Chain & Sprocket

Open by asking: how do you spin a wheel 6 inches from your motor? Gears would need a huge stack. Chain (or belt) bridges that gap. Tell them chain is the FRC default for power transmission over distance. Contrast with belts (HTD): chain handles more shock load and is rebuildable in the pit, belts are quieter and need no lube. We'll CAD chain today but the geometry logic is identical for belts.

Pass around a real chain loop if you have one. Point out the roller actually rolls into the tooth pocket — that's why it's 'roller chain.' The single most important number is PITCH. Everything (sprocket size, length, center distance) is built from pitch. Common mistake: students think more teeth = bigger pitch. No — pitch is fixed by the chain number.

Critical Fusion tip: do NOT try to model sprocket teeth by hand. Download the STEP or F3D from WCP, REV, or AndyMark and use Insert > Insert Derive / Insert McMaster-Carr. Teeth geometry is fiddly and you'll never get the involute right. The 1/2" hex bore is the FRC standard shaft — it self-aligns and transmits torque without keys. Show the difference between an 18-tooth and 24-tooth sprocket: same chain, different diameter.

Students confuse outer diameter with pitch diameter — the chain wraps the PITCH circle (through the roller centers), which is smaller than the tooth tips. The formula matters for spacing two sprockets. In practice you'll read PD off the vendor datasheet or measure it from the inserted STEP. Don't make them memorize the formula — make them understand WHY center distance is measured pitch-circle to pitch-circle.

Walk through the idea, not the heavy algebra: straight runs on top and bottom plus the wrap around each sprocket. The result must be a whole number of links, and standard chain joins prefer EVEN counts (a master link connects two outer plates). An odd count forces a half link (offset link), which is weaker. Use an online chain calculator or the vendor tool — but they must understand the rounding is what creates tension slack.

Master link (connecting link) is how you actually close a chain loop in the pit — it has a removable clip or press-fit plate. Half links let you fine-tune length by a single pitch but they're the failure point under load, so design your center distance to land on an even count. Tell the 7558 story: a half link snapping mid-match is a classic preventable failure. Design it out.

Two tensioning strategies: (1) move a sprocket via slotted holes — cheap and common for drivetrain; (2) push on the slack span with an idler or a tensioner block (REV and WCP sell these). Over-tight chain robs power and destroys bearings; too loose skips teeth and throws the chain. Demo modeling a slot in Fusion: sketch two arcs and two lines, or use a slot sketch tool, then extrude-cut. Bolt rides in the slot.

Chain-in-tube is the modern FRC drivetrain standard — the 2x1 tube becomes the structure AND the chain guide. The tube walls keep the chain from derailing and provide light tensioning. This is why our drivetrain rails are 2x1. Insert the WCP or TTB chain-in-tube assembly rather than building it. Point out the bearing blocks at each end that hold the dead axles and index the sprockets.