Thursday, January 5, 2017

Trylon Gin Pole: Review

As with everything there are things that work and things that do not. The gin pole I constructed for the Trylon tower did the job though not without some problems. I like to be open about what goes wrong since it is almost always a valuable learning experience. Refer to the earlier article on the construction of the gin pole and the objectives I set for it to better understand what follows.

The good

We might as well begin on a positive note. The primary one being that it worked quite well. Here are the main points:
  • Capacity: The gin pole easily handled weights up to 150 lb. It was also rigid and secure enough to allow some lateral force, needed when tugging on the tag line to free tower sections when they tangled with the tower or protrusions on the gin pole itself.
  • Smooth action: Operating the winch to lift large and bulky loads was surprisingly easy. The 4:1 advantage of the winch still required some sweat to lift the heaviest items yet could still be done with one arm. One time I accidentally lost my grip on the handle, allowing the load to drop and the winch to unwind. Even so the drop was slow and was easily stopped by grabbing the handle. That's another benefit of the 4:1 ratio. Adjusting the load up and down for final positioning also worked well.
  • Economics: It was really cheap. The pipe was free, and even if bought is not expensive. The hardware can be found in any hardware store. The most expensive part was the winch. For that I went with quality rather than cost. It's an investment since it will have other uses.
  • Easy to build: The machining only required a drill, hacksaw and file. No special tools or techniques were needed other than attention to detail.
The bad

Despite all of its positive points there were a variety of problems. These were all managed without undue danger or effort. Well, there was one uncomfortable moment which I will describe.

Tower hooks: I'll mention the worst problem first. The hooks I constructed worked well but did have a few important negatives. First was some difficulty in attaching the gin pole to the tower. There is a narrow gap between the keeper plate and support plate into which the tower leg must be inserted, and there are two of them that must be done at the same time. When I was holding the gin pole from the bottom, supporting its weight and striving to keep it vertical, that act was not easy. However the pole could be temporarily set on the tower diagonals so that a rest could be taken.

For about half the sections there was a small gap between one support plate and the diagonal due to the tower design. I mentioned this in the construction article. Since the kick out due to the gap was typically small I usually ignored it. That is until the time the bottom hook disengaged completely when the tower section being lifted was swung over the tower into position for splicing. The only reason the gin pole didn't swing out of control was the loaded weight of the support plate on the diagonal. But I could not lift the gin pole plus tower section to reset it in place. Instead I kicked it a few times to push into place, again gripping the tower leg. From then on I always used tie-down straps to secure the gin pole to the tower.

Weight: At 2.7 lb/ft the Schedule 40 water pipe gin pole comes in at about 35 lb (16 kg), including pulley, hooks and hardward. This can be handled by one person of ordinary ability. Anything heavier becomes unwieldy since not only must the weight be lifted and maneuvered on the tower, it must also be kept vertical or it will "get away" and cause a serious incident. It becomes a two man job.

I intend to search out lighter and stronger pipe, either aluminum or thinner wall steel with a wider diameter, for lifting the LR20 sections since the sections are 10' and so require a taller gin pole.

Height and offset: As constructed the height of the pipe up to the pulley is ~10.5'. With the amount of pipe that straddles the tower section and pipe that extends above the pulley and below the bottom hook the distance between the top of the tower and the pulley is 6.5'. Since a section is 8' the grab point is about 2' below the top of the lifted section. This is adequate and serves to reduce gin pole height and weight and therefore increase safety when handling it on the tower and pulling on the tag line.

With this design a few precautions are recommended. Because the lifted section will lean more as its centre of gravity is approached heavy sections require more effort to swing into position for splicing. This was discussed in the earlier article (link at the top of this article). Two workers up the tower is best.

A more subtle problem is that the pulley can jam against the section or lifting hook when it is pulled into position for splicing. The problem is exacerbated when the gin pole is strapped to tower for additional security since there is less play. The solution is to build an offset into the gin pole at the upper hook so that it leans back a bit from the tower rather than following the contour of the leg or tower face. I didn't do that for this gin pole and I should have. The problem did occur on 3 lifts requiring extra effort to perform the splice. I took a shortcut thinking I could get away with it on this tower and I was wrong. It could have been achieved as simply as an extra nut between the pipe and the keeper plate on the upper hook.

Winch misalignment: I carefully squared the winch to the ground so that the cable would be normal to the winch during lifting. The intent is to avoid lateral force on the winch and tower mount and to ensure the cable winds onto the spool across its full width. This is the correct procedure for straight sections that are the norm for guyed towers. However this does not work for tapered self-supporting towers.

Since a photo does not show the problem very well I drew a diagram to make it clearly evident. Notice how the cable extending upward from the winch leans left. As a result the cable naturally tries to wind onto the spool from right to left. But we want it to alternate left and right when the cable reaches the side of the spool. The problem is always present to the same degree (the angle is constant with height) although it becomes more evident (worse) as the tower rises due to the amount of cable that is spooled.

Especially for heavy loads it is necessary to manually reverse the winding towards the right by pulling the cable sideways while cranking. This is tedious and tiring. The solution is to mount the winch so that it is normal to the tower leg rather than to the ground. My tower mount system did not allow for adjustment of the winch orientation.

Winch spool capacity and cable tension: From shopping around the size of winch spools is roughly proportional to the working load rating. This appears to reflect the size (cross section) of cable for that working load. Unfortunately this limits the amount of cable that the spool can hold. For tower this is a problem since so much cable must be used, more than twice the height of the tower.

Using 3/16" aircraft cable for the lower sections the spool on my winch could only hold enough cable to get me up to 5 sections of tower. I tried using rope at that point, without the winch, but the sections were too still too heavy for me to do alone. Since I needed lots of smaller cable anyway I bought a 500' reel of ⅛" aircraft cable and cut 200' for the winch. That worked. However, note that while tidying up I wound the full length of cable onto the winch and found that was all that would fit.

Although a higher capacity winch isn't needed for the loads being lifted in future it may be needed simply for the larger amount of cable that can be held.

The other problem is one of cable tension. When taking up slack the cable does not compress on the spool. This becomes a problem when the slack is taken up and tension develops. The underlying cable on the spool is compressed by the cable, causing kinks and crushing. This must be carefully managed to avoid damage and weakening of the cable.

Tag line: I quickly relearned that all lifts require a tag line (or lines) where a fixed overhead pulley is used. This includes gin poles.

The purpose of the tag line is to avoid tangling of the load being lifted with the structure it is being hoisted along. It is natural that the load will strive to rest against the structure (tower) since that's where the pulley is affixed. There are many opportunities for tangling due to the many protrusions and opening in the tower, gin pole and load. If the load has a tendency to rotate in the wind, especially yagis, two tag lines may be needed.

Little tension is needed on the tag line, just enough to avoid unwanted contact, or any contact at all if abrasion is a concern. For example, the soft zince galvanizing coating. Too much tension is usually a bad idea with a gin pole since it does not have the lateral strength of a tower or mast. An experienced hand on the tag line is helpful to get it just right. When I was doing the work alone I allowed the load to brush against the tower and only touched the tag line when something caught. Most often it was the underside of the gin pole.

You can see the presence of a tag line in the previous article on topping the Trylon.

Abrasion: When rope is used the concern with abrasion is on the rope rather than the tower. With steel cable the concern is with the tower. Aircraft cable is very strong and substantially harder than the structural steel use in towers and far harder than the tower coating, whether it is paint or zinc galvanizing. The harder substance always abrades the softer one.

My design of the Trylon gin pole involves some abrasion of the cable on the tower, mostly at the top where there is a slight angle change in the cable from the pulley over the top member of the tower, both towards the load and the winch. There isn't a lot of abrasion but it is less than ideal. Abraded areas are prone to rusting sooner than undisturbed surfaces.

The solution is to offset the pulley from the gin pole, and even use a second pulley so that both sides of the cable clear the tower completely. This requires more fabrication and additional gin pole strength, which comes down to cost and weight. Some designs see the cable thread the interior of the gin pole which can eliminate most, though not all points of abrasion.

For the future

Gin poles are pretty simple machines. The challenges are mainly in their construction and use. I have chosen construction techniques that do not require welding or machining with tools I do not own. That may have to change if I want to avoid problems in the future.

The local tower service company offered me the free use of one of their many gin poles. While very kind of them none were suitable for Trylon towers. First, all of them were heavy, with the lightest among them almost 80 lb. Second, the hooks seemed to be designed for towers with round members, which is more common for commercial products that the formed sheet metal used by Trylon and others. They would have deformed the L-shaped diagonals.

Most recommended gin poles use L-shaped hooked where the horizontal part rests on the tower cross-member and the vertical part keeps it  from sliding out. For towers with symmetrical cross-members on each side of the legs each hook has two parts, allowing the gin pole to straddle the leg and hold its position that way rather than rely on gravity to pull it down one diagonal to rest against the leg.

This is what I attempted to accomplish in the gin pole I built for the DMX tower I put up at my Ottawa QTH. Unfortunately this is not easy to do with standard hardware. The hooks need to be formed from steel stock and welded to the gin pole. Alternatively the hooks are welded to short pipe sections that slip over the gin pole proper and bolted in place at the desired position, determined by the tower type.

The same article I link to immediately above has a diagram of a manufacturer recommended gin pole that places the pulley a few inches outboard of the pole. Its purpose is to reduce contact (abrasion and tangling) between the tower and load, perhaps obviating the need for a tag line. Of course the gin pole must be strong enough to handle the lateral forces of an offset load. In some cases the arm will pivot on the pole to ease positioning the lifted section over the tower.

I will shop for a lightweight, strong pipe as a basis for a gin pole and spend some money with a welder to build proper hooks for the tower types I have or might have in future. Considering the amount of tower construction I am planning this will be a good investment. No more improvising.

No comments:

Post a Comment

All comments are moderated, and should appear within one day of submission.