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.

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.

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.

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.

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.

A pinion gear made of AMS 6470 steel, nitrided all over, lost internal splined teeth due to wear. Spline failure of the power turbine gear caused an engine overspeed and disintegration. Excessive spline wear resulted from a new coupling being mated during overhaul with a worn gear spline. Wear on the spline teeth flanks of the coupling was attributed to severe wear on the mating gear (internal) spline teeth. The assigned cause was an inadequate maintenance procedure which resulted in a wear-damaged component being retained in the power train during engine overhaul. To prevent reoccurrence, specific inspection criteria were issued defining maximum limits for spline wear. A procedure and requirements were specified for installing the coupling and pinion gear at the next overhaul.

Three spur gears made from 8622 Ni-Cr-Mo alloy steel formed a straight-line train in a speed reducer on a rail-mounted overslung lumber carrier. The gears were submitted for nondestructive examination and evaluation, with no accompanying information or report. Two teeth on one of the gears were found to be pitted, one low on profile and the adjacent tooth high on profile. The mating gear had a similar characteristic, two adjacent teeth with evidence of pitting and the same difference in profile. It was correctly deduced that the pitting occurred because the gears were in a static position under a reverberating load for an extended period of time.