Prop fell off!!

[Bluebird19kf]

So today I was flying my kf3 and after takeoff at 300 the engine reved up uncomanded. There was no thrust. Turned towards the airport and made the field. I shut down the engine and the propeller was still spinning. After it stopped I pulled on it and it came off with the gearbox shaft. It's a b box. Anyone ever heard of this?

[Timberwolf]

That’ll get your attention. Post pics when you get the box pulled apart....interested to know what failed. Glad you and the plane made it down safe

[efwd]

Damn! how did it not separate from the engine? Glad your safe.

[avidflyer]

If the bolt holding the drive gear on the crankshaft came loose and the gear loosened on the tapered crankshaft, it would probably do what you describe. Guy I know had his prop fly off when the keeper that holds the driven gear on to the prop shaft came off. The prop went flying across the yard then. Good thing he wasn't flying it then. JImChuk

[Danzer1]

If the bolt holding the drive gear on the crankshaft came loose and the gear loosened on the tapered crankshaft, it would probably do what you describe. Guy I know had his prop fly off when the keeper that holds the driven gear on to the prop shaft came off. The prop went flying across the yard then. Good thing he wasn't flying it then. JImChuk

Possible, but for the prop shaft to come out of the housing the bearings would have to completely disintigrate. If the crank gear bolt is intact upon inspection, the likely culprits are: badly out of balance prop, prop strike or lack of lubrication of the bearings or combination of those.

In another thread you mentioned 190 hrs on the engine and stuttering in midrange. Obviously something was wrong for this to happen on a 190 hour engine. Could the stuttering actually have been vibration? If so, the problem might have been one of the above all along.

[Bluebird19kf]

I took the gearbox apart. Nothing appears to be wrong although I haven't cleaned up everything and inspected closely. The bearings feel fine. The previous stuttering problem ended up being mis-adjusted choke valvle. I am running the 68" 3 blade Ultralite Ivo prop. Is that not good with the b box? Is it really just the split washer that fits in the shaft groove holding it in? Seems crazy it is that easy for that to fail. Also, I don't think the gearbox has been inspected since new and has like 350 hrs. on it. Is it likely that the spring washers got weak and that causes the split washer thing to come out of the shaft groove?

[Danzer1]

I took the gearbox apart. Nothing appears to be wrong although I haven't cleaned up everything and inspected closely. The bearings feel fine. The previous stuttering problem ended up being mis-adjusted choke valvle. I am running the 68" 3 blade Ultralite Ivo prop. Is that not good with the b box? Is it really just the split washer that fits in the shaft groove holding it in? Seems crazy it is that easy for that to fail. Also, I don't think the gearbox has been inspected since new and has like 350 hrs. on it. Is it likely that the spring washers got weak and that causes the split washer thing to come out of the shaft groove?

I must be missing something here? The front bearing inner is pressed onto the shaft and the outer is pressed into the housing. I don't see how the bearing could be intact unless heat from free spinning (between shaft and bearing) caused the inner to expand and free itself from the shaft, allowing the shaft to come out without the inner half of the bearing. I'd look there for damage to - shaft wear, inner race wear.

[avidflyer]

I was just looking at a YouTube video of a guy overhauling a b gearbox. Itsbeen a year or more since I had one apart, and I was questioning my memory after another post was put up on this topic. Watching the video, you can see that if the keepers are removed that hold the large gear on, the propshaft can then move forward. When he reinstalls the prop shaft on reassembly, he taps it a little to get it in place, but it's not pressed on. Interesting side note on the video, is the reason for overhauling the gearbox is the prop had hit something while cruising, and all the teeth were knocked off the small gear. That would also allow the prop to freewheel, although it's not your problem. Were you able to figure why it freewheeled when you tore your gearbox apart? JImChuk

[avidflyer]

Danzer, just reread your last post, and have to take issue. The front bearing is pressed in from the inside of the gearbox case. Its held in place by a snapring. Then the propshaft is pushed in from the front. I would post a link to the video I mentioned, but I'm using a tablet cause my PC is messed up. Just search for b gearbox overhaul on utube.. It's about 16 minutes long. JImChuk

[Danzer1]

Hi Jim,

Just looked at what I think is the same video. If you look at about 6:38 he puts the front half of the gearbox in a press to drive the shaft out of the front bearing.

Then at 11:38 he puts it back in the press to put the keepers in - that is also where the shaft gets pressed into the bearing - that pressing is to 3600 Lbs. Point is - it is a sufficiently tight fit for the bearing to do the work and the shaft not to spin in the bearing.

Of course the keepers would have had to come out or disinigrate for the prop gear to come off of the shaft too the shaft to come out of the case.

https://www.youtube.com/watch?v=B_pn4XGnaHU

[Danzer1]

Just looked at it again - I am wrong on the 11:38 part. Pressing to put the keepers in does not press the shaft to the bearing. But I know the shaft has to be sufficiently tight to the bearing to prevent if from spinning in the bearing. Possible it all gets tightened (pressed) when the halves are assembled and the front an back bearings are both "squeezeed" to the shaft when the housing is torqued.

Greg

[avidflyer]

I'm sure it is a snug fit between the bearing and the shaft, but not a pressed on fit. He does press the shaft out, but says he taps it in. I think he just used the press rather than pounding on it. Still wondering why the prop was freewheeling on the airplane in question in this post. JImChuk

[72chevy]

I'm sure it is a snug fit between the bearing and the shaft, but not a pressed on fit. He does press the shaft out, but says he taps it in. I think he just used the press rather than pounding on it. Still wondering why the prop was freewheeling on the airplane in question in this post. JImChuk

Disclaimer-Just a dumb mechanic here who works on semis & has never looked inside a b-box other than the video posted and recommends having a professional make your final evaluation.

It looks like to me if the split washers came out, for whatever reason, you would get exactly what the original poster has described. From what I see the propeller shaft is driven by what I believe the youtuber described as a dog gear, the dog gear is driven by the large gear that slides on the propeller shaft. So if the split washers fail the dog gear & large gear can decouple allowing free spin, and also allow the propeller shaft to be pulled out of the front of the gear housing. I also think this failure could likely show little to no other damage other than dislodged or broken split washers, also mentioned by the original poster. A close inspection of the parts should reveal the cause of failure though. Now, without going into any details, that b-box looks like a terrible design to me and I can’t believe they sold it for aircraft use.

[avidflyer]

Now I'm wondering if I can't read and remember any thing or what. I never got that the prop came out in the original post, or did the post get edited and I never went back and reread it? Reading the first post now, I would say 72cheavy is right in his comment of what caused the problem. As far as those keepers go, that's pretty much the same design that hold valves in place on every 4 stroke engine. The guy I mentioned in my first reply that had his prop fly right off the plane, had a prop strike before this and never inspected anything before he ran up the engine. Is there more to the story? JImChuk

[Danzer1]

Well, I guess we all agree as that is exactly what I said yesterday morning in post #10! I guess we'll have to wait for Bluebird to confirm/deny.

Greg