Large dimension bearings and gears are generally made of surface hardened steels after induction heating or case-hardened steels. Unlike case hardened steels, induction surface hardened steels show a drastic reduction of material properties (hardness, endurance limit…) between the treated layer and the core called “metallurgical slope of the transition zone”. The combination between the use of conventional hardened depth dimensioning methods (depth greater than 2 times the maximum shear depth) and a severe metallurgical slope allows the existence of an unconventional site of fatigue cracks initiation leading to early failure.
There is no current method for sizing this phenomenon. In collaboration with users and manufacturers of slewing bearings, Cetim is thus developing a simplified sizing methodology which aims to set up easy to use design rules.
This methodology is based on the correlation between rolling contact fatigue tests performed on a twin-disc machine, which reproduced the contact between rolling elements and rings, and finite element calculations taking into account the geometry of the specimens and the gradient of mechanical characteristics.
This article presents an experimental methodology on a twin-disks test bench to reproduce this type of failure.