High Ratio Eccentric Gearbox 12099:1

This is a design for a 3D printable gearbox with an insanely high gear ratio in just a single stage.

It was originally designed as a telescope drive motor that does not need microstepping and as such is meant to fit to a NEMA 17 motor.

WARNING: I have not printed this gearbox yet. There might still be problems with tolerances or assembly.

Required Parts

• 4x 626 Bearings
• 1x 6202 Bearing
• 1x M3x20 Bolt

How it Works

The design of this gearbox is a modification of a planetery gearbox. The input sun gear is replaced by a hollow gear and the stepped planets are reduced to a single one with almost the same number of teeth as the ring gears. With this, the transmission ratio is proportional to the number of teeth of one of the gears (the number sia lmost the same for each) squared.

While the insane gear ratios achievable with this design seem very desirable, it is mostly unusable for power transmission. The efficiencies are so bad, that almost all of the power is wasted internally. The input torque i snot far away from the output torque and the gearbox approximates a clutch.

The cycloidal and harmonic drive gearboxes are kinematically equivalent improvements on this design. For the harmonic drive, a wave generator instead of the eccentric shaft works together with an elastic planet to get a higher contact ration and remove all backlash.

Cycloidal gearboxes improve the efficiency by eliminating all sliding contacts with the help of bearings. This makes them backdrivable, which this gearbox is not.

More information on this type of high ratio gearboxes can be found on my website

TODO:

• print & assemble
• test strength
• test accuracy and efficiency

Print Settings

Printer Brand:

ELEGOO

Printer: 

ELEGOO MARS

Supports:

Yes

Notes:

This gearbox is very small, with teeth of module 0.5. Therefore it is advisable to print it on a resin printer. Additionally, the tolerances must be kept low.
To position the parts, they should be pritined upright on the buildplate, with the symmetry axis aligned with the z-axis. Try to keep regions that hae to be very accurate, e.g. the shaft couplings on the top. I did not print them directly on the buildplate, but used supports.

For some parts,such as the ring gear and the housing, this can cause problems with layer seperation from the suction cup effect. My remedy for this was to use a slow lift speed and heating of the printer to 40°C.

How I Designed This

If you want to design such a gearbox yourself, you have to watch out for some things. Because the difference in the number of teeth is so small, interference between the inner and outer gears is a problem. This can be fixed by using a profile shift or increasing the tooth difference a bit. A higher tooth count difference comes at the cost of increased eccentricity, therefore more vibrations.

Because in my case, the main limit to the maximum output torque was slipping gears because of shaft deflection, it was important to keep the distance between the shaft bearings short and use a large diameter. Furthermore, the shaft has to be split, which required a stiff coupling between both halfes. I chose to insert an M3 boltto transfer the bending moments and at the same time locate the second bearing on the shaft. Another remedy i used, was to use a shallower pressure angle for the gear teeth. While nowadays almost all gears use a pressure angle of 20°, smaller pressure angles have some advantages, such as lower tangential forces which translate into smaller bending moments. Because i used 3D printing, it is easy to use an arbitrary pressure angle, as i am not limited by the available tooling.

To fix the imbalance of the gear, it is useful to use a counterweight for the eccentric planet. I omitted these for my purpose as a gear for an equatorial mount, since the input side is rotating so slowly.