Gear Failure

Grinding Burn – Gear Failure

Case-carburized gears are mildly distorted during heat treatment and require aggressive grinding to achieve final part tolerance. This article will discuss grinding burn detection methods, appearance, and failure mode causes.

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

An overview of grinding burn

Case-carburized gears are mildly distorted during heat treatment and require aggressive grinding to achieve final part tolerance, during this process abrasive grains on the grinding wheel can become blunted due to mechanical and thermal stress, as well as physicochemical wear.

Grinding burn, also referred to as grind temper, describes any surface layer 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 lead to cracking.

Such cracking may occur immediately after manufacture, or once the gearbox is in service. Regardless, these cracks can proceed rapidly to severe failures like 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.

What causes grinding burns?

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

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

Such errors can result in thermal loads so high as to thermally induce material alteration at localized points. There are two forms of such material alteration:

Tempering – when a localised area is overheated 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 overheating while grinding case-carburised gears.

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Appearance of grinding burns

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.

These cracks tend to be shallow and appear across the tooth surface in a distinctive crazed, meshed pattern. As they 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 the manufacturing process, prior to commissioning.

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, this failure 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 therefore a comprehensive study should be carried out to identify the root cause.

Upon detection, 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.

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Detectability

Method

Detection Efficiency

Notes

Nital etching

Easy

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.

Visual inspection

Easy

Burn marks, cracking and macropitting resulting from grinding burn is often visible to the naked eye or with low-magnification.

Borescope inspection

Easy

Burn marks, 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

Medium

As grinding burn damage progresses and macropitting or fractures appear, it will become detectable by vibration.

Oil debris sensor

Hard

Grinding 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 analysis

Hard

Grinding 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.

SCADA data

Not applicable

SCADA does not aid detection of grinding burn.

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Nital etching

Nital etching is a non-destructive testing method used to detect grinding burns in gears. It should be carried out on all ground gears by a qualified inspector as part of the manufacturing process. Nital is a solution of alcohol and nitric acid, which can be used to etch carbon steels and reveal their microstructure. When inspecting for grinding burns, the etched gear surfaces will display distinct characteristics:

A flat, uniform grey colour slightly darker than before etching if no issues are present.
Dark, black lines if localised tempering has occurred due to grinding burns.
Re-hardened areas will not react to the etching and will appear white and shiny, typically surrounded by black tempered zones.

While nital etching significantly improves the identification of grinding burns before a gearbox is put into service, it does not completely eliminate the problem. In some cases, severe grinding burns may occur during the second-to-last stage of the finishing process, and any surface evidence of this damage might be removed in the final stage, making it undetectable by nital etching. Additionally, due to the various underlying causes, a gear manufacturer’s past success in mitigating grinding burns does not guarantee future success. Consequently, grinding burns remain a leading cause of gearbox failure.

Other methods, such as magnetic particle inspection, eddy-current testing, or dye-penetrant inspection, can also be used to detect grinding cracks either during manufacturing or in service.

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Severity rating

Rank 1 N/A

Detection:

N/A

Recommended action:

N/A

Rank 2 N/A

Detection:

N/A

Recommended action:

N/A

Rank 3 N/A

Detection:

N/A

Recommended action:

N/A

Rank 4 Although it can vary in appearance and may not seem severe, grinding burn is an inherent manufacturing error and will progress quickly. Immediate action should be taken upon confirmation of the failure, regardless of current appearance.

Detection:

Visual, borescope, vibration, nital etching, magnetic particle, eddy-current, dye-penetrant

Recommended action:

Consider 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.