Gear teeth Failure Modes – Gear Defects :

Gear is the most essential element of power transmission prefer for short distance. It is a very economical and very effective way of power transmission .it is used almost all engineering purpose for power transmission. A gear is a machine element designed to transmit force and motion from one mechanical unit to another.

Gear Failure occurs in different modes. Many modes of gear failure are there, for example fatigue, impact, wear, or plastic deformation. Of these, one of the most common causes of gear failure is tooth bending fatigue. Fatigue is the most common failure in gearing. Tooth bending fatigue and surface contact fatigue are two of the most common modes of fatigue failure in gears.

Modes Of Gear Teeth Failure – Types Of Gear Defects : 

1. Bending failure.

• Gear tooth behaves like a cantilever beam subjected to repetitive bending stress. The tooth may crack due to repetitive bending stress
• To avoid such failure, the module and face width of the gear is adjusted so that the beam strength is greater than the dynamic load.

2. Pitting.

• It is a surface fatigue failure due to repetitive contact stresses. Pitting starts when total load acting on the gear tooth exceeds the wear strength of the gear.
• To avoid the pitting, the dynamic load between the gear tooth should be less than the wear strength of the gear tooth.
• The initial or corrective pitting is a localized phenomenon, characterized by small pits at high spots. Such high spots are progressively worn out and the load is redistributed. Initial pitting is caused by the errors in tooth profile, surface irregularities, and misalignment.
• The remedies against initial pitting are precise machining of gears, adjusting the correct alignment of gears so that the load is uniformly distributed across the full face width, and reducing the dynamic loads.
• 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.

There are two types of Pitting. They are

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. One has to study the surface of the gear tooth to identify the micro pitting. They appear as very small dots that 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.

3. Scoring.

• It is a lubrication failure. Inadequate lubrication along with high tooth load & poor surface finish results in the breakdown of the oil film and causes the metal to metal contact.
• This type of failure can be avoided by properly designing the parameters such as speed, pressure, and proper flow of the lubricant so that the temperature at the rubbing faces is within the permissible limits.
• Excessive surface pressure, high surface speed, and inadequate supply of lubricants result in the breakdown of the oil film. This results in excessive frictional heat and overheating of the meshing teeth.
• Scoring is a stick-slip phenomenon, in which alternate welding and shearing takes place rapidly at the high spots. Here, the rate of wear is faster.
• Scoring can be avoided by selecting the parameters, such as surface speed, surface pressure, and the flow of lubricant in such a way that the resulting temperature at the contacting surfaces is within permissible limits.
• The bulk temperature of the lubricant can be reduced by providing fins on the outside surface of the gearbox and a fan for forced circu1ation of air over the fins.

Scoring is due to the combination of two distinct activities: First, lubrication failure in the contact region and second, the establishment of metal to metal contact. Later on, welding and tearing action resulting from metallic contact removes the metal rapidly, and continuously so far the load, speed, and oil temperature remain at the same level.

The scoring is classified into initial, moderate, and destructive.

INITIAL SCORING

Initial scoring occurs at the high spots left by previous machining. Lubrication failure at these spots leads to initial scoring or scuffing. Once these high spots are removed, the stress comes down as the load is distributed over a larger area. The scoring will then stop if the load, speed, and temperature of oil remain unchanged or reduced. Initial scoring is non-progressive and has corrective action associated with it.

MODERATE SCORING

After initial scoring if the load, speed, or oil temperature increases, the scoring will Spread over to a larger area. The Scoring progresses at a tolerable rate. This is called moderate scoring.

DESTRUCTIVE SCORING

After the initial scoring, if the load, speed, or oil temperature increases appreciably, then severe scoring sets in with heavy metal torn regions spreading quickly throughout. Scoring is normally predominant over the pitch line region since elastohydrodynamic lubrication is the least in that region. In dry running, surfaces may seize.

4. Abrasive wear.

• It is a surface damage caused by particles trapped in between the matting teeth surfaces.
• This type of failure can be avoided by providing filters for the lubricating oil or by using high viscosity lubricant oil which enables the formation of thicker oil film and hence permits easy passage of such particles without damaging the gear surface.
• Foreign particles in the lubricant, such as dirt, rust, weld spatter, or metallic debris can scratch or Brinell the tooth surface. Remedies against this type of wear are the provision of oil filters, increasing surface hardness, and the use of high viscosity oils. A thick lubricating film developed by these oils allows fine particles to pass without scratching.

5. Corrosive wear.

• It is due to chemical action by the improper lubricant or sometimes it may be due to the surrounding atmosphere which may be corrosive nature. To avoid this type of wear, proper anti-corrosive additives should be used.

Gear teeth Failure Causes and Remedies :

1) Bending failure-

• Causes: Every gear tooth acts as a cantilever. If the total repetitive dynamic load acting on the gear tooth is greater than the beam strength of the gear tooth then the gear tooth will fail in bending.
• Remedies- Module and face width of the gear is adjusted so that the beam strength is greater than the dynamic load

2) Pitting-

• Causes: Surface fatigue failure which occurs due to many repetitions of Hertz contact stresses, failure occurs when the surface contact stresses are higher than the endurance limit of the material. It starts with the formation of pits that continue to grow resulting in the rupture of the tooth surface.
• Remedies- Dynamic gear tooth load the of gear tooth between the gear tooth should be less than the wear strength

The following guidelines should be adopted to minimize the onset of pitting in gear units:

• Reduce contact stresses by optimizing gear geometry.
• Proper heat-treatment (very high hardness increases brittle with low hardness reduces surface strength) of gears. Carburizing is preferable from an economic point of view.
• Gear teeth should have smooth but hard surfaces to reduce the chance of generation of surface cracks.
• Use proper quantities of cool and clean lubricant with desired additives.

3) Scoring-

• Causes – The excessive heat is generated when there is excessive surface pressure, high speed, or supply of lubricant fails. it is a stick-slip phenomenon in which alternate shearing and welding takes place rapidly at high spots.
• Remedies – By proper designing of the parameters such as speed, pressure, and proper flow of lubricant, so that the temperature at the rubbing faces is within the permissible limits.

4. Abrasive wear-

• Causes – The foreign particles of lubricants such as dirt, dust or burr enter between the tooth and damage the form of tooth.
• Remedies- by providing filters for the lubricating oil or using high viscosity lubricant oil which unable the formation of thicker oil film and hence permits easy passage of such particles with ought to damage of gear tooth surface.

5) Corrosive wear-

• Causes – Due to the presence of corrosive elements such as additives present in the lubricating oils.
• Remedies- proper anti-corrosive additives should be used.

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