Functionality of Mechanical Advantage and Considerations

Written By: Lance Piatt

The Mechanical Advantage (MA) systems used in rescue incorporate one of the most elegant simple machines known to man, the pulley. While it is Newtonian Physics that explains how pulleys work, we owe the Sicilian mathematician Archimedes for its invention. History tells us that Archimedes boasted to his king that with enough pulleys he could move any given weight. The king put him to the test and it is said that he, single-handedly, moved a freight ship up the shore.

As rescuers, we must always remember that raising operations have a higher risk potential than lowering operations. There is often a tendency to overpower the system. This occurs by either building a system with too much mechanical advantage or by having too many haulers on the system. Experience will teach you which system is appropriate given the available manpower.

There are several factors to consider in deciding which MA system to build. Keep in mind that MA systems are more complex and have more inherent risks than lowering systems. Always consider the possibility of lowering rather than hauling. While there are times when lowering might entail a longer carry out or more difficult terrain to negotiate, when factoring in the work involved in hauling, a lowering operation may still be more efficient. Lowering also carries the advantage of allowing gravity to work for, rather than against the operation.

When building an MA system, remember to factor in system inefficiencies. MA systems are identified by their Theoretical Mechanical Advantage (TMA), which is the MA generated in a perfect world with no friction. Practical Mechanical Advantage (PMA) is the actual MA generated after friction is factored into the system. For example, when using a 5:1 (pronounced five-to-one) MA system to haul a 500-pound load, the real-world hauling effort will exceed 100 pounds; this is due to friction.

Mechanical advantage systems always involve trade-offs. While applying an MA system to a rescue load will multiply hauling power, the speed at which the load moves will decrease proportionally. Just as a lower gear in a motor vehicle generates greater torque, or power, at the drive wheels at the cost of speed, MA systems multiply the pulling power of rescuers, but the load moves proportionally more slowly. In addition, as MA increases, the amount of rope that must be pulled through the system also increases proportionally. The speed of a hauling operation is often less of a concern than the amount of rope that must be pulled through a high MA system to move the load.

There are several ways to minimize the effects of friction on MA systems. The friction of the rope rubbing across various surfaces is potentially one of the greatest inefficiencies in rope systems. The use of secondary directional anchors can help guide the ropes through the most efficient routes.

In addition to decreasing friction, padding contact points and edges can also protect system components from damage. Using purpose-made devices such as rollers and high-density plastic pads can greatly reduce friction and increase MA system efficiency.

Remember, no pulley is 100 percent efficient; pulleys always introduce some friction into the MA system. Use good pulleys of the proper diameter, but do not overuse them.

Other inefficiencies in our MA systems result from excessive angles. Maximum efficiency occurs when ropes entering and exiting pulleys are parallel to one another. As these angles increase, the mechanical advantage decreases. Often, change of direction (COD) pulleys can minimize these angles but the COD pulley itself will introduce additional friction.

When deciding which MA system to build for a two-person rescue load, technicians must consider several factors. How many haulers are available? How much friction is on the mainline? Can that friction be managed with padding, rollers, etc.? How heavy is the load?

In general, strive to use the fewest number of haulers on the lowest ratio MA system that will get the job done without risk of injury to the haul team. If hauling becomes more difficult during an operation, do not simply add more haulers; stop and find out what has created the additional load. The load may be caught on an obstruction. Adding additional haulers without addressing the cause of added resistance could cause equipment failure or could injure the rescuer.

Constant communication between the haul team and the load is vital.

Peace on your Days



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