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:
https://www.instructables.com/member/the_burrito_master/
and you can visit Imperar and his k'nex supercar made with sketchup here:
https://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.

Step 1: Step 1

1. Overview
2. Build
3. Build
4. Build
5. Align the sections from 2. and 3. like so.
6. Attach with the sections from 4.

Step 2: Step 2

1. Overview
2.-6. Build.
7. Attach to Step 1.
8. Attach to Step 1.
9. Attach these pieces. Be sure it's on the correct side.
10. Attach this piece.
11. Attach the axle on the opposite side. You will need to reconnect a few pieces.
12. Attach the blue rod to the white connector and add a red rod to the yellow/green/white connectors.

Step 3: Step 3

This step will be short, and shortly explained, so you can enjoy putting the top half of the tank on at once. The large piece you'll be making is symmetrical. 2. and 3. are the opposite sides of the same piece.
1. Overview
2.-3. Build
4. Orient and attach.

Step 4: Step 4

Now it's time to build the last piece, the turret, which took me some time to figure out. I made a few minor changes.
1. Overview
2. Build.
3. Build. This section is symmetrical.
4. Add to the previous section.
5. A few more pieces, and you're done.
6. Connect the turret to the rest of the tank.
7. Add four red gears.
8. Add four more red gears. Look at the pictures in the intro if you need to.

Add the chain by making a long line of links and running the motor in the proper direction to drag it along (or turn the axle if you haven't attached a motor yet). I didn't count how many links there were on each track, but the key is making it small enough so that it doesn't fall off easily, but large enough so that it doesn't heavily restrict the shape of the track. There should be some slack if you lift the tank.