(Schmalzried et al., 1992). Although joints do not wear out, they produce many millions of wear particles, and it is the adverse biological reaction to these particles that leads to bone resorption and to loosening and failure (Shanbharg et al., 1994; Howie et al., 1993). Currently, there is considerable attention focused on the wear, wear debris, and adverse biological reactions in artificial joints (Fisher, 1994) and this is discussed in Section 41.6. Over the last 40 years, several different bearing couples have been used in joint replacements: metal on metal, ceramic on ceramic, polytetrafluoroethylene (PTFE) on metal and, polyethylene on metal. By far the most frequently used bearing couple has been ultrahigh-molecular-weight polyethylene (UHMWPE) articulating on ceramic or metal femoral heads in the hip (Figure 41.10), and metal femoral condyles in the knee. There has been good agreement between the clinical measurement of wear and in in vitro simulators in UHMWPE cups in the hip. For smooth polished metal femoral heads, wear rates of about 40 mm 3 /year have been recorded both in vivo and in vitro , and this corresponds to a linear penetration of 0.1 mm/year. Hence, with this wear rate, it would take an 8-mm-thick cup 80 years to wear out. Wear is, however, accelerated by damage to the femoral head, oxidative degradation of the UHMWPE, and also by the use of larger-diameter heads and cups. Clinical and laboratory studies have shown that damage to the femoral head and aging of the polyethylene can independently accelerate the wear by a factor of 2; and hence, with these combined effects, it is possible to obtain wear rates of between 100 and 200 mm 3 /year clinically (Livermore et al., 1990). Explant studies have shown osteolysis associated with cumulative wear volumes of 500 to 1000 mm 3 and therefore it is not surprising that the incidence of wear debris-induced osteolysis increases markedly after 10 years. Clinical evidence indicates that the wear rate of UHMWPE in the knee is not so high due to less multi-directional kinematics; nevertheless, other wear-related failure mechanisms, such as fatigue and delamination, can lead to early revision. Alternative bearing materials for the hip, such as alumina ceramic on alumina ceramic (Sedel, 1992) and cobalt chrome alloy pairings (Muller, 1995), have been used clinically in the hip in limited numbers. The wear volumes are typically 10 to 100 times less than UHMWPE clinically and this has prompted renewed interest in the development of second-generation hard-on-hard bearings (Figure 41.11). It has FIGURE 41.10 Schematic diagram of the artificial hip joint. FIGURE 41.11 Volumetric wear rates of UHMWPE on ceramic metal on metal and ceramic on ceramic in a hip joint simulator. © 2001 by CRC Press LLC (Schmalzried et al., 1992). Although joints do not wear out, they produce many millions of wear particles, and it is the adverse biological reaction to these particles that leads to bone resorption and to loosening and failure (Shanbharg et al., 1994; Howie et al., 1993). Currently, there is considerable attention focused on the wear, wear debris, and adverse biological reactions in artificial joints (Fisher, 1994) and this is discussed in Section 41.6. Over the last 40 years, several different bearing couples have been used in joint replacements: metal on metal, ceramic on ceramic, polytetrafluoroethylene (PTFE) on metal and, polyethylene on metal. By far the most frequently used bearing couple has been ultrahigh-molecular-weight polyethylene (UHMWPE) articulating on ceramic or metal femoral heads in the hip (Figure 41.10), and metal femoral condyles in the knee. There has been good agreement between the clinical measurement of wear and in in vitro simulators in UHMWPE cups in the hip. For smooth polished metal femoral heads, wear rates of about 40 mm 3 /year have been recorded both in vivo and in vitro , and this corresponds to a linear penetration of 0.1 mm/year. Hence, with this wear rate, it would take an 8-mm-thick cup 80 years to wear out. Wear is, however, accelerated by damage to the femoral head, oxidative degradation of the UHMWPE, and also by the use of larger-diameter heads and cups. Clinical and laboratory studies have shown that damage to the femoral head and aging of the polyethylene can independently accelerate the wear by a factor of 2; and hence, with these combined effects, it is possible to obtain wear rates of between 100 and 200 mm 3 /year clinically (Livermore et al., 1990). Explant studies have shown osteolysis associated with cumulative wear volumes of 500 to 1000 mm 3 and therefore it is not surprising that the incidence of wear debris-induced osteolysis increases markedly after 10 years. Clinical evidence indicates that the wear rate of UHMWPE in the knee is not so high due to less multi-directional kinematics; nevertheless, other wear-related failure mechanisms, such as fatigue and delamination, can lead to early revision. Alternative bearing materials for the hip, such as alumina ceramic on alumina ceramic (Sedel, 1992) and cobalt chrome alloy pairings (Muller, 1995), have been used clinically in the hip in limited numbers. The wear volumes are typically 10 to 100 times less than UHMWPE clinically and this has prompted renewed interest in the development of second-generation hard-on-hard bearings (Figure 41.11). It has FIGURE 41.10 Schematic diagram of the artificial hip joint. FIGURE 41.11 Volumetric wear rates of UHMWPE on ceramic metal on metal and ceramic on ceramic in a hip joint simulator. © 2001 by CRC Press LLC (Schmalzried et al., 1992). Although joints do not wear out, they produce many millions of wear particles, and it is the adverse biological reaction to these particles that leads to bone resorption and to loosening and failure (Shanbharg et al., 1994; Howie et al., 1993). Currently, there is considerable attention focused on the wear, wear debris, and adverse biological reactions in artificial joints (Fisher, 1994) and this is discussed in Section 41.6. Over the last 40 years, several different bearing couples have been used in joint replacements: metal on metal, ceramic on ceramic, polytetrafluoroethylene (PTFE) on metal and, polyethylene on metal. By far the most frequently used bearing couple has been ultrahigh-molecular-weight polyethylene (UHMWPE) articulating on ceramic or metal femoral heads in the hip (Figure 41.10), and metal femoral condyles in the knee. There has been good agreement between the clinical measurement of wear and in in vitro simulators in UHMWPE cups in the hip. For smooth polished metal femoral heads, wear rates of about 40 mm 3 /year have been recorded both in vivo and in vitro , and this corresponds to a linear penetration of 0.1 mm/year. Hence, with this wear rate, it would take an 8-mm-thick cup 80 years to wear out. Wear is, however, accelerated by damage to the femoral head, oxidative degradation of the UHMWPE, and also by the use of larger-diameter heads and cups. Clinical and laboratory studies have shown that damage to the femoral head and aging of the polyethylene can independently accelerate the wear by a factor of 2; and hence, with these combined effects, it is possible to obtain wear rates of between 100 and 200 mm 3 /year clinically (Livermore et al., 1990). Explant studies have shown osteolysis associated with cumulative wear volumes of 500 to 1000 mm 3 and therefore it is not surprising that the incidence of wear debris-induced osteolysis increases markedly after 10 years. Clinical evidence indicates that the wear rate of UHMWPE in the knee is not so high due to less multi-directional kinematics; nevertheless, other wear-related failure mechanisms, such as fatigue and delamination, can lead to early revision. Alternative bearing materials for the hip, such as alumina ceramic on alumina ceramic (Sedel, 1992) and cobalt chrome alloy pairings (Muller, 1995), have been used clinically in the hip in limited numbers. The wear volumes are typically 10 to 100 times less than UHMWPE clinically and this has prompted renewed interest in the development of second-generation hard-on-hard bearings (Figure 41.11). It has FIGURE 41.10 Schematic diagram of the artificial hip joint. FIGURE 41.11 Volumetric wear rates of UHMWPE on ceramic metal on metal and ceramic on ceramic in a hip joint simulator. © 2001 by CRC Press LLC 42 Technologies for Machinery Diagnosis and Prognosis 42.1 Introduction 42.2 Failure Prevention Strategies Learn from History • Study the Asset • Maintain the Asset 42.3 Condition Monitoring Approaches Vibration Monitoring • Oil Monitoring • Thermal Monitoring • Nondestructive Evaluation • Corrosion Monitoring • Performance Monitoring 42.4 Tribo-Element Applications Fluid Film Bearings • Rolling Element Bearings • Gears • Seals 42.5 Equipment Asset Management 42.1 Introduction For as long as there have been engineered structures and mechanical systems, there have been maintenance issues, uncertainties regarding reliability, and failures. Yet, the impact of such occurrences has significantly changed as manufacturing has moved away from the manual labor of the Industrial Revolution to the machinery of today’s technology-driven society. With an ever-increasing reliance on expensive and complex machines, machinery failures significantly affect company profits, largely due to the loss of equipment availability, the cost of spare componentry, the risk of injury to people, and the possibility of damage to the environment. The response of such pressures from industrial concerns and government agencies has been to demand that maintenance systems minimize the risks of equipment failure. In turn, this has spurred technology advances in providing a means to monitor and assess the condition of tribological elements and mechan- ical systems, rather than waiting until failures occur or replacing parts as a matter of routine. The aim of this chapter is to present these technologies for machinery diagnosis and prognosis in terms of failure prevention strategies and condition monitoring approaches, and suggest how they can be applied so as to lead to the effective management of equipment assets. 42.2 Failure Prevention Strategies Machine failures happen in many different ways and for many different reasons. To prevent their occur- rence at an inopportune time, a strategy can be employed, based on learning from past events, under- standing present performance, and adopting a cost-effective maintenance approach. Richard S. Cowan Georgia Institute of Technology Ward O. Winer Georgia Institute of Technology © 2001 by CRC Press LLC Glossary A abrasion, n . A process in which hard particles or protuberances are forced against and move along a solid surface. Note: This term sometimes is used to refer to abrasive wear. See also: abrasive wear. abrasive erosion, n . Erosive wear caused by the relative motion of solid particles that are entrained in a fluid, moving nearly parallel to a solid surface. See also: erosion. abrasive wear, n . Wear due to hard particles or hard protuberances forced against and moving along a solid surface. actual contact area, n . See: real area of contact. additive, n . In lubrication, a material added to a lubricant for the purpose of imparting new properties or enhancing existing properties. Main classes of additives: viscosity index (VI) improvers, anti- corrosive, anti-foam, antioxidant, anti-wear, detergent, dispersant, extreme-pressure (EP), and boundary lubrication additives. adherence, n . The physical attachment of material to a surface by either adhesion or by other means of attachment that results from the contact of two solid surfaces undergoing relative motion. Note: Adhesive bonding is not a requirement for adherence because such mechanisms as mechanical interlocking of asperities can also provide a means for adherence. See also: adhesion. adhesion, n . In frictional contacts, the attractive force between adjacent surfaces. Notes: (1) In physical chemistry, adhesion denotes the attraction between a solid surface and a second (liquid or solid) phase. This definition is based on the assumption of a reversible equilibrium. (2) In mechanical technology, adhesion is generally irreversible. adhesion, mechanical, n . Adhesion between surfaces produced by the interlocking of protuberances on those surfaces. See also: adherence. adhesive wear, n . 1) Wear by transference of material from one surface to another during relative motion due to a process of solid-phase welding. Note: Particles that are removed from one surface are usually either permanently or temporarily attached to the other surface. 2) Wear due to localized bonding between contacting solid surfaces leading to material transfer between the two surfaces or loss from either surface. See also: transfer. air bearing, n . A bearing using air as a lubricant. See also: gas lubrication. angle of contact, n . In a ball bearing, the angle between a diametral plane perpendicular to a ball bearing axis and a line drawn between points of tangency of the balls to the inner and outer rings. angle of incidence, n . The angle between the direction of motion of an impinging liquid or solid particle and the normal to the surface at the point of impact. angular-contact bearing, n . A ball bearing of the grooved type, so designed that under no load, a line through the outer and inner raceway contacts with the balls forms an angle with a plane perpen- dicular to the bearing axis. anti-friction bearing, n . 1) Commonly, a bearing containing a solid lubricant. 2) Roller bearings or rolling element bearings. apparent area of contact, n . In tribology, the area of contact between two solid surfaces defined by the boundaries of their macroscopic interface. (Contrast with real area of contact.) Syn: nominal area of contact. See also: real area of contact, Hertzian contact area. Francis E. Kennedy Dartmouth College Peter J. Blau Oak Ridge National Laboratory © 2001 by CRC Press LLC . Although joints do not wear out, they produce many millions of wear particles, and it is the adverse biological reaction to these particles that leads to bone resorption and to loosening and failure. Although joints do not wear out, they produce many millions of wear particles, and it is the adverse biological reaction to these particles that leads to bone resorption and to loosening and failure. Although joints do not wear out, they produce many millions of wear particles, and it is the adverse biological reaction to these particles that leads to bone resorption and to loosening and failure