The Influence of Slide–Roll Ratio on the Extent of Micropitting Damage in Rolling–Sliding Contacts Pertinent to Gear Applications

Abstract

Micropitting is a type of surface damage that occurs in rolling-sliding contacts operating under thin oil film, mixed lubrication conditions, such as those formed between meshing gear teeth. Like the more widely studied pitting damage, micropitting is caused by the general mechanism of rolling contact fatigue but, in contrast to pitting, it manifests itself through the formation of micropits on the local, roughness asperity level. Despite the fact that micropitting is increasingly becoming a major mode of gear failure, the relevant mechanisms are poorly understood and there are currently no established design criteria to assess the risk of micropitting occurrence in gears or other applications. This paper provides new understanding of the tribological mechanisms that drive the occurrence of micropitting damage and serves to inform the ongoing discussions on suitable design criteria in relation to the influence of contact slide-roll ratio (SRR) on micropitting. A triple-disc rolling contact fatigue rig is used to experimentally study the influence of the magnitude and direction of SRR on the progression of micropitting damage in samples made of case-carburised gear steel. The test conditions are closely controlled to isolate the influence of the variable of interest. In particular, any variation in bulk heating at different SRRs is eliminated so that tests are conducted at the same film thickness for all SRRs. The results show that increasing the magnitude of SRR increases the level of micropitting damage and that negative SRRs (i.e. the component where damage is being accumulated is slower) produce more micropitting than the equivalent positive SRRs. Measurements of elastohydrodynamic film thickness show that in the absence of bulk heating, increasing SRR does not cause a reduction in EHL film thickness and therefore this cannot be the reason for the increased micropitting at higher SRRs. Instead, we show that the main mechanism by which increase in SRR promotes micropitting is by increasing the number of micro-contact stress cycles experienced by roughness asperities during their passage through the rolling-sliding contact. Therefore, the asperity stress history should form the basis of any potential design criterion against micropitting.

From Hertzian contact to spur gears: analyses of stresses and rolling contact fatigue

“…The overpressures resulting from the micro-geometrical parameters have to be regulated with surface finish improvement. OILA has given the same conclusions through empirical investigations [7]. -A representative gear damage simulation is made possible with this model.…”

“…Then, formulation (6) comes directly from calculations so as to isolate the number of cycles to fatigue n i . The dimensionless version is given in (7).…”

“…When a more realistic representation of classical surfaces in contact is wanted, the introduction of a rough component is needed. Surfaces microgeometry is indeed highlighted in literature [7,26,27] as a major parameter to take into account so as to correctly forecast micropitting. So, a sinusoidal roughness geometry (one frequency 1/l) is set [28].…”

“…DT 5,re is computed from the following method: -The total number of oscillations for each GB is estimated with the rainflow algorithm (each extrema is identified in Fig. 12 by a full circle) -For each oscillation in each GB, a local DT i,j (i being the index of simulation and j the oscillation one) is estimated with the minimum-maximum method -The global number of cycles N i,re is then calculated with equation (18) -Finally, the equivalent DT i,re is obtained from N i,re and equation (7).…”

“…Among them, pressure, roughness, friction and slide-to-roll ratio are predominant. More recently, RYCERZ [7] has also identified slide-to-roll ratio as a major influencer in micropitting occurrence and particularly on the rough micro-contact stress cycles. This asperity stress history is highlighted as a potential basis for a micropitting criterion.…”