Planetary gear set has a sun but this gear set is without a sun. Also, it has double plannet(s). One ring is fixed while the other is the output. Motor rotates the planet(s) around the common center of the rings. There could be a single planet (like in cycloidal gear) or more planets (like in planetary gear).

Advantages

• High gear ratio
• Less parts
• More ratios possible
• High torque
• Does not need special components (like in cycloidal gear)
• Bi-directional (you can swap input/output)

Disadvantages

• Axial output configuration suffers from torsion
• Torsion avoidance configuration has side output only or requires counter balast

Gear ratio

1 : dr2 / (dr2 - dr1 * ds2 / ds1)

ds1 .. diameter (number of teeth) of the 1st spur gear
dr1 .. diameter of the 1st/fixed ring
ds2 .. diameter of the 2nd spur gear
dr2 .. diameter of the 2nd/output ring

Negative number represents counter-rotation.

There's a Python script to find proper gearing for the specified gear ratio.

Examples

• Prusa Nextruder (1:20): 5, 6, 7, 8
• LDO Sherpa Mini Extruder (1:5): 4, 6, 8, 10
• hours (1:12): 6, 7, 13, 14