3D Printed Gearbox: How to Design Your Own Box

If there’s a box around gears, it’s typically there to hold something in or to keep something out. You may want to hold in gear lubricant that would otherwise spray everywhere as the gears spin. You may want to keep out dust and debris so the gears run efficiently or keep out fingers for safety. In this sense, a wristwatch enclosure is a gearbox, protecting the small, delicate gears inside.

The specific mechanical function of a gearbox is to position the gears relative to each other and provide structural support. If you’re not worried about keeping things in or out of the gears, you may not need a full box at all, and so the “box” can be reduced to a simple frame that provides the mechanical functions mentioned. Thus, when we say “gearbox”, much of the actual box may not be useful for the particular application.

Before getting into how to make a 3D printed gearbox, let’s look at a few general details, such as nomenclature, best practices and rules of thumb, and the process of putting together a simple gear train. You can then apply this knowledge to any application requiring gears. Use this as a starting point to build your knowledge further.

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Below are a few important terms to know in relation to a simple gear train:

• Speed ratio: Imagine you have a gear on a drive shaft rotating at a particular speed. For your application, you need an output shaft to rotate at another speed higher or lower than that of the first. By a series of connected gears between the driveshaft and an output shaft, you can take your driveshaft rotational speed and convert it to the needed rotational speed at the output shaft. This is called “speed ratio”, calculated by the ratio of drive gear tooth count to output gear tooth count.
• Torque ratio: As with the speed ratio, a gearbox provides a means to convert available torque at the drive shaft to needed torque at the output shaft.
• Gear train (or gear set): The series of gears connecting the drive gear to the driven or output gear. From gear to gear in the train, each gear is handing off rotational direction, speed, and torque to the next.
• Driving gear: The first gear on the drive shaft, connected to the power source.
• Driven gear: Any gear after the driving gear.
• Idler gears: Gears that do nothing but reverse rotational direction between their neighbors.
• Shaft, axis: The rotational support structure connecting the gear to the gearbox.

The following terms are used in making gears and gearboxes. You’ll see them in the Onshape Spur Gear application we’ll use later.

• Pitch diameter (or pitch circle, base circle): The diameter of the gear, used for spacing gears. This is not the same as the outside diameter of the gear.
• Module: Pitch circle diameter divided by the number of teeth.
• Pressure angle: Usually 20° or 25°, though 14.5° has been used in the past.

For gears to mesh correctly, they must have the same module and pressure angle.

Below are a list of gears and common applications for each. Remember though, we are makers and disrupters, so feel free to bend the rules a little…

• Spur gears: In a spur gear, the ridge of each tooth is parallel to the axis of rotation. In this way, teeth are used to transmit rotation from one shaft to another parallel shaft. In 3D printing, spur gears are often employed by extruders to drive the filament into the hot end. Many versions exist, including Wade’s Geared Extruder, a well-known extruder in the RepRap community.

• Helical gears: Here, the teeth are inclined to the axis of rotation. Helical gears can be used in the same way as spur gears but run quieter because the teeth engage more gradually.
• Bevel gears: Bevel gears can be spur or helical, but the teeth rest on a base resembling a cone rather than a cylinder. The resulting angle can be used to transmit rotation between intersecting shafts.
• Worm gears: These gears transmit rotation between non-parallel and non-intersecting shafts. The worm gear looks like a screw and is the driving gear.

• Rack and pinion: These two parts consist of a standard gear (spur or helical) and a flat component with teeth. Together, they work to convert rotational motion to linear motion or vice versa.
• Internal gear and pinion: Like a rack and pinion, except that the rack is effectively curved around the standard gear, forming a loop and looking something like a gear “cut-out”. This setup is getting a bit more complex.

Along with gear types, it’s worth mentioning the involute gear profile. Essentially, this is a gear tooth design that is optimized to reduce noise during operation and maximize efficiency of the gear train. Sounds neat, looks nice – the involute is a combination of form and function we can all appreciate.

You can check out online references such as Engineers Edge to learn more about the design of the involute gear. For now, it’s enough to say, if possible, stick to the involute gear design and avoid the noisier, less-efficient straight-toothed gear.

How will you use your 3D printed gearbox? Think about what you want to do, whether that’s changing rotational direction, shaft speed, or shaft torque – all are usual reasons to employ a gearbox. Note that there are some limitations, such as 3D printing materials and 3D printer dimensional accuracy and capacity. 

With these limitations and the intention of your gear box in mind, a great resource to begin with is All3DP’s article on 3D printing gears, which among other things, touches on

• 3D printed gear test files and use
• the relationship between gear size, tooth strength, and 3D printer capacity
• gear thickness, materials, and strength
• rules of thumb for gear train design

Naturally, some of that information is again repeated in this article.

Another great resource for 3D printing gears is “A Practical Guide to FDM 3D Printing Gears” on Instructables. For gear train math, see the article “Gear train“, which describes mathematical details for simple, compound, and other gear set types. See also an article from WikiHow, “How to Determine Gear Ratio“.

For a 3D printed gearbox, you’ll need to model the gears in a CAD program. In this tutorial, we use the Spur Gear feature in Onshape. Of course, there are other gear design tools out there, including Gear Generator and Fusion 360. Fusion 360’s Spur Gear Creator even has a free 30-day trial.

As mentioned before, we need to consider what we want to do with this gearbox. For this project, let’s make a gearbox that will decrease the shaft speed of a motor by half. If, for instance, the input motor shaft speed is 500 rpm, then the output shaft speed should be 250 rpm. This will come as a result of the tooth count ratio  between the input and output gears connected to these shafts.

Let’s add an additional gear between the input and output shafts for a total of three gears. This will allow the input and output gears to have the same rotational direction.

The first gear, connected to the motor drive shaft, is the driver. The middle gear is the idler, which is simply there to reverse the direction of rotation. The third gear, connected to the output shaft, is the driven gear.

Below are the inputs necessary for the Spur Gear feature in Onshape:

• Gear 1
  • Number of teeth: 12
  • Pitch circle diameter: 30 mm
• Gear 2
  • Number of teeth: 20
  • Pitch circle diameter: 50 mm
• Gear 3
  • Number of teeth: 24
  • Pitch circle diameter: 60 mm

All of these gears will have a pressure angle of 20°, module of 2.5 mm, be extruded to a depth of 2 mm, and have a bore diameter of 5 mm. (The bore is the hole for the shaft). The other items in the input window for each gear can be left as they are.

Note that the gear tooth ratio from the input (Gear 1) to output (Gear 3) shaft is 12/24 = 0.5, which is also the speed ratio. Recall our original goal was to decrease the input shaft speed by half. Our 500 rpm input rotation multiplied by the 0.5 speed ratio gives 250 rpm output rotation. Just what we wanted!

The image shows the pitch circle diameters highlighted around each gear. The pitch diameters of any two mating gears are tangential to each other. Use the pitch circle diameters to position the gears for proper tooth engagement between gears.

Using the pitch circle diameters for each gear noted earlier, the vertical distance from Gear 1 to Gear 2 is 40 mm and the horizontal distance from Gear 2 to Gear 3 is 55 mm.

Finally, the reason we’re here! In fact, the act of choosing and realizing your gearbox isn’t as complicated as you might think. Basically, it comes down to what you need, but the possibilities are endless.

The most important thing is positioning the gear shafts where you need them to be. For a simple project or a as an initial prototype, a board with properly positioned bolts or nails will do the trick. Have any old Lego sets lying around? Use them! 

Using the information from the previous section, position your axes. Then, install the gears on their respective shafts. Give the completed gear set a test spin to see the speed ratio at work. 

For every full revolution of the smallest gear on the top left as shown in the image, the largest gear on the bottom right will turn only halfway. Because of the middle idler gear, the input and output shafts will rotate in the same direction. Hopefully, this initial “gearbox” is everything we hoped for.

As mentioned earlier, the nature and look of a gearbox depends on what it’s being used for. It might be to protect the gears from external forces or particles, or it might be to integrate the gear system into a larger working mechanism. Regardless, the most basic function performed by any gearbox is to properly position its gears such that they operate properly with one another.

To keep things simple, the gearbox we’ll design here will focus on these basic functions: holding together and enclosing the gear set we designed above. We’ll design an enclosure in CAD that will provide structure for the gear shafts and all-round protection for the gears. Once again, we’ll use Onshape.

We also have to ensure that the features we add below are dimensioned to allow the gears to fit together well and able them to spin freely. Keep in mind that you can change many of the dimensions stated below to fit your needs.

The image shows the pitch diameters of each gear along with the pitch diameter dimensions and the distances between each shaft axis. The optimal distance between shafts of connecting gears is found by adding the gear’s pitch radii (half of the pitch diameter).

For example, the shaft spacing between Gear 1 and Gear 2 = 30 mm / 2 + 50 mm / 2 = 40 mm.

First, we’ll make the backplate of the gearbox to position and support the gear set. The dimensions shown in the image are general and so can be changed to your liking. The image also details the positions of each shaft relative to the backplate edges – again, change these dimensions as you like.

An important aspect to consider when inserting shaft holes in the black plate is clearance. In particular, the holes in the plates need to be a little larger than the shaft to allow it to spin freely. Towards this end, the holes for the shafts will be 5.5 mm to hold and give clearance for the 5-mm shafts.

Our next step is to extrude a shaft through the center of each gear. We need each shaft to fit tightly in its gear’s central hole so that, when the shaft is turned, so does the gear. The shaft also needs to be long enough to protrude out of one or both sides of the gearbox. We’ll use the extension to turn the gear by hand.

Because the central hole in the gear is 5 mm, we’ll make the shaft 5 mm in diameter for a tight fit, and 20 mm long. This length will allow some protrusion of the shaft through the front- and backplates as shown in the image.

The next step adds four walls to enclose the gears. As with the gear shafts, the gears themselves need room to move uninhibited. These enclosing walls will act as a spacer to hold the front and back plates apart by the thickness of the gears plus a little more for clearance.

The gears are 2 mm thick, so the spacer can be 3 mm thick to give some clearance.

Finally, it’s time to produce a frontplate. As with the backplate, this plate will need 5.5-mm-clearance holes for the gear shafts. The frontplate in the image is made to be translucent to show the gears within.

Again, what we’re looking at here is a very simple design. Naturally, if you have prior modeling experience, it’s very easy to expand upon these simple principles, for example adding vent-like slots or external mounts or clamps to your front- and backplates. Once you’ve dealt with the necessary bit – supporting your gears – the possibilities are endless!

Here, we’ve designed three main components of a gearbox: backplate, spacer, and frontplate. From here, 3D printing is no different than with any other design.

If desired, the entire box can be 3D printed. Alternatively, just the spacer can be 3D printed while the front and back plates are made out of sheet material. In the latter case, the dimensions used above in designing the backplate can be used to position the holes in the plate material you use.

If the entire gearbox is to be 3D printed, print the backplate and the spacer as one print. In Onshape, this is done by a boolean union operation to join the backplate and the spacer. Then, assemble the gears in the box and add the frontplate to enclose it all.

Worried that your design is too small or complex to function properly after printing? In that case, you might want to consider using a 3D printing service. Find the right provider (and price) with Craftcloud, All3DP’s 3D printing price comparison service!

Now that the insides of a gearbox are not such a mystery, you can take the tools and knowledge outlined here to make a 3D printed gearbox of your own design!

(Lead image source: 3dsets.com)