General

I decided to develop gearbox with ratio from 1:25 to 1:35 to archive 180 degrees per second with full torque (i hope).

Small backlash is acceptable (at least for robot leg actuator)

All designs will be created in Openscad.

Design

I  had a few experiments in the past: So my favorite designs are: 

1) Spur Pinion  double gears organized in 3 gears per stage (each stage is a 3 spur-pinion gears at triangle corners) with ring gear as output: Openscad drawings and visualization will be uploaded soon.

This type of gearbox have 1:33 ratio (can vary, but not limited at max)

2) Cycloidal - i experimented in past, I got 1:25 ratio (strong limit for 70x70 cm footprint), but my past experiments had some Issues (mostly because of missing bearing at critical places)

Best ideas for Cycloidal you can see here: 

https://hackaday.io/project/165653-3d-printed-robot-joint-with-active-compliance

https://hackaday.io/project/167855-simple-cycloidal-robot-leg-for-quadruped

Gearbox exploded view concept

Key features: 

• Ratio: 1:33.1852 (1st stage 1:2.66, 2nd 1:2.666 output is a 1:4.66)
• Output gears module can be bigger then stage modules
• Gear height can be different for each stage
• Place to embed as5600 i2C absolute rotary sensor (2$ on aliexpress)
• Extra stage can be added without problems (1 extra stage and you can get 1:88 ratio)
• Only output stage is rotate (all double gears are rotates on the static common shafts)
• Each stage have 3 points of contact (less backlash and more torque)

Gearbox disassembly video

Assembled gearbox

Actuator test without absolute encoder

Gearbox test with absolute encoder