Cube 3D printer community

Hey guys, could you please help me out.  I have a cube 2nd gen trying to use bulk hack with makerbot pla 1.75mm. The problem is that half way through printing, the filament starts to get stripped by the gear. If i loosen the gear, then the black and white stripped gear stops spinning because it cant grip the filament. I know its trial and error but can someone pls tell me how to set the correct tension of it using a spanner (e.g. place the filament first and then put the bolt on) Pls help guys. It would be much appreciate. Cheers !