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.

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.

A spiral bevel gear set in the differential housing of a large front-end loader moving coal in a storage area failed in service. The machine had operated approximately 1500 h. Although the failure involved only the pinion teeth, magnetic particle inspection was performed on each part. The 4817 NiMo alloy steel pinion showed no indication of additional cracking, nor did the 4820 NiMo alloy steel gear. The mode of failure was tooth bending fatigue with the origin at the designed position: root radius at midsection of tooth. The load was well centered, and progression occurred for a long period of time. The cause of failure was a suddenly applied peak overload, which initiated a crack at the root radius. Progression continued by relatively low overstress from the crack, which was now a stress-concentration point. This was a classic tooth bending fatigue failure.

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.

The gear of a spiral bevel gear set broke into three pieces after about two years of service. The gear (made of 4817 steel) broke along the root of a tooth intersected by three of the six 22-mm diam holes used to mount the gear to a hub. Fatigue progression for about 6.4 mm at the acute-angle intersections of three mounting holes with the root fillets of three teeth was revealed by examination of gear. Cracks at the intersections of the remaining three mounting holes and the adjacent tooth-root fillets were revealed by magnetic-particle inspection. Through hardening at the acute-angle intersections of the mounting holes and tooth-root fillets was revealed by metallographic examination. Design of the gear and placement of the mounting holes, which resulted in through hardening, were concluded to be the contributing factors to the fatigue failure of the gear.

Several teeth of a bevel pinion which was part of a drive unit in an edging mill failed after three months in service. Specifications required that the pinion be made from a 2317 steel forging and that the teeth be carburized and hardened to a case hardness of 56 HRC and a core hardness of 250 HRB. Two teeth were revealed by visual examination to have broken at the root and fatigue marks extending across almost the entire tooth were exhibited by the surface of the fracture. Cracking in all the tooth was showed by magnetic-particle inspection. The pinion was concluded to have failed by tooth-bending fatigue. Spalling was also noted on the pressure (drive) side of each tooth at the toe end which indicated some mechanical misalignment of the pinion with the mating gear that caused the cyclic shock load to be applied to the toe ends of the teeth.

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.

The failure of a spiral bevel gear from the transmission of a helicopter was discovered when the transmission was removed after an in-flight incident. Two adjacent teeth from the carburized AISI 9310 steel gear were found to have undergone fatigue failure. Internal initiation occurred in a region depleted of chromium and nickel. This condition coincides with a microstructural inhomogeneity consisting of large, soft ferrite grains. Its origin was probably contamination of the solidifying ingot during the consumable vacuum arc remelting operation.