RAISING A TILT-UP TOWER WITH A WINCH
This tower once came crashing to the ground because I somehow didn't properly connect the cable to the towing hooks on the truck. Pretty idiot proof I thought it was, but hey, we all have our idiotic moments. The accident totally destroyed the windmill but most of the tower survived, so here it is again with only modest repairs. The winch is the one critical addition that I hope will prevent a future accident.
Here I have put a little axial flux genny on it with a blade that I hacked out of a 2x6 pine board. It does work in strong winds, but it is usually ornamental. Who knows when Murphy will get hold of me next! Adding this winch could just introduce more means of failure that I haven't recognized yet.
Here is a photo of a foolish "rush" job that I did in July. As soon as I was done I started to have my doubts. I could raise the tower with the winch on this pier, but it broke as soon as I pulled on the tower by hand. When I did that I was careful to have the tower raised only an inch above the cradle - exactly what it landed on as the pier gave way. Ripped it out of the ground and started again.
The real pier is much larger and I used a lot of re-bar, too. I think I mixed a total of 14 bags of concrete. I used a former only at ground level to make the base square - underneath it's a giant blob 4 feet down.
The winch is only supposed to be mounted with three 3/8" bolts. A little wierd but when I run the numbers I have a big safety margin, and the winch itself also is supposed to have a safety factor of 4. I wasn't about to cast 3/8 ready-rod into concrete so I put four pieces of 1/2" threaded rod into the concret pier instead. Here is the adapter that I made to mate the 4-bolt pattern to the 3-bolt winch. I just happened to have a heavy-wall 6x1 tube that's been sitting around as scrap and suddenly I had the perfect use for 6 inches of it!
Below you can see how I've layed out the winch now. Instead of just winding the cable in a straight line or below the gin pole, I chose to redirect the cable to one side. It keeps me close to see what's going on yet away from all the cables as it goes up. I am also running out of room, so staying within the fenced-off square is a plus. I bought a big pulley for this and anchored it right under the gin pole.
This winch has a "shepherd's hook" eye, so I put a rod with a loop into the power drill and crank away. I tried it with a cordless drill the first time, but that just made the poor battery hot! Drained it flat while the tower was only half-way up! Better not do that again, eh?
Another observation while raising and lowering with this winch is that the cable wraps up on one side more than it should before the cable traverses back across the drum. Once the cable has wound a second wrap on one side, that extra cable then slides over suddenly. The resulting jerk to the cable causes an abrupt jerk on the tower!!! I can still hear the "twang". This happened once and my heart literally skipped a beat.
So I rigged up a fairlead which should keep the cable from wanting to bunch up at the ends of the drum. I haven't tested it enough to know yet if it really prevents this. Lacking any structure nearby to mount the fairlead, I put together some bracket arms to hold it. Here I was lucky to have mounted the winch on the adapter box because now I could bolt the fairlead arms to it.
TOWER RAISING LOADS
Since I went to a lot of trouble to design this tower, I know its weights and dimensions pretty exactly. The tower itself weighs 310 pounds, and I made provision for up to 250 pounds of weight on the top. It is 45 feet tall and the gin pole is 21 feet long. Here is a diagram with some general dimensions. It doesn't exactly match my tower because I drew it only for the sake of estimating loads a long time ago.
When starting to raise the tower, the loads are the greatest. Multiply the weight of the tower and all of the things attached to it by the distance each one is from the pivot:
310 Lb x 22.5 feet = 6,975 foot-pounds (pipe) 250 Lb x 45.0 feet = 11,250 foot-pounds (windmill) 33 Lb x 22.5 feet = 750 foot-pounds (electric wire and guys)
The total of all of these moments is 18,975 foot-pounds.
To raise the tower, an equal and opposite moment must be applied. It is done by pulling on the tow cables, which go from winch to the end of the gin pole and other cables attach on the tower itself. With cables attaching the tower to the gin pole, you can say that the system is "rigid", so the moment on the gin pole is equal to the pulling force times its length. In this case:
18,975 foot-pounds / 21 feet = 904 pounds
The tow cable doesn't pull at right angles to the gin pole. It makes something of a 45 degree angle down to the ground. This magnifies the tension in the cable (the cable must pull more to apply the same horizontal force on the gin pole).
904 pounds / cos (45 deg) = 1278 pounds
This is the tension on the cable that the winch must apply to hold the tower off of the ground. The tension will go down as the tower goes up because the weight of the tower applies more and more over the pivot. There are things that have not yet been considered and they are important for SAFETY.
I use a "bounce" factor to find the tension load. Anyone who has towed up a tower knows that it's a wiggly wobbly affair until it is upright and the guys are tight. Then you get events like the winch cable bunching up, and who knows what else can happen. I use a bounce factor of 1.5
1278 pounds x 1.5 = 1920 pounds Design Working Load for the Winch.
For this tower, then, the safe working load rating of the winch cannot be less than 2000 pounds.
Every tower is different, and I've simplified the calculation just a little for clarity. When actually raising a tower, the weight of the gin pole plays its part, and as it goes up the angle the tow cable makes with the ground changes a lot. When raising some towers, a heavy gin pole can actually pull ahead of the tow cable when it's near vertical and swing the tower the rest of the way up, maybe too abruptly! When this happens the gin pole bumps on the ground or on its anchor and gives the tower a wicked backlash.
Good luck, and work carefully!