discussion, the difference between RCW and RCF is that, in RCF, surface fatigue is the damage accumulation process that eventually results in wear particle formation. Rolling contact fatigue may continue for hundreds, thousands, or even millions of cycles before the first wear particles are removed. Furthermore, the corners of pits or other RCW damage features may act as nucleation sites for additional fatigue cracks and spread the damage across the surface. Because RCF and RCW are so closely related, the causes and effects of both processes will be discussed. The magnitude of the effects of RCW varies from one tribosystem to another. Sometimes, a component can sustain appreciable RCW damage before its function is impaired; other times, loss of performance immediately results from the first spall. For example, a guide roller in a hot metal bar-handling system may sustain considerable RCW, but this wear may be unimportant as long as the component continues to function adequately. In the case of ultraprecision ball bearings for missile guidance systems, however, a very small spall may cause the center of mass of the rapidly spinning bearing to shift, resulting in significant guidance errors. Rolling contact is frequently accompanied by slip or sliding. The complex motions experienced by tribocomponents in many types of rolling contact situations produce at least a small percentage of slip or sliding. Pure rolling is probably the exception rather than the rule in the diverse applications of rolling components. For example, Fig. 1 is a schematic profile of meshing spur gears. When the gear teeth first touch, there is a measure of sliding plus rolling. When the contact point coincides with the pitch circle, there is pure or nearly pure rolling (depending on the accuracy of gear alignment or lateral vibration). Past this point, slip again occurs between tooth surfaces; it reaches its second maximum just at the point where the surfaces separate. Slip can result in scuffing or adhesive wear damage to the mating surfaces if the lubrication is inadequate. Therefore, proper lubricant and surface treatment selection is important to minimize the deleterious effects of slip in many types of rolling contact arrangements. Lubrication and wear of rolling-element bearings is discussed in the article "Friction and Wear of Rolling-Element Bearings" in this Volume. Fig. 1 Engagement of gear teeth in a gear set Physical Signs of RCW Gears and bearings are the components most commonly subjected to RCF and RCW. Rolling mill rolls also frequently experience RCW (see Ref 1 for a thorough discussion). Surface hardening, refinishing, or hard coating is sometimes used to attack the problem. Rolling contact wear is often manifested by defects on the rolled product that replicate the worn portions of the roller surfaces. See the article "Metalworking Lubricants" in this Volume for a discussion of lubricant selection. Gears. As previously mentioned, the wear modes experienced by gears usually involve both rolling and slip or sliding. Dudley (Ref 2) has described the wear of gear teeth, noting that the relative amount of sliding increases as the number of teeth increases. Furthermore, the wear is not generally a function of the relative sliding velocity along a tooth face; however, at very high speeds, scoring can occur, and the friction losses in gear sets are strongly affected by sliding velocity. Hypoid gears, worm gears, and spiroid gears can run relatively well even when appreciably worn; consequently, gear designers do not always use wear as a critical design parameter. Major considerations in gear design are that: • Gear tooth stress does not exceed a critical value for desired life • Gear materials are of the right kind and quality • The form and finish are adequate • The lubricant and lubrication system are adequate • The system is adequately protected from rust and contaminants • Pitting fatigue life versus contact stress and elastohydrodynamic lubrication conditions However, determination of the size of the gear unit that is, the pit ch diameter, gear ratio, and the face width often must involve consideration of the Hertzian conditions necessary to avoid pitting failure of the gear face within the design life. Several types of pitting are recognized with regard to gear surface fatigue failures: • Initial pitting. Surface fatigue that usually occurs as a narrow band just below or at the pitch line at the beginning of component operation and that stops after the asperities have widened sufficiently (worn- in) to carry the load adequately • Destructive pitting. Usually starts below the pitch line and spreads in number and size of pits until the gear shape is rendered unusable Other terminology has been used for describing surface damage in gears. Terms include micropitting, surface origin pitting, subsurface origin spalling, subcase fatigue (also known as case-crushing), scuffing, and plastic flow. Spalling is another type of wear mode that occurs in RCW of gear teeth. Spalling occurs sporadically, its frequency and exact location being statistical. The relatively large particles produced by surface spalling can cause damage if they embed in the contact surface or find their way into clearance-critical portions of gears or bearings. Dudley (Ref 2) states that both destructive pitting and spalling seldom happen at less than about 10,000 cycles, but notes that plastic flow (primarily in metallic or polymeric gears) can occur at a relatively low number of cycles if there is an overload on the contact surfaces. In summary, there generally are three types of gear wear: (1) normal or polishing wear, (2) moderate wear that is not necessarily destructive to gear life, and (3) destructive wear. For more details, see the article "Metalworking Lubricants" in this Volume. Rolling-Element Bearings. As with gears, pitting, smearing, and spalling are important RCW manifestations in rolling-element bearings. The design and wear characteristics of rolling-element bearings are discussed in the article "Friction and Wear of Rolling-Element Bearings" in this Volume and in Ref 3. The depth of the spall tends to be related to the location of the maximum Hertzian shear stress below the surface. The fatigue spalling life of a bearing, L, is usually defined in terms of the first appearance of a spall and is generally based on the ratio of the equivalent dynamic load, P, to the load capacity, C, obtained from the manufacturer: (Eq 1) The exponent p depends on the type of bearing. For ball bearings, p = 3; for roller bearings, p = 10/3. In rolling bearings, the design and operating conditions significantly affect the location and rate of RCW damage. For example, a given bearing ball may experience a complex rotational path as it moves between the inner and outer bearing races. A given point on the surface of the ball may not be stressed once per revolution of the inner race, but rather, because of compound motions, may bear a Hertzian contact stress only occasionally. The center of the bearing race groove, on the other hand, is repeatedly stressed each time a ball rolls by. Therefore, the race tends to accumulate localized RCF damage cycles more rapidly than the balls. Damage in rolling-element bearings can result from a complex combination of radial loads, axial loads, eccentric loads, thrust loads, and internal (cage/retainer) loading effects. A thorough treatment of the failure of rolling-element bearings, including not only rolling contact wear but also fracture, may be found in the article “Failures of Rolling-Element Bearings” in Failure Analysis and Prevention, Volume 11 of ASM Handbook, (Ref 4). The mechanisms of surface distress are also discussed in a collection of papers published in 1985 (Ref 5). Widner and Littmann (Ref 6) also provide a comprehensive treatment of bearing damage analysis. Rolling Contact Fatigue Testing Because RCF is a primary process for the production of RCW, some of the methods that investigators have developed to test the response of materials to RCF conditions similar to those they may experience in service are of interest. A particularly useful reference is Rolling Contact Fatigue Testing of Bearing Steels (Ref 7). In the introduction to this book, the editor states, "In building any moving machine, it is of primary concern to obtain a long endurance life of the rolling bearing. In designing a bearing life test, on the other hand, a long testing time should be avoided. The test must be accelerated so that results can be obtained within a reasonably short period of time. Unfortunately, the shorter the test becomes, the farther the simulation departs from real conditions of application." Twenty-one papers in this collection describe testing machines, methods, and effects of processing and microstructure on RCF damage. Table 1 provides a guide to the RCF testing methods described in Ref 7 and 8 and Fig. 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, and 12 illustrate the testing arrangements listed in the table. A variety of RCF machines have been developed, ranging from one-of-a-kind research instruments to commercially manufactured bearing test rigs. Table 1 Summary of RCF testing methods Method Description Ref NASA five-ball testing apparatus (Fig. 2) Four lower balls, freely rotating 90° apart in a separator, simulates the kinematics of a thrust-loaded bearing; the contact angle can be varied; vibration sensor detects failure in unattended tests; low-(cryo) and high-temperature testing (to 1000 °C, or 1830 °F) 7 (a) Flat-washer testing apparatus (Fig. 3) 16 retained balls rolling in a circle on a flat washer with a 75 mm (3 in.) OD, 50 mm (2 in.) bore, and 6.4 mm ( in.) thickness; 4.17 GPa (605 ksi) contact stress; 1500 rev/min; filtered lubricant delivery system; piezo sensor detects vibration 7 (b) Unisteel testing apparatus (Fig. 4) Flat washer on retained balls; hanging dead-weight load; contact stress approximately 4.5 GPa (650 ksi); 1500 rev/min; drip feed of lubricant; vibration detection system; thermocouples monitor temperature (typically 50-60 °C, or 120-140 °F) 7 (c) Rolling contact testing apparatus (Fig. 5) Two hemispherically ground, toroidal rollers loaded against a round bar; 40:1 ratio of roller diameter to bar diameter; 2.7-5.5 GPa (390-800 ksi) contact stress; 12,500 rev/min; drip-feed lubrication; velocity- vibration sensor 7 (d) Ball-rod testing apparatus (Federal- Mogul) (Fig. 6) Three 12.5 mm ( in.) balls loaded against a rotating 9.5 mm ( in.) OD center rod; 3600 rev/min; spring load on opposing tapered retaining rings; accelerometer coupled with a shutdown device; drip- feed lubrication; stress per ball typically 6 GPa (870 ksi) 7 (e) Cylinder-to-ball testing apparatus (Fig. 7) Symmetrical arrangement of two 19 mm ( in.) balls rolling on a 12.5 mm ( in.) OD captive cylinder; coiled-spring load through a multiplying lever; small cylinder rev/min = 22,677; splash lubrication; maximum contact stress, 5.8 GPa (840 ksi) 7 (f) Cylinder-to-cylinder testing apparatus (Fig. 8) Symmetrical arrangement of two 12.5 mm ( in.) cylinders on two 20 mm (0.8 in.) OD captive cylinders; coiled-spring load through a multiplying lever; small cylinder, cpm = 20,400; splash lubrication; maximum contact stress less than 4.4 GPa (640 ksi); vibration sensor terminates test 7 (g) Ring-on-ring testing apparatus (Fig. 9) Crowned rings rolling on their peripheries; ring diameters of 50 and 53 mm (2 and 2.1 in.) provide "no- slip" condition, but various degrees of slip are possible by changing ring diameters; typically 2000 rev/min; contact ratio measured by electrical resistance; contact stress range typically 0.98-3.9 GPa (140- 570 ksi) 7 (h) Various types See review article of RCF and full-scale bearing testers 7 (i) Multiple bearing testing apparatus (Fig. 10) Deep-groove ball bearing design; typically 3000 rev/min; four bearings on a single center shaft; maximum contact stress, 2.9 GPa (420 ksi); accelerometers on the outer housing monitor failure 7 (j) Rolling four-ball testing apparatus (Fig. 11) Top ball drives three lower balls in a tetragonal arrangement; lower balls free to rotate in the cup; all balls 12.5 ( in.) diam; upper ball spindle speed, 1500 rev/min; 5.9 kN (1325 lbf) load applied vertically 7 (k) High-speed four-ball Same arrangement as above, but speeds of 15,000-20,000 rev/min; operating temperatures often exceed 7 (k) testing apparatus (Fig. 11) 100 °C (210 °F) (Plint machine) "AOL" vertical testing apparatus (not shown) 11 retained balls clamped between two flat washers; thrust load; recirculating lubricant system 7 (k) Inclined ball-on-disk testing apparatus (Fig. 12) Spindle-held 20.5 mm (0.8 in.) ball rolling on a disk; up to 800 °C (1470 °F); ball speed up to 7200 rev/min; disk speed up to 3600 rev/min; variable slide/roll ratios; traction measurements; designed for ceramics 8 Further information can be found on the following pages of Ref 7: (a) p 5-45, (b) p 46-66, (c) p 67-84, (d) p 85-106, (e) p 107-124, (f) p 125-135, (g) p 136-149, (h) p 150-165, (i) p 169-189, (j) p 206-218, (k) p 219-236 Fig. 2 NASA five-ball RCF testing apparatus. See Table 1. Fig. 3 Flat-washer RCF testing apparatus. See Table 1. Fig. 4 Unisteel RCF testing apparatus. See Table 1. Fig. 5 Rolling contact testing apparatus. See Table 1. Fig. 6 Ball-rod RCF testing apparatus. See Table 1. Fig. 7 Cylinder-to-ball RCF testing apparatus. See Table 1. Fig. 8 Cylinder-to-cylinder RCF testing apparatus. See Table 1. Fig. 9 Ring-on-ring RCF testing apparatus. See Table 1. Fig. 10 Four-bearing RCF testing apparatus. See Table 1. Fig. 11 Four-ball RCF testing apparatus. See Table 1. Fig. 12 Ball-on disk RCF testing apparatus. See Table 1. Mechanisms of RCW [...]... Impact Wear of MgO Single Crystals, Wear, Vol 58, 1980, p 28 3-2 99 16 G Levy and J Morri, Impact Fretting Wear in CO Based Environments, Wear, Vol 106, 1985, p 9 7- 1 38 17 I.R Sare, Repeated Impact-Abrasion of Ore-Crushing Hammers, Wear, Vol 87, 1983, p 20 7- 2 25 18 R.G Bayer, Impact Wear of Elastomers, Wear, Vol 112, 1986, p 10 5-1 20 19 P.A Engel and D.L Millis, Surface Topography Study in Impact Wear, Wear, ... Trans.), Vol 18, 1 975 , p 27 9-2 89 12 S.L Rice, Reciprocating Impact Wear Testing Apparatus, Wear, Vol 45, 1 977 , p 8 5-9 5 13 S.L Rice, The Role of Microstructure in the Impact Wear of Two Aluminum Alloys, Wear, Vol 54, 1 979 , p 29 1-3 01 14 S.L Rice, H Novotny, and S.F Wayne, Characteristics of Metallic Subsurface Zones in Sliding and Impact Wear, Wear, Vol 74 , 198 1-1 982, p 13 1-1 42 15 T Sugita, K Suzuki, and Y... ASME Wear of Materials Conference, American Society of Mechanical Engineers, 1985, p 57 2-5 77 16 Mark's Standard Handbook for Mechanical Engineers, E.A Avallone and T Baumeister III, Ed., 9th ed., McGraw-Hill, 19 87, p 8-1 10 to 8-1 11 Impact Wear Peter A Engel, Department of Mechanical and Industrial Engineering, State University of New York at Binghamton Introduction IMPACT WEAR can be defined as the wear. .. Vol 75 , 1982, p 42 3-4 42 20 V Veronesi, Wear Phenomena in Impact Printers A Scanning Electron Microscopy Study, Wear, Vol 55, 1 979 , p 26 5-2 76 21 D Tabor, The Hardness of Metals, Oxford University Press, 1951 22 P.A Engel, J.L Sirico, and T.H Lyons, Impact Wear Theory for Steel Specimens, Wear, Vol 23, 1 973 , p 185 -2 01 23 D.D Roshon, Testing Machine for Evaluating Wear by Paper, Wear, Vol 30, 1 974 , p 9 3-1 03... ferrous materials for rolling-element bearings include AISI 52100 steel, 440C stainless steel, and M-50, M-50 Nil, M-1, M-2, and M-10 tool steels The temperature limit for 52100 and 440C is about 250 °C (480 °F), and molybdenum-containing tool steels such as M-1, M-2, and M-10 may be used up to about 500 °C (930 °F) Highreliability bearings are also made from vacuum degassed and vacuum induction melting/vacuum... 17 5-2 00 14 0-2 70 16 5-2 40 References 1 J.A Schey, Tribology in Metalworking, American Society for Metals, 1983, p 249 2 D.W Dudley, Gear Wear, Wear Control Handbook, M.B Peterson and W.O Winer, Ed., American Society of Mechanical Engineers, 1980, p 75 5-8 30 3 L.B Sibley, Rolling Bearings, Wear Control Handbook, M.B Peterson and W.O Winer, Ed., American Society of Mechanical Engineers, 1980, p 69 9 -7 26... Contact, Wear, Vol 7, 1964, p 53 5-5 59 6 P.L Ko, The Significance of Shear and Normal Force Components on Tube Wear due to Fretting and Periodic Impacting, Wear, Vol 106, 1985, p 26 1-2 81 7 K Wellinger and H Breckel, Kenngroessen und Verschleiss beim Stoss Metallischer Werkstoffe, Wear, Vol 13, 1969, p 25 7- 2 81 8 R.S Montgomery, The Mechanism of Percussive Wear of Tungsten Carbide Composites, Wear, Vol... treatments, and range of surface hardnesses Table 2 Typical ferrous gear materials Material Steel A-1 through A-5 AISI 4140 AISI 4340 Cast iron Nodular iron Malleable iron (pearlitic) Surface treatment Minimum surface hardness HRC HB Through-hardened and tempered Flame- or induction-hardened Carburized and case-hardened Nitrided Nitrided As-cast Annealed, quenched and tempered 5 0-5 4 5 5-6 0 48 46 180 -4 00... Hoo, Ed., STP 77 1, ASTM, 1982, p 35 8-3 79 13 A.V Olver, H.A Spikes, and P.B McPherson, Wear in Rolling Contacts, Proceedings of ASME Wear of Materials Conference, American Society of Mechanical Engineers, 1985, p 25 4-2 72 14 N.P Suh, The Delamination Theory of Wear, Wear, Vol 25, 1 973 , p 111 15 D Zhu, F Wang, Q Cai, M Zheng, and Y Cheng, Effect of Retained Austenite on Rolling Element Fatigue and Its Mechanisms,... Impact Friction and Wear of Polymeric Materials, Wear, Vol 73 , 1981, p 21 3-2 34 3 P.A Engel, H.C Lee, and J.L Zable, Dynamic Response of a Print Belt System, IBM J Dev., Vol 23 (No 4), 1 979 , p 40 3-4 10 4 P.A Engel et al., Review of Wear Problems in the Computer Industry, J Lubr Technol (Trans ASME), Vol 24 (No 100), 1 978 , p 189 -1 98 5 A.W.J DeGee, C.P.L Commissaris, and J.H Zaat, The Wear of Sintered . of Ref 7: (a) p 5-4 5, (b) p 4 6-6 6, (c) p 6 7- 8 4, (d) p 8 5-1 06, (e) p 10 7- 1 24, (f) p 12 5-1 35, (g) p 13 6-1 49, (h) p 15 0-1 65, (i) p 169 -1 89 , (j) p 20 6- 218, (k) p 21 9-2 36 . steel, and M-50, M-50 Nil, M-1, M-2, and M-10 tool steels. The temperature limit for 52100 and 440C is about 250 °C (480 °F), and molybdenum-containing tool steels such as M-1, M-2, and M-10 may. Flame- or induction-hardened 5 0-5 4 . . . A-1 through A-5 Carburized and case-hardened 5 5-6 0 . . . AISI 4140 Nitrided 48 . . . AISI 4340 Nitrided 46 . . . Cast iron As-cast . . . 17 5-2 00