There is an old adage “If the only tool you have is a hammer, every problem becomes a nail”.  In rescue, we often fall into the trap of thinking we only have one or two tools, and as a result the options we have for solving a specific problem are consciously limited.  Fact is, you may have a lot more tools, and thus more options, than you thought you had.

I wound up beside a “seasoned citizen” awhile back in a timber rigging class – this was back before timber rigging became passé (nowadays, a suggestion of using timber usually gets you the kind of look reserved for politicians and other snake oil salesmen).

We were rigging gin poles, and he remarked that he learned gin pole techniques in the army.  The army?  I asked him to elaborate and he said that when they got tanks stuck in Korea, they would use a tall tree and lots of cable to construct basically the same system we were using 4×4 timber for.

In the last few years, I’ve only seen a few A-Frames rigged, and those were for a high directional or attachment point.  However, a A-frame can also provide a considerable amount of leverage or, to use our language, mechanical advantage.  Specialized tools like the new TerrAdaptor system can easily be used to construct a-frames or gin poles in addition to tripods.  Fast, light, and back-packable, these systems, coupled with a knowledgeable and open-minded TR, can vastly expand the options you have.

To paraphrase Archimedes, mechanical advantage (MA) is nothing more than exchanging force for distance: if you pull a rope or push a lever one foot and the load moves one foot, you have 1:1 MA.  If you pull or push twice as far as the load moves, it a 2:1, and the MA continues to grow as your distance increases for the same load movement.

So what is the mechanical advantage of an a-frame?  It varies – at exactly 45 degrees it’s 1:1.  As the frame moves toward vertical, the MA increases.  Therefore, if you calculate the MA of an a-frame at any given angle, you actually are computing the starting MA.  Let’s use 30 degrees of luff, which is a good starting point.  Let’s also assume you use 16’ timber and by the time you wrap it and open it up, you have 12’ between the ground and the attachment point.  The resulting A-Frame will look like this from the side:

Now, I could bore you with cosine laws and Pythagorean theorem, but just trust me – If we luff our 12’ a-frame over at a 30-degree angle, the attachment point will be 6’ to the right and 10.4’ from the ground.  If I load the frame so that it starts lifting at 30 degrees, then I will pull 6 feet of rope to move the load 1.6 feet vertically (12’ – 10.4’).  Therefore, my mechanical advantage is 6:1.6, or 3.75:1

Keep in mind that the above example is a perfect world: it doesn’t take into account rope stretch, system compression, or my inability to accurately determine a 30-degree angle in the field.  However, I feel comfortable that if I get around 30 degrees of initial luff, I can count on 3:1 from my A-frame – use a z-drag to get it upright and your total MA approaches 9:1 at the start of the pull.

This sounds complicated, but it’s not: you don’t count moving ropes every time you rig a piggyback system, because you already know it’s 4:1.  Likewise, if you build an a-frame and luff it 30 degrees, expect 3:1 MA on the initial pull.  If you really want to remember something, remember 0.866.  Figure out the distance between the pivot and the attachment point, and that distance times 0.866 will be the distance from the ground to the attachment point.  Subtract that from the pivot/attach distance and you have the lift height, again assuming 30 degrees of luff.  To use our above example, 12 x 0.866 = 10.4.  Then, 12 – 10.4 = 1.6, which is the lift height (rounded).

One important point: remember that as the frame goes toward horizontal, the MA decreases – to the point where you can break a General Use rope trying to pick up 100 pounds.  Here are some examples of the MA for different angles from vertical:

  • 15 degrees: 7.67 to 1
  • 30 degrees: 3.75 to 1
  • 45 degrees: 1 to 1
  • 60 degrees: 1 to 3.75
  • 75 degrees: 1 to 7.67 (e.g. 767 pounds of force to move 100 pounds)
  • 90 degrees: infinite (i.e. you’re going to break something)

Now go try it yourself.  Don’t worry about a TerrAdaptor yet – get some 4x4s and dig up a vintage copy of Nethercutt’s “The International Manual of Basic Rescue Methods”.  Laugh at the old pictures, but respect what these guys did with a fraction of the equipment and the accumulated base of knowledge that we take for granted.  You’ll come away a more versatile and skilled Technical Rescuer.

 

Luke