The increasing demand for quiet power transmission in machines, vehicles and generators has created a growing demand for a more precise analysis of the characteristics of gear systems. In addition, the success in engine noise reduction promotes the production of quieter gear pairs for further noise reduction. In the process of gear design, the pressure angle of gear plays a vital role. It is because pressure angle simultaneously affects the base circle radius of the involutes profile and the minimum number of teeth to avoid interference varies with pressure angle. Moreover, the Lewis form factor used in Lewis equation to account for bending stress at tooth root depends upon the number of teeth and pressure angle. Higher value of pressure angle has greater length of contact, whereas smaller value of pressure angle gives quietness in operation. Nowadays, a trend is followed in using different pressure angles at driving and costal side of gear tooth. The work involves experimentation using strain gauge and 3D photoelasticity techniques to study the effect of pressure angle on bending stress at critical section of helical gear. The results obtained from the experimental analysis have been compared with theoretical and finite element method results.

The principal type of gear tooth failure is scoring, pitting, corrosive wear, abrasive wear and bending failure. Out of these failures, some can be avoided by taking proper care during design of gear. Bending failure takes place when beam strength of gear tooth is less than bending stress at roots of tooth of gear. Beam strength of gear tooth depends upon Lewis form factor, and this factor varies with pressure angle and number of teeth. The resultant force between two meshing teeth depends upon pressure angle; also incremental dynamic load depends upon form of teeth that is pressure angle. The gears with high pressure angle appeared to be better suited to low speed high load, grease-lubricated conditions compared to gears with perfluoroether-based space greases. High pressure angle spur gears (35° pressure angle) running at high speed provide improved performance with similar bending and contact stress over more traditional pressure angles (20°). The higher the pressure angle, the lower the temperature increase of the lubricant across the gearbox while being tested at identical conditions in aero space jet-lubricated gear boxes. Lewis equation used for theoretical bending stress analysis at root of tooth of gear is based on various assumptions. It does not consider material properties and stress concentration. Further, for the analysis of helical gear, it is considered as imaginary spur gear. Therefore, the study of the effect of pressure angle on bending stress at tooth root is of great importance. In this paper, an experimental and finite element analysis is carried out for bending stress at critical section of helical pinions of different pressure angles and the results are compared with theoretical results. Experimental analysis has been carried out by using three-dimensional photoelasticity and strain gauge technique. Stress freezing photoelasticity is a responsive and inexpensive tool for stress analysis in the industrial environment (Robert, 1969). For finite element analysis, ANSYS software is used. In this study, three gears of different pressure angles of full depth 14.5o, 20o and 30o are considered and the other parameters.

Mechanical Engineering Journal, Pressure angle, Helical gear, Bending stress, Strain gauge, Photoelasticity.