High-end robots require actuators with high torque and high precision. The popular choises are Harmonic Drives (strain wave) or Cycloidal Drives. Both offer compact reduction and low backlash, which makes them more suitable than regular spur gears or planatery gear sets. A newer and less popular option, for now, is the RV reducer. This combines the cycloidal drive with a set of spur gears to adress the weakest part of the RV reducer: the single eccentric shaft. After seeing a video on this, I was mesmerized and i wanted to design one of my own. This will come in handy for my next project: A scara robot for a CNC plasma cutter.
First-off, a bit more info on the working of a RV-Reducer. The main part is a cycloidal disk which needs to have a very specific shape. This disk gets placed in a circle of small pins, which form the outer gear. By wobbling the cycloidal disk in just the right way, it will jump over a single pin. Meaning the reduction ratio is equal to the amount of pins. This wobbling is caused by an eccentric shaft. In regular cycloidal gears, the eccentric shaft is directly attached to the motor and the output is coupled to the cycloidal disk with an additional set of pins or bearings. In the RV-reducer, the cycloidal disk is held in place with three eccentric disks. In this case, the outer gear forms the output. Because the eccentric shafts are moved off center, and because they now come in three, the RV-reduces can handle significantly higher torsional loads and shocks. Now, the only part missing, is the coupling to the motor. The trick here is a set of spur gears centered between the eccentric shaft. This gives an addition reduction stage without taking up a lot of additional space.
Common RV-Reducer design can have around 30 pins for the cycloidal reduction, and around 3:1 reduction on the spur gears. This gives a total reduction of 90:1. The design also has very low friction, meaning the actuator is often ever backdrivable. This is especially desirable in the trends of collabrative robots. One point of criticue though, is that the spur gears introduce backlash again. However, the spur gears come before the cycloidal reduction. This means that any backlash is scaled down by the factor of 30 making it quite insignificant.
First off: cost. But then why choose this type of reducer? Well, with an ideal cycloidal gear all teeth are in contact with the outer pins at all times. This splits the loads across multiple teeth and increases the maximum shoch loads. This is especially beneficial when using softer materials, such as the 3d printed version. It also helps to even out any inaccuries in its shape. has many advantages compared to simple spur gears, this gives it a high shock absorptions, and is a large advantage. Especially when this gear is made of softer materials, such as in a 3d printed version. They often have very low backlash and friction, making them back-drivable even at very high reductions.
The challenge was to create a design that is easy to print on regular FDM printers. This means that the XY plane will have the best accuracy, and surfaces that require supports will have the lowest accurcy. The design acounts for this by always putting important dimensions away from suported surfaces, and by minimizing the need for supports in the first place.
The shape of the cycloidal disk needs to exactly match the shape traced by rolling a small circle around a larger circle. The best way to make this shape is by simply entering the corresponding equations. This can be done in CAD (like Solidworks) using a parametric equation. If you want to replicate this, simply paste in the following:
Each eccentric shaft holds 4 bearings and those cant really slide onto this shaft. Therefore the shaft must be split into multiple pieces. I wanted these to lock the bearings in place and had to go through various versions to achieve this. The tollerances of these parts are also very important because any offsets can cause the cycloidal disk to bind up. I started off by printing these parts in resin but to make the design more accesible for other makers, I wanted it to be printable on FDM printers as well. The final design I ended up with is actually very simple as it splits eccentric shaft lengthwise. The spur gear is added on seperatly. An important point I noticed: Use multiples of three for the spur gear teeth count. Other wise they wont line up and you need threeversions of the eccentric shaft each slightly offset from the other. A bolt through to middle, as well as the bearings, hold everything together a offer additional reinforcements of the shafts.
The final design is published on my GitHub. There you will find the required files and info to build your own.
Sidenote: I also used this RV-reducer design to join a competition of the Design for Additive Manufacturing. The poster for that also sumirizes important benefits of the design. You can find that poster here: ...