The "critical point test" (aka "scissors test") requires that if any one item fails, nobody should be injured.
A "bend" is a knot that joins two ropes.
A "hitch" is a type of knot that must be tied around another object.
The "whistle test" requires that if all rescuers let go, nobody should be injured.
Friction is the resistance to motion that occurs when two objects touch. During a rescue, friction allows you to slowly lower a litter through a brake bar rack and to maintain control while rappelling. Even the ability to control how fast a rope slides through your gloved hand depends on friction. When descending, friction is your friend.
Friction can also be a thief, stealing energy as you try to raise a load. It resists movement as your rope is pulled over rocks and the edges of buildings, through carabiners and pulleys, and eventually finds its way to your subject. And the angle that the rope encounters other objects has a profound effect on the friction. For example, if the rope in your raising system must bend 90 degrees over the edge of a building, it will typically double the force required to raise the load. When raising, friction is your enemy.
Regardless of whether the friction is helping you while lowering or making raising more difficult, the energy consumed by friction is not lost—it is converted into heat. The heat is initially absorbed by the objects that are rubbing against each other and it is subsequently dispersed into the air. The smaller the objects that generate the friction, the hotter they will become. For example, given the same friction and forces, a small tube-style descent control device will become much warmer than a large brake bar rack, simply because the rack has more mass to absorb, and more surface area to dissipate, the heat.
|Typical Friction Percentages|
|Brake Bar Rack||75% - 98%|
|Carabiner (180° bend)||40% - 60%|
|Figure 8 DCD||90% - 95%|
|Pulley (ball bearing)||5%|
|Pulley (bronze bushing)||30%|
|Rescue 8 DCD||80% - 95%|
All illustrations on this website were created using the vRigger software.