A nylon oil-lubricated driving gear showed pitting upon visual inspection. The pitting produced numerous surface microcracks. Further investigation (visual inspection and 37x images) supported the conclusion that failure occurred in association with large-scale fragmentation (frictional wear). No recommendations were made.

In an industrial application, 24 speed-increaser gearboxes were used to transmit 258 kW (346 hp) and increase speed from 55 to 375 rev/min. The gears were parallel shaft, single helical, carburized, and ground. The splash lubrication system used a mineral oil without antiscuff additives with ISO 100 viscosity. After about 250 h of operation, two gearboxes failed by bending fatigue. Investigation showed the primary failure mode was scuffing, and the earlier bending fatigue failures were caused by dynamic loads generated by the worn gear teeth. Testing of a prototype gearbox showed that the failure resulted from several interrelated factors: the lubricant viscosity was too low causing high temperatures; no antiscuff additives were used; a gearbox designed as a speed reducer was used as a speed increaser (the designer selected a long-addendum tooth for the pinion); the gear teeth were not provided with a coating or plating to ease running-in; and the gears were not run-in properly under reduced loads. The case suggests that such gear failures can be avoided if designers and operators recognize that the lubricant is an important component of a gearbox and appreciate that gear design requires the consideration and control of many interrelated factors.

An investigation was conducted to determine what caused a bearing sleeve in a locomotive turbocharger to fail. The sleeve, which is made of nitrided 38CrMoAl steel, fractured at the transition fillet between the cylinder and plate. Visual examination revealed significant wear on the external surface of the cylinder, with multiple origin fatigue fracture appearing to be the dominant fracture mechanism. Metallurgical examination indicated that the nitrided layer was not as deep as it was supposed to be and had worn away on the outer surface of the sleeve, exposing the soft matrix underneath. This led to further wear and an increase in friction between the sleeve and bearing bush. Fatigue crack initiation occurred at the root fillet because of stress concentration and large frictional forces. Insufficient nitriding depth facilitated the propagation of fatigue cracks.

The main shaft in a locomotive turbocharger fractured along with an associated bearing sleeve. Visual and fractographic examination revealed that the shaft fractured at a sharp-edged groove between two journals of different cross-sectional area. The dominant failure mechanism was low-cycle rotation-bending fatigue. The bearing sleeve failed as a result of abrasive and adhesive wear. Detailed metallurgical analysis indicated that the sleeve and its respective journal had been subjected to abnormally high temperatures, increasing the amount of friction between the sleeve, bearing bush, and journal surface. The excessive heat also softened the induction-hardened case on the journal surface, decreasing its fatigue strength. Fatigue crack initiation occurred at the root fillet of the groove because of stress concentration.

A brittle-like crack propagation caused failure in a rubber office-chair roller. A crack initiated from the inside of the roller and propagated in a discontinuous brittle-like fashion, as indicated from the evolution of concentric fracture striations. Compressive fatigue was a dominant mode of loading. Nevertheless, the fracture surface of the failure-causing crack suggested a tensile-stress component was involved in driving failure.

Friction and wear are important when considering the operation and efficiency of components and mechanical systems. Among the different types and mechanisms of wear, adhesive wear is very serious. Adhesion results in a high coefficient of friction as well as in serious damage to the contacting surfaces. In extreme cases, it may lead to complete prevention of sliding; as such, adhesive wear represents one of the fundamental causes of failure for most metal sliding contacts, accounting for approximately 70% of typical component failures. This article discusses the mechanism and failure modes of adhesive wear including scoring, scuffing, seizure, and galling, and describes the processes involved in classic laboratory-type and standardized tests for the evaluation of adhesive wear. It includes information on standardized galling tests, twist compression, slider-on-flat-surface, load-scanning, and scratch tests. After a discussion on gear scuffing, information on the material-dependent adhesive wear and factors preventing adhesive wear is provided.

An investigation of wear and failure of babbitt bushes was completed in this study. The results showed that wear at dry sliding of babbitt obtained by plasma spraying was less than that of babbitt in the as-cast state and after a deformation heat treatment. The failure of babbitt bushes was caused by a simultaneous and interrelated exhibition of fatigue and wear processes that depend considerably on cohesion strength between the bush and the bearing base and accumulation of defects on the contact surface between the bush and the shaft.

This article considers the main characteristics of wear mechanisms and how they can be identified. Some identification examples are reported, with the warning that this task can be difficult because of the presence of disturbing factors such as contaminants or possible additional damage of the worn products after the tribological process. Then, the article describes some examples of wear processes, considering possible transitions and/or interactions of the mechanism of fretting wear, rolling-sliding wear, abrasive wear, and solid-particle erosion wear. The role of tribological parameters on the material response is presented using the wear map concept, which is very useful and informative in several respects. The article concludes with guidelines for the selection of suitable surface treatments to avoid wear failures.

This article reviews the abrasive and adhesive wear failure of several types of reinforced polymers, including particulate-reinforced polymers, short-fiber reinforced polymers (SFRP), continuous unidirectional fiber reinforced polymers (FRP), particulate-filled composites, mixed composites (SFRP and particulate-filled), unidirectional FRP composites, and fabric reinforced composites. Friction and wear performance of the composites, correlation of performance with various materials properties, and studies on wear-of failure mechanisms by scanning electron microscopy are discussed for each of these types.

Plastics or polymers are used in a variety of engineering and nonengineering applications where they are subjected to surface damage and wear. This article discusses the classification of polymer wear mechanisms based on the methodologies of defining the types of wear. The first classification is based on the two-term model that divides wear mechanisms into interfacial and bulk or cohesive. The second is based on the perceived wear mechanism. The third classification is specific to polymers and draws the distinction based on mechanical properties of polymers. In this classification, wear study is separated as elastomers, thermosets, glassy thermoplastics, and semicrystalline thermoplastics. The article describes the effects of environment and lubricant on the wear failures of polymers. It presents a case study on nylon as a tribological material. The article explains the wear failure of an antifriction bearing, a nylon driving gear, and a polyoxymethylene gear wheel.

A 58.4 cm (23 in.) diam heavy-duty brake drum component of a cable-wound winch broke into two pieces during a shutdown period. Average service life of these drums was two weeks; none had failed by wear. The drums were sand cast from ductile iron. During haul-out, the cable on the cable drum drove the brake drum, and resistance was provided by brake bands applied to the outside surface of the brake drum. Friction during heavy service was sufficient to heat the brake drum, clutch mount, and disk to a red color. Examination of the assembly indicated that the brake drum would cool faster than its mounts and would contract onto them. Brittle fracture of the brake drum occurred as a result of thermal contraction of the drum web against the clutch mount and the disk. The ID of the drum web was enlarged sufficiently to allow for clearance between the web and the clutch mount and disk at a temperature differential of up to 555 deg C (1000 deg F). With the adoption of this procedure, brake drums failed by wear only.

An air blower in an electric power plant failed unexpectedly when a roller bearing in the drive motor fractured along its outer ring. Both rings, as well as the 18 rolling elements, were made from GCr15 bearing steel. The bearing also included a machined brass (MA/C3) cage and was packed with molybdenum disulfide (MoS 2 ) lithium grease. Metallurgical structures and chemical compositions of the bearing’s matrix materials were inspected using a microscope and photoelectric direct reading spectrometer. SEM/EDS was used to examine the local morphology and composition of fracture and contact surfaces. Chemical and thermal properties of the bearing grease were also examined. The investigation revealed that the failure was caused by wear due to dry friction and impact, both of which worsened as a result of high-temperature degradation of the bearing grease. Fatigue cracks initiated in the corners of the outer ring and grew large enough for a fracture to occur.

Rolling-element bearings use rolling elements interposed between two raceways, and relative motion is permitted by the rotation of these elements. This article presents an overview of bearing materials, bearing-load ratings, and an examination of failed bearings. Rolling-element bearings are designed on the principle of rolling contact rather than sliding contact; frictional effects, although low, are not negligible, and lubrication is essential. The article lists the typical characteristics and causes of several types of failures. It describes failure by wear, failure by fretting, failure by corrosion, failure by plastic flow, failure by rolling-contact fatigue, and failure by damage. The article discusses the effects of fabrication practices, heat treatment and hardness of bearing components, and lubrication of rolling-element bearings with a few examples.

Failure analysis results were employed to identify a better alloy. Chipper knives used in the field to chip logs failed frequently. The knives were made of alloys with a composition of Fe-0.48C-0.30Mn-0.90Si-8.50Cr-1.35Mo-1.20W-0.30V. The development of tougher alloy steel with superior properties was initiated. The nominal composition of Fe-0.50C-0.30Mn-0.40Si-5.00Cr-2.00Mo was developed which achieved the goals of edge retention, resistance to softening under frictional heating, wear resistance, ease of heat treatment, dimensional stability in heat treatment, grindability, and low alloy cost. A chip harvester made from this composition was tested in field with older composition knives. It was found that the new knives outperformed the older knives. The key to the development was interpreted to be careful study of a number of failed knives with different problems used in different types of operations.

An oil scavenge pump was found to have failed when a protective shear neck fractured during the start of a jet engine. Visual inspection revealed that the driven gear in one of the bearing compartments was frozen as was the corresponding drive gear. Spacer wear and thermal discoloration (particularly on the driven gear) were also observed. The gears were made from 32Cr-Mo-V13 steel, hardened and nitrided to 750 to 950 HV. Micrographic inspection of the gear teeth revealed microstructural changes that, in context, appear to be the result of friction heating. The spacers consist of Cu alloy (AMS4845) bushings force fit into AA2024-T3 Al alloy spacing elements. It was found that uncontrolled fit interference between the two components had led to Cu alloy overstress. Thermal cycling under operating conditions yielded the material. The dilation was directed inward to the shaft, however, because the bushing had only a few microns of clearance. The effect caused the oil to squeeze out, resulting in metal-to-metal contact, and ultimately failure.

The flanged bearing bush carrying the drive shaft of a feed pump suddenly fractured after about two years of service. The chemical composition was normal for high chromium ledeburitic cast steel, which was corrosion and wear resistant as well as refractory. For unknown reasons the rotating shaft came into direct contact with the flange. Mechanical friction caused a rise in temperature on both contact surfaces. This mutual contact lasted long enough for the temperature in the contact zone to exceed 1200 deg C, at which the flange material became softened or molten. As a result, considerable structural changes took place on the inner wall of the flange. Thermal stresses and excessive mechanical loads due to smearing of the flange material then led to fracture of the flange.

Wear, a form of surface deterioration, is a factor in a majority of component failures. This article is primarily concerned with abrasive wear mechanisms such as plastic deformation, cutting, and fragmentation which, at their core, stem from a difference in hardness between contacting surfaces. Adhesive wear, the type of wear that occurs between two mutually soluble materials, is also discussed, as is erosive wear, liquid impingement, and cavitation wear. The article also presents a procedure for failure analysis and provides a number of detailed examples, including jaw-type rock crusher wear, electronic circuit board drill wear, grinding plate wear failure analysis, impact wear of disk cutters, and identification of abrasive wear modes in martensitic steels.

Engineered components fail predominantly in four major ways: fracture, corrosion, wear, and undesirable deformation (i.e., distortion). Typical fracture mechanisms feature rapid crack growth by ductile or brittle cracking; more progressive (subcritical) forms involve crack growth by fatigue, creep, or environmentally-assisted cracking. Corrosion and wear are another form of progressive material alteration or removal that can lead to failure or obsolescence. This article primarily covers the topic of abrasive wear failures, covering the general classification of wear. It also discusses methods that may apply to any form of wear mechanism, because it is important to identify all mechanisms or combinations of wear mechanisms during failure analysis. The article concludes by presenting several examples of abrasive wear.

Fretting is a wear phenomenon that occurs between two mating surfaces; initially, it is adhesive in nature, and vibration or small-amplitude oscillation is an essential causative factor. Fretting generates wear debris, which oxidizes, leading to a corrosion-like morphology. This article focuses on fretting wear related to debris formation and ejection. It reviews the general characteristics of fretting wear, with an emphasis on steel. The review covers fretting wear in mechanical components, various parameters that affect fretting; quantification of wear induced by fretting; and the experimental results, map approach, measurement, mechanism, and prevention of fretting wear. This review is followed by several examples of failures related to fretting wear.

Erosion of a solid surface can be brought about by liquid droplet impingement (LDI), which is defined as "progressive loss of original material from a solid surface due to continued exposure to erosion by liquid droplets." In this article, the emphasis is placed on the damage mechanism of LDI erosion under the influence of a liquid film and surface roughness and on the prediction of LDI erosion. The fundamentals of LDI and processes involved in initiation of erosion are also discussed.