FRC Power Transmission: Chain vs Belt, Sprockets, Pulleys, and Tensioning

Every FRC mechanism starts the same way: a motor spins fast with very little torque, and you need it to move something useful. A gearbox trades that speed for torque, but the gearbox output shaft is almost never in the same place as the wheel, roller, arm, or elevator you actually want to drive. Bridging that gap — getting rotation from shaft A to shaft B a few inches away — is the job of power transmission, and in FRC it almost always comes down to one of three tools: roller chain, synchronous (timing) belt, or gears/direct drive.

Choosing well matters. The wrong chain size snaps in a match. An under-tensioned belt skips teeth and throws off your odometry. An over-tensioned chain eats bearings and bends shafts. This guide walks through the real hardware — pitch, tooth counts, load ratings, and tensioning — with the actual numbers you need, so you can make these calls on purpose instead of copying last year's robot and hoping.

The three ways to move power a short distance

Before diving into chain-versus-belt, it helps to place all the options on one map. Each one carries rotation from an input shaft to an output shaft, but they differ in efficiency, packaging, and how forgiving they are.

• Gears (direct mesh): Two gears touching directly. Highest torque density and stiffness, near-zero backlash if well made, but the two shafts must sit at a fixed, precise center distance (the sum of the pitch radii). Great inside a gearbox, awkward for spanning a chassis.
• Roller chain + sprockets: A metal chain wrapping two sprockets. Cheap, rebuildable link-by-link, tolerant of dirt and misalignment, and forgiving of imperfect center distances. Heavier, louder, needs lubrication and tensioning.
• Synchronous belt + pulleys: A toothed rubber/urethane belt on toothed pulleys. Light, quiet, clean, and zero-slip when tensioned. But belts come in fixed lengths, so center distance is constrained, and they tolerate less shock load than chain of similar size.

A useful rule of thumb: gears for the shortest, highest-load spans (inside gearboxes); belts for clean, light, quiet reductions; chain for long spans, high shock loads, and anywhere you want to adjust length in the pits. The rest of this article is about picking correctly between the last two, because that's where teams make the most avoidable mistakes.

Pitch: the one number that defines a chain or belt

Pitch is the distance from one tooth (or roller, or pin) to the next. It is the single most important spec because a chain, its sprockets, a belt, and its pulleys must all share the same pitch to mesh. Mixing pitches simply does not work.

Roller chain sizes

FRC uses two ANSI roller chain sizes, and their names encode their pitch in eighths of an inch:

Those pitch and roller dimensions are the ANSI B29.1 standard values (per the AmesWeb and ISC roller-chain dimension tables — note that #25 and #35 are technically bushed chains, so the listed "roller diameter" is the bushing outside diameter), and the working-load and tensile figures are the numbers West Coast Products publishes for the chain it sells (per the WCP FRC Build System "Sprockets & Chain" docs). The distinction between the two load numbers is critical:

• Tensile strength is where the chain physically breaks. You never design to this number.
• Working load is the continuous tension the chain is rated to carry safely. This is your real budget.

Notice #35 carries roughly 2.7× the working load of #25 for about double the weight per foot. That gap is the whole reason both sizes exist.

WCP also lists a #25H ("heavy") chain — same 0.250" pitch, but thicker side plates giving a 209 lb working load and 1,146 lb tensile. It runs on standard #25 sprockets, so it's a drop-in strength upgrade when #25 is marginal but you don't want to jump to #35's size and weight.

Timing belt profiles

Belts are named by tooth profile and pitch. In FRC you'll see two families (per the gm0 power-transmission docs and the WCP "Belts & Pulleys" docs):

The HTD (High Torque Drive) profile has a deep, rounded tooth that seats firmly and resists ratcheting under the hard, reversing loads a drivetrain sees when you slam direction. That's why the WCP docs call the HTD profile "ideal for FIRST Robotics Competition applications." GT2 uses a shallower, more precise tooth optimized for positional accuracy at lighter load — perfect for a flywheel or the first reduction inside a gearbox, but easier to skip if you overload it. A common mistake is running a GT2 belt on an HTD pulley (or vice versa) because the pitches happen to be close; the tooth shapes differ, so they wear badly and skip. Match the profile, not just the pitch.

Belt width sets load capacity for a given profile. WCP offers both HTD 5mm and GT2 3mm in 9 mm and 15 mm widths; wider is stronger. A 15 mm HTD belt is a standard choice for a serious drivetrain, while 9 mm is fine for lighter mechanism reductions.

Sprockets, pulleys, and the pitch diameter formula

A sprocket (for chain) or pulley (for belt) is defined by its pitch and its tooth count (N). From those two numbers you can compute the pitch diameter (PD) — the effective diameter of the circle the chain or belt rides on — with one formula that works for both chain and belt:

PD = (N × pitch) / π

That's it. The teeth sit on a circle whose circumference is exactly N pitches long, so the diameter is N × pitch ÷ π. A worked example each way:

• An 18-tooth #25 sprocket: PD = (18 × 0.250) / π = 1.432 in.
• A 24-tooth HTD 5mm pulley: PD = (24 × 5) / π = 38.20 mm (≈ 1.504 in).

Pitch diameter — not the outside diameter you'd measure with calipers over the teeth — is what you use for ratios and for laying out shaft positions in CAD. Getting comfortable with this formula in a good CAD package saves enormous frustration when your belt "won't fit" because you sketched to the wrong diameter.

Ratios work exactly like gears

The reduction across a chain or belt stage is just the ratio of tooth counts (equivalently, of pitch diameters):

Ratio = N_driven / N_driver = PD_driven / PD_driver

An 18-tooth sprocket driving a 36-tooth sprocket is a 2:1 reduction — output turns half as fast with twice the torque, minus losses. Because it's tooth count that matters, you can compute this without ever touching pitch diameter. This stacks on top of your gearbox reduction; the belt or chain stage is often the last reduction, sitting between the gearbox output and the wheel. If ratios feel shaky, the full walkthrough lives in our gear ratios explainer.

One practical constraint: very small sprockets and pulleys engage fewer teeth and wear faster. For chain, going below roughly 15 teeth increases the "chordal" (polygon) effect and stress per tooth, so many FRC sprockets start in the mid-teens. For belts, a small pulley means fewer teeth in contact, so the load per tooth climbs and skip risk rises. When you need a big reduction in a tight space, GT2's finer pitch lets you fit more teeth on a small pulley than HTD.

Center distance and belt/chain length

Here's the fundamental difference in how the two systems handle geometry.

Chain is granular and adjustable. You buy chain by the foot and break it to length with a chain tool, adding or removing whole links (and half-links, which let you change length by a single pitch instead of two). If your center distance is a little off, you re-tension or add a half-link. This makes chain forgiving and pit-friendly — you can literally re-cut it between matches.

Belt is fixed. A belt has a specific tooth count and therefore a specific length. You cannot shorten it. That means the belt length and the two pulley tooth counts together dictate the exact center distance, and you have to design your shaft positions around belts that actually exist. For two equal pulleys, WCP gives a handy shortcut for the belt tooth count you need:

Belt teeth ≈ 2 × center-distance-in-pitches + pulley teeth

and the minimum center distance (pulleys nearly touching) is:

CC_min = ((N₁ + N₂) / (2π)) × pitch

The full chain center-to-center formula is genuinely ugly (it involves a square root of the tooth-count difference), so nobody computes it by hand — every team uses a calculator or CAD. What you should remember is the healthy range: keep the center distance roughly 30–50× the chain pitch, and never beyond about 80× pitch (per the WCP chain docs). Too short and the chain barely wraps the small sprocket; too long and the chain sags and whips. For a #25 chain (0.250" pitch), 30–50× pitch is about 7.5 to 12.5 inches — a good target for a typical drivetrain module.

Wrap angle: the quiet killer

Both chain and belt need enough of the small sprocket/pulley wrapped to engage teeth reliably. The guidance from gm0 is the same for both:

• Absolute minimum: 90° of wrap on the smaller sprocket/pulley.
• Best practice: 180° or more.

Wrap gets tight when a small pulley drives a much larger one at a short center distance, or when an idler pushes the belt off the pulley. If you're skipping teeth even at correct tension, check wrap angle before anything else.

Tensioning: tensioners vs. adjustable mounts

A chain or belt at the wrong tension is the number-one cause of transmission failure in FRC. Both extremes hurt:

• Too loose: the chain/belt skips teeth ("ratchets"), the mechanism jerks, position control drifts, and for belts you lose the zero-slop property that made you choose belt in the first place. A skipping drivetrain belt will destroy your autonomous path following because the wheels no longer track the encoders.
• Too tight: enormous side load on the shafts and bearings. The WCP docs warn that excess tension increases friction and wear and drives premature component failure — the added side load lands squarely on the bearings and shafts, and it also makes the motor work harder. You can bend a 1/2" hex shaft or blow out a bearing this way.

The field-test for "about right," per gm0: push on the middle of the span. It should move slightly under gentle pressure, not feel bar-tight and not flop loosely. For belts, WCP even suggests intentionally loosening intake/conveyor belts by 0.015–0.020" of center distance so they don't over-load light bearings.

There are two mechanical strategies to set and hold tension:

1. Adjustable mounts (moving the shaft)

Design one of the two shafts so its whole center distance can move — typically with slotted bolt holes or a plate that slides. You set the belt/chain on, slide the shaft until tension is right, and lock the bolts. This is the cleanest, most reliable method because it keeps the belt path straight and both spans doing useful work. It's the default for a well-planned drivetrain: mount the gearbox on slots and slide it. The downside is you must design the adjustability in from the start — you can't retrofit a slot easily on a finished plate.

Chain-in-a-tube and cam-style eccentric spacers are variations on the same idea: they change the effective center distance to take up slack.

2. Tensioners (pushing on the span)

When you can't move a shaft — fixed center distance, or a long run — add an idler that presses on the slack span to take up the extra length. Two flavors:

• Chain: a spring-loaded or bolt-adjusted idler sprocket, or a hard nylon/UHMW slider block the chain rides against. Because chain only pulls in one direction under a given load, you tension the slack side.
• Belt: a smooth idler pulley (flat idlers usually press on the back of the belt; toothed idlers can engage the tooth side). Keep idlers on the slack span and as large as practical — a tiny idler bends the belt sharply and shortens its life.

Idlers cost you a little efficiency and wrap, and a spring-loaded tensioner can pump under shock load, so prefer an adjustable mount when the geometry allows and reserve tensioners for fixed-shaft situations.

When to use chain vs. belt vs. gears

Here's the decision boiled down. None of these is a hard law — good teams break them deliberately — but they're a solid default.

The WCP application guide lines up with this: plain #25 for intakes, conveyors, and turrets; #25H for drivetrains with wheels under 6" (and elevators); #35 for drivetrains with 6"+ wheels, wrists, and arms. For an arm or elevator, the reversing and holding loads are exactly the punishing case where HTD's deep teeth or #35's working load earn their weight. For an intake that eats floor debris, chain's dirt tolerance and easy adjustment often win over a cleaner belt.

Two more considerations that tip the balance:

• Weight and packaging: belts and their pulleys are noticeably lighter than steel chain and sprockets, and run without lube. On a weight-critical robot, belt-in-tube or plate-mounted belts save real ounces.
• Efficiency: a properly tensioned belt is quiet and efficient; chain has slightly more friction and needs lubrication to stay efficient. Neither dominates enough to decide on its own.

Remember that the transmission choice interacts with the rest of the design — shaft size, bearing load, and packaging all feed back into your overall robot design process. If a belt forces a shaft span so long it deflects, that's a real problem you can check with a tool like the structural deflection calculator before you cut metal.

Common failure modes (and what actually causes them)

Most transmission failures trace back to a handful of root causes. Learn to recognize them fast in the pits.

1. Skipping / ratcheting teeth. The mechanism jerks and loses position. Cause: under-tension, insufficient wrap angle (< 90°), too-small sprocket/pulley, or worn teeth. Fix tension and wrap first.
2. Thrown or derailed belt/chain. It walks off the pulley/sprocket entirely. Cause: shaft misalignment (the two shafts aren't parallel/coplanar), missing flanges on pulleys, or badly under-tension. Align shafts and use flanged pulleys or retaining hardware.
3. Snapped chain / stripped belt. Sudden total loss of drive. Cause: exceeding the working load (undersized chain for the shock load), a fatigued master link, or a manufacturing flaw. Step up a chain size (#25 → #25H → #35) or a belt width, and inspect master links each event.
4. Bent shaft / destroyed bearing. Cause: over-tension, which slams a huge side load into the bearings, or too-small a shaft for the load. Back off tension to the "slight deflection" feel and verify shaft/bearing sizing.
5. Rapid tooth wear / stretch. Cause: chronic under-tension letting teeth climb, no lubrication (chain), contamination, or a tiny idler bending a belt too sharply. Chain "stretch" is really pin/roller wear elongating the pitch — replace worn chain, don't just re-tension indefinitely.

A quick pit checklist that catches most of these: shafts parallel? tension gives slight deflection? wrap ≥ 90°? master link seated and clip facing the right way? no missing/cracked teeth? Running that five-item check before every match prevents a shocking fraction of mid-match transmission deaths — it belongs right next to your battery and connection checks in your pre-match routine.

Putting it together: a worked drivetrain example

Say you're building a 4" wheel drivetrain and your gearbox output shaft sits about 9 inches from the wheel shaft. That center distance is 9 / 0.250 = 36 pitches of #25 chain — comfortably inside the 30–50× sweet spot, so #25 is geometrically ideal. Your wheels are under 6", which puts you in #25/#25H territory: WCP specifically recommends the heavier #25H for drivetrains with wheels under 6", but a light 4" drive runs fine on standard #25's 154 lb working load — and because #25H is a drop-in on the same sprockets, you can upgrade later if you want more margin.

You pick an 18-tooth sprocket on the gearbox output and an 18-tooth sprocket on the wheel: a 1:1 pass-through (the reduction is all done in the gearbox), each with PD = (18 × 0.250)/π = 1.432". You mount the gearbox on slotted holes so you can slide it to tension the chain — no idler needed. You break the chain to length, add a half-link to dial in the last bit, set tension to a slight deflection, and confirm the chain wraps well over 90° on both sprockets. That's a clean, serviceable, well-reasoned transmission — the kind of decision-making that separates robots that finish matches from robots that limp off the field.

Frequently asked questions

Is chain or belt better for an FRC drivetrain?

Both are proven; it's a tradeoff, not a winner. Chain (#25/#25H for small wheels, #35 for 6"+ wheels or heavy defense) is cheaper, tolerant of dirt and shock, and adjustable in the pits with half-links. HTD belt (9 mm or 15 mm) is lighter, quieter, cleaner, and has zero slop when tensioned, which helps odometry and path following. Many top teams run belts for the weight and precision; many equally strong teams run chain for durability and serviceability. Pick based on your priorities: weight and precision lean belt, ruggedness and adjustability lean chain.

When should I use #35 chain instead of #25?

Step up to #35 when the load is high or the shocks are severe. Per WCP's guidance, use plain #25 for intakes, conveyors, and turrets, #25H for drivetrains with wheels under 6" (and elevators), and #35 for drivetrains with wheels 6" and larger, plus wrists and arms. #35 has roughly 2.7× the working load (419 lb vs. 154 lb) for about double the weight. If #25 is marginal but you don't want #35's bulk, #25H is a drop-in heavier chain (209 lb working load) that runs on standard #25 sprockets.

How do I know if my chain or belt tension is right?

Push on the middle of the span with a couple fingers. It should deflect slightly under gentle pressure — not feel bar-tight, and not flop loosely. Too loose skips teeth and loses position; too tight overloads bearings and can bend shafts. Set tension by sliding an adjustable (slotted) mount if you have one, or with an idler tensioner on the slack span if the shafts are fixed. Re-check after the first few runs, since new chain and belts settle in.

What's the pitch diameter formula for a sprocket or pulley?

PD = (N × pitch) / π, where N is the tooth count and pitch is the chain/belt pitch (0.250" for #25, 0.375" for #35, 5 mm for HTD 5mm, 3 mm for GT2 3mm). This is the effective diameter the chain or belt actually rides on — use it for CAD layout and shaft spacing, not the outside diameter you'd measure over the teeth. For example, an 18-tooth #25 sprocket has PD = (18 × 0.250)/π = 1.432".

Why does my belt keep skipping teeth even though it feels tight?

Tension is only one of three culprits. Check wrap angle — the belt needs at least 90° of contact on the small pulley (180° is better); a small pulley at a short center distance can starve the wrap. Check shaft alignment — non-parallel shafts let the belt walk and ratchet. And check that you're not overloading a light-duty profile — GT2 is meant for low loads, so if a GT2 belt skips under drivetrain-level torque, switch to a deeper HTD 5mm profile or a wider belt rather than just cranking tension higher.