What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Threaded rod question

M

Maxrate

Well Known Member
Patron
Vans gives you the option of welding or riveting the threaded rod ends. An AP friend voiced concerns that now the inside is air tight after welding the ends on. After searching several threads I was unable to resolve if this has been an issue or not to those who have chosen to weld their aileron rods. I did as the manual said to do and completely primed the inside and let it dry for several days before having it TIG welding :confused:
 
I, too, have heard concerns expressed about making the tube airtight, though I don't know why this is bad. Did the AP tell you why this is an issue? I considered that the moisture in the trapped air could cause corrosion, but I decided that there would be insufficient moisture to cause a real problem. When I had my rods welded, I put a couple of cc's of oil in the tube prior to welding the last threaded end, but had no real basis for doing so....
 
My last project was a welded steel tube fuse biplane. The standard procedure for welded steel tube structures is to drill a #30 hole in the tube then squirt some oil in the tube. There are several ways this can be done. One way is to put some heated oil in the tube, then roll it around to oil the entire inside of the tube. You can feel the tube being heated by the oil as it spreads. When you are happy with your oil spreading set the tube hole side down over night to let the excess oil drain out, then stop up the hole with a sealing pop rivet. If you are the the type to want to be different you can fill the tube with helium and seal. This evacuates the oxygen and makes the tube lighter. I would think this helium method would not be practical since the part is so small. On the flip side I have a piece of 4130 tube that I have been using as a poker for my grill since 1995. It has been outside on with the grill that long and has not rusted away yet.
 
Charles in SC said:
....you can fill the tube with helium and seal. This evacuates the oxygen and makes the tube lighter. I would think this helium method would not be practical since the part is so small....
Click to expand...

Helium will leak through the tiniest spaces and will invariably leak out. It makes a good leak detector for that reason but it's difficult to make a tight enough structure to take advantage of it. You can buy a helium detector for finding these leaks. Some people pressurize tube structures and install a pressure gauge so they'll have something to inspect.

Certain older helicopters had their blade spars pressurized with a gauge. It was easy to check the gauge on preflight. That was back in the early '70s and I don't know if that's still being done. I think dry nitrogen was the preferred gas for this.

Dave
 
At work, we often pressurize large weldments with dry nitrogen, add a pressure gauge and monitor during inspection cycles. This might be a little over-the-top, even for RV builders.
 
Chemistry of oxidation

Water by itself does not oxidize metal, so water is not the problem in terms of rust/corrosion. It is the oxygen that does the work, although any water present facilitates the chemical reaction.

Unless you have an actual leak in your welded structure (thus the pressure gauge that is mentioned in earlier posts), the amount of corrosion that can occur inside a sealed structure that contains air is minimal. Once the available oxygen inside the tube is used up by the rusting process, no further rusting can occur. Therefore, if you have a good airtight weld, it should last virtually forever (or at least until it rusts through from the outside). The purpose of putting oil into a closed structure is to allow the oil to "rust," that is, to oxidize rather than the metal. This is the same idea as the sacrificial anode (zinc) that the car makers put on cars years ago.

Greg
 
Ok guys. I just drive planes for a living. I'm not Carl Gustav Jacob Jacobi :D I guess what I want to know is now that my tubes are welded how many out there that have welded tubes have ever had any issues. Vans only has a blurb in the plans that says rivet or optional weld. Nothing more. If I were to drill a #30 hole to fill with oil what would be the best place to do this. I think my AP friend is concerned that altitude changes " pressure changes" might cause the tube to bend or distort. I just don't see this being an issue. These planes are only flown at 10 to 12 thousand feet max. Has anyone out there ever seen one of these rust through, etc.... Thanks
 
Last edited:
Maxrate said:
. If I were to drill a #30 hole to fill with oil what would be the best place to do this
Click to expand...

Inch or two from one end, but like Greg said.....

. I think my AP friend is concerned that altitude changes " pressure changes" might cause the tube to bend or distort. I just don't see this being an issue.
Click to expand...

"Neither do I" would be an understatement ;)
 
I do not think RVs have been around long enough for rust throughs to be a problem yet. One problem aircraft part that comes to mind are the wing struts on the Piper Cubs types that were 50+ years old. Oil treating the tubes inside would be a long term prevention move.
 
You must log in or register to reply here.
Back
Top