The single most common cause of gear failure is inadequate lubrication, improper selection, lubrication quantity, and application technique. Proper lubricant is one of the most important factors in achieving safe, fault-free service. Viscosity is the primary consideration when selecting lubricants. Open gears, operating at very slow speeds (15 rpm), may require viscosities as high as 2,200 cst. (AGMA 9005-E). In parallel, as we are faced with situations of mixed friction or limit lubrication, damage prevention depends directly on the quality and type of lubricant to be selected. A low quality lubricant has a direct effect on the formation of damage such as scuffing, or abrasive wear, and on the formation of corrosion. The consequences of a lubricant shortage usually influence deformations such as the development of waving, hot or cold creep. The flank surfaces of new (or damaged) gears are not smooth. At a microscopic level they appear as a sequence of peaks and valleys, that is, roughness or roughness. In the stress diagram on the tooth we have:
Pure rolling stress on the line of pitch diameter, which decreases in the direction of the ends of the tooth root and head respectively.
Friction stress that has a theoretical value of zero on the line of primitive and maximum diameter at the head and foot of the tooth respectively, contrary to the magnitude of the rolling stress.
When meshing occurs, contact is limited to the peaks of opposite asperities between both surfaces of the respective pressure flanks. Using conventional open drive lubricant, these surface asperities or irregularities are generally sheared off, or subjected to a “weld and fracture” cycle, leading to further damage and even greater surface irregularities.
Therefore, it can be considered that due to the lubricant “cushion” effect, using high base oil viscosities (both asphalt and non-asphalt bases with tenacious film) currently used in the market, a totally efficient and safe lubrication is not achieved, especially as regards protection against pitting, in “limit” service conditions.
Therefore, a highly effective chemical and physical protection layer is required, which prevents the contact points from being subjected to high specific loads and forming welding damage or surface wear.
We can consider that the ideal situation is achieved with lubricant formulations that not only have a high viscosity base oil, and a package of extreme pressure and antiwear chemical additives, but also contain the addition of a combination of solid additives, which interpose between the ridges. As a result, the effect of this formulation causes these ridges to give way and be pushed into the “valleys.”
The end result is an improvement in the smoothness of the meshing surfaces. When cleaning during a service stop, the surfaces with a “mirror shine” can be observed. This process, also known as “flattening”, results from the combination of high specific loads in the presence of an effective anti-wear barrier. The direct consequence of damage to existing pitting is the rounding of the edges and stopping their progress.
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