Use safety glasses when operating the PLA Spring Motor Demonstrator. While I've operated the mechanism well over a hundred times without a failure, I always recommend erring on the side of caution as the spring could snap and send pieces flying where they shouldn't be flying, and that's never a good thing.
UPDATE: Try "Spring New.stl" as opposed to "Spring.stl". Some have had problems with the original, hopefully the new one will solve those problems.
Carefully study "Assembly.skp". Note that the .skp file should be used as an assembly diagram and not be used to print parts as the parts in the .skp file may not be the final parts as found in the .stl files.
Print all PLA parts from the .stl files, print 3 of "Gear Idler.stl". Print "Knob.stl" and "Spring.stl" with Makerware "Standard" settings modified for 100% infill, the remaining parts using Makerware "Standard" settings with the default infill.
Test fit and trim, file, sand, etc. all parts as necessary for smooth movement of moving surfaces, and tight fit for non moving surfaces. Depending on the colors you chose and your printer settings, more or less trimming, filing and/or sanding will be required. The idler gears must spin freely on their respective shafts, and the main gear must spin freely on the hub of "Spring.stl" (prior to pawl installation). Also, "Knob.stl" must spin freely in the frame, while also remaining a tight fit in "Spring.stl".
Assemble as per "Assembly.skp".
Lubricate moving surfaces with light machine oil.
Comment if questions.
Hope you like it!