Tech Lic. Me 53 Acta Polytechnica Scandinavica. Stockholm 1970

Abstract:

Stresses in external involute spur gears is the subject of this work. New non-dimensional parameters are derived and calculated for gear contact pressures and gear bending stresses as well. These non-dimensional stresses are based on equal centre distance, equal power transmitted at equal angular velocity of pinion, and equal gear ratio. The analysis of these shows the advantage of using modified gears.

A very interesting result is that from contact pressure point of view there is an optimum condition using about 20 teeth on the pinion, provide it is sufficiently modified. This is true for almost every ratio.

Bending stresses increase with the number of teeth. This demands the use of modified gears, as those permit smaller number of teeth.

A numerical method of solving plane elastic problems within curved boundaries is introduced. This numerical method is then used to solve e gear stresses as a problem of elasticity. Also principal stresses are calculated and their difference are plotted. A theoretical fringe pattern is compared to a photoelastic test. The agreement between theory and experiment is remarkable. Theoretical stress concentrations have been calculated and compared to earlier known experimental stress concentration factors. Here again the results differ very little.

Fatigue tests showed good agreement with calculated contact pressures and given material properties. These tests also showed that surface strength can be improved, in certain cases, by stubbing the gear wheel.

A gear breakage test also verified the tangential stresses at the fillet curve. The fracture occurred at the predicted load.

Modern steel materials can after hardening stand much higher contact pressures. The risks of having bending breakage will then be greater. Some plastic materials are also sensitive to bending stresses. These facts call for complete calculations of accurate gear bending stresses.