Handbook of Lubrication Episode 2 Part 6 pps

FIGURE 2. Section through babbitt thickness showing advanced stage of fatigue cracking in babbitt with cracks extending to bond and intersecting. (Magnification × 75.) (From Burgess, P. B., Lubr. Eng., 9(6), 309, 1953. With permission.) 478 CRC Handbook of Lubrication FIGURE 1. The effect of lining thickness on babbitt fatigue strength. (From Szeri, A. Z., Ed., Tribology: Friction, Lubrication, and Wear, Hemisphere Publishing, Washington, D.C., 1980. With permission.) Copyright © 1983 CRC Press LLC machinery in which the bearings are used and is in the form of weld spatter, grinding wheel abrasive, and foundry sand. Other debris can enter the bearing from the environment. 3 Regardless of the source, the debris can cause scoring and tracking of the bearing surface and embedding of the debris in the surface. Journal scoring can also occur, but its severity is dependent upon the relative hardness of the debris and journal materials. In some cases the debris becomes completely embedded in the bearing material and results in only super- ficial wear damage with no significant effect on bearing performance. In other cases, severe wear damages both the bearing and journal. Increased wear and ultimate failure of the bearing occurs. Figure 5 shows circumferential scoring, tracking, and embedded debris in a 76-mm (3-in.) diameter tin-base babbitt bearing with an embedded debris particle at the end of a score mark. Self-propagating mechanical wear by debris can also occur. One such type of wear has Volume II479 FIGURE 3.Sketch showing babbitt fatigue initating wiping damage. FIGURE 4. Babbitt fatigue in 178-mm (7-in.) diameter turbine bearing with thick babbitt layer. 1,2 Copyright © 1983 CRC Press LLC been termed wire-wooling or machining-type wear because the debris generated resembles wire wool or metal turnings by a machine tool. 4,5 In this type of wear the journal is usually more severely worn than the bearing. Severity of the damage is dependent upon the chromium content of the journal steel. High chromium, e.g., 12% chromium, is particularly susceptible to this type of wear damage as seen in Figure 6. Figure 7 is the mating bearing which was used. The damage is initiated by debris embedded in the bearing material which generates additional journal steel debris and forms a hard steel scab in the babbitt surface. This scab acts as a tool which further propagates the wear and often results in catastrophic damage. Simple scoring damage on a journal can sometimes be incorrectly identified as wire-wooling or machining-type wear, but in many cases is not self-propagating. Another type of mechanical wear results from self-loading or radial binding of a bearing and its journal. Jamming of debris in the bearing clearance, too tight a radial fit, or dimen- sional interference from differential thermal expansion 6 can cause high wear of both bearing and journal. Figures 8 and 9 show a graphite bearing and its journal which were worn by self-loading. Both axial and circumferential cracks occurred in the graphite bearing. Electrical Damage Wear is sometimes experienced in rotating machinery as a result of the passage of current between the bearing and its mating surface, journal, or thrust runner. 7,8 Sparking between the surfaces causes pitting damage to both surfaces. Pits on the harder journal or runner surfaces are usually considerably smaller than those on the bearing. Multiple pits, closely spaced, produce a frosted appearance of the surfaces; and the removal of fused metal particles cause the surfaces to be rough. The rough surfaces produce further wear by mechanical abrasion. An additional consequence of sparking is deterioration of the lubricant and possible contamination of the lubricant and the lubricating system by spark debris. In extreme cases, the passage of current can cause an increase in the temperature of the parts which may damage the bearing or the lubricant. Volume II481 FIGURE 7.Damaged babbitt bearing used with journal of Figure 6. 1,2 Copyright © 1983 CRC Press LLC Regardless of their size, electrical pits have a characteristic appearance. The bottoms are rounded and have a smooth, shiny, melted appearance (Figure 10). Usually the periphery of the pit at the bearing surface has a ridge of melted metal. In some cases this ridge is worn away by contact with the journal. For cases in which wiping damage has been su- perimposed, electrical pitting can be identified as the cause by examining the harder journal surface which operated against the bearing. Volume II 483 FIGURE 10. Electrical pitting in tin-base babbitt produced by high current. (Magnification × 15.) (From Boyd, J. and Kaufman. H. N., Lubr. Eng., 15(1), 28, 1959. With permission.) FIGURE 11. Electrical pitting in tin-base babbitt produced by low current. (Magnification × 15.) (From Boyd, J. and Kaufman, H. N., Lubr. Eng., 5(1), 28, 1959. With permission.) Copyright © 1983 CRC Press LLC Figure 12 shows schematically some sources of bearing currents in rotating machinery. Table 1 reviews these sources and methods of eliminating or reducing the bearing currents. Because bearing currents may be produced by a variety of conditions, no single method of measuring potentials is suitable in all cases. Table 1 indicates the best location for taking measurements. In the case of the dissymmetry effect, measuring the potential between the shaft and one bearing may be unreliable. This is because both bearings are in series with the generated emf and a temporary large resistance in the one bearing may make the potential across the other bearing negligible. The most reliable method is to measure the potential between the extremities of the shaft. Using this method on ordinary electrical apparatus with 484CRC Handbook of Lubrication FIGURE 12.Principal sources of bearing current in rotating machinery. (From Boyd, J. and Kaufman, H. N., Lubr. Eng., 15(1), 28, 1959. With permission.) Copyright © 1983 CRC Press LLC 486 CRC Handbook of Lubrication Table 1 (continued) PRINCIPAL SOURCES OF BEARING CURRENT AND METHODS OF CURRENT CONTROL a See Figure 12. Copyright © 1983 CRC Press LLC journal bearings, it was found that potentials less than 300 mV cause no significant electrical damage. The corresponding value for machines with ball and roller bearings is 100 mV. Since electrostatic effects are greatly influenced by humidity and by surface conditions, the measurement of electrostatic potential is apt to be extremely erratic. Absence of elec- trostatic potential during a set of measurements does not necessarily mean that such potentials are not present under other conditions. Generally speaking, electrostatic potentials ordinarily do not produce sustained currents of large magnitude. The intermittent charging and dis- charging, however, can eventually produce enough bearing damage to cause failure. The erratic nature of electrostatic potentials makes it difficult to set practical limits for satisfactory operation. It is known that peak voltages of 20 V or more can produce bearing damage. Reducing the voltage to the order of 1 V by some form of grounding device ordinarily eliminates the trouble. The main methods of eliminating or reducing damage due to bearing currents include: (1) eliminating the source, (2) insulating the machine parts, (3) grounding the shaft, and (4) modifying the machine design. Damage from Thermal Effects The physical properties of some materials, such as the tin-base babbitts can differ along different axes of the grains making up their structure. Such anisotropic properties, coupled with differences in orientation of grain axes, can result in grain distortion when thermal cycling is imposed. This effect is shown schematically in Figure 13. If this grain distortion occurs in a babbitt bearing, the journal can contact the distorted or raised grains and result in slight wear or burnishing. This produces a mottled appearance of the bearing surface as shown in Figure 14. Mottling is usually not detrimental to bearing performance. However, Volume II 487 FIGURE 13. Sketch showing effect of thermal cycling on tin-base babbitt grains. 1,2 FIGURE 14. Surface mottling on 178-mm (7-in.) diameter bearing due to anisotropy of tin-base babbitt. 1,2 Copyright © 1983 CRC Press LLC in some cases of severe grain distortion, cracks can occur in the babbitt surface along the grain boundaries as fatigue from thermal cycling. Another thermal phenomenon results from the reduced strength properties of babbitt with increasing temperature. At elevated temperatures babbitt will undergo creep with rippling of the surface and subsequent wiping. 9 WIPING Wiping is the smearing or removal of bearing material from one point and the redeposition at another point on two surfaces in sliding contact. Superficial wiping in which bearing performance is not significantly affected can occur from either a temporary overload or temporary loss of lubricant. If either the overload or loss of lubricant is of long duration, severe damage frequently results. Bearing misalignment often results in wiping damage. Wiping damage is shown in Figure 15 on a 178-mm (7-in.). diameter bearing. Babbitt smeared by wiping is shown in the developed view of Figure 16. 488 CRC Handbook of Lubrication FIGURE 15. Wiping damage on a 178-mm (7-in.) diameter bearing. FIGURE 16. Developed view of babbitt smeared by wiping. 1,2 Copyright © 1983 CRC Press LLC Wiping sometimes is the indirect result of blistering at the interface of babbitt metal bonded to steel. This is a rare occurrence caused by hydrogen gas, trapped in the steel during manufacture, later diffusing to the interface where sufficient pressure is developed to cause the babbitt to blister and to be wiped by the journal or runner. Figure 17 is a section through babbitted steel which shows the inclusions in steel through which the gas can migrate to cause a blister. Figure 18 is a section through a blister showing the separation at the bond. Volume II489 FIGURE 17. Section through babbitted steel showing babbitt blister formation at locations above inclusions in steel. Copyright © 1983 CRC Press LLC [...]... were pockets of lead removed by the corrosive attack Figure 22 is a metallographic section showing copper corrosion in a copper-lead bearing The white copper grains at the bearing surface have been chemically attacked Copyright © 1983 CRC Press LLC Volume II 493 FIGURE 24 Enlargement of fretting corrosion of bronze bushing of Figure 23 (Magnification × 15.)1 ,2 FIGURE 25 Cavitation erosion of a 44-mm... as a participating body in ISO work, offers the most complete and authoritative set of standards for use in this country A great majority of the so-called inch series of bearings have been superceded by metric series which fit a number of well-established boundary plans.1 Of particular note, a new series of tapered roller bearings has achieved a reduction in the multiplicity and complication of sizes...Volume II 491 FIGURE 20 Corrosion and fatigue of a leadedbronze railroad diesel bearing.1 ,2 FIGURE 21 Section through a copper-lead bearing showing lead corrosion (Magnification × 150.) (From Burgess, P B., Lubr Eng., 9 (6) , 309, 1953 With permission.) shows leaded bronze corrosion combined with fatigue on a 22 9-mm (9-in.) diameter railroad diesel bearing Figure 21 shows a metallographic section... diameter aluminum bearing.1 ,2 EROSION Erosion is the removal of material from the bearing surface by fluid action which results in the formation of voids or pits in the surface It can be produced by changes in the direction of flow of high-velocity fluid streams or by the abrasive action of debris in the fluid stream as it impinges on the bearing material Cavitation erosion is a type of erosion in which the... may be (1) machined out of solid, (2) stamped and formed, (3) fabricated and fastened by cold heading or riveting, or (4) molded of one or two elements Since retainer contact with the roller or ring involves some sliding, a lubricant should be chosen which is compatible with the nature of the contact as well as the material of the retainer Radial and Angular Contact (Figure 2) Tapered roller bearings... function of these bearings In all cases of angular contact, spherical or tapered roller thrust bearings, the relation of thrust to radial load must be carefully controlled to ensure that the bearing does not come apart Manufacturers’ recommendations must be carefully adhered to Mounted-Bearing Units A significant proportion of rolling element bearings are supplied in integral housings with seals which offer... Cavitation erosion is a type of erosion in which the formation and collapse of gas bubbles in the lubricant produces high localized pressures which result in fatigue pitting of the bearing surface Figure 25 shows erosion damage on a 44-mm (1.15 in.) diameter aluminum bearing from a high-speed gas compressor Figure 26 is an enlargement of the damage Copyright © 1983 CRC Press LLC Volume II 495 ROLLING ELEMENT... 4) have lube fittings separated from the main lubrication system of the bearing Some applications are so severe that frequent and heavy relubrication is relied on to purge the system of contaminants and to exclude water vapors due to “breathing” where intermittent operation is encountered ROLLING BEARING STANDARDS Boundary Dimensions The great majority of the world rolling element bearings are in compliance... useful life of most machinery is often extended by overhaul, at which time replacement of bearings may be essential At overhaul the availability of new bearings may be a real problem and any selection of a bearing which does not fit standard published boundary dimensions must include a plan for resupply In some industries the replacement bearings are procured at the time of original manufacture and then... standard .2 Copyright © 1983 CRC Press LLC Volume II 499 FIGURE 5 Graphical representation of the diameter and width series for radial bearings (From Metric Ball and Roller Bearings Conforming to Basic Boundary Plans, ANSI/AFBMA Standard 20 , Anti-Friction Bearing Manufacturers Association, Arlington, Va., 1977 With permission.) designs requires careful consideration and prior evaluation The useful life of . Figure 26 is an enlargement of the damage. Volume II493 FIGURE 24 . Enlargement of fretting corrosion of bronze bushing of Figure 23 . (Magnification × 15.) 1 ,2 FIGURE 25 . Cavitation erosion of a. developed view of Figure 16. 488 CRC Handbook of Lubrication FIGURE 15. Wiping damage on a 178-mm (7-in.) diameter bearing. FIGURE 16. Developed view of babbitt smeared by wiping. 1 ,2 Copyright. N., Lubr. Eng., 15(1), 28 , 1959. With permission.) Copyright © 1983 CRC Press LLC 4 86 CRC Handbook of Lubrication Table 1 (continued) PRINCIPAL SOURCES OF BEARING CURRENT AND METHODS OF CURRENT CONTROL a See