Aluminum tubing strength and fittings

A

Aurelia

New member
How well would 2 lengths of 7/8 alum tubing hold 175 lbs horizontally on a 10 foot span. Would it flex more than an inch or two? Also, could one use stainless fittings with alum tubing or should I stick with nylon? I notice nylon rail fittings and stainless fittings but not so much in the way of aluminum fittings. I am trying to put together a couple of slides for a hard dinghy but I am not sure about these details only having experience with stainless 1in material and fittings. I like the lighter weight of alum if the strength is enough.

Greg
 
Aurelia":35w5rgji said:
How well would 2 lengths of 7/8 alum tubing hold 175 lbs horizontally on a 10 foot span. Would it flex more than an inch or two? Also, could one use stainless fittings with alum tubing or should I stick with nylon? I notice nylon rail fittings and stainless fittings but not so much in the way of aluminum fittings. I am trying to put together a couple of slides for a hard dinghy but I am not sure about these details only having experience with stainless 1in material and fittings. I like the lighter weight of alum if the strength is enough.

Greg
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Hi Greg!

I'm not an engineer, but I kinda think 175lbs/2, or 87.5 lbs in the middle of a 10 foot span of 7/8" aluminum tubing would collapse it, though it also depends on the wall thickness of the tubing, of course, and how it was made (welded or extruded), and whether it was an alloy, etc. Also, the weight of the dinghy might be spread out along the tube, and not centered in the middle of it.

Stainless is many times stronger than aluminum, and I'd prefer it over aluminum a lot, both for its strength and its beauty. Stainless fittings on stainless tube is in a whole different league than aluminum and nylon, IMHO. Replace the screw down fittings with ones welded on, and now we're really talking security and headed towards military specs! (After a few more tests and x-rays.)

If I wanted to use aluminum, I'd go with a much larger diameter, at least 1-1/2", heavy walled to prevent collapse, use an alloy, like 6061 or 6063 with T-6 (temperature hardening), and an Alodine or anodized finish to fight corrosion.

But then, the guy who was my mentor in these things was in charge of fabricating missile tubes for nuclear submarines, and the engineering safety factor was more important than the weight!

Then too, I' ve broken enough aluminum spinnaker poles to know not to make them too marginal in the strength department.

You might want to describe your project a little more throughly, so others can see how the dinghy's weight will load up the railing, if possible.

Good Luck!

Joe. :teeth :thup
 
The other thought is aluminum (or stainless) pipe vs tubing. The pipe is considerably stronger. Here is a quote from one of the suppliers on the subject:

"Pipe is generally more rigid than tube, and is usually produced in heavier wall thicknesses.
Pipe is specified by a nominal dimension which bears little or no resemblance to the actual dimensions of the pipe. 1" Schedule 40 pipe, for instance, has an actual OD of 1.32", a wall of 0.133", and an inner diameter of 1.049". Tube dimensions are actual dimensions.
Pipe fittings are sized to meet pipe sizes, but not tube sizes. A 1" schedule 40 nipple will fit correctly on a 1" schedule 40 pipe, but not on a 1" OD tube. "

There is also Schedule 80 pipe--which is stronger than the Schedule 40.

You have to use fitting specific for that pipe of tubing you are using. I would not consider using nylon fittings for what you are involved with. Also avoid pot metal fittings (which are often used with aluminum tubing).

I'll go back and look at the other thread--I am not sure that I completely understand what you are doing--and how the balance of this will work (including your presence on the fore deck.
 
I guess that everyone is aware that aluminum and stainless corrode when joined together? Especially in salt water.

By the way, the load that tubing/pipe will bear is a structural analysis, and that's a straightforwart solution.

Boris
 
Sounds to me like you have a serious engineering exercise ahead of you.
The selection of aluminum alloy in order to address issues of both bending strength and corrosion should be considered as should be the finish. Anodizing was mentioned although I believe I'd only consider hard anodizing for both corrosion considerations and friction if you're planning to slide a load on it.
With respect to plastic fittings, the strength is typically addressed by designing to distribute loads over large areas since stress cracking from concentrated loads is a common failure mode with plastic parts. And don't forget to account for UV exposure when selecting the appropriate resin.
If you find this list somewhat discouraging just remember that there are a a number of reason why stainless is commonly used in marine applications.

Good luck with your project,

Paul Priest
J.C.Lately
Sequim, WA
 
I will be going with stainless to keep it simple and strong. I may try to omit the span from the rack to the bow and just use tensioned tubular webbing for that horizontal movement. That would change the bow rail assembly but I am thinking through that option now.

Greg
 
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