[Bob Steele]

I used a similar method in 2003. But be careful
the slots are in the same direction that the pull of the chains will be..
So when the motor pulls it could slide the screw to the loose side. It will be difficult to tighten the screws enough to keep this secure. You might add a second screw (and spacer) to each side directly in the slot.

If you really jam your motors on... (in quick acceleration or in changing from one direction to the other....) you can put a great deal of force in the same direction as the slots...if one side is less tight than the other you will get misalignment and you will throw chains...

You might want to consider something that moves the motor in and out perpendicular to the pull of the chain... this would not have as great a tendancy to loosen. There are many ways to keep tension on chains..
I am sure if you do a search you will find others.

Testing is the key... keep track of how tight you tighten the bolts... (use a torque wrench) Don't count on tightening as tight as you can without being able to know how tight that really is.... (this can vary between people and between situations...) Reproducibility and testing the system under these shock loads will be necessary for you to see if this will work for your situation.

Just be wary of the nature of this design... As I said... back when we were using drill motors and a plastic transmissions this method caused us a great deal of grief...and matches with our robot traveling in a circle on the field while dangling a chain...

Good luck in your design!!