-: Forms of Teeth :-
In actual practice fallowing are the two types of teeth commonly used -
1- Cycloidal teeth
2- Involute teeth
1- Cycloidal teeth:- A cycloidal teeth is the curved traced by a point on the circumference of a circle which rolls without slipping on a fixed straight line .
Cycloidal teeth are of two types -
( i ). Epicycloid
( ii ). Hypocycloid
( i ). Epicycloid:- When a circle rolls without slipping on the outside of a fixed circle , the curve traced by a point on the circumference of a circle is known as epicycloid .
( ii ). Hypocycloid :- If a circle rolls without slipping on the inside of a fixed circle , then the curve traced by a point on the circumference of a circle is called hypocycloid .
2- Involute teeth :- An involute of a circle is a plane curve generated by a point on a tangent , which rolls on the circle without slipping .
Gear Tooth Failures
or
Causes of Gear Tooth Failure
The different modes of failure of gear teeth are as follows :-
1- Bonding Failure
2- Pitting
3- Scoring
4- Abrasive wear
5- Corrosive wear
1- Bonding Failure :- 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 i.e. the gear tooth will break , this is known as bending failure of gear tooth .
To avoid such failure , the module and face width of the gear is adjusted so that the beam strength is greater than dynamic load.
2- Pitting :- A failure occurs when the surface contact stresses are higher than the endurance limit of the material , with the formation of pits which continue to grow resulting in the rupture of the tooth surface is known as pitting .
Pitting is the surface fatigue failure which occurs due to many repetition of Hertz contact stresses .
To avoid the pitting , the dynamic load between the gear tooth should be less than the wear strength of the gear tooth .
3- Scoring :- A stick-slip phenomenon in which alternating shearing and welding take place rapidly at high spots is known as scoring .
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 .
4- Abrasive wear :- When the foreign particles in the lubricants such as dirt , dust or burn enter between the tooth and damage the form of the body , then this failure is known as abrasive wear failure of tooth.
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 .
5- Corrosive wear :- The corrosion of the tooth surface is mainly caused due to the presence of corrosive elements such as additives present in the lubricating oils .
To avoid such type of wear , proper anti-corrosive additives should be used .
Cycloidal teeth are of two types -
( i ). Epicycloid
( ii ). Hypocycloid
( i ). Epicycloid:- When a circle rolls without slipping on the outside of a fixed circle , the curve traced by a point on the circumference of a circle is known as epicycloid .
( ii ). Hypocycloid :- If a circle rolls without slipping on the inside of a fixed circle , then the curve traced by a point on the circumference of a circle is called hypocycloid .
2- Involute teeth :- An involute of a circle is a plane curve generated by a point on a tangent , which rolls on the circle without slipping .
Gear Tooth Failures
or
Causes of Gear Tooth Failure
The different modes of failure of gear teeth are as follows :-
1- Bonding Failure
2- Pitting
3- Scoring
4- Abrasive wear
5- Corrosive wear
1- Bonding Failure :- 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 i.e. the gear tooth will break , this is known as bending failure of gear tooth .
To avoid such failure , the module and face width of the gear is adjusted so that the beam strength is greater than dynamic load.
2- Pitting :- A failure occurs when the surface contact stresses are higher than the endurance limit of the material , with the formation of pits which continue to grow resulting in the rupture of the tooth surface is known as pitting .
Pitting is the surface fatigue failure which occurs due to many repetition of Hertz contact stresses .
To avoid the pitting , the dynamic load between the gear tooth should be less than the wear strength of the gear tooth .
3- Scoring :- A stick-slip phenomenon in which alternating shearing and welding take place rapidly at high spots is known as scoring .
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 .
4- Abrasive wear :- When the foreign particles in the lubricants such as dirt , dust or burn enter between the tooth and damage the form of the body , then this failure is known as abrasive wear failure of tooth.
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 .
5- Corrosive wear :- The corrosion of the tooth surface is mainly caused due to the presence of corrosive elements such as additives present in the lubricating oils .
To avoid such type of wear , proper anti-corrosive additives should be used .
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