Machine Design
Gear Drive
Law Of Gearing : The law of gearing states that " the common normal to the tooth profile at the point of contact should always pass through a fixed point called pitch point in order to get constant velocity ratio."
Gear Tooth Failures :
There are two basic modes of gear tooth failure :
(a) Breakage of tooth due to static and dynamic loads.
(b) Surface destruction.
The complete breakage of the tooth can be avoided by adjusting the module and face width, so that the beam strength of the gear tooth is more than the sum of static and dynamic load.
The surface destruction or tooth wear is classified as follows :
(i) Abrasive wear : Foreign particles such as dirt, rust etc. can scratch the tooth surface. Remedies against this type of wear or provision of oil filters, increasing surface hardness and use of high viscosity oils.
(ii) Corrosive wear : The corrosion of the tooth surface is caused by corrosive elements such as extreme pressure additives present in the lubricating oils and foreign materials due to external contamination. These elements attack the tooth surface, resulting in fine wear uniformly distributed over the entire surface.
(iii) Initial pitting : The initial pitting is a localized phenomenon characterized by small pits at high spots. Initial pitting is caused by the errors in the tooth profile, surface irregulation and misalignment. The remedies against initial pitting are precise matching of gears, adjusting the correct alignment of gears so that the load is uniformly distributed.
(iv) Destructive pitting : Destructive pitting is a surface fatigue failure which occurs when the load on the gear teeth exceeds the surface endurance strength of the material. This type of failure can be avoided by designing the gear in such a way that the wear strength of the gear tooth is more than the sum of static and dynamic load.
(v) Scoring: Exclusive surface pressure, high surface speed and in adequate supply of lubrication result in the breakdown of the oil film. This results in excessive frictional heat and overheating of meshing teeth. It 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 limit.
Gear Tooth Failures :
There are two basic modes of gear tooth failure :
(a) Breakage of tooth due to static and dynamic loads.
(b) Surface destruction.
The complete breakage of the tooth can be avoided by adjusting the module and face width, so that the beam strength of the gear tooth is more than the sum of static and dynamic load.
The surface destruction or tooth wear is classified as follows :
(i) Abrasive wear : Foreign particles such as dirt, rust etc. can scratch the tooth surface. Remedies against this type of wear or provision of oil filters, increasing surface hardness and use of high viscosity oils.
(ii) Corrosive wear : The corrosion of the tooth surface is caused by corrosive elements such as extreme pressure additives present in the lubricating oils and foreign materials due to external contamination. These elements attack the tooth surface, resulting in fine wear uniformly distributed over the entire surface.
(iii) Initial pitting : The initial pitting is a localized phenomenon characterized by small pits at high spots. Initial pitting is caused by the errors in the tooth profile, surface irregulation and misalignment. The remedies against initial pitting are precise matching of gears, adjusting the correct alignment of gears so that the load is uniformly distributed.
(iv) Destructive pitting : Destructive pitting is a surface fatigue failure which occurs when the load on the gear teeth exceeds the surface endurance strength of the material. This type of failure can be avoided by designing the gear in such a way that the wear strength of the gear tooth is more than the sum of static and dynamic load.
(v) Scoring: Exclusive surface pressure, high surface speed and in adequate supply of lubrication result in the breakdown of the oil film. This results in excessive frictional heat and overheating of meshing teeth. It 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 limit.
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