8 inch tubing for 17ft turbine

[Kid]

I cam across some 8" tubing with 1/8" wall for $10/ft and as of now it is the cheapest I have found. Originally I was quoted by a company the 10" dia for $18/ft so I figure the 8" is a pretty good deal. My question is if I should use it on a 17ft wind turbine 60ft in the air? Does the extra 2 inches make that much of a difference?

[DanB]

I wont say here.. it is smaller than what I'd suggest and I guess that's just based on intuition.  10" tube works out on my 20'machine so perhaps it'd be fine but I wished Id gone bigger on that one now.  2" does make a difference, my understanding is that if the thickness remains the same, then the rigidity of the tower is related to the square of the diameter.  So 10" stuff is about 40% more rigid if I'm right there.

[ghurd]

You may find some explanations of the relationships searching the internet about arrows.

Diameter, wall thickness, and length.

G-

[greend88]

It probably also depends on whether your going to guy it off too.  I know someone who just had a 16' turbine installed on a 75' free standing tower.  the tube is 16" 3/8 thick pipe.  The foundation is probably 8' x 8' x 10' deep maybe a lil shallower can't remember.  And it has an engineers stamp on it too.  

[Kid]

I would definitely guy it off and was thinking 3/8" guy wire.

[Kid]

Why do you wish you would have gone bigger?

I will be doing a little more research on this issue before I purchase the tubing so I don't get stuck with something I regret.

The person who has this 8" tubing also has some 6" with 1/8" wall that is galvanized for about the same price that I was thinking of using for a 10ft turbine but thought it might be a little overkill. (Isn't there a formula for this?)

[DanB]

of course you would have to guy it, just below the tips of the blades.  I expect 8" tubing would be OK... but if something goes wrong (like a blade comes off or ice causes a severe imbalance) then maybe there would be problems.

My own 20' turbine does fine on 10" tubing... until things get really out of balance then it looks a bit spooky!  Which is why I feel that it's worth spending a little bit more money for a lot more rigidity.

I would be a bit scared to put such a large machine on thin walled 8" tubing but I am not an engineer.

[SparWeb]

Strength of a tube increases with the cube of the diameter.  10 inch is actually 58% to 59% stronger than 8" with the same wall thickness.

Stiffness is a similar property (but don't mix them up), which goes up with the 4th power, so there's even more advantage than you guessed - it's about 2x stiffer.

You could make up for the flexibility of the 8" by using more guy wire sets, but that doesn't fix the increased bending stress you have in the stub above the highest set of wires to the tower.

[SparWeb]

Kid,

It's the 1/8" wall the bugs me more than anything.  If you were to (for example) weld lugs directly to the side of the tube, then the guy wire loads could pull the tube out of round.  

In answer to Dan's point, I did the math and the 10" gives you a LARGE boost in both strength and stiffness of the tower.  

The difference in cost is 500 dollars.  I think you get value for that money, for an increase in weight of 25%.

[fabricator]

The Dans 17' machine is 10" tube 12ga wall, so Kids 8" tube is slightly thicker wall, Kid, where in west MI are you? I work in Muskegon and can get 10" 12ga wall for about 250 bux a stick through my employer, if you are interested I can get a solid price number for you.

[nekit]

I'm planning on using 8" -.125 wall on my 17' unit, but I got the tubing for free. Hard to pass up. I'm planning on guying it well. I think it should work. I guess I'll find out.