Raising a farm windmill tower

[kapkichi]

TL;DR Summary

want to understand the mechanics/critical points of picking up a windmill tower with a crane.

I have a retirement project in restoring an old farm windmill and am close to being ready for the install. I want to pick the 35' tower and mill head together in one operation using a crane. My last statics class was 45 years ago so I'm a little rusty. I know the mill weight of 250 lbs will add stresses during the lift that the tower wasn't designed for. The tower is about 700 lbs. I believe my concerns are the moment introduced by the mill in the horizontal position and the combined axial/bending stress on the 2 lower tower legs while it is being tipped up to vertical. My thought was to pick the tower 3.5' down from the top which is the bearing for the mill post. I can't lift from the mill head because it only sits in the tower by gravity.
My questions
Does my pick point limit the moment arm to 3.5 feet or should I be looking at the point where the first girt and tension diagonals are, 5' down from the top.
Is the horizontal position the worst case scenario for moment or somewhere else along the 90 degree rotation to vertical.
I'm thinking by using a crane (vs. jib beam and winch technique) with the pick point always vertical that should lessen the axial forces in the lower tower legs. Is 45 degrees the worst case for this. What would a FBD look like to analyze that? The original design doesn't have horizontal girts around the bottom. I have added those on all 4 faces to help distribute the lower tower leg load to all 4 legs. Once the tower is hanging vertical I will attach leg anchors and socket into bored holes. one of the windmill forums offered some ancedotal advice suggesting the lower legs be lifted while the crane is picking to vertical (could due that with tractor on the lower girt) but I fail to see the value in doing that. I have attached a sketch to help visualize
Any advice, sketches of a FBD on how to further analyze would be welcome

 

[Flyboy]

Forgive the potentially dumb question, but… why do you have to tip the tower up with the mill head attached? If you have sufficient lift capacity and height to tip the tower upright with the head on it, it would seem that you could tip the tower itself, then hoist the mechanical kit as a second, separate lift. Sure, you have to send someone up to the top for final attachment, but it would reduce the stresses on the structure.

 

[sophiecentaur]

Sure, you have to send someone up to the top for final attachment,

'Working at Heights' regulations and training would come into the problem. Really best avoided if you don't have a qualified rigger on hand.

TL;DR Summary: want to understand the mechanics/critical points of picking up a windmill tower with a crane.

My thought was to pick the tower 3.5' down from the top

I have only experience of lifting and lowering a 10m mast (90kg) and have done it single handed. I used an A frame (calculated leg lengths and starting angle for optimum forces) , pivoted about a line through the base so it's simpler than your problem. But I would probably think in terms of more than one lifting line, giving the turbine a separate line to reduce torque on the tower.
But would the static weight of the turbine be greater than the maximum expected force from the wind? If not then I can't see there would be any problem with a single pick point just above the centre of mass of the ensemble.

TL;DR Summary: want to understand the mechanics/critical points of picking up a windmill tower with a crane.

I can't lift from the mill head because it only sits in the tower by gravity.

That could be a worry. It suggests that the turbine was lifted onto an already erect tower. Do you know how they did the job? Some pins would avoid problems of the turbine rocking out of the top of the tower.

 

[Baluncore]

I have moved a few wind pumps alone, and have not needed a crane. A crane might make it easier, but it is an unnecessary cost. If you lift the tower with a crane, you might need a couple of temporary diagonals to handle compression from the bottom feet to the first girts. You will also have to check the lubrication and oiler rings are in place.

The best first investment is in a gin pole for the top of the tower. That makes fitting and removing the head, vanes or tail an easy operation. Attach a vertical tube at the top that a gin pole can seat in. The gin pole should be an inverted 'J' with a pulley block. The gin pole rotates in the tube from over the wind-shaft to outside the tower. Remove the gin pole when the wind pump is in operation.

To raise the tower, chain the lower two feet to the near footings. Mount a 20ft 'A' frame, (shear legs), with feet at the foot of the tower, also attached with the chains.
A rope or wire passes from the top of the tower to the top of the 'A' frame, to the tractor draw bar. Use the tractor to raise the tower. The high point of the 'A' reduces stress on the bottom of the tower during the lift.

If you did not raise the tower with the head fitted, lift the head and vanes with the gin pole.

 

[Nik_2213]

IIRC, the safest approach with truss-base hinged at back is to have a sufficiently tall A-frame --As mentioned above !!-- to convert that initial low-angled, inefficient pull to something with much better 'mechanical advantage'...

IIRC, from my 'yachty' reading, the A-frame may either re-direct the main haul-line, so stand between winch and task, typically over hinge-line, or be a stand-alone hoist to get the 'spar' to advantageous angle for the main winch to accept. This may straddle mast near its c/g, be it bare or top-loaded. Sundry control guys essential, of course, of course...

FWIW, you probably need a 'tail' guy-line to control later stages of main lift, as tower c/g approaches footprint of base and 'Dire Lord Murphy' unleashes 'butterfly effect'. Best make that two, set diagonally...

Converse of task is 'Spanish Windlass' & Co, where even a child applying 'minimal' side-ways force to a taut tow-line may significantly contribute to un-ditching car: Been there, done that...

 

[kapkichi]

Forgive the potentially dumb question, but… why do you have to tip the tower up with the mill head attached? If you have sufficient lift capacity and height to tip the tower upright with the head on it, it would seem that you could tip the tower itself, then hoist the mechanical kit as a second, separate lift. Sure, you have to send someone up to the top for final attachment, but it would reduce the stresses on the structure.

agree that's the prefeered way structurally. 2 issue- to plumb and level the tower exactly over the well head I want the the mill and well rods attached. the tower isn't perfectly straight. The way the mill post seats into bearing cup I would need to mobilize a bucket truck while the mill is suspended to facilitate that connection. much easier to insert and do own ground