Search Results for Rolling-contact wear

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.

Butterfly-shaped microstructural features in tempered martensite in an otherwise clean steel suggested that overloading led to premature spalling of a coal-crushing plant taper bearing. Extensive rolling contact fatigue occurred because of the overload condition. The crusher was designed to handle soft lignite coals but had been used to crush hard deep-mined anthracite coals.

Two intermediate impeller drive gears (made of AMS 6263 steel, gas carburized, hardened, and tempered) exhibited evidence of pitting and abnormal wear after production tests in test-stand engines. The gears were examined for hardness, case depth, and microstructure of case and core. It was found that gear 1 had a lower hardness than specified while the case hardness of gear 2 was found to be within limits. Both the pitting and the wear pattern were revealed to be more severe on gear 1 than on gear 2. Surface-contact fatigue (pitting) of gear 1 (cause of lower carbon content of the carburized case and hence lower hardness) was found to be the reason for failure. It was recommended that the depth of the carburized case on impeller drive gears be increased from 0.4 to 0.6 mm to 0.6 to 0.9 mm to improve load-carrying potential and wear resistance. A minimum case-hardness requirement was set at 81 HRA.

Failure occurred in the teeth of a case-hardening Ni-Cr-Mo alloy steel spur gear in the transmission system of heavy duty tracked vehicles. The defects were in the nature of seizure on the involute profile. Scrutiny of the transmission system showed there might be choking in the lubricating oil line. Such would cause seizure of the gears and damage. The incidence of such defects stopped after corrective measures were taken.

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.

Two fuel injection pump gears that were nitrided in a cyanide bath were submitted by the engine manufacturer for examination of hardness distribution and failure analysis. The gears showed signs of wear after only comparatively brief operation. They were made of normalized unalloyed steel C 45 (Material No. 1.0503) according to DIN 17200 and were normalized. Gear 1 with 1905 h of operation showed at one side pittings on both flanks of the teeth as well as incipient fractures. Gear 2 with 1713 h of operation also showed at one side incipient fractures of the nitride layers at the outer part of the teeth. The nitride layer did not stand up to the high and one-sided compressive stress applied in this case and could not prevent pitting. It could even have accelerated the wear by the incipient break down. Gas nitriding at greater depth under application of a suitable special steel or case hardening would have been better under these circumstances.

The radial-contact ball bearings (type 440C stainless steel and hardened) supporting a computer microdrum were removed for examination as they became noisy. Two sizes of bearings were used for the microdrum and a spring washer that applied a 50 lb axial load on the smaller bearing was installed in contact with the inner ring for accurate positioning of the microdrum. The particles contained in residue achieved after cleaning of the grease on bearings with a petroleum solvent were attracted by a magnet and detected under a SEM (SEM) to be flaked off particles from the outer raceway surface. Smearing, true-brinelling marks, and evidence of flaking caused by the shifting of the contact area (toward one side) under axial load, was revealed by SEM investigation of one side of the outer-ring raceway. The true-brinelling marks on the raceways were found to be caused by excessive loading when the bearing was not rotating or during installation. It was concluded that the bearings had failed in rolling-contact fatigue. The noise was eliminated and the preload was reduced to 30 lb by using a different spring washer as a corrective measure.

A spiral bevel gear and pinion set that showed "excessive wear on the pinion teeth" was submitted for analysis. This gear set was the primary drive unit for the differential and axle shafts of an exceptionally-large front-end loader in the experimental stages of development. There was no evidence of tooth bending fatigue on either part. Several cracks were associated with the spalling surfaces on the concave sides of the 4820H NiMo alloy steel pinion teeth. The gear teeth showed no indication of fatigue. The primary mode of failure was rolling contact fatigue of the concave (drive) active tooth profile. The spalled area was a consequence of this action. The pitting low on the profile appeared to have originated after the shift of the pinion tooth away from the gear center. The shift of the pinion was most often due to a bearing displacement or malfunction. The cause of this failure was continuous high overload that may also have contributed to the bearing displacement.

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.

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.

A ‘worn-out’ spiral bevel gear and pinion set was submitted for examination and evaluation. This was a spiral bevel drive set with the gear attached to a differential. The assembled unit was driving a new, large, experimental farm tractor in normal plowing and tilling operations. The primary failure was associated with the 4820H NiMo alloy steel pinion, and thus the gear was not examined. The mode of failure was rolling contact fatigue, and the cause of failure improper engineering design. The pattern of continual overload was restricted to a specific concentrated area situated diagonally across the profile of the loaded side, which was consistent on every tooth.

Drastic reduction in the service life of a production gearbox was observed. Within the gearbox, the axial load on a bevel gear (8620 steel, OD 9.2 cm) was taken by a thrust-type roller bearing (3.8 cm ID, 5.6 cm OD) in which a ground surface on the back of the bevel gear served as a raceway. Spalling damage on the ground bearing raceway at five equally spaced zones was disclosed by inspection of the bevel gear. The bearing raceway was checked for runout by mounting the gear on an arbor. It was found that the raceway undulated to the extent of 0.008 mm total indicator reading and a spalled area was observed at each high point. The presence of numerous cracks that resembled grinding cracks was revealed both by magnetic-particle inspection and microscopic examination. Spalling was produced by nonuniform loading in conjunction with grinding cracks. As corrective measures, the spindle of the grinding machine was reconditioned to eliminate the undulations and retained austenite was minimized by careful heat treatment.

One end of an axle shaft containing the integral spur pinion was submitted for examination, along with the report of a tooth pitting failure. The spur pinion, integral to the axle shaft, operated in a medium-size, off-highway truck at an open-pit mine, for “a relatively short time.” Only the pinion head had been returned. The shaft portion had been torch-cut away. Chemical analysis along with the microstructure confirmed the specified material was SAE 43BV12 Ni-Cr-Mo alloy steel. The mode of failure was surface contact fatigue through the shear plane subsurface at the lowest point of single-tooth contact. The cause of failure was tooth-tip interference from the mating gear teeth. Because the mating parts within the assembly had not been returned or examined, unanswered questions remained.

The rim of a gear wheel of 420 mm width and 3100 mm in diam broke after four years of operation time in a sheet bar three-high rolling mill. The rim was forged from steel with about 0.4C, 0.8Si and 1.1Mn. The rim started to break in the tooth bottom from a fatigue fracture which extended from the gear side to more than half the rim width. A second incipient failure commenced from the opposite tooth bottom. Both fractures joined below the tooth of the rim. Both incipient cracks were fatigue fractures with several starting points, all located in the transition between tooth flank and tooth bottom. The remaining failure was a fine-grained ductile fracture. It was found that the teeth were not supported uniformly over the entire width and were thus overloaded on one side. The transition from the tooth flanks to the tooth bottom was sharp-edged, causing a tension peak there. The tooth bottom was machined only roughly. Also, the yield point was a little bit too low.

A major cause of failure in components subjected to rolling or rolling/sliding contacts is contact fatigue. This article focuses on the rolling contact fatigue (RCF) performance and failure modes of overlay coatings such as those deposited by physical vapor deposition, chemical vapor deposition, and thermal spraying (TS). It provides a background to RCF in bearing steels in order to develop an understanding of failure modes in overlay coatings. The article describes the underpinning failure mechanisms of TiN and diamond-like carbon coatings. It presents an insight into the design considerations of coating-substrate material properties, coating thickness, and coating processes to combat RCF failure in TS coatings.

An AISI 4320 H transfer gear shaft that was part of a transmission sustained severe surface damage after 12 h of dynamometer testing at various gearing and torque loads. The damage was characterized by generalized wear and spalling. Examination of a cross section of the shaft that intersected undamaged, burnished, and surface-spalled zones revealed no anomalies in the chemistry, microstructure, or hardness that could have caused the damage. The physical evidence suggested that the operable mechanism was contact fatigue caused by misalignment of the shaft in the assembly.

A bull gear from a coal pulverizer at a utility failed by rolling-contact fatigue as the result of continual overloading of the gear and a nonuniform, case-hardened surface of the gear teeth. The gear consisted of an AISI 4140 Cr-Mo steel gear ring that was shrunk fit and pinned onto a cast iron hub. The wear and pitting pattern in the addendum area of the gear teeth indicated that either the gear or pinion was out of alignment. Beach marks observed on the fractured surface of the gear indicated that fatigue was the cause of the gear failure. Similar gears should be inspected carefully for signs of cracking or misalignment. Ultrasonic testing is recommended for detection of subsurface cracks, while magnetic particle testing will detect surface cracking. Visual inspection can be used to determine the teeth contact pattern.

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-contact fatigue (RCF) is a common failure mode in components subjected to rolling or rolling-sliding contact. This article provides a basic understanding of RCF and a broad overview of materials and manufacturing techniques commonly used in industry to improve component life. A brief discussion on coatings to improve surface-initiated fatigue and wear is included, due to the similarity to RCF and the increasing criticality of this failure mode. The article presents a working knowledge of Hertzian contact theory, describes the life prediction of rolling-element bearings, and provides information on physics and testing of rolling-contact fatigue. Processes commonly used to produce bearings for demanding applications are also covered.

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.