Saturday, October 1, 2016

Unbending Bent Tubes

When disaster strikes we often find ourselves with a pile of scrap aluminum and steel. After shedding a tear or two it is time to dive into the scrap heap and decide what can be salvaged and what must be trucked away for disposal or recycling.

A word about risk

Of course we want to reuse as much as possible, it just isn't always wise. In these matters I find it helpful to reflect on the old cliche: "if you have to ask the question...replace it." Unfortunately the less you know the more doubts you will have. Or if you thrive on risk you simply skip the question phase and jump into salvaging almost everything while prudence looks on in horror.

A little knowledge combined with a little ingenuity is perhaps best: you strive for economy yet know that there are important limits to what can be salvaged. Aluminum tubing can be expensive and is a prime candidate when an antenna turns into scrap.

The danger is that aluminum and its alloys fatigue easily when bent. Every bend weakens the metal, including the bend to return it to its original shape. Tempered alloys, including those used in aluminum tubing, weaken even more when bent. You must think very carefully whether the risk is worth it.

The general rule I use with aluminum tubing is to try and save a tube with a gradual curve and discard one with a visible kink, especially a kink that changes the cross-section from a circle to any other shape, be it an ellipse or something more extreme. The application also matters: a tube that is at the end of a yagi element or light duty boom mostly only supports itself and is more amenable to rescue.

Consider the following tube:

This is a 1.5" O.D. tempered aluminum alloy tube (most likely 6061-T6) with a 0.058" wall that telescopes with similar tubing in ⅛" steps. It comes from the boom of my old A50-6 yagi that I plan to resurrect in modern form. The bend dates to when the temporary wire antenna mast of which it was a part collapsed.

In my judgment this tube is a good candidate for saving. Even if only for the practice it is worth making an experiment of it. The replacement cost is small if the straightening attempt fails.

Tough tube

Aluminum alloy tubing of this diameter is surprisingly tough. Try to bend it over your knee and it's your knee that is more likely to bend. Some casual attempts to straighten it soon after the disaster went nowhere so I cast it aside for the time being.

When I recently took down my towers I recognized an opportunity. The 1.5" schedule 40 steel pipe coupled to the frame of the house (used as a tower guy anchor) looked perfect for what I had in mind. There are two important reasons. First, the pipe was effectively an immovable object for the force I would need to apply. Second, the I.D. of the pipe is 1.61" (the 1.5" designation is a nominal figure) into which the tube would slide in nicely and largely conform to the shape of the tube.

The second point is worth some elaboration. To straighten the tube it is necessary to apply a force greater than the tube's yield strength. Unless the tube is secured against a similarly curved surface there will be stress risers that will result in kinking or even collapse. The 1.61" I.D. is in a way even better than 1.5" I.D. since it allows a portion of the curved section of tube to be firmly couple to the pipe. That is, slide it into the pipe until you encounter resistance.


We are not quite ready to proceed to bending. We need a way to apply the right force at the right place. Let's look at a tool for helping with that task: the snipe.

The bending force must be applied close to where the tube is secured to the pipe so that we can ensure that the only point that yields is in the vicinity of that coupling point. If we grab the tube by its end, for maximum leverage, the result will be unpredictable, and likely destructive. Yet if we hold the tube closer to the coupling point we almost certainly don't have the required body strength.

Pipe snipe
What we need is a snipe. Most often it takes the form of a long steel pipe that is placed over the handle of a tool to multiply leverage. Too often a snipe is used inappropriately (easy to do) resulting in a broken tool, broken machinery or, worse, a broken body. The approach we are going to take is more moderate and will permit good control of the force.

My snipe is another 1.5" schedule 40 steel pipe. It is long enough for the required leverage yet no so long that damage is easy to inflict on the tube. As with the fixed pipe it has a shape that reduced the risk of stress risers.

Gauging the force being applied is not obvious. If you push too hard you will feel the tube bend. You don't want that since it invariably means you've applied too much force and probably damaged the tube. If you are too careful the tube will not bend at all.

Experimentation with increasing amounts of force is required. It is necessary to inspect the tube after each push, which though tedious is mandatory.

Inspect the tube carefully: the bending will occur in the exposed area between the two pipes. Keep the pipes separated by about 6" so that the stress is distributed in a way to straighten several inches of tube at a time. Several bending actions will be required. Be sure to push exactly against the curvature. Align the tube so that the curve is exactly vertical or horizontal so that you can more easily judge the direction of applied force.

Almost straight
Start from one end of the bend and work gradually toward the other. Slide the tube further into the fixed pipe as each bend is accomplished. Check carefully after each bend to make sure the tube is straightened over that portion of the tube. When you believe you're done put a straight edge on all sides of the tube to confirm that you were successful. You'll have to decide how exact is sufficient to your use of the tube.

Almost straight

When I was done I inserted the tube back into the boom. The tube was straight but the boom was not!

I hadn't noticed before that the next telescoping tube (1.625" O.D.) also had a bend, although it is much less than that in the end tube. That will require a different setup since it will not fit the steel pipe I have on hand.

For the moment that is inconvenient so it must await a future opportunity. This is not something to rush into since use of the wrong size pipes can wreck the tube.

Final words

Bending aluminum tubes in the manner I described is risky but can be worth the attempt if the damage is within reason, as I described earlier in the article. In this case the tube is inexpensive so I was prepared to take a risk knowing I could buy a new one if I damaged it further.

For tubes in more structurally critical applications I would advise you to be safe, skip the experiment and buy a new tube. The same advice applies if you simply cannot be certain that it is safe to straighten the tube.

Then there's steel. That is a topic worth its own article. I have one planned on that subject focussing on damaged towers.

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