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Lubricating Oils
Published in Don M. Pirro, Martin Webster, Ekkehard Daschner, Lubrication Fundamentals, 2017
Don M. Pirro, Martin Webster, Ekkehard Daschner
Micropitting occurs on surface-hardened gears and is characterized by extremely small pits. Micropitted metal typically has a frosted or a gray appearance. This condition generally appears on rough surfaces and is exacerbated by use of low-viscosity lubricants. Slow speed gears are also prone to micropitting owing to the presence of thin oil films. Micropitting may be sporadic and may stop when good lubrication conditions are restored following run-in. Therefore, maintaining adequate lubricant film thickness is the most important factor influencing the formation of micropitting. Higher speed operation and smooth gear tooth surfaces also hinder the formation of micropitting. The FZG micropitting test according to FVA 54/7 consists of a load stage test and an endurance test. Test gears type C-GF run at a circumferential speed of 8.3 m/s and a lubricant temperature of 90°C (194°F) or 60°C (140°F). The load and the test periods are varied.
Ashless Phosphorus–Containing Lubricating Oil Additives
Published in Leslie R. Rudnick, Lubricant Additives, 2017
Corrosive wear occurs when the metal surfaces react with their environment to form a boundary film, whereas fatigue wear is the process of the fracture of asperities from repeated high stress. Micropitting is an example of this form of wear, which is the subject of considerable investigation today. Micropitting is the result of plastic deformation of the surface that eventually causes the fracture of the asperity, leaving a small pit in the surface. Ploughing wear arises when a sharp particle is forced along the surface, leaving a groove behind, whereas adhesive wear is the tendency of very clean surfaces to adhere to each other. However, this action requires the generation of fresh surfaces during the wear process, perhaps by plastic deformation. It is now thought that this mechanism is much less prevalent than was earlier believed [64].
Gear Load Capacity Calculation Based on ISO 6336
Published in Stephen P. Radzevich, Dudley's Handbook of Practical Gear Design and Manufacture, 2021
Daniel Müller, Nadine Sagraloff, Stefan Sendlbeck, Karl Jakob Winkler, Thomas Tobie, Karsten Stahl
Micropitting is a surface fatigue phenomenon, which is the result of numerous small surface cracks. It occurs in Hertzian rolling and sliding contacts under unfavorable lubrication regimes in EHL-contacts and usually on materials with a high surface hardness. It can be optically detected as a gray and dull area on affected surfaces. The main influence factors on micropitting are the lubricant, the flank surface roughness, the gear geometry and the operating conditions.
Influence of Black Oxide Coating on Micropitting and ZDDP Tribofilm Formation
Published in Tribology Transactions, 2022
Mao Ueda, Hugh Spikes, Amir Kadiric
Micropitting is a common type of surface fatigue damage caused by stress fluctuations that occur due to asperity interactions as the contacting bodies move over each other. These asperity stress cycles result in initiation of numerous tiny surface fatigue cracks, which then propagate until small fragments of material detach from the surface. The damage manifests itself as numerous small pits, tens of micrometers in size, that give the surface a frosted appearance and can result in substantial loss of material and eventual component failure. Micropitting is one of the most important failure modes in gears and rolling bearings because of increases in power density of mechanical systems and the introduction of low-viscosity lubricants, with consequently thinner lubricant films and more severe asperity interactions. A number of factors have been shown to promote micropitting, including low specific film thickness and high counterface roughness (1), high friction (2), high slide–roll ratio (SRR) (3), and inadequate running-in due to fast initial tribofilm growth rate (1, 4). In practical situations, where the lubricant formulation and contact conditions are imposed by numerous factors, it is not always possible to control these parameters with the sole aim of preventing micropitting. The application of black oxide (BO) coating has recently been suggested as an effective, alternative way to alleviate micropitting (5).