Nitrile O-ring belt drive - suggested stretch percentage..?

[Wal]

Hello all,

I'm currently building a small parts tumbler in which I intend to tumble aluminium parts amongst 3mm porcelain beads.

Some pics below:



Although the 12v 350mA 20RPM motor spins the tub reasonably well (provided that the static friction remains low) I'm thinking of adding an identical motor below it and connecting it via a nitrile o-ring belt so that it's 'helping out' the motor that's directly driving the tub, like so:



Now, let's say that an O-ring with an inside diameter of 100mm fits perfectly (un-stretched) around the two couplers:


I'm guessing that this wouldn't be ideal as there'd be very little traction or transfer of torque - so my question is: how much stretch percentage should I introduce to the o-ring for it to do its job as a conduit between the two motors? 10%? 20%?

Cheers!

Wal.