear, however, tends to be progressive unless operating conditions are changed. The affected areas become rough and torn, which can trigger further damage. It may escalate to severe adhesive wear or lead to other failure modes, such as micropitting, macropitting, or abrasive wear.

Severe adhesive wear, also known as scuffing, is not regarded as a failure mode that leads to sudden, catastrophic failure. While it doesn’t progress as quickly in gears as it does in bearings, and the risk of seizure is low, it can pave the way for failures like macropitting or fracture.

Detectability

Discussion

Proper run-in of gears during the manufacturing load test is crucial for preventing adhesive wear. During the test, the gearbox lubrication system should be equipped with a particulate counter, and the gearbox should be run in 25% incremental load steps. At the start of each load step, the particulate count will rise before gradually decreasing as the rough surfaces left by gear tooth profile grinding are smoothed out.

Severe adhesive wear is rare in wind turbine gears because the appropriate run-in procedure is well established. Along with a proper run-in, adhesive wear can be further reduced by using high-hardness materials and ensuring adequate lubrication with anti-scuffing additives. Since adhesive wear is caused by sliding motion, it is most likely to occur in high-speed stage gears.

Severity rating

Rank 1 Either no or some light micro-scale markings from adhesive wear during run-in. Superficial with little to no depth. Damage arrests after run-in.

Detection:

Borescope

Recommended action:

None – run turbine as normal.

Rank 2 Visible patches of adhesive wear near the roots and tips of gear teeth. Can be progressive.

Detection:

Visual, borescope

Recommended action:

Run turbine. Continue to monitor during routine inspections. Investigate suitability of lubricant and additives.

Rank 3 Severe adhesive wear (scuffing) covering the substantial amounts of the gear teeth.

Detection:

Visual, borescope, vibration, SCADA

Recommended action:

Review situation and either run, curtail or stop turbine in short term. Increase inspection frequency. Begin scheduling replacement.

Rank 4 Severe adhesive wear (scuffing) covering the majority of the tooth flanks. Evidence of progression to macropitting. The addendum may be visibally deformed.

Detection:

Visual, borescope, vibration, SCADA

Recommended action:

Stop turbine and schedule replacement.