I've been using carbon fiber rod for pushrods lately. For the 1/2A stuff, I use .070 solid rod epoxied to brass ferrels. On short runs, they work without support, but need a midlength guide to prevent bowing on longer runs. There is essentially zero stretch. However, the rod is too light to use on .15 sized and up.

1/8" OD pultruded tubing seems to be about right with sufficient stiffness for larger models. However, I wasn't sure how well I could get the 1/16" wire to adhere in the ends or what might be the best adhesive. So the experiment ---

Pultruded CF Tubing: Purchased from Kitebuilder.com
ID: .068
OD: .125
Grams: 10.13
Length: 48"
Price: 2.95

I prepped the CF tubing with acetone wiping the outside and using an acetone soaked balse stick to swab the inside. I roughed up 1/16" wire with 80 grit and wiped with acetone. Brass ferrels about 3/16" long were cut and prepped. I made up two test rods, one using Elmers ProBond polyurethane and the second with 30 minute Hobby Poxy II. The wire was inserted 3/4" in both and both cured 24 hours.

I drilled a 1/8" hole in a 2x2 deep enough so that about 1/4" of tubing below the ferrel was exposed -- I wanted to see how well the wire was retained and not break the rod off. I set the 2x2 on a scale and pushed down on the wire end with a steel plate. At about 80 pounds, the epoxied wire was slowly pushed into the rod -- it was not an immediate breakdown, just slid in. I was able to load the ProBond up to 130 pounds, but it held.

Frankly, I was surprised that the ProBond beat the epoxy and that it held up at 130 pounds (tested it twice at 120 and once at 130). Bottom line is that either adhesive will produce a bond strength greater than any of the other parts on the model can withstand.

Since CombatPigg has his Thrust-O-Meter, I guess I can label my bathroom scale a Push-O-Meter.

That's very good information, thanks for posting it. I am about to use CF pushrods for aileron interconnects on my Dave Patrick Ultimate, so this test came at a perfect time for me. From what I've read, JB Weld is the adhesive of choice for this application; I wonder how it would compare to the Polyurethane.

Something to consider. The epoxy is likely only in the area between the the wire and the tube whereas the ProBond may have expanded into the area behind the wire creating a hard plug that you were essentially trying to push the wire through. Would a pull test take as much force?

That's pretty amazing strength! That's right up there with the strongest hobby servos you can get.

The ProBond did expand into the tubing -- my original intent was to make up a tube with one end epoxied and one with ProBond and pull on each end to see which failed first. However, there was a small amount of ProBond showing in the end of the open tube. Rather than try to clean it out and have the epoxy perhaps not hold as well, I made up another tube with epoxy. But, even if I can get 70 pounds of pull strength, I think that would be sufficient except for the largest model.

I will try another test with JB Weld and ProBond this weekend -- if I epoxy first and poly second, then I can try the pull test. Another adhesive that is supposed to be pretty good is the newer black or rubberized CA, but I don't have any to try.

Either way, the ProBond is impressive.

Andrew, why not go with a slightly bigger tube? A tube is always stiffer than a solid rod and you can go bigger with the same weight. Might not need to worry about a support for the longer runs.

I found a CF tube that is a nice tight fit over 2-56 threaded rod, also. I have been having trouble finding 2-56 aileron horn brackets. The last ones that I bought were either CG ,GP or DB. You know what I mean, the little nylon fittings that connect the servo clevises to the aileron torque rods. I can only find them for #4 and #6 thread now.

CP,

DuBro makes the nylon connectors for 2-56, but you have to buy the whole torque rod set to get them. I've been using a piece of inner ny-rod with a hole drilled in it for 2-56 torque rods. Here's a pic of what they look like before the holes are drilled.

Thanks for showing me that and confirming that the 2-56 knuckles are history. I bought a SIG nylon C/L bellcrank and have been making my own knuckles out of it. I just put on my best pair of glasses and hack the pieces out with a razor saw, and drill and tap the material. The park flyer hardware is too small, and the 1/2A aileron hook up kit costs more than what I have invested in most of my planes!

I've also made them out of servo horns.

I agree -- this sounded like the most plausible explanation and the existence of a hard plug would certainly compromise what I was trying to find out. I did a depth check tonight and if there is a plug, it's less than 1/16" thick. So the ProBond may indeed be that tough -- which still surprises me. The JB Weld is setting up now -- I will ProBond the other end in the morning and try pulling.

Patrick --

You're absolutely correct and if I were using larger rod and tubing, the tubing would be the best choice. However, at the diameters I'm using, the pultrusion process is apparently not capable of producing very thin walled tube -- maybe for similar reasons that we have big shaft holes in the small diameter APC props. In any event, the first available tubing size is more than twice as heavy as an equivalent length of .070 rod -- so I'm kind of stuck with more weight or some support. But, I can always combine the support with a mid-fuselage former.

I made up my aileron interconnects today using the same size CF tube that Andrew used. Since these are less than 9" long, and for the sake of simplicity, I ran a piece of stainless 2-56 threaded rod all the way through. I figured that a couple of extra grams wouldn't hurt on a .40 sized model and I wouldn't have to worry about it being able to pull out. I didn't have any aluminum or brass tubing in the right size so I made aluminum ferrules and glued everything together with CA.

Pull Test Update -- 6/5/05

I made up another pushrod -- 2" of pultruded 1/8" OD CF tube, 3/16" long brass ferrels, 1/16" wire pins. The wire was well roughed up and inserted 3/4". Everything was cleaned with acetone and one end was glued with Elmer's Probond (polyurethane that activates with water); I used JB Weld on the other end. The JB weld had set up about 43 hours; the ProBond has about 30 hours.

I could not manually pull the pins out. I wanted to have some sense of how much weight it would withstand, so I suspended a pair of vice-grips from a treelimb, snapped a second pair on the other end of the pushrod and stood on a scale. I weigh 210 -- so I pulled on the pushrod until the scale read 100 pounds -- I'm assuming that the rod was carrying about 110 pounds of pull. I figured if something broke, I could have a face full of vice-grip so I stopped. I loaded the rod to 110 pounds twice.

Results:

30 min. epoxy failed at 80 pounds of push.
ProBond loaded to 130 pounds of push without failure (a check showed no plug of ProBond behind the wire -- good point by Bipe)

JB Weld and ProBond did not fail at 110 pounds of pull.

So, with a little care in cleaning and surface prep, either JB Weld or ProBond joints will give adhesion strength greater than the structural strength of most of our planes. I'm very impressed with the performance of ProBond -- one part and easy cleanup. One note of caution: Choose your carbon fiber tubing with an ID close to the wire size to avoid too much of an adhesive bridge.

Thanks for doing the pull test. It proves that ProBond is more than adequate for this application and is a good replacement for JB Weld.