YS91ST rear bearing spins on crank.

[Hardcarve1]

I have had 20 tanks through the YS91 and was well aware of the rear bearing problems and made sure I had the spare OS bearings for their replacement.
I just thought that most of the problems were because of lean runs and/or hard work but after my 20 tanks of treating it like a baby the shakes and bearing noise just got to much. You now somethings wrong when a spectator (female) questioned the funny noise coming out of the heli.
So I pulled it down after todays flight and inspected through the rear back plate and noticed some surface rust. It was when I rotated the crank I noticed that the inner race on the rear bearing was not rotating with the crank. I then proceeded to completely disassemble the motor and sure enough the back end away from the crank pin was shinny as well as the inside of the bearing. From my experience I would normally find that the surface of the crank were the bearing sits would have a discoloration from electrolysis or corrosion. I put the OS bearing on and it was a much tighter fit and much smoother.

Is the bearing spinning on the crank normal ?

As for the bearing, true to form the balls and outer race were pitted. I treated this thing with respect which was highlighted by the fact that only a slight orange colour residue was left on the top of the piston (CP LV) and no bluing around the exhaust port. I wiped the top of the piston with a clean cloth and it came up like new so I,m sure I had not abused the motor.
Alex

[capebob]

Is the bearing spinning on the crank normal ?


A resounding no to that question. I've had extra bearings sitting around here for a while now, but haven't had the occasion to use them yet. I guess I've been lucky but not lucky enough to win any fuel. :shock:

Bob

[conrod]

Ok..I had my fan loosen up on my new 91 and became paranoid about the brass collet dust gettingin the front bearing. Soo...I ordered a new front and rear high speed bearing from boca. After 4 flights I noticed that it was harder to turn the motor over by hand (using the fan) along with it running a little warmer. Pulled it out and noticed the same thing! the crank spun inside the new rear bearing when rotated to a certain position. Like yours, the crank had the polished look...not blue from overheating. Soo...I've put the old (5 gallons) rear bearing back in and was thinking of sending the boca one back. I measured the ID and both were the same. It's only .005" larger than the crank including at the polished area where it rides. My original bearing didn't look pitted, just looked new. Haven't tried it out yet. Even considered using red locktite to lock the inner race on, but discarded that idea. I'm going to run it and if it blows up, I'll send it back. Might have to call PS to find out what's going on in the mean time. Let me know what you find out as I too am very curious.
conrad

[Hardcarve1]

I will assemble the motor and see how it goes. I have never been happy with the smoothness of this motor and even when I fist started it up it was very rough at idle. My only experience has been with the OS 61 motors and this YS is very ordinary compared to them. I will give it one more month of playing around with and if it's still rough in the bin it will go and back to OS. I will check the fan again but I have checked this twice on a magnetic balancer and spinning up with compressed air it showed extremely little vibration.
I will keep you guys informed.
Alex

[conrod]

put the stock one back in as mentioned previously and it rocks!
Don't know what the deal w/ the boca brg was, but gonna leave it alone and fly the piss out of it!
conrad

[Hardcarve1]

I put new OS bearings in the YS 91 and it goes OK. Can't say that it's any smoother so I have been leaning it out and trying to see if tuning is the issue. The thing sure does haul ass and is very easy to tune but I still find it has heaps of vibration.
Alex

[conrod]

What's your top end and needle settings?
what muffler/pipe?
Where is carb barrel when hovering?
What headspeed/gearratio?
Clue us in and maybe we can help.

[Rodney]

Bearing problems. I had the rear bearing fail in my YS91.
The bearing has no rust or pitting but the cage that holds the balls broke and went everywhere. I sent it back to ys. Waiting on there reply as to weather they will replace it. Motor has never been ran lean. Ran breakin settings until Jason Krause tuned it at Birmingham so I know that was not the cause. There was no markes on the piston at all. Other than where cage part got up intake port and marked it on top.

[Hardcarve1]

I pulled the engine apart because the vibration is just a joke and noticed when I removed the back of the case that the new OS bearing was also spinning on the crank. When I removed the crank their is further evidence of polishing on the crank surface. The intolerance would get worse as the temperature increases and this slop is what I'm sure is giving me the bad vibes.. I have ordered a new crank and will see how that turns out.
When all this is done I will be sending the crank back to YS with an invoice of expenses.
Alex

[conrod]

latest update...Mine is still running strong with original rear bearing.
Leaning it out some will smooth things up a bit.
Freakin ROCKS! I love this motor!!

[WillJames]

but I have checked this twice on a magnetic balancer and spinning up with compressed air it showed extremely little vibration.


Did you dial indicate the fan on the motor? Amazing how much vibration even a couple thousandths can cause, even if your fan is perfectly balanced off the engine.

I to have had a crank that spins in the rear bearing and I used red locktite. Was this a bad thing? The motor ran fine after that. It did not run any different that I could tell. I gave it away as a parts engine when I was done with it because I cracked the case where the muffler bolts on in a doinked auto, so it was not really tested over a long time after I used the locktite. Probably just a few gallons before the doink. It was an OS engine.

[Hardcarve1]

I purchased 3 sets of OS rear bearings as each one had a different fit to my new YS crank. I didn't want any problems with this new crank so I spent the money making sure the bearing fit was tight. I finally found the right bearing and put the crank it in the freezer and heated the engine and bearing up to temp. With the different temps the crank just slid down the onto the hot bearing and after a few seconds when the bearing started to heat up it locked the crank hard. I could not pull the crank out when it cooled down.
I pulled the back plate of this engine after about 30 flights just to see how the bearing was and guess what? it also now spins on the brand new crank. The inside race of the bearing is polished along with the crank shaft. So I put this down to how these engines must just run like this and don't worry about it any more.
By the way I found the vibe problem, I went along to a heli setup day and let the Xcell field rep look at my heli fly. As soon as he saw the hover he said that my blades were the problem. I found this hard to believe because I had checked them and they seemed very good when I checked the balance 4 times. He gave me a loan of his SAB blades and I went for a fly, My machine flew with only a hint of a vibe which was tuning related.
I will never ever buy again this popular brand of blade as it cost me many hundreds of $ trying to chase down the vibration and I almost gave up heli's because of it. I fly SAB blades now.
As for the spinning rear bearing in the YS91, I give up as the motor runs so well I will just live with the problem.
Alex

[conrod]

chordwise balance Is important....maybe more so than spanwise and total weight.
:wink:

[Hardcarve1]

I agree, span and chord balance where checked and where extremely close.
The problem I have been told is that the blades may have been removed from their mold when still green and not fully cured. If you put them on a flat suface they seem very flat and looking down the leading and trailing edges they also look straight. It must be the profile is different between each blade and different lift is being produced between the blades.
Great for mixing paint.
Alex

[DavidH]

agree, span and chord balance where checked and where extremely close.


Chordwise needs to be better than extremely close. Chordwise needs to be the exact same.

David

[conrod]

Hey...maybe some will realize that if CW is out, then their spanwise is out too when flying the blades :)
Lead/Lag will be off if CW is out.
RotorPro is the way to Go!