It happened!!!!

[KTMDirtFace]

Not sure "hammering away" at it is valid. at least on mine.. But yea I don't like that slop. when I first got the heli i thought there was something wrong with the maingear assembly.

I put a brand new mainshaft in took off in a hover.. switched to idle up and it snapped.

[DoubleCH]

Originally Posted by 3vw

Number 1, blue locktite won't do a damn thing to take up the slop between the undersized bolt & the oversized hole. The best thing is to get the two to mate/fit properly.

Then it's probably another testimony OWB helps. Before maiden I filled the jesus bolt hole on the main shaft with blue loctite before putting on the bolt and made sure to take up the slop by holding belt gear and rotate main rotor CW before letting loctite dry overnite. After some flights I removed the jesus bolt and blue loctite was still holding up. Remember I had OWB mod BEFORE maiden. But since then I disassembled and reassembled main gear lots of time I decided to forget about putting loctite on.

Number 2, if the material is truly the fatigue factor (too soft) and not the lack of material around the hole then the hole size won't make much difference... If you can appreciate my point.

I strongly believe it's more about too little material around jesus bolt than too soft material. That's why suggested what I did about not recommeding drilling out jesus bolt hole.

If the shaft itself is (now) strong enough to begin with it won't snap because I choose to remove a few more thousands for a proper fitting screw IMHO.

Doesn't matter if you use the strongest material in the world. If there's too little material for strength any material will break.

I can only hope everyone will look into what I shared here and find the new shafts that HH is now selling. Maybe then this snapping issue will become more rare than common.

All the main shafts I've tried (original one, 2 as replacement in 1 pack sent from HH) have sloppy jesus bolt holes. I still have another pack of 2 sent from HH that I haven't opened to check yet.

[DoubleCH]

Originally Posted by toys2cars2toys

Yes the shaft is weakend by over drilling and maybe by the sound of it, poor hardening

but stop the frame flexing totally and almost all of the problem will be taken away?

What loads are on the weak part of the shaft?
- bending due to frame flex
- twisting due to torque
- axial elongation due to vertical flight loads

I think if you can eliminate frame flex, then the shaft (albeit poor) will be strong enough against the other forces - in flight at least, maybe not in a crash

Of course even with that said, it would be much better to have a stronger shaft too

The problem is that bearing 1 and 2 are locked together, but there is relative movement to the 3rd, the only way that the shaft can accommodate this relative movement is by bending, the fulcrum of the bending force is right at the point of the hole

If is was just a case of failure due to weak over drilled shafts, then why aren't we seeing the same amount of failures in the hole the holds the head on?

I think main shaft snap is caused by all of the above combined, but I think most importantly frame flex combined with main rotor out-of-balance/track and wobbly main gear can significantly accelerate metal fatigue. Imagine the can-opener effect. You flip the lid back-and-forth to break the final bit. Imagine on the main shaft. A significant enough out-of-balance main rotor and wobbly main gear are like trying to bend the main shaft back and forth every 3000+ times every minute. A main shaft that's weak enough to be bendable just a tiny bit could be asserted such repetitive force 12,000 times per flight. That most likely explains why a brand new main shaft can snap in its very 1st flight.

[DoubleCH]

Originally Posted by kpod

I think the acceleration and deceleration forces cause the small bolt to twist back and forth in the large hole, hammering away at it until it fails. The owb eliminates the deceleration hammering. Filling the gap would help too. The frame flexing could accelerate the damage as well. Most heli's that I've had don't have that third bearing though so i'm not sure that its hurting anything.

Base on KTMDirtFace's experience snapping a brand new main shaft on maiden, perhaps it's indeed more about vibration that keeps bending the main shaft 3000+ times per minute. Perhaps my OWB mod helps not by eliminating whiplash stress on the jesus bolt but by the fact that OWB will allow main gear to twist a bit freely on main shaft (i.e. self-centering effect at speed) to reduce the force trying to bend the main shaft every 3000+ times per minute.

The other helis with 2-bearing main shaft support like Align 450 Pro runs jesus bolt below main gear. This probably further prove that it's more because of bending of the main shaft than whiplash stress on the jesus bolt that accelerates metal fatigue.

Actually, from all the pics of snapped main shaft, the snap point looks more like metal fatigue than pry-bar effect from jesus bolt.

[syzygyQ]

Encouraging to read a report that the main shafts have been seriously improved.

My original stock main shaft looks noticeably weak and cruddy on the inside, with an appearance of thin veneer on top. There's probably some simple name or descriptive term for this material known to metallurgists. My knowledge is restricted to to a gut feeling of disappointed recognition when I see that kind of weak interior exposed on a broken metal part that looked good on the outside. I believe I've seen it before in cheap tools that have snapped.

My initial belief was that the main shaft was engineered to fail, but got over done so that the shafts would fail under regular use. Perhaps the "new" shafts will tend to retain their integrity under the strain of regular operation.

I suspect thought that even if made with high strength steel, the main shafts will continue to be an impact fuse. A good way to test this hypothesis would be to bind a main shaft in a vise, attach something like vise grip pliers to the other end, brace the shaft so that a blow on the vise grips would impart only rotatioal pressure on the shaft, and then strike the vise grips with a rubber mallet. If the shaft readily snaps at the bolt hole on moderate impact, then we would have confirmation that the "new" main shafts continue to be fuses.

[DoubleCH]

Like I've suggested just above, I believe that the rotational force is just one smaller contributing factor to snap the main shaft compare to the constant bending force from:
1. a wobbly main gear without a OWB to buffer the twist and,
2. an out-of-balance/track main rotor

especially when KTM can snap a brand new main shaft on the 1st hover, 3000+ times bending back-and-forth every minute and 12,000+ times in just one flight.

[toys2cars2toys]

If it's possible to remove all frame flex so that all 3 bearings are rigid, then the forces due to an out of balance head will not reach the point where the lower hole is drilled

In that case bad balance would probably show up as a top bearing failure

[347mont]

I had mine snap on me too the other day. Did a back flip and snap, same common place through the

bottom bolt hole (main gear). First main shaft snapped went 1 minute into my 2nd hover with this bird.

The MS failure the other day was the 2nd time it has snapped on me. Its a bummer crashing because of

mechanical failure, I'd rather crash because I screwed up lol. The way my 300x has been flying lately

I really do have a soft spot for it, its ripping up the sky and so smoothe with the 2214-13 and is a power house.

Too bad build quality was off from the start ie weak MS crappy plastic frame, motor that will vibe sooner or later etc.

[nartac]

I can't say if I'm lucky or because I did some preemptive measures from events you guys mentioned. I have a lynx frame stiffener on as well did DoubleCH OWB mod and haven't had any crashes except for the one caused by a brown-out. Since installing a cc 10 bec running at 6v, no more BO since. I do think the three additions that I did have a lot to do with how well my 300x has been flying along with well balanced main blades and tail

[syzygyQ]

I'm going to add the OWB bearing to my to do list of projects to try.

Are these the right parts?

-stock main shaft
-this T450 OWB
-item 50415K162 or equivalent for the shim
-stock 300X main gear

To do the job I need to cut the shim to the length of the bearing, is that right? What's the best way to do this -- would a dremel tool with a cutting attachment be ideal? What about using a dollar store hack saw?

Apart from the shim cut, is there anything else likely to be involved that goes beyond standard wrenching?

Thanks!

[DoubleCH]

You need a OWB hub from 450X. It comes with OWB. If you're lucky and get a good OWB you don't need to buy the Align OWB. You need to drive the OWB up inside the OWB hub (best if you heat the aluminum hub first).
https://www.helifreak.com/showthread.php?t=503141

I also filed down the 450X hub for extra clearance to the lower frame deck around the bottom main shaft bearing. You use 4-to-5mm sleeve adapter if you have OWB sleeve for 450X 5mm main shaft (or any other 450 heli) but you'll need to machine it as the ridge on the one for 450X or any other 450 heli is usually not in the right place of the right size. Or you can just get a 4-to-6mm stainless steel tube 50365K311 on the link you posted. Wow that's pricey. You'll need to cut the thick tube and machine the end to make it smooth. I don't think that's the way to go. I remember nartac saying he bought some tube that's supposed to be 4mm ID but wouldn't fit. Either way it requires machining work at least on the 450X OWB hub. I'm planning on a new version that requires no machining but it still needs more than just simple wrenching. If you can wait, you'll certainly hear from me soon on how my new version goes.

[steve54]

So I have not broken my shaft since i put a lynx frame stiffener on. I had 4 or 5 shaft breaks, 2 resulting in frame rip aparts, so i got the stiffener and wala, no more broken shafts. I do change out the main gear hub every time i crash because i think it gets out of true vary easy due to the soft material and causes vibes, i have a bunch of crashes now, darn servos failing in flight and hitting my batt cutoff at bad times, 5 or 6 and have only bent the main shaft like a normal 450 would in a crash, i saw a pic of the lynx auto rotation hub but havn't seen anything since. I sure would like to try one of those out

[syzygyQ]

DoubleCH -- I'll be interested in the non-machined OWB project.

Steve54, usually heli pilots can avoid LVC through lipo monitoring. What kind of lipo charger do you use?

If you've got "good" lipos that your 300X is cutting out on, I would suggest that you consider an ESC upgrade. I upgraded to the YEP 45A ESC. It's cheaper than the stock ESC, more powerful, a high quality build, and has configurable extra slow spool up, governor mode, and the option to configure your LVC very low, or even turn it off (which I've done). Turning off LVC is risky, but it's a manageable risk, and puts a stop to premature LVC. The YEP 45A ESC feeds 6V to the servos, which shouldn't be a problem, as I believe the stock servos are sold for use on other Blade helis that take 6V stock.

YMMV, and there's also other good ESCs out there, including ones that take BL Heli (great if you can get the programming tool).

[steve54]

It hits the cuttoff well past the timer, i somtimes am having too much fun and fly till i hit it, usually around 6 min with the pulse 1350, i have the align 35A esc on her now. when i land at 5 min i am always ok but usually put about 1100 mah back in, my esc hits its cutoff with these pulse 1350 at around 1250mah so it is holding its voltage prettygood.