Gear drive is used to transmit power from one shaft to another when distance between two shafts is very small. There are several failure mechanisms for spur gears. Bending failure of the teeth is one of the main failure modes. In this research work, various stress relieving features are studied. Single circular holes, two circular holes, one circular hole in combination with elliptical hole are used to study the effect of these features in stress reduction. Modeling is done with drafting software AutoCAD. Finite element based software ANSYS is used for this work. Initially using AGMA method bending stresses were determined analytically. A good agreement was found between analytical & finite element results of bending stress.

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Study of Circular and Elliptical Holes as aStress Relieving Feature in Spur Gear

A gear is a component within a transmission device that transmits rotational forces. Gears are commonly used for transmitting power. Gear teeth failure due to fatigue is a common fact observed. Even a small reduction in the root tensile stress results in great raise in the fatigue life of a gear. They develop high stress concentration at the root and the point of contact. The repeated stressing on the fillets causes the fatigue failure of gear tooth. For many years, gear design has been improved by using better material, hardening surfaces with carburization and heat treatment, and shot penning to improve surface finish etc. Few more hard work have been made to improve the durability and strength by changing the pressure angle, using the asymmetric teeth, varying the geometry of root fillet curve and so on. The majority of the above systems don't ensure the compatibility of the current rigging frameworks. This work presents the possibilities of utilizing the stress redistribution techniques by introducing the Stress relieving features in the stressed zone to the advantage of reduction of root fillet stress in spur gear.

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Modeling and Reduction of Root Fillet Stress in Spur Gear Using Stress Relieving Feature

This work presents the possibilities of percentage of reduction in the root fillet stress of spur gear by introducing circular stress relieving feature of various sizes at different locations. Two categories of systematic analyses are carried out using finite element model of spur gear. In the first category of analyses emphasize is given to determine the maximum root fillet stress which is required as reference to determine the stress reduction factor. In addition to this, the effect of number of teeth, pressure angle, and basic parameters of rack cutter on root fillet stress is investigated. In second category which is the prime focus of this work; the effect of introducing geometric stress relief feature is carried out. Circular stress relief feature of different size at strategic locations on a spur gear tooth are introduced. This reduces the root fillet stress. The magnitude of the load shared by a gear tooth depends on the position of the point of contact of teeth engagement along the line of action. It also depends on contact ratio. High contact ratio gear teeth pairs are subjected low load in comparison low contact ratio gear for transmitting same amount of torque. A tooth is released from the load as soon as it disengages with the mating gear tooth and it is taken by the next conjugate tooth. Therefore the gear teeth are subjected varying (fatigue) load. If a gear fails by bending fatigue, the failure is catastrophic and occurs with little or no warnings. The maximum root fillet tensile stress is one of the key factors which determine the fatigue life of the gear. Therefore the magnitude of the maximum value of the root fillet tensile stress can be treated as the index of fatigue life of gear. Even slight reduction in root fillet stress leads to a large increase in fatigue life of the gears designed for the stress level above endurance limit. This type of design consideration is very common, where safety and light weight are the principal design criteria. Therefore for all the reasons cited above, this work is of more practical importance. The program is coded in ANSYS Parametric Design Language (APDL). It automates the task of creation of model, meshing, applying boundary conditions, choosing the appropriate density of the mesh depending on the stress gradient. It also provides a means for defining the diameter and location of the circular stress relief features.

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Mr.Nagesh Kamanna

Papers

Empirical relations to predict the probablepercentage of reduction in root fillet stress inspur gear with circular stress relief feature