Failure Analysis for High Speed Gears

In this paper, the failure analysis of high speed gears driving air blowers in starch manufacturing processes was investigated in order to find the failure reasons and so over coming them in the future. The gear velocity was 21000 rpm and manufactured from carburized steel (18CrNiMo7-6) DIN No. (1.6587). The gear module was 2 mm and number of teeth was 23 teeth. The gear has failed within 3 weeks of first time operation. Visual investigation has carried out and a set of photos was taken. The teeth of the gear have undergone a bending deflection accompanied with surface wear. The operating conditions (environmental effects, lubrication, loads, assembly and alignment) were revised. The design data was reviewed according to the standard gear design recommendations and also the manufacturing data were checked according to the design data. Additionally, the material analysis, micro hardness measurements and micro structure were investigated. A set of errors were found. The analysis showed that the gear material selection was wrong because the carburization is not suitable for small gear modules. Also, the carburizing depth is not suitable to the gear of module 2 mm. Moreover, the heat treatment process has a set of faults. The micro hardness measurements and micro structure revealed that there was decarburization at surface with depth of 50 to 70 microns due to the presence of decarburization elements such as hydrogen and oxygen from the moisture inside the furnace. Also, there is an excessive retained austenite due to the slow cooling rate in the quenching operation which did not avoid the CC curve nose. The quenching temperature of the gear was less than 700 Co. There was also an existence of bainite, which reduces the fatigue strength. Although, the recommended values of surface hardness of gear teeth must be more than 56 HRC, the hardness was about 52 HRC at surface of teeth and 42 HRC between teeth at roots, which may reduced the Hertzian contact stress resistance of teeth. Therefore, as a recommendation, the nitriding as a case hardening material must be selected with at least 0.5 mm thickness and of hardness more than 60 HRC for better wear resistance and contact strength for modules of less than or equal 2 mm.