To: 212 Man
The US Navy and the US Air Force suffered quite a few shaft separations on their 212s and their 412s. The phenomena of separations were attributed to movement of the transmission in relation to the combining gearbox. This misalignment would cause the shaft couplings to align with the respective attach points (Transmission and combining gear box) and the shaft would have to find a position of neutrality which caused it to bow in order to maintain alignment with the couplings. If in the process of this bowing the shaft mass went out of balance the shaft would begin to whip and in doing so, it would disconnect at one of the two ends and it would immediately begin to flail. Another thing that would contribute to this condition is if the shaft were already out of balance.
When the 214 was introduced they had problems related to the Noda Matic suspension system. The nodal beam allowed the transmission to move up and down in relation to the fuselage. This caused a misalignment between the engine and the transmission. The couplings on the short shaft would accommodate the misalignment. This movement of the couplings caused them to pump out the internal lubricating grease. This grease would plate out on the inlet bell of the engine and sand and other small-ingested material would stick to the grease. This build up would cause a disruption in the inlet flow and result in compressor surges or compressor stalls. I was associated with the 214 program for over two years and in that time they tried several fixes including a change in the internal “O” rings on the couplings. The only solution they found that worked was frequent engine washes and frequent refilling of the couplings. I don’t know how they eventually solved the problem.
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The Cat