۱٫ Introduction

۲٫ Theoretical background: method for calculating the global dynamic factor KAV

۳٫ Practical case: accessories gearbox for aerospace application

۴٫ Results and discussion

۵٫ Conclusions

References

The present paper aims to propose a method, in ISO Standard environment, in order to calculate a single global dynamic factor KAV, replacing both KA and KV, in case of gears subjected to variable velocity and loading conditions. This procedure, based on the Miner damage rule, allows to process a given load spectrum and to calculate a value of the equivalent tangential force that includes all dynamic effects; this force value is useful for bending and pitting calculation of the service life. In this work a practical case for bending strength is presented, based on a recorded flight mission, referring to a gear box for aerospace applications. Obtained results in terms of equivalent forces and global dynamic factor values have been compared to those calculated by means of the classical ISO Standard formulae, based on the corresponding experimental data.

Introduction

Classical design formulae available in literature to determine the load capacity of spur and helical gear drives are intended, both for pitting resistance and bending strength, to establish uniformly acceptable methods to assess the corresponding life estimation. The most common approach, widely described in ANSI/AGMA Standard 2001-D04 [1] and in ISO 6336-1 Standard [2-6], compares the calculated maximum stress values (tensile and contact, for bending and pitting respectively) to the permissible ones representing the limit value for stresses (tensile and contact) derived from material tests using meshing gears as test pieces. Referring as an example to the bending case [1, 3], discussed in detail in the present paper, the permissible bending stress σFP provides damage curves characterized by the nominal stress number σFlim and by the life factor YNT, and then corrected using the relative influence factors for notch sensitivity, surface roughness, size. For as concerns the calculated stress, the tooth root stress σF is the maximum tensile stress at the surface in the root and it can be obtained by multiplying the nominal tooth root stress (which is the maximum local principal stress produced at the tooth root) by the so called overload factors. Overload factors [1-4] are generally influence factors, independent each other, that aim to take into account uneven overloading conditions of gears.