GAS EB Tail Transmission Dogbone Coupler Problem – Need Advice

[Dr.Rivet]

I’ve had 3 gallons through this machine since new and noticed that tail hub had a rotational play of about +-20 deg (40 deg total). Play has been increasing gradually over time and machine has not been flown very hard, just stall turns with couple of pirouettes on top. Today I removed the tail boom and found the problem, which is derlin dogbone coupler (part # 9010) with dimples worn into the grooves where dogbone pin sits. I run a driven tail, 611 gyro with whatever servo it comes with and a tail push-pull upgrade. Please let me know if this is normal or should I replace the coupler. I can see that coupler grooves have dimples formed only on one side where the torque is applied which is logical to me. Sorry can’t take a picture. I’ve tried but pictures don’t show clearly the damage I’m describing above.

Thank you for your responses!

[cbergen]

It is normal over time to wear in this fashion. I usually replace the delrin coupler when the play causes about 1/2" of rotational movement of the Tail Blades before taking up the slack.

It would probably last longer, but at some point the ends will meet, and the dogbone will complete the circle...:)

[Dr.Rivet]

thank you Chris for fast reply!

Can I slide the tail boom out about 3mm to avoid new coupler replacement for now?

Is there a metal one available?

[kelly steed]

I have that same proplem some times,when I run my tail blades deep in the snow and hit ice,when I stripe a main gear because the tails digging a hole,any kind of tail abuse starts to cause slop.I think It,s a good idea to change it,gives you a chance to do a tail to transmison inspection.

[cbergen]

3mm may be a bit much, you only have so much depth in the coupler.

A metal one would NOT be a good idea as the dogbone is also metal......

[rbort]

Replace it with a new one, but this time around fill the delrin coupler with white lithium grease before inserting the dogbone. This will help to minimize the wear damage.

-=>Raja.

[rkeith2]

There is no specifications in Bergen instruction manuals to place any grease into the coupler or on the dogbone assy.

I would have to pose the question as to what life increase of the coupler would be gained at the wear points from the forces placed upon the dogbone pin from rotational forces with a good Gyro / Servo combination in piro stops and reversing as well as normal correction.

[rbort]

These are not manual things but experience "updates" that people who fly a hell of a lot come up with.

What's a good name for them...? maybe "fringe improvements"?

Maybe some day they might be adopted into the manual :)

-=>Raja.

[cbergen]

Or there may be reasons why NOT to do something that YOU would not be aware of.....

Kind of like some of Ace's suggestions.....

In this case however, some lubrication wouldn't necessarily HURT anything, But I don't see a benefit either as the parts are not rubbing against each other except on installation or removal.

The continual pressure IN ONE DIRECTION is causing the wear ON ONE SIDE, so the two parts are spinning together...

We also "FLY A LOT". The difference is we are flying different configurations, different pieces parts, testing them, different models, different powerplants, under a lot of different conditions. All in an effort to find the best way, the best materials, to test our configurations. Does that mean that we don't take outside advice? Of course not. But we also look at from where the advice came from. Some one who is knowledgeable about our machines first, then can suggest an improvement from their experience building, setting up, AND flying our machines.

I take exception to the innuendo that we or our sponsored pilots do not "Fly A Lot".

[rbort]

I know nothing! But seriously, he was looking for an answer and I gave it to him. Grease reduces friction. The parts are rattling around in there and the grease dampens that. Vibration from the motor is what is wearing the delrin coupler from contact of the pins on it.

Try some grease and see if your service miles vary. As you said, it doesn't HURT anything, it can only help! Don't be shy to accept suggestions, improvements often come from modeler's experience at the field :) Yes Ace may not have experience, agreed, but if I take my experience and divide it by 100 its still probably more than Ace's so maybe I do know something?

Incidentally Chris, I'm flipping through May\June 2008 Rotary Modeler that I got at MHA and is that YOU on page 76 flying the Bergen Turbine in front of Greg Alderman? What year was that?? You look a little different...maybe fresh out of the service? You ought to get a copy of that and post the centerfold on your office wall!!

-=>Raja.

[cbergen]

If your delrin coupler is "rattling around" in there, you have other issues. The ball is a tight fit into the coupler.

[Dr.Rivet]

Thanks for all the advice! I'll replace the coupler after this weekend and will fly the old one next three days. Dimples are quite far away from each other and three days of flying should not make a difference. As far as Raja's suggestion goes, I'll try grease on a new coupler, but I don't think it will hurt or help anything. The dimples are formed by static pressure from the pin which exceeded Derlin's Elastic Compression Stress Allowable (i.e. Fcy). Only viable solution I see is to increase contact area by increasing diameter and length of the pin and to increase OD of the coupler thus making grooves deeper and wider to accommodate larger pin.

[Dr.Rivet]

Now that I thought about it, coupler is the cheapest part in the trany (vs gears) and probably is best left that way so it fails during blade impact.

Any other thoughts?

[rbort]

Now that I thought about it, coupler is the cheapest part in the trany (vs gears) and probably is best left that way so it fails during blade impact.

Would be nice but it never fails during blade impact. You'll strip the gears before you fail a coupler.

I'd say fill the new couplers with grease and see how things go for you - that's how I do it.

-=>Raja.

[Dr.Rivet]

Raja,

Do you own a Bergen? I thought you were a Spectra G and an older Xcell (I don't mean MS Excell, just forgot the name) guy...

I'll fill it with grease, thanks. I don't see any negative effect since it will be isolated from dirt penetration.

[rbort]

But a Bergen is essentially very similar to Xcells especially the tail drive coupler in question. I work on/maintain Bergens and fly them for other people though, like the Bergen Twins for a company and the Turbine observer for another fellow.

-=>Raja.

[lperagallo]

Guys,

I had an incident with the power train of the turbine and put it in last night. Upon inspection I found that the tail and torque are fine but the dog one coupler was broken.

So if you stop those tail blades quickly, the coupler will snap!:oops:

Lou

[rbort]

WHAT is going on??? Every time I turn around you've crashed again. Man, didn't you use up all your crash allowances for the year already? Sorry to hear about the crash...

About the delrin coupler, I've actually never broken one in 10+ years time of flying. They seem bullet proof but with your bad luck run, I think you're having all the possibilities!!?!

Still waiting for you to visit Boston someday and give me a call, would love to give you some "northern hospitality".

-=>Raja.

[lperagallo]

Raja,

Yeah, another mechanical issue with the turbine. I ended up dumping the heli because I had just started a turn into final when it let loose and I didn't have control of the heli with the power coming on and off like someone was pumping the throttle. Couldn't keep it straight and since it was pointed at about four guys on the flight line I decided I'd put in down fast rather then risk an accident.

As JC says If I didn't have bad luck, I'd have no luck at all. This is just making up for a relatively crash free last summer!

Lou

[rbort]

So what exactly happened? Instead of the delrin dogbone getting loose from the setscrew the delin coupler (you mean the white thing that you insert the dogbone into) broke in 1/2? You'd hardly expect that especially with smooth turbine power.

-=>Raja.

[cbergen]

Again Raja, you are assuming things of which you have no knowledge.....

Our Delrin coupler is black for instance, and he stated that it broke when it crashed!

The problem was with the power train of the TURBINE, which was recently rebuilt, but we haven't determined the exact cause yet.

Don't make the mistake of trying to guess what happened, and posting your "guesses" based on your "experience"...

[lperagallo]

Raja,

The dogbone coupler (delrin) broke after I put it in on the side and stopped everything very abruptly. It wasn't the cause of the crash, the turbine gear box failed.

Lou

[bcook01]

Lou

Was it bearings that caused the failure?

Sorry to hear about the prang..:shock: I question if you really need any more "experience".

Appears you really are aquainted with Murphy.

Bob

[lperagallo]

Bob,

I still haven't pulled it apart yet, but discussions with Wren indicate a failure in the bearing tunnel, not an actual bearing. There is a redesigned tunnel made out of SS which is stronger and can hold the bolts better. I'll post what I find once I get a chance to pull it apart.

Lou

[bcook01]

Lou

The tunnel is between the turbine and the gearbox?

how many hrs?

tnx

Bob