SIZE EFFECT IN BENDING STRENGTH OF CARBURIZED GEAR TEETH

This paper deals with the bending strength and the size effect of carb urized fine module gears. The fine module test gears (m = 1- 1.5) are made of a low-carbon alloy steel, and they are carburized to have vari ous effective case depths. The bending fatigue tests are performed usi ng these gears. From the test results, the AGMA recommendation of the effective case depth is confirmed to be valid in view of the bending s trength. The SN curves are obtained and they are expressed by the expe rimental formulas. The fatigue strength of fine module gears are compa red with the strength of gear of m = 5, and the size factor is evaluat ed. In order to make clear the size effect, the fracture mechanics is applied to the fatigue test results. The fatigue life is assumed to be the crack propagation process, and the maximum initial length of crac k is evaluated. This crack length is proportional to the thickness of nonmartensitic layer at the fillet. The size effect is discussed on th e basis of stress intensity factor and the thickness of nonmartensitic layer.