Monday, June 3, 2013

Tower Bolts

Here's a question that more hams (and OTA TV enthusiasts, for that matter) ought to ask themselves: are you smarter than the engineers and technicians that designed and manufactured your tower?

Most would, correctly, say 'no'. Yet too many go on to behave as if they do believe they are smarter, or at least smart enough to tamper with the tower. This is a bad idea. The bolts that hold together the sections of a tower are a good study since it is one of the common items that people will substitute with inappropriate hardware.

The 30' secondhand Golden Nugget tower I recently brought home is a good case in point. Take a look at the adjacent photograph of 3 sets of bolts and nuts.
  • The one at the top appears to be one of the originals that was supplied new with the tower. It's a bit rusty with age and a few years of burial but still quite usable. High-strength carbon steel bolts are often like that, rusting more slowly than their inexpensive brethren. I don't know what class of strengthening (and hardening) it has since the markings on the bolt head are mostly illegible and perhaps proprietary.
  • The middle set is stainless steel, which was bought and installed by the tower's previous owner. Notice a couple of things here. First, there is no lock washer. Second, the shank diameter is ¼" rather than the ⅜" of the original bolt.
  • The bottom set I just purchased. The set is class (or grade) 5 with a ⅜" shank and 2" long, which is about ¼" longer than the original. The important differences are the longer section of unthreaded shank and coarser threads. I still need matching lock washers.
In that earlier article I mentioned that I was not entirely comfortable with the quality of the tower. While I didn't say why at the time -- only mentioning the rust -- the major points were that the tower sections were not linearly aligned and the overlap where they mate was compressed enough that separating sections was difficult. On further inspection the sections look fine. It's those stainless steel bolts and their application that were at fault.

Towers are not piston engines and do not need the same machining precision for their bolt holes. Even so there is no accident to their design. There is enough play in the tower leg bolt holes to allow the installer to drive the bolts home while dangling up in the air even when all the legs are not simultaneously aligned, but not so much that the there is significant play once the bolts are in place. Leaving ⅛" of play is too much. Not only did the sections slip and slide over time due to weather the threads on the bolts were galled by the weight and motion of the tower legs against the threads.

Stainless steel is also the wrong material. Avoiding rust is nice but never at the cost of tower integrity. The least-expensive stainless steel hardware tends to be harder than carbon steel bolts but is not of the required strength.

In the past I have gone to the source and purchased replacement bolts from the manufacturer. As far as I know they all do this since it is a common need in the field. This time I bought standard bolts of the approximately correct type to save time (and probably money) with the confidence that they will work out fine for this light duty tower. As a general rule I don't recommend this.

To do the subject justice I will delve into a little more detail. Look again at the picture with the 3 bolt and nut sets. Notice the shank wear just under the head of the original bolt. That is caused by the weight of the tower sections pressing down on the bolt shank. Yet the threads are not damaged, just rusty. The opposite is seen on the middle, stainless steel bolt where the shank is pristine and the threads are galled just inside of where the nut was positioned.

What causes the different wear pattern? In the former case the shank is properly sized so there is no room for play even if the nut is loose. Thus there is no wear on the threads at the inside of the legs. The shank shows compression wear due the tower weight, but the bolt is strong enough to withstand the wear.

The stainless bolts are harder but because they are undersized the bolt will get twisted up and down and (slightly) side to side by action of the wind. Hard or not, the threads will gall and, being harder than the tower, the tower leg bolt holes will get worn, and that wear is visible on closer inspection. No amount of bolt tightening will help since if you torque the nuts too much the tubular legs will yield inward and loosen the nut.

It is the purpose of the lock washer to hold the nut in place with the nut properly torqued. The leg doesn't distort and the lock washer, by cutting into both the nut and tower leg face keeps the torque steady. In this application the correct lock washer must be selected, which is one with multiple locking flanges on the interior of the washer, up against the bolt shank where the nut can work on it. The lock washer should be the same hardness as the nut or it won't cut. Split ring and exterior lock washers must not be used since neither can properly accomplish the job.

This is where the coarser thread of the bolts I purchased have an effect. With the coarser thread there is a greater change in torque for an equal angle rotation of the nut. So if the nut rotates by the effect of weather or improper installation there is greater risk that the lock washer will cease to operate, promoting further loosening of the nut. Although the difference is modest, and probably not a problem, I will periodically check the hardware once the tower is up. Of course I would do that anyway. It is something I do before working on someone else's tower, especially one of uncertain pedigree. I've encountered many towers with several or many loose bolts!

You should gather from all this that the tower is supported by the bolt shank, in particular the outside end without threads, not by compression of the tower legs by torquing the nut. This is also true of towers like the DMX series and Trylon with formed sheet metal legs. The tower rests on the bolt and the compression holds everything in the correct position.

As a further example, the adjacent picture shows two sets of class-5 ½" bolts, nuts and lock washers, where one bolt has a custom tapered shoulder for DMX towers and the other with a standard, fully-threaded shank. In this case you must use the custom bolt to properly align and mate the sections, so order them if you need replacements. I've seen many hams use the standard variety, but they cannot hold the tower sections in proper position and will almost certainly loosen over time.

The bolt, nut and lock washer each play their unique role. This is only possible if they are all of the correct type. If you're at all uncertain and you need new hardware buy new from the manufacturer or their dealer. An improperly installed tower can kill.

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