Designing Planetary Gearboxes in Fusion 360 for Robotics

In this instructable, I'm going to show you how to design your first planetary gearbox in Fusion 360, and at the end, I'm going to 3d print it to see how it works. Thru the article I'm also going to explain how gears work and suggest some design decisions you might want to take while designing your own gearbox.

- 3D printer

- Some filament

- few nuts and bolts

- Fusion 360

What are gears? What they aren't is a better question, but gears at their simplest are simple machines and they are basically levers just more circular.

Let's compare levers and gears so we can see why. If you have a lever that is two times longer on one side than the other and you move your lever on the longer side you will move two times the distance you would if it was 1:1 but the force on the other side force is two times higher and movement is the same as on the 1:1 lever. If you push the other side you will have to inflict double the force but the other side will move double the distance you did. Now when it comes to gears if you have a 1:1 gear ratio which means you have the same amount of teeth on both sides and turn one side the other will receive the same amount of force as the other one and will do the same amount of rotations same as our lever from before. Now if we have a gear ratio of 2 which means one gear has double the teeth than the other as the lever has double the length on one side, now when we turn the bigger gear one time the smaller one will do two rotations. Why? Well, the bigger gear has double teeth of the smaller one so if we go back to the 1:1 example we know that the same number of teeth will result in the same number of rotations so half of the turns ie half of the double of teeth will turn the smaller gear one time and a full turn will double that but because we doubled the rotation we are going to need double the force on the bigger gear and now we can see the similarity with the lever. And just to confirm that everything matches if we turn the smaller gear two times the big one will turn one but the force will be doubled so that is why gears are lever and thus simple machines. Footnote: I am just talking about spur gear, although all gears are similar in how they work spur gears are the simplest among them. Also, this isn't how real-life gears work 100% because there is always friction which turns some energy to heat, and other factors that make gears now work 100% as I described them.

Photo used

Planetary gears are often used in robotics to reduce the speed of the motor and increase the torque because in motors as it's easier to achieve higher speeds than torque so we use high-speed motors and then reduce that speed to get high torque. These gearboxes can provide big reductions in a small space so they are often used in the robotic arm and leg joints since we only need 180-270 degrees of rotation.

Planetary gears consist of 4 basic parts, on the picture we can see all of them named. Planetary gearboxes can have one output and one input but all the parts can be used as inputs and outputs. If we immobilize the ring gear and then turn the sun gear the carrier will rotate and it's the output, this is one of the most common configurations used In robotics. Second common use case is to use the ring gear as our output but to do that we need to immobilize the carrier and planet gears from turning but planet gears still need to be able to turn, this configuration is commonly used in 3d printed robotic arms since its very easy to integrate the ring gear itself into the arm and thus simplifying the design and making it thinner. Some other less common use cases are to use the sun gear as output either by turning the ring gear or carrier.

To better understand how these gears work and to help you design your own I suggest you take a look at this website. Here you can see a planetary gear animated and you can interactively change up parameters to see how it impacts the gearbox and outputs.

Credit for the photo used.

I'm going to design a planetary gear with 4 planet gears and I'm going to use the carriage as the output with screws going out of it as an attachment point. I decided I want a 4:1 gearbox so I used a 20 tooth module 1 gear for sun and planet gears and 60 tooth gear to cut the ring gear.

Fusion 360 has this cool feature of add-ins which can be found under the tools section. Fusion is preloaded with a lot of add-ins but we are going to use the spur gear generator.

When you open the generator it will ask you some questions and I'm going to lead you thru all of them:

Standard - here I chose metric since I use it and this tutorial is in metric so you should choose metric, but if you chose imperial standard your module will be changed to diametral pitch which is pretty much the same thing as the module just its not cross-compatible. Also, your measurements will be in inches and not millimeters.

Pressure angle - I don't want to get too into gear math so just don't change this one.

Module - determines the size of the gear, if you have a 100 tooth gear and a 6 tooth gear if they have the same module they mesh but if they have a different module they wont work. Module is the ratio between diameter (d) and the number of teeth (N) of the gear. Simply put higher module will make bigger gears with bigger teeth and diameter and small module will make smaller gears.

Number of teeth - This is the number of teeth you want your gear to have (N).

Backlash - is something every gear must have since we don't live in a perfect world and we cant produce perfect gears we need to allow some wiggle room and this setting will do exactly that. For making plastic gears because after some time gears will just make their own backlash. I haven't experimented a lot with this setting but 0.5 seems to work ok.

Root fillet radius - This is already shown on the photo Fusion provides, basically if you were to cut out a circle between two teeth of the gear this is the radius of that circle. Fusion won't let you create a gear with a radius higher than X for a specific module and will show that on the screen, you don't need to worry about this option for 3d printed gears.

Gear thickness - is pretty self-explanatory, it's the thickness of the gear.

Hole diameter - also self-explanatory, it's the diameter of the hole in the middle of the gear. You can leave this at 0 and then just extrude a hole you need later.

We are going to create a 60 tooth module 1 gear and then we are going to create a cylinder 70mm in diameter and then we are going to cut out the gear from the cylinder.

Next, we are going to make sun and planter gears, all of these are module 1 and 20 teeth. The sun gear will need something to drive it, I have added a cylinder extension with a hole for the stepper motor shaft. At this point, you can also add a backplate to the ring gear to support the sun gear.

Note: You should integrate a bearing for the sun gear so plastic is not rubbing on plastic, I'm not doing that since this is just an example and I only have big bearings on hand.

Carrier is one of the tricky ones to design, but to keep it simple you will need to create 4 shafts(in my case I just extruded a hole in the planet gears and then extruded back an about 1.5% smaller cylinder that goes thru the hole in the gear and then connected it to my carrier) and then you will connect them to the carrier, these shafts must allow planter gears to rotate and thus they cant be hard connected so gear must be able to rotate around the shaft so I make it around1.5% smaller than the hole. In my carrier, I have integrated hexagon cutouts on the gear side of the carrier so I can pass 5 M5 bolts thru the carrier and then attach something to them later on the outside. You can just place nuts in the cutouts so you can screw something from the other side.

Note: You should really have integrated bearing in every gear for the shaft since friction from rubbing is really bad and brings down the efficiency a lot.

Last part we need is something to support the carrier, this is not always mandatory but this time I included it. This lid very simple just a flat prate with a circular wall and a hole. Something to note is that you would add a bearing between the carrier output and the lid to support it and reduce friction.

I have printed and assembled the gearbox, it does work and the ratio is ok but I had to do a lot of sanding to remove friction, and still, there is a lot of friction.

This design really needs bearings to make it actually work and having a shaft as an input and then using a coupler is probably a better idea since I had a lot of slipping between the motor shaft and the sun gear. Another thing to consider is to use another material like PETG or ABS because some lubricants will attack PLA and destroy it over time.

If you are designing something that is actually useful like a robot leg then consider using double helix gears instead of spur gears which are harder to use but have many benefits.

Finally, if you want to learn more here are some links:

- Gear basics

- How spur gears are made

- 3D printed planetary gearbox with double helical gears

- Even more info on gears and different types of gears

Thank you for reading and consider giving me a vote for the simple machines competition :)