The effects of hard turning and deep rolling process on surface integrity and fatigue life of deep groove ball bearings

It is well known that the raceways of ball bearings are often damaged due to the harsh operating conditions. The fatigue life of the bearings is directly related to the surface and sub-surface characteristics of their raceways. Currently, surface integrity of an inner ring raceway is ensured by grinding process in conventional bearing production. In this study, the effects of hard turning and deep rolling processes on surface integrity and fatigue life of inner ring raceway of deep groove ball bearings were investigated. The inner rings of the 6208-type deep groove ball bearings were employed as workparts during the tests. This study consists of two important steps. In the first step, the effects of cutting conditions and tool geometry on cutting forces, surface integrity, noise level of bearings were studied in hard turning of the inner ring raceways (62 HRC). In the second step, the performances of grinding (GR), hard turning (HT) and hard turning+deep rolling (DR) processes applied to raceways of inner rings are compared to each other in terms of surface integrity, noise level and fatigue life. Experimental results in the first step showed that the passive force was significantly affected by the effective rake angle and the magnitude of contact area at the toolworkpiece interface. Another finding of the study also is that minimal surface roughness, roundness error, compressive stresses substantially affecting the fatigue life of bearings are achieved by V-35 insert. Therefore, the bearings with inner rings hard turned by V-35 insert were employed for evaluations in the second step. A remarkable point in second step is that although DR process under excessive pressures produces higher compressive residual stresses in the deeper layers of the work surface, DR bearings have a lower fatigue life than HT and GR bearings. HT bearings present longer fatigue life due to their inner ring raceways having high compressive stresses, low roundness error and surface roughness. Keywords: AISI 52100, hard turning, bearing, residual stresses, fatigue life, deep rolling