GEAR FAILURE

Failure Modes in Gears

Gear Systems or gear trains tend to play a very vital role in all industries and also in our day to day life. Any failure to the gear system leads to the total system failure.Now let us look into the various modes by which the gears fail:
The various major areas by which the gears tend to fail are
 * Shock Loading Failures
* Fatigue Failures
* Failures due to Wear
* Failures due to Scuffing

SHOCK LOADING

1. Breakage by sudden shock loads: When the gear system is subjected to a sudden shock load more than it is designed for, it instantly breaks. Though this cannot be identified prematurely, utmost care can be taken to avoid such mishaps.

FATIGUE FAILURES

1. Bending Fatigue:

 Gears based on their rotational motion and the heavy amount of load they carry are subjected to fatigue. Due to the application of load on the gear teeth, the gear tooth is subjected to bending. When the same is repeated for a very huge number of cycles Bending Fatigue failure of the gear sets in. The fatigue in the gears induces the formation of cracks in the root of the gear tooth which propagates with each rotational cycle of the gear and ultimately leads to the failure of the gear tooth.

2. Pitting: 

This is a major cause of gear failure accounting for nearly 60% of the gear failures. Pitting is the formation of craters on the gear tooth surface. These craters are formed due to the high amount of compressive contact stresses in the gear surface occurring during transmission of the torque or in simple terms due to compressive fatigue on the gear tooth surface. 

a) Micro Pitting: 

These are basically formed due to

•   Inherent Errors in the gears
•  Presence of water in the lubricant that is lubricating the gears
•  Wrong viscosity selection of the lubricant used.

Visually, micro pitting is not so clearly visible at the first go.They appear as very small dots which one can feel when he runs his finger over the gear tooth.This sort of pitting normally tends to make the gear useless and damages the whole gear system.

FAILURES DUE TO WEAR

1. Moderate Wear:

This is a most common type of wear mode caused mainly due to lubricant contamination. The other contributors to this are limitations of lubricant viscosity, gear speed, and temperature

2. Abrasive Wear:

 This is normally characterized by scratch marks on the tooth surface when we look at it visually. This type at certain times is misjudged as Micro Pitting marks due to the similarity in the visualization. Abrasive wear is caused due to the presence of metal particles from the gears and bearings, weld spatter, scale, rust, sand and dirt in the lubricant of the gears

3. Corrosive Wear: 

As the name says this type of wear regime occurs due to corrosion action on the teeth of the gears. This type of wear regime is visually looked at as spotty dots appearing scattered on the tooth of the gear. The common causes for this type of wear are

• chemical action of active ingredients in the lubricant like acid,
•  presence of moisture, foreign materials in the lubricant
•  extreme-pressure additives added to the lubricant which react with the gear teeth

Scuffing damage occurs on gear teeth if they are operated with an inadequate lubricant film between the teeth or when the viscosity of the lubricant is very less. They can be identified by ripples on the contacting gear tooth surface. This type of failure is also characterized by a burning smell of the lubricating oil during running and also development of burn marks on the contact teeth surface. This can be prevented by proper selection of the lubricant and monitoring of the gearbox temperature at equal intervals to ensure that the temperature does not exceed the maximum running temperature of the lubricating oil.