Grasas lubricantes

Downloaded from Reliabilityweb.com on the web at http://www.reliabilityweb.com UNDERSTANDING THE BASICS OF GREASE
Since lubricating oil can oxidize, so can the base oil in grease. When the grease oxidizes, it usually darkens; there is a build-up of acidic oxidation products, just as in other lubrications. These products can have a destructive effect on the thickener, causing softening, oilbleeding, and leakage. Because grease does not conduct heat easily, serious oxidation can begin at a hot point and spread slowly through the grease. This produces carbonization and progressive hardening or crust formation. All things considered the effect of oxidation is more harmful in grease than in oil. The rate of oxidation is mainly dependent on the temperature. Here it is good to understand thatif a soapbased grease is heated, its penetration increases only very slowly until a certain critical temperature is reached. At this point the gel structure breaks down, and the whole grease becomes liquid. This critical temperature is called the DROP POINT. Grease, when heated above its drop point and then allowed to cool it usually does not fully regain its grease-like consistency, and itsperformance subsequently will be unsatisfactory. Therefore, it follows that at no time should the drop point be exceeded. The problem with multi-purpose greases is that at least one of the agents will exceed its drop point thus adversely affecting the soaponification process. Grease has a maximum temperature at which it can safely be used. Therefore, it follows that it must also have a minimumtemperature. This minimum temperature is the point where the grease becomes too hard for the bearing, or other greased component, to be used. Again, the base oil of the grease determines the minimum temperature. Obviously, the base oil of the grease for low-temperature service must be made from oils having a low viscosity at that temperature. The temperature limits for use of greases are therefore,determined by drop point, oxidation, and stiffening at low temperatures. Grease can be divided into 6 general categories or types; i.e., mixtures of mineral oils and solid materials, heavy, asphaltic-type oils blended with lighter oils, extreme-pressure greases, roll-neck greases, soapthickened mineral oils and multi-purpose grease. MINERAL OILS MIXED WITH SOLIDS These types of greases are very heavylubricants for specialized applications. Such greases lubricate roughfitting machine parts operating under heavy pressures or loads at relatively slow speeds. Examples of equipment that will typically use this type of grease include concrete mixers, bearings and rollers on conveyors and heavy construction equipment. HEAVY ASPHALTIC-TYPE OILS BLENDED WITH LIGHTER OILS These types of lubricants areclassified as greases but are actually thick, heavy oils used to lubricate opentype gearing and wire rope. A primary advantage of these oils is that they form a heavy protective film when heated or painted on surfaces and then allowed to cool. Lighter oil is typically blended with the heavy oils in order to improve the pour point of the oil. EXTREME-PRESSURE GREASES The unique characteristic of thistype of grease is that it contains additives to improve firm strength under various applications. In essence, film strength provides the resistance of the lubricant to being torn apart, thus preventing metal-to-metal contact of the equipment being lubricated. A film is formed by a chemical reaction of the metal to the additives in the grease. The chemical reaction is usually brought about (oraccelerated) by pressure exerted on the grease, creating heat.

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A few of the additives used in EP greases include compounds containing parts of chlorine, phosphorus, active and/or passive sulfur, chlorinated waxes and phosphates. Zinc...