VEX Differential Guide
After I posted my video on ratchets, many people asked me to make a video on differentials as well. In this guide I go over how a differential works, and 4 different examples of how you could use one.
credit to tilden obvs
The Core Design
I’m calling it this because the only implementations of this design have involved 2 mirrored copies, causing 2 spinning “cores” to be formed in the chassis. Last year, one of the VEXmen teams used a design like this to share power between their drive and their tilter. They used 393 motor bearings to decrease friction. While this does make the system slightly smoother, it’s not necessary in my experience.
Either the shafts connected to the motors, or the cores themselves, spin depending on the difference in speed between the two motors.
If both motors spin the same way, the shafts will spin, spinning the wheels. The chain connected to the shafts force both shafts to always be locked in their rotation, regardless of how the motors spin.
If the motors both spin in opposite ways, however, the chain that locks the shafts together prevents the shafts from spinning at all, as equal forces are acting trying to spin the shaft either way. Instead, this forces the cores themselves to spin, and the meshing gears close the differential.
Here’s an example of this kind of differential:
It uses one of the outputs to power the drive, and the other output is diverted to power the lift. We can see that the lift uses the power of all 4 motors, as does the drive.
The 4-Bar Design
This design includes a 4-bar linkage and a ton of gears.
Systems similar to this have been useful in both Tower Takeover, to share power with a tilter, and In The Zone to share power with a Mobile Goal lift. To the best of my knowledge, a 5-digit team from Colorado that I’ll leave unnamed pioneered this design.
On a system with 4 gears, the outside gears spin opposite ways. When the outside gears are forced to spin the same way, the gears lock up, and don’t spin.
Because the 4-bar is allowed to pivot, instead of snapping the gears, as a locked-up system like this usually would, the bars simply pivot, and the pinions crawl across the big gears.
When the pinions spin in opposite ways, however, the gears spin smoothly, and the wheels on the chassis spin.
Here’s the example that I made for this style of differential:
The 4-Bar could act as a Mobile Goal lift, effectively allowing the same 4 motors to power both the drive as well as another necessary robot function. Due to the absurd amount of gears on this drivetrain, it’s not the smoothest to drive. If I were to rebuild this, I would shorten the chassis, and reduce the amount of gears.
The Hood Design
I am not exactly sure what to call this design, as I have seen it used for things other than hoods, but no other name really applies.
I first saw this type of differential on some early designs this season.
This differential works by having two motors always spinning in the same direction, but at slightly fluctuating speeds.
One motor is on a bar that pivots around a shaft with the main flywheel, while the other is powering said flywheel.
When both motors spin at the same speed, the flywheel is powered by 2 motors. When one is slightly faster or slower than the other, the power of the flywheel decreases slightly, but the bar can be moved to a new position.
Here’s a basic example:
This type of differential could be used for a moving hood, such as one where you want to angle the shot from a flywheel or one where you want to let a ball out of the system entirely.
Here’s an example of a differential of this type more relevant to this season:
Differential Swerve Drives
hese are complex gear trains and chassis that allow for full 360-degree movement with full power in every direction.
The wheels are in “pods” that rotate, similar to the wheels on a shopping cart. The difference in the speed of the two motors determines if the wheels will be moving, or changing orientation.
Swerve drives in general aren’t used much in VEX, as they often pose more challenges than they create due to their complexity.
On the other hand, swerve drives are used quite frequently in FRC, as their robot’s requirements and constraints are very different than the ones in VEX.
Here are some examples made by Kyle from team 81818X of some differential swerve chassis. I wasn’t able to build an example for these, as I do not own enough of the necessary gears to build it.
You can see the humongous amounts of gears in the drive, and how much space they take up. Huge thanks to team 81818X for letting me use these renders in this guide.
Using differentials can be an effective way of sharing motor power between multiple different subsystems on your robot.
As some people have requested I do, I’m sharing all the CAD files for all the robots and mechanisms built in this video. These include Pack and Go zip files for inventor assemblies, Blender project files (where applicable), and some renders. Here they are:
Remember that there are many many more ways to use a differential on your robot than the designs that I mentioned. These are only a few of the most common examples.
I hope this guide helped you, and have a great Change Up season.