Also I'll suggest that my first intuition regarding this is that you would need a very stiff spring.
if the spring is... anti-stiff (what's the word for that?
)... then under load it could allow the chain to lose tension somewhere along it's length, which could cause the chain to slip off.
without having much experience with chain tensioners, my intuition tends to lean towards an idler gear that can be moved in order to tension the chain, and then can be locked into place. this could be designed to be fairly simple.
currently my team has one axle on each chain loop mounted on slots so it can be tensioned pulled away and tension the chain. this is somewhat problematic since the axle is moving and thus is less than perfectly secure (even when locked into place).
I'd continue working on this idea and developing ways to tension chains..
maybe try making a list of possible solutions, and then make a pro/con list for each solution...
-Leav