By Michael Tain
Mechanical advantage is, simply put, the use of a system of ropes and perhaps pulleys to multiply an input force, creating a greater output force than could otherwise be generated — and tree crew members use the mechanics of the system to their advantage. Many arborists have used a very simple mechanical advantage system where only rope is used to pull over trees. The pull line, attached to the top of the tree to be moved, is passed around another tree, a knot or hitch of some sort is formed in the standing part of the line between the two trees, and the running end of the line is passed through this loop. By pulling on the line running through the loop users have achieved mechanical advantage — though of a somewhat crude form — as a great deal of the user’s input force is lost to the friction of the pull line going around the anchor tree and to the rope-on-rope contact at the loop. Obviously, mechanical advantage systems can have a great deal of value in a variety of tree care operations, from pulling over a tree against its lean in a felling scenario to lifting a horizontal branch that has an obstacle or target beneath it. However, it can, and does, have requirements, advantages and disadvantages of which users must be aware.
Advantage or Disadvantage: Tree crews want to exert the greatest amount of force, whether it be lift or pull, on the object to be moved, not on the anchor point for their particular system. Thus, knowing how to determine the most advantageous setup of the system is critical. Additionally, output is going to be adversely effected by friction in the system, so reducing friction as much as possible is in the user’s best interest. Pulleys are an excellent way to do this, and although it is impossible to remove friction completely, the use of pulleys in a system will lessen the friction involved greatly. A simple way to determine how much mechanical advantage is being provided is to count the parts of line at the moving pulley. For example, three parts of line at the moving pulley would indicate an advantage of 3 to 1. This scenario would indicate that an input force of 200 pounds would be multiplied by three, resulting in roughly 600 pounds of output force, though some would be lost to friction. Users who set up a system in such a manner that more lines are acting on the anchor pulley than the moving pulley have rigged to disadvantage and are actually pulling harder on the anchor than on the object they wish to move.
Input force: Although trucks, skid-steers and other forms of motorized equipment are often used in tree care operations to provide pulling or lifting force, this use is ripe with the possibility of catastrophic failure, and should be avoided if possible in felling or rigging operations. Simply put, it is extremely difficult — if not impossible — to estimate how much force is being put on the tree being pulled over, the branch being lifted, or the rope and equipment being used when generating force with motorized equipment. Most individuals can pull their body weight for a short period of time, though this is impacted by other factors such as footing, grip, etc. Thus, users of mechanical advantage systems employing crew members as an input force can fairly accurately estimate what forces the tree and their gear is experiencing, and perhaps avoid “blowing up” ropes, treetops and slings.
Prusik Minding Pulley (PMP): Prusik minding pulleys are pulleys specifically designed and formed to “self-tail” a Prusik or other gripping hitch on the pull line. They are typically somewhat bell shaped, and come with a varying number of sheaves for the rope to travel on. The use of PMP’s in mechanical advantage systems gives users the security of a Prusik or other hitch in place to grab the pull line should they lose their grip or footing, or simply need to rest, thereby not releasing/losing the pull or lift already generated on the tree or branch.
Moving Pulley Attachment: The moving pulley is the one that is attached in some manner to the pull or lifting line, and should be the pulley that has the largest number of parts of line at it — thus generating the most mechanical advantage on the object to be pulled or lifted. This pulley could be attached to the line by tying some form of midline knot. However, this technique should be avoided if possible, due to the inherent strength loss created by tying knots in a line and its inability to be adjusted should the system “two block” or be brought all the way together without enough lift or pull being generated. Another method of attachment is the use of a mechanical rope grab or ascender; and although simple and easy to employ, this method should also be avoided, if possible, due to its tendency to start to actually tear the cover of the rope under excessive loads. The use of a VT or some other form of gripping hitch is the best option in this application. They will minimize strength loss, provide fairly easy adjustability, and tend to slip under excessive load, not the best of outcomes, but better than torn strength compromised ropes.
Mechanical advantage systems can make many daily tree care tasks much easier and efficient, but only if they are used correctly and care has been taken to examine each component of the system closely. There are many more factors involved in their use than those discussed here such as fiddle blocks, the amount of line required for a system, etc. But these basic concepts provide a starting point for exploring the joys of working smarter, not harder.
Michael (House) Tain is a contract climber, splicer, educator and writer associated with North American Training Solutions www.northamericantrainingsolutions.com and Arbor Canada Training and Education www.arborcanada.com He is currently located in Lancaster, Ky., and can be reached via e-mail at firstname.lastname@example.org