tight chains

[mikemat]

From my understanding of your mechanism, I'd imagine what you are seeing is the chain transferring torque to the gearbox. If you are using it to tension elastic, all the force the elastic has needs to go through the chain to the gearbox, and it only runs through one side because chain always pulls. If you chain up 2 wheels and pull them in opposite directions, you should see the same thing happen. It shouldn't be a problem because it's just the normal function of a chain amplified under the extra tension.