Pinion/Clutch Bell Stripped in flight

[Funky]

I flew 4 tanks through my Stratus today. The heli was flying great all day long. On the last tank I suddenly lost power while I was inverted and wrecked it. There was a really loud screaching noise before I lost power. I was confused because I heard the motor reving back up after the screach so I thought I had regained power but instead I just bled all of my headspeed and then it fell out of the sky. The clutch bell is stripped where the pinion screws into it. The heli wasn't even really stressed when it let go. I had just rolled it over and was hovering inverted for about a second and it suddenly let out a loud shreak and started losing power immediately. I am worried because I don't know what could have caused it. Everything on the heli is still spinning smoothly but the clucth bell is completely stripped out where the pinion connects. I tried searching for this problem but I can't find anything. Has anyone seen this problem before? Any guesses on why it would just let go? I have flown about 3 gallons through the heli before this problem.

Does anyone have a HD Clutch Bell/Pinion assembly cheap :roll:

[carey shurley]

can you post a photo please?

[Funky]

Did you want pics while it is still together or once I pull the bell out? Here is a pic of the bird now. Would you like a different angle/side/zoom? BTW, the bevel gear was stripped after the crash, I still had tail control on the way down.

http://www.helifreak.com/attachment.php?attachmentid=35394&stc=1&d=1200180736

Here are some other pics from today that I like a little better :roll:

http://www.helifreak.com/attachment.php?attachmentid=35397&stc=1&d=1200180866

http://www.helifreak.com/attachment.php?attachmentid=35396&stc=1&d=1200180866

[Funky]

I don't know if this is related, but all day I was getting a strange grey residue on my head. I wiped it off after I noticed it, but there was more on the head after the next flight. I haven't seen this before today.

http://www.helifreak.com/attachment.php?attachmentid=35398&stc=1&d=1200181438

http://www.helifreak.com/attachment.php?attachmentid=35399&stc=1&d=1200181438

[dogg5306]

Did you check your clutch? What about the clutch bell lining?

[Funky]

I haven't started pulling it apart yet. From looking at it inside the bird, it looks ok. The clutch bell spins freely around the clutch. I rotated the cluth with my fingers and it looks good and is not hanging up on anything.

It is very confusing. If I was in the middle of a flip or something that was stressing the heli, at least that could have possibly caused the failure... but it and was stationary in an inverted hover when it let go :confused: The heli was flying perfectly all day until the clutch bell/pinion stripped :( It definately stripped in flight and not after the crash. We all heard the screaching noise before the wreck and if I hold the pinion and spin the clutch bell, it makes the same screaching noise (although much quieter).

[Funky]

Here are a few more pics. If anyone sees anything or wants to see something something from a different angle, please let me know. I still don't see anything that looks like it obviously caused it :confused:

BTW, the 2 large gouges on top of the clutch bell have been there since I bought the heli and were not caused today. The bearing inside the clutch bell is still smooth and doesn't feel damaged. I also checked the 3 bearings in the bearing block above the clutch bell and all 3 bearings are still very smooth. All 3 mainshaft bearings are also good.

http://www.helifreak.com/attachment.php?attachmentid=35458&stc=1&d=1200201417

http://www.helifreak.com/attachment.php?attachmentid=35459&stc=1&d=1200201417

http://www.helifreak.com/attachment.php?attachmentid=35461&stc=1&d=1200202522

http://www.helifreak.com/attachment.php?attachmentid=35460&stc=1&d=1200201417

http://www.helifreak.com/attachment.php?attachmentid=35462&stc=1&d=1200202522

Here is a better look at the clutch bell and pinion after cleaning them up

http://www.helifreak.com/attachment.php?attachmentid=35476&stc=1&d=1200210124

http://www.helifreak.com/attachment.php?attachmentid=35477&stc=1&d=1200210124

[rstacy]

It looks to me like the cause was a loose pinion in the clutch bell that wasn't addressed in time..
The pinion continued to vibrate in the bell as you ran the engine and take away more material from the bell.
The residue that you noticed on the head was a mixture of clutch bell aluminum and exhaust oil.
Eventually so much material gets eaten away that there isn't anything left to hold the pinion.

[Funky]

Thanks Ray, I really appreciate your input :thumbup:

Wouldn't the pressure of the motor turning the pinion keep it tight though? Although, this definately sounds like a possibillity and your explanation makes a lot of sense. I have never had the bell/pinion apart so I have no idea what condition the threads were in for previous flights.

BTW, I only use normal mode to spool up and then I go into gov mode at 1900 rpms right after takeoff so rpms should be somewhat consistant for my flights. This bird ran VERY smooth too. Definately the smoothest running heli I own. The fan was dialed in to less than .001 runout. There has been no fuel dancing or foaming in the tank to make me look for possible vibe issues.

[DavidH]

Funky,
Just my opinion. I have looked at the pictures and have no idea why the pinion would strip the threads out of the clutch bell.
As to the pinion not being screwed tight in the clutch bell. I don't think that was the problem. The pinion has normal right hand threads. When the motor and clutch are turning the clutch bell. They will be turning the clutch bell the direction that will tighten it against the pinion. I have just hand screwed a pinion in a clutch bell and installed it in the heli. Then when I would try to change the pinion, it would be tight. I never apply loctite to pinion threads when I have installed them in a clutch bell due to this reason.
The residue you see on the head is from the plastic fan shroud rubbing against the head.
You take the fan shroud off and you will see a shiny spot on the cylinder head where the fan shroud has been vibrating against it.
I use that same cylinder head on my OS 91 CSpec. I had a friend machine the top fin off the head and then cut 4 mms off each side of the head. I can run the head with the shroud fully enclosing it without any problems.

David

[rstacy]

David,
I agree with everything you said and yes the engine should tighten the pinion but you left out the part about what happened to his clutch bell?

[Caleb Phillips]

If it was way over tightened, then it would be way over stressed. Maybe the previous owner overtightened it and since it's usually not one of the things you worry about when buying a used heli, you didn't know. I've never seen this happen before so that's my best guess.

[Funky]

Luckily it doesn't sound like a normal problem at all. I couldn't find a single similar problem while searching several sites. If I am the only one I am happy to chalk it up to an unfortunate fluke that shouldn't happen again. The heli has been flying like a dream so I can't wait to get it fixed and back in the air. I hate it when it looks like this :(

http://www.helifreak.com/attachment.php?attachmentid=35565&stc=1&d=1200274077

FYI, the hammer was only used for beating myself in the head after wrecking my #1 bird :arggg:

[Caleb Phillips]

I didn't wanna ask at such an unfortunate time for you, but what was the damage report? It doesn't look too bad. Also, how are your aluminum part so shiny?

[Caleb Phillips]

One more thing, I don't believe you needed to split your fan shroud in half. :wink:

[Funky]

The damage is pretty light. The spindle has a very slight bend. The clutch bell obviously needs replaced. The bevel gear that drives the tail is stripped. The horizontal fin is broken. One of the main blades and one of the tail blades both have a pretty good sized chip out of them. There are also a few small chips of paint missing on the new canopy, but they aren't too bad. Overall the damage is very low. I am really lucky I got it right side up before it hit. It came down hard on the skids, bounced and then came down on its side. For how hard it hit the ground, I was really expecting a lot of damage. This is one tough bird!! I just hope nothing got bent. This heli flew so smooth before, I hope that doesn't change.

... and I pulled off the fan shroud so I could clean up that residue on the head. That motor is new with about a gallon through it, so I have to keep it looking good :roll: The fan and pipe are also new with this motor which is probably why they still look so good. Also, despite buying the heli used, it is still under a year old from when it was originally purchased as a new kit. It was VERY clean and didn't look like it had much flight time before I purchased it.

[Caleb Phillips]

Yeah, I've had two crashes on the Stratus. First one broke a boom and bent the flybar and the second totaled it. My crashes always seem to be hit or miss. If I know a crash is eminent, I can usually hit t-hold in time and everything will be ok. Other times, the heli just seems to hit the ground to quick for me to have reacted. Glad to hear it worked out for you though. Check your ladders and make sure they don't have any cracks in them. Something else to look for before every flight is the pin connecting the TT to the delrin coupler (just under the elevator servo) isn't broken. Everything else is kinda standard. :thumbup:

[DavidH]

David,
I agree with everything you said and yes the engine should tighten the pinion but you left out the part about what happened to his clutch bell?

Nope I didn't leave it out. LOL

Funky,
Just my opinion. I have looked at the pictures and have no idea why the pinion would strip the threads out of the clutch bell.


Funky and me have exchanged a couple of PM's. I told him in the PM's that it was just probably a fluke and something that happened is unexplainable. I have not heard of the problem before,
As for the residue on the cylinder head, I believe that is from the fan shroud rubbing on the head. I have seen that same residue before and that was what it looked for when the shroud was rubbing the head. I use the same cylinder head on my CSpec motor. I had someone machine off the top fin and take 4 mm off each side of the head so the MA fan shroud would fit over it. That was just a better way to me to handle the problem than cutting most of the fan shroud away.

[carey shurley]

just out of curiosity, any idea how many flights may have been on the model before you got it?

I'm asking only to establish a timeline of how many flights from the original factory assembly before this happened

[Funky]

According to the previous owner, it was only run as a nitro for around 2 galons before it was converted to electric. When I bought it, he was running it as an electric and it wasn't using the clutch bell assembly at all. Since I bought it, I have put almost 3 gallons through it. Here is a pic of the bird right before I bought it.

http://www.helifreak.com/attachment.php?attachmentid=35588&stc=1&d=1200281235

[carey shurley]

probably no way to know for sure the history behind these parts. As David said though, its VERY unusual to see something like you've experienced. These parts are carefully primered and coated with thread lock. Its very hard to get them apart.

At this point I'd send the whole driver/clutch/bell/pinion/bearing block to the shop and have them look at it. They can replace the bell and check out everything else and you'll be good to go.

[Funky]

Thanks for the advice. Like you said, there is no real way to know how long the parts have been run, or how they were treated, etc. My heli had the typical "2 gallons" as a nitro and a short period as an electric..... just like every other motor/pipe/servos for sale, they have all only been run for "2 gallons" :roll:

I sent an email to MinAir to see if they have ever heard of the problem. If I can buy the replacement parts through them I most likely will do that. If they offer to look at them, I will gladly send them the stripped clutch bell if there is any chance of finding a possible cause to the problem.

[Funky]

For anyone that is curious, MinAir emailed me back today and said they have never seen this problem and It was most likely caused by someone removing the pinion at some point and possibly cross threading it when it was reinstalled. This sounds very likely to me, especially considering the scratches on top of my clucth bell prior to the crash which look like they would be caused by a set of pliers/channel locks.etc.

I will replace the parts and hope for the best :wink:

[DavidH]

Glad they emailed you back. Sounds feasible. The previous owner could have removed the pinion when doing the conversion to electric.
Don't know why the scratches are on the bell. When I have removed a pinion, I wrap a rag around the pinion and use the pliers where the pinion enters the bearing block.

David

[Dr.Ben]

Every once in a while MA gets some parts where the machine shop has used a tap that is dull or otherwise beyond its useful life. When a tap is dull, it'll cut very shallow, weak, and crappy threads. The pinion may have appeared to thread in at the shop with no incident, but there may not have been enough metal to hold it securely under load. I would concur with David that the metalic oil is from the head rubbing. Pull the shroud, and you should see the contact point.

The pinion should be OK. The bell will need replacing. That pinion should have been securely loctited into the upper clutch bearing block. If it was free, that was a problem unto itself.

Ben Minor