Lack of lubrication certification for helicopter gearboxes

[SASless]

Originally Posted by megan

At times thought the same SAS, thinking of good friend Jerry Hardy.

Too true....he was such a good guy.....frequently think of him!

[riff_raff]

Originally Posted by SASless

......What about the aircraft with MGB's that are not able to contain a gear failure and afford a safe landing? How many single point failures do we tolerate that can result in a catastrophic result?
No...there shall never be a fault free aircraft....but we should hope to see them as failure tolerant as possible.

SASless-

The discussion of legacy Russian helicopter main gearbox designs jogged my memory. Consider the massive (8,000 lb) Mi-26 main gearbox design shown below. Instead of epicyclic stage(s) for the final drive, it uses a torque-split configuration with 16 small diameter pinions driving a stacked pair of large diameter external bull gears. One major reason for using this configuration is to allow more room for a large diameter rotor shaft.

Regarding your question shown above asking about the capability to "contain a gear failure" so that it does not fully disable the gearbox, I thought you might find the following of interest. There was a paper published by the AHS back in 2008 about a trade study conducted by Sikorsky comparing an advanced epicyclic configuration to a split-torque configuration (somewhat similar to the Mi-26 MRGB) for the new CH-53K main gearbox. One conclusion from the study was that the split-torque configuration was less susceptible to damage from FOD being trapped between the output stage gears. Here's a quote from page 8 of the paper:

"Third, materials such as FOD (foreign Object debris) and wear debris could either be trapped inside or drop into a multiple-stage planetary system due to the planetary motion of those pinions between the ring gears the sun gears, which more likely could cause damage to those rotating parts.
The split-torque MGB design configuration, on the other hand, eliminates the above concerns……….Besides, the trapping of FOD in the split-torque MGB is less likely since all herringbone pinions stay outside the bull gear."


The approach Sikorsky engineers are discussing is slightly different than what I believe you were thinking of, but the goal is similar. That is preventing debris produced by a failed component from causing further damage. Sikorsky chose to use a split-torque configuration for the new CH-53K instead of the epicyclic configuration used on the previous H-53 models, Based on this paper, it seems apparent that Sikorsky engineers were considering the type of problem you brought up back in 2008.

The paper is worth reading if you can find a copy. It's titled, "Trade study on different design configurations of the CH-53K main gearbox".

[Concentric]

Originally Posted by riff_raff

Finally, there were a couple proposed requirements that seemed a bit optimistic. For example, the Class 3 failure criteria (Imminent Failure) on p.18 states "the efficiency of the gearbox may be reduced by up to 10%.". A decent new build MRGB probably has an input/output efficiency of ~96%. So a gearbox on the verge of imminent failure after the loss of lube test cycle must still demonstrate an input/output efficiency of >86%. I don't think this particular requirement is realistic.

Riff, you might find this short presentation (download) from Leonardo interesting. If I am reading it right, that quoted 96.5% seems to refer to the MGB running after loss of lubricant (LoL) though it is not clear where in the timeline of the test that was recorded. Could that possibly be after 50 minutes LoL?

[riff_raff]

Concentric, it says "an overall transmission efficiency up to 96.5% at min cruise power (LoL test condition)".

A couple things to consider with this claim. What losses are included in that overall efficiency number? Were the oil pumps not operational? At what point during the LoL test was that peak efficiency number recorded? Having little oil flow within the gearbox housing would result in reduced gear windage losses and lower viscous losses in the rolling element bearings, at least during the very early stage of the testing.

One unusual thing about the overall efficiency of rotorcraft main gearboxes is the effect load has. Most MRGB losses are two types. Speed dependent and load dependent. For example, rolling element bearings have both viscous (oil churning) losses which are primarily speed dependent, and contact losses (sliding, skidding, etc) which are more load dependent. If the MRGB is operated at max speed and low load, the speed dependent losses remain high and only the load dependent losses drop. And a rotorcraft MRGB driven by turboshaft engines typically operates within a very narrow speed range close to maximum, with the input gear stages turning at high speeds.

With this in mind, it becomes apparent that if you compare two MRGBs operating at the same speed, but one at high load and one at low load, the one at high load should have better overall efficiency. If the 96.5% efficiency at cruise power number in that presentation was recorded at the end of a LoL test, that would be very impressive. And would indicate that the gearbox suffered very little damage during the test.