repeated R 90 tail gear failure

[secondchildhood]

I'm hopeing the Helifreak gang can help out with this problem. My heli instructor and flying buddy recently switched to a R90 from another brand 90 looking to save a few bucks on crash cost. He is a "HARD CORE" 3 D flyier and facing the fact that when you push it to the limit every time, you're gonna have to rebuilt occasionally;^)
The problem is that although we've followed the minimal instruction in the Raptor manual on setting up the tail gears he has had two really bad crashes (in 2 weeks). Both were caused by tail gears basically shucking or melting the teeth and locking up. Today when it happened he had 5 fantastic flights prior and was flyin it like he stole it. The high loads he was putting on the tail in backward flight, piro flips, and funnels finally took it's toll and whack, there went another tail gearbox exploding. It literally blew the tail box off the back of the boom. Needless to say , it's now wadded up pretty bad again. He's a heck of a good flyier, and he accepts the fact that they all get bent up now n then, but he is now beginning to question his choice in the R90. 2 Crashes in 2 weeks caused by the same mechanical problem is kinda bumming him out.

On the first box that failed, he set up the gears with no play , but not "tight". One shim on each side. After that didnt work, he set the second set of gears up with one shim on the yoke side of the gear, and none on the pinned side. It was slightly looser than the first, but still showed pretty much full contact of tooth mesh. I'd guess that if you ran a piece of newspaper through the mesh it would then be "tight". I know that there are plenty of guys flying their R90s even harder than my buddy and havent heard of any tail gear problems until now. I assume it's just some kind of setup problem and we're evidently missing something important.
Any help would be highly appreciated! I owe this guy big time, and could never repay him for all the help he has given me. Finding a fix to this problem would be a small start though :^)

Thanks
Dwight

[HelicopterJohn]

I believe I heard from someone a while back where they were putting more than one tail shaft support inside the tail boom to better stabilize the tail tube. It is also possible that the tail tube support inside the tail tube could be moving and causing the tail tube to start a whipping action. I have not flown mine in any hard 3d stuff like you described but I would hope others would possibly comment on the multiple tail shaft support to see if there is any merit as a possible solution.

One other thing you could do is to e-mail Alan Szabo and see if he has anything to offer.

Hope this helps in some small way.

:)

[secondchildhood]

He's running two support bearings in the boom tube, trisected, per the manual.
I was mistaken on the shims. Turns out that on the second gearbox failure he was running NO shim washers on either side. Still same results.
7 flights total per gearset then bang.
I know there's got to be a fix for this, or we're just setting the gears up wrong. After watching the Szabo videos, he's punishing his much more than my buddy so he must have it figured out :^) If someone has Alan's email perhaps they could send him a link to this thread. His comments would be highly appreciated.

Thanks again,
Dwight

[steve9534]

I talked to Alan Szabo about the tail gears and he claims he's never had a failure. I've had one set fail in flight and another that was chipping off pieces of the teeth, but never actually failed. I fly in some fairly thick and tall grass so the tail is always hitting the grass when I sit the heli down, but I don't know that I actually hit the t/r in the dirt on either set of gears. What I am now doing is to use at least one and usually 2 shims on the left side of the output shaft between the gear and the left side bearing which serves to tighten up the mesh. The gears may be a little notchy at first, but since doing this, I've not had a failure. Hope this helps. steve.

[WillJames]

I fles 3 Raptor 60's and then 90's for almost 2 years and never had a tail gear failure that did not follow a tail strike or crash. I ran 3 bearings in my R90's and 2 offset in my R60's. Also, pull the tailboom out a mm or 2 helps keep the torque tube running true. Once it starts whipping, you better land and get it sortred out as TT whip will cause you troubles all over the machine. Did he notice any tail vibs, specifically up and down, but also could be side to side before he had his trouble? What I did was add a 3rd bearing block to the boom, and also locktite the bearings to the tube before inserting into bom, nad never had TT whipping again. I ran the carbon TT's in my R60's.

HTH,

[alexander]

No problems here either but we apply grease to the gearbox. I run minimal gear lash also and no shims.

This is most likely your problem below:

If you shove the boom into the frames too far or the shaft is too long by position in the tail boom then it will shove the forward TR gearbox pinion aft into the output shaft pinion and burn up the gears. Assemble the gearbox half to the boom with the shaft installed and see if the gear is loaded aft. If it loading the pinion aft then pull the shaft forward in the tail boom.

stephen

[secondchildhood]

I dont think he was having any vibes in the tail, at least none when in hover, as his machine is smooth as silk. Of course when under full tilt 3D loading it could be doing them then and not be noticeable to the eye.
Adding the extra bearing in the torque tube sounds like a good idea to help prevent whip. I'll mention that to him. He sets the tube into the frame full depth then pulls it back out a 1/16th or less so as to not preload the gears. We will check the position of the driveshaft in the tube and insure that it is not preloading the tail gears. The fwd gears are origional and have survived all this with no signs of wear, which really suprises me. Steven, I am curious as to what kind of grease you use on the nylon gears. I would think that might help, but am concerned about the chemical reaction between the grease and the nylon.
Once this problem is sorted out he's going to love the R90. He says it flys as good as his previous (high$$$) did and will be much more economical to fix when the inevetible "oops" happens.

Thanks for all the input guys! I really appreciate it.

Dwight

[alexander]

hi,

Mobil 28 because I have it but automotive wheel bearing grease should be fine. I've tried other lithium type greases successfully and not had a problem with similar plastic gears in other machines like century falcon. I had the same question years past about the gears being weakened by the solvent in grease, but so far it has not bitten me. I remember my old H2 triple wack-a-saki years ago had a plastic oil pump gear submerged in transmission fluid so with that reasoning I proceeded. Model fuel pump gears are plastic too and take a bath in methanol fine. Many gasoline carbs on lawnmowers seem fine these days. There are silicone greases available which might be fine too. The thing is you don't want the grease too thick and add frictional heat. I fill the gearbox half ways or better.

stephen

[WillJames]

I used white lithium grease on mine, but I have heard a lot of people say not to grease them. It worked for me, but you know how that goes.....

Best of luck sorting out the problem!!

[secondchildhood]

As Alexander mentioned, there is lots of nylon and plastic used submerged in very hot oil and transmission fluid on your modern day autos. I can see why its not recommended though because using grease on the exposed gears would collect so much sand, dirt and debris that it would be shooting yourself in the foot there. Inside the TR housing would keep the contaminants out and in most cases a little lube is a good thing. Possibly the dielectric silicone grease that a lot use on the rubber dampeners might be a good compromise. I dont think theres any petroleum in it since it is recommended for rubber etc.

Whatever ends up working, when we're satisfied we've fixed the problem, I will post the results .

Thanks again, Helifreak Rules!

Dwight

[steve9534]

The problem is that he's not using the shims correctly and the mesh is too loose. You need to use at least one and maybe two shims on the left side of the gear that's on the output shaft. If you can run any paper of any thickness between the gears, it's too loose. I spend 3/4's of my flying time going backwards or sideways, and since shimming the gears as described, have not had a failure. The gears will not turn perfectly smoothly, but will be a little "notchy" when first assembled this way, but will loosen up after 2 - 3 flights. If you put any shims at all on the right side of the gear, it will be too loose, and failure is pretty much gauranteed. I always grease my gear box, the gears may not need it, but the bearings will love you for it. Hope this helps. steve.

[secondchildhood]

As promised, here's the follow up on what fixed the problem.

He has burned a couple gallons since the last tail gear failure so we're assuming that it is fixed. He thrashes it pretty hard, so I'm pretty satisfied the problem has gone away.
Heres what the final combo was.

Replace the forward gears that drive the tailshaft . They looked good but had a little slop in them-- not much, but some. It was suggested that this may have beat on the tail gears due to the slop.

Made sure the tail driveshaft was straight. Brand new one from TT was 15 thou out. He used an arrowshaft straightening tool with dial indicator to do this.

Added the 3rd bearing to the driveshaft to prevent whip.

Set all gears up "snug" with no freeplay, but not "tight" or notchy.

Boiled all the new gears before installing them. This was a new one on me, but a local Raptor guy said to put em in a pot of water, bring it to a boil, then shut the heat off and let them slowly cool in the boiling water until the water is near room temp. (He swears it makes them stronger, and has been doing it for years) :dontknow Whether this made any difference or not I have no idea.

No lube of any kind.

At least it appears to be fixed now .

I guess "whatever works for ya" I know most folks never have this problem, but if ya do, this might help you trouble shoot it.

Dwight

[WillJames]

Thanks for the follow-up Dwight. I am glad you guy sgot things sorted out. I had never heard of boiling the gears. Interesting.

Thank you!!

[LuckyLandings]

Boiling the gears would harden the material as it cools I would think.

[steve9534]

The plastic material tends to get "dehydrated" and brittle, much like an old piece of leather. I'd read of the airplane guys boiling their nylon props to make them more resilient and less likely to break, but had not thought of doing this to the gears. I think I'll try it with the next set. Thanks for the info. steve.

[Mike Fortin]

I have been flying Raptor 60's, 70's and now 2 90's for about 1 1/2 years and have yet to have any tail gear fail. I don't boil them but I do add a touch of lithium grease to the gears.

Well, good luck with the new set.

[secondchildhood]

5 gallons thru the machine and still going strong! And this weekend he beat on it like a redheaded stepchild. Extreme flipping piros, tictocs, funnels etc in 25 mph winds. He shows his heli no mercy. I's say it is definetly fixed.

I'm betting that the slop in the front gears, along with the shaft being out a tad was the origional culprit. Be sure and check your tail driveshaft for being straight before installing it.

Dwight

[AMainMaker]

I have a R90 and have had the same set of gears in it for over 5 gallons of good 3d. I did finally toast them in a tail touch and brok one ofthe gears in half. In that whole time I have never used any shims in the front or the rear set of gears. there is some backlash too, and it hasn't seemed to affect flight in any way.I do run the 3rd bearing on the torque tube though. That seemed to get rid of alot of vibration and shakes I had in the past.
Jason