tube thickness calculations

Building up a long scope and needing to know dimensions for pipe. Assuming 20 pounds at each end of an aluminum tube 170" long and about 11 inches ID and supported at the center (balance point). What size tubing (wall thickness) do I need? Or what formulas needed to figure this out?? I would assume most tubing is 6061????

Also where to buy??? The lens cell is 10 7/8" OD.


Thanks in advance.

Kreig McBride said:

Building up a long scope and needing to know dimensions for pipe. Assuming 20 pounds at each end of an aluminum tube 170" long and about 11 inches ID and supported at the center (balance point). What size tubing (wall thickness) do I need? Or what formulas needed to figure this out?? I would assume most tubing is 6061????

Also where to buy??? The lens cell is 10 7/8" OD.


Thanks in advance.

Hi:

There are some basic calculations you could do based on bending deflections of a tube under such loads. These would be based on the moment of inertia (=(r0^4-ri^4)*pi/4 for a tube), the length and the stiffness of Aluminum, 10,000,000 psi for all alloys. My calcuations show that a 11 inch Tube with a 0.10" wall would deflect about 0.012inches at the center under the specified loads including self-weight. In general, most any tube will be stiff enough, I think.

What is more critical is the ability of the tube to hold its shape and avoiding "beer canning." There are also equations one can use to determine such things, unfortunately I am not at my office so I don't have this stuff at my finger tips and picking the proper loading is not so easy.

So in general, what I would do is use a combination of calculation and common sense. I would look at the bending to be sure it did was OK, and then I would look at some 11 inch tubes and get a feel for how the wall thicknesses available affect their stability. And at some point I would see what others have used...

Big help huh???

jon isaacs