We may classify failure of gears into four categories: 1 – Pitting, 2 – Wear, 3 – Plastic flow 4 – Breakage. The visual aspect of the various adversity and failure modes can differ between gears that have through hardened and those that have surface hardened teeth.
3. Abrasive wear
• Abrasive wear is the removal of material from a surface by a
harder material impinging on or moving along the surface under
load.
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4. Pitting
• Pitting or pitting corrosion on gears is a form
of wear and tear.
• It can be identified by darker patches or
shallow indentations on the toothed surfaces.
• These irregularities cause friction, resulting in
lots of heat being generated.
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5. Scoring
• Scoring refers to transfer of metal from one component to another
under sliding contact.
• This process is caused by lack of adequate lubrication under
extreme pressure.
• In gears, pitting occurs when continuous high-pressure forces act
on the surface of the gear teeth.
• Pitting, in turn, can cause scoring.
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6. Common Causes of Gear Failure
• Moderate wear:
• This type of where leaves contacts patterns that show the metal has been affected in the
addendum and the dedendum area.
• Issues with inadequate lubrication commonly cause it, but it may also be due to
contamination in the lubrication as well.
• Excessive wear:
• This is wear that has continued to be a problem until a significant amount of material has
been affected on the surfaces.
• You may see pitting on the surface with excessive wear, typically caused by not seeing the
first wear early enough, and it continues to progress.
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7. • Abrasive wear:
• In gears that are showing signs of abrasive wear, it may appear as radial scratch marks,
grooves or some other identifier that would show contact is a problem.
• One of the most common reasons why abrasive wear takes place is due to foreign bodies in
the lubrication.
• This issue could include metallic debris from the bearings or gear system, rust, sand or weld
spatter.
• It is common in new systems before the filter can clean the system.
• Corrosive Wear:
• Chemical action deteriorates the surface of the gear, such as through acid, additives or
moisture in the lubrication oil.
• As the oil breaks down, the chemicals that exist in the lubricant attack the surfaces.
• It tends to result in uniform, fine pitting on the two surfaces.
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8. • Pitting:
• A problem with pitting can be labeled as either initial, in which the surface is
experiencing small pits to destructive, in which the pits are larger in diameter.
• Initial pitting may be a problem with the gears not fitting together properly.
• Destructive pitting is typically an issue with surface overload.
• Breakage:
• It is possible for the entire tooth or a piece of the tooth to break away.
• It often leaves evidence of the focal point of the fatigue that led to the break,
which results from any number of issues, including high stress or excessive
tooth loads.
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9. • Frosting:
• This issue usually shows up in the dedendum area of the driving gear.
• The wear pattern gives a frosted appearance, which are many micro pits on the surface.
• Frosting is a common issue when the heat breaks down the lubrication film.
• Spalling:
• Although it is similar to severe pitting, the pits tend to be shallow and larger in
diameter.
• Additionally, the area that is showing spalling does not tend to be uniform.
• It is a common problem when high contact stress exists.
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