This annular gear was designed with the Bricklink part designer https://www.bricklink.com/v3/studio/partdesigner.page.
The custom annular gear part is originally designed by projunk available online: http://www.thingiverse.com/thing:1257736 under the Creative Commons - Attribution license 3.0.
Disclaimer: LEGO® is a trademark of the LEGO Group of companies which does not sponsor, authorize or endorse this project.