Choosing Haul Systems
In making choices as to which haul system to use the haul team leader must actively integrate the qualities of the various mechanical advantage (MA) systems, see chart, with the physical conditions at hand and the resources available. Generally the most efficient system will be the simplest but will in turn require the greatest number of haulers, i.e. the 1:1. To determine if this is an appropriate choice one needs to be able to asses the “force” requirements for the job at hand and link those requirements with the “force” available, i.e. the force that your team will be able to exert over the period of the lift.
It seems to be generally accepted that the average rescuer can exert a force of 50 lbf although some studies have shown average individuals “grip strength” to be more in the neighborhood of 30 lbf.
If we have 10 haulers then we can can expect a sustained output of between 300 and 500 lbf, which, given a perfectly frictionless system would be capable of raising a 300 to 500 lb load. If the raise is relatively short then the 500 lbf choice in team assessment will probably suffice, but in the case of a long raise in which the team will be required to exert themselves over a prolonged period of time the 300 lbf force assessment will be alot easier on our haulers and is probably more appropriate.
We next need to determine what the actual load will be. If we have a patient in a basket with an attendant, we can assume that we probably have a combined weight of between 400 and 600 lbs. If we are sure that this weight is close to 400 lb then our team’s “force” will probably be adequate for a short haul using a 1:1 MA system. If on the other hand the combined weight is closer to 600 lb, it’s a long raise, or there is significant friction in the system, then a 2:1 MA system will be required. The source of this friction might be directional pulleys, edge rollers or even rope pads placed at the lip and other areas where the rope makes contact with potentially damaging surfaces.
In setting up our 2:1 MA system we need to take into account the length of the raise and be sure that our haul line is at least twice the length of the raise. Another consideration is the possibility of the two halves of the haul line becoming twisted around each other. One cause of this is from basket rotation in a free drop. This will greatly increase the friction in the system possibly to the point of making the system unworkable and therefore dangerous. A furthur danger can come from branches or other objects becoming caught in between the haul line and the pulley attached to the load as it rises. If these conditions are felt to be a significant risk, then a single haul line can be “ratched” up using a 2:1 MA.
It should be noted however that this scenario shares the same disadvantages inherant with higher MA systems. These systems must periodically be stopped and reset. Once the haul is started with a simple 1:1 or 2:1 MA, the load can be raised continuously until it reaches the top. This can save time and give the patient a smoother ride, two important goals in a rescue environment.
However if our resources are not sufficient to accomplish the lift with either a 1:1 or 2:1 MA then a 3:1, 4:1 or higher MA will be required. Space can be a determining factor in choosing a system; if your haul team is small because there is no room for more bodies, you will need to use a higher ratio MA system.
Both 3:1 and 4:1 MA systems have disadvantages and advantages over one another. As previously mentioned both have to be “ratcheted” or “reset”. Besides being more equipment intensive they also introduce greater losses into the lifting system through friction (see chart). A further disadvantage of the 4:1 MA is that it must be “reset” after only traveling half of the distance it requires to be set up in: i.e. the 50% collapse rating in the chart. This can be a real problem in a tight space, creating the necessity for very short lifts between “resets”. A 3:1 MA or a “Jigger”* would probably be a better choice in this case since they both collapse completely. The standard “Z rig” which has a 3:1 MA has the distinct advantage of requiring only one rope to operate while the 4:1 MA or Piggy-Back requires two. In the end however the higher MA systems may be the better choice simply because of their greater lifting capacity using fewer people.
A good rule of thumb is to use the simplest system that will allow your resources to accomplish the task: K.I.S.S.
Carroll C. Bassett
NOTE: I have purposely left out any reference to belay systems, progress capture devices, etc. in an effort to concentrate on some of the considerations for choosing a haul system. Their absence in this discussion does not imply their lack of importance; it is simply beyond the scope of this work.
* “Jigger”: A 4 or 5:1 MA system composed of two double sheave pulleys rigged one above the other. When the rope emerges from the pulley attached to the anchor, the system has a MA of 4. If the system is inverted and the rope emerges from the pulley attached to the load, the system has a MA of 5.
Common Mechanical Advantage
|System Friction * :||0%||5%||10%||10%|
|Force (L=load):||1L||1/2L + 5%L||1/3L + 10%L||1/4L + 10%L|
|Collapse % :||100||100||100||50|
|* Good low friction pulleys generally are in this range, some are much worse. At lower loads, bearings are more efficient than bushing but at higher loads this can reverse. Also the force to bend kermantle rope increases with load (inner fibers sliding over each other under increasing tension). The addition of edge rollers and directional pulleys will also contribute to system friction and should be considered.|