Gear Root Bending Strength: A New Multiaxial Approach to Translate the Results of Single Tooth Bending Fatigue Tests to Meshing Gears

Franco Concli,Lorenzo Maccioni,Lorenzo Fraccaroli

doi:10.3390/met11060863

Abstract

Developing accurate design data to enable the effective use of new materials is undoubtedly an essential goal in the gear industry. To speed up this process, Single Tooth Bending Fatigue (STBF) tests can be conducted. However, STBF tests tend to overestimate the material properties with respect to tests conducted on Running Gears (RG). Therefore, it is common practice to use a constant correction factor fkorr, of value 0.9 to exploit STBF results to design actual gears, e.g., through ISO 6336. In this paper, the assumption that this coefficient can be considered independent from the gear material, geometry, and loading condition was questioned, and through the combination of numerical simulations with a multiaxial fatigue criterion, a method for the calculation of fkorr was proposed. The implementation of this method using different gear geometries and material properties shows that fkorr varies with the gears geometrical characteristics, the material fatigue strength, and the load ratio (R) set in STBF tests. In particular, by applying the Findley criterion, it was found that, for the same gear geometry, fkorr depends on the material as well. Specifically, fkorr increases with the ratio between the bending and torsional fatigue limits. Moreover, through this method it was shown that the characteristics related to the material and the geometry have a relevant effect in determining the critical point (at the tooth root) where the fracture nucleates.

Highlights

Gears are widespread components commonly used for transferring mechanical power between noncoaxial rotating shafts [1]
The σEq Running Gears (RG) is the maximum Huber–Mises stress recorded in the OPSC condition in RG simulations while σEq Single Tooth Bending Fatigue (STBF) is the maximum Huber–Mises stress observed when the applied force has achieved its maximum value in STBF simulations
While the RG and STBF are equivalent from the perspective of the standard, by considering the tensile stress induced by bending and the effect of the compression force, it emerges that the loading conditions could significantly differ

Summary

Introduction

Gears are widespread components commonly used for transferring mechanical power between noncoaxial rotating shafts [1]. According to [4], the repeated contacts between gear flanks lead to fatigue failure modes such as scuffing [5], wear [6], pitting [7], and micropitting [8]. Whereas the repeated pulsating bending loading of the teeth root leads to a failure mode called Tooth (Root) Bending Fatigue (TBF) [9]. The crack propagation leads to the tooth root breakage, and to the endangerment of the entire system [11]. For this reason, the TBF is considered one of the most dangerous failure modes, and it could potentially lead to catastrophic consequences [12].

Methods

Results

Conclusion