Analysis of failed collector pinions of a Helicopter

Madan, M and Bhuvana, R and Sujata, M and Venkataswamy, MA and Bhaumik, SK (2011) Analysis of failed collector pinions of a Helicopter. Project Report. National Aerospace Laboratories, Bangalore, India.

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The RH and LH pinions of main gearbox pertaining to a helicopter were sent to the laboratory for investigation. Examination showed spalling on the loading flank of two teeth of the LH pinion. Detailed study carried out on the samples from the damaged teeth discovered that the spalling initiated at the bottom of the dedendum, and that the subsurface cracks propagated progressively upward. Initiation of spalling was observed all along the bottom of the dedendum in all the teeth. This was found to be due to excessive contact stresses resulting from tip biting. In general, the evidences on the flank surface suggested considerable sliding of the pinion instead of rolling. The meshing of the pinion did not appear to be healthy. It encompassed the entire tooth height right up to the top land. It may, however, be noted that no gear/pinion is designed to have such contact patterns. Hence, it is imperative that the elastic deformation of the tooth under full load was more than that of the anticipated design value. Also, when the load on the flank is excessively high, it would promote sliding rather than rolling, as seen in the present case. The RH pinion did not have any observable damage though it also showed meshing and contact patterns similar to that seen in case of LH pinion. However, metallographic study conducted on the cross section of the teeth indicated initiation of subsurface cracks on the loading flank of this pinion as well. The microstructure, composition and the surface hardness of the teeth were found to be satisfactory. The case depth on the loading flank was found to be in the range 0.78 to 0.89 mm for LH pinion and 0.78 to 0.87 mm for RH pinion. In summary, it is stated that the LH pinion has failed by spalling. Study also showed evidences of initiation of spalling in the RH pinion. Investigation revealed that all factors required for spalling were present, namely, unfavourable meshing, high load, and tip biting. From the analysis, it appears that deficiencies other than metallurgical factors were responsible for the premature spalling in the pinions.

Item Type: Proj.Doc/Technical Report (Project Report)
Uncontrolled Keywords: Helicopter;Collector pinion;Spalling; microstructure; Retained austenite;Grain size;Hardness
Subjects: AERONAUTICS > Aeronautics (General)
CHEMISTRY AND MATERIALS > Chemistry and Materials (General)
Division/Department: Materials Science Division, Materials Science Division, Materials Science Division, Materials Science Division, Materials Science Division
Depositing User: Ms. Alphones Mary
Date Deposited: 18 Jul 2011 10:59
Last Modified: 18 Jul 2011 10:59

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