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Overrolling of solid par­ticles can produce surface indentations of raceways in rolling-sliding lubricated contacts. These indentations are known to increase the risk of contact failure. The failure process may be influenced by the amount of sliding present in the contact, gears being different from rolling [bearings online]. In order to increase the understanding of the failure process and to develop preventive measures, a detailed analysis of the dynamic behaviour of indentations in rolling-sliding heavily loaded lubricated contacts has been carried out. 

The effect of pressures, stresses and surface distress around the indentations has been analyzed and the prediction of damage compared with experimental results. Studies of the influence of sliding in the contact have shown that for low sliding conditions, typical for rolling nsk bearings, the failure process is dominated by film thickness reduction and surface distress around the shoulders of the indentation, while for high sliding (gears), a fatigue process due to superposition of two waves of pressure ripples is more likely.

Surface geometrical disturb­ances (e.g., scratches, indentation marks, etc.) are known to be a source of damage in rolling-sliding heavily loaded lubricated contacts. These contacts generally work under elastohydrodynamic lubrication (EHL) conditions, but sometimes mixed lubrication can take place. The contacts are found in many machine elements, including gears and rolling bearings. Surface imperfections can generate local film thickness fluctuations and pressure ripples, causing stress concentrations. Surface stress induced by geometrical features passing through Hertzian-like (EHL) contacts have been studied by many researchers in the past with the use of numerical, experimental and semi-analytical methods. 

Current industrial trends of downsizing, low energy consumption and higher operating temperatures tend to increase the surface stress in rolling bearings. Thinner film thicknesses, higher (or very low) speeds and dynamic conditions in rolling bearings are common. All these factors are likely to increase the risk of bearing failure from raceway imperfections such as indentations. Oil cleanliness in bearings is an important factor to maximize durability. This is why the SKF bearing life model and the ISO 281 standard include lubricant cleanliness as an input parameter in bearing life calculations. In a previous article the effects of lubricant cleanliness and its impact on bearing life have been discussed; here, a more detailed description of the indentation mark and lubrication conditions of the bearing raceway contact will be considered.

Many authors have previously worked on the topic using lubricated and dry contacts and low and high sliding values, giving a rather unclear overall picture of the failure mechanism. Cheng et al. carried out an experimental investigation on crack development in the region of artificially produced indentations in rolling-sliding contacts, such as dents and grooves under EHL lubrication conditions with a sliding nachi bearings factor S = ±0.24, i.e., S = 2 (u2-u1)/(u2+u1). The ana­lysis was complemented with some numerical simulations using non-Newtonian fluid that showed that sliding changes the position and magnitude of the pressure ripple around the indentations.

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