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Engine making metal...

rocketbob said:
I don't think the root cause is a gear manufacturing/hardness problem. Since you don't have mags installed the right mag idler gear could be ringing on every power pulse and beating the crankshaft gear to death. I would remove the idler gear on the right side since it no longer serves a purpose. The damage to the gear appears to be consistent with this sort of thing and maybe that's why there's no metal being shed by the gear. Essentially the crankshaft gear could be getting shot peened to death. Which would explain the lack of metal in the filter.
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Bob,

Unless the two gears are of different hardness, shouldn't the idler gear show similar damage? (the same impact force is felt on both surfaces). Also I'm not sure I see how this explains the earlier lack of metal in the filter; it sure looks like there is metal missing from the gear. Are you saying the gear wore down too slowly to produce noticeable metal?
 
I can't say what kind of shape the idler gear should be...just don't know. But I can say that the wear, if you want to call it that, is not like the wear I've seen on crankshaft gears, or any gears in the accessory case. Usually they are polished smooth and thats what I would expect to see here. With the raised metal on the sides of the worn area on the teeth make me think the gear was getting pounded.

Someone correct me if I'm wrong here but it appears the BACK side of the teeth are worn, not the front side that are under the most pressure.
 
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I was an Engineer at and electric motor/gearbox manufacturer for several years. That wear is not normal. Assuming hardness is correct, something else is wrong. Usually that kind of wear is caused by incorrect clearance/backlash or support bearing failure. I would have the case checked very closely and make sure the idler shafts are secure and ok. I would consider replacing the idler gears also.

There is a Service instruction that deals with reworking that end of the crank to get the correct fit for the gear. The crank shop can do part of it and you will have to perform the other part when you install the gear.
 
rocketbob said:
I can't say what kind of shape the idler gear should be...just don't know. But I can say that the wear, if you want to call it that, is not like the wear I've seen on crankshaft gears, or any gears in the accessory case. Usually they are polished smooth and thats what I would expect to see here. With the raised metal on the sides of the worn area on the teeth make me think the gear was getting pounded.

Someone correct me if I'm wrong here but it appears the BACK side of the teeth are worn, not the front side that are under the most pressure.
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Bob, it is the pressure side that is worn. But, there is some evidence that the back side has some polishing.
 
Gear hardness

I tested both the crankshaft and cam gears on the Rockwell C scale. I do not have any idea what the "bulk" hardness should be, and I also do not know what sort of hardening/heat treat they normally see (case, induction, etc.). Anyway, several points were taken in the hub area of the gear and several on the sides of the teeth. The data points on the edge of the teeth could tend to understate the hardness, as the structure might deform somewhat there.

Crank gear hub - Rc 24.9, 24.7, 26.1, 26.3, 24.1
Crank gear teeth edge - Rc 28.7, 26.1, 27.4

Cam gear center area - Rc 31.2, 32.1
Cam gear teeth edges - Rc 39.4, 38.5, 38.0, 38.1, 41.0

This may be meaningless, and I might get a section done to get a scan of microhardness through the thickness of a tooth.
 
Back in the AIR!

After 10 weeks and about as many thousands of $$, I'm back flying! Ended up doing a major on the lower end, since it was apart, as well as new cylinder assemblies. I also rebuilt the fuel servo and distributor, starter and alternator. All new hosing as well.

It is like having an old friend back.
 
RV7ator said:
Alex,

Was there a root cause determination for the gear failure?

John Siebold
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John, that would not be possible, as one would have to have full traceability of the part (they weren't marking parts then), and one would have to have full manufacturing history of the part as well. My slightly informed opinion - the hardening was not properly done or through hardening was not done properly, not sure which path is the design intent. The hardness numbers reported earlier seem too low. Interesting theories aside, I doubt some mysterious combination of things on my specific plane had anything to do with it.
 
Congrats on getting the engine running again. What a mystery hey? Pretty sure old Murphy is laughing is buttocks off. :)
 
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