Failure Modes

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The following article provided by RBB Engineering

Five Common Gearbox Failure Modes, and How to Identify Them

Repairing or replacing a failed wind turbine gearbox is an extremely expensive undertaking. Gearbox failures are one of the largest sources of unplanned maintenance cost in the wind industry. When a failure occurs, it is important to correctly identify the failure mode, so that the appropriate actions can be taken to reduce the likelihood of a reoccurrence of the same type of failure. Gearbox failures can be caused by fundamental design issues, manufacturing defects, deficiencies in the lubricant or lubrication system, excessive time at standstill, high loading, and many other reasons. A correct failure mode diagnosis is the first step in identifying the steps that can be taken to prevent additional failures. Five of the most common gear and bearing failure modes, along with tips on identification and potential means of prevention, are provided below.

Micropitting

Micropitting on Gear Teeth. Photo by GEARTECH, 2014

Micropitting can affect both gears and bearings, and failures due to micropitting are very common in wind turbine gearboxes. Micropitting occurs when the lubricant film between contacting surfaces is not thick enough, and when the surfaces have high amounts of sliding action. Micropitting results in a frosted or matte finish surface in affected areas as seen in the figure above. Micropitting related failures can be prevented by changing lubricant type, or by reducing component surface roughness.

Macropitting

Macropitting on Gear Teeth. Photo by GEARTECH, 2014

Macropitting can also affect both gears and bearings. Macropitting occurs when the contact stress in the gear or bearing exceeds the fatigue strength of the material. Wind turbine gears and bearings are typically designed for a 20 year service life, and macropitting that occurs before the end of the design life is an indication that one or more design assumptions, such as contact stress, material properties, lubricant condition, or applied load, were not met. Macropitting results in craters on the gear tooth or bearing ring (or roller) surface. Beach marks due to the presence of corrosion and lubricant in the crack are sometimes present, and indicate a fatigue progression process. Macropitting failures can be prevented by reducing loads, improving gear and bearing profiles to reduce stress, using cleaner steel, or by increasing material strength, through alloy selection or heat treatment process.

Bending Fatigue

Bending fatigue. Photo by GEARTECH, 2014

Bending fatigue is a failure mode that affects gear teeth. Bending fatigue failures occur when the stress at the root of the gear tooth exceeds the capability of the gear material. This can be due to excessive loads, incorrect heat treatment, inclusions in the steel, or a notch in the root of the tooth. The appearance of the fracture surface will vary depending on whether the failure was high or low cycle fatigue. Features such as ratchet marks are occasionally present, and indicate multiple crack origins. Bending fatigue failures can be prevented by decreasing load, increasing gear material strength, or by optimizing the gear root fillet geometry.

Fretting Corrosion

Fretting Corrosion on Gear Teeth. Photo by GEARTECH, 2014

Fretting corrosion can affect gears and bearings. It is a surface wear phenomenon that occurs when two contacting surfaces have small oscillating relative motions, with no lubricant film between the surfaces. It often occurs in wind turbine gearboxes due to transportation, or to spending extended periods of time with no rotation. Fretting corrosion can be identified by the presence of ruts along the lines of contact, along with the presence of reddish brown or black wear debris. Fretting corrosion can be prevented by minimizing the amount of time that a gearbox spends without rotating or by improving transportation conditions, depending on the cause of the fretting corrosion.

Axial Cracks

Axial Cracks on Bearing Inner Ring, Photo by RBB Engineering, 2014

Axial cracking is a phenomenon that occurs in bearings, almost always on the bearing inner ring. Failures of this type have become very common in wind turbine gearboxes, and were the subject of an article in the June 2013 issue of North American Windpower. The cracks develop in the axial direction, perpendicular to the direction of rolling. Axial crack failures are most likely to occur in through hardened bearings. Axial crack failures can be prevented by using case carburized bearings, ensuring that the appropriate amount of retained austenite is present, applying a black oxide coating, and ensuring the correct level of interference fit exists between the bearing inner ring and the shaft on which it is mounted.

The failure modes described herein are some of the most common wind turbine gearbox failure modes, but there are many others. They include case-core separation, plastic deformation, scuffing, polishing, adhesion, abrasion, subcase fatigue, erosion, electric discharge, cavitation, corrosion, and several additional forms of cracking. Regardless of the type failure mode, correct identification is key to preventing a recurrence of the failure. Identifying symptoms promptly is also very important, as some types of failure can be prevented from progressing and causing serious damage through the implementation of the right corrective action. For example, fretting corrosion caught in the early stages can be prevented from becoming severe enough to result in a failure by ensuring that the gearbox is rotated enough to restore the lubricant film between gear and bearing contact surfaces. If micropitting is present as a result of degradation of the lubricant properties such as viscosity and additive content, action can be taken to change out the oil before the damage becomes severe enough to result in a failure of the gearbox. If the damage is due to excessive loads, the turbine can be derated to extend the life of the gearbox until a crane is onsite for a different issue, reducing the cost of the crane mobilization.

A resource for identifying each of the failure modes discussed in this article, along with root cause and steps that can be taken to prevent each type of failure from occurring can be found at http://gearboxfailure.com, a resource for gearbox reliability improvement. The site features an “Ask An Expert” page, where readers can submit gearbox reliability questions and photographs of damage, and leading experts will respond with diagnostic information and advice on how to prevent failures.


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