The real root stress of a cup-type flexible spline of the strain wave gearing (commonly known as a harmonic drive) is assumed to be composed of six kinds of stresses. They are the stress σ
T produced by the tooth load, not influenced by the deformation of the rim ; the stresses produced by the tensile and bending nominal stresses, σ
CT and σ<CB>, respectively, in the circumferential direction of the flexible spline : the stresses Produced by the tensile and bending nominal stresses, σ
AT and σ
AB, respectively, in the tooth trace direction : and the stress produced by the nominal shear stress τ
s acting on the rim. On the basis of the experimental nominal stresses and the theoretical calculation of the root stress concentrations using the three-dimensional finite-element method, the real root stresses are estimated. The main results are as follows. The maximum root stress lies on the center of space in the center part in the tooth trace direction. About 90% of the stress in the tangent direction of the tooth profile is produced by tensile and bending nominal stresses, σ
CT and σ
CB, respectively, in the circumferential direction of the flexible spline, and about 10% of that is produced by the tooth load.
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