During overhaul of a primary drive helicopter Spur Gear, which also serves as the outer race for an integral spherical
bearing, magnetic particle inspection detected an approximately 0.3-inch long crack indication on the raceway surface.
Subsequent metallurgical evaluation disclosed that the indication was a 0.028-inch deep, pre-existing grinding crack
that had not been detected by the manufacturer during final inspection. A sample re- inspection of product that was
currently in storage that had been shipped from this manufacturer revealed that additional components with grinding
cracks. In an effort to determine if the processing cracks posed a safety of flight concern, several gears that contained
cracks were designated to undergo a rigorous dynamic bench test. However, in order to insure that the bench test
produced the maximum amount of relevant test data, it was necessary to document, nondestructively, all of the grinding
crack dimensions (location, lengths, and depths) prior to the start of the test. The locations and lengths of the cracks were
determined using a specially modified magnetic rubber inspection technique, which could reliably document crack
lengths as short as 0.006 inch. A fixture was developed that made it possible to produce a magnetic rubber casting of the
gear raceway, while simultaneously performing up to thirty, one second central conductor shots. To nondestructively
estimate crack depths, a unique, highly sensitive, laboratory eddy current inspection technique was developed. Using a
limited number of data points, a“response-to- depth” curve was generated that predicted the maximum crack depths to
+/-0.002 inch. Using these two techniques, the“status” (i.e, growth)of each crack was monitored at various stages of the
bench test. At the completion of the bench test, a detailed destructive metallurgical evaluation of each of the test gears was
conducted and documented. The post-test data confirmed the accuracy of the nondestructive eddy current estimates, and
provided additional data that will make future crack depth predictions more accurate.
AGMA 02FTM5-2002 pdf download
PS:Thank you for your support!
