carbon fibre pushrods

Hello mellow fellow modelers,
Got some 3/16 carbon pushrods with the carbon steel ends from Tom Morris, upon checking the fit, there seems to be no clearence so that the epoxy will be pushed down into the rods.. How do I keep enough epoxy in the rod tubes that I have a good (and safe!!) fit. So that nothing comes loose.. Am I making my question clear?? Sure don't want to do something wrong and cause a crash!!
This sure is a good board for modeling!!!!!!
Gil:P

Hi Gil:

I noticed that same tight fit on the last parts I got from Tom also.

I use a small needle file to rough-up and create a sliding fit.

I also file longitudinal grooves into the metal ends to prevent them from rotating.

It takes very little epoxy to form a shear bond. But mine were so tight that the tubing split when I pushed the end in.

Make certain you wrap the end of the finished rod with kevlar or dacron thread and coat with epoxy or Cya. the failure mode is that the rod can split and the end comes out.

I also drill a small hole (about 1/16 dia) about 1/2" from the end of the rod. When filled with epoxy, it forms a "key" to help prevent a pull out.

Hope this helps.

I, like you, was concerned about not having enough epoxy to fully adhere the two parts together. On the composite ends, I drilled a small hole (off center enough to miss the 4-40 threaded rod) and put a pin through it up to the head and bent the other end over ann cutit off just past the bend and then epoxied the pin in place also. On the metal ends, I rough sanded a small area on two sides (enough to feel the roughness and then applied the epoxy all around both inside and outside and immediately pushed the two parts together. I have never had a failure but also have not seen any evidence that they needed the "extra" preparatoin that I did. I haven't heard of anyone having a failure of Tom's stuff.

Bigiron

I also got some CF pushrods from T. Morris. I measured wrong and had to make my own from CF tubing. I find that the CF is sensitive to splitting, even using a Dremel cut-off wheel. And any resistance putting the wire into the CF causes splitting.

I go back to my old standby: 3/16" or 1/4" square balsa, spiral wrapped with silkspan tissue saturated in Ambroid. The strength is in the outer layer, like monocoque. They don't bend and are light weight. Sometimes, modern stuff isn't always the best.

I cut the pushrods to length then force fit a 1" length of aluminum tubing over the outside of the carbon fiber tubing at each end, effectively making a ferule which prevents the carbon fiber tube from splitting. A drop of CA locks the aluminum tubing in place. It is then an easy task to use a drill and bit to get the proper size hole for the pushrod ends to slip into. JB Weld completes the assembly.
Chris

Fellows,
These are sure some good suggestions for the make up of the ends !!
This board has some great modelers that are willing to share their knowledge, and that is another thing I like about flying stunt!!
Again fellows, thanks a lot...
Gil

First off, you should ALWAYS use J-B Weld when assembling CF pushrods and threaded ends. These rods are manufactured by Central Hobbies. We have been using them for about six years. When we first started using them we consulted the guys at Central Hobbies and this is what they told us: "use 24 hour J-B Weld when assembling the rods, nothing else". At that time they had sold over 1000 sets and never had a failure when they were assembled properly.

As for getting the J-B Weld inside. I simply put some on the rod end with a toothpick and begin pushing it down in, then removing the end and pushing it back in again. I do this several times and it always works. When you put the second in together the air inside the tube will try to push it back out, so it it necessary to make sure there is some pressure against it to keep it in place. I will also rotate the rod a couple of times as it is drying to make sure there is an even distribution of the J-B Weld.

Steve,

As cool as CF pushrods are, I think maybe they are overkill. I went to the archery shop and got them to give me a bunch of busted fiberglass arrowshafts. These are so incredibly strong, I can't see why anything else is needed.

The CF kits I got earlier were all the same diameter as the fiberglass ones, and were noticeably HEAVIER. Yes, I don't doubt they are stronger. But the fiberglass is more than adequately strong. So, CF seems to be an overkill, and extra expense as well.

The same argument can be applied to wrapped balsa pushrods - if they are adequately strong, what else is needed?

Seems like you could go to 1/8" CF pushrods or something, maybe you already are..

[photo not recovered: 3e69bb3870f12ad5.jpg]

"If it is not right do not do it; if it is not true do not say it." -Marcus Aurelius

For my Combat models, I've been using aluminum brazing wire. 1/16" for 1/2-As, 3/32" for the bigger planes (including Fast Combat models. Brace well, don't bother with clevises (we don't need no steenking clevises ).

>
> Hello mellow fellow modelers,
> Got some 3/16 carbon pushrods with the carbon steel ends
>from Tom Morris, upon checking the fit, there seems to be no
>clearence so that the epoxy will be pushed down into the
>rods.. How do I keep enough epoxy in the rod tubes that I
>have a good (and safe!!) fit. So that nothing comes loose..
>Am I making my question clear?? Sure don't want to do
>something wrong and cause a crash!!
> This sure is a good board for modeling!!!!!!
> :P :P Gil:P :P

Ron B.
F4Fguy

While it may seem counter-intuitive,more clearance and more adhesive is not indicated in most cases.The ideal fit for epoxies (and most other adhesives) is a slip fit with minimal clearance.The best bonds are those which put the bond line in shear.The more the separation,i.e. the thicker the epoxy,the weaker the joint.A good adhesive bond is a molecular phenomenon and is stronger than the adhesive itself.Roughening the surface helps,not because it "locks" the joint,but because it promotes wetting of the surface,and cleans the surface to promote the molecular attraction.Smooth,chemically clean surfaces would be better if they could be obtained.Epoxy with higher strength is only of benifit where you have poor fits and/or dirty conditions making it necessary to depend on the epoxy itself as the shear element.

The above is true of almost all adhesives,even Ambroid,or Testor's cement.Minimal clearance,with just enough glue to fill the joint will give the greatest strength,always.Glue "fillets" only add weight.Cross drilling will aid in installing the second end,but it also introduces a stress riser which does more harm than good.The keying effect is putting a plug of epoxy in shear,which is not as strong as the carbon rod or the end.Properly done,an epoxy joint will develop very nearly the full strength of the weakest material joined.This strength falls off rapidly as the joint clearance increases beyond a few thousandths of an inch.

I use a 5.5mm CF arrow shaft with the thin walled aluminium tube inside. For the end fittings I drill and tap a close fitting alloy plug and Locktite it in place.

Larry,

What a spoil sport you are! If we are paying top dollar for CF push rods, surely they are better than all the clap trap--wrapped balsa push rods indeed!--you have used in your models.

I don't doubt your experiences, but it could be the CF rods you weighed were merely 'glass rods with a dash of CF tossed in, or maybe just some carbon black to give them that desired CF look.

The rods from Central Hobbies (and UHP, and Tom Morris) are the Real Deal. I have been using the 3/16" size, although Central offers larger and (I think) smaller diameters. The Central rods actually are CF, quite light, very stiff. Toss in end fitting which are threaded 4-40, the left-hand thread end fitting from Morris and we're talkin' the best control system parts ever.

For you guys having trouble with the CF tubes splitting while being worked, first wrap the area where you will be cutting with masking tape.

And do pay close attention to Ron's posting. You actually want *less* clearance between rod and end fittings, not more. Quit hacking up stuff which is already fully to specification!

Gil, I have been using Tom Morris' CF rods and ball ends for two years now and have never had a failure. Tom's stuff is the best on the market for the money. Smooth and reliable. Just follow Tom's instructions and use JB Weld and NOT epoxy. Sonny