The durability of gear and disc specimens - Part II: Post failure examination and gear-disc correlation

In this paper the authors present a post failure examination of the gear an d disc tests reported in Ref (1) on a standard carburized steel (BS S156), a temper resistant carburized steel (AISI M50NiL), and a carburized mid nit rided temper-resistant steel (AISI M50NiL Duplex). An analysis of the relationship between the gear and disc tests is also pre sented. The fatigue lives of the gear tests correlated well with the chamfe red disc tests. However, this was thought to be fortuitous, due to the diff erent stressing of the specimens. The fatigue lives of the crowned disc tes ts gave longer lives than the gears for equivalent stresses. As the slide-r oll ratio was increased towards that of the highest point of single tooth c ontact (-0.26) the correlation improved. Ar a slide-roll ratio of -0.28 on the disc specimens where the correlation of pitting life might have been ex pected to be the best, scuffing of the disc occurred, possibly due to rite higher temperatures. Several factors thought to be responsible for the diff erence in lives between gear and disc specimens have been discussed. The ro ughness and slide-roll ratio have been deemed the most important.