This Split Ring Compound Planetary Gear System Can Achieve Super High Reductions

The idea

Trying to move items that require a lot of torque can be very difficult for most motors out there, as they are normally rated to handle up to a mere half Newton-meter of torque, such as in the case of the ubiquitous NEMA 17 stepper motor. That's where adding gearboxes can help, as they act as levers to convert some of the distance generated by the motor into increased leverage over the load being moved. Maker and YouTuber Levi Janssen has created several of these gearboxes throughout the years, but his most recent design is far more capable than previous attempts.

How gears work

Gears operate on the principle of turning one wheel with teeth cut into it to transmit motion into another wheel with teeth. By varying the size of the wheels and number of teeth on each, the rate of rotation and torque provided can be drastically adjusted to provide a mechanical advantage. For example, an engine will spin very quickly but not have much turning force (torque), so a transmission is used to convert the distance moved by the spinning engine into more power for the wheels. However, gearboxes can take up a lot of space, so being able to achieve higher ratios in a smaller space can be highly desirable for a mechanism that fits directly onto a motor. That's why Janssen went with a compound planetary gear system that uses a series of gears to reduce the rotations from the stepper motor's shaft to the outer drive ring by a large amount.

Designing the planetary gears

The compound nature of the split ring design means there are two "layers" in this gearbox. The bottom layer works as a normal planetary gearset, with the central shaft turning five planetary gears that sit just inside of the outer stationary ring. When these planetary gears rotate around, they slowly move the top layer which only has 49 teeth compared to the 50 of the bottom one. Because of this, the bottom layer has a ratio of 3.5:1 and the top has a ratio of 49:1 for a total reduction of 171.5:1 which is incredible for something this compact.

Fabricating and driving the assembly

To take the design from Fusion 360 and make it a reality, Janssen printed each part on his Ender 3 V2 printer with some minor finishing work. For the motor, he went with a special kind of stepper motor that houses a driver and servo converter onboard. This means the stepper always knows where it's positioned and can rotate to exactly the right position without the host microcontroller having to perform these calculations.

The bottom gear ring was mounted directly to the NEMA 23 stepper motor with a few screws. Next, the planetary gears were slotted into position along with the pinion gear that takes motion from the motor's shaft and sends it to the gearbox. Finally, the output gear ring was slipped over the entire assembly with a bearing to make sure everything stays smooth.

Testing the system

Janssen wanted to see just how far he could take this design, so he set up a series of increasing weights mounted to an arm that would test the amount of torque produced. The first item to be lifted was a two pound water bottle, which the mechanism easily moved with no problems.

After that was a five pound weight that caused the stall torque current to increase by a mere 20mA with no other negative effects. Last to be tested was a ten pound weight, and this caused the mechanism to fail due to a lack of rigidity on the planetary gear teeth.

Overall, this design works really well considering how compact it is, and it will be exciting to see what improvements get made over time. You can see how he built this split ring compound planetary gear mechanism in his YouTube video here.