Tower Pipe Sizes

[domeguy]

I'm finally to the point of getting materials for my tower.  My plan is to erect a 40' tower that pivots about the top of a 20' fixed tower.  When up the assembly will have two sets of three guy wires - one at the pivot point and the other just below the top.    Since the 40' pivoting piece will be unsupported over its length while raising and lowering it's been suggested that I go with larger diameter pipe than the more common non-pivoting tower.  I'll have a 10' Dan-built machine on the top.

My plan is to put the 20' sections together with fittings that nest over the main pipes.  I wanted to use 6" schedule 10 pipe for the long pieces with rather thick 7" stock being used to mount guys, transition to the smaller pipe at the top, join the two long pieces together, attach the pivoting hardware, etc.  Unfortunately, 7" pipe is kind of rare.  More commonly available is 5" schedule 10, with 6" schedule 80 for the fittings.

So, what do you think, folks?  Does any of this make sense?  Does 5" schedule 10 (0.135  wall thickness) seem  strong enough to resist excessive bending while being raised and lowered?  The 6" schedule 80 should nest over the 5" pipe with about 0.100 of play.

[SparWeb]

Are you an engineer?

The stress calculations that would tell you the answer are fairly basic, if you know how to draw and analyze a free-body diagram.  

I have a "thing" about broad questions about towers.  It's one thing to make a generator, that may or may not produce electricity, in your garage.  It's quite another to weld together a bunch of 400 pound pieces of pipe and hoist them all over your head.  Simultaneously.

There are tower designs, available for free on this site, that take out a lot of your guess-work.

[domeguy]

Thanks for your comments Steven.  Yes, I'm an engineer, just not the right kind of engineer.  But I do know that you're pretty close to the weight of the complete assembly at just under 400 lbs.  It was not my intent to hoist the whole thing up at one time.  That would be quite a challenge.  As far as the design is concerned, I have my reasons for not wanting to build the more normal type of tilt-up tower that is well described in various places in this site as well as in Dan's other sites.  I'm also pretty sure that a 40' piece 5" schedule 10 pipe will bend when it's supported in the middle - I just don't have a feel for when the deflection becomes failure.

[gizmo]

Hi domeguy.

Your tower sounds very like my tower, only taller. I used 70mm pipe, 4mm wall. 4 guy wires at the pivot, and another 4 near the top. Two of the top guys are disconnected before lowing the tower. The bottom section was 3 meters tall, and the top section 7 meters, pivoted at 4 meters, giving a 7 meter tower when up.

I did find the tower would bend a lot, about 400mm deflection, scary to look at. So I added a brace about 500mm long at the pivot point, and then run a cable from top to bottom of the top 7m section over this brace, and tensioned this with a turnbuckle. The photo on this page will explain http://www.thebackshed.com/windmill/FORUM1/forum_posts.asp?TID=497&PN=1

There is still some bow in the tower when its at 90 degrees, but no where near as much as before.

Glenn

[dinges]

The link below contains an introduction to structural engineering (statics). Reading/studying this should give you an idea on the various aspects of designing structures.

http://instruct1.cit.cornell.edu/courses/arch264/text/struct-elem-scr-sp07rev01.pdf

Now, personally, after 4 years of mechanical engineering school, I don't feel up to the task of designing towers, apart from the simplest ones (guyed pipes).

Succes,

[scorman]

I am in the process of building a tilt-over tower  ( not tilt-up ) and have a couple of interesting links:

this thread was on GreenPower:

http://www.greenpowertalk.org/showthread.php?t=646&highlight=tilt+tower

he uses pipe not unlike yours, I am more into the Rohn type design that I have seen the hardware for a 60 foot unit not far away.

I posted a few diagrams:

http://i145.photobucket.com/albums/r203/scorman1/Wind/antenna_tilt1.jpg

a real engineering spec here:

http://i145.photobucket.com/albums/r203/scorman1/Wind/rohntiltover.jpg

the trick to not bending is simple and as shown in the first thread ....you need a  1/4 inch cable attached below the turbine near the top, to a anchor at the pivot point and a standoff somewhere in the middle ...sort of like a suspension bridge or crane cabling

the tower should simply be supporting a static compression load ie the weight, and guys take the lateral load from the wind pushing sideways if it is not freestanding

the tiltover is made easier if there is a hefty counterbalance weight near the ground, so that the winch is doing very minimal work ie the whole assembly is "balanced" on your pivot tower

Northeast has a bunch of photos and a few videos on youtube:

http://www.northeastwindenergy.com/tower-22.jpg

http://www.northeastwindenergy.com/Tower-Videos.html

hope this helps

Stew corman from sunny Endicott

[wpowokal]

This troubled me when I first read it, so I waited to see what others contribuited. Now assuming I understand your goal of a tower of total height of 60', 40' of which is 6" pipe hinged on a fixed 20' tower, and on top of this a 10' turbine. Stop me here if I got it wrong.

But you won't be raising it all at once?, Ok I assume the fixed tower is in place and the top 40' is then raised. But this is achieved without any support while raising and lowering one assumes some sort of cantileaver is proposed, oh and there is a joining piece at 40'. And once vertical one set of guy wires as close to the turbine as practical, another at 20'.

My problems....., it will bend beyond its modules of elasticity (which forms part or most of your question). Lets just assume you manage to raise it, and secure the guy wires, along comes a wind (3 days later) and we have the force acting on the 10' dia prop pushing this turbine away from the wind. The fulcrum point(top guy attachment point) is say 5'6" from the top which leaves 34'6" bowing in the oposite direction.

I admire your enthusisim and applaud you asking but.... oh well just keep all dogs and children clear.

Then I most probally have totally misunderstood your proposal.

allan down under

[domeguy]

Thanks for all of the helpful comments and links.  I see that I was not very clear when I tried to describe my tower.  There is a 20 foot tall fixed tower that is held up with three guy wires.  The top of this 20 foot tower is attached to the middle of the 40 foot tower.  When the tower is erect it will be 45 feet tall, with the top 5 feet being used to mount the Dan-built alternator / turbine assembly.  This is held up with another three guy wires that are attached just under the swept area of the blades.  So the 40 foot tower is really fixed in position at the top with three guys, and the middle with three guys (it is attached to the 20 foot tower here), and at the bottom with a clamp of some sort that is yet to be designed.

My plan to initially erect the tower is to first raise and guy the 20 foot tower.  I will leave a hoisting cable across the top of this tower attached to the middle of the 40 foot tower.  With the bottom end of the 40 foot tower somehow constrained by the base of the 20 foot tower I'll use a pickup truck to raise the 40 foot tower.  I think the hoisting cable will need to pass through a large eyelet of some kind to keep it from falling to the side.  When the 40 foot tower is temporarily held in place with the hoisting cable I'll tighten the three guys coming from the top of the 40 foot tower to the same anchor points used to hold up the 20 foot tower.  Then I can insert the pin that connects a bracket at the middle of the 40 foot tower to the middle of the top of the 20 foot tower.  With the pin in place, I can remove the hoisting cable.

To lower the turbine for maintenance I simply un-anchor one of the top guys and use a light-weight cable attached to the bottom of the 40 foot tower to control the speed.  I think only a light-weight winch will be required to control the descent.  Since the entire assembly is on a hillside, I don't need to lower the turbine head to the level of the tower base, but the head will be lower than the pivot point at the  top of the 20 foot tower.

Since I described the pipes I intended to use I've found better pipe sizes, I think.  Our local seller of pipes and steel stocks 6" schedule 40 as well as 6" 12 guage steel tubing which will mate with about 1/16" clearance.  I would then use the 6" schedule 40 pipe, which has a wall thickness of about 0.25" for fittings and the 12 guage tubing for the long pieces of the tower which are all 20 feet long.  I think about 1 foot of overlap, pinned with a bolt or welded will adequately reinforce the stress points.

Sorry for being so long-winded...

-Lee

[SparWeb]

A picture is worth 1000 words.

BTW, if 40 feet of 5" pipe bends under its own weight, then so will 40 feet of 6" pipe.  Think about it...

As you can see, many agree that this topic comes with disclaimers.

[TomW]

sparweb;

I think a 40 foot 2 foot tall I beam will "bend" or deflect under its own weight if you support it in the center.

Just the view from here.

Cheers.

TomW