Minimal extruder housing for use with the Printrbot Gearhead drive gears, 1.75mm filament, and a NEMA14 motor.
The secondary gear is inserted from the side, and the additional M3 screws or setscrews hold the gear in place and adjust the tension. It's designed to use a bowden coupler from RobotDigg. Make sure the setscrew doesn't stick out on the primary drive gear, especially if using a motor without a D-shaft.
Filament is Emerald Green PETG from Atomic Filament.
There is also a NEMA17 version and info about several other dual drive bowden extruders I've just uploaded on thrinter.com