Here is a design I have been working on for a 3:1 geared bowden extruder capable of pushing flexible filament such as TPU.

I printed everything with 25% tetrahedral infill in cura. 4 top/bottom/wall shells. 0.2mm layer height. Some of the fittings are a bit tight and some parts may need some cleaning after printing. Use supports everywhere 10% lines in cura as they are quite easy to remove and help with some parts.

1: Attach the gearbox part to the bowden motor mount (I use: https://www.thingiverse.com/thing:2369063) attached to your frame using either 4xM3x10MM bolts or other suitable bolts.
2: Fit bearings into main body and top cover, they should push all the way in with a little persuasion.
3: Fit M5 bolt into spur gear and push all the way in. Slide one of the washers on after the spur gear.
4: Push M5 bolt with spur gear attached into bearing in main body from rear.
5: Fit Mk7 extruder gear to M5 bolt and line up with filament holes in main body, it can help to mark where this lines up with the holes and file a flat spot for the grub screw but this is not essential.
6: Assemble arm, put M3 nut into hole in handle and thread 20-25mm M3 bolt in to it from the other side. Put other M3 nut into hex hole on adjuster cog and bolt onto the 20-25mm bolt. Push larger bearing onto the stub at the other end of the arm and then push the stub on arm retainer into the top of the bearing.
7: Attach arm bushing to main body with M3x10mm Bolt from rear. You may need to clean this part up a bit so it fits nicely into the hole on the arm. Slide assembled arm onto the bushing once attached and insert spring between the adjuster and hole on the main body.
8: Fit top cover to main body using 4xM3x10mm bolts. Put M5 washer on M5 bolt on top of bearing in top cover then bolt M5 nylock nut onto M5 bolt. Do not overtighten. The bolt should be able to turn freely with no pressure on the arm.
9: Fit assembled extruder to gearbox using 4xM3x10mm bolts, paying attention to the orientation of the push on gear for the stepper motor. It will only run one way. Attach the pneumatic couplings to main body. You may need to use a spanner as they are a tight fit at first.
10: Calibrate extruder steps per mm. Mine is 316.66 for PLA and 327.59 for TPU.

Leave any comments and suggestions in comments section please :)

Required parts:
13x M3x10mm Cap head bolts
1x M3x20 to 25mm Cap head bolt
2x M3 nuts
1x M5x45mm bolt
1x M5 nylock nut
2x M5 washers
1x Stock Anet A8 extruder spring. (about 8.5mm diameter and 20mm long)
1x Mk7 extruder gear. 13mm tall, 12mm OD, 10.6mm ID, 5mm bore
2x PC4-M6 pneumatic couplings
1x 608 bearing (I used 608-ZZ)
2x 625 bearings (I used 625-2RS)

https://www.youtube.com/watch?v=BB9uH5Oiohk

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