This is the_burrito_master's k'nex tank, based on the E.U. tank in Battlefield 2142. He built it originally, and I simply copied it from pictures and then made an instructable using Google Sketchup. Imperar created the individual k'nex pieces in sketchup. I simply put them together. You can find the 3d model here:http://sketchup.google.com/3dwarehouse/details?mid=3d44ba4e984ee8bf7036b270f2e25c91
and you can visit the_burrito_master for more k'nex stuff here:http://www.instructables.com/member/the_burrito_master/
and you can visit Imperar and his k'nex supercar made with sketchup here:http://www.instructables.com/member/Imperar/
The tank is propelled by a motor, and is durable enough to drive off the 2 foot table it's pictured on without breaking. The chain does not jam into the gears, mainly because of a few green connectors guiding it at the bottom and the gears' elevation from the ground. The turret can be made to turn with a motor, but I didn't happen to have an extra motor, so you'll need to figure that out on your own, as well as adding a motor if you don't have the same one I do.
If any part of the instructable is unclear, let me know and I'll add some extra views of the section. Or, if you would prefer it, download Sketchup and the tank's file and look at it from any perspective your heart desires.