A "Cam" is a protrusion on one side of an axle that causes something touching the cam to moving closer to, and then further away from, the axle as it rotates.
Simple cams can easily be made with K'NEX by attaching a number of Clip with Hole end side-on to a rod.
The photo shows a model in which:
1. A handle may be used to turn a red rod (axle) on which 4 clips have been attached.
2. The rotation of the handle causes one end of a second red rod to rise and fall (as in a trip hammer).
Wider cams can be made by using more connectors, and cams with a greater rise and fall may be made by attaching orange 2-way connectors rather than clips with hole end.