A mechanical part, which supports the moving part, is termed a mechanical bearing and can be classified into rolling (ball or roller) bearings and sliding bearings. This article discusses the failures of sliding bearings. It first describes the geometry of sliding bearings, next provides an overview of bearing materials, and then presents the various lubrication mechanisms: hydrostatic, hydrodynamic, boundary lubrication, elastohydrodynamic, and squeeze-film lubrication. The article describes the effect of debris and contaminant particles in bearings. The steps involved in failure analysis of sliding bearings are also covered. Finally, the article discusses wear-damage mechanisms from the standpoint of bearing design.

This article is dedicated to the fields of mechanical engineering and machine design. It also intends to give a nonexhaustive view of the preventive side of the failure analysis of rolling-element bearings (REBs) and of some of the developments in terms of materials and surface engineering. The article presents the nomenclature, numbering systems, and worldwide market of REBs as well as provides description of REBs as high-tech machine components. It discusses heat treatments, performance, and properties of bearing materials. The processes involved in the examination of failed bearings are also explained. Finally, the article discusses in detail the characteristics and prevention of the various types of failures of REBs: wear, fretting, corrosion, plastic flow, rolling-contact fatigue, and damage. The article includes an Appendix, which lists REB-related abbreviations, association websites, and ISO standards.

The case study presented in this article details the failure investigation of an M50 alloy steel bearing used in a jet engine gearbox drive assembly. It discusses the investigative steps and analytic tools used to determine the root cause, highlighting the importance of continuous, thorough questioning by the investigating activity. The combined analyses demonstrated that the bearing failed by a single event overload as evidenced by bulk deformation and traces of foreign material on the rolling elements. The anomalous transferred metal found on the rolling elements subsequently led to the discovery of overlooked debris in an engine chip detector, and thus resulted in a review of several maintenance practices.

An aero engine failed due to the misalignment of the ball bearing fitted on the main shaft of the engine. The aero engine incorporates two independent compressors: a six-stage axial flow LP compressor and a nine-stage axial flow HP compressor. The bearing under consideration is a HP location bearing and is fitted at the rear of the nine-stage compressor. It was supposed to operate for at least 5000 h, but failed catastrophically after 1300 h, rendering the engine unserviceable. Unusually high stresses caused by misalignment and uneven axial loading resulted in the generation of fatigue crack(s) in the inner race. When the crack reached the critical size, the collar of the race fractured, causing subsequent damage. The cage also failed due to excessive stresses in the axial direction, and its material was smeared on the steel balls and the outer race.

A heat transport pump in a heavy water reactor failed (exhibiting excessive vibration) during a restart following a brief interruption in coolant flow due to a faulty valve. The pump had developed a large crack across the entire length of a bearing journal. An investigation to establish the root cause of the failure included chemical and metallurgical analysis, scanning electron fractography, mechanical property testing, finite element analysis of the shrink fitted journal, and a design review of the assembly fits. The journal failure was attributed to corrosion fatigue. Corrective actions to make the journals less susceptible to future failures were implemented and the process by which they were developed is described.

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.

An aluminum bronze bushing that serves as a guide in a crimping machine began to fail after 50,000 cycles or approximately two weeks of operation. Until then, typical run times had been on the order of months. Although the bushings are replaceable and relatively inexpensive, the cost of downtime adds up quickly while operators troubleshoot and swap out worn components. Initially, the quality of the bushings came into question, but after a detailed analysis of the entire crimping mechanism, several other issues emerged that were not previously considered. As a result, the investigation provides information on not only better materials, but also design changes intended to reduce wear and increase service life.

A ball bearing in a military jet engine sustained heavy damage and was analyzed to determine the cause. Almost all of the balls and a portion of the outer race were found to be flaking, but there were no signs of damage on the inner race and cage. Tests (chemistry, hardness, and microstructure) indicated that the bearing materials met the specification requirements. However, closer inspection revealed areas of discoloration, or nonuniform contact marks, on the ID surface of the inner ring. The unusual wear pattern suggested that the bearing was not properly mounted, thus subjecting it to uneven or eccentric loading. This explains the preferential nature of the flaking on the outer race and points to an assembly error as the root cause of failure.

A tri-lobe cylindrical roller bearing was submitted for investigation to determine the cause of uniformly spaced axial fluting damages on its rollers and outer raceway surfaces. The rollers and raceways were made from premium-melted M50 and M50NiL, aircraft quality steels often used in bearings to minimize the effects of orbital slippage and rolling-contact fatigue. The damaged areas were examined under a scanning electron microscope, which revealed a high density of microcraters, characteristic of local melting and material removal associated with bearing currents. Investigators also examined the effect of electrical discharge on crater dimensions and density and the role that thermoelectric voltage potentials may have played.

An automotive front-wheel outer angular-contact ball bearing generated considerable noise shortly after delivery of the vehicle. The inner and outer rings were made of seamless cold-drawn 52100 steel tubing, the balls were forged from 52100 steel, and the retainer was stamped from 1008 steel strip. The inner ring, outer ring, and balls were austenitized at 845 deg C (about 1550 deg F), oil quenched, and tempered to a hardness of 60 to 64 HRC. Investigation (visual inspection) supported the conclusion that failure was caused by fretting due to vibration of the stationary vehicle position without bearing rotation. Recommendations included improving methods of securing the vehicle during transportation to eliminate vibrations.

Two blade-detachment failures in large (600 kW) wind turbine generators were investigated. In the first case, bolt failures were established as the initial failure event. A fatigue crack reached a critical length, fast fracture developed and was then arrested as the bolt unloaded. Crack growth resumed when loading increased with cracking or fracture of adjacent bolts. The problem was identified as one of insufficient preload on the bolts. In the second failure on a different unit, a retaining nut on a blade assembly split, allowing a roller bearing to slide off a shaft and a blade to separate at its attachment hub. The failure was observed to be by fatigue. It was determined that pieces of the outer retaining rib (or flange) on the bearing inner cage had fractured by fatigue and were trapped between the nut and the bearing, producing excessive cyclic loading on the nut by a wedging action as the blade pitch adjusted during a revolution. Fatigue of the rim occurred as a result of inadequate lubrication in the bearing, which led to load transfer across the rollers, onto the rim.

The shaft-and-bearing assembly in a freon compressor was subjected to severe pounding and vibration after six years of service. After about one year of service, the compressor had been shut down to replace a bearing seal. One month before the shaft failed, a second seal failure occurred, requiring the collar, spacer sleeve, seal, roller bearing, and lock washer to be replaced. The shaft was made of 4140 steel, heat treated to a hardness of 20 to 26 HRC. The seal, bearing, and lock washer were commercial components. Investigation (visual inspection, 4.5x images, x-ray diffraction, hardness testing, and microscopic exam) supported the conclusion that shaft failure was initiated by fretting between the bearing race and the bearing surface on the shaft because of improper bearing installation. Once clearance was established between the bearing and the shaft, the shaft began pounding on the inner bearing race, causing final failure of the shaft surface. Recommendations included proper fitting of the shaft and bearing race to preventing movement of the bearing on the shaft. Also, the lock washer and locknut must be installed properly.

The presence of secondary, branching intergranular stress-corrosion cracking in a type 440C stainless bearing caused the analyst to overlook the real culprit, which was a mechanically-initiated, primary transgranular crack that propagated through the steel's hard chromium carbide. Failure was actually caused by overload. Had the original conclusion been accepted, a relatively exotic alloy would have been specified. In another case, brass heat exchanger tube failure was automatically attributed to attack by an acidic cleaner, and a decision was made to stop using the solution. A more thorough analysis showed failure was caused by tube vibration. In a third case, a type 304 stainless steel bellows in a test loop was thought to have failed because of chloride stress corrosion. The report concluded with a recommendation that carbon steel be used as an alternative bellows material. Caustic, not chloride, stress corrosion was the culprit. Had material substitutions been made on the original premise of countering chloride stress corrosion, most of the loop's highly stressed components would have eventually failed.

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.

On 22 Feb 1997, one of the arms on an amusement park ride became detached from the central pylon, allowing the passenger carriage at the end of the arm to fall to the ground. Detachment of the arm was found to have occurred as a result of fracture of the cast steel bearing cap and the retaining bolt, which then allowed the axle to move out of its housing. The cap material was a cast 0.25% carbon steel. The retaining bolt failed in the threaded region, with no evidence of fatigue. One of the reinforcing straps welded to the outer surface of the cap had also fractured as the result of fatigue. Evidence of a weld repair could be seen at the location of the fracture. The bearing cap, cap 2L, failed by fatigue initiated from the corners of the two threaded holes. One of the two reinforcing straps then failed by overload, while the second one became detached by fracturing through the welds. The bolt then failed by overload. The cracking in cap 2L was not an isolated occurrence. Cracking was detected at the same location in almost all the other caps on the same device. The cracking had been in progress for considerable time. To prevent the reoccurrence of such a failure, recommendations are given for a more-rigorous inspection protocol.

Three bearing bosses from the cover of scrap shears were sent in for examination. They had torn off the base plate to which they had been welded by fillet welds all around. Two of these were examined. They showed entirely the same symptoms. The bosses had broken away on three sides along the welds. The cleaved fractures in the burned notches propagated partially above and partially below several incipient cracks which may have been fatigue fractures. Metallographic sections showed that the fractures had occurred either at the burned notches near the transition from the weld to the sheet, or else they ran in the sheet material next to the weld. The quality of the welds could not be judged because the opposite fracture pieces to which they adhered had not been sent in. It was concluded that the breakaway of these bosses was at least favored by overheating and hardening.

Nodular cast iron crankshafts and their main-bearing inserts were causing premature failures in engines within the first 1600 km (1000 mi) of operation. The failures were indicated by internal noise, operation at low pressure, and total seizing. Concurrent with the incidence of engine field failures was a manufacturing problem: the inability to maintain a similar microfinish on the cope and drag sides of a cast main-bearing journal. Investigation supported the conclusion that the root cause of the failure was carbon flotation due to the crankshafts involved in the failures showing a higher-than-normal carbon content and/or carbon equivalent. Larger and more numerous cope side graphite nodules broke open, causing ferrite caps or burrs. They then became the mechanism of failure by breaking down the oil film and eroding the beating material. A byproduct was heat, which assisted the failure. Recommendations included establishing closer control of chemical composition and foundry casting practices to alleviate the carbon-flotation form of segregation. Additionally, some nonmetallurgical practices in journal-finishing techniques were suggested to ensure optimal surface finish.

Randomly selected dictating-machine drive mechanisms, which contained small ball bearings, were found to exhibit unacceptable fluctuations in drive output during the early stages of production. It was indicated that the bearing raceways were being true brinelled before or during installation of the bearings. The preinstallation practices and the procedures for installing the bearings were carefully studied. It was revealed that during one preinstallation step, the lubricant applied by the bearing manufacturer was removed and the bearing was relubricated with another type of lubricant prior to which the bearings were ultrasonically cleaned in trichloroethylene to ensure extreme cleanness. Equally spaced indentations resembling true brinelling were revealed by careful examination of the bearing raceways. It was concluded that the ultrasonic energy transmitted to the balls brinelled the raceways enough to cause fluctuations in machine output. Solvent-vapor cleaning was employed as a corrective technique for removing bearing lubricant.

A drawing plant which processed steel wire of designation 105 Cr 2 for ball bearings had losses due to crack formation and wire breakage during drawing. To establish the reason for the breakage, seven fractures were submitted for investigation with contiguous wire segments on both sides of the fracture of 300 mm each. Missing in the lamellar surface structure, with the exception of the remnants of a coarse network, were the pre-eutectically precipitated carbides to be expected in this steel. Surrounding the ferritic region in the surface structure, a ring of lamellar pearlite is seen, which turns into the granular annealed structure towards the core. The described structural phenomena were noted in all of the seven fracture regions. Their intensity always decreased with increasing distance from the fracture. Surface decarburization caused the formation of lamellar pearlite during annealing. This investigation further revealed that the localized decarburization and pearlite formation was present already in the rolled wire in uneven distribution over the entire coil length.

The support bearing of a hydrofoil vessel failed after only 220 h of operation. The bearing consisted of an outer ring made of chromium-plated AISI type 416 stainless steel and an inner ring with a spherical outer surface made of AISI type 440C stainless steel, with a plastic material, bonded to the outer ring, between the two. The inner ring was found to have failed in four places. The two metallic rings were allowed to come in contact with each other by the disappearance of the plastic material. It was revealed by examination of the fracture surfaces of the inner ring that the failure was caused by fatigue initiated in corrosion pits (caused by seawater). The fracture was found to be transgranular. It was recommended that the inner and outer rings should both be made from the more corrosion resistant 17-4 PH (AISI type 630) stainless steel.