115T main gear fail investigation - HeliFreak
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Old 02-03-2017, 01:35 AM   #1 (permalink)
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Default 115T main gear fail investigation

I went to the field for one or two flights. I wanted to take the heli in the air, but the engine did not want to spin – like a lean. Of course – the needles were set to about +5C and it was -7C and the engine/fuel were cold.
I little richen the L needle and everything seemed to be fine (4strocking at about 12k for time to time, smooth running at 13.5k).

I turned on governor to push the engine to 13.8k and started flying. When was descending down under little negative pitch, the tail started fast oscillate from right to left. I compensated it by increasing cyclic load and I heard a “chrrrr”.

Because I was over a field and there was about 40cm of snow, I very carefully moved back oved landing zone. I was ready to cut the throttle anytime.

I landed safely and checked the heli (I thought that I stripped an umbrella tail gears during the tail oscillation). The main gear was stripped at two (2) places!



I started investigation:
• No bearing failure – correct gear mesh
• Pinion was OK
• Checked spark plug for lean running – it was OK.
• I noticed bubbles under the teeth on the gear, which was stripped (it was gear from HobbiKing for Assalut 700)

Root cause identification:
There were more issues, which came into play.
• The root cause are the bubbles under the teeth.
• Next supporting fact for the failure is very cold weather
• Next supporting fact is, that I did not readjust the governor limits (GV-1) and due the richer settings, the gov. could cause the tail oscillation.

I have to say, that I have over 50 flights with main gear in different condition (include cold weather, hard fly, tail oscillation, etc.… ). But all above mentioned facts were not present at one time…

Here are the “bubbles” (I will explain it later):


The temperature:


When all of the facts appeared together, the main gear failed!
Well, why there are the bubbles? Frankly speaking, those are not bubbles of air, but there was created a vacuum voids. Why?
Because of error in molding process. If you are molding a plastic part, you must supply next material during cooling period, because the material is shrinking during cooling period. The problem is, if you are not able to supply more material (e.g. the a gate seal got frozen, insufficient backpressure, bad cavity(-ies) design, etc.…) and due to the process in the plastic, there will be the voids and it means less material integrity.

Well, I had Align 115t CNC main gear (HN7019D) at home ready to change it, I did it. I test it during the same condition (include tail oscillation and hard cyclic load in low temperature) and everything was OK…

For comparison, here are the gears (Assalut 115t Molded and Align 115t CNC made):



I hope is might help to somebody to avoid similar situation.
If/when I have time, I will cut the Assalut 115t gear and will check the bubbles/voids occurrence.
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Old 02-03-2017, 06:55 AM   #2 (permalink)
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Nice save!
After having clone parts fail years ago, I have stuck with name brands.

Big difference in the support webs too, align uses more plastic with larger radi making it stronger. Also the polymers may be different.
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Old 02-03-2017, 07:00 AM   #3 (permalink)
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Bubbles like that indicate poor quality control in their injection molding process. I would not use Assault gears again and stick with the Align.

I have over 100 flights on my Align 115T gear, including flights in similarly cold temps, with no issue. One difference however is I run lower rpms - 12000-12900 at the engine.


BTW, the "CNC" align gear is injection molded also. They start with a molded blank for a general shape and just CNC mill the teeth to get an accurate mesh. Few gears are 100% completely CNC'd from a billet.
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Old 02-03-2017, 07:30 AM   #4 (permalink)
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Of course. I had problem to get Align 115t gears when I build the heli, therefore I ordered some Assalut ones. When Align "D" gears arrived, I prepare it for change, but I did not realize it.

But as I said – the Assalut one worked fine for over 50 flights. But I do not have any power plant there – I have stock 240RC Zenoah. If it is properly tuned, it has enough power to spin with 640x59mm blades…

If I find some time, I will cut second Assalut gear as well (I had 2 ones).
I have 2 pcs of Align HN7019A as well (it should be molded as well) – maybe, I will cut one as well. I will see.

You are right, the “D” are not fully made on CNC, but there is a footprint from CNC tool on the surfaces. Looks like not only tooths are CNC made.
If only the basic shape is molded, it means that there is enough space for hot plastic to flow to the outer diameter of the gear.



Of course, a big role play a molding tool – how is optimized. And injection process – how is optimized and monitored.
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Old 02-03-2017, 04:58 PM   #5 (permalink)
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I don't know what shipping would be but GrandRc here in the US has the Align 115T in stock.
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Old 02-03-2017, 06:06 PM   #6 (permalink)
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100 bucks its brittle plastic
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Old 02-03-2017, 06:16 PM   #7 (permalink)
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"it was gear from HobbiKing" LOL or is this not hobby king?
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Old 02-04-2017, 12:38 PM   #8 (permalink)
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Quote:
Originally Posted by Kap'n Krunch View Post
100 bucks its brittle plastic
Actually now that I think about it this is a good point. Looking at the photo again the molded Assault gear has a very shiny finish... Hope they didn't cheap out and make it out of something like ABS, that would grenade when it gets cold....
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Last edited by jharkin; 02-04-2017 at 03:32 PM..
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Old 02-14-2017, 05:29 PM   #9 (permalink)
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This place has moulded delrin 115T gears for $8. It's being sold for use with the Morphis gasser conversion. Probably better quality than the Gartt / Hobbyking gear.

http://gasserheli.com/Estore.php
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115t main gear fail, helibug 600




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