As a rough guide the cam exerts up to four times the force that you the climber can create in a fall outwards onto the side of the crack.
In doing so the greater the force in a fall the greater the hold force of the cam. It is therefore essential you place the cam into solid rock, and be wary of using them on superficial flakes that might be prized off in by the force of a fall. Secondly the cams come in many sizes and the tolerance to what size cam will fit where is important.
First off to stay in place the cam converts downward force into outward force.
This means the rock needs to be much more solid than it would if you were placing a hex or wire.
As if you over or under cam the placement the integrity of the placement is compromised.
As a guideline try and get the lobes of the cam to be cammed between 25% and 75% of their range.
Camming devices and Friends were a breakthrough in climbing design and are mini engineering wonders that most climbers covert.
They are by far the most misused item on a climbing rack as there are many things to consider when placing.
There is more advice on cams in How To Climb Harder, which you can buy from the link below.However if you understand these aspects then cams can offer great protection in places that wires or hexes simply do not fit.First off you need to understand the mechanic is a basic way, as these show one of the limitations of all coming devices.Finally cams will walk into a crack if the rope moves it when you are climbing as such extending a cam can be vital, especially if the cam might walk into a crack that widens out deeper into the crack. Similarly it may walk into a place that makes it hard or impossible to remove, which can be costly.
Other important things are to get all four lobes in contact with the rock and make sure the stem is pointing in the direction that any load will occur if you fall off.
As a basis for specifications for the testing of spring loaded camming devices used as rock climbing anchors, two successive models of these devices are developed.