Moving power: gears, chain and belts

The motor is almost never exactly where the work needs to happen. It is bolted where it fits, and the wheel, roller or arm is somewhere else. Power transmission is everything in between.

FTC robots use three ways to connect one shaft to another: gears, roller chain and timing belt. All three obey the same ratio rule you learned in the last lesson. What differs is how far they can reach, how much maintenance they need, and how forgiving they are of a layout that keeps changing.

The rule all three share

Count teeth on the driven side and divide by teeth on the driving side:

ratio = driven teeth / driving teeth

A 15-tooth sprocket driving a 45-tooth sprocket is a 3:1 reduction. So is a 20-tooth pulley driving a 60-tooth pulley, and a 30-tooth gear driving a 90-tooth gear. The output turns one third as fast with (up to) three times the torque, whichever you use.

One difference matters when you wire up the motor: two meshing gears turn in opposite directions, while two sprockets on a chain or two pulleys on a belt turn the same direction. If your mechanism runs backwards after switching from chain to gears, that is why — fix it with one line in code, not by rebuilding.

Gears

Gear mesh

The overlap between the teeth of two gears. Too loose and the teeth wear and skip; too tight and they bind, waste power and grind.

Gears connect shafts that are close together. They need no tensioning: once the centre distance is right, they just work.

Centre distance

Two gears only mesh properly at one specific distance between their shafts. For metric gears, that distance comes from the module (millimetres of pitch diameter per tooth):

centre distance = (teeth₁ + teeth₂) × module / 2

Two module-1 gears with 20 and 60 teeth:

centre distance = (20 + 60) × 1 / 2 = 40 mm

On a fixed-hole system like goBILDA channel, the hole pattern decides which gear pairs land on a correct centre distance, so you choose from the combinations that fit. On extrusion like REV's, you slide one shaft until the gears mesh, which makes any ratio possible but means you are responsible for the mesh.

Gear rules worth memorising

• Only mesh gears of the same pitch or module. Different tooth sizes will not mesh, no matter how close they look.
• Do not mesh a plastic gear with a metal gear. The metal one eats the plastic one.
• Plastic gears can strip at the bore under high torque. REV sells a hex hub strengthener for exactly this.
• Gears are compact. A big reduction takes far less space with gears than with sprockets, because gear teeth are much smaller.

Roller chain

Pitch

The distance from one chain pin (or belt tooth) to the next. Sprockets and chain must have the same pitch to work together.

Chain connects shafts that are further apart, and its length is adjustable: you can break it and rejoin it at a new length, often without taking anything off the robot. That makes chain the forgiving choice while a design is still changing week to week.

Two pitches are common in FTC:

They are not interchangeable — a #25 sprocket will not run 8 mm chain.

The cost of chain is maintenance. Chain stretches as it wears, so a run that was tight in October is loose in January. Plan a way to re-tension it: a slot to slide one shaft along, or an idler sprocket you can move.

Timing belt

Timing belt

A toothed, fibre-reinforced rubber belt that engages matching teeth on a pulley. It does not slip the way a smooth belt can, so the two shafts stay "in time".

Belts are lighter and quieter than chain, they do not stretch much, and when correctly tensioned they have essentially no slop. Game Manual 0 recommends belted drivetrains, while noting belts are harder for brand-new teams to get right. The catch: a belt is a closed loop of fixed length. You cannot shorten it. You design the shaft spacing around a belt you can buy.

Common belt profiles in FTC include HTD 5 mm (goBILDA's main timing belt) and GT2 3 mm (REV's DUO system), among others. As with chain, the pulley and belt profile must match — an HTD pulley will not drive a GT2 belt properly even if the pitch is the same.

Worked example: designing to a whole-tooth belt

You want a 2:1 reduction with a 20-tooth pulley driving a 40-tooth pulley on 5 mm pitch belt, with the shafts about 100 mm apart.

First, the pitch diameter of each pulley — the circle the belt's teeth effectively ride on:

pitch diameter = teeth × pitch / π
d₁ = 20 × 5 / π ≈ 31.83 mm
d₂ = 40 × 5 / π ≈ 63.66 mm

The belt length needed at exactly 100 mm centres:

L = 2C + π(d₁ + d₂)/2 + (d₂ − d₁)² / (4C)
  = 200 + 150.0 + 2.5
  ≈ 352.5 mm  →  352.5 / 5 ≈ 70.5 teeth

No one sells a belt with half a tooth. Pick a real belt and move the shafts to suit it:

So you design the shafts 98.7 mm or 101.2 mm apart — not 100 mm. Then check your vendor actually sells that belt length.

Run your own numbers in the belt drive calculator. It rounds to whole-tooth belts for you and warns you when too few teeth are engaged on the small pulley.

Wrap and tension: why chains and belts skip

Two things keep a chain or belt from jumping teeth.

Wrap is how far around the sprocket or pulley the chain or belt reaches. Game Manual 0's guideline: at least 90° of contact, and ideally 180° or more. Small sprockets next to big ones lose wrap fast, which is why the small one is usually the one that skips.

Tension has to be right in both directions:

• Too loose and it skips teeth under load or falls off.
• Too tight and it adds friction, wastes power, loads the bearings and can overheat the motor.

A quick hand check: press on the middle of the run. It should move a little without much resistance. If it barely moves under a gentle push, it is too tight.

Choosing between them

A sensible rookie path: prototype with chain, where you can change lengths in minutes, and switch to belt once the layout stops moving and you can model the centre distances properly.

Next

Every one of these systems ends at a shaft. Next: how to make that shaft spin freely, stay put, and actually turn the thing mounted on it.

Sources and further reading

• Game Manual 0 — Gears
• Game Manual 0 — Roller Chain
• Game Manual 0 — Timing Belt (includes a belt calculator)
• Game Manual 0 — Drivetrains (belt, chain and gear recommendations for drivetrains)
• goBILDA — Timing belts and pulleys
• REV DUO — Timing Belts and Pulleys (GT2 3 mm system, flanges and tensioning)
• REV DUO — Sprockets and Chain