PSI - Issue 2_A

G. Meneghetti et al. / Procedia Structural Integrity 2 (2016) 3185–3193 G.Meneghetti/ Structural Integrity Procedia 00 (2016) 000–000

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4. Conclusions A twin-disc test rig for contact fatigue characterization of materials was developed. The device is aimed to reproduce the most influencing operating conditions of gears at the most critical point along the tooth profile with regard to pitting damage. To allow the execution of complete tests without the need for inspection by an operator, the rig has been provided with a vision system conceived to automatically detect the onset of pitting on the surface of specimens. The tests are stopped automatically when a given percentage of worn surface is obtained. The criteria used to design specimens in order to recreate as much as possible the operating conditions of a specific planetary gear box have been as follows: the stress field below the most critical point of contact of gears with regard to pitting has been recreated by using a proper crown radius on one of the discs; moreover, since the low operating speed of gears prevents the formation of a EHD film, the absolute sliding speed was chosen as the main kinematic parameter to be recreated in discs. Such choice is in contrast with most of the existing literature which claims the SRR as the relevant parameter, but in applications where speeds are high enough to form an EHD film of lubricant, which is not the present case. Although preliminary tests on specimens with different geometries showed encouraging results, a test campaign must be performed to prove the correlation between gears and specimens durability, the topic being still debated in the existing literature. It should be pointed out that durability is expected to be greater for disc specimens than for gears because of the regular geometry of the specimens that reduces the risk of geometric stress concentration due to local reduction of the curvature radius. Moreover, in twin-disc tests the expulsion of debris from the contact area is easier because of the smaller contact length in axial direction of disc with respect to gears, which should result in lower risks of indentation and then creation of possible sites for crack nucleation. Finally, the lubrication of discs surfaces might be favoured by the higher rolling speed, even if a full separation between the crests of asperities is not expected. Acknowledgements The authors would like to acknowledge Carraro S.p.a. for the financial support to the specimens manufacturing. References AGMA 925-A03 Effects of Lubrication on Gear Surface Distress, 2003. Ahlroos, T. et al., 2009. Twin disc micropitting tests. 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