This paper deals with the bending strength and the size effect of carburized 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 various effective case depths. The bending fatigue tests are performed using these gears. From the test results, the AGMA recommendation of the effective case depth is confirmed to be valid in view of the bending strength. The SN curves are obtained and they are expressed by the experimental formulas. The fatigue strength of fine module gears are compared with the strength of gear of m = 5, and the size factor is evaluated. 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 crack is evaluated. This crack length is proportional to the thickness of nonmartensitic layer at the fillet. The size effect is discussed on the basis of stress intensity factor and the thickness of nonmartensitic layer.

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