Volume 18 - Friction, Lubrication, and Wear Technology Part 4 doc

Fig. 2 Circulating oil lubrication system Circulating systems are usually fitted with filtration systems to remove bearing wear and environmental contamination from the lubricant. The use and maintenance of clean oil is of utmost significance in obtaining extended bearing life. Generally, increased filtration is beneficial to bearing lifetime if the filter is properly chosen and maintained. A 10 m (0.4 mil) filter is a good choice. The sump should be of adequate capacity to contain oil for at least 30 min in order to provide settling of contaminants. Oil Jet. The amount of oil for very high speed operation should be minimized to limit bearing temperature rise. A useful method for this case is oil jet lubrication. As shown in Fig. 3, a jet of oil under pressure is directed at the side of the bearing, between the inner ring outside diameter and cage inside diameter. The oil jet velocity must be high enough (>15 m/s, or 50 ft/s) to penetrate the turbulence generated by the bearing rotation. Fig. 3 Oil jet lubrication of rolling bearings Oil mist lubrication has been found to be useful in either high-speed applications or those in which the bearing housing is surrounded by water and grit. Mist lubrication is suitable for both horizontal and vertical shafts. Very small, accurately metered amounts of oil are directed at the rolling element by compressed air. By minimizing oil quantity, operation can occur at a lower temperature and higher speed than is possible using any other method of lubrication. Oil consumption is consequently low, and the air flow prevents the entrance of grit and subsequent wear. Several atomizer designs are available from a variety of manufacturers. Single-pass lubrication systems can be effective when minimum bearing friction is essential, loads are low, and speeds are moderate. A minimum spray of fluid mist is delivered to the bearing contacts. Churning and resultant losses are eliminated, and low usage permits discarding the lubricant after a single pass. One-time exposure to the contact environment relaxes the oxidation and property requirements of the lubricant. Tests have shown that trace quantities of injected lubricant at 1 h intervals are sufficient to keep precision spindle assemblies running at friction torque levels that are unobtainable by any other method. A disadvantage is that the generation of oil mists outside the bearing enclosure must be strictly limited to meet health and safety regulations. Mineral Oils The predominant chemistry for rolling bearing lubricants is refined mineral oil, which generically refers to a product of petroleum crude. Chemically, these oils are composed of a large number of paraffinic, naphthenic, and aromatic groups, combined into many distinct molecules. Also present are trace amounts of molecules containing sulfur, oxygen, or nitrogen. On an elemental basis, the composition of petroleum oils is consistent: carbon, 83 to 87%; hydrogen, 11 to 14%; and a remainder composed primarily of sulfur, nitrogen, and oxygen. The exact molecular makeup of a petroleum-base stock is very complex and is dependent on its specific origin. With regards to lubrication, petroleum-base stocks are characterized by the chemistry of the distillates obtained. Therefore, it is common to speak of paraffinic, naphthenic, and mixed crude oils. Aromatics seldom predominate in lubricant oils. Modern distillation, refining, and blending techniques allow the production of many oil products from a given base stock. However, because of subtle variations, some base stocks are more desirable for lubricant formulation, as discussed below. Paraffinic-base stocks have a favorable viscosity-temperature relation for lubrication. Such fluids are generally low in undesirable trace components. Naphthenic-base stocks do not contain paraffinic waxes and are better suited for certain low-temperature uses. These base stocks also have lower flash points and are more volatile, compared with paraffinic oils. Two common industrial lubricants are the rust and oxidation (R&O) inhibited and the extreme-pressure (EP) oils. R&O inhibited oils are often formulated with additional additives, such as antifoam and antiwear agents. These products are generally suitable for use between -20 and 120 °C (-4 and 248 °F). They are often employed in applications where bearings and gears share a common lubricant reservoir. Extreme-pressure oils are usually R&O inhibited products with an EP additive to generate a lubricating surface that can prevent metal-to-metal contact when the fluid film fails. Two main strategies exist in formulating EP oils. One is to use active sulfur, chlorine, or phosphorus compounds to generate sacrificial surfaces at the contacts. These surfaces will then shear, rather than weld, upon contact. The second approach employs a planar solid to impose between the contact surfaces. Of course, both contacting situations occur only when there is insufficient fluid film to separate them. EP oils are used either when bearing loading is high or where shock loading exists. Normally, EP oils are used between - 20 and 120 °C (-4 and 248 °F). Some cautions are necessary when such products are used. EP solids can reduce internal bearing clearances, causing wear and, possibly, failure in certain bearing types. These additives can also be lost upon filtration. Some sulfur-chlorine-phosphorus additives are corrosive to bronze and nylon cages and accessory items. Synthetic hydrocarbon fluids are manufactured from chemical precursors, rather than the petroleum-base stocks that constitute mineral oils. Whereas a large number of molecules exist in mineral oils, the number and type of molecules in synthetic hydrocarbons are strictly controlled by the manufacturing process involved. The ability to pick and choose components allows the production of a petroleum fluid with optimum properties for lubrication. One commercially important type is the polyalphaolefin (PAO) fluids that are widely used in turbine lubricants, hydraulic fluids, and grease formulations. PAO fluids show very high viscosity indexes, compared with refined mineral oils, which means better viscosity retention at elevated temperatures. Synthetic hydrocarbon lubricants exhibit superior thermal and oxidation stability over conventional lubricants, permitting higher operating temperatures. Other improved properties include flash point, pour point, and volatility characteristics. Although synthetic, the materials are compatible with refined petroleum lubricants because of the similar chemistry involved. Viscosity of Lubricants The most important property of a lubricant under normal conditions is viscosity. This applies both to fluid lubricants and to the base fluids in grease formulations. By definition, viscosity is the resistance to flow. For the purposes of this article, viscosity is the factor of proportionality between shearing stress and the rate of shear. Very simply, increased viscosity relates to an increased ability of the lubricant to separate contacting microsurfaces under pressure. This separation is at the heart of lubrication for rolling bearings. Viscosity is usually measured kinematically per ASTM D 445. This test measures the time required for a measured volume of fluid to pass through a standard length of capillary tube under the force of gravity. Standardized test temperatures for rolling bearing lubricants are between 40 and 100 °C (104 and 212 °F). Many alternative viscosity determinations exist and are of utility when either very viscous fluids or low temperatures are involved. The ISO VG classification is universally used to designate lubricant viscosity grade. This classification is based on the ISO 3448-1975(E) standard. Simply put, an ISO VG 32 lubricant has an approximate viscosity of 32 mm 2 /s (32 cSt) at 40 °C (104 °F). A range of viscosities is defined for each grade in the ISO standard. ISO grades run from VG 2 to VG 1500. The derived quantity, viscosity index (VI), is often encountered. This dimensionless number reflects the effect of temperature on kinematic viscosity. The higher the VI for a fluid, the smaller the viscosity loss with increased temperature. A typical paraffinic mineral oil base lubricant will have a VI from 85 to 95. Polymers can be added to mineral oil base stocks to obtain a VI of 190 or more. The shear stability of these additions, as well as the actual effect in the microcontact, is open to question, and the VI of such fluids generally deteriorates with time. Many of the synthetic fluids have VIs that far exceed those of mineral oils. ASTM D 567 describes the method of calculating VI from kinematic viscosities at two temperatures. Selection of Proper Viscosity for Petroleum Oil Lubricants. Figures 4 and 5 can be used to obtain a minimum acceptable viscosity for a bearing application. With known values for the pitch diameter, d m , and the rotational speed, Fig. 4 can indicate the minimum suitable viscosity at the bearing operating temperature. Figure 5 can then be used to relate this viscosity to the standard reference viscosities for ease of selection. Figure 5 can also be used to determine the actual viscosity of a petroleum oil with a VI of 85 at a given temperature if its standard viscosity data are known. Fig. 4 Calculation of minimum required viscosity Fig. 5 Viscosity-temperature relation for mineral oil base lubricants with a Vl of 85 Example of Viscosity Calculation. A bearing has a bore diameter, d, of 340 mm (13.6 in.) and an outside diameter, D, of 420 mm (16.8 in.). Thus, its pitch diameter is 380 mm (15.2 in.). It is operating at 70 °C (160 °F) and at 500 rev/min. What is the minimum acceptable viscosity under these conditions? As shown in Fig. 4, the required kinematic viscosity is at least 13 mm 2 /s (13 sSt). Remembering that the operating temperature is 70 °C (160 °F), it can be seen, in Fig. 5, that the required viscosity of an oil at 40 °C (104 °F) is at least 39 mm 2 /s (39 cSt). When estimating operating temperature, it is useful to remember that the oil temperature is from 3 to 11 °C (5 to 20 °F) higher than the bearing housing temperature. If a lubricant with a higher than required viscosity is used, an improvement in bearing fatigue life is expected. However, because increased viscosity will raise the operational temperature, there is a practical limit to the lubrication improvement that can be obtained by these means. When unusually high or low speeds, or heavily loaded conditions, or unusual lubrication circumstances are encountered, the bearing manufacturer should be consulted. Types and Properties of Nonpetroleum Oils Many types of "synthetic" fluids have been developed in response to lubrication requirements that are not adequately met by petroleum oils. These requirements include extreme temperature, fire resistance, low viscosity, and high viscosity index. Table 1 lists typical properties of various lubricant base stocks and indicates application areas for finished products of each type. As is the case for petroleum oils, many additive chemistries have been developed to enhance the properties of these fluids. Table 1 Typical properties of lubricant base fluids Flash point Pour point Base fluid Density, g/cm 3 Viscosity at 40 °C (104 °F), mm 2 /s Viscosity index °C °F °C °F Volatility (a) Oxidation resistance (a) Lubricity (a) Thermal stability (a) Application range Mineral oils . . . . . . . . . . . . . . . . . . . . . 5 5 5 5 Paraffinic 0.881 95 100 210 410 -7 19 . . . . . . . . . . . . Naphthenic 0.894 70 65 180 356 - 18 0 . . . . . . . . . . . . Mixed base 0.884 80 99 218 424 - 12 10 . . . . . . . . . . . . VI improved 0.831 33 242 127 261 - 40 - 40 . . . . . . . . . . . . Polyalphaolefin 0.853 32 135 227 441 - 54 - 65 4 4 5 4 Standard lubricant components, either as oil or grease base Esters . . . . . . . . . . . . . . . . . . . . . 3-5 3-7 3-6 4-7 Diabasic 0.945 14 152 232 450 <- 60 <- 70 3 5 5 5 Polyol 0.971 60 132 275 527 - 54 - 65 . . . . . . . . . . . . Tricresyl phosphate 1.160 37 -65 235 455 - 23 -9 3 3 3 7 Polyglycol ether 0.984 36 150 210 410 - 46 - 51 3-5 7 4 5 Silicate 0.909 6.5 150 188 370 <- 60 <- 76 3 7 6 4 Jet turbine lubricants, hydraulic fluids, heat transfer products; used as bases for low- volatility, low-viscosity grease; phosphate esters fire resistant Silicones . . . . . . . . . . . . . . . . . . . . . 1 1 7 1-5 Dimethyl 0.968 100 400 >300 >572 <- 60 <- 76 . . . . . . . . . . . . Phenyl methyl 0.990 75 350 260 500 <- 60 <- 76 . . . . . . . . . . . . Chlorophenyl 1.050 55 160 288 550 <- 60 <- 76 . . . . . . . . . . . . Perfluoroethyl 1.230 44 158 >300 >572 <- 60 <- 76 . . . . . . . . . . . . Phenyl ether . . . . . . . . . . . . . . . . . . . . . 1 1-3 5-7 1 Low-viscosity 1.180 75 -20 263 505 10 50 . . . . . . . . . . . . High-viscosity 1.210 355 -74 343 650 5 41 . . . . . . . . . . . . High-temperature/low-volatility applications, either as oil or grease base; lightly loaded bearings; excellent thermal stability Perfluoroalkylether 1.910 320 138 . . . . . . 3-7 1 5-7 1-3 Extreme-temperature fluid; used in very low 32 26 volatility applications (a) The value 5 characterizes highly refined mineral oil. Values of <5 reflect superior performance, whereas values >5 reflect inferior performance to mineral oil with respect to lubrication properties. Before discussing the general synthetic classes, it should be noted that the use of such lubricants requires a thorough understanding of the application requirements. The favorable properties of some synthetics are obtained only with unsuitable performance characteristics in areas such as load-carrying ability and high-speed operation. Similarly, many very high temperature fluids developed for military applications have very short service lifetimes, compared with commercial requirements. Polyglycols are often used as a synthetic lubricant base in water emulsion fluids. This class of fluids includes glycols, polyethers, and polyalkylene glycols. Properties of the class include excellent hydrolytic stability, high viscosity index, and low volatility. The most prevalent usage is as a component of fire-resistant hydraulic fluids. Phosphate esters have poor hydraulic stability and a low viscosity index. Because an outstanding characteristic of these fluids is fire resistance, they are often used as hydraulic fluids in high-temperature applications, such as aerospace. Dibasic acid esters are a family of synthetic base stocks that are widely used in aircraft turbine applications and as a basis for low-volatility lubricants. They are synthesized by reacting aliphatic dicarboxylic acids (adepic to sebacic) with primary branched alcohols (butyl to octyl). Some are available from natural sources, such as castor beans and animal tallow. Characteristic properties of these fluids are low volatility and high viscosity index. Polyol esters that are formed by linking dibasic acids through a polyglycol center are suitable as high film strength lubricants. Blends of dibasic esters, complex esters with suitable antiwear additives, VI improves, and antioxidants are used to form the current generation of jet engine lubricants. Generally, these products show excellent viscosity-temperature relationships, good low-temperature properties, and acceptable hydrolysis resistance. Elastomeric seals used with these materials must be chosen carefully, because many standard rubbers will suffer attack. Silicone fluids (organosiloxanes) exhibit outstanding viscosity retention with elevated temperature and are functional under conditions of extreme heat and cold. These fluids are the basis for many high-temperature (200 °C, or 392 °F) lubricants. In addition to favorable viscosity-temperature characteristics, volatility is low and both thermal and oxidation resistance characteristics are excellent. As a family, these fluids exhibit good hydrolytic stability. If very high temperatures are avoided, these fluids are inert with most elastomers and polymers. However, oxygen exposure with high temperature can result in gelation and loss of fluidity. The lubrication properties of these fluids are not impressive when compared with other classes of lubricating fluids. Typical applications are in electric motors, brake fluids, oven preheater fans, plastic bearings, and electrical insulation. Silicate esters represent a mating of the previous two lubricant fluid types. As a class, these fluids possess good thermal stability and low volatility. They are used in high-temperature hydraulic fluids and low-volatility greases. Fluorinated polyethers are the highest-temperature lubricating fluids commercially available. Although distinct chemical versions are marketed, all of these fluids are fully fluorinated and completely free of hydrogen. This structural characteristic makes them inert to most chemical reactions, nonflammable, and extremely oxidation resistant. Products from these oils show very low volatility and excellent resistance to radiation-induced polymerization. The products are essentially insoluble to common solvents, acids, and bases. The density of these oils is approximately double that of conventional petroleum oils. Products of this chemical family are used to lubricate rolling bearings at extremely high temperatures from 200 to 260 °C (392 to 500 °F). Other application areas include high-vacuum operations, corrosive environments, and oxygen-handling systems. As would be expected, the cost of these oils is very high. Grease Lubrication Greases consist of two major components: a fluid phase and a thickener system that determines the consistency of the product. Although thickeners can comprise a variety of materials, all provide a large specific surface area to retain oil. Mineral oils predominate, but the fluids used in grease formulations encompass the spectrum of lubricating fluids. The resulting product behaves like a semisolid, releasing oil at a controlled rate to meet the requirements of the rolling bearing. The fluid phase of the grease is either gradually degraded by oxidation or lost by evaporation, centrifugal force, and other factors. In time, the grease at the contacting bearing surfaces becomes depleted. [...]... >230 >44 6 150 302 >200 90 >250 180 >250 180 240 >392 1 94 >48 2 356 >48 2 356 46 4 150 60 140 121 150 100 121 >225 >250 >43 7 >48 2 150 150 Water resistance(a) Loadcarrying capability(a) Corrosion protection(a) °F Lowtemperature limit °C °F 177 350 -3 0 -2 2 G-E G G-E 302 140 2 84 250 302 212 250 177 77 177 150 177 121 150 350 170 350 302 350 250 302 -3 0 -2 0 -3 0 -3 5 -3 5 -2 0 -3 0 -2 2 -4 -2 2 -3 1 -3 1 -4 -2 2 E E G-E... 170 350 302 350 250 302 -3 0 -2 0 -3 0 -3 5 -3 5 -2 0 -3 0 -2 2 -4 -2 2 -3 1 -3 1 -4 -2 2 E E G-E F-G G-E P-F G G-E G G-E F-G G F-G G E G-E E G-E E P-F E 302 302 177 177 350 350 -2 0 -2 0 -4 -4 E E F F E G-E P, poor, F, fair, G, good; E, excellent Relubrication intervals for rolling bearings depend not only on the lubricant type and environmental conditions, but on specific design features, which vary among bearing... ASTM D 217 is used to generate the penetration values upon which the greases are classified Table 2 NLGI penetration grades NLGI grade 000 00 0 1 2 3 4 Penetration (60 strokes)(a) 44 5 -4 75 40 0 -4 30 35 5-3 85 31 0-3 40 26 5-2 95 22 0-2 50 17 5-2 05 5 6 (a) 13 0-1 60 6 5-1 15 Per ASTM D 217 The consistency of rolling bearing greases should not dramatically change with temperature or with mechanical working Greases that... agents Phenolic materials, such as 2 ,4, 5-trichlorophenol, destroy bacteria directly Materials such as 1,3-di(hydroxy-methyl )-5 ,5dimethyl-2 , 4- dioxoimidazole, upon being added to water-based metalworking lubricants, release formaldehyde slowly to keep bacteria in check Materials such as 2,2-dibromo-3-nitrilopropionamide are useful for controlling bacteria, fungi, and yeast There are over 50 commercially... Rolling Mill Lubricants, Lubr Eng., Vol 42 (No 12), 1986, p 74 0-7 50 A.D Cron and J Fatkin, Graphite in Lubrication: Fundamental Parameters and Selection Guide, NLGI Spokesman, Vol 53 (No 4) , 1989, p 13 7-1 47 M Fukuda, T Nishimura, and Y Moriguchi, Friction and Lubrication in the Fabrication of Titanium and Its Alloys, Metalwork Interfaces, Vol 5 (No 3), 1980, p 1 4- 2 1 K Glossop, Copper Wire Drawing Lubricants,... indicating tool-work-piece surface contact with localized asperity welding and associated friction spikes The addition of the filmstrength additive eliminates friction spikes and gives an overall low and smooth frictional force trace Fig 3 Crossed-cylinders frictional force showing 1 .18 mm2/s (1.8 × 1 0-3 in.2/s) linear paraffin (curve A), 2 .49 mm2/s (3.9 × 1 0-3 in.2/s) linear paraffin (curve B), and 5% methyl... coolant and that prevent corrosion of the tool, workpiece, and lubricant handling system are also commonly used In water-based metalworking lubricants, special materials are employed to disperse or solubilize the oil-additive package in water Additives commonly used in metalworking formulations are described below Film-strength additives adsorb on tool-workpiece surfaces and prevent direct metal-to-metal... good lubricating and heat removal qualities and are widely used in most metal removal operations and many metal forming operations Microemulsions are clear-to-translucent solutions containing water; a hydrophobic liquid, that is, an oil phase; and one or more emulsifiers, which are often referred to as surfactants and co-surfactants Microemulsions in which water is the continuous phase and oil is the... metals, is known as "built-up edge." This problem results when some of the workpiece welds to that part of the tool that is in contact with the chip Portions of the built-up edge eventually detach, come between the tool and the workpiece, and blemish the workpiece finish Built-up edge is controlled through film strength and EP additives, which react at the interface to prevent welding, and by choosing an... and reduces metal transfer In full fluid film lubrication, there is no contact between the tool and workpiece, and force is transmitted from the tool to the workpiece through the lubricant film In this case, the lubricant film supports the load In the thin-film regime, there is partial contact between the tool and the workpiece, and both the lubricant film and the film strength or EP additives support . . 3-5 3-7 3-6 4- 7 Diabasic 0. 945 14 152 232 45 0 < ;- 60 < ;- 70 3 5 5 5 Polyol 0.971 60 132 275 527 - 54 - 65 . . . . . . . . . . . . Tricresyl phosphate 1.160 37 -6 5 235 45 5 - 23. grade Penetration (60 strokes) (a) 000 44 5 -4 75 00 40 0 -4 30 0 35 5-3 85 1 31 0-3 40 2 26 5-2 95 3 22 0-2 50 4 17 5-2 05 5 13 0-1 60 6 6 5-1 15 (a) Per ASTM D 217 The consistency. 350 -3 0 -2 2 G-E G G-E Barium complex >200 >392 150 302 177 350 -3 0 -2 2 E G-E E Calcium 90 1 94 60 140 77 170 -2 0 -4 E G G-E Calcium complex >250 > ;48 2 140 2 84 177