Gears

Gears change the speed and torque coming out of a motor. FRC motors spin very fast with relatively low torque, and most mechanisms need the opposite (slower speed, higher torque), so almost every motor on your robot goes through some kind of gear reduction before driving anything.

Gear ratios: the core concept

A gear ratio describes the tradeoff between speed and torque. When you reduce (which is what you're doing almost every time in FRC), the output spins slower than the input but with more torque.

The ratio is calculated by dividing the number of teeth on the output gear by the number of teeth on the input gear:

Ratio = output teeth / input teeth

So if a 20-tooth gear drives a 60-tooth gear, the ratio is 60/20 = 3:1. That means the output spins 3x slower with 3x the torque.

For multi-stage reductions (which is how planetary gearboxes work), you multiply the ratios of each stage together. For example, stacking a 3:1 stage and a 5:1 stage gives you 3 x 5 = 15:1 total reduction.

Higher ratio = slower output, more torque. Lower ratio = faster output, less torque. Almost everything on an FRC robot needs a reduction somewhere between 4:1 and 100:1 depending on the mechanism.

Planetary gearboxes

In FRC, you should avoid designing custom gear trains with loose spur gears. Instead, you buy a planetary gearbox that bolts onto your motor and gives you the reduction you need in a compact package.

A planetary gearbox uses a central "sun" gear, several "planet" gears around it, and an outer ring gear. You don't need to understand the internals in detail because you're using them as a sealed unit. What matters is how to pick the right one and configure it.

How a planetary gearbox works. Notice how the input (green) spins a lot faster than the output (red).

MAXPlanetary (what we use)

The MAXPlanetary from REV is a cartridge-based modular planetary gearbox. It's the standard on 4123 and on most competitive FRC teams right now. Here's why:

Cartridge system. Each reduction stage is a sealed cartridge that snaps in. You can swap ratios in minutes without rebuilding anything. Available cartridges are 3:1, 4:1, 5:1, and 9:1.

Stack stages for higher reductions. Need 15:1? Stack a 3:1 and a 5:1. Need 25:1? Stack a 5:1 and a 5:1. Need 45:1? Stack a 5:1 and a 9:1. Up to 3 stages can be stacked.

Compatible with our motors. MAXPlanetary works with Kraken X60, Kraken X44, NEO, NEO Vortex, Falcon 500, and CIM motors. Since we run CTRE Krakens, this is important.

1/2" hex output. The output shaft is standard 1/2" hex, which is compatible with all standard FRC pulleys, sprockets, wheels, and hubs.

Two mounting options. Face mount (bolt through the front plate) or side mount (bolt through the housing). This gives flexibility in how you package the gearbox on your mechanism.

REV's Maxplanetary gearbox. The different stages are contained within cartridges.

Common MAXPlanetary configurations

Application	Typical ratio	Cartridge stack	Notes
Intake rollers	4:1 to 9:1	Single 4:1 or 5:1	Intakes usually want moderate speed with enough torque to grab game pieces
Shooter flywheel	1:1 to 3:1	Single 3:1 or no cartridge	Shooters want high speed, so you use little or no reduction
Elevator	9:1 to 25:1	5:1 + 5:1 or 9:1 + 3:1	Elevators need high torque for lifting
Arm pivot	25:1 to 100:1	5:1 + 5:1 + 4:1 or higher	Arms need very high torque, especially with long reach. Often combined with an external belt/gear reduction as well.
Climber winch	25:1 to 60:1	5:1 + 5:1 or 9:1 + 5:1	Needs to lift the full weight of the robot

These are starting points, not gospel. Always verify your ratio with the JVN calculator or ReCalc before committing. Plug in the motor, ratio, load, and wheel/spool diameter, and the calculator will tell you whether the mechanism can actually do what you need.

How to pick the right ratio

1

Define what the mechanism needs to do

How fast does it need to move? How much force or torque does it need to produce? For a drivetrain this is "what speed in ft/s" and "how much pushing force." For an elevator this is "how fast does the carriage need to travel" and "how heavy is the carriage plus game piece."

2

Open JVN or ReCalc

ReCalc is the easiest starting point. Select the mechanism type (linear, rotary, drivetrain) and fill in the motor, number of motors, gear ratio, and load parameters.

3

Try different ratios and check the output

The calculator will show you output speed, force/torque, current draw, and time to speed. Adjust the ratio until the output meets your requirements without drawing excessive current (which means the motor is working too hard and will overheat).

4

Pick the MAXPlanetary cartridge stack that gets closest

Since cartridges only come in 3:1, 4:1, 5:1, and 9:1, you can't hit every exact ratio. Pick the combination that gets close. If the calculator says you need 18:1 and you can stack 4:1 + 5:1 = 20:1, that's close enough. If you need a ratio between what two cartridge stacks give you, you can add an external belt or chain stage to fine-tune it.

Other gear types you might encounter

Spur gears

Spur gears are flat gears with straight teeth that mesh when placed on parallel shafts at the right distance apart. You'll see these inside swerve modules, in some COTS gearboxes, and occasionally in custom reductions when a team needs a specific ratio that a planetary can't provide.

For two spur gears to mesh properly, the center-to-center distance between the shafts needs to be correct. Gear vendors and online calculators will give you this number based on the gear sizes. If you're using spur gears in a design, get the center-to-center distance right in CAD and verify it before manufacturing.

For most mechanisms on 4123, you won't need to design a custom spur gear reduction. The MAXPlanetary handles it.

WCP's COTS inverted gearbox. The spur gears are on the outside here.

Bevel gears

Bevel gears transfer rotation 90 degrees (from one axis to another). They show up occasionally when you need to change the direction of a power transmission path for packaging reasons. You're unlikely to design one yourself, but you might see them inside COTS products.

VersaPlanetary (WCP/VEX)

The VersaPlanetary is an older modular planetary gearbox that's been in FRC for a long time. It still exists and is still sold, but it was designed around older motors (BAG, MiniCIM, 775pro, CIM) and uses a different form factor than the MAXPlanetary. We use MAXPlanetary because it was designed from the ground up for modern brushless motors (Kraken, NEO) and has better torque density for its size. You'll see VersaPlanetaries on other teams' robots and in older documentation, so it's good to know what they are, but we don't use them.