Main Rotor Shaft Failure

[JustPlaneChris]

What diameter is the main shaft?

This is bringing up bad memories from some of the horrible bad old days of trying to hop up micro helis with a 3mm main shaft....

[GSPI]

The main rotor shaft is 4mm in diameter.

[ukpaul]

Worse than that, 3.95mm according to my guage lol, with a 2mm hole through the middle!

[GSPI]

I was giving my main rotor shaft the benefit of the doubt. I would say a hair under 4mm in diameter. I have been trying to figure out the shear stress required to snap this shaft and with the hole you have more deflection which depending on the makeup of the shaft could be the reason the bolt shears the shaft rather than the other way around.

[salmans21]

Just wanted to add that I've broken 2 main shafts in only 4 flights on my Heli. I have only flown my heli in idle up 2 (I prefer the straight 75 throttle curve). Both shafts broke after aprox. 1 and a half batteries (6 min) of flight time. Both times the heli started flying strangely so I brought it into a hover about a foot off the ground over tall grass. I haven't damaged any parts (not even a blade strike) on either shaft break. Heli actually landed on it's skids both times. On the second shaft break I did notice that my ball link that is over the motor had a nice chunk missing from it (I think it must have hit the motor when the shaft broke and allowed enough play for it to bump against the motor). I have flown another battery through the heli and have already started to feel the increased jitteriness that preceded the other 2 shaft breaks. I estimate that I'm about 2 minutes of flight time away from breaking another main shaft and will attempt to record the shaft break while on the phone with Horizon tomorrow afternoon. I took my heli to the hobby shop where I bought it and had the owner (30 years experience) comb through it. He said that the problem is in the blades. Although they balance correctly, when he spooled the heli up without the main blades there was little to no vi ration in the tail. When he reattached the main blades and spooled it up we could see a pretty significant vibration in the tail. He thinks that part of the problem is coming from the wooden blades but he says there has to be more to it than that. He checked belt tension and beastx settings, blade pitch, and blade balance. I'm really disappointed and think that at this point the only thing that would make me happy would be a replacement heli. I can't believe that I sold my 3 month old mini Titan e325 beastx, savox servos all around for this problematic heli!

[GSPI]

Vibration at a high frequency can cause fatigue and eventual shaft failure. I also have to wonder if you actually have a frame fracture present too.

[GSPI]

I started the tear down on my 300X. Preliminarily I see that the case hardening is fairly deep and a little uneven on this shaft. It had what appears to be fatigue failure of the inner material so far at around the bolt hole. I'll be looking at that under the scope tomorrow. Early indication is that the way the failure occurred appears to be from torsional force and possibly a two stage failure that still has to be determined once under the scope.

Also eyeballing the head assembly with the head on the SR that is on the bench... the shaft size is the same so maybe that could work too!?

adding photos for documentation of the tear down.

[jaydekay]

Originally Posted by GSPI

Starting this thread to document the crash on flight #9 of my 300X due to an apparent main rotor shaft failure...

Is your blade tracking perfect? Any vibes from bad tracking could easily snap that shaft over time.

Not to insult your intelligence, but I have a feeling that some newer guys (which you may not be) are overlooking their tracking on this heli and it could definitely be the cause for both vibes and busted shafts.

Good luck.

[GSPI]

Having spent so much time with the SR and working out blade tracking issues on it after belting its tail, the tracking was one of my first checks on this heli along with gear mesh, linkage travel, programming and so on. I am learning this heli like I did the SR and the 130X and the others, each one has a quirk or two so if you find them, you can deal with them.

I'm glad you are posting these suggestions though as that may help others too so you are not insulting my intelligence by saying it. I am trying to figure out what happened in the flight to cause this shaft to snap so that I can avoid the situation in the future and maybe give other HF members some ideas in what to watch for, it's all good. I have been an on again off again heli guy for at least 15 years but have been building all types of boats, cars, and planes from scratch and kits for the last 35 years so I'm not unfamiliar with problems but I'm always open to suggestions.

I also have been looking over other posts here on the site to help me get a full picture of what is going on with the 300X and I have a few comments that have brought interest to what I'm looking at. One is Dylwad is testing alternative motors and he notes how much power the 300X takes for such a small bird which makes me think about the loading going on in the mechanics of the belt, bearings and internal friction generated from possible frame loading which could contribute to why this has to take so much force to make it work and can also contribute to the fracturing of the shaft. The blade tracking that you brought up also is a factor as well as blade balancing and possibly the mass from the wood blades vs carbon or plastic fiber maybe. To note, I think you mentioned something about the cord of the wood blades being a problem too. There was also something about the temperature affecting the expansion and contraction of the blades. Then there is the fact that someone also stated that they are having the plastic sheeting on the blades starting to come off which will also generate vibrations.

There are many factors here so I'm open to looking at all of them to see what the issue is and so that's why I also say, keep bringing up what else you think might be a problem.

[GSPI]

Notation for my thread: https://www.helifreak.com/showpost.p...3&postcount=24

I have a marked up part of the B300X manual describing what I am finding and it is looking more and more as if there is an issue with uneven frame flexing going on causing this fracturing of the main rotor shaft. More information to follow.

[toys2cars2toys]

Originally Posted by GSPI

Notation for my thread: https://www.helifreak.com/showpost.p...3&postcount=24

I have a marked up part of the B300X manual describing what I am finding and it is looking more and more as if there is an issue with uneven frame flexing going on causing this fracturing of the main rotor shaft. More information to follow.

Shame we can't merge the 2 threads, I've just posted my thought in the other thread, in summary my feeling is that if it was blade or balance related, on any other heli, this would show with issues in the top bearing or the shaft between the top bearing and head

I agree, Isolate the shaft from the flexing frame and you should isolate it from the stresses that are causing it to break

[GSPI]

Ok, the photomicrographs of the failure of my main rotor shaft looking from the bolt attachment hole for the gear assembly up towards the rotor hub at 200X magnification. The fatigue shows a fracture of the material mainly from an axial load (Pitch up and down along the major dimension of the shaft) and then shear forces from the bolt and the remaining attached material of the shaft when the shaft was separating. Based on the failure of the material it goes to my theory that the airframe flexed and caused forces to fatigue the bolt hole and since the gear/belt pulley assembly is also jacketed in metal it created a "vise" in a way and bent the shaft during rotation and caused the main rotor shaft to flex beyond it's deflection point and broke.

To note, the dark area is the case hardening (excess of 950*F) crystalline structure of the steel shaft and the shiny area with striation marks is the shear portion of the failure when the shaft started to separate. This shaft appears to be hardened using a minimum of Nitriding method possibly even Cabonitriding due to the tempering I am seeing on the fatigued portion of the failed shaft.

I now have to look at the airframe for fatiguing and fractures.

[GSPI]

Originally Posted by toys2cars2toys

Shame we can't merge the 2 threads, I've just posted my thought in the other thread, in summary my feeling is that if it was blade or balance related, on any other heli, this would show with issues in the top bearing or the shaft between the top bearing and head

I agree, Isolate the shaft from the flexing frame and you should isolate it from the stresses that are causing it to break

Bringing over your image that you posted:



This is spot on so far with what I have found on the fracture of the shaft. The airframe does contribute and very likely causes this shaft to fail. I do not believe that the main rotor shaft material is necessarily the problem.

[toys2cars2toys]

Originally Posted by GSPI

Ok, the photomicrographs of the failure of my main rotor shaft looking from the bolt attachment hole for the gear assembly up towards the rotor hub at 200X magnification. The fatigue shows a fracture of the material mainly from an axial load (Pitch up and down along the major dimension of the shaft) and then shear forces from the bolt and the remaining attached material of the shaft when the shaft was separating. Based on the failure of the material it goes to my theory that the airframe flexed and caused forces to fatigue the bolt hole and since the gear/belt pulley assembly is also jacketed in metal it created a "vise" in a way and bent the shaft during rotation and caused the main rotor shaft to flex beyond it's deflection point and broke.

To note, the dark area is the case hardening (excess of 950*F) crystalline structure of the steel shaft and the shiny area with striation marks is the shear portion of the failure when the shaft started to separate. This shaft appears to be hardened using a minimum of Nitriding method possibly even Cabonitriding due to the tempering I am seeing on the fatigued portion of the failed shaft.

I now have to look at the airframe for fatiguing and fractures.

Amazing, that's a good macro lens you have on your Kodak! Are you planning to pass this on the HH?

I'm going to play around with frame bracing, but it's going to have to be pretty comprehensive

[GSPI]

Originally Posted by toys2cars2toys

Amazing, that's a good macro lens you have on your Kodak! Are you planning to pass this on the HH?

I'm going to play around with frame bracing, but it's going to have to be pretty comprehensive

Yes, I am planning on passing on my findings to HH.

That lens happens to be part of a Nikon compound (bi-chromatic) microscope, Nikon Instruments optics with an LED light source and a Nikon digital camera attachment... no Kodaks were harmed or used to make those photomicrographs...

Once I evaluate the airframe it will make sense where to brace it.

[honkytown]

very cool!

[toys2cars2toys]

Originally Posted by GSPI

That lens happens to be part of a Nikon compound (bi-chromatic) microscope, Nikon Instruments optics with an LED light source and a Nikon digital camera attachment... no Kodaks were harmed or used to make those photomicrographs.

I bet your partner loves your pillow talk, (sorry)

Here's my idea for a frame stiffener, shown in green, made out of CF sheet, fixed using the mountings shown, which would have to be through screws

This is without looking at my 300, some areas (such as around the motor screws) would need doubling up and relieving around the raised sections

It would need one on each side, the other side would be much harder due to the elevator servo, it could be a weaker version

The red section is a secondary idea, have a look at the flexing without shaft video, the servo frame lozenges, a fillet bracket to the flat "gyro" tray would help to stop this



It will be interesting to see how this develops, I have a 130X which has lots of little problems, most of which appear to fixable with very (very) careful set up and maintenance. Where as the 300X looked like an absolute winner (I still think it is) If the problem turns out to be frame flex related, I can't see the HH would have any option but to do an urgent update, either a new frame or strengthener

[stevebaker]

The flex in the frame appears to be causing lots of problems, namely;
broken shafts,
rubbing servo link
vibes

I now have mine stiffened using the carbon brace on the right hand frame and also Honytown's lexan mod - 3 pieces of 0.5mm lexan in between the rear servos.

I actually discovered some more rubbing on my servo link today. I was quite surprised as I have had the carbon stiffener on, early on. When I looked closely I realised the brace had come unstuck at one end.

Having now glued it back and done Honkytown's lexan mod, it feels much stiffer. I did a test hover in the garden and immediately noticed the vibes that have been plaguing me seem to have gone - I'll confirm this tomorrow at the field after I've put a few more flights on it.

[salmans21]

I just got off the phone with Horizon Hobby. They sent me a set of new main shafts after my first snapped on my second flight. I explained to them that I snapped a second main shaft on the 4th flight with the heli. I shared some of theories going around on the forums regarding the main shaft breaks. He basically completely discredited anything that I suggested from the constant vibration, COG blade issues, to frame flexing. He simply said that I must have received 2 bad main shafts!!!! and that since they sent me a package of 2 main shafts I should just fly the one I have left. When asked what I should do if that one breaks, he simply said that I would need to send the Heli in (at my expense). He is the second person at Horizon to tell me that I am the first person calling in with a broken main shaft on the blade 300x. At this point I didn't even want to speak to him any longer. I thanked him for his time and hung up.

I just ordered a set of carbon fiber blades and will look into somehow stiffening the frame. I don't think Horizon is gonna be any help with this issue!

[ukpaul]

Originally Posted by salmans21

I don't think Horizon is gonna be any help with this issue!

Maybe someone in accounts has been tapping away on the calculator and worked out what it would cost them to fix this issue