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Grinding burn

Grinding burn – Gear failure

Case-carburised gears are mildly distorted during heat treatment and require aggressive grinding to achieve final part tolerance. Grinding burn, also referred to as grind temper, describes any surface damage resulting from grinding. Error in the grinding process can cause localised tempering or re-hardening of the gear tooth surface. This results in detrimental residual surface stresses which rapidly leads to cracking. Such cracking may occur immediately after manufacture, or initiate once the gearbox is in service. Regardless, these cracks can proceed rapidly to macropitting and fracture. Nital etching can greatly improve detection and should be performed as part of the manufacturing process. As a manufacturing error, grinding burn may be a serial defect affecting multiple assets. It is cause for serious concern.

Also referred to as: Grind Temper, Grinding Error, Grinding Abuse

Causes

Case-carburised gears are ground following heat treatment. Grinding burn is caused by error in this process, some examples of which are:

  • Using improper or fouled grinding wheels
  • Improper grinding speed or direction
  • Insufficient coolant flow
  • Incorrect positioning of gear or grinding equipment
  • Pre-existing defects on the gear

Such errors can result, momentarily, in temperatures so high as to thermally induce material alteration at localised points. There are two forms of such material alteration:

  • Tempering – when a localised area is over-heated by grinding to the point tempering occurs. The area becomes softer than the surrounding material.
  • Re-hardening – in more extreme cases a localised area may be heated beyond the austenising temperature. Surrounded by comparatively cold steel, it is quenched, but not tempered. It results in a re-hardened zone surrounded by a softer, tempered zone.

In both cases, residual stress on the gear tooth surface results in drastically reduced mechanical suitability. This damage is known as grinding burn and leaves the tooth surface predisposed to cracking.

Grinding burn is caused by localised over-heating while grinding case-carburised gears.

Appearance

Residual stress from grinding burn will lead to the development of a network of cracks on the gear tooth surface. The cracking mechanism is similar to surface initiated macropitting, but distinguishable. Grinding burn cracks tend to be shallow and appear across the tooth surface in a distinctive crazed, meshed pattern. As these cracks intersect, pieces of material will be liberated and pits formed. In more severe cases, the gear tooth may have fractured.

Grinding burn can often be detected during manufacture, prior to commissioning. Its appearance at this stage is covered in the Discussion section, below. This section remarks only on the appearance of grinding burn damage in an operational wind turbine gearbox.

Progression

Grinding burn is progressive and will initially result in cracking. These cracks may appear immediately after the grinding process, or arise later from residual stress concentrations. Regardless, once initiated these cracks tend to propagate rapidly to macropitting and tooth fracture.

If identified, grinding burn is cause for serious concern. When caused by issues in the manufacturing process (rather than a material defect with one specific gear), it may affect a full production batch or more. Upon detection of grinding burn on one asset, an investigation to establish the root cause should therefore be conducted. The development of a site-wide inspection strategy in case of serial defect may be deemed appropriate.

Detectability

MethodDetection EfficiencyNotes
Visual inspection✓✓✓Cracking and macropitting resulting from grinding burn is often visible to the naked eye or with low-magnification.
Borescope inspection✓✓✓Cracking and macropitting resulting from grinding burn is often visible to the naked eye, but the low-magnification and accessibility afforded by a borescope will improve the likelihood of detection.
Vibration analysis✓✓As grinding burn damage progresses and macropitting or fractures appear, it will become detectable by vibration.
SCADA dataSCADA does not aid detection of grinding burn.
Oil debris sensorGrinding burn does not shed much debris in the early stages. A debris sensor will provide warning once macropitting or fracture develops although it will not indicate the source of the wear.
Oil sample analysisGrinding burn does not shed much debris in the early stages. Oil analysis will provide warning once macropitting or fracture develops although it will not indicate the source of the wear.
Nital etching✓✓✓Nital etching is a non-destructive testing procedure. All ground gears should be checked for grinding burn by a qualified inspector as part of the manufacturing process.

Magnetic particle, eddy-current or dye-penetrant inspection can also be used to detect grinding cracks during manufacturing or once operational.

Discussion

Nital etching is a non-destructive testing method which can be used to detect grinding burn. It should be performed on all ground gears by a qualified inspector as part of the manufacturing process. Nital is a variable solution of alcohol and nitric acid, and can be used for etching carbon steels to reveal their microstructure. In the case of detecting grinding burn, etched gear surfaces will appear as follows:

  • A flat, even grey colouring slightly darker than before etching if there are no issues.
  • Dark, black lines if there has been localised tempering due to grinding burn.
  • Re-hardened areas will not take-up the etching and will appear white and glimmering. Re-hardening occurs within, and is therefore surrounded by, black tempered areas.

While nital etching has greatly improved identification prior to gearbox commissioning, it does not eliminate the issue. There are cases, for example, where severe grinding burn may occur during the penultimate stage in the finishing process. Any surface evidence of this may be removed by the final stage, meaning nital etching would not detect the damage. Furthermore, due to the multitude of underlying causes, a gear manufacturer’s past success in mitigation does not guarantee future success. As such, grinding burn remains a leading cause of gearbox failure.

Magnetic particle, eddy-current or dye-penetrant inspection can also be used to detect grinding cracks either during manufacture or in operational.

Severity Rating

RankDescriptionDetectionRecommended Action
1N/AN/AN/A
2N/AN/AN/A
3N/AN/AN/A
4Although it can vary in appearance and may not seem severe, grinding burn is an inherent manufacturing error and will progress quickly. Action should be taken upon confirmation regardless of current appearance.Visual, borescope, vibration, nital etching, magnetic particle, eddy-current, dye-penetrantConsider stopping turbine. Begin scheduling replacement of affected gearing as it can propagate to fracture quickly. Replace gearbox if fracture has occurred and consequential damage is severe. Develop inspection strategy to check for grinding burn on other assets – could be serial defect.
1
Not applicable
2
Not applicable
3
Not applicable
4
Example of rank 4 grinding burn (a gear failure)
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