For the bearing nerds amongst us: Interesting failure mode of hybrid ceramics

[ReelClean]

An interesting  aircraft accident report on an AW139 helicopter that used hybrid ceramic bearings in the tailrotor shaft.  It shows one possible failure mode of such bearings in extreme use (excerpt of salient text):

The AAIB's investigation had immediately focused on the anti-torque system. It was discovered that the inner and outer races of both sides of the bearing had become damaged by Rolling Contact Fatigue—a type of surface damage resulting from repeated rolling or rolling and sliding contact between curved surfaces such as the race and ball in a bearing. Manufacturer Leonardo had used a hybrid bearing, which used ceramic ball bearings made from silicon nitride.

Although lighter, the ceramic balls had a higher contact pressure in the bearing. Combined with deterioration in the grease used to lubricate the bearing, this caused the ceramic balls to slide rather than roll. This ultimately led to wear and fatigue fractures that resulted in the bearing seizing.

[JasonGotaProblem]

Interesting. Thank you for sharing. Relevant question: given the problem was caused by old grease making the balls stop rolling, would it have been any different with steel bearings?

[foakes]

Being a pilot, I have a little knowledge of aircraft.

And, a few friends who fly helos professionally.

One friend in Kansas is also an inspector for a major helicopter manufacturer.

His take on helicopters is —-

100,000 parts all fighting against each other —- and vibrated to pieces at the same time —- and each built by the lowest bidder.

He goes to helo crash sites to inspect and determine what the cause was.

And he teaches aircraft mainframe & powerplant repair and maintenance to techs.

He says, in a helicopter —- auto-rotation landing is a good skill to master —- it is effective about half of the time when there is a major failure.

I have only been up in a helicopter (1) time for a morning traffic report —- and decided against ever going up again.

Amazing to use ceramic bearings!

Sort of like the tiles on the Space Shuttle.

Best, Fred

[ReelClean]

I was

Interesting. Thank you for sharing. Relevant question: given the problem was caused by old grease making the balls stop rolling, would it have been any different with steel bearings?

I was surprised by "old grease".  Every tailrotor shaft support bearing I have seen on a flingwing (granted I am talking old Bells here) had a grease nipple that was religiously serviced at the required intervals.  Maybe they have these new fangled "sealed for life" bearings (transmissions, gearboxes, Magseal -- insert name of latest mechanical wunderkit here --) that never need to be serviced! 

[jurelometer]

Not a pilot myself, just a bearing nerd.

It seems unlikely that for such a critical assembly, a hybrid ceramic bearing would be "sealed for life", unless the bearings were replaced at intervals approaching what would have been normal maintenance.

Hybrids have to be lubricated, the lubricant has a lifespan, and it is unlikely that bearings like this can be externally monitored for wear. So they have to be inspected and relubricated, or at least relubricated.

Ceramic balls in this case have the advantage of less thermal expansion, less rolling weight from the balls, and maybe a longer lifespan due to the extra hardness.  I would suspect that friction is primarily a function of lubricant. 

If a ball or race gets worn  or scratched just a teensy bit, the ball(s) are going to start going from  a rolling surface to a sliding surface with a very small surface area, which means a lot more friction and a lot more heat.  I could see how in a high load/ high velocity situation, a hybrid bearing might perform fairly well right up until the point of failure.

So to answer Jason's question: Yes, if you have a bearing that is supposed to be lubricated, and the lubricant degrades enough, and the bearing is pushed hard enough, it will fail.  The interesting thing here is that ceramic hybrids fail a bit differently  due to  the balls being hard (and therefore brittle- no free lunch).  They wont expand much, so the bearing doesn't seize the same as all steel.  Instead, the balls start fracturing.  Don't know if that makes a difference for the pilot once the failure happens and the tail rotor stops rotating. 

I was reading that they use a ceramic coating that itself is coated with PTFE for plain bearings for the main rotor shaft in some modern helicopters.  That is pretty impressive.  Gotta place your trust in science any time you are flying, I guess.

-J