To produce a camming device that works over a wide range of crack sizes, multiple rods are combined into a two dimensional surface in the shape of a cam.
This shape satisfies the criterion shown in Figure 3a below: the angle between the surface and the line perpendicular to the radius must remain a constant, the camming angle, .
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The parallel crack criterion is also important because it bounds narrowing and flaring cracks; parallel crack results define a performance limit for each regime.
As a first approximation, a camming device can be modeled as a rod wedged or ``cammed'' in a parallel crack as shown in Figure 2a below.
If the rod does not slip out, a downward tug or force applied to the upper end wedges the rod harder into the crack, causing the rod to push against the walls with a normal force.
Whether or not the rod slips out is determined by a third force, that of friction.
Figure 2b shows the forces on the rod; the friction force on the upper left end of the rod is intentionally omitted.