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Gear teeth are exposed to repeated loads every time a gear set operates. Over time, contact stress, sliding, overload, poor lubrication, contamination, or alignment problems can damage the tooth surface or tooth root.

The damage does not always look the same. Some failures begin on the tooth surface, while others develop from bending stress at the tooth root. Common failure modes include pitting, wear, scuffing, tooth fracture, and plastic deformation. Knowing what the damage looks like can help identify what went wrong and what should be checked next.

1. Gear Tooth Pitting

Pitting is a surface fatigue failure caused by repeated contact stress between mating gear teeth.

As the teeth move through the mesh, the contact stress changes continuously. After many load cycles, small cracks can develop near the tooth surface, particularly in areas exposed to high contact stress. Lubricant can enter these cracks, and pressure within the cracks can help them grow. Eventually, small pieces of material break away and leave pits on the tooth flank.

Pitting may develop as micropitting or macropitting. Micropitting creates very small surface damage and can give the tooth flank a gray or frosted appearance. Macropitting produces more obvious pits and can remove larger areas of the tooth surface as the damage progresses.

Surface finish, contact stress, lubrication, material properties, and operating conditions all affect the development of pitting.

Small amounts of pitting may have little immediate effect, but progressive pitting can increase noise and vibration. As more material is removed, the tooth profile can also change and the gear may lose part of its load-carrying capacity.

2. Gear Tooth Wear

Wear occurs when material is gradually removed from the tooth surface during operation.

One common cause is abrasive wear. Hard particles or contaminants trapped between the teeth can scratch the mating surfaces. Similar damage can occur when a harder gear surface runs against a softer one under sliding contact, especially when lubrication is poor.

As wear continues, the original tooth profile becomes less accurate and the tooth thickness is reduced. This increases the risk of tooth failure because the remaining tooth section has less strength.

Wear is influenced by several factors, including lubrication, contamination, surface hardness, sliding speed, load, and the hardness difference between the two mating gears.

A gear does not necessarily fail as soon as wear appears. The concern is progressive wear that changes the tooth geometry or reduces the tooth section enough to affect transmission performance.

3. Gear Tooth Scuffing

Scuffing is a more severe form of adhesive damage on the tooth surface.

It can occur when the lubricant film between the mating teeth becomes too thin to keep the metal surfaces apart. Under high load and sliding conditions, local contact between surface asperities can cause them to adhere briefly. As the teeth continue to move, the bonded material can be torn away.

The damaged areas usually appear as scratches, torn surfaces, or grooves running in the direction of sliding.

At high speed and high load, friction can generate enough heat to weaken the lubricant film and accelerate scuffing. At lower speeds, a heavy load can also cause problems because a stable lubricant film is harder to maintain.

Scuffing can develop quickly once the tooth surface has been damaged. Increased surface roughness then makes further damage more likely, which is why lubrication and operating conditions are particularly important for heavily loaded gear transmissions.

4. Tooth Root Fracture and Tooth Breakage

Tooth fracture is one of the more serious gear failures because a broken tooth can stop power transmission or cause secondary damage to the gear set.

Bending stress is concentrated near the tooth root, especially around the root fillet. Repeated loading can eventually produce a fatigue crack in this area. Excessive load, insufficient tooth-root strength, stress concentration, manufacturing defects, and poor surface condition can all contribute to crack formation.

The crack may grow over many operating cycles until part of the tooth breaks away.

Not every tooth fracture is caused by fatigue. A sudden overload or impact can also break a tooth in a single event. This type of failure may occur without the gradual surface damage normally associated with wear or pitting.

For this reason, gear design has to consider both tooth bending strength and tooth surface strength. A gear with a hard tooth surface can still fail at the root if the bending load is too high.

5. Plastic Deformation

Plastic deformation occurs when the local stress becomes high enough to permanently deform the gear material.

Instead of returning to its original shape after the load is removed, part of the tooth surface or tooth profile remains displaced. Depending on the operating conditions, the damage may appear as plastic flow, ridging, peening, or other forms of permanent profile change.

Heavy loads, impact loading, and insufficient material hardness can increase the risk of this type of damage.

Once the tooth profile changes, the contact pattern can also change. The load may become concentrated in a smaller area, which can increase contact stress and contribute to other forms of tooth damage.

For heavily loaded gears, material selection and heat treatment therefore play an important role in maintaining both surface durability and tooth strength.

6. The Effect of Lubrication and Contamination

Lubrication problems are often involved in more than one type of gear failure.

Insufficient lubricant, an unsuitable lubricant, contaminated oil, or lubricant degradation can reduce the protective film between the mating tooth surfaces. This can increase friction and surface temperature and accelerate wear, pitting, or scuffing.

Contamination presents another problem. Hard particles entering the mesh can scratch the tooth surface or create local indentations. Once the surface has been damaged, wear can progress more quickly.

This is why a damaged gear should not be evaluated only by looking at the visible marks on the teeth. The operating load, lubrication condition, contamination, gear alignment, material, heat treatment, manufacturing accuracy, and service conditions all need to be considered.

How to Identify Gear Failure

The appearance of a damaged gear often gives useful clues about the failure mechanism.

Pitting appears as small cavities or larger fatigue pits on the tooth flank.

Wear gradually removes material and changes the tooth profile or tooth thickness.

Scuffing produces scratches, grooves, or torn areas that generally follow the direction of sliding.

Tooth fracture is usually identified by a crack and eventual breakage in the tooth, often starting near the root.

Plastic deformation changes the original tooth profile because material has been permanently displaced.

In actual gear failures, more than one mechanism may be present. Poor lubrication, for example, can increase wear and also make scuffing more likely. Surface damage can then change the contact conditions and accelerate further deterioration.

The visible damage is therefore only part of the diagnosis. The reason behind the damage often comes from the combination of load, speed, lubrication, material, heat treatment, geometry, alignment, and manufacturing conditions.

Summary

Common gear transmission failure modes include pitting, tooth wear, scuffing, tooth fracture, and plastic deformation.

Pitting develops through repeated contact stress and surface fatigue. Wear removes material from the tooth surface over time. Scuffing is associated with adhesive damage under high sliding and unfavorable lubrication conditions. Tooth fracture is mainly related to bending stress, fatigue, overload, or impact. Plastic deformation occurs when local stress is high enough to permanently change the tooth profile.

When a gear fails, identifying the visible damage is only the first step. The next question is why that damage occurred. In many cases, the cause is related to gear design, material selection, heat treatment, lubrication, load, alignment, manufacturing accuracy, or actual operating conditions.

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