Abstract
An investigation was made to determine the causes of surface contact fatigue failure of a case hardened driver pinion located in the intermediate shaft of a reducer gearbox used in a sugar and alcohol mill. The examination of the component revealed the presence of a cemented layer substantially thicker than that generally specified for pinions devised for this application. This, associated with the massive presence of brittle threadlike carbon-rich cementite phase (Fe3C) in prior austenite grain boundaries of the pinion teeth, favored surface crack nucleation and propagation during cyclic loading, leading to spallation of the contact surface with the counterpart gear, which impaired the system's operation. Poor carburization practice was discovered as the root cause of the mechanical failure, thus demanding the implementation of a new manufacturing route to avoid problems in similar load-bearing rotating components.
Highlights
Gears are mechanical elements connected to rotating shafts whose contact surfaces must be carefully shaped to a specific profile to transmit uniform and continuous rotary motion
The pinion analyzed in this study was manufactured with DIN 17CrNiMo6 steel, cemented/ carburized in a carbon rich atmosphere to reach the required 2.6 mm deep carbon rich surface layer, according to the DIN 50190 standard[13], and subsequently quenched and tempered (Q&T) to obtain a structural piece exhibiting a hard and wear resistant surface layer allied to a very tough core to withstand repeated impact loads
A visual inspection of the damage suffered by the case hardened pinion tooth made of SAE 8620 type steel showed that the failures originated from surface contact fatigue, through pitting mechanism
Summary
Gears are mechanical elements connected to rotating shafts whose contact surfaces must be carefully shaped to a specific profile to transmit uniform and continuous rotary motion. It is well known that exceptionally high cyclic contact pressure loads develop in real contacting rotating bodies (e.g., wheel-pinion pairs) and that the relative motion between them causes simultaneous rolling and sliding This combined mechanical loading of the parts defines, to a great extent, the probability that surface contact fatigue will develop, which is why this phenomenon is commonly called rolling-sliding contact fatigue (RSCF). The main factors that contribute significantly to the development of this process are unsuitable depth of the hardened surface layer (either thinner or thicker than specified in the project stage), incorrect hardness profile in the hardened case and along the tooth height, insufficient core hardness (all of them resulting essentially from defective thermal and/or thermo-chemical treatments), occasional overloading during operation, errors in mesh due to manufacturing and assembly faults, and the occurrence of cyclic stress that induces fatigue crack growth in the material. The challenge here is, to disclose the root cause(s) which led to the case hardened pinion failure during apparently normal in-service conditions of the studied gearbox
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