[Zyrano]

We had the same problem with the these gearboxes last year, you just need to play with the spacing of the gears relative to each shaft. We were able to use some hardware store nylon spacers to make everything work. if you look closely, you'll see that some of the gears are able to interfere with each other when you push the slop one way or the other. It was somewhat fustrating as my students had to assemble and disassemble multiple times before finding the problem.