I am assembling a Four Star 60 ARF. The aileron pushrods are 4-40 with solder clevises on one end and threaded steel clevises on the other. The manual states that a jam nut needs to be screwed against the threaded clevis after everything is adjusted. What is the purpose of the jam nut?

You know? I wondered the same thing. Considering one end is soldered and the other end is attached to a control horn there is no way for anything to unscrew itself. None the less my instructor insisted on the jambnuts and what the heck, it certainly can't hurt.

Often, the threads are not perfect, meaning the 4-40 thread may not be exactly within tolerance. This is not the most expensive hardware on the face of the earth. When they are mated, the rod and the clevis, there may be some wiggle in the fit, even when it is screwed way in (but for adjustment purposes, only screw it in about half way so you have some room to adjust for fine tuning the elevator, rudder, and ailerons).

The 'check-nut' is there to tighten up against the clevis and keep it both tight and straight. If it wiggles, you could get some control surface flutter. Even a little flutter caused by a loose connection could eventually cause problems.

DS.

As CG implied it reduces wear caused by what is called "fretting". ANY slight movement of two metal parts againt each other WILL cause wear. The jam nut stops that movement.

Fretting (or fretting corrosion) refers to corrosion damage at the asperities of contact surfaces. This damage is induced under load and in the presence of repeated relative surface motion, as induced for example by vibration. The ASM Handbook on Fatigue and Fracture defines fretting as: "A special wear process that occurs at the contact area between two materials under load and subject to minute relative motion by vibration or some other force."

What he said...

DS

Thanks. I'll use the nuts. This is my first experience with steel clevises.

I want to know who Bruce really is behind that mild-mannered disguise

Ya work on REAL airplanes long enough and ya learn stuff. It just sinks in. I've seen aircraft skin "fretted" into aluminum foil. More often seen in places like engine baffling though due to high vibration area.

Been there.

Here I am in a previous life

Oh, you worked on the little ones?

OK, so I know they're bigger than ours. But not by much. And the A-380's would have been bigger. Don't know how the B-777 compares. Don't actually work on the planes much any more, mostly just repair the avionics once other mechs have removed them. BUT, I'm inside and dry.

Hey, if you did work on them you wouldn't have time for RCU!!!! Gotta keep those priorities straight there!!!

Ken