LUBRICATING OILSOils used for lubricating the bearing parts of machinery. They are usually the heavy distillates following kerosene in the fractional distillation of petroleum, between 253 and 317°F (123 and 158°C). They are separated into grades—light, medium, and heavy—depending upon the molecular weight. They are also classified as pale when yellow to reddish and dark when brownish black. The flash point ranges from 300 to 600°F (149 to 316°C), and the specific gravity usually from 0.860 to 0.940. Neutral oils are light oils obtained by distillation without cracking, and they will not emulsify in contact with water, as do the paraffin oils. They are thus desirable for crankcase lubrication and in circulating systems. Lubricating oils may be bleached with acid, and they may be mixed with vegetable or animal oils. The ideal of lubrication is to obtain a full fluid film with little clearance between the moving surfaces so that the shaft rotates on a film of oil. Hydrodynamic lubrication with pressure gives this condition. Only a boundary lubrication with contact merely on the bottom is obtained when the clearance is too great, the oil viscosity too low, the load too heavy, or the speed is too slow, so that the film does not support the shaft.
Animal oils are greasier than mineral oils, but they are acidic. Vegetable oils are greasy and have more oiliness, but they oxidize easily and are also acidic. They are likely to gum in use unless an antioxidant is employed. Vegetable and animal oils add the property of adhesion to the lubricating oil, but in no case should any element be added to an oil that will cause emulsification. Federal specifications for marine engine oil call for 15 to 20% blown, refined rapeseed or peanut oil. This lubricating oil has a flash point of 350°F (177°C). Steam cylinder oil has 5 to 10% fatty acid vegetable oils, and the flash point is 450°F (232°C). Absorbed oil is a name of a combination oil of E. F. Houghton & Co. which acts as both film and lubricant. Amlo is a trade name of a mineral oil refined wax-free, used for low-temperature lubrication. The silicones are now often used to replace lubricating oils for very high and very low temperature conditions, but in general the lubricating value is not high.
Antioxidants used in oils to reduce oxidation and minimize sludging and acid formation are usually tin compounds such as tin dioxide, tin tetraphenyl, and tin ricinoleate. Tin dust alone also has an inhibitory action. Detergents are compounded in lubricating oils for internal combustion engines in order to prevent and break down carbon and sludge deposits. High percentages of animal or vegetable oils may be added to lubricating oils for use on textile machinery. They are called stainless oils for this purpose, since such oils wash out of the textile more easily than mineral oils do. They also give lower coefficients of friction. The high lubricating qualities of the vegetable oils without the disadvantage of gumming can be obtained with mineral oils by the addition of an oiliness agent such as cetyl piperidine ricinoleate. The EP lubricants (extreme pressure) for heavy-duty gear lubrication are made with a high-quality oil compounded with a lead-sulfonated soap.
For extreme high pressure and high temperatures where oils and greases oxidize, molybdenum disulfide, MoS2, is used alone or mixed with oils or silicones. It is a fine, black powder, available in particle sizes as small as 30 (xin (0.75 (xm), which adheres strongly to metal surfaces, gives a low coefficient of friction, and permits operation up to 750°F (399°C), but it has an acid reaction and is corrosive to metals. MoS2 resembles graphite but is twice as dense. The sulfur attaches itself with a weaker electron bond on one side than the other, forming laminal plates or scales in the molecular structure which tend to split off and give the sliding or lubrication action. MoS2 may be used as a filler in nylon gears and bearings to reduce friction. It also increases the flexural strength of the plastic. MoS2, code-posited with and infiltrated into titanium nitride at Oak Ridge National Laboratory, can serve as a self-lubricating composite coating for engine and other moving parts. Molysulfide, of Climax Molybdenum, MoS2.
Tungsten disulfide is also used as a lubricant in the same way as molybdenum disulfide. The electron bond of sulfur to tungsten is stronger than that to molybdenum, and it is thus more stable at high temperatures. The tungsten disulfide of GTE Corp. is a crystalline, gray-black powder with particle size from 39 to 79 (xin (1 to 2 (xm).Liqui-Moly, of Lockrey Co., and Molykote are molybdenum disulfide lubricants. Dry-film lubricants are usually graphite or molybdenum sulfide in a resin or volatile solution. They are sprayed on the bearing surface, and the evaporation of the solvent leaves an adherent thin film on the bearing. Polytetrafluoroethylene, which resists temperatures up to 500°F (260°C), is a dry-film lubricant. It is also used as an additive to lubricating oils. Krytox perfluoropolyether lubricant, from Du Pont, is like Teflon in many ways, resists heat to at least 450°F (232°C) and is used on ball bearings.
Selenium disulfide, SeS2, will retain its lubricating qualities at temperatures to 2000°F (1093°C) and is useful for lubrication under vacuum because of its low emission of gas. Other materials used as dry lubricants are tantalum disulfide, TaS2, tantalum diselenide, TaSe2, titanium ditelluride, TiTe2, and zirconium diselenide, ZrSe2.
Hydraulic fluids for the operation of presses must lubricate as well as carry the pressure. They are mostly mineral oils, but chemicals are used where high temperatures are encountered, such as in die-casting machines. Lindol HF-X, of Hoechst Celanese Corp., is a flame-resistant hydraulic fluid with a tricresyl phosphate base. Skydrol, of Monsanto Chemical Co., for aircraft hydraulic systems, is an oily ester produced from petroleum gas. The ignition point is 1050°F (566°C), and it operates at temperatures as low as —40°F (—40°C). The Fluorolube oils of Hooker Chemical are polymers of trifluorovinyl chloride fractionated to provide grades from a colorless, low-viscosity oil to an opaque, heavy grease. They have high lubricating values, are resistant to acids and alkalies, and have an operating range from 572°F (300°C) down to very low subzero temperatures. Hydraulic fluid QF-6-7009, of Dow Chemical USA, for closed systems operating from —25 to 550°F (—32 to 288°C), is a diphenyl didodecyl silane. Refrigeration oils, for lubricating refrigerating machinery, are mineral oils refined to remove all moisture and wax. Ansul oil, of Ansul Chemical Co., is an oil of this class which remains stable at temperatures as low as — 70°F ( —57°F). Hydrolube HP-5046, of Union Carbide, is a water-glycol hydraulic fluid that can be used at pressures up to 5,000 lb/in2 (34 MPa). It has a pour point of —81°F (—63°C) and a viscosity index of 170, and it can be used over a wide temperature range with minimal effect on the bulk fluid viscosity. Environmentally acceptable hydraulic fluids are being used increasingly in agricultural equipment and other environmentally sensitive applications. These fluids, such as Mobil Oil’s EAL 224H, are highly biodegradable and relatively nontoxic to animal and fish life. Fluid life compares favorably with conventional hydraulic
fluids, and they are compatible with seal and hose materials used with mineral oils.
The nature of the bearing metals often has an effect upon the action of the lubricating oil. In highly alloyed metals, some elements act as catalyzers to oxidize the oil, or the acids or moisture in the oils may act to break down the metal. In lead-bearing metals, free magnesium causes disintegration of the lead in contact with moisture. The alkali-lead metals also tend to dissolve in contact with animal or fish oils. Normally, however, none of the white bearing metals are attacked by the animal and vegetable oils used for lubrication unless there are perceptible amounts of a freely oxidizing element present. Graphite adds to the effectiveness of a lubricating oil and can be held in suspension with a tannin. Graphite lubricants are used where continuous lubrication is difficult, for running in, for springs, or for bearings where heavy films are desired. The Dag Lubricants and Dag Dispersants of Acheson Colloids Co. comprise a large group of lubricants, lubricant coatings, and mold partings consisting of graphite or molybdenum sulfide in oils, resins, or solvents, usually applied by spray. Glydag is a solution of 10% graphite in glycerin, Castordag is a graphite in castor oil, Glydag B is graphite in butylene glycol, and Dag Dispersion 223 is molybdenum disulfide in an epoxy resin. Neolube, of Huron Industries, is graphite in alcohol. With these, the carrier liquid evaporates, leaving a film of graphite on the bearing. Polyphenyl ether lubricants are highly radiation-resistant. They lubricate after absorbing gamma-ray doses that solidify mineral oils. They are used as specialty lubricants under extreme high-temperature conditions.
There are five principal types of synthetic lubricants: polyal-phaolefins (PAOs), polyglycols, polyol esters, diesters, and phosphate esters, the PAOs being the most widely used in the United States. Compared with mineral oils or petroleum-based lubricants, the synthetics can operate over a wider temperature range [—100 to 550°F (—73 to 288°C)], reduce friction better, and are more durable. They may also reduce the risk of fire in high-temperature applications. Phosphate esters containing 8 to 12% phosphorus are inherently fire-resistant. Triaryl phosphate esters have flash points from 460 to 485°F (238 to 251°C), compared with 300 to 350°F (149 to 177°C) for mineral oils. Their autoignition temperatures are between 950 and 1050°F (510 and 566°C), compared with 500 and 700°F (260 and 371°C) for the oils. However, the synthetics are far more costly. Tricresyl phosphate ester, once widely used, has lost appeal due to its neurotoxicity. New synthesis routes have been developed, however, to create other triaryl phosphate esters that are safer. Two such synthetics of Akzo Chemicals are isopropyl
phenyl phosphates and t-butyl phenyl phosphates. Durad-620B, of FMC Corp., is a triaryl phosphate ester of superior hydrolytic and oxidative stability.
|