Yeah you nailed it with the PTFE tubing and shaping it just right underneath the bearing and the drive gear. I had to use an exacto knife to shave down the tubing so it fit just perfectly to not allow barely any gap at all. I also ended up needing to buy an E3D hotend to get consistently good prints with NinjaFlex and other TPU-like materials.
That being said once you have everything set properly, printing with NinjaFlex is IMO one of the more rewarding and impressive materials to work with. It's durable in the extreme and the little straps I designed and printed with it I use literally every single day on my coffee mug and water bottle(s)
http://www.thingiverse.com/thing:835257. I'm also convinced a person could design some incredibly cool and mutli-functional wearables with it, which I'm slowly working on.
As a printing tip if you use Simplify3D, I had to set my extrusion multiplier to 1.12 and infill extrusion width to 1.10 in order to have a solid "water-proof" surface to prints. Otherwise the infill becomes more of a fine mesh, which is cool in a way and could be useful, but I need my surfaces solid. Also I found that much over 16mm/s as a main print speed gave the best results and least amount of failures when feeding the filament.
BTW if you get into printing a lot of flexible filaments there's a Swedish filament re-seller that has it in varies hardnesses for considerably cheaper than NinjaFlex:
https://www.creativetools.se/hardwa...ies/filaments/flexible-filaments/eco-tpu-1-75