Originally Posted by
The Sultan
Riff- The 92 crash was caused by failure of the xmsn tail rotor output roller bearings which allowed the quill to move and, after it was stripped, loss of tail rotor drive. Main rotor drive was never lost.
Sultan-
In my post I stated,
"...where the main gearbox TTO spiral bevel pinion suffered catastrophic plastic failure of the gear teeth from elevated temperatures...". This is a conclusionfrom the
CAB investigation report A09A0016 sec. 1.12.3 . From the picture of the failed TTO spiral bevel pinion included in the report section linked, most notable is that all of the teeth are missing and it does appear to be the result of severe plastic deformation. However, there is also evidence of similar type damage on both of the pinion's bearing journals which would agree with your statement about the tapered roller bearings failing. Since both the gear teeth and bearing journal damage appears to be severe plastic deformation resulting from loss of mechanical strength due to the material being heated well above its tempering temperature, it seems logical to conclude a C64 pinion would have performed much better than a 9310 gear. At the tempering temperature used with C64 gears, a 9310 gear will have experienced a reduction in tensile strength of ~35%. The loss of drive to the tail rotor would seem to be the result of the TTO pinion losing all of its gear teeth. But I may be wrong.
Lonewolf_50-
ISF of gear tooth flanks definitely provides some additional scoring margin under marginal lubrication conditions such as that experienced during loss of lube operation. The reduced mean surface roughness provided by ISF improves the min hydrodynamic oil film thickness the gear contacts can safely operate at. In case you're interested, the technical term for the relationship between hydrodynamic oil film thickness and mean height of the mating gear surface asperities is called "lambda ratio".