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Tribology: Lubrication Hydrodynamics


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action occurs when the relative motion between two lubricated surfaces creates a sufficient lubricating film thick to prevent any metal-metal contact. This meant that it is minimum film thickness is slightly higher than the average amount of irregularities of both surfaces and resistance to movement is given only by the friction between the layers of lubricant, the latter which fully supports the load. In this case, the lubrication conditions are optimal and the mechanism may operate for long time without any wear, as long as they keep these operating conditions.


Hydrodynamic lubrication requires an adequate flow of lubricant to keep separate the two surfaces. When one of the elements (or both) are moving on, the hydraulic action of the oil produces a convergent wedge, which develops pressures above 50,000 psi in the work zone, avoiding metal contact between the two surfaces.

The hydrodynamic lubrication theory has its origins in the laboratory of Beachump Tower, in the 1880's in England. That made a series of plain bearings lubricated expermentos. One of these consisted of placing a shaft in an oil bath and put it to spin, the half bearing that supports it (on top) had a hole for lubrication. However, to turn the shaft the oil came under some pressure by that hole, the colcársele a stopper, he was expelled slowly being implemented. Then a gauge mounted in the hole and saw that the pressure of oil film was about twice the average pressure based on the projected area of \u200b\u200bbearing (W / LD). Later there was another series of experiments to determine the hydraulic pressure distribution along the periphery of the roller bearings. (See figure)

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expiration date yoghurts

A yogurt with no additives of proteins, starches or other stories can remain edible for months without refrigeration (if not open).

Indeed, as time passes, the fermentation of yogurt increases, so the amount of acid produced in it increases. The expiration date represents, for all practical purposes, the date on which the yogurt has become sufficiently acid so that the consumer rebound. With the expiration date, the manufacturer tries to avoid associating their brand with yogurt acids (they are uncommercial).

But the fact that yogurt has become acid does not mean or anything, which is harmful to health. Quite the contrary. In general the growth capacity of a possible contaminating microorganism decreases with increasing acidity of the medium, so the more acid it becomes safer yogurt is (provided that it remains closed). Thus, an additive-free yogurt can be kept perfectly edible for months (if you do not mind the acidity), even outside the refrigerator.

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TRIBOLOGY: TRIBOLOGY



This scheme represents an intermediate state between boundary lubrication and hydrodynamic mechanism by which all pass before reaching this condition. In this case, part of the surface roughness of both mechanisms are inserted so that only part of the burden is borne by the hydrodynamic action and the other by the boundary film that covers the irregularities of both surfaces.

An incorrect selection of oil, as well as a decrease in viscosity, may lead to the mechanism is operating under these conditions. In this case, the lubricant film thickness is equal to the average roughness of both surfaces and not enough to obtain a laminar flow. See Fig. (A) and (b). For

cylinder internal combustion engine, where the present conditions of boundary and mixed film in the top and bottom dead spots, respectively, the incorporation of additives to oils unctuous base (during the manufacturing process) to ensure a better fixing of the oil film on the surfaces, or use of solid film lubricants (graphite or molybdenum disulfide), relatively insensitive to temperature and can form resistant layers and good rubbing permanence, can reduce a significant proportion wear in regime (fully loaded) and adhesive wear at startup. The figure shows these decreases in wear from five successive trials in a single-cylinder gasoline engine, the addition of graphite or MoS2 to a pure mineral oil and oil additives.




course, the benefit obtained with the addition of a solid film lubricant base oil, in terms of adhesive wear should not be offset by the potential risks that would result from a greater tendency to deposit formation as a result of insufficient stability of the suspension of solid lubricant base oil to be centrifuged at the elbows of crankshaft or passing through the filters of the lubrication circuit. However, current techniques of preparation and selection of these products and stabilization of suspensions suitable dispersant additives, SENCIBLES reduce these dangers. Whenever these conditions are met, these products can provide effective protection during the shooting and lead to surfaces that have a low coefficient of rubbing and whose subsequent wear are weak.

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celebrate in 2011 the International Year of Chemistry , 63, proclaimed by the UN General Assembly, under the slogan: " Chemistry: our life, our future ." It also marks the centenary Nobel Prize in Chemistry awarded to Marie Curie therefore no longer an opportunity to celebrate and highlight the contributions of women to science

From the earliest times with the use and control of fires, with the preparation of the first alloy of the history, chemistry has been present in the development and evolution of humanity and just look around to see who is everywhere: our body, the clothes we wear, what we eat, everything is made of matter consisting of molecules. In short, everything is " Chemistry: our life, our future ."

However, the educational administration remains committed to maintaining the science away from the Elementary and Secondary Education. It has been five years since the APCCC saw the light and, although it may seem otherwise a look at the booths, science teaching are feeling the same problems we denounce in 2006: No spaces or material to develop practical activities in primary education. The labs are closed on High, for lack of time and teachers to teach splittings, which does not happen in other regions. Moreover, the Andalusian students 15 years get the worst scores in the PISA tests, at the State level, there are fewer scientific vocations and our students entering university with a poor scientific training. Finally, the schedule is insufficient scientific subjects in secondary and inferior to the rest of Europe, with optional at 4, holding an absurd gap between Arts and Sciences is creating a high scientific illiteracy.

Our goal, as an association, is improve scientific culture of citizens and for this, we demand a greater presence of science in our schools and colleges. To achieve this, we have proposed to the Education Authority in the following steps repeatedly, to no avail:

• Provide resources and spaces with primary schools to carry out scientific experiments.
• Reduce the number of students in all areas of Teaching Secondary or establish splittings of practice, as in many Autonomous Communities.
• increase to three sessions weekly schedule of the subjects of Biology and Geology / Physics and Chemistry of 3 º ESO, separated for the purposes of evaluation. Include
• Science as compulsory subjects in 4 º ESO, with two levels of depth, depending on the difficulties of students.
• Set as mandatory on 1 st of Bachelor of Science and Engineering, the subjects of Physics and Chemistry and Biology and Geology, to get a solid scientific background of all students, regardless of further studies. Encourage
itineraries
• 2 of Bachelor of Science and Engineering to allow the student to access the first course of the Schools and Colleges with basic training in all scientific areas.

CÓRDOBA TEACHERS ASSOCIATION FOR SCIENTIFIC CULTURE.

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Mixed Lubrication: Lubrication Limit




simpre takes place a mechanism is put in motion, because the conditions of speed, load, temperature (viscosity) or lubricant application methods are not favorable for the formation of a fluid film.

There are only a small amount of lubricant on metal surfaces, which allows this maximum interaction between the roughness of both surfaces, however, the anti-wear lubricant adtivo prevents this metal-metal friction allowing it to be of solid type. If the boundary lubrication prevails for a long time, you can break the solid film formed by the antiwear additive and presented a significant reduction in the life of the mechanism.

Under normal operating conditions, the film lubrication regime limit should disappear completely, otherwise, the lubricant has been improperly selected or is contaminated with a substance of lower viscosity.

Other circumstances that may give rise to this situation is when you vary the operating conditions of the machine, as in the case of subjecting the different mechanisms at pressures greater than the design, leading to hydrodynamic effects are totally null . Film boundary conditions are also presented immediately before the mechanism stops. Therefore, a team whose mechanical parts are subject to frequent stops and starts consuming more antiwear additive so the oil should be changed faster than those that run continuously, because these anti-wear additive only works when boot or when the mechanism stops.
adhesive wear under boundary film can be heard only during the time it takes to react with anti-wear additive metal superfiie, when the solid film formed by the antiwear additive is gone.
The lubricant film thickness (h) under a range boundary film lubrication between 0.001 and 0.05 um. In

codiciones boundary layer lubrication, the wear can be caused by corrosion, adhesion, fatigue, and plow (or abrasion), acting alone or in combination.
corrosion is initiated by the formation of a film when the surface boundary layer reacts chemically with their environment. These films may be formed by:
-metal oxides formed by oxygen or water in the air or lubricant.
-Sales resulting from reaction with a lubricant additive or oxidation products. Tine
corrosive wear occurs when the film boundary layer tends to be displaced to the edges of the surface rather than penetrate the true load bearing areas between the asperities.
adherence occurs when the surfaces are totally clean and has a rough microsoldadura between the two surfaces.
fatigue cycles is the result of high tension and compression acting on the two fences. These efforts produce plastic deformation alguan until part or all of the rough, they break or disappear from the surface. This mechanism of wear is evident, because of micropicaduras aparción on the surfaces of the elements in moviminto.

Plough (or abrasion) occurs directly when a roughness hard sharp or a third body (as a wear particle roughness strain or a film of dust) slots or scratch the surface.
All products used in lubrication contain anti-wear additives that control the conditions of boundary lubrication film, but there is market lubricants specially formulated to be used when these conditions prevail for long. The most used are:

* organic origin: are a mixture of paraffin type mineral oil and 5% to 7% of animal fat or a fatty acid. (Stearic, oleic, palmitic and lauric, etc). The acid radical has an affinity for metals, particularly iron and steel, the initial attraction being electrical in nature and the molecules orient themselves or bristle, as hair from a carpet. The effectiveness of fatty acids is due to the radical reaction between polar and metal surface, which results in a lubricating film of low shear, but adheres strongly to surfaces.

film lubricant compound or compound significantly reduces the risk of microsoldadura between the roughness of metal surfaces and therefore adhesive wear.
fatty acids are highly miscible with mineral oils, obtained lubricants and have an excellent power of boundary lubrication, and are known as oil of oiliness additives, compounds or Compound. Are effective for operating temperatures below 50 ° C.

* solid film: Most employees are grafitoy molybdenum disulfide (MoS2). Can be used in a colloidal base oil is added from 5% to 7% by volume, or powder to be added to the fat from 3% to 5% by weight. In the market get so much oil as fat, which already come with molybdenum disulfide. In the figure shows a mechanism that operates under constant lubrication conditions boundary film.

* With extreme pressure additives (EP): are made from paraffinic base and an additive package of metal type, such as phosphorus, sulfur and chlorine. Used for operating temperatures above 50 ° C.