On 31 Jan 2000, a McDonnell Douglas MD-83 airplane crashed off the California coast while en route from Puerto Vallarta, Mexico, to San Francisco. Approximately 90% of the aircraft was recovered from a depth of about 700 ft. (213 m). Among the recovered components were parts of the jackscrew assembly, including the jackscrew with an internal torque tube, the gimbal nut, and the upper and lower mechanical stops. The jackscrew was connected to the horizontal stabilizer and controlled its movement. Multiple damage features, indicative of contact with another object, were observed on the upper surface of the lower mechanical stop. Damage to the spline teeth was also observed on the lower mechanical stop. The stripping pattern and offset circumferential marks were consistent with the lower stop being at two or more skewed angles to the splines of the jackscrew during stripping. This investigation is continuing.

Impact or percussive wear is defined as the wear of a solid surface that is due to percussion, which is a repetitive exposure to dynamic contact by another body. Impact wear, however, has many analogies to the field of erosive wear. The main difference is that, in impact wear situations, the bodies tend to be large and contact in a well-defined location in a controlled way, unlike erosion where the eroding particles are small and interact randomly with the target surface. This article describes some generic features and modes of impact wear of metals, ceramics, and polymers. It discusses the processes involved in testing and modeling of impact wear, and includes two case studies.

This article discusses the generic features of impact wear on metals, ceramics, and polymers. It describes normal impact wear and compound impact wear, as well as the features of impact wear testing apparatus such as ballistic impact wear apparatus and pivotal hammer impact wear apparatus. Most mechanical components continue to be functional beyond the zero wear limit, and their usefulness is normally connected with the loss of a specific depth of material. The article reviews the zero impact wear model and some measurable impact wear models. It presents a case study illustrating the impact of wear failure on automotive engine inlet valves and seat inserts.

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 accidental overspeed condition during wind tunnel testing resulted in the destruction of a propeller rotor The occurrence was initially attributed to malfunction in the collective pitch control system. All fractured parts in the system were inspected. Highly suspect parts, including the pitch control thrust bearing set, head bolts, hub fork, and actuator rod end, were examined in more detail The thrust bearing set (52100 steel) was identified as the probable source of the uncommanded pitch angle change. A complete failure analysis of the bearing indicated that failure was precipitated by excessive heating, causing cage disintegration, plastic flow of the races and balls, and eventual separation of inner and outer races. It was recommended that the bearing set be resized to accommodate the large thrust as and that a thermocouple be added to monitor the condition of the bearing during testing.

This article first reviews variations within the most common types of gears, namely spur, helical, worm, and straight and spiral bevel. It then provides information on gear tooth contact and gear metallurgy. This is followed by sections describing the important points of gear lubrication, the measurement of the backlash, and the necessary factors for starting the failure analysis. Next, the article explains various gear failure causes, including wear, scuffing, Hertzian fatigue, cracking, fracture, and bending fatigue, and finally presents examples of gear and reducer failure analysis.

The failure of an ATAR engine accessory angle drive gear assembly caused an engine flame-out in a Mirage III aircraft of the Royal Australian Air Force (RAAF) during a landing. Stripping of the engine revealed that the bevel gear locating splines (16 NCD 13) had failed. Visual and low-power microscope examination of the spline of the shaft showed evidence of fretting wear debris; similar wear was observed on the splines of the mating bevel gear. It was concluded that the splines had failed by severe fretting wear. Fretting damage was also observed on the shaft face adjacent to the splines and on the bevel gear abutment shoulder. Additional tests included a metrological inspection of the shaft, bevel gear and support ring; metallographic examination of a section from the shaft; chemical analysis of the shaft material (16 NCD 13); and hardness testing of a sample of the yoke material. The wear had been caused by incorrect machining of the shaft splines, which prevented the bevel gear nut from locating correctly against the gear.

Cam crack failures are a common occurrence in cam-follower systems often caused by excessive loading or inappropriate operating conditions. An investigation into such a failure was conducted to assess the effect of cam crack damage on the dynamic behavior of cam-follower systems. It was shown both theoretically and experimentally that a cracked cam causes an overall reduction in stiffness. To further probe the effect, investigators derived an analytical formula expressing the time varying stiffness of a cam-follower system. They also succeeded in quantifying the relationship between crack size and stiffness, showing that cracks have an amplitude modulating effect.

Gears can fail in many different ways, and except for an increase in noise level and vibration, there is often no indication of difficulty until total failure occurs. This article reviews the major types of gears and the basic principles of gear-tooth contact. It discusses the loading conditions and stresses that effect gear strength and durability. The article provides information on different gear materials, the common types and causes of gear failures, and the procedures employed to analyze them. Finally, it presents a chosen few examples to illustrate a systematic approach to the failure examination.

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 provides a background of friction-bearing failures due to overheating. The failures of locomotive axles caused by overheated traction-motor support bearings are discussed. The article also describes liquid-metal embrittlement (LME) in steel. It examines the results of various axle studies, with illustrations and concludes with information on the simulation of the LME mechanism.

This article focuses on the types of activities required for the resolution of wear problems. These include examining and characterizing the tribosystem; characterizing and modeling the wear process; obtaining and evaluating wear data; and evaluating and verifying the solution.

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.

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.

Rolling-contact fatigue (RCF) is a surface damage process due to the repeated application of stresses when the surfaces of two bodies roll on each other. This article briefly describes the various surface cracks caused by manufacturing processing faults or blunt impact loads on ceramic balls surfaces. It discusses the propagation of fatigue cracks involved in rolling contacts. The characteristics of various types of RCF test machines are summarized. The article concludes with a discussion on the various failure modes of silicon nitride in rolling contact. These include the spalling fatigue failure, the delamination failure, and the rolling-contact wear.

A splined shaft on a wood chip-to-fiber refiner failed during equipment start-up. The shaft broke into two pieces at a location close to the end of the splined part of the shaft. The failed component showed the classical fatigue-cracking fracture face. The shaft had a diam of approximately 140 mm (5.5 in.) in the unsplined section and was made of 4340 Ni-Cr-Mo alloy steel heat treated to a uniform hardness of HRC 31. Cracks from at least seven different origins had coalesced to produce the single large crack that resulted in failure. The origins of these cracks were on the flanks of the splines. SEM examination revealed the splined shaft failed by fretting fatigue.

Two vertical coal-pulverizer shafts at a coal-fired generation station failed after four to five years in service. One shaft was completely broken, and the other was unbroken but cracked at both ends. shaft material was AISI type 4340 Ni-Cr- Mo alloy steel, with a uniform hardness of approximately HRC 27. Metallographic examination of transverse sections through the surface-damaged areas adjacent to the cracks also showed additional small cracks growing at an angle of approximately 60 deg to the surface. The crack propagation mode appeared to be wholly transgranular. SEM examination revealed finely spaced striations on the crack surfaces, supporting a diagnosis of fatigue cracking. Crack initiation in the pulverizer shafts started as a result of fretting fatigue. Greater attention to lubrication was suggested, combined with asking the manufacturer to consider nitriding the splined shaft. It was suggested that the surfaces be securely clamped together and that an in-service maintenance program be initiated to ensure that the tightness of the clamping bolts was verified regularly.

Three blades from 45,000 kW, 3,000 rpm turbine were received for examination, comprising the root of blade 28, blade 89 showing a crack in one of the root teeth, and blade 106 which was free from defects. Microscopic examination of the blade material showed it to be a ferritic stainless steel of the type commonly used for turbine blades. A number of non-metallic inclusions were present which had been drawn into threads in rolling; these appeared to consist largely of duplex silicates. The failure of blade 28 was the result of the development of a creeping crack. Magnetic crack examination of blade 89 revealed a crack in a tooth in an identical position to the start of the crack in blade 28 but on the opposite, i.e., steam inlet, side of the blade. Similar examination of blade 106 did not reveal any cracks. Cracking was associated with unsatisfactory bedding of the blade teeth on the faces of the wheel grooves. It was concluded that the blade failures were due primarily to over-loading of the individual blade teeth due to incorrect fitting in the wheel. Vibration was an important contributory factor, as it resulted in the imposition of fluctuating stresses on the overloaded teeth. Non-metallic inclusions in the blade material playing a minor part.