Originally Posted by
Concentric
From information previously posted on the crash thread, the Service Life Limit used to be 5000FH and the TBO 2000FH but the TBO is said to have been reduced to 1500FH. With a rotor speed taken as 265 rpm and the 2nd stage sun gear rotating at 820 rpm, I reckon the 51 tooth planet gear should ‘rotate’ at approximately 1013 rpm (about its own centre). To achieve the original 5000FH SLL a planet gear would have to rotate approximately 304 million times.
To answer the question asked though, in a ‘lifetime’ the planet gear may rotate anything between 607,800 and 304,000,000 times. That 'lifetime' would be based upon the directive to manually inspect all EC225 MGB magnetic chip detectors every 10 flying hours applicable, it would seem, to even brand new gears. It should be noted for fatigue considerations that cyclic bending stresses occur twice per rev of the planet gear.
Good point about the planet gear teeth being subject to a full reverse bending load cycle twice per rotation. The EC225 MGB epicyclic output stage has a fixed ring gear, sun gear input, and planet carrier output. The planet carrier is coupled to the main rotor mast, so the number of planet gear tooth bending load cycles per rotor rev would be the no. of ring gear teeth divided by the no. of planet gear teeth multiplied by two. However, I would point out that the gears used in helicopter MRGB's are typically designed for unlimited fatigue life in tooth bending. Since root fracture of a gear tooth due to bending fatigue is a failure mode you never want to occur.
Your later comment about how the magnitude of load cycles and when they occur during the component's lifespan can have different effects on fatigue life is also relevant. Here is a good explanation of the
Goodman relation.