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Pivot for tilt up three-sided tower
By elt, Section Diaries Posted on Wed Aug 6th, 2008 at 10:33:34 PM MST
minimalistic build

After looking at pictures and getting what info I could, I put this pivot together to tilt up my tower:

(tower pic here)
I know it doesn't look like much but after looking at the pictures, I took everything that I couldn't figure out what it did out of the design. :-)
There's 1" x 1/8" angle iron on 1 1/4" x 3/16" bar and a 5/8" rod. (I think that the
strap" going around the tower is part of the climb-guard. It fits nicely there so I figure that it belongs...)
It only looks like half a hinge; I'm thinking I just need two threaded J-rods on each side in the concrete; the pivot rod will go between them and then I'll bolt a bar over the top of the rod to hold the pivot (and tower) down.
- Ed.

Pivot for tilt up three-sided tower | 14 comments (14 topical, 0 editorial)

Re: Pivot for tilt up three-sided tower (3.00 / 0) (#1)
by elt on Thu Aug 7th, 2008 at 08:10:39 AM MST
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Here's a drawing of what I'm planning at the concrete base to hold the tower.

Just loosening the pivot clamp a little should allow the tower to tilt.

Originally I had thought that I'd need two rods in the concrete per side with a strap across them to hold down the pivot. Now I think that a piece of angle iron will do to contain the pivot rod. Installed as shown, the force against base while raising and lowering the tower will be towards the j-rods in the concrete, not the (slightly) loose end of the clamp, so the pivot should stay in place with just the one rod per side.

Sprained my ankle though, so it will be a while before I'm in shape to dig a hole.

- Ed.

Re: Pivot for tilt up three-sided tower (3.00 / 0) (#2)
by vfarrell on Sat Aug 9th, 2008 at 08:01:42 AM MST
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Hi Ed

What is the estimated weight and height of the tower? (thinking of the pending forces you will be applying to those two legs on the tower and the pivot rod itself during the raising of the tower)

also what is the dia of the steel dowel you used in your pivot?

"Everybody's got to believe in something. I believe I'll have another beer." --W. C. Fields
[ Parent ]

Re: Pivot for tilt up three-sided tower (3.00 / 0) (#3)
by vfarrell on Sat Aug 9th, 2008 at 12:21:23 PM MST
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opps sorry missed the part about 5/8 rod my bad..
"Everybody's got to believe in something. I believe I'll have another beer." --W. C. Fields
[ Parent ]

Re: Pivot for tilt up three-sided tower (3.00 / 0) (#4)
by elt on Sat Aug 9th, 2008 at 12:23:22 PM MST
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Hi Vic,

I figure the tower weighs a little less than 100 pounds and the mill and pipe on top of it are another 100 or a few pounds more. The tower is 24 feet; the mill with be up another 6 feet on the pipe. The tower will be bolted to my workshop about 12 feet up.

The steel pivot rod is 5/8" diameter. What I was thinking was that most anchor bolts I've seen were 1/2" or 5/8" so I figured (guessed) that the pivot didn't have to be any bigger...

Comments are appreciated!

Thank you,
- Ed.

[ Parent ]

Re: Pivot for tilt up three-sided tower (3.00 / 0) (#5)
by TomW on Sat Aug 9th, 2008 at 01:07:37 PM MST
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Ed;

I think the forces on the hinge in a tilt up are enormous. Much more than any static load while in place. I am no good at math but if you ever tilted something up from horizontal you would see how much pull it takes and it all gets amplified into the hinge.

My 30 foot 4 leg tower that I tilt up uses a 5/8 hard bolt in each leg of the hinge side as a "pin". It has a 10 foot dual rotor similar to those built by DanB. I would not even consider anything less. You can see the flex in the 3 inch angle iron legs and embedded connections to the footing when it is just coming to rest or just being lifted off horizontal. The shearing forces on the bolts must be a couple tons in my set up. There are forces at work here that are not immediately obvious.

Bigger is better, IMHO.

Just from here.

I also never stand near the base when I am lifting in case something fails I want to be well beyond where it can get at me.

Good luck with it.

Tom

If everyone is thinking alike, then somebody isn't thinking.

--George S. Patton
[ Parent ]

Re: Pivot for tilt up three-sided tower (3.00 / 0) (#7)
by elt on Sat Aug 9th, 2008 at 07:45:19 PM MST
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Hi Tom,

I looked at your towers in your gallery; thanks for putting them up. I'm quite jealous! I'd really like to go with a four-sided wide-base tower. I understand their structural advantages but for now I have to go with what I have.

You wrote:

> I am no good at math [...] The shearing forces on
> the bolts must be a couple tons in my set up.

I'm pretty good at math if I do say so myself. My problem (not being a mechanical or civil engineer) is that I don't know what formulas to use, what the numbers would mean if I got an answer, or what's "standard" or "code" even if I knew what I was doing...

So, I've done some math but whether or not it means anything or not I'm unsure:

My math says (conservatively) that the 5/8" rod has a shear strength of about 8000 pounds. Lifting the tower up can't generate that kind of force but a 250 pound sideways force at the top of the tower while it's being lifted could. But the weight of the tower and mill could only generate half that force. That 2x safety factor combined with the conservative rating of the rod gives a safety factor of about 4x. I might want a higher factor in a public setting but I think (guess) that that's enough for my backyard. Well, that's if my engineering is worth the pixel dust you're reading...

> You can see the flex in the 3 inch angle iron legs [...]

It's possible that my tower is stiffer. I haven't seen any tendency for it to flex yet.

Also, I don't think that the weight of the tower and mill (also a 10 footer) will flex the rod because the rod is supported along it's entire length and range of motion directly on the concrete.

If you can imagine that the following drawing is an end-on view of the bracket, then it shows the tower and bracket at 0, 30, 60 and 90 degrees.

This is the main reason that I didn't put the rod into a hole drilled into a stationary piece of metal, then I'd have to wonder about the strength of that piece as well. But since it's not there, it can't fail!

Well, that's my thinking-slash-design-strategy. I'm not saying that stronger isn't stronger but that would still leave me with the question of what's good enough.

Thanks again,
- Ed.

[ Parent ]

Re: Pivot for tilt up three-sided tower (3.00 / 0) (#6)
by vfarrell on Sat Aug 9th, 2008 at 04:13:33 PM MST
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If it was mine I'd beef it up a tad, and recommend guy cables at the top.
safety 1st bro

imho

-Vic

"Everybody's got to believe in something. I believe I'll have another beer." --W. C. Fields
[ Parent ]

Re: Pivot for tilt up three-sided tower (3.00 / 0) (#8)
by elt on Sun Aug 10th, 2008 at 05:47:52 AM MST
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Do you mean temporary guy wires while raising it? I think that I have clearance in my yard to do that and that would minimize the risk of a sideways force damaging anything while the tower is being raised.

Thank you,

- Ed.

[ Parent ]

Re: Pivot for tilt up three-sided tower (3.00 / 0) (#9)
by vfarrell on Sun Aug 10th, 2008 at 11:07:24 AM MST
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I would At least While raising the tower, as permanent part of the tower couldn't hurt if you have feasible anchor points on the ground to do so.

Vic
"Everybody's got to believe in something. I believe I'll have another beer." --W. C. Fields
[ Parent ]

Re: Pivot for tilt up three-sided tower (3.00 / 0) (#10)
by SparWeb (sparweb at ANTISPAM_hotmail_com) on Mon Aug 11th, 2008 at 12:06:42 AM MST
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Ed,

I have my doubts. I can bend a 36" long piece of 1/2" rod with my bare hands...
Why didn't you get a 1" pipe, which costs about the same at the hardware store? Much stiffer, stronger - more meat for the abuse a tower must take.

I looked at your pictures, and the bent pole over your garage sort-of stands out as a warning.

I can't guess how you will be raising/lowering the tower with it all beside the garage like that. Swing side to side? Pull up against the roof of the shed?
Steven Fahey

Re: Pivot for tilt up three-sided tower (3.00 / 0) (#12)
by elt on Mon Aug 11th, 2008 at 07:56:44 AM MST
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Hi Steven,

> I can't guess how you will be raising/lowering the tower

The tower will go against my workshop, a 24'x32' building, not the shed in the picture. I'll reinforce the workshop wall, like I did in the shed, and use a pulley to redirect a winch cable off to the far side.

> I can bend a 36" long piece of 1/2" rod

I can't say that these formulas are the right ones to use or that my interpretations are correct:

What I found for mild steel gives a yield strength of 40,000 psi. That'd be a strength of 40000 psi times a cross sectional area (.25" x .25" x pi) for your 1/2" rod = 7854 pounds. Grabbing it on the ends give you a 144 :: 1 mechanical advantage so it takes 7854 / 144 = 54 pounds to deform the rod. Does that sound about right? (give or take a bit?)

An inch long bolt of the same material would have the same strength (or less because of the threads) but you can't bend it in your hands; you don't have the same mechanical advantage.

> Why didn't you get a 1" pipe [?]

I looked at pipe... It all came down to thinking that whatever I fabricated, eventually it would be held to the concrete base with a 1/2" or 5/8" anchor bolt... a half inch of steel was going to be the weak link in the chain. Given that, it didn't appear to me that the other parts mattered so I did away with them.

But maybe my thinking is the weak link of the chain!

- Ed.

[ Parent ]

Re: Pivot for tilt up three-sided tower (3.00 / 0) (#13)
by SparWeb (sparweb at ANTISPAM_hotmail_com) on Mon Aug 11th, 2008 at 09:14:36 AM MST
(User Info)

Your way of looking at the narrow question of strength of materials won't get you into too much trouble, but your back-of-the-envelope calcs won't get you far when you have to deal with the larger structure, and there are many questions to answer.

Structural engineering is interesting enough that I enjoy it, but it's not everyone's cup of tea. If you want to go further, then you really must get a hold of an engineer (or at least a student) and maybe pick up an engineering textbook or two (the student would also be a good source for that).

There's a 2.5" diameter pipe rotating inside a 3" diameter pipe at the base of my tower. Bolted into the concrete through an angle. Welded together. Easy to make. Could pull a dumptruck out of quicksand. You did the same amount of work, but it won't get a VW out of a ditch.

Reducing size and weight is valuable in aircraft. Please reassure me that you don't want your tower to fly away!

Steven Fahey
[ Parent ]

Re: Pivot for tilt up three-sided tower (3.00 / 0) (#11)
by SparWeb (sparweb at ANTISPAM_hotmail_com) on Mon Aug 11th, 2008 at 12:22:34 AM MST
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Looked at your comment about the math, and maybe it looks okay to you, but you've brushed over a major load condition without analyzing it.

While lifting the tower, it is extremely vulnerable. Your winch/tractor may jerk the line - that could triple the load in an instant. A guy wire might get snagged or come up short - that could also double the load. Where's the gin pole? You don't show it, so I bet you haven't factored in the loads IT imposes on the same hinge point. There are even more things to consider than that, but those are the biggies, and I think I've made my point already.

You also missed a crucial load consideration for backyard truss towers: when your brother-in-law says "so how solid is that thing" and climbs on to "test it".
Steven Fahey

Re: Pivot for tilt up three-sided tower (3.00 / 0) (#14)
by elt on Tue Aug 12th, 2008 at 06:48:34 AM MST
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Thank you Steven, I do appreciate a check list of what must be accounted for; however, the most compelling point to me is this: I think my base is stronger than the manufacturer's tilt up base. I can look at jin poles, jerk and climbing forces but if they aren't show stoppers with their base then they won't be with mine.

The math concentrating on the base strength is to convince myself that that is so, not to make little of the overall problem.

Thanks again,
- Ed.

[ Parent ]

Pivot for tilt up three-sided tower | 14 comments (14 topical, 0 editorial)

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