Designing and Building a Working Differential Gear

Have you ever taken a turn in a car and wondered how the inside wheel rotates more slowly than the outside wheel without skidding? The mechanical system that enables this behavior is called a differential gear.

In this project, I designed, 3D printed, and assembled a fully functional differential mechanism to better understand how rotational speed and torque are distributed purely through mechanical means. This Instructable walks through the design process, CAD modeling in Fusion 360, 3D printing considerations, assembly, and testing, with a focus on making the mechanism accessible, modifiable, and educational.

Materials:

1. PLA or PETG filament (PLA recommended for ease of printing)
2. Small screws (3m recommended)
3. Optional: a small piece of wood or other board
4. Optional: metal shaft or printed shaft (8 mm or similar)
5. Optional: small bearings (for smoother rotation)
6. Optional: Washers

Tools:

1. Fusion 360
2. 3D printer
3. Slicer software
4. Hex keys or screwdriver
5. Light lubricant (optional)

Before designing the mechanism, it is important to understand how a differential works.

This differential consists of:

1. A central carrier, driven by an input shaft
2. Two side (output) gears
3. Multiple bevel “spider” gears that allow relative motion between outputs

When both output shafts experience equal resistance, they rotate at the same speed. When one output is constrained, the spider gears rotate, allowing the opposite output to spin faster. This behavior—allowing different angular velocities while maintaining an average input speed—is the defining feature of a differential.

This mechanism is essential in vehicles because wheels must travel different distances while turning.

Why does this design work?

In this design, all bevel gears share the same pitch and pressure angle so torque is transmitted smoothly across perpendicular axes. When both outputs experience equal resistance, the spider gears remain stationary relative to the carrier, causing both outputs to rotate together. When resistance differs, the spider gears rotate on their shafts, redistributing angular velocity without interrupting power flow. Proper gear alignment and clearance are critical—misaligned bevel gears will bind and prevent differential motion. This is very important to consider when we begin designing.

For these next few parts I will be using Fusion 360 to model, assemble and simulate the differential, while other CAD softwares may be suitable, I highly recommend using Fusion 360 as it is the most straight forward and versatile software for this project.

Before we create our gears please take note of the following:

1. The diameter of the shafts you are using (mine were 2.3mm)
2. The build-plate of your 3D printer

This differential was designed parametrically in Fusion 360. The following parameters can be adjusted without redesigning the assembly:

1. Shaft diameter
2. Gear size and tooth count
3. Overall scale of the mechanism

When you design yours you should be able to adjust these based on your constraints rather than the ones I had.

Bevel Gears

Bevel gears are required so motion can transfer between perpendicular axes. Fusion 360’s gear tools simplify this process, but creating them manually is also possible.

Helpful references:

1. YouTube: X4on6tWQv8Q
2. YouTube: 9ikFtQ1XkUM

Spider and Output Gears

1. Both should be bevel gears meshing at 90°
2. Output gears should have a tight bore to grip the shaft (I used 2.5 mm holes)
3. Spider gears should rotate freely (I used 3 mm holes)
4. These are the smallest gears and should be designed with print resolution in mind

Carrier and Input Gear

1. The carrier gear should be at least 1.5× the diameter of the spider gears
2. It must rotate freely around the output shaft
3. Integrated supports hold the spider gears in place
4. The input gear meshes directly with the carrier gear and should have a tight shaft fit

To support the shafts, I designed simple rectangular stands with shaft holes. To reduce print time and allow flexibility, I mounted these supports to a wooden base. The entire structure can be printed as one piece if desired, but modular supports make alignment easier.

Each component was exported as a separate STL file.

Printed Parts

1. 2 output gears
2. 2 spider gears
3. 1 carrier gear
4. 1 input gear
5. 5 shaft supports

Printing Notes

1. Gears were printed flat to maximize tooth accuracy
2. No supports were required
3. Tolerances were slightly oversized to accommodate real-world prints

Printer Settings Used

1. Layer height: 0.2 mm
2. Infill: 20–30%
3. Walls: 3 perimeters
4. Material: PLA
5. Supports: None
6. Printer: Creality Ender 3 V3 SE

Allow parts to cool completely before removal to avoid warping.

I chose PLA for ease of printing and dimensional accuracy. While stronger materials could support higher torque such as PETG, my design prioritizes visibility and smooth motion for educational purposes.

Below I have attached my STLs if you want to print specifically mine.

Before assembly:

1. Remove any print artifacts or stringing
2. Clean gear teeth if needed
3. Test-fit shafts and bearings
4. Light sanding can improve smoothness

If gears bind during rotation, this is usually caused by leftover material or bad clearance. Slightly enlarging bores or sanding gear faces resolves most issues. I widened a few holes with a drill to ensure the axle fit.

I recommend at least a 0.3mm face offset specifically for the gears if they are not working

1. Insert a shaft through the carrier gear supports and mount the spider gears with beveled faces inward
2. Align one output gear with the carrier and insert the output shaft
3. Insert the second output shaft and gear
4. Install the input gear on its shaft
5. Mount the assembly using the shaft supports and secure them to the base

Ensure all gears mesh freely before final tightening.

To test functionality:

1. Rotate the input shaft while holding both outputs → both outputs rotate together
2. Hold one output → the other spins faster
3. Rotate one output → the other rotates in the opposite direction

If your mechanism behaves this way, you’ve successfully built a working differential!

A short video is included in this step demonstrating differential behavior with one output held stationary.

Designing and building this differential helped me better understand how complex motion can emerge from simple constraints. While each part is straightforward on its own, the system only works when geometry, alignment, and tolerances are considered together. Small adjustments in Fusion 360—such as gear spacing or bore clearances and especially face offsets— had a noticeable impact on how smoothly the mechanism behaved once printed.

Modeling the differential parametrically in the variable studio in Fusion made it easy to iterate and adapt the design for 3D printing. Instead of treating this as a fixed model, I approached it as a system that can be scaled, modified, and improved. With further iterations, I would explore stronger materials, clearer visualization of gear motion, and higher-precision components.

Overall, this project taught me a lot on CAD modeling complex mechanical systems and their fabrication and integration into the real world. hope this project helps make differential mechanisms more accessible and easier to understand