Many was the time a gaggle of sixth grade engineers and I would stand by the white board sketching out a gear assembly in an effort to design a structure that would duplicate the movement needed for a project. This is one area where I always wished I had deeper training so I could have more ably guided my students. I did the best I could by having samples of mechanisms and photos of what other students had done. (And the trusty whiteboard.)
I saw an article about a site called Kinematic Models for Design Digital Library (KMODDL) at Cornell University. What a great resource! They have photos and movies of a variety of mechanical models. Are you trying to describe a universal joint to a group of seventh graders? Navigate to this site and you can show them a model of a universal joint. You can also show them a video of the machine in action.
If I still ran a tech lab, I'd build a challenge or two around a few of these descriptions. Possibly have students reproduce some of the them using their Lego kits. If you like this kind of stuff, this site will become a favorite.
[Story source frome Makezine:KMODDL - Kinematic models for design digital library: http://blog.makezine.com/archive/2007/12/kmoddl_kinematic_models_f.html?CMP=OTC-0D6B48984890]
[Image: Grabbed from KMODDL/Universal Joint; http://kmoddl.library.cornell.edu/model.php?m=240]