Belts & Pulleys

Belts are the default way we transfer motion on 4123. They're lighter than chain, quieter, don't need lubrication, and don't stretch over time. Unless a mechanism specifically requires the higher torque capacity of chain or gears (like a heavily loaded pivot arm), try belts first.

Beyond power transmission, belts are also used to move game pieces through the robot (conveyor-style intake paths, indexers, feeders). The same principles apply to both use cases.

The FRC standard: HTD 5mm

All of our belt runs use HTD 5mm pitch belts (sometimes labeled GT2 5mm). The "5mm" refers to the distance between teeth. Belts come in two common widths:

Width	Use for
9mm	Light to moderate load mechanisms: intakes, indexers, feeders, conveyor paths, game piece transport
15mm	High load applications: drivetrains, elevators, heavy mechanism reductions

When in doubt, 15mm is the safer choice. It costs slightly more weight but holds significantly more torque before skipping teeth.

How belt sizing works

Three numbers define a belt run: the tooth count of each pulley, the belt tooth count, and the center-to-center distance between shafts. These are all related, and you typically work through them in this order:

1

Pick your pulleys

The tooth count on each pulley determines your reduction ratio, just like gears. A 24-tooth pulley driving an 18-tooth pulley gives you a 18/24 = 0.75:1 ratio (speed increase). A 18-tooth driving a 36-tooth gives you 36/18 = 2:1 (reduction).

Smaller pulleys save space but the belt wraps around fewer teeth, which makes it more likely to skip under heavy load. Try to keep the smaller pulley at 16T or above.

2

Find a belt that works

Belts only come in specific tooth counts (roughly every 5 teeth). You can't order a custom length. So you need to find a belt tooth count that gives a workable center-to-center distance for your design.

Use the #BeltCTC_5mm() function in Onshape (from the Origin Cube featurescript) or ReCalc to calculate this. You give it the belt tooth count and both pulley tooth counts, and it gives you the exact center-to-center distance.

In practice, you often start with a target C-C distance, try a few belt sizes, and pick the one that's closest. Then you design the mechanism around that exact C-C distance rather than trying to force a specific distance.

3

Verify in CAD

In Onshape, use the Origin Cube featurescript's #PulleyPD_5mm() function to draw correctly sized pulley pitch circles in your layout sketch, and the #BeltCTC_5mm() function to drive the C-C dimension. This captures design intent so that if you change a pulley size, the C-C distance updates automatically.

Use the Belt and Chain Gen featurescript to generate a 3D model of the belt for visual verification.

Design around the belt, not the other way around. Since belts only come in fixed lengths, it's almost always easier to adjust your mechanism's shaft spacing slightly to match an available belt than to try to find a belt that matches an arbitrary distance.

3D printed pulleys

We 3D print most of our pulleys rather than buying COTS ones. This gives us flexibility to use any tooth count, any bore, and any width without waiting on shipping or being limited to what's in stock.

How to generate them in Onshape:

• Use the Robot Pulley featurescript (from FRCDesignLib) or the pulleys available in MKCad

• These generators let you set tooth count, bore (1/2" hex, 3/8" hex, round), width, and whether to include flanges

Important: use a metal insert for the bore. 3D printed hex bores strip out easily under load. Press a metal hex insert (available from WCP and Thrifty Bot) into the pulley so torque transfers through metal instead of plastic. A cheaper alternative is to order laser-cut hex inserts from a service like Fabworks, which costs about $1 each in bulk.

Material: PETG is fine for most pulleys. Use nylon or CF-nylon for pulleys in high-torque applications.

Flanges: Add flanges (the raised lips on each side of the pulley) to keep the belt from walking off. This is especially important on longer belt runs.

Prototyping trick: print pulleys to match your belts

During prototyping, you might not have the right belt length for the C-C distance you want. Instead of ordering a new belt and waiting for it to ship, you can print pulleys with different tooth counts to change the required C-C distance so it matches a belt you already have.

For example: you have an 80T belt on hand and your prototype needs a belt run between two shafts. Print a few pulley sizes (18T, 24T, 30T), try them with the 80T belt, and see which combination gives you a C-C distance that works for your prototype. This lets you keep iterating without stopping to order parts.

Tensioning

A belt that's too loose will skip teeth under load. A belt that's too tight adds unnecessary force on the bearings and shafts. It should be taut with very slight deflection when you press on it with a finger.

Method	How it works	When to use
Slotted mounting holes	One of the pulleys (usually the motor) mounts on slotted holes. Slide it to tension (usually with a cam), then tighten.	The default. Simple and reliable. Use this whenever possible.
Idler pulley	A small unpowered pulley that presses against the back (flat side) of the belt to take up slack.	Use when slotted holes aren't practical, or for very long belt runs where stretch matters.

You don't always need to have a tensioning strategy in your design before manufacturing. However, if you design a belt run with fixed C-C and no slotted holes, you have no way to adjust tension. The belt will either be too tight or too loose, and you won't be able to fix it without remaking parts. Therefore it is very helpful to test C-C distances through prototypes.

Belt uses on the robot

Power transmission

The most common use. A belt connects a motor (through a gearbox) to the shaft that drives a mechanism. Examples:

• Gearbox output to drivetrain wheel shaft

• Gearbox output to intake roller shaft

• Gearbox output to elevator spool

• Motor to shooter flywheel

Moving game pieces