Saturday, July 19, 2014

Notes on Gin Pole Mechanics

If you've ever erected a tower you are likely familiar with gin poles. The alternative, raising tower sections is to do this by brute force. I've done this many times, but only with a reliable partner. This is not recommended practice even if it is common among amateurs. Other alternatives are typically expensive and inconvenient: cranes and helicopters (sky hooks are yet to be invented).

Rather than borrow or manufacture a tower-specific gin pole to raise my DMX tower I chose to fabricate a simple gin pole that, while a bit unconventional, is up to the task. My reasons were to avoid the need for a welder (and the associated expense of parts and labour) and simply for the joy of it. Now that I have one it is available for other projects.

DMX gin pole recommended
by the manufacturer
Before we look at the gin pole itself it would be helpful to understand what we are asking it to accomplish. It is vitally important that it is up to the demanding task of safely and effectively lifting heavy and bulky tower sections. Failure can result in serious injury or death, or at least damage to property.

Forces on the gin pole

If the tower section weighs 25 kg what is the force on the gin pole when it is lifted off the ground? The correct answer is not 25 kg.

When the tower section is in the air and not moving, and both sides of the rope are vertical, there is an equal downward force on the side of the rope being pulled. This doubles the force on the gin pole to 50 kg.

An additional downward force is required to move the section upward. The greater the force the faster the section rises. This force must also be supported by the gin pole. Another consideration is acceleration: the force needed to change the upward velocity of the tower section: F = ma, according to Newton.

If you pull on the rope hard enough (large accelerating force) you can lift yourself off the ground. Less amusing is that the acceleration can break even a strong gin pole. Never jerk hard on the rope; use gradual, fluid movements when lifting tower sections.

Vectors, not scalars

As a physicist would tell you: force is a vector, not a scalar. What this means is that calculation of the net force requires taking into account the directions of the contributing forces. You can't simply add together the magnitudes of X and Y. When you have that section hanging in the air the scalar net force is 25 + 25 = 50 kg, but that is only a valid sum if both forces are in the same direction.

Since you cannot easily stand directly underneath the gin pole when pulling on the rope the net force will be in a direction other than straight down. This not only stresses the gin pole it requires more scalar force (how hard you pull on the rope).

When you pull on the rope the gin pole will bend towards you. This creates bending stress along the length of the gin pole and lateral stress on the bottom attachments between the tower and gin pole. Since the tower section is hanging vertically the additional force causing the stress must be coming from you pulling on the rope. That is, in addition to the force that is equal to the weight of the tower section you must pull harder (additional force) so that the force causing the aforementioned stress, when added to the weight of the tower section, equals the pulling force. Not only are you working harder that extra effort is creating a serious safety risk.

If at any point on the gin pole the stress exceeds its yield strength it will fold or split. This is something we do not want to occur.

Let's assume that you are standing where the angle between the pulley and the rope in your hand is 30°. Just as in the guy station/wire calculation there is a lateral force component additional to the 25 kg vertical (downward) force. The scalar value is tan 30° x 25 = 14.4 kg. The tension of the rope in your hand is therefore SQRT(25² + 14.4²) = 28.9 kg. When you pull on the rope to raise the section (as discussed above) this value is even higher, as is the lateral force.

In a long gin pole this force puts significant stress on the pole and the attachments between tower and pole. Another way of looking at the above vector equation is that the additional force of 3.9 kg (28.9 - 25) multiplies 3.7x to a lateral force at the top of the gin pole of 14.4 kg. That is, if you stand a little way from the gin pole and you pull hard you can easy gain enough mechanical leverage to destroy the gin pole, and all the mayhem that entails.

Another area of concern is where the bottom of the gin pole attaches to the tower (typically at two points). The top attachment acts as a fulcrum for the lateral force of 14.4 kg at the pulley. If the two attachment points are on the tower are 1 meter apart and the gin pole is 4 meters long the lateral force on the bottom attachment point is 4 x 14.4 = 58kg. This is in addition to the vertical force of 25 + 25 + lifting force + acceleration force.

Bottom line: stand as close to the tower as possible when manually lifting tower sections with a gin pole, and wear a hard hat. Gin poles can and do break. Don't turn yourself or your friends into statistics.


The major components to be selected in any gin pole include:
  • Pole: More than anything this determines the strength and weight of the gin pole. A steel cylinder (tube or pipe) provides the optimum balance between strength and weight. Wall thickness and heat treatment, if any, determines yield strength. The longer the pole the greater the stress on it, so the stronger it must be. It must be long enough to securely attach to the tower and project high enough above it to, at a minimum, place the pulley above the lifted section's centre of gravity.
  • Pulley: The pulley should be labelled with its working load rating. A pulley without a rating should be avoided. For light duty tower work the rating should be a minimum of 100 kg but even so avoid a pulley that is rating less than 150 to 200 kg. I recommend avoiding one with a centre axle that is secured to the housing in rivet style. The axle should have wide flanges and/or clips outside the housing body that cannot easy break or wear through. Get a decent match with the rope diameter you'll be using.
  • Tower attachment brackets: As with the other components the brackets must support all the loads on the gin pole. Since the brackets are at stress points the pole can break at the brackets if they are improperly joined, allowing the pole to fracture or bend. The brackets must also ensure that the gin pole cannot slip off the tower structural members on which it rests, that the tower at the attachment can deal with the additional load, and do not require more than two hands (!) to position and secure the gin pole.
  • Rope: Match the rope diameter and the pulley, but ensure that the rope's working load strength is at least twice the forces involved (4x the weight of the heaviest tower section). If the rope is too small for the pulley there is a chance that the rope can get trapped between the wheel and housing. You don't ever want that to happen. I can assure you from experience it is difficult and dangerous to free a trapped rope from a pulley that is holding a tower section in mid-air and is out of hand's reach above you.
In summary, the objectives in selection of gin pole components include:
  • Light enough to be safe and easy enough to be maneouvered  by one person on the tower, with optional assistance by ground crew. I generally do this sort of tower work with at least one other person, but I decided to design the equipment and process in this instance to permit me to raise the tower on my own as a challenge to myself.
  • Withstand all calculated forces with a large safety margin.
  • Easy to construct, or at least fall within a comfortable budget.
With all the forgoing in mind I fabricated a gin pole for the DMX tower. It is pictured at right.
  • I used material on hand to keep the cost low since I do not know if it'll get used again, other than to eventually take this tower down. The 7.5' 18 gauge fence rail used to be the replacement bottom section of my version 2.0 antenna mast for the inverted vee. A 4' length of heavy gauge angle aluminum is attached at the top with muffler clamps. The pulley is fastened at the top, on the side facing the tower. Total length is 10.5' (the photograph perspective altered the apparent length ratio between the pole and angle stock).
  • A close-up of the pulley fastened to the angle stock is shown above. It is rated at 200 kg (440 lb) and was bought at Canadian Tire (of all places) for $7. It comfortably accommodates ⅜" and ½" rope without binding. The pulley mounting has just enough free pivoting to respond to the load. It should turn so that the rope enters and exits in the plane of the wheel as the load shifts. Its motion does not allow the rope to be trapped between housing and pole or to spin the pulley and twist together the two sides of the rope.
  • 5/16" Grade 5 hardware is used to fasten the pulley and the two tower brackets. I used ⅜" nuts as spacers for the pulley mount, and allow room for the brackets to comfortably grab and rest on the tower's X-braces. The rectangular pieces are galvanized steel, thick gauge construction brackets that hold the X-braces between the pole and brackets.
  • I did not weigh the gin pole but it is very light. I can manipulate it on the tower with just one arm.
  • There is an important difference between this gin pole and the design Wade (Delhi) recommends (see earlier picture). The "official" gin pole uses two vertical rods at each bracket which surround the tower leg and rest on the top of an X-brace on both sides of the leg. Mine uses one vertical rod at each bracket and these fit the bottom of X-braces on the left side of a tower face. This is important since the distance between X-braces on a DMX tower is not constant, varying between 36.5" and 37.75" on those I measured. As a result typically only the top bracket on both gin pole designs rests on an X-brace (the brackets are 36.5" inches apart), and therefore the bottom brackets can move sideways. On the official design the tower leg prevents this motion. On mine the free bracket must be tied to the tower so that it doesn't pivot. I use a $2 heavy-duty rubber tie-down which is easy to attach and remove with one hand.
  • Another difference is the total height of the gin pole. The official design is 15' tall, for the purpose of picking up a tower section from the top. This is not strictly necessary since it is only mandatory to pick up a tower section from above its centre of gravity. This allows for a shorter and lighter device, and one that is stronger for the same steel gauge. That is why I can get away with using 18 gauge steel rather than the recommended 16 gauge. However it causes the section to stray further from the vertical, requiring the section to be manually rotated when dropped into the lower section.

Tower erection is dangerous. You can be killed or seriously injured by nothing more than a moment's inattention. Erecting a tower on your own without requisite experience is foolish. Don't do it. Get experience working with others on tower projects before going solo.

That said, here are a few points on safety relevant to this project.
  • Have a plan for emergencies. If something breaks and stuff falls are you standing in a safe place? If a snag occurs up in the air do you have a way to secure everything while you effect a solution? Is a partner nearby to watch over you, and help if necessary? Are you carrying a phone in case you can't move and need to call for help? These are uncomfortable thoughts but you must think about them before you begin. This goes for any tower work, no matter how many people are involved.
  • If you have someone helping you do you trust them? If you tell them "do this" or "don't do that" will they listen and obey? Of equal importance, will you take direction when someone points out a flaw in your plan or execution? Learn to listen to what others are telling you. Stubbornness and pride can kill.
  • Wear a certified hard hat and sturdy shoes when you are underneath any heavy equipment, tools, dangling tower sections, etc. These many not save you from injury but that injury will be reduced, and you get to live a little longer.
  • Test your equipment before starting. Look over those forces described earlier and then, with the gin pole near the ground, lift some test weights a short distance, and try it at various rope angles. Do this even with borrowed or bought equipment, not just for (as in my case) homemade equipment. For one test, since I weigh 55 kg, I could subject the gin pole to 2x+ the weight of a tower section by grabbing both ropes and lifting myself off the ground.
  • If you must climb the tower while a tower section is in the air keep your body and hands away from the gin pole and climb on a side of the tower that is not under the tower section.
  • Plan, don't improvise. Know exactly what will happen, what to do when the wrong thing happens, and lay out all the needed tools and parts where you'll need them. Clean up afterwards.
  • Use a rope cleat on or near the tower to easily secure a load with one hand and no knots. I typically bolt a cleat at shoulder height on my own towers.
The anti-climax: lots of planning and preparation makes for short work

Be safe out there
To close off this article I'll post one more picture. Above is the third section (DMX-4) dangling in the air, ready for me to climb up and bolt it into place. The lift rope is secured to a cleat and then tied off to the tower for redundancy. I did have to climb the tower once during lifting when the section got caught on a temporary rope guy. My homemade equipment performed well.

That tower section is now attached, the bolts torqued to spec, and the first set of guys attached to the "ugly" guy station. The guys are not yet at final tension but they have been adjusted to bring the tower into good vertical alignment. There are now 3 more sections (half the tower) to go, plus one more guy station.

Lifting and securing the tower section took only 30 minutes, including the time taken to unsnag it from the guy rope. It took longer, 90 minutes, to attach and tension the guys and plumb the tower.

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