Chain & Sprockets

We default to belts for most applications, but chain is the right choice when the torque demands are high enough that a belt would skip teeth. Pivot arms, heavy elevator lifts, and certain drivetrain configurations are the most common places where chain shows up on our robots.

When chain makes sense over belt

Use chain when...	Why
The mechanism transmits very high torque (arm pivots, heavy lifts)	Chain has a higher torque capacity than belt at a given width. A #25 chain run won't skip under loads that would strip teeth off a 15mm belt.
The belt keeps skipping teeth despite proper tensioning	This is a sign that the load exceeds what the belt can handle. Switch to chain.

For everything else (intakes, shooters, indexers, light to moderate reductions), belt is preferable because it's lighter, quieter, doesn't stretch, and requires no maintenance.

Chain sizes

Size	Pitch	When to use
#25	0.250"	The standard for most FRC chain applications. Strong enough for the majority of mechanisms.
#35	0.375"	Significantly stronger (roughly double the working load of #25) but heavier. Use when #25 isn't holding up or the load is extreme.

If you're not sure, start with #25. Only move to #35 if you have a specific reason.

Steel vs. aluminum sprockets: We highly recommend steel sprockets, especially for sprockets with low tooth counts. In 2023 we sheared the teeth off aluminum sprockets because the torque concentrated on fewer teeth was more than the aluminum could handle. Steel sprockets are heavier, but the weight difference is small and the reliability difference is significant. If you're running a sprocket with fewer than 20 teeth or the mechanism is high-torque, use steel.

Sizing a chain run

Chain sizing works similarly to belts, but with a key difference: chain can be cut to any length (by adding or removing links), so you're not constrained to fixed lengths the way you are with belts. This means you can set your center-to-center distance first and then cut the chain to match.

Ratio math is the same as belts and gears. Output sprocket teeth divided by input sprocket teeth gives you the ratio. A 16T sprocket driving a 48T sprocket is a 3:1 reduction.

Use even link counts. Chain is made of alternating inner and outer links. Using an even number of links means you can close the loop with a standard master link. An odd link count requires a half link, which is weaker and should be avoided when possible.

Minimum sprocket teeth: Keep the smaller sprocket at 14T or above. Fewer teeth means less chain wrap, which increases the chance of skipping under load. 16T or higher is better.

In Onshape: Use the #ChainCTC_25() and #ChainCTC_35() functions from the Origin Cube featurescript to calculate exact center-to-center distances. Use the Belt and Chain Gen featurescript to model the chain in 3D.

Tensioning

This is the biggest practical difference between chain and belt. Chain stretches over time (technically the pins wear, which makes the effective length increase). A belt run you set up once stays tensioned. A chain run will loosen and needs a way to be retensioned.

Method	How it works	Notes
Slotted bearing blocks	The shaft sits in slotted holes. A cam (an eccentric bolt or lobe) pushes the shaft to the correct tension position, then you tighten the bearing block bolts to lock it in place. This is the method used on gearboxes like the WCP Rotation SS, where an eccentric cam built into the mount lets you dial in tension by rotating the cam before clamping down.	The most common and most reliable method. The cam makes tensioning repeatable rather than a guess-and-check process.
Inline turnbuckle & Spartan Tensioner	A variable-length link that replaces one section of the chain. Turn the turnbuckle to shorten or lengthen it.	REV sells a #25 chain turnbuckle. Good for runs where moving a shaft isn't practical.
Spring-loaded tensioner	A sprocket or roller on a spring that pushes against the slack side of the chain.	More complex, but automatically compensates as the chain wears. Hardly used in FRC (typically because the other 2 options work good enough).

Every chain run needs a tensioning strategy designed in from the start. If you design a chain run with fixed center-to-center and no adjustment, the chain will go slack after a few hours of use and you'll have no way to fix it without remaking parts.

Assembly and maintenance

Master links

A master link connects the two ends of the chain into a loop. It's a removable link with a clip plate that slides over the pins. Install the clip with the closed end facing the direction of chain travel so it doesn't get knocked off during operation. Master links are the most common failure point on a chain run, so double-check them before every competition match.

Alignment

Chain is more sensitive to sprocket alignment than belts are. Both sprockets need to be on parallel shafts with the teeth in the same plane. If the sprockets are even slightly offset, the chain will wear unevenly, make noise, and eventually derail. Check alignment visually by holding a straightedge against the faces of both sprockets and verifying they're in the same plane.

Common failures

Failure	Cause	Fix
Chain skip	Insufficient tension, worn sprockets, or small sprocket with poor chain wrap	Retension, replace worn parts, use a larger sprocket (16T minimum)
Derailment	Sprockets misaligned, chain too loose, or no chain guide	Align sprockets, retension, add chain guides on the slack side
Master link failure	Clip plate installed backwards or not fully seated	Install clip with closed end facing direction of travel, verify before every match
Excessive stretch	Normal wear over time, accelerated by high load and poor lubrication	Retension using your built-in adjustment. Replace chain if stretch exceeds what your tensioner can take up.