Torque & Speed

Open with a wrench analogy: torque is how hard you twist a bolt, speed is how fast your hand spins. A NEO free-spins ~5676 RPM but its raw torque is tiny — you can stop the shaft with your fingers. A drivetrain needs the opposite. Ask: 'What happens if you bolt a wheel straight to a motor?' Answer: it spins uselessly fast and stalls instantly under load. That's WHY gears exist.

This is the single biggest idea of the lesson. Gears are a TRADE, not a magic boost. If a student says 'just add gears to get more power,' correct it: gears move power along the torque-speed curve, they never create it. Use the seesaw image — push one side down, the other goes up. Real losses: each gear mesh is ~95-98% efficient, so a 3-stage reduction loses ~10%.

Demo the SpurGear add-in: Utilities > Add-Ins > SpurGear, or the built-in Fusion spur gear generator. Generate a 20DP 12T pinion and a 60T gear, set the same pressure angle (20deg) and module/DP so teeth mesh. Stress: ratio is teeth in : teeth out. Common mistake — students measure diameter instead of counting teeth, or mix DP between two gears so they don't mesh.

You can't always do a huge reduction in one mesh — a 50:1 single stage would need an enormous gear. So we stack. Multiply, don't add: 3:1 then 3:1 is 9:1, not 6:1. In Fusion, model each shaft as its own component and use a Rigid joint to lock the pinion to its gear on a shared shaft. Real example: a VersaPlanetary or 3-stage WCP gearbox.

Walk the math slowly. Free speed is the no-load number; real loaded speed is lower (~80% is a decent rule of thumb for drivetrain). Kraken x60 ~6000 RPM, NEO ~5676 RPM, NEO 550 ~11000 RPM. Point out: you NEVER reach free speed under load — that's why we also care about the stall/loaded end of the curve, which the calculators handle.

FRC is a units minefield: COTS gears are imperial (in-lb, 20DP), but motors are often spec'd in N-m. Show Document Settings > Units in the Fusion browser to set inch or mm. A classic error: computing drivetrain speed in RPM and forgetting to convert through wheel circumference to get ft/s. Wheel speed (ft/s) = RPM × wheel circumference (ft) ÷ 60.

These free tools (reca.lc and the JVN design calculator) turn the math we just did into a real design check. They also flag current draw — if your ratio pulls too many amps you'll brown out or trip a breaker. Workflow: pick a target (e.g. 15 ft/s drivetrain), let the calculator tell you the ratio, THEN build that ratio in Fusion. Calculate first, CAD second.

This is the Fusion equivalent of Onshape Variables. Modify > Change Parameters > '+' under User Parameters. Drive your SpurGear tooth counts from these params so changing drivenT from 60 to 48 re-cuts the gear automatically. Teach naming with no spaces. Common mistake: typing a raw number into a feature instead of referencing the parameter — then nothing updates downstream.