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HomeRC & Power✈️Aircraft🚁HelicopterEngines Plugs Mufflers Fuel › How did my YS 91SR-3DS break the crankshaft?
01-14-2013 02:00 AM  8 years ago
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rrKey Veteran

Morgan Hill, CA. USA

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How did my YS 91SR-3DS break the crankshaft?
WTF!?! YS 91SR-3DS with maybe 70 to 100 flights.
First flight of the day and the crankshaft breaks.
I flew it 5 flights 3 days before and nothing had changed.

Why did this crankshaft break?

Everything was going normally.
I am about 3 min into a normal 5 min flight. Good smoke trail, good power, no hard 3D.
Then, it misses a few beats, and then picks up. Misses a few more beats and starts to sound labored.
I've heard that sound before so I hit throttle hold and do a wonderful autorotation back to my take off spot.
The engine died when I hit throttle hold and was not idling when I landed.
I go back to my box, and the motor seems a little hot - but the starter shaft turns and it doesn't seem too bad.
I spin the motor with the starter - seems normal.
I hook up the glow and it starts okay - idles okay. Humm.
I shut down and check a few things - seems okay.
I start it back up, walk out to the box, spin up and lift into a hover.
Everything seems okay - I land, check the backplate - normal temp.
Spin it up, climb into a hover, hold the hover for about 20 seconds and it starts sound labored (loading up) and missing. Then, it stops.

When I try to turn it by hand, it is stiff and tight.
I try to spin it with the starter - and it is really tight.

When I took it apart on the bench, the counterweight & main bearing came out as a unit.

The last real "crash" was about 3 months ago when a linkage broke on the cyclic. But I had throttle hold engaged when it hit the ground.
01-15-2013 08:21 AM  8 years ago


South africa

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just a guess :

Cant see the crack clearly , but a half educated guess would be some fatigue there , some thing like this does not happen on the day it breaks , , how old is the motor ?, has it had a few "Hydraulic locks"
trying to turn the motor over with the piston locked over time will cause this , also possibly if there is a "Stress raiser" point along the crank at to point that it broke , check the crack line very carefully , is there a point that the diameter changes , even very slightly , or even a machining mark on the inside that made a "deepish scratch" ? that coupled with a few hydraulic lock conditions and possibly even a slightly over hardened shaft all come together to give you what you have , A 2 piece crank!

If there is any evidence of machining marks along the crack line , I would send it off to YS for a "manufacturing defect" claim
01-15-2013 07:03 PM  8 years ago

rrKey Veteran

Morgan Hill, CA. USA

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The motor isn't too old - maybe 70 to 100 flights over a 2 year period.

I never had a hydro-lock - that I know of. I use a Hobbicon 12v starter with 4s LiPo power - I doubt it would pack enough juice to cause any damage if it did hydro-lock.

I would go along with the stress riser - but I don't know from where.

I separate the broken part from the bearing and see if I can detect anything - scratches it dent or something.
01-15-2013 07:12 PM  8 years ago
Ace Dude



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I recall seeing previous posts on YS cranks snapping. It might be helpful to research some of those threads to see what the outcome was.  
01-19-2013 08:20 PM  8 years ago

rrKey Veteran

Morgan Hill, CA. USA

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I did a search on "crankshaft" and couldn't find anything in this section. Maybe it is on a different site?

I am going to take it with me to the LHS. The owner is a nitro guru.
Other than that, I guess I could call YS support in Reno and see what he thinks went wrong.

It is clear now that the labored engine sound and the high heat (after the first engine cut-out) was related to the broken back-half of the crankshaft and the main bearing moving back and rubbing the steel backplate.

It is just so weird that the engine would keep running pretty well even with a fractured crankshaft. I have no idea how long it was cracked / fractured. I figured it would just quit.

Every other internal part looks fine. All I would need to do is buy a new crank and have the engine running in 20 min.
01-19-2013 10:24 PM  8 years ago

rrElite Veteran

east coast

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I remember this happening to the old YS 90's. a friend had his break and he wasn't even doing hard 3d, in fact it broke during a loop. Get in touch with YS and see if they can help out.
01-19-2013 11:05 PM  8 years ago


Cookeville, TN

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Most likely an inclusion in the material caused the start of it and time took care of the rest. When the cranks are manufactured they are rough machined in the soft state, then once all of the rough machining is complete they are hardened and then ground to final size and shape. After the hardening process the parts are stress relieved. If the heat treatment and draw is correctly performed there will be little or no internal stresses and the final grinding will bring all of the critical bearing fits to final size while still maintaining the overall hardness spec of the material.

Typical hardess of a crank is in the high 56-58 Rc. To compare this hardness to something that everyone can relate to, a good quality file is 62+Rc in hardness. If the file will file the end of what is left of your crank this will give you an indication of how hard YS was able to get it during heat treat.

I have seen some cranks use a case hardening technique which is a very old method of imparting a hard surface to the steel and then machining off just enough material to size the part. This is not typically how a model engine crank is manufactured.

Part 107.
The only ZERO flight hour certificate in the world.
It's like getting a driver's license without the driver's test.
01-19-2013 11:28 PM  8 years ago

rrElite Veteran

Chicagoland area

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I experienced three broken cranks on YS 50's in the past. Very frustrating.Enjoy things that money can buy IF you don't lose the things money can't buy.
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