Chain and Sprockets

How it works

Chain transmits power between two sprockets the way a bicycle does. It tolerates misalignment, handles shock loads well, and is easy to repair. In FRC two sizes dominate, named by the ANSI standard:

• #25 chain — pitch (link spacing) 0.250“. Lighter and smaller; the standard for most drivetrains and mechanisms.
• #35 chain — pitch 0.375“. Fatter and harder to derail, but significantly heavier. Used where loads are high.

Ratios with sprockets

A sprocket ratio works exactly like gears: driven teeth ÷ driving teeth. A 12T driving a 24T sprocket is a 2:1 reduction.

Chain wrap and idlers

The chain must wrap far enough around the smaller sprocket to engage enough teeth. The common rule is at least ~120 degrees of wrap on the small sprocket; a sprocket ratio of about 3:1 or less guarantees this regardless of center distance , while higher ratios in a single run risk dropping below 120 degrees and skipping teeth. For larger reductions, add an idler sprocket to increase wrap, or split the reduction across two stages.

Tensioning

Chain stretches slightly with use and must be tensioned. Common methods:

• Sliding bearing blocks / tensioner cams (e.g., WCP cams) that move one shaft to take up slack.
• Idler sprockets/tensioners pressing on the slack span.
• Designing the center-to-center distance so a whole number of links fits snugly (using a chain-length calculator).

Master links

You join a chain into a loop with a master link (a removable connecting link). Carry spares: a thrown chain mid-match is a classic failure, and a master link lets you re-join quickly in the pit. Many teams instead use riveted or factory-pressed loops for runs they never plan to open.

Chain vs. belt vs. gears (quick guide)

• Chain: rugged, forgiving of alignment and distance, but heavier and can throw if loose.
• Belt (GT2/HTD): quiet, light, no lubrication, very efficient, but requires precise center distance and is less tolerant of debris.
• Gears: most compact and efficient for short distances, but require tight tolerances and the correct center distance.

Key takeaways

• #25 chain has 0.250“ pitch (lighter); #35 has 0.375“ pitch (heavier, harder to derail)
• Keep ~120 degrees of wrap on the small sprocket; a ~3:1 or smaller ratio guarantees it, otherwise add an idler
• Tension chain with sliding blocks, cams, or idlers, and carry spare master links for serviceable runs

This lesson was adapted from learnfrc.com.