Heat-shrinking aluminum tube 1

[wrangler007]

I am working on a prototype project to join two sections of 1/2-inch OD 3/8-inch ID Alloy 6061 tube by insertion of 7/8-inch long steel coupler nuts driven into the tube ends and then screwing them together with 1/4-20 steel threaded rods. I am also trying some Alloy 6063, and some nominal wall thickness of 1/16-inch.

I am heating the aluminum with a Mapp gas torch as I drive the nuts in, thinking that the aluminum will "shrink" to tighten the fit upon cooling. I have bent a few tube ends already in trying this process

Question #1: will the aluminum shrink upon cooling, improving the strength of the joined sections to lateral (pulling) stresses?

Question #2: will the aluminum joint be weaker as a result of the heating than simply driving the steel nuts in with a hammer?

 

[MikeHalloran]

You haven't told us how strong the joint has to be, how many you want to make in how much time, how long the finished joint has to maintain its strength, or what environment it has to live in.

The mapp gas is just annealing the aluminum, or leaving it in some uncontrolled erratic state.

At room temperature with relatively clean parts and 12h of cure time, you'd probably do better with Gorilla Glue between the 1/4-20 rods and the aluminum tubes.




Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[wrangler007]

Thanks for the reply, Mike. I won't be making many, because the application is a vertical antenna for hobby (ham radio) use. I'm driving the coupler nuts into the ends without adhesive to maintain electrical continuity between two connected sections. I have been able to produce some strong-enough joints, because the application does not apply very much stress.

I have used the heat from the MAPP torch to expand the tube while driving in the nuts, expecting that when quenched (in water), the aluminum would shrink tightly around the nuts; but then again, I don't know much about the properties of aluminum prepared this way. I have used alloy 6061-T, 6063-T832, and unknown stock tube from Home Depot in my experiments.

What I'm after is a stronger joint vs. a weaker one by using the heating and quenching process. Obviously I'll stop using heat if it just weakens the aluminum.

 

[MikeHalloran]

If you think of the antenna section as a lever, where one arm is the length of the tube, and the other arm is the distance from the tube's periphery to its center, i.e. its radius, you will appreciate that a small lateral force applied to the tube, as by wind, can generate a large axial force. It's not really a low stress situation.

Anyway, I would suggest tapping the ends of the tubes 7/16-20 and joining them with a 7/16-20 fine thread headless bolt. If deformation makes you feel better, try joining the tubes with a 1/8" pipe nipple; I'd use a steel hex nipple from a hydraulic supply house.

I'd also use Loctite on the threads, so that they won't loosen, and to exclude oxygen from the metal to metal interfaces so they can't oxidize.



Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[IFRs]

It is most likely that the strength of the aluminum will suffer, it may become brittle, it's temper will fade, it will oxidize and be prone to corrosion. How hot are you heating it? Threading in thinwall tube is problematical. Good luck!

 

[MikeHalloran]

One other possibility; join the tubes with a Swagelok butt connector. Ask the guy at the hydraulic supply house to instruct you in proper tightening; do not overtighten.



Mike Halloran
Ft. Lauderdale, FL, USA

 

[unclesyd]

Take a trip to the local big box hardware or a fastener supply store and look at the expansion anchors. You cna expand them to get any type grip you need.

I would look at a double expansion or a drop-in. You can get these out SS or galvanized

 

[wrangler007]

Thanks for all of the replies; you all have given me a lot of ideas.

First off, my name is Ted Bruce, and my amateur radio callsign is KX4OM. I hold an Advanced class license, and I've been a ham since 1960. I also gained the FCC 1st Class Commercial Radiotelephone Operator license with Radar Endorsement in 1979.

The antenna I'm designing is for hobby use and non-commercial. The design is to be freely shared with the ham radio community. It is an 8-foot long portable vertical antenna that breaks down into 2-foot long sections for transit. Removable coils wound on CPVC and attached with 1/4 x 20 hardware to the coupler nuts inside the tube allow operation on several bands. It is not designed for permanent installation.

The idea of using brass pipe nipples is one adopted by Phil Salas, AD5X, a EE and communications and antenna designer. Also, James Bennett, KA5DVS designed an antenna (of which a version is now a commercial product) that used two 1-foot long 1/4-inch diameter aluminum rods, tapped with 1/4 x 20 thread at the ends and joined by the same coupler nuts I'm using. His designed used a thin 6-foot Radio Shack whip as the extension of the antenna above the removable coils to an overall length of 8 feet. When I built that antenna, I found that tapping the rods was not only difficult to achieve and created weak points at the point that they were joined by the coupler nuts. I used mine during a contest back in May in horrible weather, and it blew over and bent the cheap Radio Shack whip and the aluminum rod sections as well.

In coming up with a hopefully better solution, I recalled from my Navy Nuclear Power program training that for the same material and mass, a tube structure is stronger than a rod. The 2-foot long solid rods I used initially per the published design are insubstantial. I can bend them and wrap them around my hand, in fact.

Another ham down here in Georgia and I have been corresponding lately on this topic. His antenna is 5/8-inch aluminum with stainless steel couplers, and at least part of his design uses internal treading of the tube.

Based on y'alls comments, I think I'll give up on the work-hardening or annealing/quenching approaches to get coupler nuts locked mechanically inside the tubes, since it's not a reliable or repeatable process. I'll take a look at the Swagelok butt splices; I used to do a lot of instrument tubing work with stainless and copper tubing and Swagelok fittings back in my younger days. I'm not familiar with the butt splces, though; I'll have to investigate that.

Also, the expansion fittings that I'm familiar with are the Hilti products used to anchor pipe supports, but they are single-ended. I'll check to see what exists in that area as well.

In this application, the strength of the joint is more important than electrical conductivity, since I use washers between the tube sections to lock them, and RF current flows on the outside surfaces of the tubes due to the skin effect. I'm no longer pursuing the metal-to-metal conductivity inside the tube as a design objective. Mike, your Gorilla Glue suggestion is looking more and more practical, since it swells after it is applied, and is tough! I've also tried JB Weld metallized epoxy, but it doesn't swell as much (if at all).

So thanks again to all who replied, and 73 (best wishes in ham radio lingo)

 

[IFRs]

You should really look at backpacking tent poles. They are strong, durable and flexible. Mine vary from 5 feet to 12 feet long and all fold down to a bundle about 18" long. They are shock corded inside and "snap" together.

 

[MikeHalloran]

I became KN2TPI in 1958, at age 13. I didn't pursue it because I didn't have fifty bucks for a radio, and I'm not nearly as talkative in person as I am online.

I agree that that tent poles deserve a real hard look.



Mike Halloran
Pembroke Pines, FL, USA

 

[IFRs]

Tent poles are typically available in anodized 7xxx aluminum and fiberglass. The high end tent manufacturers use reinforced ends, these babies are tough!! Even your local Army/Navy store will have them, some are lower technology but may suit your purposes.

 

[unclesyd]

Here is another approach you might want to try. For a really strong connection you may be able to double them.

This looks like the manufacturer.

http://www.mdionline.net/

Connectors for sale.

http://www.gpistore.com/index.asp?PageAction=VIEWPROD&ProdID=108