An SLCD is made of four lobes that are usually relaxed outwards. As the trigger of the SLCD is pulled, the width of the lobes decrease. then when the trigger is released, the springs that are inside of the cam push the lobes outwards against the crack thus holding the cams in place. The cam is then pulled out by using the trigger to force the springs to tighten, thus pulling the lobes back, and the SLCD is extracted fro the wall.
If a person were to fall, there are four different forces acting on each lobe. Each lobe is connected to an axel, and the axel is connected to the trigger where the rope s then clipped through a carabiner. Force of friction acting up, the force of the person falling, the Normal force of the wall pushing against the lobe, and the force of the lobe pushing against the rock due to the spring force. The normal force and the force of the lobe are equal to each other. In order for the am to hold, the frictional force must be equal to or greater than the force of the person falling.
We are taught to focus on the camming angle when w place a cam into the wall. This is becaus the camming angle is directly proportional to the coefficient of friction. If u=tan(o), then the bigger the angle of camming (from 0-90) then the stronger the force is.
The optimal camming angle for a black diamond cam is 15 degrees, resulting in an angle between the lobes of around 60-90 degrees. If we have to high of a ration of normal force to force downwards, then the cam will deform and may end up slipping out of the rock. Too small, and the cam will also slip out of the rock. If placed correctly, cams can hold in between 5kN to 35kN!There is small room for error, but lots of room for fun!
All figures were taken from:http://www.indiana.edu/~iubphys/undergraduate/theses/Thesis_Phillip_Anuta.pdf