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.

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.

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.

An aircraft was grounded when illumination of the transmission oil-pressure light and an accompanying drop in pressure on the oil-pressure gage was reported by the pilot. No discrepancy in the bearing assemblies and related components was revealed by teardown analysis of the transmission. The center bearing of the transmission input-shaft ball-bearing stack had a broken cage and one ball was found to have been split into several pieces. Several scored balls and flaking damage in the raceways of the inner and outer rings was observed. The origin (area in rectangle) was oriented axially in the raceway and was flanked by areas of markedly different-textured flaking damage. Stringers of nonmetallic inclusions were revealed at the origin during metallographic examination of a section parallel to the axially oriented origin. Thus it was concluded that the failure was caused by contact fatigue mechanism (flaking) activated by the subsurface nonmetallic inclusions.

Inner rings of spherical roller bearings out of full hardening ball bearing steel 100 CrMn 6 (Fe-1C-1.5Cr-1.1Mn, Material No. 1.3520) failed in service. Due to the cracks, parts from the middle flange broke or the rings failed in radial direction completely. All the cracks and fracture originated from the middle flange. In all of the three rings one flank showed heavy wearing and scouring. The cracks started from the edge of this flank with the cylindrical mantle surface of the middle flange. The cracking resembled fatigue cracking. However, in a fine-grained hardened steel such as this, fracture faces due to stress-cracking and overload fracture look the same. Metallographic examination showed the failure of the rings was a result of repeated heating and rapid cooling of the surface due to the grinding of the bearings on one flank of the middle flange. The stress-cracks (grindcracks) spread in steps which finally led to the breaking off of parts from the middle flange and complete failure of the rings.

The drive-shaft hanger bearings failed after 300 to 400 h in service. The shaft, supported by labyrinth-sealed single row radial ball bearings of ABEC-1 tolerances, was made of aluminum 2024-T3 tubing (2.5 cm diam and 1.2 mm wall thickness). The bearings were lubricated with a paste-type mineral-oil lubricant (containing molybdenum disulfide and polytetrafluoroethylene particles) or grease conforming to MIL-G-81322 (containing thickening agent and synthetic hydrocarbons) and had two-piece spot-welded retainers. On visual examination, the balls were observed to be embedded in the inner-ring raceway which had been softened by the elevated temperatures reached during the failure. Broken retainers and worn and bent out of shape seals were found. Penetration of gritty particles, water and other corrosive agents and leakage of lubricant out of the bearing permitted by the worn seals was observed. It was concluded that overheating was caused by lubricant flow was permitted by wear of the labyrinth seals. Positive rubbing seals and MIL-G-81322 grease lubricant were found to have longer life than those with the labyrinth seals and mineral-oil-paste lubricant on testing under simulated environmental conditions and were installed as a corrective measure. Importance of dirt free supply and drainage of oil was discussed.

Factors which may lead to premature roller bearing failure in service include incorrect fitting, excessive pre-load during installation, insufficient or unsuitable lubrication, over-load, impact load vibration, excessive temperature, contamination by abrasive matter, ingress of harmful liquids, and stray electric currents. Most common modes of failure include flaking or pitting (fatigue), cracks or fractures, creep, smearing, wear, softening, indentation, fluting, and corrosion. The modes of failure are illustrated with examples from practice.

A needle bearing from a filling and seating machine for milk cartons became unusable due to corrosion and fracture of a ring after only four weeks of operation of the machine in a Finnish milk packing plant. These bearings were subject to corrosion by water condensates in this type of environment because of constant temperature changes, and they normally are replaced after eight months. The bearings were lubricated by a molybdenum sulfide paste. Judging by their structure the needles probably consisted of ball bearing steel. They showed corroded initial cracks of the pitting type, i.e., shear-fatigue fractures due to excessive surface pressure. The needles too were overstressed by compression. It seemed that the higher pressure necessary for the pressing of thicker paper accelerated the corrosion, which lead to the crack initiations of the parts and possibly also to impaired lubrication. The machine manufacturer therefore switched to bearings with shells of a complex bronze.

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.

A 52000 bearing steel valve guide component operating in the fuel supply system of a transport aircraft broke into two pieces after 26 h of flight. The valve guide fractured through a set of elongated holes that had been electrodischarge machined into the component. Analysis indicated that the part failed by low cycle fatigue. The fracture was brittle in nature and had originated at a severely eroded zone of craters in a hard, deep white layer that was the result of remelting during electrodischarge machining. It was recommended that the remaining parts be inspected using a stereoscopic microscope and/or a borescope.