Machinery Lubrication

Machinery Lubrication May-June 2017

Machinery Lubrication magazine published by Noria Corporation

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Page 49 of 76 | May - June 2017 | 45 what is called boundary contact conditions. This support involves wear and friction- control additives. The base oil and additives are carefully blended together to produce the specific lubricant product (either oil or grease), which is formulated to mitigate the anticipated boundary conditions. The lubri- cant then possesses film strength and boundary lubrication properties. Film Strength Film strength can be described as the lubricant's ability to lessen the effects of friction and control wear by means other than the film thickness. As mentioned, the viscosity is the primary contributor to film thickness during hydrodynamic and elas- tohydrodynamic lubrication. When the base oil viscosity is insufficient to over- come metal-to-metal surface contact, the base oil and additive chemistry work together to create a surface protection mechanism. During these boundary condi- tions, boundary lubrication is also influenced by the chemical and physical properties of the mechanical surfaces and any contributing environmental factors. Even when loads and temperatures are higher and relative surface velocities are lower, the film strength is improved. Unlubricated Surface Interactions If you were to observe contacting mechanical surfaces on a molecular level, you would see that they can be relatively rough, even if they are machined to be very smooth and appear that way to the unaided eye. This could be compared to how the earth looks like a perfect sphere from the perspective of an astronaut in space but is brimming with mountains and valleys of all heights and depths when viewed by someone standing on the earth's surface. This is relevant because when two unlu- bricated metal surfaces come in contact, the actual contact area will be substan- tially less than the apparent contact area. The surfaces will only come in contact where these "microscopic mountains" called asperities are the tallest and reach to the other surface, preventing lower asperities from making contact. These asperity surfaces can then elastically deform based on the corresponding shear strength of the metals. Thus, the real contact area will increase proportionally with an increase in load because the initial contact points will elastically deform first and more contact points will connect. Friction Friction, the resistance to sliding motion of interacting surfaces, is subject to several influencing parameters or processes. Most people consider the roughness of the surface as the primary contributing param- eter for friction. However, when considering that the real contact area may be less than 1 percent that of the apparent contact area, the actual roughness becomes much less relevant. The significant process contrib- uting to friction is a result of the adhesive bonds occurring at the atomic level of asperity contact. Wear Generation In conditions where there is inadequate lubricant film thickness between the metal surfaces, the asperity contact points can lead to cold welding, which is the prerequi- site for adhesive wear. The adhesion at these asperity points undergoes a work-hardening process, which strengthens the material. Thus, the shear point happens at layers below the asperity contact point where the metal has not been strengthened. As the metal shears, the asperity tip is then either transferred to the other surface or broken off as an abrasive particle. Adhesion is often seen as the initial form of mechanical wear, but as abrasive particles present themselves (either from wear or from an external source), abrasive wear can become more destructive. This form of abrasion is called three-body abrasion, whereas two-body abrasion is caused by cutting or gouging of sharp surface contact points. During rolling contact, surface fatigue can occur. Fatigue mechanisms stem from cracks propagated at the surfaces or up from layers under the surface that contain inclusions or other impurities. The high ML

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