Originally Posted by
turboshafts
in any case as long as there is a rigid fit between rotor mast and 2nd stage planet carrier it will take up radial load.
the bearing in the bottom of the conical housing will take up some load.
that was the principle of the load distribution that we just talked about!
even if you say there is enough radial play in the planetary bearing/gear to completely unload the 2nd carrier of any radial load from the rotor mast
it will be taking up the load as long as it is driven
That is not what I am saying. Picture yourself, the AC with rotor mast and MRA, but without the MGB. Is it a rigid construction that could carry the load of the AC? No. That is what I am talking about. If the MGB is a rigid part of the MRA and MGB fails shattering the housing, the Rotor will detach. In order to do this, you need to make a gearbox with a stiff housing. Possibly add 7-10 susp bars. The stiff housing is increasing load on individual gears and reducing lifetime of them, but if it could allow a total break-up of MGB without sacrificing the rigidity of the rotor assembly, in my view that would be a life-saving solution because it would allow to autorotate without the MGB. In the same way as twin-engine, emergency lubrication etc are needed.
There is no saying against a multiple failing MGB cannot be a rigid part of the AC airworthy capability.
So I dont say that either susp bars, conical housing or gears broke first. I am saying the should never break together.
From what I can see in the online images available, the mast bearing arrangement in the conical housing appears to be a back-to-back pair of preloaded tapered roller bearings. This bearing arrangement provides excellent stiffness and load capacity in radial/axial directions, and also for moments out of the plane of rotation. The spline joint between the lower end of the rotor shaft and the output epicyclic stage planet carrier is likely not a "rigid" connection. These types of spline connections (fillet root, side fit) are usually made with a small amount of clearance between the mating teeth, and have a small amount of radial play when assembled. A small amount of radial play in the spline joint would seem necessary to prevent an over-constrained condition in the rotor mast bearing set.