The paper pertains to analytical study of the major reasons that cause tooth failure in gearing having small
teeth number. Below, gearswith small teeth numberarereferred to STN–gears. Fortheanalysisofgeartooth
failure, tooth contact stresses, and the combined contact/shear stresses (CS–stresses) are investigating.
The study is based on the in–depth analysis ofthe gear tooth loading and on accounting ofvariation in time of
the tooth loading and ofother gear parameters in various phases oftooth meshing. The earlier developed by
theauthorDG/K–method 1 ofsurfacegeneration is used in thestudy. Forthecomputation ofthecontactloads
of parallel–axis gears, a novel load distribution model is proposed in the paper, as well as a novel analytical
method for computation (a) of contact stresses, and (b) of combined CS–stresses in STN–gearing is
discussed. The CS–stresses are caused by simultaneous impact of (a) contact stresses together with (b)
stresses caused by the pinion/gear tooth profile sliding. To use the developed method of computation,
minimum of preliminary assumptions are required. While developed for use in STN–gearing, the method is
also recommended for the accurate computation of stresses in gearing having regular teeth number. Using
commercial software MathCAD/Scientific, a computercodes forcomputation ofboth ofcontactstresses, and
of the combined CS–stresses are worked out. The reported results of the research could be used for
extending of the existed standard AGMA 908–B–89 to the area of gear drives comprised of STN–gears, e.g.
comprised of gears having teeth number less than N g = 1 2. The developed method for the computation is
ready to put in practice.

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