11/10/2023 0 Comments Viscosity chemistry vocabulary![]() The most common example of boundary lubrication includes bearings, which normally operate with fluid film lubrication but experience boundary lubricating conditions during routine starting and stopping of equipment. Boundary lubrication occurs whenever any of the essential factors that influence formation of a full fluid film are missing. The bulk flow properties of the liquid play little or no part in the friction and wear behavior.Īs mentioned, boundary lubrication is effective when a complete fluid film does not develop between potentially rubbing surfaces, the film thickness may be reduced to permit momentary dry contact between wear surface high points or asperities. The friction and wear in boundary lubrication are determined predominantly by interaction between the solids and between the solids and the liquid. The oil film has become so thin in Zone 1 that there is no hydrodynamic contribution and only boundary lubrication which is defined by Campbell in 1969 as the lubrication by a liquid under conditions where the solid surfaces are so close together that appreciable contact between opposing asperities is possible. The contact surfaces of bearings and journals must be smooth and free from sharp surfaces that will disrupt the fluid film. The operating speed must be sufficient to allow formation and maintenance of the fluid film. ![]() ![]() The fluid must be distributing itself completely within the bearing clearance area. The fluid must be adhering to the contact surfaces for conveyance into the pressure area to support the load. The fluid viscosity must be high to maintain adequate film thickness to separate the contacting surfaces at operating speeds. ![]() The contact surfaces must meet at a slight angle to allow formation of the lubricant wedge. The formation of fluid film is influenced by the following factors: Another way of handling the heat dissipation is to use commercially available additives to decrease the viscosity's temperature dependence which are known as viscosity index improvers. One method of accomplishing this is to cycle the lubricant through a cooling reservoir in order to maintain the desired viscosity of the fluid. Special attention must be paid to the heating of the lubricant by the frictional force since viscosity is temperature dependent. Hydrodynamic lubrication is an excellent method of lubrication since it is possible to achieve coefficients of friction as low as 0.001, and there is no wear between the moving parts. The surfaces have to be such that a converging wedge of fluid can develop between the surfaces, allowing the hydrodynamic pressure of the lubricant to support the load of the shaft or moving surface. The surface geometry is also very important. The distance between the two surfaces decreases with higher loads on the bearing, less viscous fluids, and lower speeds. We have to be very careful that the distance between the two surfaces is greater than the largest surface defect. Too low of a viscosity jeopardizes our system though. Only friction present in a hydrodynamic lubrication system is the friction of the lubricant itself, it would make sense to have a less viscous fluid in order to minimize friction: the less viscous a liquid the lower the friction. There are four essential elements in hydrodynamic lubrication, a liquid, relative motion, the viscous properties of the liquid, and the geometry of the surfaces between which the convergent wedge of fluid is produced. Hydrodynamic Lubrication is often referred to as stable lubrication. In zone 3 is the zone of hydrodynamic or fluid film lubrication where there is no wear because there is no contact between the surfaces. With the proper design, in fact, this very sliding motion constitutes the means of creating and maintaining that film. Except under some conditions of reciprocating motion, this is not necessarily true at all. It would be reasonable to suppose that, when one part slides on another, the protective oil film between them would be scraped away. Whether the sliding occurs on flat surfaces, as it does in most thrust bearings, or whether the surfaces are cylindrical, as in the case of journal (plain or sleeve) bearings, the principle is essentially the same. Hydrodynamic lubrication is the more common, and it is applicable to nearly all types of continuous sliding action where extreme pressures are not involved. In the latter, the oil film is not sufficient to prevent metal-to-metal contact. ![]() In the former, a continuous full-fluid film separates the sliding surfaces. Basically, lubrication is governed by one of two principles: hydrodynamic lubrication and boundary lubrication. ![]()
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