Windmill tower installation

Resurrection of an old windmill tower.

By Matt Erickson KK5DR

(fully erected windmill tower)

I'm sure many hams have thought of it, but few ever actually do it. The old windmill standing in an open field or next to a farmhouse. Each seems very sturdy, and they are. Think about it, how many times have you seem a windmill tower blown over by a storm? Not many I would say. These towers stand for decades facing fierce windstorms with a mill that has huge wind loads, vibrations and torque placed on the tower. These towers are very sturdy.

There are several types of windmill towers, made by many manufacturers, I'm no expert on these towers but there are two basic types of windmill tower, (a) four leg, (b) three leg.

A few years ago I acquired a three leg type, 43' tall tower from a friend who removed it from his property where it had stood for 50 years or more. The mill had been removed many years before. It is constructed of fully galvanized steel angles and flat strap, as well as stamped clips. It is all bolted together, no rivets or welds. There is an integral climbing ladder up one side which goes from the ground level to the bottom of the top section. The base section is about 20' tall, with legs spaced equally at 8' and tapering up to about 4.5' at the top of the base section. This section weighs about 300#. I had to fabricate new anchor embed legs since the tower had been cut off at ground level when removed. These legs are 2" square tube 4' long with a 6 x 6", 1/2" thick plate welded on the bottom for embedding in concrete. I welded these legs to the tower legs.

The mid section of the tower is about 15' tall, tapering from 4.5' to about 11" where the top section connects. The mid section weighs about 200#. The top section is 5' tall taper from 11" to 7". this section weighs about 50#. There is a cast iron mast plate at the top with a 2" bore and set screws. I fabricated an additional mast plate that is bolted in about 2.5' below the top plate, it also has a 2" bore and set screws. This should secure a mast very well. No rotor plate on these towers, but if needed, one can be fabricated one easily and bolt it in near the bottom of the top section.

I consulted with an engineer, who concurred with my design for the installation of the base section, so I proceeded to undertake the task. First, I dug three holes, spaced 8' apart in the form of an equilateral triangle. Two of the legs are placed facing into the dominate storm wind direction (which in this area is from the NW). Wind against the tower tends to "lift" the windward side placing it in tension, and compress the leeward side leg. So, with two legs on the windward side, there is twice as much weight load to "lift". A quick calculation showed me that 10-20 tons of force per leg would be required to pull the anchor out of the ground.

Each of the holes I dug are 3' diameter and 4' deep ( I used a mini excavator to do this). I then placed the assembled base section upright into the holes (using the excavator for lifting). After the base is set in the holes on top of a layer of bricks at the bottom of the holes, I checked the base for level, which required a small amount of shims under two legs, this put the section within 1/2 degree of perfect level on all legs. Then I placed a layer of bricks around the bottom of the hole to create a cup for the concrete. Using the excavator bucket I mixed up a "wet" batch of sack concrete which will flow into and around the bricks locking them into a matrix. I mixed half a batch each time and in between the pours I tossed in some pieces of iron bar to re-enforce the anchor, then poured the final batch. 360# of concrete per hole. I placed an 8' long ground rod in each hole before re-filling them with the dirt I had previously removed. The picture below shows the installed base section, as well as the mini-excavator I used for most of the work. The fill dirt can be seen heaped up around the legs, this is done to allow for settling and compression of the dirt over the next several months.

Using a "wet" mix of concrete means I got better flow and adhesion of the concrete, but it also means the cure time is much longer, so the base section will be allowed to set undisturbed for a week.

Each of the ground rods will be connected to the leg of the tower base by a short piece of 1/0 welding cable. A giant crimp connector bolts the cable to the tower leg, and a brass compression clamp connects the cable to the rod. The tower leg and the rod are coated with corrosion inhibiting, electrically conductive grease, which is waterproof, and should insure a low resistance connection for many years.

The rest of the tower is very straight forward, bolting together. The use of lock washers and thread-locking fluid are highly recommended. Before applying the thread locker or final torque on each bolt, the section should be checked for alignment. Alignment is pretty simple, using a tape measure, pick a fixed point such as a bolt on one end of the section, then measure the distance to a similar point on the opposite end, but directly diagonal to it, measure the same points on the other side of the section. When both sides read the same, the section is aligned and ready for final torque and thread-locker. Then proceed to the next section.

There is almost no ham antenna that can't be held by a windmill tower that has been properly installed and anchored. These towers are tough, some having been standing on the prairie for a hundred years with a giant fan on top. If you have the chance to acquire one, it is a great self-supporting tower, a bit of work to install, but super heavy duty.

The last picture is a view from the ground looking up the ladder to the top. It stands fully constructed, and solid as a rock, it should be standing long after I am gone.

73 de Matt KK5DR