Bob Yeager

My Feedback: (5)

This is almost too strange to be true! I have had two carbon fiber pushrod end failures. Both failures were on the left aileron pushrod where the pushrod end screws into the DU-BRO Safety Lock Kwik-Link. Both breaks occurred at the exact end of the Kwik-Link. Both failures occurred after approximately three flights each on my Uproar ARF, not exactly a high performance aircraft. The hinge point is located directly over the hinge gap. The servo connection is a DU-BRO swivel ball link. It is my feeling that if there is any flexing of the pushrod assembly, it would occur at the hinge point. In both cases I was luck enough to recover the aircraft and land safely.

I am attempting to tap into the vast knowledge base here on RCU in the hope that someone can shed some light on this situation and tell me what I am doing wrong. Any and all input will be greatly appreciated. Thank you for viewing this post and any insight into this situation which you can provide.

Tall Paul

I think your threaded end at the surface stripped as noted in this thread.
http://www.rcuniverse.com/forum/m_40...tm.htm#4067904
I wouldn't use those things, now that I've seen this.

red head

Call DU - BRO and ask if there has been any other problems with this item that they know of. They are a good Honest company and if there is a problem they NEED to know. ENJOY !!! RED

da Rock

Looks like the two parts that failed did so where the threading stops on that shaft. What is that part made of? Is it metal?

And the shaft that appears to be threaded surely is a small diameter. Everything else looks robust. That area looks feeble. No wonder there could easily be a stress concentration that'd overcome that small diameter. And the diameter is less where the threading is. And stress risers happen where the threading stops.

If you have any more of those fittings, why not give them a test by hand. Try to break them.

Looks like each end of the rod has the same fittings. Since they're adjustable, and you don't need to adjust both ends, did you have one end screwed tight? If so, the stress on both fittings that failed would have been at the same location, right where the threading stops.

Test all that you've got in use now, while you can. or.....

BTW, if I were you, after I'd trimmed the model, and gotten those connecting rods set at the length they'll probably remain for the rest of the model's life, I'd epoxy those "gaps" solid. You're not going to need to adjust again. And it'd be a waste to throw out something that can be easily strengthened.

Bob Yeager

My Feedback: (5)

Many thanks to Tall Paul, red head, and darock for your input. The part that failed was the rod end at the point where it is completely "captured" by the Kwik-Link threaded channel. This is a rather poor description, but I seem to have a problem at the moment accurately describing the actual failure point. The actual failure point was in the threaded section of the rod end, not where the threads end.

In any event, here is an update. Just out of curiosity, I threaded a 2-56 nut on a new rod end. It was a very SLOPPY fit. I then threaded the nut on a Harry Higley 2-56 tap. This was an acceptable fit. Evidently these rod ends have a thread which is slightly smaller than true 2-56.

This is what I may have done. I have no memory of doing it, but I may have tapped the Kwik-Link with the Higley tap, thus creating a slightly oversize threaded channel in the Kwik-Link. There may have been just enough "slop" in the threaded channel in the Kwik-Link to allow the rod end to flex just enough to fatigue the threaded portion of the rod end. This is all that I can come up with at the moment. I will assemble a new rod without tapping the Kwik-Link and see what happends.

I did try to break the undamaged rod end with thumb pressure without success. It doesn't seem possible that this tiny amount of "slop" would be enough to fatigue the rod end to the point of failure, but this hobby is a constant learning experience. Again, many thanks to all who replied.

da Rock

Bob,
The pattern boys at my field were talking about a problem they had some time back. Seems that a certain make of engine is notorious for very strong vibration. And seems that make has a history of beating aileron servos to death. Seems there was some reason the aileron servos got the worst of it but they didn't know what that was. However, they all knew the story 'cause most of them had lost servos when they switched to that brand of engine. And all of 'em knew a similar story about a certain brand of shock absorbing mount that did the same thing to aileron servos.

I just helped run a pattern meet this last weekend and overheard the discussion, so don't acutally know if I even got the hearsay right.

But what I do know for sure is that a sloppy linkage to one of our model surfaces is an invitation to flutter. And I also know for a fact that flutter is one POWERFUL force, even in our little old toy airplanes.

fwiw

Digger44

My Feedback: (4)

Just wondering, are these 2-56 ends made by Du-bro. I purchased some from AbellRC that were 4-40 and they seem very strong.

tmproff

My Feedback: (5)

The ones from ConnectorsRC.com are made of 303 Stainless steel with 4-40 threads. They're very strong and will not break at the threads....

gene6029

Bob, i use the 4-40 links made by du-bro on everything includeing All my 35 & 40% planes. Been useing them for years & this is the first I've heard of any failures. It wouldnt hurt to switch from 2-56 to 4-40, since you have such a short push rod length & their wont be any significant weight difference. I make my own push rods also, useing carbon fiber rod & 4-40 threaded rod JB welded together. I do use a ball link at the servo end though.....Gene

Lnewqban

Can the vertical horn rotate easily?

If not, the breaking point is seeing a flexing load coming from the transverse movement of the servo horn.

It fails there because that is the weakest point of the mechanism.

TFF

Just some thoughts; is the ball link binding at full throw? And although I have the same arms on some of my planes, I did not like the way the long links pulled; it seemed to me that it wants to pull, in a line, the control surface before it wants to rotate on the hinges; I moved my threaded part in and re-drilled the hole.

jaka

Hi!
Vibration!
Lets see how your engine is mounted and prop!

Bob Yeager

My Feedback: (5)

Good Morning All:
Thank you all for the fresh responses to this old thread. I obtained these rods and ends from Abel RC in Billings, Mt. After the second failure I replaced the control rod with an old steel cheapie that I had already made up which was resting in one my junk boxes. No further problems. What I am unable to understand is why the two failures on the one side, and no problem with the other aileron which is set up exactly the same way?

I am still flying this old girl on an almost daily basis, with no further problems. The CF pushrod on the right aileron continues to function perfectly. She is my "Back Seat Beater" airplane. I would not care to estimate how many flights this aircraft has. I do know that she is starting to show her age!

I believe that Inewqban probably has the most logical explanation. There must have been some type of twisting motion involved which stressed the rod end at the threads.

Regards To all, and thank you for the input!

Stick 40

Hi,
I am setting up a PT-17 with carbon fiber push rods and someone put me on to the ones from www.darrolcady.com and I order all of it from them.

I put together a test of a 2 ft. piece of 1/8" carbon fiber tube using 2-56 threaded rod with the IC-2000. I did it just the way they told me to. Then I hung a 43 lb. cement block on it for a week. Its still good, did not pull out a bit. I am very impressed with this stuff. I did put a 2-56 nut up against the carbon fiber and alum. tub, just in case but that had nothing to do with the cement block and a week of pulling on it.

This IC-2000 is great stuff, I will be using it a lot.

sticks

carlgrover

This.

I never run aileron servos mounted like that. The servo should be rotated 90 degrees so it is in the same plane as the control horn. That eliminates the side to side motion. The servo mounting position may not be the number 1 reason the pushrod is breaking but it certainly isn't helping. You might not be able to rotate the servo in this plane but for future reference I wouldn't do it like this again.

If you can find Bill Newman's excellent book, "RC Control Systems" at a swap meet, it's explained in there.

carl

Weasel Keeper

My Feedback: (16)

I think Lnewqban nailed it by asking this. I've never liked this style of control surface horn for this reason. You are relying on the control horn post to not rotate AND that the nylon part does not turn on the threads under compression. If either happen, you develop a lateral force on the control rod.