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Old 20th Oct 2015, 00:00
  #120 (permalink)  
riff_raff
 
Join Date: Sep 2007
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I'm wondering if there are any implications for LoL situations with a split torque design
SA- That's actually quite an interesting question.

The answer to your question would depend on the specific configuration of split torque transmission. And since the number of potential split torque configurations is quite large, let's just compare the split torque and improved planetary designs considered for the 53K in terms of LoL operation.

First, compare two well known cases where LoL caused overheating and structural failure of a gearbox component, which resulted in loss of MRGB function, and finally loss of the aircraft with everyone aboard. There is the 1982 CH-47 accident in Mannheim and the 2009 S-92 accident in St. John's. What both accidents have in common are the components that failed first after LoL - a spiral bevel pinion and the tapered roller bearings supporting it. With the CH-47 MRGB, the spiral bevel pinion was part of the interconnect drive between the F/R rotors. With the S-92 MRGB, the spiral bevel pinion was part of the tail rotor drive. However, where these two cases differ is the extent of the LoL condition. With the CH-47, there was only LoL at one point in the circuit. The oil jet for the failed spiral bevel pinion bearings became clogged with debris. With the S-92, there was a filter housing leak that resulted in loss of all lube oil.

When comparing the two 53K MRGB configurations, it's important to note that they both use spiral bevel gear meshes for the engine inputs and tail rotor drive. So one design characteristic that has proven to be an issue with LoL conditions is still present in both gearboxes.

The two gearbox components most affected by LoL are bearings and gears. During LoL conditions the greatest concern is scuffing/scoring producing excess heat buildup at the contact surfaces, locally weakening the material, and causing local yielding or structural failure. Lots of design/analysis effort goes towards addressing this issue. The overheating problem can be especially difficult with the bearing inner race surfaces and rolling elements since they have very limited conductive heat transfer ability to surrounding structures. The outer bearing races are usually not an issue since they are in contact with large/cooler housing structures.

The one advantage I can see with the split torque design vs the improved planetary in terms of LoL performance is elimination of the highly loaded planet gear bearings. These planet bearings are mounted in a rotating carrier structure and are fairly well thermally isolated. On the other hand, all the gearshaft bearings (outer races) in the split torque design are mounted in fixed structures with good conductive heat transfer paths.

Designing a helo MRGB to provide the necessary operational capability under LoL conditions is no simple task. It involves expertise in heat transfer, metallurgy, tribology and kinematics. Things would also be much easier if weight wasn't such a big concern.
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