By Michael “House” Tain
The term mechanical advantage can, and does, mean many different things in a variety of fields, industries, and applications. But for the tree care profession it is a fairly straightforward concept; one that once properly understood and implemented can make tasks involving the movement of heavy things much easier. The application of mechanical advantage in tree care operations — whether they are pruning, rigging or removal activities — allows those who use it correctly to “multiply” the input force being brought to bear on the load in question. This “multiplication” can be achieved in several ways. But typically the simplest and most commonly used method is the use of blocks/pulleys and additional rope to magnify the input force. However, as with any method used on this planet, the existing laws of physics do apply; and a failure to understand the principles, advantages, and limitations of this “force multiplier” can lead to catastrophic consequences.
A common confusion among users of mechanical advantage is how much the input force is being multiplied. An example of this would be a tree care professional attaching a line to a log, running it through a pulley, then attaching the other end of the line to a truck, with the assumption that mechanical advantage has been gained and they are actually pulling “harder” on the log than they would have been otherwise. The reality is that no advantage has been gained in this scenario as described — all that has occurred is that the line has been redirected. In fact, depending on the angle of the line exiting the pulley, the system is in all likelihood putting far more force on the anchor the pulley is attached to than either the log or the truck. A roughly 2-1 advantage could be gained by attaching the line to a suitably sturdy anchor point, running the line through a pulley attached to the log or load to be moved, then attaching the other end of the line to the truck or suitable input force. Each part of the line is seeing whatever input force is being put in resulting in the load to be moved seeing roughly twice the input force. Although a 2-1 advantage can be gained in this manner, it can be quite hard on pulleys — particularly in the case of pulling a tree over in one direction or another during felling operations. This technique can be used without a pulley simply by passing the rope around a suitable branch attachment point or the trunk of the tree; but a great deal of force is lost to friction, and the bark/wood can be quite hard on the rope. When using more complex mechanical advantage systems the easiest method to determine the amount of force multiplication is often to count the number of parts of line at the moving block or pulley. For example, if there are three parts of the line at the moving block, then the input force is being multiplied by three — or a 3-1 mechanical advantage exists.
A 5-1 mechanical advantage system with the load to the left and the anchor point to the right. Counting the parts of line at the moving block will roughly determine the multiplication of force.
All photos courtesy of Michael “House” TainMechanical advantage can be gained and generated simply by using the line that is attached to the load to be skidded/lifted/pulled with the addition of appropriate pulleys, anchor points and control methods. But users must keep in mind that to multiply the input force, rope is going to be used up. For example, a 3-1 system will require roughly three feet of line for every one foot it will move. Thus, some users may wish to employ available existing systems that are added to or “piggybacked” on the rigging line. These systems are often called fiddle blocks, and come with their own line for input force and generating mechanical advantage. They also often have an integrated cam or rope grab of some sort to allow tree crew personnel to relax once an appropriate amount of force has been applied, without having to constantly hold the load. In situations where the fiddle blocks may come together or “two block” prior to the load being lifted or moved as much as desired, operators should have a separate anchor point (such as a Port-a-wrap) set-up to stabilize and hold the load while the fiddle blocks are disengaged and reset. The Good Rigging Control System (GRCS) can provide mechanical advantage, though of a different nature, through its integrated winch. This device provides an advantage of 22-1 when the handle is turned in one direction, and 44-1 when turned in the other during lifting/pulling/skidding operations.
A Prusik used with a PMP to provide control and rope grab capability at the anchor point.A properly designed and set up mechanical advantage system should not be exposed to the dynamic shock loads that a normal rigging system would be. So the use of pulleys rather than the heavy-duty arborist blocks is certainly acceptable, although blocks can also be used if desired. The ability to use pulleys allows climbing arborists access to a variety of types, forms and functions as long as the safe working loads and appropriate uses are kept in mind. Of particular value will be features such as multiple sheaves, which allow a variety of turns of the line, more turns generating more force; and Prusik minding capability (commonly called PMP or Prusik Minding Pulleys) which allow the use of a Prusik as a constant load tender. PMPs typically have a very distinct bell shape with flat sides and corners that help tend or mind the Prusik when in use — thereby eliminating the need for another crew member to monitor the Prusik and adjust it as needed.
As can be easily seen once one understands the concept of mechanical advantage, forces strong enough to break gear, ropes or pieces/parts of trees can be easily generated with these systems. For that reason, it is recommended that tree crews try to avoid the use of trucks, skid steers, or other mechanized equipment as an input force. Most individuals can pull or generate a force roughly equal to their body weight for a short period of time; this allows for a rough approximation of how much force is being applied to the system and the load being pulled or lifted. For example, two 200-pound crew members pulling on a 3-1 system will generate approximately 1,200 pounds of pull/lift force on the load. Even this rough approximation is not possible when motorized equipment is involved, which can lead to easily exceeding the breaking strength of any and all of the components in the system.
Control of the load is a key issue in the majority of tree care operations, for both the safety of personnel and to avoid property damage. Any mechanical advantage system employed should also have some form of control component. Readymade systems such as fiddle blocks or a GRCS will most likely have an integral control mechanism of some sort. But, as mentioned earlier, users must look ahead and have an additional control method in the event of “two block.” Mechanical advantage systems created from the actual pull/lift/load line should use added control measures such as Prusiks or other hitches in conjunction with PMPs to ensure control in the event a crew member slips or simply needs a break from pulling. These hitches, properly designed and implemented, will hold the load static without any loss of lift or pull.
A Valdotain Tresse used to attach a Prusik minding pulley (PMP) to the line in a mechanical advantage system.Attachment points can certainly be easily created in a pull line by tying knots, but the resulting loss of strength in the line and the inability to move the attachment point both outweigh this ease of creation. The use of rope grabs or ascenders to attach pulleys to the line should also be avoided, because this can create sharp bends in the rope decreasing strength — and, in extreme cases, even tear the cover of the rope. The best option for attachment is a hitch of some sort tied with a separate piece of cordage around the rope with the pulley or device attached to the ends of the hitch. Hitches such as a Valdotain Tresse (VT) have been shown to work well — though users must keep in mind the extreme loads to which these hitches will be exposed, and should increase the friction or “grab” of the hitch by increasing the number of wraps around the line (many more than in climbing applications). The use of a hitch not only decreases the amount of strength loss in the line by not sharply bending it or tearing the cover in most cases, but also allows for adjustability in the event of a “two block,” assuming a load control measure of some sort is in place.
Although this is only a brief introduction to the wonders and applications of mechanical advantage, it does provide some basic concepts and ideas to build upon. This basic knowledge, coupled with some training and experience, can quickly help tree care professionals realize that with enough rope, pulleys and suitable anchor points, the question moves from “What can we lift?” to “What can’t we lift?”
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